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2Some elements at Boulder, CO.
NIST Special Publication 972

Highly Charged Ions

Publications of the EBIT Project, 1993-2001

Atomic Physics Division, NIST, Gaithersburg, Maryland

Editors:

John D. Gillaspy

Laura P. Ratliff

James R. Roberts

Endre Takacs

Atomic Physics Division

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8420

July 2002

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Arden L. Bement, Director
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such

identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 972

Natl. Inst. Stand. Technol. Spec. Publ. 972, 319 pages (July 2002)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFHCE

WASHINGTON: 2002

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
reface

Assembled here are the publications of NIST Electron Beam Ion Trap (EBIT)

project, from its inception through the end of 2001. They are grouped thematically, as indicated in the table of contents. In order to address some of the most frequently asked

questions that are not fully answered in the papers themselves (When did this work

begin? How much did it cost? Who worked on it?) a historical introduction is included.

Other introductory material can be found on our web site at http://physics.nist.gov/ebit.

iii

ontents

Historical introduction to the NIST EBIT project - J.D. Gillaspy

Remembering Mort Levine and the beginning of the EBIT - R. E. Marrs

i

vii

eviews and overviews

J.D. Gillaspy, J.R. Roberts, CM., Brown, and U. Feldman 1 The NIST EBIT: A progress report

AIP Conference Proceedings 274 Proc. Vlth International Conference on

the Physics ofHighly Charged Ions, ed. by P. Richard, M. Stockli, and

CD. Lin, (AIP press. New York, 1993), p. 682.

J.D. Gillaspy, J.R. Roberts, CM., Brown, and U. Feldman 5

Update on the NIST EBIT

National Institute of Standards and Technology Special Publication 850,

Apr. 1993 (U.S. Government Printing Office, Washington, 1993).

J.D. Gillaspy

First results from the EBIT at NIST

Physica Scripta T7I, 99 (1997).

8

J.D. Gillaspy, Y. Aglitskiy, E.W. Bell, CM. Brown, CT. Chantler, 13

R.D. Deslattes, U. Feldman, L.T. Hudson, J.M. Laming, E.S. Meyer,

CA. Morgan, A.I. Pikin, J.R. Roberts, L.P. Ratliff, E.G. Serpa, J. Sugar,

and E. Takacs

Overview of the EBIT Program at NIST

Proceedings ofNobel Symposium 91, Physica Scripta T59, 392 (1995).

J.D. Gillaspy 17

Visible and UV light from highly charged ions: exotic matter

advancing technology

IEEE LEOS Newsletter, August 1995, p. 19.

J.D. Gillaspy 19

EBIT spectra of highly stripped ions from the visible to the x-ray

Physica Scripta T65, 168 (1996).

V
E. Takacs, L.P. Ratliff, and J.D. Gillaspy

X-ray, visible, and electron spectroscopy with the NIST EBIT

Hyperfine Interactions 108, 59 (1997).

26

E. Traebert, P. Beiersdorfer, S.B. Utter, J.D. Gillaspy, and P.O. Serpa 40

Forbidden line wavelengths and transition probabilities measured

using an electron beam ion trap (EBIT)

Proceedings of the 6th International Colloquium on Atomic Spectra and

Oscillator Strengths for Astrophysical and Laboratory Plasmas, accepted

for publication, University of Victoria, British Columbia (1998).

L.P. Ratliff and J.R. Roberts 44

Highly charged ion studies at the NIST EBIT

Trapping Highly Charged Ions: Fundamentals and Applications, ed. by

J.D. Gillaspy, Nova Science Publishers, 257 (2001).

J.D. Gillaspy 65

Highly charged ions

Journal of Physics B: At. Mol. Opt. Phys. 34 R93 (2001)

E. Takacs and J.D. Gillaspy 103

Trapped highly charged ion plasmas

Proceedings of the Non-neutral Plasmas Workshop 2001, ed. by C.F.

Driscoll, F. Anderegg, L. Schweikhard, AIP Conference Proceedings

Series (2001)

Itraviolet and visible radiation

C.A. Morgan, F.G. Serpa, E. Takacs, E.S. Meyer, J.D. Gillaspy, J. Sugar, 1 17

J.R. Roberts, CM. Brown, U. Feldman

Observation of visible and uv magnetic dipole transitions in highlycharged xenon and barium

Physical Review Letters 74, 1716 (1995).

H. Adler, E.S. Meyer, F.G. Serpa, E. Takacs, and J.D. Gillaspy 121

Fabry-Perot Spectroscopy of a visible magnetic dipole transition

T» 34+ m Ba

Proceedings of the 7th International Conference on Highly Charged Ions,

Nuclear Instruments and Methods in Physics Research B 98 585 ( 1995).

vi
F.G. Serpa, E.S. Meyer, C.A. Morgan, J.D. Gillaspy, J. Sugar, 125

J.R. Roberts, CM. Brown, U. Feldman

Anomalous Z-dependence of a magnetic dipole transition in the Tilike isoelectronic sequence

Physical Review A 53, 2220 (1996).

F.G. Serpa, E.W. Bell, E.S. Meyer, J.D. Gillaspy, and J.R. Roberts 130

Kr spectra from an electron beam ion trap

Physical Review A 55, 1832 (1996).

F.G. Serpa, C.A. Morgan, E.S. Meyer, J.D. Gillaspy, E. Traebert, D.A. 134

Church, and E. Takacs

Measurement of a magnetic dipole transition probability in highly

charged Xe using an electron beam ion trap (EBIT)

Physical Review A 55, 4196 (1997).

F.G. Serpa, J.D. Gillaspy, and E. Traebert 139

Lifetime measurements in the ground configuration of Ar"^ and Kr"*

using an electron beam ion trap

Journal of Physics B: At. Mol. Opt. Phys. 31, 3345 (1998).

J. V. Porto, I. Kink, and J. D. Gillaspy 147

UV light from the ground term of Ti-like Ytterbium, Tungsten, and

Bismuth

Physical Review A 61, 054501 (2000)

J. V. Porto, I. Kink, and J. D. Gillaspy 151

Direct imaging of highly charged ions in an electron beam ion trap

Review of Scientific Instruments 71, 3050 (2000).

-ray radiation

E. Takacs, E.S. Meyer, J.D. Gillaspy, J.R. Roberts, C.T. Chantler, 161

L.T. Hudson, R.D. Deslattes, CM. Brown, J.M. Laming, U. Feldman,

J. Dubau, M.K. Inal

Polarization measurements on a magnetic quadrupole line in Ne-like

barium

Physical Review A 54, 1342 (1996).

D.J. McLaughlin, Y. Hahn, E. Takacs, E.S. Meyer, J.D. Gillaspy 170

Radiative and inner-shell dielectronic recombination in a highly

charged barium ion

Physical Review A 54, 2040 (1996).

vii
Y. Aglitskiy, F.G, Serpa, E.S. Meyer, J.D. Gillaspy, and CM. Brown 180

The use of a spherically curved crystal spectrometer for x-ray

measurements on electron beam ion trap

Physica Scripta 58, 178 (1998).

C.T. Chantler, D. Paterson, L.T. Hudson, F.G. Serpa, J.D. Gillaspy, and 184

R.D. Deslattes

Progress towards absolute x-ray spectroscopy on the NIST electron

beam ion trap: current status and results

Physica Scripta T73, 87 (1997).

C. T. Chantler, D. Paterson, L. T. Hudson, F. G. Serpa, J.D. Gillaspy, and 187

E. Takacs

Precision x-ray spectroscopy at the NIST electron beam ion trap:

resolution of major systematic error

Physica Scripta T80, 440 (1999).

C. T. Chantler, D. Paterson, L. T. Hudson, F. G. Serpa, J. D. Gillaspy, and 190

E. Takacs

Absolute measurement of the resonance lines in heliumlike vanadium

on an electron-beam ion trap

Physical Review A 62, 042501 (2000).

E. Silver, H. Schnopper, S. Handler, S. Murray, N. Madden, D. Landis, 203

J. Beeman, E. Haller, M. Barbera, G. Tucker, J.D. Gillaspy, E. Takacs,

and J. Porto

Laboratory astrophysics and microanalysis with NTD-germaniumbased x-ray microcalorimeters

Nuclear Instruments and Methods in Physics Research A 444 156 (2000).

E. Silver, H. Schnopper, S. Bandler, N. Brickhouse, S. Murray, 208

M. Barbera, E. Takacs, J.D. Gillaspy, J.V. Porto, I. Kink, J.M. Laming,

N. Madden, D. Landis, J. Beeman, and E. Haller

Laboratory astrophysics survey of key diagnostic lines using a

microcalorimeter on an electron beam ion trap

The Astrophysical Journal 541, 495 (2000).

I. Kink, J.M. Laming, E. Takacs, J.V. Porto , J.D. Gillaspy, E. Silver, H. 214

Schnopper, S. Bandler, M. Barbera, N. Brickhouse, S. Murray,

N. Madden, D. Landis, J. Beeman, and E. Haller

Analysis of broadband x-ray spectra of highly charged krypton from

a microcalorimeter detector of an electron-beam ion trap

Physical Review E, 63, 046409 (2001 ).

viii
J.M. Laming, I. Kink, E. Takacs, J.V. Porto , J.E). Gillaspy, E. Silver, H. 224

Schnopper, S. Bandler, N. Brickhouse, S. Murray, M. Barbera, A. K.

Bhatia, G. A. Doschek, N. Madden, D. Landis, J. Beeman, and E. Haller

Emission-line intensity ratios in FeXVII observed with a microcalorimeter on an electron beam ion trap

The AstrophysicalJoumal 545, LI61 (2000).

I. Kink, J. M. Laming, E. Takacs, J. V. Porto, J. D. Gillaspy, E. Silver, H. 228

Schnopper, S. R. Bandler, M. Barbera, N. Brickhouse, S. Murray, N.

Madden, D. Landis, J. Beeman, and E. E. Haller

Microcalorimeter/EBIT measurements of x-ray spectra of highly

charged ions

Physica Scripta T92 454 (2001 ).

on-surface interactions

A.I. Pikin, C. A. Morgan, E.W. Bell, L.P. Ratliff, D.A. Church, and J.D. 23

1

Gillaspy

A beam line for highly charged ions

Review ofScientific Instruments 67, 2528 (1996).

L.P. Ratliff, E.W. Bell, D.C. Parks, A.I. Pikin, and J.D. Gillaspy 237

Continuous highly-charged ion beams from the NIST EBIT

Review of Scientific Instruments 68, 1998 (1997).

E. Takacs, Z. 6nodi-Sz-cs, L.P. Ratliff, J.D. Gillaspy, and J. Palinkas 242

X-ray emission for 3-137 keV Ar""^ impacting SiO^

Nuclear Instruments and Methods in Physics Research B 124, 431 ( 1997).

H.-P. Cheng and J.D. Gillaspy 246

Nano-scale modification of silicon surfaces via coulomb explosion

Physical Review B 15, 2628 (1997).

H.-P. Cheng and J.D. Gillaspy 255

Large scale molecular dynamics simulation of a surface Coulomb

explosion

AIP Conference Proceedings 392, Application ofAccelerators in

Research and Industry, ed. by J.L. Duggan and I.L. Morgan (AIP Press,

Woodbury, New York, 1997) p. 197.

H.-P. Cheng and J. D. Gillaspy 259

Surface Coulomb explosions: The influence of initial charge

distributions

Computational Materials Science 9, 285, ( 1998).

ix
D.C. Parks, M.P. Stockli, E.W. Bell, L.P. Ratliff, R.W. Schmieder, F.G. 271

Serpa, and J.D. Gillaspy

Non-kinetic damage on insulating materials by highly charged ion

bombardment

Nuclear Instruments and Methods in Physics Research B 134, 46 ( 1998).

J.D. Gillaspy, D.C. Parks, and L.P. Ratliff 278

Masked ion beam lithography with highly charged ions

Journal of Vacuum Science and Technology B 16, 3294 ( 1998).

L. P. Ratliff, R. Minniti, A. Bard, E. W. Bell, J. D. Gillaspy, D. Parks, A. 282

J. Black, and G. M. Whitesides

Exposure of self-assembled monolayers to highly charged ions and

metastable atoms

Applied Physics Letters 75, 590 (1999).

E. Takacs, Z. Berenyi, J. D. Gillaspy, L. P. Ratliff, R. Minniti, J. Pedulla, 285

R. D. Deslattes, N. Stolterfoht

Separation of inner shell vacancy transfer mechanisms in collisions of

1 7-4- slow Ar ions with SiOi

Journal ofPhysics B: At. Mol. Phys. 34, 1277 (2001).

L. P. Ratliff, J. D. Gillaspy 296

In situ imaging of highly charged ion irradiated mica

AlP conference proceedings, XVIth International Conference, ed. by J.L. Duggan and l.L. Morgan, (AlP press. New York, 2001), p. 935.

R. Minniti, L. P. Ratliff, J. D. Gillaspy 300

In-situ observation of surface modiHcation induced by highly charged

ion bombardment

PhysicaScriptaT92,22(2001)

X
istorical introduction to tine NIST EBIT project

The birth of the NIST EBIT may be traced to a discussion between Jim Roberts

and Dick Deslattes prior to my arrival in 1988. Jim had been searching for new research

directions for his Plasma Radiation group, and was encouraged by Dick to consider a new

device called an "Electron Beam Ion Trap" (EBIT) that had just been developed at Berkeley and Livermore (see remarks by Ross Marrs, following this introduction). Jim

traveled to California to see the original EBIT, and became convinced that NIST would

benefit from being one of the first to embrace this new technology. On the day I arrived

at NIST to join Jim's group, I was swept into a meeting in which he was already

discussing the proposal to build an EBIT at NIST.

As plans developed, Jim teamed up with Josh Silver at the University of Oxford

and Uri Feldman at the U.S. Naval Research Laboratory to have the components for two

identical new EBITs designed and machined simultaneously. Mort Levine, recently

retired from Lawrence Berkeley Laboratory, went to Oxford to lead the design work. In July of 1990, a contract was signed for $165k for one set of "EBIT parts" (including $35k

for materials) to be fabricated by the Oxford Nuclear Physics Machine Shop and

delivered to NIST. The contract was supplemented with a $13k addition near the end of

the 1-year fabrication period. Meanwhile, Jim specified the purchase of approximately

$100k in supporting electronics, vacuum pumps, and cryogenics, to be purchased in

collaboration with the Naval Research Laboratory.

After completing my postdoc work on another project in Jim's group, he hired me to lead the EBIT project in 1991. Doug Alderson began to work part-time on the project as the group technician, helping me prepare the laboratory space. In April of 1992 the box

of EBIT parts arrived. Charlie Brown, one of Uri Feldman's staff members, traveled to NIST periodically to work with me on the assembly. Eventually, we purchased another

$50k or so of additional hardware, to bring the EBIT into operation for a grand total of

approximately $330k.

Our vision was to create a facility that would last many years and that would be

flexible enough to accommodate a wide range of experiments without major revision, so Charlie and I took the better part of a year to carefully assemble and align the

components. Special attention was paid to cleanliness because we did not want to bake

the vacuum system and damage some of the internal magnet systems. The extra effort

paid off: we obtained an outgassing rate comparable to that of baked stainless steel, and

we have never had to disassemble the core of the EBIT for any reason since. We have

only opened the vacuum chamber occasionally to replace windows or to insert a fresh

electron gun. One of the last views of the inside of the NIST EBIT is shown in the

photograph in figure 1. Approximately 150 machined components make up the 6 major

EBIT subsections that are shown.

xi
Shortly after the system was assembled, the first NIST EBIT postdoc, Gino Serpa,

arrived from Notre Dame and began helping with the external wiring. A few months

later, Endre Takacs came as a guest researcher from Keith Burnett's group at Oxford and

began helping with the high voltage connections. Martin Laming visited from NRL to help install and debug some of the CAMAC electronics. Cornelius Morgan joined us from the University of Maryland, shortly after finishing his PhD, and began helping with

the interlock systems. Our first trapped ions were obtained later in 1993 and our first major scientific results were submitted for publication in 1994.

Around this time, a major expansion of our EBIT work developed. Eric Meyer

arrived from Harvard as our first NRC postdoctoral associate. Helmar Adler, a visitor

from the Princeton Plasma Physics Laboratory (and former postdoc in another project in our group) carried out some experiments with Eric using a Fabry-Perot interferometer.

Collaborations with Germany, Russia, Australia, Hungary and groups within NIST

developed, as discussed below. At the same time, however, our collaborating scientists

from NRL began to focus their work in other areas and became less involved with the

EBIT. Two NRL technicians, Glenn Holland and Greg Boyer, continued to work on the

construction of a metal ion injector for our EBIT, however. Recently, NRL participation

has increased, as Martin Laming has become active in the planning and data analysis for

the microcalorimeter collaboration discussed below.

In 1994, Elmar Trabert began a series of visits from Germany to carry out

measurements of excited state lifetimes. Towards the end of this work (1998), we also

did the first spatial imaging of the ion cloud inside our EBIT. Support for this

collaboration was from a NATO International Scientific Exchange Grant and, later, the

Alexander von Humboldt Foundation. Trey Porto arrived at the end of this period (1998)

as an NRC postdoc and took the lead in developing the ion cloud imaging work.

Also 1994, Yefim Aglitskiy joined us half-time as a senior guest researcher from

Russia. Yefim's colleague, Anatoly Faenov, also came for a short visit to do an x-ray

spectroscopy experiment. The bulk of our x-ray spectroscopy, however, was done in

collaboration with Dick Deslattes' group at NIST (which included Larry Hudson, Al

Henins, Joe Pedulla, and Chris Chantler). After taking a position as a lecturer at the

University of Melbourne in Australia, Chris returned periodically to carry out various x- ray experiments with us. We are currently co-PI's on a grant supporting this work from

the Australian Research Council.

With a "competence" funding award in 1994 from the office of the NIST Director

(Ray Kammer, retired), our research was expanded to address the emerging field of

nanotechnology. Laura Ratliff was hired as the second permanent NIST EBIT staff

physicist that year. In 1995 we extracted beams of highly charged ions from the top of our EBIT. Bob Schmieder, from Sandia National Lab, loaned us an air-operated atomic

force microscope (AFM), and encouraged his postdoc, Dan Parks to join us as an NRC

associate. Cornelius Morgan began the design of an ion beamline, and Alexander Pikin

later joined us from Russia (by way of Stockholm) to lead that effort. Ed Bell arrived as a JILA postdoc to help put the beamline designs into practice. By 1996, our 8 meter

xii
beamline was fully operational and producing record fluences of highly charged ions for

experiments on surfaces. Joe McDonald and Joachim Steiger visited from Livermore to verify the beamline performance with an independent set of diagnostic hardware. The

following year, Christiana Ruehlicke visited from Livermore, to carry out some

experiments on protein fragmentation that required our very high beam fluences.

Some of our early ion-surface work was done in collaboration with Skip Berry and

Stacy Mogren at the National Security Agency, using the analytical instruments that they

had available at the Microelectronics Research Laboratory in Columbia, Maryland. By

1997, we had submitted our first ion-surface results for publication, and we purchased

our own UHV AFM and scanning tunneling microscope. By 1998, this microscope was

coupled to the EBIT beamline, and a new NRC postdoc, Ronnie Minniti, had developed a vibration isolation system and begun experiments with the microscope. By the following

year, we submitted for publication the first atomic-scale images obtained in-vacuo of

features induced by highly charged ions.

Between 1995 and 1998, Endre Takacs was back in Hungary, but a U.S. -Hungarian

Joint Fund grant made it possible for him to make periodic visits to NIST and bring, at various times, several of his students (Attila Bader, Zoltan Onodi-Szucs, Csilla Szabo,

Zoltan Berenyi, and Gabor Kusper). In 1999, Endre returned full time on a contract with

MIT. It was during this later period that Csilla Szabo joined us again on a Fulbright

Fellowship.

Jim Roberts retired from NIST in 1999, and I assumed his position as group leader.

In that year, we also began to deploy the EBIT to support the x-ray astronomy missions

of NASA, in collaboration with Eric Silver and Herb Schnopper from the HarvardSmithsonian Center for Astrophysics and Marco Barbera from the Observatory of

Palermo (Italy). Simon Bandler was the key postdoc from Harvard in this effort, and

Frank Defreze was the supporting technician. Ilmar Kink joined us during this time on a

postdoctoral fellowship from Sweden and worked together with Endre Takacs on this and

various other experiments. The astrophysics work is currently supported by a NASA

grant, for which we are co-investigators. Jim Roberts continues to work part time on the

development of EBIT instrumentation.

Our most recent NRC postdoc, Jason Sanabia, joined us in 2001 from the University

of Maryland and is working on ion-surface studies. Also this year, Hiro Tawara came

from Japan (by way of Kansas State) as a senior guest researcher.

In addition to the people mentioned above, research has been carried out at the

NIST EBIT by several other PhD students, Andrew Black (Harvard University) and

David Patterson (University of Melbourne), undergraduates, David Fanning

(Massachusetts Institute of Technology), Mark Scafonas (Saint Joseph's University),

Cornelius Jackson (Norfolk State University), Chad Riland (University of Maryland),

Vivik Goel (Rice University), and Lashondria Dixon (Oklahoma State University and

Southern University), and local high school students (Jessica Thomas, Brent Cezairliyan,

Melissa Hao, Sam Jahanmir, Daniel Bates, and Gregory Vieira).

xiii
A number of additional people have repeatedly worked short-term on various aspects

of the our EBIT program, including several NIST staff members (Jack Sugar, Yong-Ki

Kim), outside visitors working in the lab (Ed Magee, Dave Knapp, John Seely, George

Doschek, Andreas Bard, Gene Livingston, Curt Riemann, Jan Lorincik, David Church,

Thomas Stoelker, Reinhold Schuch, Tony Calamai) and providing theoretical support

(Hai-Ping Cheng, Nico Stolterfoht, Jacques Dubau). Others, participating remotely, are reflected in the co-author lists of the reprints contained in this document.

No list of EBIT team players would be complete without including the local

management who have supported this work and provided us with their encouragement

over the years: Wolfgang Wiese, Chief of the Atomic Physics Division at NIST, and

Katharine Gebbie and Bill Ott, Director and Deputy Director of the Physics Laboratory at NIST.

My personal thanks go to all of you for daring to join in the EBIT adventure.

John Gillaspy,

Gaithersburg, MD; November, 2001

xiv
XV

RI Xemembering Mort Levine and the beginnings of the

EBIT

Mort Levine died in February 2001. During the period 1985 - 1986 Mort and I worked together on the development of the first electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory. At the time, Mort was an employee of

Lawrence Berkeley National Laboratory collaborating with several physicists at LLNL,

including myself. This was the heyday of Star Wars and x-ray lasers at Livermore, and it became clear that more accurate measurements of energy levels and electron-ion collision cross sections in highly charged ions were necessary for the successful design of x-ray

lasers. In this environment we knew that funding and management support would be

available at Livermore for the development of tools for such measurements. We were

aware of the production of very high ion charge states in an electron beam ion source (EBIS) in Russia, and the observation of x-rays from electron-ion collisions inside an EBIS in France. (We even had a private joke about exploiting the Russian "highlycharged-ion gap" to obtain funding.)

The conception of the EBIT was preceded by an attempt to use an EBIS at Berkeley for the study of highly charged ions. The Berkeley EBIS had been built as a cyclotron ion source and then abandoned, and was available at the time. It was a room

temperature device that was unable to produce very high charge states. The solenoid

magnet was made from split coils, which made it possible to look into the electron beam

from the side. We started by studying the EBIS rather than using it. We built an imaging

mirror for soft x-rays in an attempt to measure the size of the trapped ion distribution.

We also measured rf emission with a probe inside the vacuum tank, and recorded

extracted ions. Mort led these measurements, and Bob Schmieder (from Sandia at Livermore) and I drove over to Berkeley to participate. Mort had the most experience in

plasma physics, and it seemed to me that he already knew that the performance of this

EBIS would be degraded by plasma instabilities and that he was just looking for the

evidence, which soon arrived. The lesson from this experience was that we needed to find a new tool for studying highly charged ions.

The experience with the Berkeley EBIS was our training ground. It gave us the

knowledge and confidence to build a new type of machine. Bob Schmieder began the

construction of a cryogenic EBIS at Sandia, and Mort and I began the construction of the

EBIT at LLNL, which provided the money. The EBIT is the marriage of a cryogenic

Penning trap with an EBIS electron beam. It was intended to be a small x-ray source for

spectroscopy of trapped ions. It was not intended to be a source of ion beams, although it was later used for that purpose. We started the design by running computer calculations

of magnetic fields and the propagation of space-charge-dominated electron beams. Mort

found an excellent cryogenic engineer at Berkeley who worked with us and helped turn our "physics design" into a mechanical design. As a result, most of the EBIT parts were

fabricated in the Berkeley shops and delivered to Livermore for final assembly.

xvii
Our experiences on the first day of EBIT operation are an example of the

interaction of careful planning and the unexpected. We knew that the weak x-ray signal

from trapped ions would be overwhelmed by bremsstrahlung if even a tiny fraction of the

electron beam (or secondary electrons) struck solid material in the field of view of the x- ray detectors. Hence the drift tubes were designed with a larger diameter in the central

section that is viewed by x-ray detectors. On the first day of operation we turned on the

electron beam with no ion-trapping voltage applied to the drift tubes and looked for x-ray

background with a Si(Li) detector, hoping that it would be small. We were relieved to find only a small x-ray count rate as we made adjustments to maximize electron beam

transmission. Later, as we examined the x-ray spectrum, we were at first surprised to find x-ray lines at an energy greater than the electron beam energy, then excited as we

realized the explanation. These lines turned out to be dielectronic recombination lines

from highly charged barium ions. The barium came from the cathode of the electron gun,

and the ions were trapped in an axial well generated by the space charge of the electron

beam in our drift tube geometry. So we made our first highly charged ions by accident on the first day of operation, which the laboratory notebook records as 29 October 1986,

arguably the "birth" date of the EBIT. As I remember it, Mort and I walked out to the

parking lot together at the end of the day; as the evidence of our success began to sink in,

Mort turned to me and said "we did it," and so we had. Highly ionized barium, first made that day by serendipity, soon became the object of the first of many atomic physics

measurements.

Mort continued working on the Livermore EBIT for several years after its initial operation. The machine that we first operated was a bare-bones version of what the

facility later became as our understanding of the operation and capabilities of the EBIT

grew and as more people joined the program and came from elsewhere as users. As we

understood problems, such as high voltage breakdown caused by electrons trapped in

magnet fringing fields, we modified parts to fix them. We built x-ray spectrometers to measure transition energies and invented ways to measure ionization cross-sections. One

of the key EBIT concepts is the role of evaporative ion cooling, which Mort recognized

and championed. We eventually developed an adjustable light gas cooling system, which

is routinely used at Livermore to optimize the ionization balance and x-ray count rate. Without it, the production of the highest ion charge states would not be possible. After

his retirement from LBNL and the EBIT program at Livermore, Mort served as a consultant for the design of the twin EBITs at Oxford and NIST.

Mort Levine was a colorful, enthusiastic, and unconventional person filled with

many ideas and proposals, not all of which were accepted by his colleagues. However

Mort challenged and stimulated those of us who worked with him. We discussed

everything. It was an environment in which we could explore different approaches and

change course if things didn't work. That is how the EBIT was developed, and it was

fun.

Ross Marrs

Livermore, CA; October, 2001

xviii
Reviews and overviews

THE NIST EBIT: A PROGRESS REPORT

J.D. Gillaspy* and J R. Roberts

National Institute of Standards and Technology

Atomic Physics Division, Gaithersburg. MD 20899

and

CM. Brown and U. Feldman

E. O. Hulburt Center for Space Research

Naval Research Laboratory, Washington, D. C. 20375

The invention of the Electron Beam Ion Trap (EBIT) at Livermore^ has

marked the emergence of a versatile new source for the spectroscopic study of

highly charged ions. An EBIT can produce virtually any highly charged state of

any atom on the periodic table, trap the ions in a controlled environment virtually

free from inherent sources of systematic error, and selectively excite transitions

with a well-defined monochromatic electron beam. During the past five years, the

Livermore research program has successfully demonstrated many of the

capabilities of the EBIT.^ NIST and NRL are now in the process of constructing

an EBIT facility on the east coast, at the NIST Gaithersburg site. In addition to

benefiting our own research, the facility will also expand the availability of EBITbased experiments for the scientific community in general.

The design of the NIST EBIT was created by Mort Levine and is based on

the Livermore prototype which he helped develop. Notable changes include the

addition of several field penetrators to drain electrons from peripheral Penning

traps which were unintentionally formed by stray fields from the primary trap. The

buildup of electrons in these peripheral traps is believed to have nucleated

electrical breakdown and thus limited the ionization stage in the prototype EBIT at Livermore. The electrodes between the electron gun and the drift tubes have also

been modified, as have the high voltage feed lines to the drift tubes which now

enter from below rather than from above.

A detailed schematic diagram of the NIST EBIT is shown in figures 1 and

2. Projected operating parameters are given in Table 1. Currently, all primary

components are on hand and assembly is in progress. We will bring the machine

on-line during the coming year.

'Bitnet: Gillaspy@NBSenh, Internet: Gillaspy@enh.NIST.gov

682 © 1993 American Institute of Physics

NIST-1
J. D. Gillaspy et al 683

Figure 1. Cross-sectional side view of the NIST EBIT. Numbered components

include electron gun assembly (1), high-voltage feed for drift tubes (2), transition

electrode (3), superconducting magnet coil former (4), liquid nitrogen shield (5),

collector magnet (6), and einzel lens (8). Overall height is approximately 1 meter.

NIST-2
684 TheNISTEBIT

\

\

i s

j rM.iOT«in vote •<»•

»0*1 AT iOVC* LC^L

com awi niK «o ct-»-n

Figure 2. Cross-sectional view of the NiST EBiT, in the plane of the trap region.

The distance t)etween outer large flange faces is 27 cm. Seven of the ports will be equipped with a variety of windows to allow external observation from the

visible to the x-ray regions of the spectrum.

Table 1 . Projected characteristics of the NIST EBIT

Electron Beam Energy

1 . 700-50.000 eV

Electron Beam Resolution 50 eV

Electron Beam Cun-ent 10-200 mA

Electron Beam & Trap Radius 30 microns

Trap Length 2 cm

Number of Trapped Ions 40.000

Ion Density 7x10" cm ^*

Electron Density 4x10'* cm-^

Current Density 5800 A/cm*

Vacuum <1x10"torr

Maximum Trapping Time 5 hours

NIST-3
J. D. Gillaspy et al. 685

REFERENCES

1. M.A. Levine, R.E. Marrs, J.R. Henderson, D.A. Knapp, and M.B. Schneider,

Phys. Scr., T22, 157 (1988).

2. An essentially complete collection of the publications from the Livermore

EBIT program through April 1992 is available as report UCRL-ID-1 10491

from the National Technical Information Service, U.S. Department of

Commerce, 5285 Port Royal Rd., Springfield, VA 22161.

NIST-4
UPDATE ON THE NIST EBIT

J.D. Gillaspy* and J.R. Roberts

National Institute of Standards and Technology

Atomic Physics Division, Gaithersburg, MD 20899

and

CM. Brown and U. Feldman

E. O. Hulburt Center for Space Research

Naval Research Laboratory, Washington, D. C. 20375

I. Introduction to EBIT.

A decade ago, the Electron Beam Ion Source (EBIS) was hailed as the most

advanced method for producing highly charged ions [1]. A few years later, the

EBIS concept was reevaluated and optimized for use as a spectroscopic source.

The new device, the Electron Beam Ion Trap (EBIT) [2], produced a high density

of ions in a relatively small volume with radial access ports. With this new

geometry, plasma instabilities were removed and very high electron beam densities

were achieved. These improvements boosted both the spectral brightness and the

maximum attainable charge state, and furnished the atomic spectroscopy

community with an important new laboratory source.

The spectra produced with an EBIT are very pure, consisting of lines from

only a few charge states of essentially a single atomic species. The choice of

charge state can be continuously varied by adjusting the electron beam energy.

The ability to literally "dial up" a particular charge state greatly simplifies the

identification of spectral lines. Virtually any highly charged state of any atom on

the periodic table is accessible in this way.

In addition to operating the EBIT in a static mode where the electron beam

energy is held constant and a spectrum is collected in order to determine accurate

wavelengths, one can also use the precise and rapid control of the electron beam

energy to carry out temporal studies of the emitted photons. This feature allows

one to directly determine excited state lifetimes and to map out dielectronic

recombination cross-sections.

A summary of the advantages of EBIT over conventional sources include:

1 . Relatively few charge states.

2. No Doppler shifts

3. Insignificant Doppler broadening.

4. Insignificant density effects.

5. Very high charge states attainable.

6. Control over dielectronic satellites.

7. Slit-like source dimensions (60 urn x 2 cm) ideally

suited to spectrometers.

8. Very little background signal.

NIST-5 145
1A6

1

9. High accuracy (21 ppm) wavelength measurements

demonstrated.

10. Ultrahigh vacuum (10"^° Pa) maintained by cryogenic

environment.

11. No cascade contamination in lifetime studies.

Although the spectral brightness of an EBIT is much weaker than that from

powerful sources such as Tokamaks, the photon flux from an EBIT is nevertheless

quite adequate for spectroscopic work when reasonable effort is taken to make

use of efficient modern detection methods. A typical x-ray spectrum recorded with

a solid state detector on an EBIT may only take a few seconds to collect. Very

high precision studies of weak lines using crystal detectors may require many

hours of data collection, however. Even in such cases the brightness is not

usually of overriding concern since the source is under local control and operates

reliably for long periods of time. Because the source is run by computer control,

ions can be dumped from the trap and reloaded automatically at periodic intervals.

II. Present status of superconducting EBIT facilities.

The only full-scale EBIT devices in operation at this time are those at

Livermore. The mechanical components for the NIST EBIT were made in Oxford

England where an identical device is being put into operation [3]. The NIST EBIT

will go into operation in Gaithersburg, Maryland during the coming year through

a collaborative effort with the Naval Research Laboratory. An EBIT project is

reportedly underway in Japan, although information about this project is scarce.

A major EBIT project has been underway in the former Soviet Union at Dubna.*

Charge states as high as fully stripped Uranium are being actively pursued

at Livermore where the prototype EBIT has been upgraded to operate above 150

keV. The NIST EBIT will begin operations under 50 keV, with possible upgrades

in the future. Even at 50 keV, most of the states of any ion can be accessed.

The design of the NIST EBIT was developed by M. Levine and is based on

his highly successful original work with the Livermore prototype. Detailed

drawings, a discussion of the improvements that have been made, and a table of

the projected operating conditions will be published in the proceedings of the Vlth

International Conference on the Physics of Highly-Charged Ions (HCI-92).

"Bitnet: Gillaspy@NBSenh, Internet: Gillaspy@enh.NIST.gov

[1]. H. Winter, The Production of Multiply Charged Ions for Atomic Physics

Experiments" in Atomic Physics of Highly Charged Ions, R. Marrus (Ed.),

1983.

NIST-6
J. D. Gillaspy ec al. 147

MA Levine, R.E. Marrs, J.R. Henderson, D.A. Knapp, and M.B. Schneider,

The Electron Beam Ion Trap: A New Instrument for Atomic Physics

Measurements," Phys, Scr., 122, 157 (1988).

J. Silver, private communication.

Y. Aglitskiy, private communication.

NIST-7
Physica Scripta. Vol. T71, 99-103, 1997.

First Results from the EBIT at NIST

J.D. Gillaspy'

Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Received July 25, 1994; accepted August 2. 1996

Abstract

The Electron Beam Ion Trap (EBIT) facility at the National Institute of Standards and Technology (NIST) is now operational and producing atomic

physics data. This report gives an overview of the design, operation, and

performance of our EBIT. A preview of some of the first atomic physics

results is given to illustrate the capabilities of the facility.

1 . Introduction and background

The remarkable success of the Livermore group in constructing the first EBIT [1] and quickly obtaining a wide range of

significant atomic physics results has stimulated a number

of other laboratories to begin similar programs. Groups at the Clarendon Laboratory at Oxford, the Naval Research

Laboratory in Washington D.C., and the NIST Physics Laboratory in Gaithersburg Maryland, were among the first to

build EBITs following the initial Livermore success. In the summer of 1990, NIST contracted the Physics Department

at Oxford to fabricate a duplicate set of EBIT components

in parallel with the set for the Clarendon lab. M. Levine was

brought in as a consultant to design a slightly modified

version of the original Livermore EBIT and two sets of mechanical parts were produced under the direction of A. Holmes in the Nuclear Physics machine shop at Oxford. By

the first quarter of 1992, one set of components was delivered

to J.D. Silver's group at the Clarendon Lab and the other

set of components was shipped to NIST where final assembly

and integration with electronic components provided by NRL

was begun by the NIST-NRL collaboration [2]. By the end

of August 1993, the NIST-NRL EBIT was running with

strong beam currents and producing clean X-ray spectra from

neon-like barium, thereby demonstrating that the Livermore

EBIT technology is readily transferable to other groups with no prior expertise with EBIT/EBIS devices. Our EBIT has

subsequently been put through about a dozen test and ex- perimental runs, some of the results of which are surveyed

in the sections below. Parallel progress by the Oxford group

has been described elsewhere [3]. A program to build another cryogenic EBIT was carried out in Russia [4]. Plans for additional large EBITs are un- derway in Japan [5], Germany [6], and perhaps Connecticut

[7] and China as well. A number of small-scale "warm"

EBITs, employing non-superconducting magnets, have also been constructed at various sites around the world including

Russia [8], Japan [9], and France [10].

Work performed with: Y. Aglitskiy, E. Bell, CM. Brown, C. Chantler, R.D. Deslattes, U. Feldman, L. Hudson, J.M. Laming, E.S. Meyer, C.A. Morgan, J.R. Roberts, F.G. Serpa, J. Sugar, E. Takacs. The author's

internet address is: Gillaspy@enh.nist.gov

2. Design and operation

Detailed scale drawings of our EBIT have been published in an early progress report [2] where the differences with the

original EBIT are pointed out. Considerable room for im- provement in the cryogenic design remains since the device continues to consume over 3 1/h of liquid helium. A schematic

(not to scale) electrical diagram is shown in figure 1. The

system of electrodes can be conceptually grouped into three

sets: the electron gun, the drift tubes (DTs), and the collector assembly. The electron gun is a commercial dispenser-type

unit with a 3 mm diameter curved cathode surface. Additional information and a diagram showing the internal structure of

the electron gun can be found in ref. [1 1]. The DTs are shown

in a detailed scale drawing in figure 2. They maintain thermal

contact with the liquid helium bath through an annular sap- phire insulator mounted in a compression fitting at the top

of the DT assembly. The lower annular insulator is made of

alumina. The shape of the outer shield is designed with two

considerations in mind: reduce the electrostatic stress on the

insulators, and reduce the possibility of having unintentionally trapped Penning electrons outside the DTs, a condition which might trigger high voltage breakdown. Although the

electron gun and collector assemblies are presently referenced

to earth ground through external connections, the internal mounting is entirely insulated from case ground, so the gun

Collector Magnet

Extractor

*

I I CollectorI u

I I I—Suppreaaor- u

>\, ^ Shield —

Upper Drift Tube —

•Center Drill Tube —

-Lower Drdt Tuba —

rVl-S/C Magnet —

-Snout- -Anoda

I \-!, Cathode D _lua>^^ Flllmant ^—Bucking CoU —

Fig. I Schematic diagram of EBIT electrodes and magnets.

Physica Scripta T71

NIST-8
100 J.D. Gillaspy

7

1

^^.^ ^"^^^

f/g. 2 Detailed scale drawing of EBIT drift tube assembly with high voltage

insulators attached. The overall height of the assembly is 25.5 cm.

can be tied to the collector and floated down to negative

high voltage in order to obtain greater electron beam energies

in the future. The transition electrode helps guide the electrons through a small hole in the grounded cryogenic shield between

the gun and the DT assembly. The primary magnet which

surrounds the DTs produces a highly uniform 3 T field at 164 A. The collector and electron gun assemblies are sur- rounded by separate magnets which can be adjusted to cancel out the stray field from the superconducting magnet. The

collector magnet is cooled by Hquid nitrogen and the electron gun magnet is cooled by a chilled perfluorinated liquid which

can withstand high voltages. The snout is a copper plated

steel plate which helps shape the magnetic field lines near

the electron gun. During initial operation, spectra were obtained with all drift tubes connected to a single high voltage power supply,

in which case the axial trap is formed solely by the variation

in space charge potential between the DT and the electron beam due to the different inner diameters of the drift tubes. This provides a modest axial trapping potential of about 50

volts for 100 mA of beam current at 5 keV (the trap depth

varies linearly with the beam current and inversely with the square root of the beam energy in this case). In order to increase the trap depth further, the two end drift tubes (end

caps) are tied together and run a few hundred volts further from ground than the center drift tube. The X-ray signal

strength as a function of end cap voltage is shown in figure

3. Although the DT shield is presently tied to the center DT, in future ion-extraction or injection modes of operation

it will be operated a few hundred volts closer to ground in order to fully dump the trap. At present we have connected

the snout electrode to the anode, although somewhat im- proved performance might be achieved if they were separately adjustable.

Tuning the EBIT is required in order to obtain optimal

beam current and maximize the X-ray signal strength. Tuning is initiated by setting the electrode voltages and magnet

300 ^ g 250^

(A '^200-

150-

0)c 100-

2 50-

I ^ 0

100 200 300 400

End Cap Potential [V]

500

Fig. 3 Typical X-ray count rate as a function of end cap voltage. Data shown

is the total count rate (number of counts in the peak channel multiplied by the number of channels corresponding to our detector-hmited FWHM of 300 eV)

for the strongest neon-like barium line (centered at 4.9 keV) as observed with a 0.13 cm^ SiLi detector located 18 cm from the trap center. The EBIT was

running at 125 mA electron beam current at 5 keV energy.

currents to the typical operating values shown in table I, and

then slowly raising the anode voltage while monitoring the increasing beam current on the collector. The snout current, the key indicator of beam tuning quality, is constantly monitored and kept below 20 |iA. The anode voltage is interlocked

to shut down (trip) automatically if the snout current rises above 80 (lA. Particularly during tuning, but even during

normal operation, there are occasional unexplained bursts of current which lead to automatic shutdown. The snout is capacitively coupled into an oscilloscope which monitors oc- casional pulses of current which are below the trip threshold, but which seem to be correlated with trip frequency. With

optimal tuning, the trip frequency can be of order 1/hour, but since the machine can be reset within a few seconds of

a trip, reasonable operating environment can be obtained even with trip frequencies as high as 1/minute. When the cw

snout current approaches 20 ^A, the EBIT is fine-tuned by

adjusting the electrodes and magnets (primarily the transition voltage, bucking coil current, suppresser voltage, and focus). This fine-tuning lowers the snout current, after which the anode voltage can be raised to increase the beam current

further. The procedure is iterated until no further gain is obtained. Once the proper settings for maximum beam cur- rent are obtained, the system can be varied from low to high

currents without retuning. The experience at Livermore is that sets of horizontally oriented external shim coils around

the DT region can be of critical importance for tuning, but

Table I. Typical electrode voltages and

magnet currents for production of neonlike barium spectra. Bucking Coil 18 A

Filament 6.3 V

Focus -5 V

Transition 5.6 kV

Superconducting Magnet 164 A

Center Drift Tube 8 kV

Upper Drift Tube 8.5 kV

Lower Drift Tube 8.5 kV

Shield 8 kV

Suppresser 680 V

Collector 2 kV

Extractor -1 kV

Collector Magnet 7 A

Physica Scripta T71

NIST-9
First Results from the EBIT at NIST 101

% 200

20 40 60 80

Electron Beam Current [mA]

100

Fig. 4 Typical X-ray count rate as a function of electron beam current Conditions are the same as in figure 3, except for the end caps which were held

at 250 V. The straight line is a guide to the eye.

we find that when our EBIT is optimally aligned, the use of

shim coils is unnecessary (minimum snout current obtained

at zero shim current). This alignment is achieved by fine tuning the position of the DT assembly with the beam on, using a set of spring-loaded guide wires which are externally

adjustable. The importance of successful tuning for spectros- copy is clear from the plot of photon count rate versus current shown in figure 4. The present ranges of operating parameters are listed in table II. At low energies, as the DT voltage approaches the anode voltage, it becomes difficult to maintain snout currents below 20 |iA, and the beam current must be reduced. Figure

5 shows a summary of typical working beam currents obtained during a number of different days. The envelope of

these currents is observed to roll ofT below a few keV.

It is well-known that spectroscopic signals from heavy

ions (like barium) can be significantly enhanced by injecting

lower mass elements (like nitrogen) into the EBIT in order

to promote evaporative cooling [12]. A fortuitous small

leak between our EBIT vacuum chamber and the liquid

nitrogen lines to the collector walls seems to have provided

nearly the ideal level of gas injection to cool our trapped

barium ions. Later injection of additional nitrogen gas from a small atomic beam directed in through one of the

side ports allowed only a modest additional increase in signal strength.

Additional introductory information about EBITs in general can be found in references [1, 13-14].

3. First Atomic Physics Results

Three atomic physics experiments are presently producing

results from our EBIT in parallel: precision X-ray wavelength

measurements, study of polarization of emitted X-rays, and

observation and study of visible and near-uv light from magnetic dipole transitions within the ground term. The first two

experiments employ Bragg crystal spectrometers in the Johann geometry [15] to study X-rays emitted from neon-like barium. The third experiment uses a scanning grating monochomater to study Hght from titanium-like and vanadium-like

barium and xenon. In addition, we have a high-efficiency

solid state (SiLi) detector monitoring the broadband X-ray

emission from EBIT during all experiments. Examples of the

spectra produced are shown in figures 6 and 7. An overview

and discussion of the significance of our present work is

^ 160- <£ 140-

120-

100-

80-

60-

40-

20-

0-

c

d> o

E

raa> m

c

o

u

HI 23456789 Electron Beam Energy [keV]

—

r

10

Fig. 5 Typical operating values of electron beam current as function of beam

energy.

discussed below; detailed results will be presented in future

publications.

To date, all precision X-ray wavelength measurements in EBITs have been made relative to other spectral lines which

are also observed in EBIT and assumed to be known through

ab initio calculations or by independent measurements in sources with very different X-ray production mechanisms

than that in an EBIT. Variation between different types of

sources, particularly with the strong polarization effects present in EBIT, has sometimes Hmited the accuracy of previous

work, as described in ref. [19]. This fact, along with the desire for a measurement which is fully independent of theory,

suggests the need for an improved approach. In the first experiment mentioned above we are attempting to use an

external calibration source which viall allow a direct tie to absolute length through the NIST measurement chain [16]. We are presently completing an extensive study of systematic

errors associated with this method, and will soon have results to compare with unpublished [17] relative measurements

taken on an independent EBIT at Livermore.

As mentioned in the previous paragraph, polarization ef- fects due to the uniaxial electron beam can affect the inter- pretation and analysis of spectra in an EBIT. Additional

polarization effects due to Zeeman shifts from the magnetic

field are typically negligible for energetic X-ray transitions, but can become significant for visible and uv transitions. Previous studies have dealt with the X-ray polarization issue by first collecting data at one polarization, then modifying

the spectrometer mounting to collect data with an orthogonal

polarization at a later time [18]. In the second experiment

Table II. Present performance parameters for the EBIT at NIST. The electron beam radius is assumed

(from measurements taken on the Livermore EBIT)

and the last two items are calculated. All other quantities are measured. Beam Energy 0.8-33 keV Beam Current 0-155 mA

Beam Monochromaticity < 90 eV

Magnetic Field 3 T

Trap Length 3 cm

Ions Trapped Ba, Ar, N, Kr, Xe Maximum Charge State Vacuum at Top Flange < 10 ' Pa (7x10 " torr) Beam Radius 30 |im Electron Density 8x10" cm ' (@ 155 mA, 5 keV)

Current Density 5,500 A-cm' ((c5 155 mA)

Physica Scripta T71 NIST- 10
102 J.D. Gillaspy

2 200- c^ 100H

4.92 4.94 —

r

4.96 4.98

120- S 100 • c 80-

60'

40-

20-

0-1

I I I I I } 5 6 7 8 X-ray Energy [keV] 10 11 12

Fig. 6 Typical X-ray spectra of neon-like barium obtained from EBIT. The

broadband spectrum was taken with a high-efficiency SiLi detector while spectrum in the inset (200x magnification on the horizontal axis) was taken

with a high-resolution Bragg crystal spectrometer, and shows transitions to the ground state from the J=\ excited states 2p~'3/23d5/2 (largest peak) and

2p ' i/23s (smaller peak) fully resolved.

mentioned above, we are using two essentially identical Bragg

crystal spectrometers to observe EBIT simultaneously in two

orthogonal polarizations. With this configuration we do not need to correct for time variations in source operating conditions between measurements of the two polarizations. Another important consideration regarding polarization has to do with the population of levels by cascade from higher

levels. This cascade feeding competes with direct population

from electron-impact excitation and can have a large effect on the observed polarization [19]. Because the electron beam

energy greatly affects the cascade feeding, the observed polarization can have a significant dependence on electron beam

energy, and we are presently studying this dependence.

The last of our first three atomic physics experiments is carried out in a new spectral range for EBIT: the visible and

near-uv. All previously reported spectroscopy in an EBIT

has been at very short wavelengths, typically around 0.2 nm

and below, although occasionally as high as 1 or 2 nm. By

imaging the EBIT trap volume onto the slit of a grating

spectrometer, we have been successful in obtaining spectra

in the 320-450 nm range, and soon we expect to be able to extend these results out towards the infrared. The remarkable

dearth of structure in these spectra over a broad wavelength

range (fig. 7) greatly simplifies the identification of the lines. Although relatively rare, these long wavelength transitions can serve as important diagnostics both for the EBIT itself and for technologically important devices such as Tokamaks.

4. Future machine upgrades

In parallel with ongoing atomic physics experiments, we are constantly upgrading our EBIT to obtain more capabilities

and improved performance. We are rapidly achieving higher

voltages, and are almost certainly not yet at the Umits of our machine in its present form. We have begun to put the system

1000

-| 800- 600-

400- A 200- 1"" 1 1 392 393 394 395

1400'

1200'

1000'

tn3 800' oo

600

400

200 —I

340 —I '

1

'

1 '

1

360 380 400 420 Wavelength [nm]

Fig. 7 Typical visible/near-uv spectra of titanium-like barium obtamed from

EBIT. The large peak is from the }—2-3 magnetic dipole transition within the ground term. The inset (lOx magnification on the horizontal axis) shows a

spectrum taken in the region of the large peak.

under computer control so voltages can be rapidly switched

through preprogrammed sequences. A vacuum arc ion source (MEVVA) is being constructed which will allow us to inject

and trap metal ions. We are beginning plans to build an

"event mode" [20] data acquisition system which will allow

efficient measurement of excited state lifetimes and cross

sections for various atomic processes. We are also working

to develop a new X-ray spectrometer for use on EBIT. Finally, we have recently begun a substantial effort to build an ion

extraction beamline in order to investigate potential technological applications of highly charged ions interacting with

surfaces. In parallel with this work, we [21] are planning to retrap ions into an external Kingdon trap in order to make

possible additional atomic physics experiments.

Since this manuscript was submitted, substantial progress has been made on the three experiments and plans referred

to in section 3 and 4. For an overview, see J.D. Gillaspy et al, Physica Scripta, T59, 392 (1995), and for specifics see E. Takacs et al., Phys. Rev. A, 54, 1342 (1996) for X-ray

polarization, C.A. Morgan et al., Phys. Rev. Lett., 74, 1716

(1995) and F.G. Serpa et al., Phys. Rev. A, 53, 2220 (1996),

for visible and near-uv light and MEVVA development, and

A.I. Pikin et al.. Rev. Sci. Am., 67, 2528 (1996) for ion beamline development.

References

1. Levine, M.A., Marrs, R.E., Henderson, J.R., Knapp, D.A. and

Schneider, M.B. Physica Scripta T22, 157 (1988).

2. Gillaspy, J.D., Roberts, J.R., Brown, CM. and Feldman, U. "Proc. Vlth International Conference on the Physics of Highly Charged Ions"

(Edited by Richard, P., Stockli, M., Cocke, C.L. and Lin CD ), 274,

p. 682 (AIP press, New York, 1993).

3. Silver, J.D. et al. Rev. Sci. Instrum. 65, 1072 (1994).

4. Aglitskiy Y. and Ovsyannikov V., private communication.

5. Ohlani, S., private communication.

6. Fussmann, G., private communication.

Physica Scripta T71

NIST-11
First Results from the EBIT at NIST 103

7. Smith, W.W., private communication

8. Artsiferov P.S. and Movshev, V.G., Z, Phys. D21, S317 (1991).

9. Kravis, S., this issue.

10. Briand, J. P. this issue

11. Knapp, D.A., Marrs, R.E., Elhott, S.R., Magee, E.W. and Zasadzinski,

R., Nucl Instrum. Meth A334, 305 (1993).

12. Schneider, M.B., el al. "International Symposium on Electron Beam

Ion Sources and Their Application" (Edited by Hershcovitch, A.), 188,

p. 158 (AIP Press, NY, 1989).

13. Vogel, D., "Design and Operation of the Electron Beam Ion Trap", UCRL-ID-104990 (National Technical Information Service, Springfield, VA, 1990).

14. Marrs, R.E., "Atomic, Molecular, and Optical Physics: Charged

Particles" (Edited by F. B. Dunning and Randall G. Hulet), 29A, p.391 (Academic Press, San Diego, 1995).

15. Brennan, S., el al.. Rev. Sci. Instrum. 60, 2243 (1989).

16. Deslattes, R.D., Nuc. Instrum. Meth. B31, 51 (1988).

17. Beiersdorfer, P., private communication.

18. Henderson, J.R., el al., Phys. Rev. Lett. 65, 705 (1990).

19. Cowan, T.E., el al., Phys. Rev Lett. 66, 1150 (1991).

20. Knapp, D.A., Z. Phys. D21, S143 (1991).

21. Yang L. and Church, D., "Proc. Vlth International Conference on

- the Physics of Highly Charged Ions" (Edited by Richard, P. Stockli, M., Cocke, C.L. and Lm CD ), 274, p. 549 (AIP press, New York,

1993).

NIST- 12

Physica Scripta T71
Physica Scripta. Vol. T59, 392-395, 1995 Overview of the Electron Beam Ion Trap Program at NIST

J. D. Gillaspy,' Y. Aglitskiy.^ E. W. Bell,^ C. M. Brown,* C. T. Chantler, R. D. Deslattes, U. Feldman,* L. T. Hudson,

J. M. Laming,* * E. S. Meyer, C. A. Morgan,* A. I. Kkin,^ J. R. Roberts, L. P. Ratliff, F. G. Serpa,* J. Sugar and E. Takacs^

National Institute of Standards and Technology, Gaithersburg, MD 20899, U.SA.

Received August 19, 1994; accepted February 1, 1995

Abstract

This paper surveys the ongoing physics experiments at the Electron Beam

Ion Trap (EBIT) facility at NIST, with particular attention paid to the underlying physical principles involved. In addition, some new data on the performance of our EBIT are presented, including results related to the determination of the trap width, ion temperature, and number of highly charged ions in the trap.

1. Introdoction With the Electron Beam Ion Trap (EBIT) at MST now fully operational, a number of different experiments are progressing in parallel. This paper presents a survey of our current work and gives a preview of future plans. While an earlier paper [1] focused more on the general operation and per- formance of our EBIT, the present paper will focus on specific experiments. Detailed descriptions of the experiments

surveyed here will appear in separate publications when the works are complete. Some additional material on machine

performance is presented in the final section of this paper.

2. Smrey of experiments

2.1. Tests ofbound-state qtumtion electrodynamics (QED)

The importance of precise X-ray wavelength measurements

for testing QED in the presence of the strong electric fields found in highly charged ions has been extensively reviewed

' Internet address: Gillaspy@enh.NIST.gov

' University of Maryland, CoQege Park, MD 20741

' Joint Institute for Laboratory Astrophysics Boulder, CO 80309.

* Naval Research Laboratory, Washington, DC 2037S.

' Texas A&M University, College Sution, TX 77843.

' University of Notre Dame, Notre Dame, IN 46SS6.

' Institute of Nudear Research (ATOMKI), Debrecen, Hungary.

* Sachs Freeman Associates Inc. Landover, MD 2078S.

in the Proceedings of Nobel Symposium 85 [2]. The use of an EBIT to measure QED effects in highly charged ions under conditions free of Doppler shift corrections (the main

systematic effect in most previous experiments) was pioneered by Beiersdorfer and colleagues [3]. Ideas for further EBIT work have been proposed by Silver [4]. Our activity

in this area is guided by the theoretical work of Mohr [5]

and Kim [6] and the experimental work of Deslattes [7]. Additional theoretical guidance is provided by outside groups such as that of Indelicato [8], Safronova [9], Dubau

[10], and Drake [11]. Our particular interest in twoelectron systems grows out of our desire to extend to very high charge states our previous work in neutral helium [12]

and along the lower third of the helium-like isoelectronic sequence [13-15].

The fractional contribution of QED to binding energies

typically increases quadratically as the charge state is increased with fixed electron number, and as the first reciprocal power of the principal quantum number (l/n) as n

is reduced with fixed nuclear charge. The Z-scaling can be

understood simply by combining the elementary Coulomb

and angular momentum behaviour of electronic wavefimctions with the fact that large QED effects occur when an

electron comes within a Compton wavelength of the nucleus

(thereby allowing the infinite bare charge of the electron to become unscreened by the virtual positron-electron pairs

which form the structure of the vacuum of space) (Table I). As a prelude to our primary work in this area, we have

undertaken a study of systematic errors associated with

measuring the resonance lines of neon-like barium

{Q = 46+) with the aid of an external cahbration source. The X-ray spectrum of barium takes on a particular importance in EBITs because of its presence in the cathode of the

electron guns frequently employed in such devices. If no

other ions are injected, the trap will automatically fill up

with barium, providing a convenient source of X-ray spectra

Table I. Dependence of various atomic quantities on nuclear charge (Z). Some of the notes

in the right-hand column refer to semiclassical argumentsfor circular electron orbits, but the

results hold true generally. For a more precise analysis, refer to Ref. [20], for example

Physical parameter Z-Scaling Notes

Angular momentum (r x p) None (Fundamentally quantized) Linear momentum (p) Z (From T - p*/2m below, and rp » const) Kinetic energy {T) Z/r (Coulomb-like, from the virial theorem) Coulomb energy (U) Z/r (DefinitioD) Bohr radius (r) r~l/Z (Consequence of fint two items) Coulomb wavelength (/) 1/Z' (ftv > (/; scaling holds for An > 0 transitions) Electron traction at nucleus (S) Z» (l/r* density of electronic wavefunction) QED wavelength shift z* (Z*S from perturbation theory) Fractional QED shift z* (Relative to Coulomb energy)

Physiea Scripta T59

NIST- 13
Overview of the EBIT Program at NIST 393

for diagnosing the (>erformance of the EBIT. Although

pjeriodic trap-dumping can be used to remove the barium

when it is unwanted, without this action barium provides a

natural background spectrum whenever other elements are being studied in EBIT. With sufficient prior study, this natural background spectriun can be used as a built-in cali- bration reference. Although many spectroscopic lines from highly charged

barium ions have been studied by the Livermore group

[16], precise wavelengths for the strongest lines observed in that work (the 2p-3d transitions in neon-like charge states) have not been published. We are presently measuring these

lines using Bragg crystal spectrometers calibrated with

external X-ray sources. QED effects for certain transitions in neon-like barium are predicted to become significant if wavelengths are measured to an accuracy of 0.1% or better

[9].

2.2. X-ray polarization

Typically one associates polarized emission of photons with

the presence of a strong external field which provides a quantization axis. Although the magnetic field in EBIT is "strong" by conventional standards (3 tesla), it is weak with

regard to the very intense internal fields present in highly

charged ions. It is interesting to compare the external field to the relativistically induced magnetic field that an electron

sees as it orbits through the strong electric field provided by

the nucleus. Because the Bohr radii which characterize highly charged ions are very small, the electrons are moving

at velocities approaching the speed of light and the relativistically induced magnetic fields are strong. For an elec- tron in the outermost shell of a neon-like bariimi ion, the induced fields are of order 7,000 tesla, and scale with the 4th

power of Z. This 7,000 tesla field is the order of magnitude

one would need to apply extemaUy in order to affect the energy levels to an extent comparable to the spin-orbit split- ting. With this in mind, it is easy to understand why highly

charged ions axe relatively imafifected by fields of order 3

tesla. An alternative way of looking at the problem is to note the ratio of a 3 tesla Zeeman shift to the transition energy. For the 5 kev resonance lines of neon-like barium,

this ratio is of order 10'*' to 10~', a sufficiently small number that there is Uttle hope of resolving it with present

day X-ray spectroscopic techniques, and even if one could

do so it would be fundamentally blurred by the natural widths of the lines themselves. This blurring of the lines typically has a strong dependence on nuclear charge

(Av/v = Z^, for El transitions). Although the presence of the magnetic field is often negligible for analyzing the emission of X-rays in EBIT, the uniaxial nature of the electron beam is highly significant, and

actually provides a strong quantization axis for the pol- arized emission of radiation. Because atomic electrons are

excited by impsict with free electrons in the beam, momentum transfer considerations leave the ion in a preferentially

aUgned state. Upon spontaneous emission, the aUgnment of

the ion is reflected in the polarization of the emitted radiation. The situation is more complex if the electron beam is tuned to a sufficiently high energy to populate levels above

the upper level of the transition of interest In this case, cascade feeding becomes important, and one must take into

account all the various paths through the magnetic sublevels which feed the transition. Because increasing the elec- tron beam energy can bring higher levels into consideration

and change the cascade pattern, the polarization of the emitted radiation can depend significantly on the beam

energy. We are presently engaged in theoretical and experimental

studies of this dependence of X-ray polarization on electron beam energy. Results of this study are important for testing the most sophisticated calculations of electron-impact exci- tation cross sections which contain information about the magnetic quantum subievels. The analysis of polarized

X-ray emission is being increasingly recognized for its importance in diagnosing technological devices such as Tokamak reactors as well.

Z3. Visible light spectroscopy In a recent review paper [17], Marrs has remarked that one

of the most exciting future developments in EBIT will be the extension of spectroscopy into the visible range of the spec- trum. We have successfully done this by observing forbidden

(Ml) transitions within the ground term of titanium-like barium and xenon using a grating monochromator [18].

These transitions are of particular interest for their use in the remote monitoring of ion temperatures by Doppler

broadening in large future tokamak fiision machines. Because the hght detected in these experiments arises from transitions between very close-lying levels, the wavelength is sensitive to small Zeeman shifts which are usually

negligible in X-ray spectra. For the Ml transitions studied

in our work, the Zeeman shifts appear at the 10 ~* level of

resolution, rather than the 10" '-10"' level which is typical

for the X-ray transitions discussed above. When used to detect Doppler shifts, there are other

advantages, though less fundamental, which visible hght

spectroscopy has over X-ray spectroscopy. Although the

fractional resolution required to detect Doppler shifts from

a moving ion is independent of wavelength, the availability of advanced refractive and reflective optics for visible Ught

allows one to achieve both high efficiency and high

resolution simultaneously. This opens up new opportunities,

and makes the detection of Doppler shifts feasible with relative ease. For example, we have recently appUed FabryPerot inteiferometry to achieve a resolution sufficient to see Doppler-blurred Zeeman broadening from ions in the trap

(see Fig. 1 and Section 3.3 below).

The present accuracy of our measurement of the wavelengths of the visible light is sufficient to reveal large dis- agreements (~4%) with ab initio Dirac-Fock theoretical predictions [19]. Our present identification of these lines is confirmed by the isoelectronic behaviour of fitted calculations using the Cowan code [20]. This experiment should

challenge theorists to improve their understanding of how

electron-electron interaction affects level separations in a

regime not widely addressed previously.

2.4. Ion-surface interactionsfor technology

The interaction of highly charged ions with surfaces has

been studied for many years at accelerator facihties, and at

least one company has come to produce a successful commercial product from this work [21]. Experiments with

highly charged ions at accelerators, however, typically

Physiea Scripta TS9

NIST- 14
394 J. D. Gillaspy et al.

r 1 I ' I 1 25 50 75 100 125

Plate Spacing [arbitrary units]

Fig. I. Fabty-Perot ioteiferometry (scanning mode) of the Ml transition near 400 mn in the ground term of titanium-like barium. The free spectral range (distance berween peaks) is approximately 0.3 nin. The end cap

potential on EBIT was 300 V.

involves high kinetic energy impact with surfaces. The use of EBITs (or EBISs [22]) to generate highly charged ions at

rest in a small-scale laboratory environment offers new

opportunities. Efforts in this area are underway at Liver- more [23], Kansas State [24], and Hiuchi Ltd. [25], as this technique may have application in the novel fabrication of

technological devices. We have begun a concentrated program to assess the feasibility of using slow highly

charged ions to fabricate an active electronic device such as a quantum dot diode. In collaboration with the recently

formed Advanced Lithography Group consortium of industrial labs [26], we are also looking closely at the potential

which highly charged ions may hold for future generations

of projection lithography techniques.

1 New detail on EBIT device operation

Several earlier papers describe the operation of EBIT

devices [1, 17, 27-28]. In this section we discuss several important parameters for which detailed data have not been

published.

3.1. Trap size Although it has often been said that the trap length for EBITs of the Berkeley-Liveimore design is 2 cm, the actual

length of the center drift tube if often longer (in our case,

3 cm). The external view of our trap is restricted by a 1.5 cm

diameter hole in the surrounding Uquid helium shield, so a

typical detector will collect light from a section of the trap

which is approximately 2 cm in length due to parallax at the

edges. Using pinhole images of X-ray emissions, the Liver- more group has measured the width of the electron beam to be 70 Mm [28] but the width of the trapped ion cloud should

be larger since the ion orbits are predicted to extend outside

of the electron beam (Fig. 2). The X-ray imaging experiment

is not affected by this because excited state lifetimes for

typical X-ray transitions are of the order of femtoseconds,

and therefore the ions travel less than 0.1 nm (a small frac- tion of an ion orbital cycle) before emitting a photon. By

studying visible-wavelength photons from long-lived states, we are able to obtain data which determine the full spatial extent of the ion cloud. In this case, the ions can move

through many orbital cycles before emitting a photon. To

obtain data on the width of the ion cloud, we focused the

center of EBIT onto the entrance slit of a grating monochromator which was mounted on a large precision transFig. 2. Numerical simulation of an ion orbit through an electron beam

with Gaussian density profile. Conditions: charge •= 30 proton charges. Mass • 100 proton masses, magnetic field = 3 T, electron beam diameter

containing 86% of charge = 60^m (shown as shaded area in figure), drift tube diameter <« 1 cm, space charge potential " 200 V, initial coordinates and velocity: = *o " lOjun, bj - r, = 4.0 x 10*m/sec (corresponding

to an ion temperature of 1 keV).

lation stage. After adjusting the monochromator to be

spectrally centered on one of the Ml transitions, we moved

the spectrometer along a direction perpendicular to the line of sight into EBIT and monitored the strength of the observed signaL The result, shown in Fig. 3, is consistent with a 180 pm wide cloud of trapped ions, after correction

for optical demagnification of the lens system It should be

possible to use this technique to monitor changes in the ion

cloud width as the ion kinetic temperature is lowered by

evaporative cooling.

32. Number of ions in the trap The longstanding and vexing question of how many ions are

in the trap has still not been definitively answered. Previous

estimates based on calculation from spectroscopic data

taking into account the solid angle of observation and other

parameters, or on ion-extraction measurements, have

yielded widely scattered values with large uncertainties. Recent work at Livermore using ion-cyclotron resonance

[29] of the trapped ions may eventually yield highly reliable values, but present uncertainties seem to be as high as a

factor of 10 [30]. In the meantime, the best estimate may be

achieved from well-characterized measurements of an iso- lated spectral line. Since the location of a radiative recombination line can be adjusted by varying the electron beam

60-1 FWHMslTOuffl

I I 200 400 600

DisplaMiiMnt [utn]

Fig. 3. Signal strength of visible Ught from Ti-like barium as a function of

transverse position of a grating spectrometer.

Physiea Scripta T59

NIST-15
Overview of the EBIT Program at NIST 395

energy in the EBIT, one can move such lines to a well- isolated region of the spectrum and obtain clear and clean

results. Even more importantly, the radiative recombination

cross sections can be calculated very accurately. With the number of EBIT laboratories growing, we suggest that a standardized procedure be adopted to compare the per- formance of different machines, as well as individual machines under varying circumstances. To this end, we

present here detailed results for measurements taken on a

particular radiative-recombination Une in neon-like barium

(Fig. 4), and infer a best estimate of 310000 ions in the trap

from the following formula which relates the rate of photons

detected (Np) to the number of neon-like barium ions (N) in the trap: N = nr^qNJifOs TIX ME, ^Vdtu] " *

where e is the efRciency of the detector, T is the transmission of the X-ray window at the photon energy, /is the frac- tion of ions in the trap which are unobscured, / is the

electron beam current, r is the spatial FWHM of the elec- tron beam, q is the charge of the electron, A is an overlap

factor which corrects for the spatial and temporal overlap of

the ion cloud with the electron beam, do(e, d)ld£l is the

differential cross section for emission of radiative- recombination photons (summed over the five sodium-like n = 3 levels contained within the finite resolution of the

detector), £ is the electron beam energy, d is the angle of

observation with respect to the electron beam, and Q is the

solid angle subtended by the detector. For our case,

6 = 100% (SiLi detector @ 9keV), T = 100% (0.007" thick Be foU at 9keV), /=0.67, / = 100mA, r = 35(mi

£1 = 5.8 X 10"*str (13mm* detector at 18cm distance), A = 18% (estimated for 1 kev, Ba*** ions with 250 V applied

end cap potential), N'p=10.86s~^ (obtained by binning

data into 10 eV wide channels for 20 minutes and fitting the peak with two Gaussians and a subtracted linear background), da/dfl = 19.9 x lO'^'m^/sr at £ = 5.69 keV

and e = 90° [31].

3.3. Ion temperature

The question of ion temperature - whether there is one, to begin with - is controversial due to the complex nature of

the trap dynamics. If we assume an ion temperature, then

the spectral linewidth observed can put an upper bound on

its value. With adequate understanding of instrument pro8000-

6000-

300-^ 200-

100-

0. [/\.

&5 9.0 9.5

I I I 3 4 5 6 7

Electron Energy [keV]

Fig. 4. X-ray spectrum show the radiative recombination line used to esti- mate the number of ions in the trap. The electron beam energy was set at

5.69 keV.

files and proper account for Zeeman broadening, the ion temperature can be accurately determined. Our preliminary

Fabry-Perot data (Fig. 1 above) corresponds to an ion temperature below 1 keV. This ion temperature is a factor of 10 lower than the axial trap depth for an ion with charge 34-»- contained by 300 V end cap potentials, suggesting the pres- ence of strong evaporative cooling by interaction with lower

charge state ions which see a shallower trap.

Acknowledgements

We thank Helmar Adler for collaboration in obtaining the Fabry-Perot

data, and Yong-Ki Kim and Peter Mohr for theoretical support

References

1. Gillaspy, J. D.. submitted. Proceedings of the Vlth International Symposium on Electron Beam Ion Sources, Physica Scripta, 1994.

2. "Heavy-Ion Spectroscopy and QED Effects in Atomic Systems" Proceedings of Nobel Symposium 8S, (Edited by I. Lindgren, L Martinson, R. Schucfa) Physica Scripta T4^ 1993. p. 1-269, Royal Swedish Academy of Sciences, Stockholm, Sweden (1993).

3. Beiersdorfer. P. a al., Phys. Rev. A41, 3453 ( 1990).

4. Silver, J. D. a ai.. Rev. ScL Inst., 65, 1072 (1994).

5. Mohr, P., "Physics of Highly-Ionized Atoms" (Edited by R. Mamis)

(Plenum. 1989). p. 111.

6. Kim, Y.-IC Physica Scripta, T47, 54 (1993).

7. Deslattes,R.D., Physica Scripta, 746.9(1993).

8. Indelicato, P., Goroeix, O. and Desdaux, J. P., J. of Phys. B20, 651

(1987).

9. Safronova, U. L, Safronova, M. S. and Brucfa, IL, Physica Scripta, 49, 446(1994).

10. Inal,M.K.andDubau,J..J. Phys.B20.4221(1987).

1 1. Drake, G. W. F.. Adv. At MoL Phys. 18. 399 (1982)l 11 Sansonetti.C J. and Gillaspy. J. D., Phys. Rev. A4S,R1 (1992).

13. DesUttes, R. D, Beyer. H. F. and Folkmann. P.. J. Phys. B17. L689

(1984).

14. Chantler, C. T., D. PhiL Thesis, Oxford University (1990).

15. Livingston. A. E., Galvez. E. J., Zacariaa. A. S. and Serpa, F. G, AIP

Conference Proceedings 274, "Vlth International Conl on the Physics of Highly Charged Ions" (Edited by P. Richard, M. Stockli, C L. Cocke and C D. Lin) (AIP. New York 1993). p. 402.

16. Hutton. R.aal^ Phys. Rev. A. 44. 1836 (1991).

17. Mam. R. E.. "Electron Bnm Ion Traps" submitted to Meth. Exp.

Phys., Livermore preprint *UCRL-JC-117432 (1994).

18. Morgan. C A. eta:.. Phys. Rev. Utt^ 74, 1716(1995).

19. Fddman, U.. Indelicato, P. and Sugar. J., J. Opt Soc Am. B8, 3

(1991). 20. Cowan, R. D., "Theory of Atomic Structure and Spectra" (University of California Press, Berkeley 1981).

21. Fleischer. R. L., Price, P. B. and Walker, R. M., "Nuclear Trades in Solids" (University of California Press, Berkeley 1975), p. 564.

22. Donets, E D., "The Physio and Technology of Ion Sources" (Edited by L G. Brown) (WUey, New York 1989), p. 245.

23. Schneider, D., Briere, M. A., McDonald, J. and Bieisack, J., Radiation

Effects and Detscu in Solids 127, 113 (1993). 24. Parks, D. C, Bastaz, R., Schmeider, S. W. and Stockli, M., J. Vac. Set and TcchnoL, in press (1994). 25. Mocfaiji. K.. Itabashi. N, Yamamoto, S., Ochiai, L and Okuno, IC

Jpn. J. AppL Phys.. in press (1994).

26. Loscfaner, H. et aL, Microlithography Worid, Spring, p. 4 (1994).

27. Levine, M. A. «ai, Physica Scripta, T22, 157(1988).

28. Levine, M. A. et a/., "International Symposium on Electron Beam Ion

Sources and Their Applications" (Edited by A. Hershcovitch) (AIP. New York 1989), p. 82. 29. Beiersdorfer, P, Beck. B, Elliott, S. and Mans, R, Rapid Commun. in Mass Spect. 8, 141 (1994).

30. Schweikhard, U et oL, Rev. Sd. Instrum., Liveimore preprint UCRL-JC-116808, (1994). 31. Saloman, E B., Hubbell, J. H. and Sco'fidd. J. R. At Data NucL Dau

Tables 3S, 1(1988).

Physica Scripta TS9

NIST- 16
Hot Topic

Visible and UV light from Highly Charged Ions:

Exotic Matter Advancing Technology

J. D. Gillaspy

Atomic Physics Division, Physics Laboratory.

Technology Administration,

U.S. Department of Commera.

National Institute ofStandards and

Technology

Gaithersburg. MD 20899-0001

.Gillaspy<^enhJ^IST.gov

In a few laboratories scattered around

the world, small devices the size of a

filin g cabinet are producing an exotic form of maner which may play a role in several future advanced technologies. These types of devices. Electron Beam Ion Traps (EBITs) [1], are capable of producing ions of virtually any

possible diarge state of any atom on

the periodic table. The very highest

charge states have strongly altered properties which make the ions not only cui interesting curiosity, but also potentially usefuL Alternative ways of producing the exotic highly charged

ions (HCIs) require huge devices which are only affordable by a large national or corporate effort. The fairly recent development of EBIT technol- ogy brings R&D work with the most

extreme HCIs down to a level which is affordable by the individual investiga- tor or small private company.

Of the various potential applica- tions for HCIs, one of the most proven

is as an optical diagnostic for hot plas- mas. Analysis of emission speara from HCIs has long been used as a remote probe of the Sun. but this technique

has now been brought down to Earth

as scientists and engineers move closer

to developing an electrical power generator wfiich uses the same relatively safe and efficient process by which nature produces sunlight: nuclear fusion. Actively harnessing the power of the sun by creating a miniattire 'solar plasma in a bottle' here on earth, has been the focus of a major national ef- fon for years [2]. Through an antici- pated international collaboration, the ongoing fusion effort appears to be on

the brink of reaching commercial vi- ability in the early next century. Al- though the tiny EBIT device can pro- duce plasmas in excess of 10,000,000

degrees Kelvin, as high as that in the Sim or in fusion test reaaors, the EBIT

caimot contain enough material to it- self be a useful energy production deAugust, 1995 T IEEE LEGS NEWSLETTER

vice. The core confinement chamber

inside EBIT is only about as wide as a human hair, and a centimeter or two

long, while fusion confinement chambers are large enough to walk around

in. In addition, the density of the EBIT

plasma is of order 10e9/cc, much less than that required for commercial

power production. One of the things that the EBIT can do very well, however, is provide a crisp, dear, and uncluttered laboratory view of the inner workings of nature in one of its more

exotic and relatively inaccessible

states: the highly charged plasma.

Knowledge gained from EBIT studies can then be direaly applied to im- prove fusion plasma diagnostic work.

A spedfic example of how EBIT can be used to help in the fusion effon is il- lustrated by some recent work we did involving the measurement of visible and near-ulcravlolet light emission

from HCIs. Although visible light is emined copiously when neutral atoms

are exdted by thermal or electronic processes, the same is not true for HCIs. As atoms lose more and more of their outer electrons, the exposed re- maining shells appear so tighdy bound

that the quantum energy level separations are thousands of eiearon-volts, rather than just a few electron-volts as they would be for separations which

give rise to visible light emission. As a

result. HCIs emit primarily Ln the Xray region of the spectrum.

X-ray spectrometers are routinely

deployed in fusion devices in order to monitor and diagnose conditions in the plasma. Information about plasma

temperature and internal magnetic

field, for example, can be obtained

non-invasively from the Doppler and

Zeeman widths of x-ray emission line

profiles. It would be highly desirable to replace the x-ray spectrometers with

visible light spectrometers in the fu- ture, however, for several reasons.

First, one could then employ optical

telecoramimications technology, fiber optics in particular, to carry out truly remote diagnostics. With x-ray spec- troscopy, the instruments must be lo- cated dose to the confinement vessel walls in areas of high radiation which

make them physically inaccessible to personnel dtxring operation. Second,

spectroscopic techniques are much

more advanced in the optical region, so more effident and higher resolution measurements will be possible. Third, thin metal-foil x-ray windows used in

test reaaors may become impractical

to use in scaled-up commerdal ver- sions of the reaaors. while thick but

transparent optical windows may offer an alternative. Finally, there are some

fundamental advantages, such as the faa that Zeeman shifts appear as a

larger fraction of the total wavelength

when one observes optical emission

from dose-lying levels rather than x- ray emission from widely separated

levels. All of these facts point towards

optical diagnostics as the method of the future. The problem is: how does one get the HCIs to cooperate and give

off the appropriate optical emission?

While ordinary visible light transi- tions are squeezed into the x-ray regime

as the charge state is inaeased isoelectronically, RP and microwave transitions experience a similar shortening of wavelength, and can be squeezed into the vis- ible and ultraviolet. Furthermore, transi- tions which would ordinarily be

forbidden by the ordinary selection rules of quantimi mechanics begin to become more and more allowed as higher order

processes are brought into play in the presence of the strong internal electric Belds which are charaaeristic of HCIs. Both of these facts open up possibilities for observing visible light from HCIs, even when the prindple emission is limited to the x-ray regions of the spectrum.

While the typically strong depend- ence of wavelength on charge state can open up new possibilities, it can

also lead to serious problems. If one

finds a spedal case where a highly

charged ion emits visible light, the wavelength will rypically.shift rapidly into the deep ultraviolet as the charge

state is varied even slighdy along an

isoelearonic sequence. Since a vari- ation in plasma temperature will alter the charge state, it would be desirable

to have a single isoelearonic sequence which emits in the visible for a wide

range of charge states. In order to have this, however, it would be necessary to violate nature's rule that wavelengths scale rapidly with charge.

A few yean ago Uri Feldman from

the Naval Research Laboratory teamed

up with Jack Sugar and Paul IndeliNIST-17
cato at the National Institute of Standards and Technology to carry out an

extensive search for an exception to nature's rule [3]. Using one of the most sophisticated computer codes

available to simulate the atomic struc- ture of highly ionized matter, the sci- entists sifted through scores of possibilities until they found the needle in the haystack. The code prediaed that certain ordinarily forbidden transitions between the One structure levels within the ground term of titaniumlike ions—atoms stripped of all but the 22 innermost electrons—would pro- duce visible light over a wide range of charge states for elements across the

entire upper half of the periodic table. These transitions are forbidden be-

'

cause, to first order, the elearic field component of an electromagnetic wave cannot connea states of identical parity. Instead, the magnetic com- ponent of the wave is required. Generally the magnetic component is rather

ineffective, but. as mentioned above, the ordinary 'rules of thumb' are highly distoned in HCIs. 'Forbidden'

transitions begin to be 'allowed' at rates which scale in certain circumstances as rapidly as the tenth power

of the charge. In extreme cases, the (formerly) 'forbidden' transitions in HCIs can proceed even more rapidly

than the 'allowed' ones. The special lines prediaed in Ti-like ions remained an untested curiosity until early last year when the EBIT at

NIST was deployed to see if they really could be seen and whether they had

the unusual propenies which were

prediaed. In a recent issue of Physical

Review Leners [4], we reponed our

findings that not only were the unusual lines clearly observable, but they were even more 'visible' than ex- peaed. writh wavelengths shifted ap- proximately 5% back tovrard the red from their prediaed location in the

near-UV. In subsequent work, we

have raised the charge state by up to 10 elemenury charges, and find that although the wavelength offset is preserved, the scaling w\xh charge is very

similar to that prediaed. We are presently measuring the lifetimes of the upper levels in order to further characterize this sequence. The reason for the wavelength offset has puzzled every theorist we have spoken to, in- cluding a cadre of the best who were

present at last year's Nobel Symposium

on Trapped Charged Particles [5]. This

is just one of the presentiy unexplained

puzzles which the highly charged form

of Nature's 'fourth state of maner'[6]

(the plasma) presents us with. Acknowledgments!

I wish to thank my colleagues and

postdocs listed on publications 1, 3, 4, & 5 below, as well as my Division

Chief, Wolfgang Wiese. and Dtreaors,

Katharine Gebbie and Ray Kammer,

for making this work possible.

For further reading

1. 'The Elearon Beam Ion Trap',

R. E. Marrs, P. Beiersdorfer. and D. Schneider, Physics Today, 47, 27

(1995).

2. 'Princeton Tokamak begins ex- periments with tritium-deuterium

plasmas', B. Schwarzschild. cover photograph and news article. Physics Today, 47, 17 (1994).

3. 'Magnetic dipole line from U

LXXI ground-term levels prediaed at 3200 A', U. Feldman, P. Indelicate,

and J. Sugar. J. Opt. Soc. Am. 8, 3 (1991).

4. 'Observation of Visible and uv

Magnetic Dipole Transitions in Highly

Charged Xenon and Bariiun', C. A. Morgan. F. G. Serpa. E. Takacs, E. S. Meyer. J. D. Gillaspy, J. Sugar, J. R. Roberts, C. M. Brovm, and U. Feld- man. Phys. Rev. Lett., 74, 1716 (1995).

5. 'Overview of the EBIT Program

at NIST', J. D. Gillaspy, Y. Aglitskiy E. W. Bell. C. M. Brown. C. T. Chantler,

R- D. Deslattes, U. Feldman, L. T.. Hudson. J. M. Laming, E. S. Meyer. C. A. Morgan, A. L Pikin. J. R. Roberts, L. P. Radiff. R G. Serpa, J. Sugar, and E. Takacs, Proceedings of Nobel Symposium #91 (in press, Physica Scripta,

1995).

6. "The Fourth State of Maxten

plasma dynamics and tomorrow's tech- nology' B. Bova (New York, St. Martin's Press, 1971).

NIST- 18
Physica Scripta. Vol. T65, 168-174, 1996 EBIT Spectra of Highly Stripped Ions from the Visible to the

X-Ray

J. D. Gillaspy'

Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Received October 3, 1995: accepted in revised form December 13, 1995

Abstract

This paper provides an updated review of the capabilities of the Electron Beam Ion Trap (£BIT) as an advanced light source for atomic spectros- copy. Recent developments extending the spectral range to over five orders

of magnitude in wavelength are highhghted. Distinguishing features that make EBIT a powerful tool both for simplifying spectral analysis and for making precise and accurate measurements are discussed. Various limi- tations of the machine are also discussed, with the intent to provide an

introductory guide for scientists who may be planning EBIT spectroscopy experiments for the first time.

1. What is an EBIT?

For the purpose of this paper, one may think of the EBIT as

a high density monochromatic electron beam of variable

energy, housed in a vacuum chamber small enough to fit in a closet. Atoms directed through the electron beam are ionized and subsequently trapped in an orbit around and

through the beam. Subsequent ion-electron collisions excite

transitions, causing the ions to fluoresce. More detailed descriptions of the operation and design of the EBIT can be

foimd in the literature [1], including a popular account with

color photographs [2], or on the World Wide Web EBIT

home pages:

http :/ /physics.nist.gov/MajResFac/EBIT/ebit.html

http :/ /www-phys.llnl.gov/N_Div/ebit.html

2. The spectral range aod simplicity of EBIT spectra

Since the last ASOSALP conference three years ago [3], the EBIT has continued to demonstrate itself as an increasingly

powerful and versatile light source for atomic spectroscopy.

Two developments in particular have dramatically expanded the spectral range of available data from EBIT sources.

First, the development of the SuperEBIT at Livermore [4] has led to the production of spectra from energetic elec- tronic transitions which emit all the way into the soft gamma-ray region of the spectrum (see Fig. 1) [5]. Second,

the demonstration at NIST [6, 7] that an EBIT can be an

effective source for visible and near-UV spectroscopy (see

Fig. 2) of forbidden fine structure transitions in highly

charged ions has extended the range of EBIT data to span

over five orders of magnitude in wavelength.

One of the striking features of data taken with an EBIT is the often remarkable simplicity of the spectra. This is observed even out to both extremes of the available spec- trum, as can be seen in both the soft gamma-ray data from

Livermore and the visible and near-UV data from NIST. In

180 200 220 240 260 280 300 320 340

X-Ray Energy (kiV)

Fig. 1. Spectrum showing that an EBIT can produce the highest possible charge states, up to and including U*' ^ which emits photons with energy

in excess of 300 keV (wavelengths <0.004 nm) [S].

1000-^ 800-

600 • 400-

200 « '-I J\.1 1 1 392 393 394 395

1400-

1200-

1000- a1 aoo> 5

600-1

400

200

I —I—•—T—"—I—' I

' 340 360 3U 400 420

' Internet address: John.gillaspy@NIST.gov

Physica Scripta T65

T "—I r- 360 3U 400

Wavalangth [nm]

Fig. 2. Spectrum showing that an EBIT can also be used as a source for

visible-light spectroscopy of highly charged ions. Here light from Ba^*'^ extending up to nearly 400 am in wavelength is measured [7]. Inset shows

the average of 10 higher-resolution (narrow slit) scans of S minutes each

over the region where the strong line appears

NIST- 19
EBIT Spectra of Highly Stripped Ions from the Visible to the X-Ray 169

the NIST barium data for example (Fig. 2) [7], only a single

line appears over a very broad spectral range (320-440 nm).

In both cases, a large pan of the simplicity of the spectra anses from the purity and selectivity of the source, as will be

illustrated more fully in the remainder of this paper.

3. How close to the ideal can an EBIT get ?

3.1. Sketch of an ideal light source An ideal spectroscopic light source would emit photons

from a single atomic species, which has been prepared in a

single charge state of any value, under conditions which

selectively excites a single transition, with high brightness,

and low density (to avoid collisions and absorption), with a

small source size, running in continuous (rather than pulsed)

mode, yet capable of being rapidly switched, and be inex- pensive and easy to build. All real sources fall short of this

ideal, sometimes for fundamental reasons (high brightness

and low density, for example, are often contradictory). It has

been suggested, however, that the EBIT goes further toward

meeting all of these requirements than any other source [8]. The remainder of this section will address how close an EBIT source can approach the ideal spectroscopic light source. Section 4 will then address ways in which the per- formance of an EBIT falls short of the ideal.

3.2. An EBIT can produce any charge state With electron beam energies near 2(X)keV, the SuperEBIT

has recently produced spectra from radiative recombination

onto bare uranium {Q = 92 + , see Fig. 1) [5]. During the

past eight years, a great deal of data has been taken from

"midrange" charge states (Q = 40-60) where the EBIT runs

with ease. Recently, data have been taken at NIST on

charge states as low as Q = 1 (molecular nitrogen ions) [9]

where the EBIT functions like a high-tech version of a

simpler electron beam device which was built for just such a

purpose decades ago (see Fig. 1 in Ref. [10]). So far, EBITs have been used by the Livermore and NIST

groups to obtain data from at least 44 elements (N, F, Ne,

Al, Si, S, CI, Ar, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge,

Br, Kr, Rb, Y, Mo, Sn, Xe, Ba, Hf, Ta, W. Re, Ir. Pt, Au, Pb,

Bi, Pr, Nd, Gd, Er, Tm, Yb, Th and U) and for at least 21

isoelectronic sequences (bare, H-like, He-like, Li-like, Belike. B-like, C-hke, N-hke, O-like, F-like, Ne-like, Na-like,

Mg-like. Al-like, Si-like, Sc-Uke, Ti-like, V-like, Co-like, Nilike, Cu-like).

3.3. The EBIT has charge state selectivity

By varying a single knob, an EBIT can be adjusted to preferentially produce a selected charge state. Although other

charge states are generally present, this varies depending on

the proximity of nearby levels. The production of a single

charge state with 97% purity has been demonstrated [11]. Even for non-optimal (open shell) configurations, it is possible to obtain a sizable degree of selectivity as evidenced by

the visible hght data from NIST (see Fig. 3). Detailed computer simulations which pedict the degree of charge state selectivity for all stages of ionization of all ele- ments and under a wide range of conditions have been

installed at all three of the major EBIT facilities (see Section

6 below) in order to help plan experiments and interpret observed results. These simulations are based on the origFig. 3. Plot showing how charge states can be preferentially tuned in or out of the observed spectrum by adjusting the EBIT [6]. Shown is the variation in the intensity of two visible-light spectral lines from Xe''* and

Xe^' * as a function of electron beam energy. Because the excitation, fed by

cascade from above, is approximately constant, the dependence of intensity on electron beam energy reflects the change in charge state abundance

directly.

inal models from Livermore [12], but contain various

refinements.

3.4. The EBIT has excitation selectivity

By adjusting the electron beam energy above or below an

excitation threshold, spectral lines within a given charge

state can be turned on or ofif. This is true not only for di- electronic recombination lines, but also for those directly excited by electron impact. Figure 4, for example, shows

bow a strong line in beliumlike iron can be extinguished by

lowering the electron beam energy by 1% and how a second

nearby line can then be extinguished by lowering the elec- tron beam an additional 1% [13]. In this regard, the EBIT

electron beam can be used much in the same way that a

laser is used to selectively excite a transition, except in the case of the EBIT the direct excitation cross-section has only a sharp lower threshold, instead of a symmetric resonance

structure. The degree of selectivity is limited by the energy spread of the electron beam, which is typically SOeV,

although it has been made as low as 16 eV [14] by compromising other aspects of the EBIT performance.

3.5. The EBIT provides a third dimension to spectroscopy

While the spectroscopy of highly charged ions using ordi- nary light sources typically consists of a simple two- dimensonal data set composed of intensity vs. wavelength,

the monochromatic nature of the electrons in the EBIT

allow the speara to be expanded along a third axis : electron beam energy. In this simple fact lies the key to understanding why an EBIT is such a useful spectroscopic source. Almost all the features of an EBIT can be traced to this single aspect of its design, and fully understanding a 3- dimensional plot of EBIT data (see Fig. 5) [see also Fig. 5 in Ref. 15] is a key test of understanding the EBIT in general.

The full significance of this third dimension is not easy to appreciate without careful consideration, particularly since many spectroscopic experiments need only sample a partic- ular slice of the full three-dimensional phase space. Some of

the unusual features of the EBIT which arise from the new

Physica Scripta T65 NIST-20
170 J. D. Gillaspy

1,845 1J50 1JS5 1.860 1.665 1.870 Waveiengtfi (A)

i:6<5 1.850 1.BS5 1.860 1.665 1.870 Wavelengin (A)

1^5 1.850 1.855 1.860 \£eS 1.870 Wavelength (A)

Fig. 4. Plot showing bow, withm a given charge state, spectral lines can be tuned in or out of the spectrum by adjusting the EBIT to bring the electron beam energy above or below the threshold for impact excitation [13]. Here,

in the X-ray spectrum of hehum-like iron (Fe"*), lines "x" and "z" are extinguished by lowering the electron beam energy by (1%) (b) and 2% (c) from the 6.74 keV value used to produce (a). Because the beam energy is far above the 2 keV threshold to produce the helium-like iron, the charge state distribution remains approximately constant.

dimension added by the electron beam energy are described

in the remainder of this subsection. The origin of the process by which a spectral line is formed can often be determined simply by observing its behavior as the EBIT control knobs are adjusted. No theoretical input or detailed understanding of the system is needed for this level of analysis. If a Une appears abruptly

and then disappears just as abruptly when the electron beam energy is varied by an amount equal to its energy

width, then the line is probably a dielectronic recombination

satellite. If a line moves continuously in photon energy as

the electron beam energy is varied, it is probably a radiative recombination Une. A judicious placing of the electron beam

energy can localize and move radiative recombination emission out of the way, or turn off undesired satellites.

Physica Scripta T65

x-ray afwrgy |li*V)

Fig. 5. Plot showing how spectra can be studied not only as a function of photon energy, but also (simultaneously) as a function of electron beam

energy. The extra dimension provides an additional degree of control and

an added axis for analysis which Ues at the heart of many of the features of EBIT.

The tunable beam energy can also be used to enhance the

effective resolution of a system which uses a low-resolution

detector. Figure 4 in Ref. [16], for example, shows a spec- trum with 50 eV resolution, taken with a solid state detector which has resolution of typically 200 eV. When viewed in three dimensions, the dielectronic recombination peaks can

be seen to be asymmetric, with the widths along the photon

and electron energy dimensions being 200 eV and 50 eV,

respectively. Selecting information along the proper "cut" with an EBIT allows features to be resolved which would

otherwise be fully blended.

3.6. The EBIT can be considered to be a bright source

(sometimes)

The spectral brightness of the EBIT is perhaps the most

misunderstood aspect of the device. When reviewing the lit- erature, one can even find these apparently contradictory

statements, both from knowledgeable members of EBIT

research groups: "EBIT is a relatively weak X-ray source"

[17], and "EBIT is a relatively bright source for emission

spectroscopy" [8]. Both statements are correct, when taken from the appropriate point of view. In order to remove the

subjectivity which necessarily comes with terms such as "bright" and "weak", I focus here on a more objective measure: how long does it take to acquire a useful spec- trum? The answer depends on the experiment, but can vary from a second to several days, as described in more detail in the remainder of this subsection.

It is worth pointing out that signal strengths have

improved dramatically since the first year of EBIT operation (around 1988) so anecdotal experience from that period should be considered atypical by current standards. Improvements have included both increases in beam current and the application of more efficient spectrometer geometries [18]. In the case of X-rays observed in low resolution (SiLi

spectrometer with spectral resolution 2% at 7 keV), one can

obtain useful spectra in one or two seconds, fast enough for real-time feedback when adjusting the EBIT to optimize its performance (see Fig. 4 in Ref. [19]). The count rate in a

single spectral peak can be over 100s~

NIST-21
EBIT Spectra of Highly Stripped Ions from the Visible to the X-Ray 171

For visible light observed with moderate resolution (scanning grating monochromater with spectral resolution 0.2% at 400 nm) one can obtain useful spectra in 5 minutes

(see inset of Fig. 2) even without the advantage of parallel detection. The count rate per spectral line can be over

10 s-'. For high resolution X-ray spectroscopy, one can obtain

high quality data in 20 minutes to an hour (see Fig. 6). The

count rate per spectral line can be over Is"'

3.7. The EBIT source shape can be optimal for many

spectroscopic studies The EBIT source shape is slit-like, approximately 3 cm long

(the central 2 cm of which are visible from outside the EBIT

vacuum chamber) and a fraction of a mm wide. The trans- verse dimension can vary depending on the excited state life- time and the ion temperature. For the very fast lifetimes associated with X-ray transitions, light is effectively limited

to the region of excitation determined by the electron beam

itself (60 nm diameter, see Fig. 5 Ref [20] for a direct measurement of the electron beam width using X-ray imaging). For visible and UV transitions with lifetimes longer than the time needed for an ion to traverse the electron beam (2 ns

for 1 keV barium crossing a 60 jim electron beam), emission

is spread over the entire trap region which can be 200 |im in diameter (see Fig. 3 of Ref. [19] for a measurement of the

visible trap width). The narrow transverse dimension of the trap helps to increase its brightness, while at the same time

allowing it to be efficiently imaged into a spectrometer (for example, the entrance slit of a grating spectrometer).

3.8. The EBIT is a very clean source With a base level of evacuation 13 orders of magnitude

below atmospheric pressure (10"* Pa or 10~'°Torr) and a

trap region surrounded by surfaces cooled to liquid helium

temperature, the EBIT runs under conditions of very high

400

4675 4725

Energy (eV)

4775

purity. Spectra are observed only from elements injected into the trap.

3.9. Doppler shifts are typically negligible in an EBIT

Although the ion temperature inside an EBIT can reach values that compare to the hottest plasmas produced on

earth (1 keV « lOOOOOOOK), the velocities are stUl only a small fraction of the speed of light (u/c = 0.01% for 1 keV

barium) so Doppler broadening of typical X-ray transitions

is smaller than both the natural linewidth and the typical

instrument resolution. Furthermore, since the ions are trapped and excited in a symmetric potential at rest in the lab frame, there is no residual Doppler shift. At Livermore,

ultra-high resolution X-ray spectrometers have been used to measure the Doppler broadening in select X-ray transitions

(see Fig. 1 of Ref. [21]) while at NIST we have used a lowresolution optical interferometer to measure the Doppler

broadening in visible light transitions (see Fig. 2 of Ref

[22]). With evaporative cooUng techniques, ion temperatures as low as 0.07 keV (50 times below the electron beam energy) have been obtained [21].

3.10. The EBIT is a sufficiently low density source that line profiles are unperturbed

Electron densities in an EBIT are about 4 x 10'^ cm"^,

comparable to that found in solar flares and Tokamak

fusion devices (see Fig. 2, Ref. [23] for a comparison with other Ught sources). The number density of highly charged

ions inside an EBIT is about 4 orders of magnitude lower

than the electron density. This results in conditions in which

self-absorption and collisional effects are negligible.

3.11. The EBIT is a steady, CW source EBITs have been put into operation for extended experi- ments which continue 24 hours a day, 7 days a week. A

single bunch of atoms can be injected and held in the trap

for hours (see Fig. 7) [20] before the intensity of emitted

light drops suQiciently to require reinjection of another

bunch. Bunches can be injected at repetition rates of order

1 Hz if necessary.

3.12. The EBIT can be rapidly switched

Lifetime measurements are examples of experiments in which rapid switching times are desirable. Both electron beam energy and current in the EBIT can be switched,

allowing the subsequent temporal decay of the intensity of

the emitted light to be monitored in order to determine the

Fig. 6. Spectrum showing that EBIT is a bright enough source to allow

high resolution X-ray spectra to be obtained in moderately short periods of

time. This spectra was taken in 20 minutes with a von Hamos spectrometer

with 0.06% energy resolution [18].

40 60 Time (mtnutes)

Fig. 7. Plot showing that ions can be trapped for hours inside EBIT. Here

a single bunch of gold ions was injected into EBIT at r = 0, stripped to a

high charge state (Q = 37 + ) in a fraction of a second, and then held in the trap for more than an hour as the intensity of the light emitted from the n ~ 2-3 transition lines was monitored. The 1/e decay time estimated firom the data is approximately 4 hours.

Physica Scripta T65 NIST-22
172 J. D. Gillaspy

lifetime of an excited state. In the case of current switching,

the ions remain trapped by the 3T magnetic field present

inside the EBIT. At Livermore, electron beam energies have

been switched with amplitudes of order 1 keV on time scales of order 10ns to measure Lifetimes between 6ns [24] and

90 |is [25] with X-ray spectroscopy. At NIST, electron beam

currents have been switched off to measure lifetimes of order

1 ms with visible light spectroscopy [26].

3.13. EBIT devices are relatively affordable and easy to build Although construction of a new EBIT facility requires a sig- nificant investment in time and money, this effort is small compared to that of any other device that can approach its capabilities. A full-scale EBIT device can be fabricated for a few hundred thousand dollars, a figure which roughly

matches the cost of the operating electronics and instrumentation. Development time can be avoided because EBITs

are now available from commercial sources. Future develop- ments can be expected to lower the cost greatly (designs are being developed, for example, for new EBITs which do not

require any magnetic field [27]). For experimenters who

wish to use existing EBIT devices in a collaborative way,

facilities are available in several locations around the world

(see Section 6 below).

4. Limitations The previous discussion has emphasized how close to ideal conditions one can achieve with an EBIT. In typical practice, the situation may not be much different, but there are some limitations which should be kept in mind.

Lack of independence of all the variables can be one of

the most troublesome limitations. Varying the electron beam energy, for example, not only adjusts the charge state, but also the range of levels accessible for excitation and

(slightly) the trap depth. These limitations can be largely overcome by electronically automating the system to slew

the electron beam rapidly between different energies, one

energy to create the charge state, for example, and another

to probe it. Such a system is available at all the major EBIT

facihties. Duty cycle factors reduce the efficiency of data collection in this mode of ojjeration.

Another compromise involves the amount of gas injected

for evaporative cooling of the ion temperature - too much

gas, and the vacuum level may be reduced enough that colli- sions will degrade the charge state purity. At the same time,

evaporative cooling may be essential since it can improve

the signal rate by orders of magnitude by allowing the ion

density to be increased. Generally however, a good compromise can be reached in which the signal level is greatly

enhanced while the charge state distribution is modified

only slightly. With present EBIT designs, the maximum obtainable

electron beam currents diminish below 2 kV (see Fig. 4, Ref.

[7]), limiting the production of low charge states in contin- uous trapped mode of operation. Useful data in CW trapping mode have been obtained for electron beam energies

down to 0.67 keV [28]. Active [29] or passive transient methods can be used to circumvent the problem, but if one's primary goal is to study low charge states, other light sources should be considered as alternatives.

Physica Scripta T65

At the other end of the charge-state spectrum, it should

be noted that work with the highest charge states (primarily

the production of bare and hydrogenic charge states of heavy elements and the excitation of helium-like charge

states in the heaviest elements) requires both a high energy (>130keV) electron beam and a vacuum level significantly

beyond that of normal EBIT operation. While an EBIT can

ionize a neutral atom up to a charge state of 50 4- in a small

fraction of a second, getting to 92+ can require several seconds, and signal rates can be weak. While an EBIT can produce and hold hundreds of thousands of ions with

charge states of 50 + , the number of bare uranium atoms

produced and held so far is over four orders of magnitude

lower. Experiments that approach the upper end of the EBITs capability require an increasing amount of patience, but are feasible (the hydrogenic uranium data in Fig. 1 was

acquired in just over one day using a high-efficiency Ge

detector). As mentioned in Section 4, charge state selectivity

depends on the proximity (in energy) of the next ionization

stage, and is not always near 100%. In an openshell system

such as U*^* in which the ionization energy is 8keV and

the adjacent ionization stages are within O.lSkeV, one can

expect the charge state distribution in the EBIT to have a fuU-width-half-maximum of 4, and a full-width-tenth- maximum of 7 (see Fig. 4b, ReL [30]). This is only modestly

better than that expected using the beam foil method to produce similar charge states in gold ions [31], although in the case of gold some of the nearby ionization stages are separated by much larger energies.

Another factor that must be considered with regard to charge state balance is that when the electron beam energy

is scanned, the presence of localized dielectronic recombination (DR) resonances can abruptly modify the charge

state distribution. Care must be taken to avoid such reso- nances when their presence could affect the results of a measurement, as is often the case. This task can be complicated

by the fact that the electron beam energy can be unintentionally changed slightly when the current is adjusted, even if the drift tube voltages are held absolutely fixed (since the current affects the space charge correction to the electron beam energy [1]). The dependence of charge state distribu- tion on proximity to DR resonances makes CW measure- ment of the DR profile as a function of electron beam

energy a recursive problem. Virtually all of the problems

mentioned in this paragraph, however, are overcome in the Umit that the electron beam is scanned rapidly compared to the charge distribution relaxation time using "event-mode"

techniques as described in the literature [15]. With regard to excitation selectivity, one must keep in mind that upper levels can be populated not only by direct

excitation, but also by recombination (dielectronic or

radiative) either directly to the upper level or to higher lying

levels which may then allow cascade through the upper level of interest Because of this, one cannot guarantee that a line

will be extinguished simply by lowering the electron beam

energy below the threshold for direct excitation. Either theoretical analysis or experimentation may be required to determine whether a particular line can be fully extinguished

by varying the electron beam shghtly.

Although the presence of a third dimension (electron

beam energy) to aid in the analysis of spectra can be a great

NIST-23
EBIT Spectra of Highly Stripped Ions from the Visible to the X-Ray 173

tool, it also generates a large amount of phase space to probe. In cases where the desire is to collect as much data as possible in the shortest time available, the selectivity of an EBIT may not be fully desirable. Some experimenters may

prefer a dense spectrum of lines over the selectivity and

spectral purity provided by an EBIT. Electronically scanned modes of operation, however, can effectively collapse the third dimension and simulate any temperature distribution

(as long as one neglects processes involving the simulta- neous presence of two electrons of two different energies).

Conversely, the resolution obtained along the third dimension may not always be as high as desired. Dielectronic recombination resonances often have structure which cannot be fully resolved with the electron energy spread which is typical in an EBIT. This is one type of measurement in which accelerator based storage ring methods

which employ electron coolers offer clear advantages over

existing EBIT capability. Another limitation due to the EBITs finite electron beam resolution is that very close lying dielectronic recombination satellites which arise from

high-lying Rydberg levels cannot be switched off [32]. The times to acquire useful spectra mentioned in Section

3.6 above might all be multiplied by a factor of ten or more

in order to correspond to a high statistics data set appropriate for a precision measurement. With sufficient instrument

resolution, however, very high precision wavelength measurements can be obtained without many photons.

Although the slit-like source shape might be considered

ideal for use with a ruled-grating spectrometer, some Bragg

reflection X-ray spectrometers would be even better served by a point source with the same total ffux. The elemental purity provided by the ultra-high vacuum is compronoised for at least one element due to the necessity to dope the electron gun cathode with barium in order to lower the work function. The cathode nms at high temperatures and is in a direct line of sight with the trap center. On time scales of many seconds, the trap acoimulates sig- nificant amounts of barium which can show up in the spec- trum. When this is a problem, the situation can be avoided

ahnost entirely by periodically emptying the trap and

rapidly refilling it with the desired element before the barium has time to reaccumulate; reduction of data accu- mulation rates due to the duty cycle for this procedure can

be made negligible.

Although an EBIT can run uninterrupted for long

periods, it does not always do so. More typically the operation is punctuated by occasional bursts of secondary

current which inevitably end up appearing on the electron gun anode or auxiliary electrodes in that general proximity.

These occurrences trip automatic shutdown protection in the power supplies, but recovery is typically immediate

(seconds).

5. Future improvements As a relatively recent development, the application of EBITs

to their full capabilities is still in a rapid growth phase. A

number of further improvements can be expected in the coming years, including the development of VUV spectros- copy with an EBIT, laser cooling and probing of ions in an EBIT [33, 34], Fourier-transform ion cyclotron resonance

probes to precisely determine and control the number of

ions in the trap [35], faster switching times to measure processes on faster time scales, more efficient element injection,

higher brightness, higher beam currents, and larger magnetic fields.

6. EBIT facilities worldwide

There are presently three major EBIT faciUties which are

active: Livermore [1], NIST [19], and Oxford [34]. The

Livermore facility has been producing atomic physics data

for 8 years and presently houses three EBITs of various

sizes (an EBIT, a SuperEBIT, and a mini-EBIT). The Liver- more team has published nearly 100 papers, the first 42 of which are reprinted in a compilation available from the National Technical Information Service [36]. The NIST

EBIT facility began producing atomic physics data during

the past year [6], and the Oxford facility is expected to produce data in the near future [37]. A SuperEBIT facility was nearing completion in Russia (Dubna) several years ago, but the effort is presently

dormant [38]. A major EBIT facility is under construction

in Japan (Tokyo) [39], and another is being planned in Germany (Berlin) [40]. Proposals have been made for at

least one more facility in the U.S. (Connecticut) [41] and in Germany (Dresden) [42], France [43], and China.

References

1. Levine, M. A., Mans, R. E., Hendeisoa, J. R., Knapp, D. A. and Schneider, M. B., Pbysica ScripU T22, 137 (1988).

2. Mans, R. E, Beieisdorfer, P. and Schneider, D, Phys. Today 47, 27

(1994).

3. Lekione, D. S. and Sugar, J. (Editors), Physica Sciipta T47, (1993).

4. Knapp, D. A., Mam, R. E., Ellioitt S. R., Magee, E. W. and Zasadzinski, VL, NucL InsU. Metb. A334, 305 (1993).

5. Mans, R. E.. Elliott. S. R. and Knapp, D. A.. Phys. Rev. Lett 72. 4082

(1994).

6. Morgan. C. A. a al^ Phys. Rev. Lett 74, 1 7 1 6 ( 1995).

7. Gillaspy, J. O., Proceedings of the Vltb International Symposium on

Electron Beam Ion Sources and Their Application, Physica Sciipta, to be published.

8. Margohs, H. S., "Studies of Highly Ionized Atoms Using an Electron Beam Ion Trap", Ph-D. thesis, Oxford, 1994.

9. Serpa. F. G. « oi, to be published.

10. Srivastava, B. N. and Miiza, I. M. Phys. Rev. 176, 137 (1968).

11. Chantrenne, S., Beieisdorfer, P., Cauble, R. and Schneider, M. B.. Phys. Rev. Utt 69. 265 (1992).

12. Pcnetiante, B. M.. Bardsley. J. N.. DcWitt D, Qark, M. and Schneider, D., Phys. Rev. A43. 4861 (1991).

13. Beiersdorfer, P. et al.. Proceedings of the ICPEAC XVU Conference,

Brisbane. July 1991, p. 313.

14. DeWitt, D. R. et al., Phys. Rev. Lett 6S, 1694 (1992).

1 5. Knapp. D. A. et al., Phys. Rev. A47, 2039 ( 1993).

16. Beiersdorfer. P., Phillips, T. W., Wong. K. L.. Maris, R. E. and Vogel, D. A., Phys. Rev. A46. 3812 (1992).

17. Beiersdorfer, P. and Waigelin, B. J, Rev. ScL Instrum. 65, 13 (1994).

18. Beiersdorfer. P. et al.. Rev. Sci. Instrum. 61. 2338 (1990).

19. Gillaspy, J. D. et al.. Proceedings of Nobel Symposium 91, Physica ScripU T59. 392 (1995).

20. Schneider, M. B. et al.. International Symposium on Electron Beam

Ion Sources and Theii Application, (Edited by A. Hershcovitch), AIP

Conference Proceedings No. 188, (AIP, New York 1989), p. 8Z

21. Beiersdorfer. P., Decaux. V, Elliott, S. R., Widmann, K. and Wong. , K.. Rev. Sci. LismmL 66. 303 (1995) 2i Adler, H. et al. NucL Instr. Meth. B98. 581 (1995). 23. Martinson, L, Rep. Prog. Phys. 52, 157 (1989). 24. Crespo Lopcz-Unutia, J., Beieisdorfer. P.. Widmann, K. and Decaux.

v.. Reported at 199S DAMOP, Confeience, and to be published.

Physica Scripta T65

NIST-24
174 J. D. Gillaspy

25. Wargelm, B. J., Beiersdorfer, P. and Kahn, S. M., Phys. Rev. Lett. 71, 2196(1993).

26. Serpa, F. G. et al.. to be published.

27. Thomae, H., Becker, R, Bongers, H. and KJeinod, M., Nucl. Instr. Meth., 398,577(1995).

28. Wargelin, B. J, A Study of Diagnostic X-ray Lines in Helium-like Neon Using an Electron Beam Ion Trap, Ph.D. thesis. University of

California at Berkeley, 1993.

29. Decaux, V. and Beiersdorfer, P., Physica Scripta T47, 80 (1993). 30. Schneider, D. et al., Phys. Rev. A44, 3 1 19 ( 1 99 1). 3 1. Chandler, G. A. et ai, Phys. Rev. A39, 656 (1989). 32. Beiersdorfer, P. et ai, Z. Phys. D21, S209 (1991). 33. Beiersdorfer, P., Proc. of the 15th International Conf. on X-ray and

InnerShell Processes, AIP Conference Proceedings No. 215, (Edited by T. A. Carlson, M. O. Krause and S. T. MansonXAIP, New York

1990), p. 648.

34. Silver, J. D. et al.. Rev. Sd. Instnun. 65, 1072 (1994).

35. Beiersdorfer. P. et al., Nucl. Instr. Meth. B98, 558 (1995).

36. "Selected- Pubhcations from the Electron Beam Ion Trap Program at Lawrence Livenaore National Laboratory" (Edited by R. E. Maris), UCRL-IDl 10491, available from National Technical Information

Service, U.S. Department of Commerce, Springfield, VA 22161, as document DE92-018-232.

37. Silver, J., private communication, 1995.

38. Ovsyannikov, V. P., et aU Rev. ScL Instrum. 65, 1 132 (1994).

39. Obtani, S., private communication, 1994.

40. Fussmann, G., private communication, 1994. 41. Smith, W. W., Pollack, E. and Kessel Q. C, private commimication.

1995.

42. Soff, G., private commimication, 1995.

43. Briand, J. P., private communication, 1994.

Physica Scripta T65

NIST-25
Hyperfine Interactions 108 (1997) 59-72 59

X-ray, visible and electron spectroscopy with the NIST

EBIT

E. Takacs^•^ L.P. Ratlilf ^ and J.D. Gillaspy'^

" Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899,

USA

^ Experimental Physics Department, Kossuth University, Bern ter 18/a, H-4026 Debrecen, Hungary

An overview is given of recent activities at the NIST electron beam ion trap (EBIT)

facility. The machine has been operational for almost three years. Important characteristics and demonstrated capabilities of our EBIT are presented. Selected results include experiments

with trapped highly charged ions (X-ray and visible spectroscopy), and with extracted ions

(ion-surface collision studies).

1. Introduction

Activity at the EBIT facility in Gaithersburg, MD at present can be divided into

two broad categories. One category is associated with the spectroscopic investigation

of the electromagnetic radiation emitted by highly charged ions created and trapped

within the machine. In these measurements the ions are studied in situ at the place

where they are created. The spectra exhibit the properties of the trap (hot plasma

confined by magnetic and electric fields), the interaction between the electrons and

the ions (excitation, ionization, and recombination processes), and the ions themselves

(atomic structure). The second group of activities are associated with the extracted

ions. In this case we use the EBIT as an ion source in which the highly charged

species are created and then extracted for outside measurements. The extraction and

beam transport is done by several electrostatic ion-optical elements and a magnet,

which is used for separating the specific charge-state needed for the experiment. After a brief introduction to the machine, some selected results will be presented to give

examples of the latest activities and to illustrate the capabilities of the EBIT laboratory

at NIST

© J.C. Baltzer AG, Science Publishers

NIST-26
60 E. Takdcs et al. / X-ray, visible and electron spectroscopy

2. The NIST EBIT facility

2.1. General

The electron beam ion trap at the Gaithersburg, MD site of the National Institute

of Standards and Technology (NIST) was built jointly by NIST and the Naval Research Laboratory (NRL). There are earlier papers describing the properties of the

machine [1,2], so only a brief introduction will be presented here.

The EBIT was built along the lines of the original design of the first Livermore

machine [3] with a few modifications. The electron gun used can provide up to 160 mA of electron beam current. The beam compression at the drift tube region

is accomplished by a pair of liquid helium cooled superconducting Helmholtz coils

capable of producing magnetic fields up to 3 T. The voltage applied to the central

drift tube determines the energy of the electron beam at the interaction region. The

electrostatic trap potential in the axial direction can be up to 500 V deep. The highest

demonstrated voltage applied to the central drift tube compared to the potential of

the electron gun (ground potential) in our machine is 32 kV. Thus, the electron beam

energy should be enough to strip off all but two electrons from atoms with atomic

numbers up to about sixty and to create neon-like ions of all the naturally occurring

elements.

Neutral gases are injected into the trap through one of the side ports facing the

central drift tube. Ionized metal atoms - produced by a metal-vapor-vacuum-arc (MEVVA) ion source placed on the top of the EBIT - are injected along the axis of

the machine parallel to the magnetic field lines. Electromagnetic radiation emitted

from inside the EBIT is transmitted through beryllium and quartz window side ports

for X-ray and visible spectroscopy, respectively. Examples from the palette of ions

which has been created and studied in our machine include different ions of nitrogen,

argon (up to 18-I-), krypton (up to 34-f-), xenon (up to 48-f-), barium (up to 46-1-), iron,

gadolinium, and neodymium.

2.2. The extraction system

Although highly charged ions created and trapped inside the EBIT offer a great

number of research opportunities (e.g., [4-14]), there are other experiments which

require the ions to be extracted from the trap. These experiments include the study of

ion-surface interactions by measuring surface damage, electron emission, and X-ray

emission. Ion-gas interactions such as charge exchange and recoil-ion experiments

may be studied as well. Recapturing ions into secondary ion traps is necessary for

another important class of atomic physics experiments which can not be done in the

presence of the electron beam (lifetime measurements of long-living excited-states,

cold ion dynamics studies, and precision mass measurements, for example). For many

of these types of experiments, data for highly charged ions are very sparse or nonexistent. Finally, extracted ions may also be used to reveal important information about

NIST-27
E. Takdcs et al. / X-ray, visible and electron spectroscopy 61

the conditions inside the EBIT and, therefore, provide complementary information for

spectroscopic studies being performed on ions trapped inside the machine.

A detailed description of the extraction and transport system can be found in [15].

An updated schematic of the beam line for the NIST EBIT is shown in fig. 1. Ions can be extracted from the trap into the beam line continuously or in pulses, typically

10 fxs to 100 ms long. The first ion-optical element in the beam line is a 90° electrostatic bender. There are other electrostatic elements to steer and align the beam

(deflectors, einzel lenses) before it enters the charge-to-mass analyzing magnet. There

is a four-jaw slit in the focal point in front of the magnet and a two-jaw slit in the

focal point behind the magnet serving as entrance and exit slits of the analyzer. The

entire system is fabricated from ultra-high-vacuum compatible materials and can be

baked to at least 100°C. Vacuum pumps maintain a pressure in the beam line of about

10~^ Pa (10~^ Torr) or better to prevent significant losses from the ion beam due to charge-exchange collisions with residual gas in the beam line.

Ion beams of several highly charged ion species have been transported through the

beam line and into the experimental chamber after the analyzing magnet. Measurements of the ion-beam current have been performed using two Faraday cups - located

after the first electrostatic deflector and after the entrance slit of the analyzing magnet - which show that no ions are lost in this section of the beam line. Figure 2 shows

an example of a charge-to-mass scan for 7.8 keV per charge Xe ions with the EBIT

operated in a continuous-extraction mode.

Presently [16], the NIST EBIT beam line can deliver a continuous stream of

3,000,000 charge-to-mass selected highly-charged ions (Xe^^) per second into a spot

3 mm in diameter. This number corresponds to a 21 pA beam current and exceeds any

previously reported values of continuous-ion-beam currents extracted from an EBIT

by several orders of magnitude. Since this beam current is 10 times higher than the

value our group recently reported [15], we expect even further improvements. The

system has also been operated in pulsed-beam mode. Operating at 2 Hz repetition rate, a change of the pulse width from 2.8 ms to 0.012 ms changes the time average current

from 9.6 pA to 1.8 pA, respectively. This mode of running - important for retrap

experiments - boosts the instantaneous beam current by several orders of magnitude.

3. X-ray spectroscopy

There are three X-ray instruments installed on the beryllium window side ports of

the EBIT for wavelength and energy dispersive X-ray spectroscopy. Two of them are

identical Johann-type bent crystal spectrometers for precision X-ray wavelengths and

polarization studies. A description of the spectrometers and detectors can be found in

refs. [17,18]. As a generic diagnostic tool and for measurements which require higher

detection efficiencies but where the energy resolution is not so critical we use a Si(Li)

detector.

NIST-28
E. Takdcs et al. / X-ray, visible and electron spectroscopy

i

Fig. 1 . The extraction and ion-transport system of the NIST EBIT facility. It consists of several electrostatic steering and focusing elements and a charge-to-mass analyzer magnet. At present the system can

provide efficient transfer of highly charged ions into experimental chambers on two separate beamlines.

NIST-29
E. Takdcs et al. / X-ray, visible and electron spectroscopy 63

I Ir I I t*— —I—I—I I I i^-f I

I . , , V-w^ , , I 0.108 0.109 0.110 0.111

Analyzing Magnet Field (T)

Fig. 2. Charge-to-mass analysis scan for 7.8 keV xenon ions. The isotopes in different charge states are depicted.

3.1. Inner shell dielectronic recombination in Sc-like barium

Dielectronic recombination (DR) is a resonant phenomenon and one of the fundamental processes in electron-ion collision physics. Due to the usually high relative cross sections corresponding to the DR process it plays an important role in determining the charge-state balance of ions in plasmas. The DR proceeds as

e + A^^ii)

^^(^-^)+(d)**^A(^-')+(f)* -f hu. (1)

The first step involves a simultaneous continuum-to-bound and bound-to-bound

electron transition. The bound-to-bound transition gives the resonant character of the

process. The resulting doubly excited autoionizing state can relax by photon emission

and the characteristic radiation is the signature of the process. According to eq. (1)

DR can be detected by either measuring the X-ray photons Qiv) [19] or measuring the

recombined ions A^^"')"*" [20].

We would like to point out that measurement of the polarization or the angular

distribution of the emitted X-rays provides information on the magnetic sublevels

involved in the process. This information remains hidden in experiments where the

detection is based on the number of recombined ions.

We started to explore the capabilities of our EBIT for measuring DR resonance

processes by investigating the phenomenon in highly charged many-electron barium

ions [19]. We used our Si(Li) detector to capture X-rays from resonant DR processes

involving 2p 3d excitations of Sc-like barium (Ba^^"*") and Ti-like barium (Ba^"^"*").

The X-ray spectra shown in fig. 3 include several small peaks and a large peak which is

NIST-30
64 E. Takdcs et al. / X-ray, visible and electron spectroscopy

C

3

OU

Energy (keV)

E-Beam Energy from 2440 eV to 2350 eV

Fig. 3. The X-ray emission in Sc-like and Ti-like barium at around 4.6 keV becomes strong at a certain electron impact energy, a distinct signature of the dielectronic recombination process [19]. (DE: direct

excitation, RR: radiative recombination, DR: dielectronic recombination.)

NIST-31
E. Takdcs et al. / X-ray, visible and electron spectroscopy 65

Strong only at certain electron beam energies. The strong X-ray emission line observed

at 4.6 keV appears when the electron beam energy is tuned close to 2.3 keV. At this

beam energy the Ti-like and the Sc-like ions of barium are the main ions present in

the EBIT. Calculations show that the smaller peaks (labeled RR) are from different

radiative recombination (RR) processes and the large peak is due to the resonant inner

shell excitation of a 2p electron to one of the 3d levels. The simultaneous measurement

of the well-separated RR and DR lines allows the deduction of relative cross sections.

The calculations for the cross sections, done at the University of Connecticut, were

based on the angular momentum average (AMA) approximation which is supposed to be justified when 3d levels are involved and also when the electron-beam energy spread

is as large as a few tens of electron volts, as in our case. Including model calculations to account for our trap conditions (charge-state distribution balance, electron-ion overlap

factor) we got good agreement between the measured and calculated cross sections.

Details of the experimental and theoretical results can be found in [19].

In our EBIT the spread of the electron-beam energy was about 40-50 eV and the

precision of the energy was 1-2 eV. Although this broad energy distribution seems to be

several orders of magnitude larger than those reported at electron cooler facilities, we

would like to note, however, that the center-of-mass impact energies are much larger

in EBITs, too. This allows the observation of X-rays from inner-shell DR excitation

processes, which are rarely observed at storage rings where the measurements usually

extend up to only a few tens of eV center-of-mass energies. Using the scaling law for

the energy spread given by [21] it can be shown that at such high energies the energy

spread is "only" about ten times less in storage rings compared to EBITs.

3.2. Polarization ofNe-like barium X-rays

Polarized X-ray emission occurs when the electron charge cloud (wavefunction) of

the ion - corresponding to the initial state of the transition - shows spatial anisotropy.

This spatial anisotropy is quite common in astrophysical plasmas. In solar flares, ions

and atoms can be excited by electrons moving along fixed magnetic field lines, which

give rise to a preferred direction in space. Other examples include supernova shock

waves, polar aurorae, and jets in active galactic nuclei. In a laboratory environment

polarized X-ray emission can be observed in crossed beam and beam foil studies. In an EBIT the excitation of the ions also occurs in a spatially asymmetric way by the well

collimated and directed electron beam. Since the spatial orientation of the electron

cloud of the ion is sensitive to the excitation process, the polarization measurements are suggested as a method to pinpoint the presence or the importance of certain excitation

channels.

For the polarization measurements our two Johann-type crystal spectrometers were

operated simultaneously, with their respective reflection planes perpendicular to each

other. The direction of observation was at 90° relative to the electron beam direction.

For a Ge(220) {Id = 0.400 nm) crystal the Bragg angle is close to 45° for X-ray

wavelengths around 0.28 nm. For these wavelengths the crystal is a near-perfect

NIST-32
66 E. Takdcs el al. / X-ray, visible and electron spectroscopy

polarizer. In the setup described above, the spectrometers simultaneously measure

the parallel and perpendicular polarization components of the radiation. The intensity cross calibration of the two spectrometers can be done by comparing the measured

intensities of a suitable unpolarized line.

In our case, we studied the electron beam energy dependence of the polarization of

the [(2p3/2)~* 3si/2]j=2 to [2p^]j=o magnetic-quadrupole (M2) X-ray emission in Nelike barium. The energy of this line is around 4563 eV corresponding to a wavelength

of 0.272 nm. Although the transition has a relatively small Einstein ^-coefficient

(about 3.0 X 10^ s~') the relative intensity of the line is comparable to that of the

high Einstein A-coefficient lines. This is because of the strong cascade feeding of the

upper level of the transition (driven by electron impact excitation to higher levels) and

the fact that the number of decays per unit channel are bottlenecked by the electron

impact excitation rates rather than the decay rates. For the intensity cross calibration

we used the nearby [(2p3/2)~' 3si/2]j=i to [2p^]j=o electric-dipole (El) line. Our

model calculations show that this line is unpolarized to a very good approxirnation.

Figure 4 shov/s our results. In the electron impact energy range from 5.0 keV

to 7.8 keV, the M2 line shows a strong negative polarization. We attributed the big

change of polarization near 5.0 keV, as well as near 5.1-5.3 keV, to the presence

of resonant excitation processes. These are not included in our calculations which

show a general agreement with the trend of the data at the non-resonant electron beam

energies. Details of the experiment can be found in 122].

4. Visible light spectroscopy

The well developed visible light spectroscopic techniques that serve as excellent

tools for observations of electronic transitions in neutral atoms usually cannot be applied to the study of highly charged ions. The reason is that the energy differences

between the same levels on an isoelectronic sequence increase rapidly with the nuclear charge Z (e.g., for El transitions the energy scales with Z^). Although visible

transitions in highly charged ions are thus hard to find, there is a strong need to locate

them for practical applications, in particular for high-temperature plasma diagnostics.

Such plasmas are present for example in the solar corona and in tokamaks. Local ion

temperatures, bulk plasma velocities, direction and strength of local magnetic fields can all be inferred from Doppler and Zeeman broadening and shifts of spectral lines

originating from these environments.

4.]. Observation of visible transitions in highly charged ions with a grating

monochromator

We experimentally found visible transitions in highly charged barium and xenon

which were theoretically predicted by Feldman et al. [23]. They predicted that there

should be an unusual magnetic dipole transition between levels of the ground-term

NIST-33
E. Takdcs et al. / X-ray, visible and electron spectroscopy

Resonance excitation

67

/ \

LNO LOO

I I I I 'I I I 1 i_i ; I I I 1 I I I 1 I I I I II I I I I i I 5000 5500 6000 6500 7000 7500 8000

Electron Energy (eV)

Fig. 4. The overall electron energy dependence of the polarization of the [(2p3/2)~' 3si/2] j=2 —> pp**] j=()

magnetic-quadrupole line in Ne-Iike barium. The solid line represents the calculations of Inal and Dubau

which do not include resonant excitation processes [22]. In the energy ranges where the LNO and LOO

resonant excitations take place, a big change in the polarization can be seen.

configuration of Ba-^"^"^ and Xe^-^"^. One of its unique properties is that this transition

remains in the visible or near-UV region for a range of ions covering the entire upper

half {Z > 45) of the periodic table. This fact makes the transition an unmatched

candidate for hot plasma diagnosis and offers a great challenge for theorists who want

to predict the transition energy using ab initio calculations.

Experimentally, we used a 0.25-m focal length, //3.5 Ebert scanning monochromator with a blue-sensitive photomultiplier operated in photon-counting mode. The lines

appeared at 393.24 nm and 413.93 nm wavelengths in Ba^'^+and Xe^^"*", respectively.

The wavelength calibration was done by shining light from low-pressure discharge

lamps (He, Hg, Ne, Ar, and Xe) through the EBIT from a port opposite to the spectrometer and by in situ measurements of nearly neutral krypton and argon lines. This

calibration resulted in an overall 0.2 nm uncertainty for the wavelengths [24]. As,

a continuation of the first observation, our group went on to study the isoelectronic

dependence of the transition energy [25].

NIST-34
68 E. Takdcs et al. / X-ray, visible and electron spectroscopy

4.2. Fabry-Perot spectroscopy

We used a Fabry-Perot instrument for a preliminary measurement of the previously

observed 2)d^ ^D2-^D3 line in with improved resolution [26]. In our original

study using the grating monochromator the 0.8 nm width of the line was dominated

by the low resolving power of the instrument. However, if one wants to use this line

for diagnosis of hot plasmas the instrument resolution has to be increased to allow

observation of the Doppler and Zeeman effects. In the experimental setup, light from the EBIT was transmitted through a quartz

window side port and then collimated onto a plane Fabry-Perot interferometer. The

instrument was followed by a 24.5 cm focal length fringe-imaging lens and a 4 mm

central diameter aperture placed at the focal point. Behind the aperture the detection

was done by the same photomultiplier used with the grating monochromator before.

The wavelength scanning was done by precisely changing the mirror spacing using a

piezoelectric element.

By fitting the data, shown in fig. 5, to Doppler broadened model line profiles, we

deduced the temperature of the ions within our trap. We found that it is somewhat

less but close to 1 keV. This conclusion is in agreement with the measurement of

Beiersdorfer et al. [27] who used a high resolving power X-ray spectrometer to measure

ion temperatures. It also agrees with our model calculations based on the numerical

simulation of the charge state evolution within the EBIT using a code developed

by Penetrante et al. [28]. We hope that with further studies using a higher resolution

interferometer and different operating conditions of the EBIT, even lower temperatures can be achieved and measured down to the point where the Zeeman components of

the line profile become visible. At that point the effects of local magnetic fields can

be studied.

5. Electron spectroscopy with extracted ions

The extraction system described in section 2.2 efficiently transfers the ions created

in the EBIT into experimental chambers. In the following, we report our preliminary

study of the interaction of slow highly charged ions with solid surfaces by means of

low energy electron spectroscopy.

There is a need to study ion-surface collision processes both for potential technological applications and from the perspective of fundamental research regarding the

behavior of matter under the influence of extremely high and localized Coulomb fields.

The availability of novel ion sources (EBIS, EBIT, ECR) has boosted the activity in

this field of research in recent years and an enormous amount of progress was made

in understanding the details of the processes goveming the ion-surface interaction.

There were especially important measurements done with (very) highly charged ions

from an EBIT [29-31]. New exotic phenomena were reported, such as the formation of "hollow atoms" or the so-called "Coulomb explosion" on insulator surfaces.

NIST-35
E. Takdcs et al. / X-ray, visible and electron spectroscopy

FABRY-PEROT SCAN (F=1 5)

69

O

o

140

120 -

100

Relative phase {2%)

Fig. 5. Doppler broadened profile of the 3d'* ^D2-^D3 line in Ba^'*'^ measured with a Fabry-Perot

instrument. The calculated curves are the results of our simulations assuming different ion temperatures.

The temperature of the ions in our trap is less than 1 keV [25]. (Note that a mislabeling of the horizontal

axis in the original publication has been corrected; the results are unchanged.)

There are still several open questions, however, which need further and more detailed

exploration. Recent reviews of the field can be found in [32-34].

When low-energy multiply charged ions are neutralized at solid surfaces, tens to several hundred-thousands of electron volts of atomic potential energy are dissipated,

primarily via ejection of electrons. Using well-characterized surfaces, the quantitative

analysis of these electrons can give detailed information about the atomic properties

of both the projectile and the target, and about the collision mechanisms.

With the setup shown in fig. 6, a preliminary study of low energy electrons originating from the collisions of highly charged ions with surfaces was performed. The

electron spectrometer was designed and developed at the Institute of Nuclear Research

of the Hungarian Academy of Sciences, Debrecen, Hungary. During the measurements

it was placed into a magnetically shielded container to reduce the effect of external,

magnetic fields which would distract the path of low energy electrons within the

analyzer.

NIST-36
70 E. Takdcs et al. / X-ray, visible and electron spectroscopy

Highly-charged-ions

from the EBIT

X^*^^ Tandem parallel-plate

electron energy analyser

Channel electron

multiplier

Fig. 6. Schematic of a parallel-plate electron analyzer equipped with a channel-electron-multiplier detector

to study the interaction of highly charged ions with a polycrystalline gold surface.

30

5 20

JQ

<

"v> I 10

* *

i

#

•

*

1—H 1 1 1—< 1 1 1 1 1 1 ••••••

—1—1 1 1 r—t——1

10 15 20

Electron Energy (eV)

Fig. 7. Energy spectrum of low energy electrons taken with the setup in fig. 6. The low energy peak is consistent with the expected convoy electrons from Xe"^"^ colliding with a gold surface.

Electrons emerging in the grazing incidence collisions of Xe'*^'*' ions with polycrystalline gold surfaces were energetically analyzed and detected. The preliminary

spectra shown in fig. 7 suggest the presence of convoy electrons originating in the

interaction. These electrons move with the speed of the specularly reflected projectile,

because they occupy low-lying continuum energy states of the outgoing ion. In further

experiments collective surface effects (image charge, surface plasmon effects) which

distort and shift the convoy electron peEik will be studied.

NIST-37
E. Takdcs et al. / X-ray, visible and electron spectroscopy 71

6. Summary

A general overview was presented to give some idea of the activities at the NIST

EBIT facility which has been fully operational for about three years. Experiments

centered around the in situ observation of ions have used different X-ray, UV and

visible light detection techniques. The newly operational extraction system delivers

energy- and mass-analyzed highly charged ions with currents unmatched by other

instruments with similar overall capabilities.

Acknowledgements

Part of the work was done with the support of the Hungarian Science Foundation

(OTKA, contract numbers: W 015012, F 013897, F 019344). One of the authors (E.T.)

acknowledges the Foundation for the Hungarian Higher Education and Research for

the support and the award of the Zoltan Magyari Fellowship for the years of 1995-

1996. The results presented in this paper are the achievements of a large number

of people from NIST, NRL and other collaborating groups whom we thank for their

overall contributions. We thank A.I. Pikin for producing the updated schematic of the

beamline and D.J. Alderson for assistance in constmcting the magnetic shield for the

electron spectroscopy measurements.

References

[1] J.D. Gillaspy, J.R. Roberts, CM. Brown and U. Feldman, in: Proc. 6th Int. Conf. on the Physics of

Highly Charged lons-1992, eds P. Richard, M. Stockli, C.L. Cocke and CD. Lin, AIP Conf. Proc. No. 174 (AIP, New York, 1993) p. 682.

[2] J.D. Gillaspy, in: Proc. 5th Int. Colloquium on Atomic Spectra and Oscillator Strengths for

Astrophysical and Laboratory Plasmas, Physica Scripta (1996), in press; and in: Proc. 6th Int. Symp. on Electron Beam Ion Sources, Physica Scripta (1995), in press.

[3] M.A. Levine, R.E. Marrs, J.R. Henderson, D.A. Knapp and M.B. Schneider, Phys. Scr. T22 (1988)

157.

[4] R. Becker, E.D. Donets, M. Kleinod, H.S. Margolis and J.D. Silver, Rev. Sci. Instr. 67 (1996) 983.

[5] H.S. Margolis, PD. Groves, J.D. Silver and M.A. Levine, Hyp. Int. 99 (1996) 169.

[6] H.S. Margolis, A.J. Vamey, R.A. Jarjis and J.D. Silver, Nucl. Instrum. Methods B 98 (1995) 562.

[7] J.D. Silver, A.J. Varney, H.S. Margolis, PE.G. Baird, LP Grant, PD. Groves, W.A. Hallett, A.T. Handford, PJ. Hirst, A.R. Holmes, D.J.H. Howie, R.A. Hunt, K.A. Nobbs, M. Roberts,

W. Studholme, J.S. Wark, M.T. Williams, M.A. Levine, D.D. Dietrich, W.G. Graham, I.D. Williams,

R. O'Neil and S.J. Rose, Rev. Sci. Instr. 65 (1994) 1072.

[8] D. Vogel, P Beiersdorfer, R. Marrs, K. Wong and R. Zasadzinski, Z. Phys. D (Suppl.) 21 (1991)

S193

[9] K.L. Wong, P Beiersdorfer, D. Vogel, R. Marrs and M. Levine, Z. Phys. D (Suppl.) 21 (1991) S197.

[10] R.E. Marrs, M.A. Levine, D.A. Knapp and J.R. Henderson, Phys. Rev. Lett. 60 (1988) 1715.

[11] CM. Brown, U. Feldman, G.A. Doschek, J.F Seely, R.E. LaVilla, V.L. Jacobs, J.R. Henderson,

D.A. Knapp, R.E. Marrs, P Beiersdorfer and M.A. Levine, Phys. Rev. A 40 (1989) 4089.

NIST-38
72 E. Takdcs et al. / X-ray, visible and electron spectroscopy

[12] D.A. Knapp, R.E. Marrs, M.A. Levine, C.L. Bennett, M.H. Qhen, J.R. Henderson, M.B. Schneider

and J.H. Scofield, Phys. Rev. Lett. 62 (1989) 2104.

[13] D.A. Knapp, Z. Phys. D (Suppl.) 21 (1991) 5143.

[14] P Beiersdorfer, D. Knapp, R.E. Marrs, S.R. Elliott and M.H. Chen, Phys. Rev. Lett. 71 (1993) 3939.

[15] A.L Pikin, C.A. Morgan, E.W. Bell, L.P Ratliff, D.A. Church and J.D. Gillaspy, Rev. Sci. Instrum. 67 (1996) 2528.

[16] L.P. Ratliff et al., to be published.

[17] B.P Duval, J. Barth, R.D. Deslattes, A. Henins and G.G. Luther, Nucl. Instrum. Methods 222 (1984)

274.

[18] A. Henins, Rev. Sci. Instrum. 58 (1987) 1173.

[19] D.J. McLaughlin, Y. Hahn, E. Takdcs, E.S. Meyer and J.D. Gillaspy, Phys. Rev. A 54 (1996) 2040.

[20] W. Spies, O. Uwira, A. Mueller, J. Linkeman, L. Empacher, A. Frank, C. Kozucharov, PH. Mokler,

F. Bosch, O. Klepper, B. Franzke and M. Steck, in: Physics of Highly Charged Ions, eds F. Aumayr,

G. Betz and H.P Winter, Special Issue of Nucl. Instrum. Methods B 98 (1995) 158. [21] W. Zong, D.R. DeWitt, R. Schuch, H. Gao, S. Asp and E. Justiniano, Hyp. Int. 99 (1996) 325.

[22] E. Takacs, E.S. Meyer, J.D. Gillaspy, J.R. Roberts, C.T. Chantler, L.T. Hudson, R.D. Deslattes, CM.

Brown, J.M. Laming, U. Feldman, J. Dubau and M.K. Inal, Phys. Rev. A 54 (1996) 1342.

[23] U. Feldman, R Indelicato and J. Sugar, J. Opt. Soc. Am. B 8 (1991) 3. [24] C.A. Morgan, F.G. Serpa, E. Takacs, E.S. Meyer, J.D. Gillaspy, J. Sugar and J.R. Roberts, Phys. Rev. Lett. 74 (1995) 1716.

[25] F.G. Serpa, E.S. Meyer, C.A. Morgan, J.D. Gillaspy, J. Sugar and J.R. Roberts. Phys. Rev. A 53

(1996) 2220.

[26] H. Adler, E.S. Meyer, FG. Serpa, E. Takacs, J.D. Gillaspy, CM. Brown and U. Feldman, Nucl. Insrum. Methods B 98 (1995) 581.

[27] P Beiersdorfer, V. Decaux, S. Elliott, K. Widmann and K. Wong, Nucl. Instrum. Methods B 98

(1995) 566.

[28] B.M. Penetrante, J.N. Bardsley, M.A. Levine, D.A. Knapp and R.E. Marrs, Phys. Rev. A 43 (1991)

4873.

[29] F. Aumayr, H. Kurz, D. Schneider, M.E. Briere, J.W. McDonald and CE. Cunningham, Phys. Rev.

Lett. 71 (1993) 1943.

[30] H. Kurz, F Aumayr, D. Schneider, M.A. Briere and J.W. McDonald, Phys. Rev. A 49 (1994) 4693.

[31] D.H.G. Schneider and M.A. Briere, Phys. Scr. 53 (1996) 228.

[32] F Aumayr and H.P Winter, Comm. At. Mol. Phys. 29 (1994) 275.

[33] P Varga, Comm. At. Mol. Phys. 23 (1989) 111.

[34] J. Burgdorfer, C. Reinhold, L. Hagg and F Meyer, Aust. J. Phys. 49 (1996) 527.

l^ST-39
-167- FORBIDDEN LINE WAVELENGTHS AND TRANSITION PROBABILITIES MEASURED

USING AN ELECTRON BEAM ION TRAP (EBIT)

E. Trabert

Experimentalphysik III, Ruhr-Universitat Bochum, D-44780 Bochum. Germany

P. Beiersdorfer, S.B. Utter Depanment of Physics and Space Technology,

Lawrence Livermore National Laboratory. Livermore, California 94501. U.S.A.

J.D. Gillaspy, F.G. Serpa

Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, U.S.A.

1. Introduction

Several coronal lines posed a long-standing riddle to earth-bound spectroscopists, until - following up

on a suggestion by Grotrian (1937) - B. Edlen (1942) confirmed that their wavenumbers indeed

corresponded to fine structure intervals in the ground configurations of highly charged ions like Fe X and

Fe XI. This in turn caused turmoil in solar physics, because the corona must be much hotter than the

underlying chromosphere in order to produce such ions.

X-ray and EUV spectra of the sun became available after World War II, by observations from

sounding rockets and satellites. These spectra confirmed the presence of the highly charged ions.

Laboratory observation of the (electric-dipole) forbidden lines, however, had to wait for the development

of low-density plasma discharges such as the Tokamak fusion experiments, because in regular light sources, collisions would probably quench such long-lived levels. Since then, a fair number of forbidden

transitions have been observed in the laboratory, and forbidden lines are being valued for plasma

diagnostics.

While forbidden transitions in light ions are often found in astrophysical light sources, similar

transitions in highly charged heavy ions such as Kr will be important for plasma machines like ITER, in which Kr will probably be used for radiative cooling and will therefore also be available for detailed

diagnostics.

2. EBIT

Electron beam ion traps (EBIT) now offer novel ways to study such forbidden transitions:

a) EBIT can produce any charge state ion of any element, with much simpler access and more elemental

flexibility than at a large-scale fusion-oriented Tokamak or similar plasma experiment.

b) EBIT can be switched from EBIT mode (with the electron beam on) to magnetic trapping mode

(electron beam off, then working like a Penning ion trap) in less than 1 ms, permitting time-resolved

studies and atomic lifetime measurements.

In explorative studies of visible spectra from the NIST and LLNL electron beam ion traps, a variety of

ion species have been excited and stored. The spectra (see the Ar spectrum in figure 1, or Trabert et al. 1998 for spectra of Kr) are dominated by a number of forbidden lines.

NIST-40
-168- Counts

1 1 1 1 Ar

1 1^ 1 1

400 500 600

Mm

700 800

Wavelength / nm

Fig. 1.-- Composite spectrum of Ar, measured at the LLNL SuperEBIT using a Im normal incidence

spectrometer equipped with a CCD camera (from Trabert et al. 1998).

Among the transitions studied for their wavelengths are the the 3j'3/7' ^P|-^P2 transition in Si-like

Kr"" and Mo"^* and the M'^ ^D2-^D3 transition in several Ti-like ions from Xe^^"" to Au^^"" (Figure

2). The measured wavelengths, in comparison with calculated data, help to improve isoelectronic

predictions for elements which may be present in future fusion test plasmas.

For several forbidden lines (the ground state fine structure transition in Ar'^"^ and the 35^ 3p^ ^Pj-^P?

transition in Si-like Kr^'"^), transition probabilities have been measured, with about 5% uncertainties

(Serpa et al. 1997, Serpa et al. 1998, Trabert et al. 1998). This is more precise than the predictions from

most ab initio calculations (Cheng et al. 1979, Biemont and Bromage 1983, Huang 1985, Verhey et al. 1987), which usually need to be corrected for experimental fine structure intervals. However, better

spectral resolution and improved light collection will be needed (and are in reach) to reach the desired

level of 1% uncenainty.

Table 1. Results of Lifetime Measurements using EBIT

Ion Upper level Lifetime (ms)

Experiment

Lifetime (ms)

. Theory as is Theory after energy

correction

Ar'3* 8.7±0.5 NIST 9.62 a

9.51 b

9.41 c 9.58

Kr^^* 35^3/?- 'P, 5.7±0.5 NIST 6.46 d 6.78

6.3±0.3 LLNL 5.83 e 6.69

Xe^^^ -id' ^D, 2.15±0.14 NIST 2.4 f

a Verhey era/. 1987 b Froese Fischer 1983 c Cheng a/. 1979

d Biemont and Bromage 1983 e Huang 1985 f Feldman a/. 1991

NIST-41
Fig. 2. -- Isoelectronic trend of the 3d'^ 2-^03 transition wavelength in Ti-like ions.

Full line: Prediction by Feldman etal. (1991). Experimental data for Xe measured at NIST

Morgan et al. 1995, Serpa et al. 1996), for Ba at NIST (Morgan et al. 1995) and at Oxford

(Bieber et al. 1997), for Nd and Gd at NIST (Serpa et al. 1996) and for Au at LLNL

(Trabert et al. 1998). Later MCDF calculations by Beck (1997) come closer to existing

experimental results for Xe to Gd than the initial calculations by Feldman et al., but give no further predictions.

3. Conclusion

Electron beam ion traps are quite suitable for the observation of forbidden transitions in the visible and

permit lifetime measurements in the millisecond range. The precision for lifetime measurements reached

so far is about 5%. In the x-ray range (with better detection efficiency and lower detector noise), EBIT

has meanwhile yielded the first lifetime data with sub-percent precision. It is expected that after further

development work at least the 1% range of uncertainty will be reached also for forbidden transitions in the visible spectrum. This is better than most applications require and severely tests theory.

4. Acknowledgments

This research has been supported by NATO (CRG 940181) and Humboldt Foundation Germany. Work

at LLNL was performed under the auspices of USDoE (Contract No. W-7405-ENG-48). Conference

participation would not have been possible without support by the German Research Association, DFG.

NIST-42
-170-

REFERENCES

Beck, D.R. 1997, Phys. Rev. A 56. 2428

Bieber, D.J., Margolis, H.S., Oxley, P.K., and Silver, J.D. 1997, Physica Scripta, T 73, 64

Biemont, E., and Bromage, G.E. 1983, M.N.R.A.S., 205. 1085

Cheng, K.T., Kim. Y.-K., and Desclaux, J.P. 1979. At. Data Nucl. Data Tables. 24. 1 1

1

Edlen, B. 1942, Z. Astrophysik, 22, 30

Feidman, U., Indelicato. P., and Sugar, J. 1991, J. Opt. Soc. Am. B. 8, 3

Froese Fischer, C. 1983, J. Phys. B, 16, 157

Huang, K.-N. 1985, At. Data Nucl. Data Tables, 32, 503

Morgan, C.A., Serpa, F.G., Takacs, E., Meyer, E.S., Gillaspy, J.D., Sugar, J., Roberts, J.R., Brown, CM.,

and Feidman, U. 1995, Phys. Rev. Lett., 74, 1716

Serpa, F.G., Meyer, E.S., Morgan. C.A., Gillaspy, J.D., Sugar, J., and Roberts, J.R. 1996, Phys.

Rev. A, 53, 2220

Serpa, E.G., Morgan, C.A., Meyer, E.S., Gillaspy. J.D., Trabert, E., Church, D.A., and Takacs, E. 1997,

Phys. Rev. A, 55,4196

Serpa, E.G., Gillaspy, J.D., and Trabert, E. 1998, J. Phys. B: At. Mol. Opt. Phys., 31. 3345

Traben, E., Beiersdorfer, P., Utter, S.B., and Crespo Lopez-Urrutia, J.R. 1998, Physica Scripta, 58, 599

Verhey, T.R., Das, B.P., and Perger, W.F. 1987, J. Phys. B 20, 3639

NIST-43
Chapter 9

Highly Charged Ion Studies at the NIST EBIT*

L. P. Ratliff and J. R. Roberts

Atomic Physics Division, National Institute of Standards and Technology,

Gaithersburg, MD 20899-8421

Introduction

In this chapter we will discuss the apphcation of the electron beam ion trap (EBIT) as a contact

source of highly charged ions (HCIs) (see fig. 1) to various experiments conducted at the National Institute of Standards and Technology (NIST). These experiments were done in collaboration with several different

groups. We will also give a very general overview of the NIST EBIT and the instrumentation for various

types of experiments that have been conducted.

The highly charged ions in an EBIT are created when an intense beam (up to 4000 A/cm^) of electrons

collides with low velocity atoms or low charged ions. The ions are trapped by both axial and radial

potential wells. Axially, along the electron beam axis, a well is formed by a series of three insulated drift tubes. Radially, a well is created by the space charge of the electron beam itself as well as by the magnetic

field from a pair of superconducting Helmholtz coils. This essentially creates HCIs with a relatively small

kinetic energy and allows two rather unique research opportunities. First, the ions can be studied

spectroscopically while being trapped for relatively long periods of time inside the device. Second, the ions can be extracted into relatively low kinetic energy beams for collision studies. In the first category of

experiments, spectrometers attached to the EBIT analyze the radiation (visible, ultraviolet, and x-ray) that

is produced as the ions radiate as a result of excitations from collisions with the electrons. In this way, we

can measure various absolute lifetimes and transition energies as well as relative cross sections, without

many of the systematic errors due to large (relativistic) ion velocities. In the second category of

experiments, ions are extracted into relatively low kinetic energy beams that bombard gas or soUd targets.

Although the ion velocities are much higher in this case than in many laboratory atom or ion beam

experiments, they are much smaller than in accelerator experiments that deal with highly charged ions. Therefore, the EBIT allows observation of the phenomenology of colhsions where the potential energy is con^arable to, or even greater than the kinetic energy. An example is the analysis of ejected secondary

particles, such as electrons, x-rays, ions, atoms, molecules or clusters, or when surface modification.

The chapter is organized as follows: a history section describes the chronology of the development of

the NIST EBIT facility along with the experiments that have been performed there. The next section is a practical guide to the operation of the EBIT, a metal vapor arc to inject materials, and an extraction beam

line. Finally, there is a brief conclusion.

* This contribution is a US government work and not subject to copyright.

NIST-44
Fig. 1 Photograph of the NIST EBIT. The EBIT itself is held by an alununum stand with an annular yoke Also

evident in the lower right of the picture is the liquid dewar for the Si(Li) detector On top of the EBIT chamber and

attached to the ceiling is the beginning section of a transfer beamline attached to the EBIT by a 90° elbow housing an

electrostatic bender.

NIST-45
HigWy Charged Ion Studies at the NIST EBIT 259

History

In 1988 the former National Bureau of Standards, NBS, was renamed the National Institute of

Standards and Technology, NIST, and a substantial change in NISTs mission occurred as enphasized byCongress "to assist industry in the development of technology needed to inqjrove product quahty, to modernize manufacturing processes, to ensure product reliabihty and to facilitate rapid commercialization

of products based on new scientific discoveries". As a result of this enhanced mission, new avenues for research and development were investigated at NIST. The Atomic Physics Division in the Physics

Laboratory at NIST had developed an expertise in the investigation of atomic properties of ionized plasmas

mainly by spectroscopic techniques for the Nation's magnetic fiision effort. It was reasoned that the

theoretical and experimental techniques utilized in the magnetic fusion effort could be expanded to include

other inportant areas of research that would be in^ortant to assist industry in the development of

technology. Consultation with NIST colleagues lead us to contact researchers at the Lawrence Livermore

National Laboratory (LLNL) who had recently brought online a new, conqiact reservoir of HCIs that emitted x-rays nearly continuously. This source was called the electron beam ion trap (EDIT). A very promising area that this device could impact and which was generic to the new NIST mission, was that of

fundamental wavelength standards in the x-ray spectral region. Up to this time, the sources of highly

charged ions utilized in research suffered from several fundamental problems, such as large Doppler shifts, wide Doppler spectral line profiles, and highly transient emission resulting in poor S/N statistics to name a

few. After a few visits to the LLNL EDIT group, it was clear that the EBIT as a source of HCIs was unique

and could provided new opportunities for the application of NIST measurement methods and techniques,

especially in the area of high precision spectroscopic investigations of atomic (ionic) properties.

.

This situation was also seen as an opportunity to provide NIST with a unique research tool. It was

realized that this source of highly charged ions could not only provide greater precision and accuracy in x- ray wavelength standards but the possibility to determine fundamental properties of highly charged ions,

such as ionization, excitation and dielectronic recombination cross sections. These types of data were

essential in the modeling of all highly ionized media from thermonuclear reactions to the evolution of

stellar atmospheres.

The first order of business was the actual fabrication of a NIST EBIT. The EBIT program at LLNL

had produced a workable EBIT that was designed and fabricated in conjuinction with researchers and

technicians at Lawrence Berkeley Laboratory. Simultaneous with the NIST inquiries about constructing a new EBIT based on the LLNL design, scientists from Oxford University, England became interested in a

similar EBIT project for the Oxford Physics Department. It was decided at this point that NIST and Oxford

would mount a joint effort to build two EBITs at the same time. With the cooperation of LLNL, by

providing both NIST and Oxford with the original EBIT design drawings, Oxford University proceeded to

fabricate two nearly identical EBITs, one of which was dehvered to NIST in February 1992. The decision

to modify the original EBIT as httle as possible stemmed from the success of the LLNL EBIT and the idea

"if it ain't broke don't fix it". Also a concern was the unknown stability properties of the ions trapped in the ~ 4000 A/cm^ electron beam and the 3T magnetic field. However, for convenience and simpUcity there were some modifications that were included. They were: 1) a redesign of the location of the high voltage

input, 2) a modified design of the conponents for electron injection, 3) more observation ports, and 4) the inclusion of small, conducting appendages near and on the drift tube assembly. This latter modification was

intended to irrprove stability by eliminating the build up of Penning electrons in the vicinity of the drift tubes. The build up of such electrons can provide a short-circuit, causing an unprogrammed shut down of

the EBIT.

The next major undertaking was to put together the parts and make the device operational. Scientists at the Naval Research Laboratory (NRL) had demonstrated interest in collaborating with NIST

on experiments of mutual interest in high precision and high accuracy x-ray spectra. Because the uniqueness of the EBIT was apparent to the researchers at NRL, a workshop was held before construction was begun at which research staff from both NIST and NRL presented and discussed ideas that could be

applied to this new source of highly charged ions. As the result of this workshop, NRL decided to collaborate with NIST on the EBIT project by providing a portion of the electron beam control instrumentation and apparatus to make the EBIT function as a world class source of highly charged ions. It was also at this point that NIST hired a project leader and together with staff from NRL, the new EBIT and

NIST-46
260 L. P. Ratliff and J. R. Roberts

some component parts were assembled. This NIST/NRL collaboration provided the scientific community

an operational EBIT in September 1993.' The first efforts were repeats of experiments done at LLNL, to check that the modified EBIT design was operating appropriately.^ A liquid nitrogen cooled Si(Li) x-ray detector with a resolution of

approximately 175 eV and an operating range of 1 keV to 25 keV was used to observe broad spectral features of ions such as barium and xenon (see fig. 2). Barium is a contaminant given off by the heated

electron gun cathode and xenon was injected as a gas into the vacuimL These preliminary experiments

confirmed that the new EBIT was operational and performed better than expected.^ Other overview papers

of the NIST EBIT early results include refs. 4, 5 and 6.

X-ray Energy [keV]

Fig. 2 The lower graph shows broadband x-ray spectra taken with a Si(Li) detector located at the trap region. The insert

is the high-resolution spectrum taken with a Bragg crystal spectrometer showing the transitions from the J=l excited

states to the ground state in neon-like barium.

Using the same Si(Li") x-ray detector, dielectronic recombination (DR) and radiative recombination

(RR) cross sections in Ba and Ba'** were studied.^ In this DR process, a continutmi electron collisionally excites a 2p electron while it is simultaneously captured into the 3d (3s) orbital of the target ion. Then the 3d (or 3s) electron decays to the 2p vacancy of the recombined ion emitting a photon of 4.6 keV (or 4.0 keV). In addition io these DR peaks, the spectrimi shows well defined radiative recombination peaks

corresponding to emission fi^om orbitals with n = 3, 4, and 5 and a broad peak due to emission fi'om higher

orbitals (see fig. 3). The n = 3 RR peak overlaps with the 3d DR peak. The resonant nature of the DR

excitation is seen by changing the electron beam energy by 100 eV (the energy spread of the electron beam

is some tens of eV); the two DR peaks go away while the RR peaks show 100 eV shifts in energy. According to simulations of the conditions in the EBIT, most of the trapped bariimi atoms are in the charge

states, q = 34 and q = 35. Calculations of the relative cross sections for this mixture of charge states, which

NIST-47
Highly Charged Ion Studies at the NIST EBIT 261

Energy (keV)

Energy (keV)

Fig. 3 Scandium-like barium spectra taken at varying electron-beam energies. The electron-beam is varied from 2270

eV to 2440 eV.

NIST-48
262 L. P. Ratliff and J. R. Roberts

Two identical crystal spectrometers in the Johann configuratiota were used for high resolution x-ray spectroscopic studies.* The spectrometers are fitted with bent germanium crystals with 2d = 0.400 (im

(2,2,0 family of planes). For incident radiation near 4.56 keV, the Bragg angle is near 45*, making the crystal a very good polarizer. Utilizing the two crystal spectrometers simultaneously and mounting them

with their optical axes orthogonal allowed observation of polarization effects due to the directional nature of the electron beam This "two-crystal" method of measuring polarization is insensitive to certain types of

systematic errors that might have affected previous measurements. The spectral line studied was the magnetic quadrupole (M2) line, 2p* 'Sq - 2p (^P°3/2)3s[3/2]°2, in neon-like barium, Ba"**^*. To account for differences in geometry and efficiency between the two detectors, the signals fi^om the detectors are normalized using the nearby electric dipole (El) line, which is essentially unpolarized. The polarization, P,

is defined as: P = (lp„ - Iperp)/(Ipar + Ipeip). where Ip,, and Ipe,p are tihe intensities of the parallel and

perpendicular polarized light measured at 90° fi^om the electron beam axis. Aside fi^om the regions where

resonances occur in the cascade feeding of the upper state, strong negative polarization is measured in the range, 5 keV to 7.8 keV which is in good agreement with calculations (see fig. 4).' We have also experimented with the implementation of new types of spectrometers for use on an EBIT. One such

instrument is described in ref 10. The first observation of visible light from an EBIT was made in 1993. Such visible spectroscopy

from highly charged ions could be a valuable tool for diagnostics of high temperature plasmas, for exanple

in nuclear fusion reactors and in the solar corona. It is beneficial to observe lines in the visible and near uv

as they can be meastired with standard, well established, optical techniques. Of specific interest were the predictions by Feldman et al.'^ Their calculations demonstrated that magnetic dipole (Ml) line (3d* -

D3) in the Ti-like isoelectronic sequence would remain in the visible/near uv spectral region for nearly half

of the periodic table from Z = 45 to Z = 92. This would be particularly valuable to use for plasma

diagnostics. Morgan et a/" made the first measurements in this sequence and determined the wavelengths

of barium and xenon to be 393.24 (20) ^m and 413.94 (20) ^im, respectively (see fig. 5). While these

values are consistent with relativistic Hartree-Fock (HFR) calculations with a scaling factor of 93% (which

is reasonable for highly charged ions), they differ by an uinacceptable 5% from ab initio^ multiconfrgiiration, Dirac-Fock calculations. This Ml line in xenon was also observed using a Fabry-Perot

spectrometer" with a resolving power of 10*. In this case, an ion temperature of less than 1 keV was

deduced from the Doppler width of the emitted spectral line, (see fig. 6). Additional observations of Ml

spectral lines have been observed in kryptoa'* In this experiment spectral lines of low charge state ions were observed along with the HCI Ml spectra. In order to expand the range of ions available for study in the EBIT, a metal vapor vacuum arc, or MEWA, ion source was added to the EBIT. The source was designed, built and tested at NRL and brought

to MIST 1995. With the ability to inject metal ions into the EBIT, Serpa et al^^ continued the investigation

of the Ti-like isoelectronic sequence by adding measurements on neodymium (X = 375.3 (2) ^m) and

gadolinium (A = 371.3 (2) fxm). Now with these four measured wavelengths in the sequence, the scale

factors for the previous HFR calculation were refined and in^roved predictions were made for the isoelectronic series from xenon (Z = 54) to osmium (Z = 76).'' In 1995 researchers at NIST^' developed a method for using the EBIT to measure lifetimes on the order of milliseconds. This ten^oral range is difficult to measure via the standard beam foil technique as it would require an unreasonably long flight tube. In this technique, the EBIT electron beam is switched on

and off. When the beam is off, the fluorescence is monitored as a function of time. The decay curve (see

fig. 7) is fitted with an exponential to extract a lifetime. Because the transitions that feed the upper state are extremely fast, there is no significant contribution to that population during the beam-off period. Carefiil diagnostics done by our group and the group at Livermore ' show that a certain portion of the decay is in

fact due to the lifetune of the excited state of the ion. Using this technique, we measured magnetic dipole

Hfetimes for Xe"*(2.15 (14) ms), Kr^* (5.7 (5) ms) and Ar*^* (8.7 (5) ms)." "

NIST-49
Highly Charged Ion Studies at the NIST EBIT 263

10 r

^ -10

a

o

-aI

o

0^

-20

-30 -

-40 -

_L-i- J—I > I L. 4^ 5.0 5.5 6.0 6^ 7.0 Electron Energy (keV)

7.5 8.0

Fig. 4 Polarization measurements of a magnetic quadrupole transition (M2) in neon-like barium. The measurements

were done with electron beam energies in the range of 5 keV to 7.8 keV.

2000

2500

2000 -i

3 1500

H

oU

1000

320 340 360 3»5 400 420 440^

Wavelength (nm)

Fig. 5 A wavelength survey of barium (a) and xenon (b) showing magnetic dipole lines in the near ultraviolet spectral

region for (a) Ba^* and (b) Xe"* and Xc^'*.

NIST-50
264 L. P. Ratliff and J. R. Roberts

190 200 210 220 230 240 250 260

Relative Phase (deg)

Fig. 6 Numerical simulations ( ) at difiFcrent ion temperatures, TD, and data ( • ) for Fabry-Perot scans of a Ba^*

magnetic dipole spectral line at 393.2 ^m.

Time (ms)

Fig. 7 The temporal evolution (a) of the magnetic dipole spectral line intensity at 413.94 ^im from the Xe'^* and (b) of the x-rays detected by a Si(Li) detector for an optimized Xe"* charge state.

NIST-51
Highly Charged Ion Studies at the NIST EBIT 265

In collaboration with the NIST Quantum Metrology Group, experimental work has been ongoing to measure the transition wavelengths in the x-ray spectral region with absolute accuracy at the 10-30 ppm

level. There are two goals here. The first goal is to in^rove wavelength standards in the x-ray spectral

region. The second goal is to measure Lamb shifts in hydrogenic systems with heavy nuclei; for a given

number of electrons, the Lamb shift of an ion varies as the fourth power of its charge, so the magnitude of

the Lamb shift of highly charged ions can be quite large. The measurements are done with one of the' Johann bent-crystal spectrometers described previously.* The calibration procedure enploys an x-ray tube

where an electron beam is focused onto a target foil. This foil is located directly opposite the EBIT trap

region from the spectrometer entrance slit. The calibration x-rays travel through the EBIT to the spectrometer. The details of the absolute calibration are presented in ref. 19. The current status and results of this effort are also smnmarized in ref 20. Although the NIST EBIT program was begun to carry out high precision and accurate x-ray wavelength measurements, it was clear that there were other areas of research that were also exciting and

that would benefit from the unique properties of an EBIT. One of these was the idea of extracting the HCIs

from the trap and transporting them to another experimental chamber where the ions could interact with

other matter, e.g., a material siuface. In 1993 Schmieder and Bastasz^' from Sandia, published results from

experiments done on the LLNL EBIT. They in^jacted HCIs onto a plastic (CR-39) surface and formed

local damage sites. The surface was chemically etched in order to make the damage sites visible with a scanning electron microscope. This discovery led NIST to undertake the design and construction of a highly versatile transport ion beam line to systematically investigate the interaction of HCIs with surfaces. The design and construction of this beam line proceeded in parallel with the ongoing spectroscopic

experiments described previously, and in early 1996 it was essentially finished.^ This new beam line incorporated unique steering elements, movable Faraday cup detectors, several einzel lenses and an q/m

charge-to-mass separator magnet. This magnet is capable of transporting almost any charge state of any

element, except for the very lowest charge states of the highest mass elements at high velocity. For

exan^le, U^* can be transmitted with 64 keV of kinetic energy at the highest magnetic field available in the separator magnet. At the end of the beam line are two chambers. One is used excltisively for the exposure

of surfaces and the other, can be configured for a variety of experiments. With two target chambers at the end of the beam line and the spectrometers on the EBIT, it is easy to quickly switch among multiple

experiments that can remain intact semi-permanently. This has attracted many collaborators with diverse

interests to the NIST EBIT.

The performance of this beam line was outstanding and gave the highest fluxes ever observed in such a system.^^ For exan^le, we measured 21 pA of Xe three orders of magnitude more than had been

reported for continuous mode operation of any EBIT at the time.^ Previously, the time-averaged flux was

observed to be much higher in pulsed mode operation than in the continuous mode. In pulsed mode, the ions are stored in the trap tmtil a significant population of the desired charge state ion is accumulated.

Then, the voltage on the center drift tube is raised, pushing the trapped ions over the barrier presented by

the upper drift tube. In this way, ions are extracted from the trap in pulses as short as a few microseconds or

as long as tens of milliseconds or more. This however can be a disadvantage for certain experiments. For

example, experiments involving pulse-coimting electronics might saturate for high intensity count rates. We have found that the NIST EBIT works quite well in what has been termed continuous leaky mode. That

is, the upper drift tube potential is set below the lower drift tube potential and the ions with the highest

energy are constantly escaping out the upper drift tube. We find that this method gives us the highest fluxes and the most stable beams.

With the beam line in place, we set out to understand the mechanism for surface damage with

relatively low velocity, highly charged ions starting with the sinqsle concept that follows. As the HCI

approaches a surface, it starts to extract electrons from the material. Some of these electrons are bound in high lying states of the projectile HCI forming a "hollow atom". This type of atom will have many lowlying vacancies with most of the captured electrons in high lying states. The atom can decay via Auger or

radiative processes and possibly extract more electrons from the material before its actual inpact. If the surface is a good conductor, the electrons should be readily replaced from the bulk material, but an

insulator could develop a locally charged region which would "Coulomb explode" leaving a crater at the surface."-"-^* We proceeded to expose several types of surfaces with highly charged ions and image their surfaces in collaboration with Schmieder from Sandia National Laboratory. In this collaboration, the experiments were performed using the NIST EBIT facility and an atomic force microscope (AFM) from the Sandia group.

NIST-52
266 L. P. Ratliff and J. R. Roberts

We exposed the sxirfaces to the EBIT highly charged ions, removed them from the vacuum and imaged

them in air. As our first surface to study we chose CR-39 to reproduce the results seen earlier by Schmieder

and Bastasz.^' As expected, after etching in 5 N NaOH solution for 30 minutes at 50° C, we saw craters that were approximately 200 \im across using scanning electron microscope (SEM) and a field emission SEM

(see fig. 8(a) and 8(b)). These craters can be seen with the naked eye as a haziness of the surface. For subsequent experiments, we again chose insulating materials as we expected to see the largest effects with them. In order to see nanometer scale features, we required extremely smooth sanqjles, so we

chose mica, which is easily cleaved with adhesive tape. In previous studies at Livermore^^ and Kansas^*,

hillocks were seen to result from HCI bombardment on mica. In collaboration with the Kansas group, we

studied the effect of potential energy on the size of these hillocks and found that they increased

approximately as the ion's potential energy to the 3/2 power.^* (see fig. 9)

Recently, we studied a standard high-resolution resist, polymethylmethacrylate, or PMMA. We

exposed the PMMA-coated silicon to Xe^*, removed it from the vacuimi and developed it in a solution of

equal parts (by volimie) of isopropyl alcohol and methyl isobutyl ketone at room temperature for 30

seconds. Figure 10 shows an AFM image of damage sites produced by multiple ion impacts through two

dimensional array of nominally square holes; they are approximately 10 )im apart and 1 ^m in diameter.^' In another investigation, low doses of HCIs on PMMA were also done and single impact sites were imaged

with an apparent width of approximately 24 ^im. We have also investigated the effect of ion bombardment

on the surface of the crystal amino acid, L-valine. (See the figures in the chapter by Edward Parilis within

this volume.)

In collaboration with George Whitesides' group in the Chemistry Department of Harvard University,

self assembled monolayers (SAMS) were also exposed with HCIs.^° The samples consisted of silicon wafers coated with a thin titanium adhesion layer and then with 20 ^m of gold. Self-assembled monolayers

of dodecanethiol, CH3 (CH2)ii SH, are grown in solution and removed from the solution just before they

are put into the vacuunL They are then exposed with various fluences of Xe*** ions. In this way, we found

that a substantial exposure required lO" ions/cm^. This is approximately a factor of 10^ less than the fluence required for a similar exposure with metastable argon atoms which have 5000 times less potential

energy than the ions. Figtire 11 shows a proof-of-principle of HCI masked lithography using

hexadecanethiol SAMs over a 40 |im layer of gold bound to a silicon wafer with a chromium adhesion

layer. The inset in figure 1 1 shows an edge roughness that appears to be limited by the quality of the mask

tised.Using a contact, energy dispersive x-ray detector, we have measured the x-ray spectra from

hydrogenic argon impacting siUcon dioxide.^' This experiment represents the first investigation with x-ray spectroscopic methods of the effects ofbombarding surfaces with HCIs from the MIST EBIT.

An ultra high vacuima (UHV) scanning tunneling/atomic force microscope (STM/AFM) and surface

preparation chamber were added to the facility in 1997. This made NIST the only facility in the world in which one can use the very high charge state ions to expose surfaces and then image the surfaces withoul

removing them from the vacuuntL With this new capabiUty, we can observe the structures created by ion

bombardment before oxidation or environmental contamination can modify the structures. The most recent addition to the equipment for studying svufaces is a high spatial resolution (~ 1 5 jim)

movable intensified charge injection device (CID) detector system. This instrtmient images the ion beam al the location of the sample to accurately determine the spatial characteristics of the beam.

Operation

The EBIT facility can be thought of as four separate systems: 1) the EBIT itself, 2) the MEWA ioi source, 3) the beam line and 4) two target chambers and related equipment. This section is an overview o: these systems.

NIST-53
Highly Charged Ion Studies at the NIST EBIT 267

a

Fig. 8 (a) Scanning electron microscope (SEM) and a (b) field emission SEM scans of an etched plastic, CR-39. The

etched damage sights are approximately 200 |im in diameter.

NIST-54
268 L. P. Ratliffand J. R. Roberts

Q I • I 1 • i ^-1 • i • i • 1 . I

- ' - 0 100 200300400800600 700 800900 1000 1100

(Potential Energy)^ (keV^

Fig. 9 A plot of the volume of damage hillocks in mica vs. the 3/2 power of the potential energy of Xe**". The points (•) represent the data for several different charge states from 30 ^ q < 50 and the ( ) a least squares fit to the data.

EBIT

The EBIT (see fig. 12) consists of three major conqjonents: the first includes the electron source, the

drift tubes and the electron collector, the second conprises four nsagnetic field coils to shape the electron

beam, and the third embodies the necessary support equipment While it has been described in other

publications, for con^leteness sake it will be described here as well. A beam of electrons from the electron

gim travels through the drift tubes to the collector. Along the way, it is shaped by the various electric and

magnetic fields. The EBIT is mounted vertically with spectrometers mounted around its circumference at the trap region and the extracted ion beam emerges from the top.

The electron gun is a commercial unit that consists of a cathode, an anode and a focusing electrode. The cathode is heated by a filament and is operated at ground potential. Electrons are extracted from the cathode when the anode is biased, normally at about 4.2 kV, and the focusing electrode shapes the beam.

There is an electrode located near the electron gun that is normally held at the same potential as the anode.

The pmpose of this electrode, formerly called the snout, is to shape the magnetic field in the vicinity of the

electron gun as well as to act as an electrostatic element. The transition electrode is located in the narrow

opening of the groimded Uquid nitrogen shield to prevent the ions from being influenced by the ground

potential of the shield (see fig. 12).

The drift tube assembly consists of three cylindrical tubes enclosed by a cylindrical shield electrode. The electron energy in the trap region is the center drift tube voltage minus a correction due to the space

charge of the electrons and ions in the trap. This corriection is calculated by integrating Gauss' Law and, as an example, is ^ 70 eV at 2 keV trap potential and 35 mA of electron current." The collector electrode is biased at approximately +2 kV to attract the electrons that have passed through the drift tubes. A

suppressor electrode located just below the collector prevents secondary electrons created in the collector from travelling back down into the EBIT. Just above the collector, the extractor electrode is at a negative

voltage to aid in extracting the ions into a beam and to suppress the escape of secondary electrons from the top of the collector.

NIST-55
Highly Charged Ion Studies at the NIST EBIT 269

Fig. 10 An AFM image of damage sites produced by multiple ion impacts of '^^e*** on a polymethylmethacrylate

(PMMA) resist coated silicon wafer. A commercial stencil mask screened the wafer with a two dimensional array of

square holes. Two sites are shown in top view (a) and depth profile (b); expanded view of a single site is shown in (c).

NIST-56
270 L. P. Ratliffand J. R. Roberts

Fig. 11 A proof-of-principle of HCI masked lithography using hexadecanethiol SAMs. The insert demonstrates the edge roughness that appears to be limited by the quality of the mask. The squares are approximately 10 micrometers

wide.

There are four magnetic field coils. The first is an electromagnet located in the electron gun region, called the bucking coil, which nulls the magnetic field from the superconducting magnet in the electron gun

region. The next two are a pair of Helmholtz superconducting magnets that provide a field of 3 Tesla in the region of the drift mbes. This field constricts the electron beam to a filament - 60 |jm in diameter. The

fourth is the collector magnet, surrounding the collector electrode, which helps to spread out the electron

beam so that it will more uniformly distribute its energy. A metal shielded room located in the comer of the laboratory houses four electrically floating systems

for: 1) the EBIT drift tubes, 2) the collector magnet, 3) the MEWA (see section on MEWA) and 4) the ion beam imaging camera (see section on the Beamline). Each of these systems consists of an acrylic box

that houses equipment such as power supplies, electronics, an isolation transformer and fiber optic

transmitters and/or receivers. The fiber optic systems allow remote control of the power supphes located in the metal shielded room from a main control console. The voltages applied to the drift tubes (up to 500 V)

float on top of the drift tube shield voltage that varies from < 1 kV to > 30 kV by way of HV voltage

amplifiers and switching electronics. The collector magnet floats at the voltage of the collector electrode.

NIST-57
Highly Charged Ion Studies at the NIST EBIT 271

Fig. 12 A schematic cross section drawing of the EBIT showing; 1) the electron gun assembly, 2) the high voltage

feed for the drift tubes, 3) the transition electrode, 4) the superconducting Helmholtz coil former and holder, 5) the

liquid nitrogen shield, 6) the collector magnet, 7) collector suspension assembly, 8) einzel lens, 9) liquid nitrogen

shield, 1 0) liquid helium dewar, 1 1) helium dewar support

The cryogenic systems perform three functions. First, the liquid heliimi bath cools the Helmholtz

magnet pair so that it is superconducting. Second, the liquid nitrogen cools the collector magnet so that it does not overheat, as well as providing an intermediate cooled jacket for the Uquid hehima. Third, the cold

surfaces in the vacuum chamber condense gasses, reducing the base pressure from 10"' Pa (10'' Torr) (at room temperature) to < 10"* Pa (10''° Tort).

Carefiil cryogenic system design is very important because of the huimid air in the laboratory. Cold

surfaces condense the water and form ice, both of which can cause problems for the electronics and high

voltage equipment associated with the EBIT. For this reason, the liquid nitrogen feed lines are made of vacuum insulated pipe. The liquid nitrogen is transferred from a remote large dewar into a smaller dewar

near the EBIT. This smaller dewar is designed so it is automatically filled during EBIT operation, thus

guaranteeing that the intermediate jacket for the liquid nitrogen is always filled. This dewar also supplies

liquid nitrogen to the collector magnet through an automatic valve. A thermocouple on the output of the

collector magnet is used as feedback to control the valve. After the collector magnet, the liquid nitrogen is vented through a well-insulated line to the outdoors where a series of three large heat sinks warms it to its gaseous state. Unless the EBIT chamber is opened, the nitrogen shield and collector magnet are kept liquid

nitrogen temperature constantly. This requires approximately 440 liters per day of liquid nitrogen.

NIST-58
272 L. P. Ratliffand J. R. Roberts

Liquid helium is fed via a flexible, vacuum insulated transfer tube directly from a portable 250 liter storage

dewar into the EBIT cryostat reservoir. Dxiring operation, the EBIT uses approximately 3.5 liters of helium

per hour. For this reason, the cryostat is not kept at hquid helium temperature except when the EBIT is in use. Gaseous helium escapes through vent ports on the top of the EBIT. Heater tapes wound around these vent lines warm the helium exhaust and prevent water vapor condensation and icing.

MEWA

The metal vapor vacuimi arc (MEWA)^^ ion source can be used to inject nearly any metal ion into the EBIT. In the EBIT, these low-charge-state injected ions are further stripped. The MEWA consists of five stainless steel electrodes: trigger, cathode, anode, extractor, and ground shield. These electrodes are arranged coaxially and are separated by ceramic spacers. A plasma is created when a capacitor is discharged between the trigger and cathode. Ions are accelerated to the anode and the extractor shapes the ion beam. The cathode electrode material determines the ion species that is created. We have used the following materials: Ti, V, Sc, Bi, Nd, Gd, W, and Fe. The MEWA is located directly above the EBIT and the metal ions are injected through a hole in the electrostatic bender of the beam line (see on Beam Line section). To inject ions, the MEW^A is triggered

and the upper drift tube potential is lowered to permit the ions, to enter the trap region, and then quickl)

raised, to trap the ions. Typical timing sequencing and voltage values are described in detail in ref. 15.

Beam Line

A highly efficient beam line is used to transport ions extracted from the top of the EBIT.^^ At

electrostatic bender (see fig. 13) located above the EBIT bends the ion beam into the beam line. A fine wire

mesh-covered hole in the upper plate of the bender allows ions from the MEWA to be injected into the EBIT. Two sets of deflectors and two einzel lenses align and focus the ion beam into an analyzing sectoi magnet. This magnet bends the ions of a desired charge state by 90° and spatially separates the ions oi different charge-to-mass ratios. Slits located at the entrance and exit of the magnet can be narrowed b)

translator feedthroughs to transmit only one charge-to-mass ratio. After the sector magnet are a set of deflectors, a lens, and a second 90° electrostatic bender. ThL* bender also has a fine wire mesh covered hole in the lower plate so that it can be biased to either bend oi transmit the ion beam. Target chambers, for exposing surfaces to ions, are located both at the output of tht bender as well as below it Each chamber is equipped with an einzel lens, a second focusing electrode, an( a plate with a 3 mm diameter aperture. The chamber below the bender is connected to an analysis chambe

that hotises a vacuum STM/AFM used for imaging surfaces that have been bombarded with ions. To detect the ion beams, we use channel electron multipliers and Faraday cups. The electroi multipliers, located in both target chambers, are capable of counting single ions and are useful fo measuring low signal levels. They are also used for scaiming the mass distribution. In this case the ioi count is recorded while the current to the mass-analyzing sector magnet is ramped, resulting in a signal tha measures the various species and their charge states present in the beam (see fig. 14). The Faraday cup measure absolute current and provide a useful verification of the signal measured by the multipliers

Faraday cups are located in the target chambers; they are mounted on motion feedthroughs and can b

inserted into the beam just in front of the multipliers. Also, two movable Faraday cups are located in th' beam, just after the first electrostatic bender and just before the magnet; these are used to monitor the ion

through the beam line to optimize the transmission through the various elements.

NIST-59
Highly Charged Ion Studies at the NIST EBIT 273

NIST-60
274 L. P. Ratliff and J. R. Roberts

In addition to the Faraday cups and channel electron multipliers, we have two imaging detectors. Each

detector has a microchannel plate to intensify the signal from the ions. Just after the microchannel plate is a phosphor and a CTD camera to allow for output. The image from the CID camera can be viewed on a video

monitor or fed directly to a computer for analysis. One of the detectors can be mounted at various positions

along the beam while the other is presently being installed permanently in the target chamber at the end of

the beam line. This second detector is mounted on a motion feedthrough such that, when the target is removed, it can be inserted into the beam at the position of the target surface during exposure. In this way, we can know the spatial distribution of intensity along the beam near and at the target. For certain experiments, ions are decelerated (or accelerated) just before impacting the target. In this case, the target is biased at a negative (or positive) voltage relative to groimd. To get an accurate measure of the beam

profile, the imaging detector must be biased at this voltage as well. For this reason, the detector is electrically isolated and can be floated at potentials up to 10 kV.

Conclusion

In 1993 NIST successfully brought an EBIT on line based on a sUghtly modified design of the LLNL

EBIT. Within one year successful experiments were being conducted involving several different types of

experiment apparatus and collaborators from around the globe. After a five year period to establish a new

competence within the newly Congressionally mandated NIST mission, the EBIT project was made a permanent facility at NIST with a wide range of objectives. These objectives encompass basic research

studies on the physics of highly charged ions and the interaction of HCIs with surfaces. The NIST EBIT facility has proven to be a very versatile instrimient, but, we believe that we have only

scratched the surface of its true versatility. There are in^ortant research avenues that we have not yet

explored. One such avenue is high precision mass measurements (see the chapter in this volxmie by C. Carlberg). This would be a powerful technique for measuring fundamental constants, acquiring atomic

reference data, and "weighing" chemical bonds and transition energies as well as contributing to a quantum

standard of mass. In this concept for the EBIT, the ions are transferred to a precision hyperbolic Penning

trap where the electric fields are very carefully characterized and stabilized. The ions in this secondary trap move at their resonant frequencies and this motion is monitored by measuring the induced image current on

the trap electrodes. The current state-of-the-art for such measurements is on the very impressive 0.2 ppb

level.^ The use of HCIs rather than singly charged ions presents some advantages. The signal level would

increase with the charge state and the resonant frequencies would increase, thereby increasing the resolution of the mass measurement. Another area would be to study the EBIT trap itself. Both

experimental and model calculations could bear fruit on the imderstanding of the trap dynamics. This could

possibly lead to revision in the basic EBIT design or mode of operation to make it more efficient or to permit the trap dynamics to be more accurately deconvolved from the fiindamental atomic physics processes in the trap and allow more accurate measurements.

This chapter has served as an overview of the NIST EBIT project to date. Updated information on the progress of ongoing and future avenues of research can be gotten by accessing the NIST EBIT web page. http://phvsics.nist.gov/MaiResFac/EBrT/ebit.html (case sensitive). This web page also has a list of the NIST EBIT publications, related web pages as well as other information.

NIST-61
Highly Chaxged Ion Studies at the NIST EBIT 275

Xe44+

3

C

xi

tm

1.0

0.9

0.8

0.7

0.6

0.5

(0c 0.4 h " 0.3

0.2

0.1

0.01000

i

1100 1200 1300

Magnetic Field [Gauss]

1400 1500

Fig. 14 A charge state distribution ofextracted Xe ions from a scan ofthe s^tor magnet's field strength (voltage).

Acknowledgements

We would like to acknowledge the students, postdocs, guest scientists, collaborators, and NIST staff

that have contributed to the success of the NIST EBIT project (those marked with a * have worked full- time for at least 1 year). Their affiliations can be found on the NIST EBIT web page. They are:

Helmar Adler Anatoly Faenov Gene Livingston 'Laura Ratliff Yefim Aglitskiy David Fanning Jan Lorincik CurtReimaim

Doug Alderson UriFeldman Ed Magee Chad Riland

Attila Bader Walt Finkelstein Joe McDonald James Roberts

Andreas Bard Katherine Gebbie Dan McLaughlin CHiristiana Ruhlicke

'Edward Bell *John Gillaspy John Mebgailis Bob Schmieder

Zoltan Berenyi Vivik Goel Eric Meyer Ryan Sears Skip Berry Yukap Hahn Stacy Mogren John Seely

Andrew Black Mehssa Hao Ronnie Minniti Gino Serpa

Greg Boyer . Glenn Holland Cornelius Morgan Dieter Schneider

Charles Brown Larry Hudson Mike Newman Win Smith

Brent Cezairliyan M. Inal Jon OriofT Joachim Steiger

Chris Chantler Cornelius Jackson Zoltan Onodi-szucs Martin Stockli

Hai-Ping Cheng Sam Jahanmir 'Daniel Parks Jack Sugar

David Church Quentin Kessel David Patterson *Endre Takacs

Richard Deslattes Yong-Ki Kim Joe PeduUa Jessica Thomas

LaShondria Dixon Dave Knapp Alexander Pikin Elmar Traebert

George Doschek Martin Laming Ed Pollack George Whitesides

Jacques Dubau Mort Levine 'Trey Porto Wolfgang Wiese

NIST-62
276 L. P. Ratliff and J. R. Roberts

References

1. Gillaspy, J. D., Roberts, J. R., Brown, C. M., and Feldman, U., "The NIST EBIT; A Progress Report",

AlP Conference Proceedings 274, Proc. Vhh International Conference on the Physics of Highly

Charged Ions (Edited by P. Richard, M. Stockli, and CD. Lin), p. 682 (AIP press, New York, 1993).

2. Gillaspy, J. D., "First Results from the EBIT at NIST", Physica Scripta, T71, 99 (1997).

3. Gillaspy, J. D., Aglitskiy, Y., Bell, E. W., Brown, C. M., Chantler, C. T., Deslattes, R. D., Feldman,

U., Hudson, L. T., Laming, J. M., Meyer, E. S., Morgan, C. A., Pikin, A. I., Roberts, J. R., Ratliff, L.

P., Serpa, F. G., Sugar, J., and Takacs, E., "Overview of the Electron Beam Ion Trap Program at NIST", Physica Scripta, T59, 392 (1995).

4. Gillaspy, J. D., "Visible and UV light from Highly Charged Ions: Exotic Matter Advancing

Technology, IEEE LEOS, 9 (August), 19 (1995).

5. Gillaspy, J. D., "EBIT Spectra of Highly Stripped Ions from the Visible to the X-ray", Physica Scripta,

T65, 168(1996).

6. Takacs, E., Ratliff, L. P., and Gillaspy J. D., "X-ray, visible and electron spectroscopy with the NIST

EBIT', Hyperfine Interactions, 108, 59 (1996).

7. McLaughlin, D. J., Hahn, Y., Takacs, E., Meyer, E .S., and Gillaspy, J. D., "Radiative and Inner-Shell Dielectronic Recombination in a Highly Charged Barium Ion", Phys. Rev. A, 54, 2040 (1996).

8. Deslattes, R. D., "X-ray instrtmientation for analysis of fluorescent and scattered radiation". Rev. Sci. /n5/rum. 63, 1128 (1992).

9. Takacs, E., Meyer, E. S., Gillaspy, J. D., Roberts, J. R., Chantler, C. T., Hudson, L. T., Deslattes, R.

D., Brown, C. M., Laming, J. M., Dubau, J., and Inal, M. K., "Polarization measurements on a magnetic quadrupole line in Ne-like barium", Phys. Rev. A, 54, 1342 (1996).

10. Aghtskiy, Y., Serpa, F. G., Meyer, E. S., Gillaspy, J. D., Brown, C. M., Faenov, A. Ya. And Pikuz, T.

A., "The Use of a Spherically Curved Crystal Spectrometer for X-ray Measurements on Electron Beam

Ion Trap", Physica Scripta, 58, 178 (1998).

1 1. Morgan, C. A., Serpa, F. G., Takacs, E., Meyer, E. S., Gillaspy, J. D., Sugar, J, Roberts, J. R., Brown,

C. M., Feldman, U., "Observation of visible and uv magnetic dipole transitions in highly-charged xenon and barium", Phys. Rev. Lett. 74, 1716 (1995).

12. Feldman, U., Indelicato, P. and Sugar, J, "Magnetic dipole line from U-LXXI ground-term levels predicted at 3200-A" J. Opt. Sac. Am. 5, 8, 3 (1991)

13. Adler, H., Meyer, E. S., Serpa, F. G., Takacs, E., and Gillaspy, J. D., "Fabry-Perot Spectroscopy of a

Visible Magnetic Dipole Transition in Ba^^', Nucl. Instrum. and Meth B, 98, 581 (1995).

14. Serpa, F. G., Bell, E. W., Meyer, E. S., Gillaspy, J .D., Roberts, J. R., "Kr spectra from an electron- beam ion trap: 300 ^m to 460 ^m", Phys. Rev. A, 55, 1832 (1997).

15. Serpa, F. G., Meyer, E. S., Morgan, C. A., Gillaspy, J. D., Sugar, J., Roberts, J. R., Brown, C .M., and

Feldman, U., "Anomalous Z dependence of a magnetic dipole transition in the Ti I isoelectronic sequence", Phys. Rev. A, 53, 2220, (1996).

16. Serpa, F. G., Morgan, C. A., Meyer, E. S., Gillaspy, J. D., Trabert, E., Church, D. A., and Takacs, E., "Measurement of a magnetic dipole transition probability in highly charged Xe using an electron beam

ion trap (EBIT)", Phys. Rev. A, 55, 4196 (1997).

17. Beiersdorfer, P., Schweikhard, L., Lopez-Urrutia, J. Crespo and Widmann, K., "The magnetic trapping

mode of an electron beam ion trap: New opportunities for highly charged ion research". Rev. Sci. Instrum. 67,3818(1996).

18. Serpa, F. G., Gillaspy, J. D., and Trabert, E., "Lifetime measurements in the ground configuration of Ar'^* and Kr^* using an Electron Beam Ion Trap", J. Phys. B, 31, 3345 (1998).

19. Paterson, D. P., Chantler, C. T., Tran, C. Q., Hudson, L. T., Serpa, F. G. and. Deslattes, R. D,

"Absolute Calibration of and X-ray Spectrometer on the NIST Electron-Beam Ion Trap: Control,

Design and Systematics". Physica Scripta, T73, 400 (1997).

20. Chantler, C T., Paterson, D., Hudson, L. T., Serpa, F. G., Gillaspy, J. D., Deslattes, R. D., "Progress

towards absolute X-ray spectroscopy on the NIST Electron-Beam Ion Trap: current status and results",

Physica Scripta, T73, 87 (1997).

21. Schmieder, R. W. and Bastasz, R., "Surface Damage Caused by Single, Low-Energy, High-ChargeState Ions", AIP Conference Proceedings 274, Proc. K/* International Conference on the Physics oj

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Highly Charged Ion Studies at the NIST EBIT 277

Highly Charged Ions (Edited by P. Richard, M. Stockli, and CD. Lin), p. 690 (AIP press, New York,

1993).

22. Pikin, A. I., Morgan, C. A., Bell, E. W., Ratliff, L. P., Church, D. A., and Gillaspy, J. D., "A Beam

Line for Highly-Charged Ions Extracted from the NIST EBIT', Rev. Sci. Instrum., 67, 2528 (1996).

23. Ratliff, L. P., Bell, E. W., Parks, D. C, Pikin, A. I., Gillaspy, J. D., "Continuous Highly-Charged Ion Beams from the NIST EBIT', Rev. Sci. Instrum., 68, 1998 (1997).

24. Cheng, H. P. and Gillaspy, J. D., "Nano-scale Modification of Silicon Surfaces via Coulomb

Explosion", Phys. Rev. B, 55, 2628 (1996).

25. Cheng, H. P. and Gillaspy, J. D., "Large Scale Molecular Dynamics Simulation of a Surface Coulomb

Explosion", Application of Accelerators in Research and Industry, AIP conference proceedings 392,

(Edited by J. L. Duggan and I. L. Morgan), p 197, (AIP Press, New York, 1997).

26. Cheng, H. P., and Gillaspy, J. D., "Surface Coulomb Explosions: The Influence of Initial charge

Distributions", Computational Materials Science, 9, 285 (1998).

27. Ruehlicke, C, Briere, M. A., Schneider, D., "AFM studies of a new type of radiation defect on mica

surfaces caused by highly charged ion impact", Nucl. Instrum. Meth. Phys. Res. 5, 99, 52 (1995).

28. Parks, D. C, Stockli, M. P., Bell, E. W., Rathff, L. P., Schmieder, R. W., Serpa, F. G., and Gillaspy, J. D., "Non-kinetic Damage on Insulating Materials by Highly Charged Ion Bombardment", Nucl. Instrum. and Meth. Phys. Res. B, 134, 46 (1998).

29. Gillaspy, J. D., Parks, D. C, and Ratliff, L. P., "Highly Charged Ion Masked Ion Beam Lithography",

J. Vac. Sci. Technol. B, 16, 1 (1998).

30. Ratliff, L. P., Minniti, R., Bard, A., Bell, E. W., Gillaspy, J. D., Parks, D., Black, A. J. and Whitesides,

G. M., "Exposure of self-assembled monolayers to highly charged ions and metastable atoms", (to be

Submitted).

31 Takacs, E., Onodi-Szucs, Z., Ratliff, L. P., Gillaspy, J. D., Palinkas, J., "X-ray emission for 3-137 keV

Ar"* impacting Si02", Nucl. Instrum. Meth. Phys. Res. B, 124, 431 (1997).

32 Brown, I. G., Galvin, J. E., Gavin, B. F. and MacGill, R. A., "Metal vapor vacuum arc ion source", Rev. Sci. Instrum. 57, 1069 (1986).

33 Natarajan, V., Boyce, K. R., DiFilippo, F., and Pritchard, D. E., 'Trecision Penning Trap Comparison

ofNondoublets: Atomic Masses of H, D, and the Neutron", Phys. Rev. Lett. 71, 1998 (1993).

NIST-64
Institute of Physics Publishing Journal of Physics B: Atomic. Molecular and Optical Physics

J. Phys. B: At. Mol. Opt. Phys. 34 (2001) R93-R130 PH: 50953-4075(01)03620-3

TOPICAL REVIEW

Highly charged ions

J D GiUaspy

Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, MD

20899-8421, USA

E-mail: john.gillaspy@nist.gov

Received 5 April 2001, in final form 27 July 2001

Published 24 September 2001

Online at stacks.iop.org/JPhysB/34/R93

Abstract

This paper reviews some of the fundamental properties of highly charged

ions, the methods of producing them (with particular emphasis on table- top devices), and their use as a tool for both basic science and applied

technology. Topics discussed include: charge dependence and scaling laws

along isoelectronic or isonuclear sequences (for wavefunction size or Bohr

radius, ionization energy, dipole transition energy, relativistic fine structure,

hyperfine structure, Zeeman effect, Stark effect, line intensities, linewidths,

strength of parity violation, etc), changes in angular momentum coupling

schemes, selection rules, interactions with surfaces, electron-impact ionization,

the electron beam ion trap (EBIT), ion accelerators, atomic reference data,

cosmic chronometers, laboratory x-ray astrophysics, vacuum polarization, solar

flares, ion implantation, ion lithography, ion microprobes (SIMS and x-ray

microscope), nuclear fusion diagnostics, nanotechnology, quantum computing,

cancer therapy and biotechnology.

1. Introduction

Most of the universe consists of highly ionized matter (Fang and Canizares 2000). Because

it is exceedingly rare on Earth, and because we are blindfolded from observing cosmic

sources by the relatively high x-ray absorption of our atmosphere, the scientific study and

application of highly ionized matter has been limited. Atomic physics and conventional ion

beam technologies, for example, were developed in the virtual absence of laboratory access

to highly charged ions. Today, however, powerful devices are available that can preferentially

produce any ionization stage of any naturally occurring atom. Some of these devices (Levine

et al 1988) are small enough to fit on a tabletop. Others are huge devices that produce ions

travelling near the speed of light (Angert 1991, Stohlker eM/ 2000). These two extremes each

have their own particular applications, but they share the property of providing a window into a part of the universe that is still relatively unexplored and that can be expected to impact a

variety of fields.

0953-4075/01/190093+38$30.00 © 2001 lOP Publishing Ltd Printed in the UK R93

NIST-65
R94 Topical review

This review covers a broad range of research with highly charged ions by using selected

results from my own work and that of others to illustrate general concepts and provide an introduction to emerging applications. More details and breadth of coverage of the

fundamentals can be found in comprehensive earlier reviews (Martinson 1989), books (Beyer

et al 1997, Beyer and Shevelko 1999, Gillaspy 2001, Marrus 1982, 1988, Pal'chikov and

Shevelko 1995) and conference proceedings (Lindgren et al 1992, Mokler et al 1998) for

example.

After this introduction, section 2 covers the fundamental aspects of the ions themselves,

while section 3 focuses on the experimental methods of producing them, with particular

emphasis on the electron beam ion trap (EBIT). Section 4 provides an overview of a variety

of applications in basic science and applied technology. Section 5 concludes with a summary

table of the various scaling laws discussed in this paper, and some remarks about the future. The functional definition of a 'highly charged ion' (HCI) varies. To many, it refers to any atom that is missing more than one or two electrons. To others, several dozen electrons

must be missing before the label 'highly charged' is used. For the purposes of this review, I adopt the following definition: an HCI is any atom that has been stripped of a large number of

electrons {Q^ 1), so that the total energy yielded during reneutralization {Eq) is outside the

realm of ordinary experience with laboratory ions (£"0 » 10 eV).

This definition overlaps with what are frequently referred to as multiply charged or multicharged ions at the lower end of this spectrum (typically Q = 2-9, with £0 < 1000 eV),

as well as what are sometimes called very highly charged ions at the upper end of the spectrum

(e.g. Q = 92 with £0 ~ 750000 eV). The tremendous variety of HCIs makes almost every

generalized conclusion and 'rule of thumb' in the field suspect, but it also opens up an enormous

range of possibilities for discovery.

2. Highly charged ions: fundamental aspects

2.1. Isoelectronic and isonuclear sequences

To explore the full range of phase space spanned by HCIs, one might first select a particular

elemental species (atomic number Z) and then remove sequentially a number of electrons

(Q = 1, 2, 3, . . .) to produce a series of ions with the same nucleus but a variety of A^-electron

structures (N = Z — Q). Since this is how the ions are actually produced, it is a natural way

for an experimentalist to organize his data. This type of phase space cut through the (Z, Q)

space of all possible HCIs is called an isonuclear sequence.

From the point of view of theory, however, scaling laws are often easier to understand by

fixing the electronic number A^, and varying the species Z. Data presented in this way are called an isoelectronic sequence. Conceptually, one may think of an isoelectronic sequence as

starting with a particular neutral atom, and asking how the electronic properties of that atom

change as the nuclear charge is increased in a series of unit steps.

An isoelectronic sequence, therefore, is like having a 'knob' on the nuclear charge that one

can turn up, pulling the electronic cloud into a more tightly bound configuration and shrinking

the size of the atom. An isonuclear sequence, on the other hand, is like pealing the layers of an onion away, to reveal the tightly bound cores of atoms. Both approaches lead to the same

end result: an object with a highly compact spatial wavefunction (figure 1). Because there are two 'charges', electronic (Q) and nuclear (Z), the phrase 'scaling with

charge' is sometimes ambiguous. The more specific phrase 'scaling with Z' refers to an

isoelectronic sequence, while 'scaling with Q' is most appropriate for an isonuclear sequence.

Along an isoelectronic sequence, Z and Q differ by only a constant, so the two are sometimes

NIST-66
i

Topical review

Figure 1. Sciiematic representation of the relative spatial extent of the wavefunction of a hydrogenlike nickel ion (Z = 28) compared with that of a hydrogen atom. The ratio is similar to the size of

the planet Neptune compared with the size of the Sun.

Figure 2. Schematic phase space plot of the range of all possible positive ions with nuclear charge

Z and electronic charge \ Q\ < Z, up to Q,Z = 100. Examples of isoelectronic and isonuclear sequences are indicated.

used (loosely) interchangeably. In the limit of highly charged few-electron ions, Z ~ Q so

the two can be interchanged more correctly.

Isoelectronic sequences begin at Z = + 1, while isonuclear sequences begin at Q = 1. Figure 2 illustrates schematically an isoelectronic sequence and an isonuclear sequence on a

plot of the full (Z, Q) phase space for positive ions. Isotopes can be thought of as extending

along a third axis that projects out of the page. Most laboratory experience is clustered around

the bottom axis ('ordinary ions'). Negative ions (below the horizontal axis) are not shown

because only a few of them are stable and they are not dealt with in this review. All of the

positive ions, on the other hand, are predicted to be stable within the framework of elementary

quantum mechanics. In most cases this prediction holds true, although quantum field theory

describes a number of interesting exceptions that are described in sections 4.2 and 4.4 below.

In discussing general scaling laws in this paper, I will consider one-electron (hydrogen-like)

ions as the prototypical case.

NIST-67
R96 Topical review

2.2. Size

Perhaps the most fundamental and physically tangible aspect of an atom is its size (spatial extent of the electronic wavefunction). The great diversity of physical properties exhibited by

the atoms across the periodic table emerges essentially from variations in the size and shape of

the wavefunction. It is perhaps remarkable, then, that all atoms are roughly the same size: a few tenths of a nanometre. This 'typical size of an atom' is the first rule of thumb that must be

abandoned in making the shift from neutral atoms to highly charged ions. U^^"^, for example,

has a wavefunction that is nearly 100 times more spatially compact than that of a hydrogen

atom. This gives a sense of the great variation in physical properties that one can expect to

find among HCIs.

A simple isoelectronic scaling law for one-electron ions predicts that the size of the

wavefunction (Bohr radius = R) varies as the reciprocal of the ion charge,

R ~ 1/Z

~l/(e + l)~l/G for (2»1. (1)

Figure 1 is drawn to scale using this formula. When the approach to a one-electron ion is viewed along an isonuclear sequence, on the other hand, the relatively smooth scaling of size

with Q indicated above gives way to abrupt jumps at closed-shell configurations.

For very highly charged ions, the wavefunction size becomes so small that one must

begin to think about the ions in a qualitatively different way. The simplistic view of an HCI

as a tiny Bohr atom with a charge is inadequate because the relevant length scales enter a new regime. Consider the hydrogen isoelectronic sequence. In the atomic limit, the system

has planetary analogies that can be quite useful for gaining intuition. The electron orbits the

nucleus at a characteristic distance that is large compared with both the spatial extent of the

nucleus and the Compton wavelength of the electron (the fundamental length scale at which the

electron ceases to behave as a point particle with finite charge). The ordinary hydrogen atom,

like the solar system, is mainly empty space. At the high end of the isoelectronic sequence,

however, the Bohr radius (R ~ 50/Z pm) falls below the Compton wavelength (2 pm). Under

these conditions, single-particle quantum mechanics begins to break down and semiclassical

reasoning based on it becomes of limited value. Indeed, even the classical concept of a central

Coulomb potential breaks down to some extent. Quantum field theory is required to describe

the system. The resulting new picture is one in which the structure of the vacuum of space itself

is significant. The concept of empty space is replaced by one in which everything is bathed

in a sea of virtual positron-electron pairs, popping in and out of existence within the small

window of time during which the Heisenberg uncertainty principle allows energy conservation

to be violated. At short distance scales, it becomes apparent that the virtual sea of charge

from the vacuum becomes polarized and screens the actual 'bare' charge of the electron. The

closer one gets, the more the screening is avoided and the higher the electron charge appears

to be. As the electron continues to 'decloak' from the vacuum at increasingly shorter length

scales, the apparent charge that it carries rises significantly above the conventional value of

1.6 X 10~'^ C, and ultimately approaches its bare value of infinity. The conceptual jump from

a hydrogen atom to a hydrogen-like uranium ion is perhaps as great, or greater, than the jump

from a classically orbiting planet to a quantum mechanical atom. For a more comprehensive

introduction to the quantum field theory of HCIs, see Greiner et al (1985).

The reduction in size of an HCI with increasing Z results in a tremendous compression

of the wavefunction density at the upper end of an isoelectronic sequence. The above scaling

law for atomic size predicts that a hydrogen-like (A^ = 1) uranium atom (Z = 92) has an

electronic density that is 780 000 times higher than that of a hydrogen atom. This compression

NIST-68
Topical review R97

of the wavefunction into the space near the nucleus dramatically alters the relative importance

of various physical effects that determine the atomic structure. The consequences of this are elaborated on in section 2.3 below.

A hint of the wavefunction compression that is so strong in HCIs is present even in the outer

(valence) electrons of heavy neutral atoms. If an outer electron has zero angular momentum,

then its wavefunction extends to the nucleus, and the electron can be thought of as spending

a finite fraction of its time inside the orbits of the core electrons. During this time inside the

core, the valence electron feels the full (unscreened) nuclear charge, and is pulled in closer

toward the nucleus. In quantitative quantum mechanical terms, the probability density of a valence s-electron at the origin scales as Z (Foldy 1958). This linear scaling is a remnant of

the full scaling of an unscreened hydrogen-like ion. This remnant scaling is one of the reasons that parity-violation experiments are done with

heavy atoms. These atoms concentrate the valence electron wavefunction at the nucleus where

the short-range interaction caused by the exchange of a massive vector Z-boson (weak neutral

current) between the electron and a nucleon is non-zero. For neutral atoms, the observable in such experiments scales up as Z^, while for HCIs, it scales up as Z^ (Fortson and Lewis 1984).

There are also weak neutral current effects that occur between electrons (far from the nucleus),

but these are generally smaller and thus have typically been neglected in experiments with

neutral atoms (Bouchiat and Bouchiat 1974, Fortson and Lewis 1984). The compression of

the electronic wavefunction throughout an HCI should amplify this effect as well. An additional

advantage to using HCIs in parity-violation experiments is that the number of electrons can be

reduced to a small value that can be accurately handled by theory. This is important because the

experiments are based on observing interferences which require conventional atomic structure calculations to extract a quantitative value for the fundamental parity-violation parameter,

the weak nuclear charge (Bouchiat and Bouchiat 1997). Theoretical issues and proposed

experiments to use HCIs to test the parity-violating aspects of the standard model have been

presented recently in the literature (Bednyakov et al 2000, Dunford 1996, Pindzola 1993,

Schafer etal 1989, Zolotorev and Budker 1997).

2.3. Energy

2.3.1. Kinetic energy. The two different types of energies associated with ions, potential

and kinetic, are sometimes confused, particularly when discussing beams of highly charged

ions. Conventional ion beams used in the microelectronics industry or in materials science are comprised of singly charged ions with a kinetic energy in the keV to MeV range. This kinetic

energy dominates the interaction of such ion beams with surfaces, and hence the internal

(potential) energy liberated when the ion neutralizes itself upon contact with the surface is usually neglected. Since conventional ions carry only about 10 eV of potential energy (the

ionization potential), neglecting it is a very good approximation. For beams of highly charged

ions in the same kinetic energy range, however, the relative importance of kinetic energy to potential energy is reduced, or even inverted—sometimes by a large factor. The implications

of an unusually large potential energy are discussed from an applications point of view in sections 4.6.2 and 4.6.3 below.

The kinetic energy that an HCI attains when it accelerates through an applied potential

y is a factor of Q larger than it would be if it were singly charged. Modest voltages or

stray electric fields can therefore have a large effect on the ion velocity. This effect can be

exploited to produce fast ion beams relatively easily, as discussed in section 4.6.1 below. This

effect can also lead to non-intuitive results in standard electrostatic ion beam optical elements.

For example, a simple 'velocity filter', formed by passing a beam of a particular mass of ion

NIST-69
R98 Topical review

through an electric field normal to its direction of motion, and selecting only those ions that are bent by a particular angle, can actually pass ions with widely different velocities as long

as they also have widely different charge states (conventionally assumed to be 1). For HCIs

produced at rest along a potential gradient and accelerated to a different common potential, a

simple velocity filter becomes an unusual type of filter that passes only those ions produced at specific locations in space, independent of the widely different velocities that they obtain after accelerating away from their point of origin. This effect is exploited in the efficient extraction

of ion beams from the devices described in section 3.3 below.

The definition of a 'fast' and 'slow' HCI often varies, depending on an experimenter's

particular experience. A rather objective benchmark for the crossover from fast to slow is set by the orbital velocity of the outermost electrons in neutral atoms. This velocity is less than the

speed of light by approximately a factor of a ~ 1/137 (the fine structure constant). Processes

such as charge capture by an HCI from a background gas molecule take on a very different

character at collision velocities much above or below the crossover velocity of the target. In addition to the fundamental difference between fast and slow ions described above,

there is also a practical one: experiments at velocities far above the crossover velocity ('fast

ions') require the use of large accelerator facilities, while those done with 'slow ions' can be

done with table-top devices. This forms a natural division of the two research communities.

Within the accelerator community, ion velocity is often expressed in terms of the quantity

'keV/u', the ratio of the kinetic energy to the rest mass of the ion, with the mass measured

in 'u' (unified atomic mass units) and the energy measured in 'keV (joules divided by 1000

times the charge of the electron). This quantity (keV/u) is roughly equal to the kinetic energy

divided by the number of nucleons (A), but it is often referred to simply as the 'energy' of

the ions. Note that ions of the same charge but different mass, accelerated through a given

electric field, will have the same energy but may have greatly different velocities. Dividing

the energy by the mass, on the other hand, will uniquely specify a velocity factor, from which

the velocity, v, can be extracted using the following expression:

keV . T(v) JP^c^^mlc^ -niQC^

X 1.07 X 10-^ = —-4 = z u moc^ moc^

\^)

I 1 1 U 2 3 i;4 5 y6 = / ^—T- — 1 = + r + T+--

y 1 - V^/C^ 2 c2 8 c4 16 c6

where T{v) is the relativistic expression for kinetic energy (Lorrain and Corson 1970) and

keV/u is the kinetic energy per unit rest mass, as described above. Sometimes the 'u' is dropped (implied), leading to possible confusion about whether energy or velocity is being

specified. Another potentially confusing practice, particularly prevalent among the community

of 'table-top' HCI researchers, is to normalize the kinetic energy by the ion charge, rather than

the number of nucleons. The ion energy is then expressed in units of 'keV/q' {q = Q, the

dimensionless ion charge state). This is frequently done because a variety of ion charge states may be created at a fixed potential and then accelerated to another potential, resulting in a

variety of ion energies and velocities (but a single 'keV/q'). Unfortunately, again, sometimes

the denominator is implied, leading to potential confusion. As an example, to bring the various

issues discussed above together, the crossover velocity for neutral target atoms (approximately

equal to that of a hydrogen atom, vq = 2.2 x 10^ m s~'), corresponds to 25 keV/u, and can be

produced by accelerating a typical hydrogen-like atom (with AjQ^ 2.5) through a potential

difference of approximately 62 kV.

Researchers studying the interaction of HCIs with surfaces have another benchmark

velocity that defines what might be called 'ultra-slow' HCIs. This is the minimum impact

NIST-70
Topical review R99

velocity (Vm) that an ion can have, under the influence of unscreened image charge acceleration

by a surface. If one tries to perform an experiment by directing an ion beam onto a surface

with velocity v < Vm, the ions will automatically accelerate to shortly before impact. For

very highly charged ions, can be surprisingly close to the crossover velocity. For a typical

surface, the energy gain due to image charge acceleration exceeds 1 eV by approximately a factor of Q^^^ (Winter and Aumayr 1999). For the case of hydrogen-like xenon, the minimum

impact velocity is about 1 % of the crossover velocity. This 1 % scales very weakly with charge

(roughly as Z'^"^) along the hydrogen isoelectronic sequence, ranging from 0.3%-1.2% for

Z = 2-92. For a fixed atomic mass, the factor of 1% scales as Q^^'^, so for very low charge

states is many orders of magnitude less than vq. The observation of impact velocities below (down to zero) has been reported on

imperfectly conducting surfaces (Briand et al 1997b, 2000). In fact, the observation of a so-called 'trampoline effect' has been reported, in which the net surface charge reverses its sign and causes an incident HCI to be repelled backwards after reaching a distance of several

nanometres from the surface. This effect remains controversial, however, (Aumayr etal 1997,

Briand etal 1997a).

2.3.2. Potential energy. To further complicate issues, there are two types of potential energy

that are distinct for HCIs: the ionization energy and the neutralization energy. For an ordinary

ion, these two are identical in magnitude, but for HCIs they can differ by more than a factor

of ten. This is because 'ionization energy' is the energy required to remove one electron,

producing the charge Q + \ from an ion with one lower charge, while 'neutralization energy'

is the energy released in replacing all of the missing electrons. The ionization energy is most

relevant to the production of the ions, which typically occurs in a step-wise process (one

electron removed after another), while the neutralization energy is more relevant to the impact

of ions on surfaces.

The ionization energy (U) grows rapidly as the ion charge increases. Simple Coulomb

potential scaling for one-electron ions predicts that there are two factors of charge involved, one that arises directly from the increased amount of nuclear charge and another from the

reduced distance from that charge:

U{Z) ~ Z/R ~ Z/{\/Z) = Z^ (3)

For multielectron systems, the increase in U with charge along an isonuclear sequence will be

reduced and less smooth, reflecting the atomic shell structure and the variation in electronic

screening. All species of ions will approach a enhancement of ionization potential in the

high charge limit {Q Z). Figure 3 illustrates these aspects for the case of xenon. The neutralization energy (£0) is given by the sum of all the ionization energies of the

charge states at and below that of the ion. Thus the energy Eq is enhanced by two factors: the fact that it is the sum of several (numbering Q) ionization energies, and the fact that the

individual ionization energies in the sum are themselves enhanced by the factor discussed

above. Examples of the neutralization energy as a function of charge are presented in figure 4.

2.3.3. Photon energy. Another set of key energies associated with HCIs are those which

separate the various eigenstates, and hence determine the wavelength of emitted light. Although

all of the energy levels become more tighdy bound as the charge increases isoelectronically,

the deeper levels experience the largest change. Semiclassically, the concentric Bohr orbits of

the electrons are pulled in towards a Coulomb singularity with a strong gradient, like matter

falling into a black hole. The resulting tidal forces within the ion cause the energy levels to

NIST-71
RlOO Topical review

10 r

I

>,

S?u

§

•ao

1 r

0.1

Ion Charge (Q) [1.6x10" C]

Figure 3. Ionization energy of xenon as a function of ion charge Q (isonuclear sequence).

The smooth curve shows the scaled ionization potential of hydrogen, 0.01362^, for comparison

(isoelectronic sequence).

become more separated. The resulting decrease in the wavelength of the emitted photons is dramatic, as shown by the example plotted in figure 5. The various spectral regions indicated

in this plot are defined following the Physics Vade Mecum (Anderson 1981).

The rate at which photon energies scale up with charge can vary widely, depending on

the terms in the Hamiltonian which lead to the separation of the energy levels in the first place. The separation between states with adjacent principal quantum numbers (An = 1) is determined primarily by the Coulomb potential, and therefore the scaling described above

for ionization potentials also applies to the photon energies. For transitions between fine structure levels (An = 0), however, the shifts in the energy levels are dominated by relativistic effects, which cause the photon energies to scale much more strongly with charge—typically as Z\

To understand the variation in photon energy for fine structure transitions, consider the

formula for the relativistic energy of a free particle, expanded for velocities slow compared

with the speed of light,

2 4 E = vZ/^V+moV ^ moc^ + ^- + • • •

. (4)

The first and second terms give nothing new—just the rest mass and the classical kinetic

energy. The third term, however, is the relativistic variation of mass that contributes to the

fine structure shift. Because the orbital angular momentum (Z = r x p) is fixed by universal

quantum numbers, p must increase as the atomic size (or semiclassically, the radius r of

the orbit) decreases isoelectronically. For example, the velocity of electrons in the ground

NIST-72
Topical review RIOl

100

>^ 10

c

2

ca 1 N

2

0.1 :r

0.01 r

r

' ' ' 1 1 1 III --r i—|—' - ' ^-T 1 1 1 1 •

-

-

* / -

- 0 / -

: / y / y -

- y -

<>

-

-

Xenon oj.*

I

o. Uranium

° • - o •

0

• -

•

- • •

_ •

8

: •

: o

- •

•

1 1 1 1 1 lJ 1 1 i_l 1 . 1 i

5 6 7 8 9 105 6 7 8 9

19 100 Ion Charge (Q) [1.6x10 C]

Figure 4. Neutralization energy for xenon and uranium as a function of ion charge. Note, by

comparing with figure 3, that 40% of the neutralization energy of bare xenon comes from the two

deepest energy levels. This fraction ranges from 100% to 34% across the periodic table of naturally

occurring elements.

State of hydrogen-like ions is v/c = aZ. Thus, the linear decrease in atomic size with

increasing charge results in a linear increase in p, which produces the scaling described

above.

Even more illuminating is the part of the fine structure shift that is due to the spin-orbit

interaction. This shift can be understood semiclassically as arising from the magnetic dipole

interaction of the electron spin with the effective magnetic field generated in the reference

frame of the orbiting electron by its motion through the electric field of the nucleus. This

magnetic field is proportional to the vector product of the nuclear Coulomb field and the

electron's orbital velocity, both of which scale up isoelectronically with Z:

B = c-u X ~ Z X = (5)

In this expression, the electric field scales up with one power of Z because of the increased

nuclear charge, and with two additional powers of Z because of the shrinking orbital radius

which places die electron closer to the 1/r^ singularity at the nucleus. In summary, the overall

scaling of the fine structure splitting is proportional to Z^, and therefore rapidly grows to become 'not-so-fine (or small)' in comparison with the principal splitting between energy

levels (A« = 1). With these scaling laws in mind, it is easy to see how transitions that produce photons

in the visible spectral range for neutral atoms rapidly scale up with charge to produce x-ray

NIST-73
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Nuclear Charge (Z)

Figure 5. Examples of photon wavelength scaling with ion charge. Plotted are the calculated (Plante etall 994) resonance lines ( 1 s^-1 s2p, lower curve) and lines from the lowest-lying metastable triplet levels (Is2s-ls2p, upper curve) of helium-like ions.

photons in HQs. A Z increase of only a factor of six under scaling, for example, causes

a green photon aX X = 475 nm to become a 3.4 keV x-ray (A. = 0.36 nm). The case of the

Sd"* ^D2-^D3 transition on the Ti-like sequence is one of a number of exceptions: an accidental

crossing of energy levels gives rise to a nearly fiat (Z-independent) variation of wavelength

over a large portion of the periodic table (Feldman et al 1991, Porto et al 2000b).

There is another important contribution to the energy levels which scales in a way

intermediate between the Z^ dependence of the Coulomb interactions and the Z'* dependence of

the relativistic fine structure interaction: the hyperfine interaction. The origin of the hyperfine

shift is the electrodynamic interaction of the orbiting charge and/or spin with the magnetic

moment of the nucleus. The Z-scaling can be understood by considering the magnetic field generated at the nucleus by a circularly orbiting electron. This field follows directly from an

integration of Maxwell's equations (or more simply, from the Biot-Savart law) and goes up

as the inverse cube of the radius of the orbit. The linear scaling of Bohr radius with Z then

directly leads to a Z^ dependence of the hyperfine shifts. For HCIs, the absolute magnitude of the hyperfine splitting grows so large that it enters

the optical regime. High-resolution laser spectroscopy can then be used to probe the electronic

structure so accurately that the atomic physics of HCIs impacts an entirely different field,

nuclear physics. Information gained about the nucleus through very accurate measurements

of electronic structure can be made to exceed the present experimental accuracy using other

methods (Kuhl et al 1997). The hyperfine splitting in H-like bismuth has been measured using an HCI accelerator facility (Klaft et al 1994) and in H-like holmium and Li-like Bi using an

electron beam ion trap (Lopez-Urrutia et al 1996, Beiersdorfer et al 1998).

NIST-74
Topical review R103

Not all energy level shifts grow with increasing Z. The shifts due to external magnetic

fields (the Zeeman effect) for example, actually decrease. The first-order (linear) Zeeman

shift for the ground state of hydrogen-like ions has a weak Z dependence due to the effect of relativity. In the relativistic generalization of quantum mechanics, four wavefunctions

(composed of two radial wavefunctions and two Pauli spinors) are required to fully describe even a single-electron atom or ion. Although one of the radial functions is typically small,

its finite value leads to the variation with Z of the first-order Zeeman shift. The functional

dependence of this variation is given by the following correction factor (Bethe and Salpeter

If this factor is neglected, the first-order Zeeman shift is independent of Z, but even then it becomes relatively less significant in comparison to the overall energy scale as Z increases.

The absolute scale of the first-order Zeeman shift is set by the product of the Bohr magneton

and the magnetic field. The second-order Zeeman shifts (quadratic in magnetic field) scale

much more strongly, decreasing as 1 /Z^ for the ground state of hydrogen-like ions. Shifts in the energy levels due to external electric fields (the Stark effect) also decrease

with increasing Z. For non-degenerate levels, the Stark shift scales as 1 /Z. This scaling can be

understood by considering the origin of the Stark shifts as a polarization of the electron cloud

which produces an electric dipole moment, g x r, which scales as r ~ 1/Z. In non-degenerate

perturbation theory, the first-order shift scales as the matrix element of the dipole moment

operator, and thus reflects the 1 /Z scaling. For degenerate levels, such as those that arise in a simple treatment of the ground level of hydrogen-like ions for example, the first-order Stark

shifts vanish. Degenerate perturbation theory can then be used to estimate the second-order

shift, which arises from two dipole moment matrix elements, plus an energy denominator that scales as the level separations. Thesecond-ordershiftthusscalesas 1/Zxl/Zxl/Z^ = 1/Z'*.

As the field approaches zero, linear shifts are eventually regained as radiative corrections lift the degeneracy on a fine scale. In addition to the dramatic shifts in the positions of spectral lines described above, HCIs

also display large changes in the relative intensities of the lines as well. With increasing charge,

the quantum mechanical selection rules that suppress certain classes of transitions no longer

apply, and transitions that are ordinarily 'forbidden' become relatively strong, sometimes

becoming even stronger than the ordinary 'allowed' transitions (see section 2.6 below).

The energy distribution of radiation from HCIs changes in at least one more important

way: spectral linewidth. While ordinary atoms typically have very sharp and well defined

spectral emission lines, these become greatly broadened in HCIs, resulting in a spectrum

that is considerably more 'fuzzy' than that of neutral atoms. This blurring of the emission

spectrum is mandated by the Heisenberg uncertainty principle because the lifetimes of excited

state levels decrease rapidly as the ion charge is increased. The Z-scaling of the lifetime

(inverse linewidth) varies widely depending on the transition involved. The widths of hyperfine

transitions, for example, scale as Z^. Intercombination transitions (A5 ^ 0) with An = ±1,

have an even stronger scaling: Z'^. The widths of ordinary (electric-dipole) transitions scale as a more moderate Z^, but this is still significantly greater than the quadratic scaling of

energy, so the relative sharpness of the emission spectrum still blurs with increasing charge:

A£/£: = zV-Z^ = Z2.

1977):

(6)

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R104 Topical review

2.4. Electric field

HCIs provide a simple system in which electrons are subject to some of the strongest electric

fields available in the universe. Scaling up Coulomb's law as (one factor of Z for nuclear

charge, two more for reduced Bohr radius), the electric field experienced by a lone electron

orbiting a uranium nucleus is approximately 10^^ V cm"', the strongest static field available

in any laboratory. Even the most intense laser fields are several orders of magnitudes less strong than this. The conditions inside HCIs thus provide a unique testing ground for 'strongfield quantum electrodynamics' (Greiner et al 1985). In addition, the residual strong fields that extend from HCIs to long distances form the basis for the emerging field of HCI-surface

interactions, discussed below.

2.5. Angular momentum coupling

Because the outer electrons in neutral atoms interact primarily by electrostatic forces, the

Hamiltonian is independent of spin (to first order). This gives rise to the ordinary LS-coupling

scheme (Cowan 1981, Heckmann and Trabert 1989) in which eigenstates are well described

by the independent quantum numbers corresponding to the total electron angular momentum

(L) and total electron spin (5). Deviations from pure LS coupling can be described within

the context of perturbation theory by expanding the eigenstates in terms of pure LS states; there will generally be a small admixture of nearby states with different quantum numbers. For

HCIs, however, the admixture can become so large that an entirely different coupling scheme is called for. For example, there are states in magnesium-like manganese and oxygen-like copper

in which the eigenfunctions are primarily comprised of a linear combination of two LS states, with virtually identical weights (equal to within a fraction of one per cent) (Gaigalas et al 1999).

In cases like these, even discussing the situation qualitatively can become confusing. There

have been cases where the admixtures were so large that theorists and experimentalists thought

they were comparing results for a particular state when, in fact, they were talking about two

physically different states. When the problem of state identification was clarified, entries in tables were switched and agreement between theory and experiment improved (Froese Fischer

and Jonsson 2001).

The required change in coupling scheme is made clear by considering the relative

importance of electrostatic (electron-electron) and relativistic effects (e.g. spin-orbit

interaction). The spin-orbit interaction grows as Z^, as discussed above. The weaker

Z^ scaling of electron-electron interaction can be appreciated by considering that (1) it is inversely proportional to the distance between the two electrons and (2) the area of space that

adjacent electrons on a spherical Bohr orbit have available to pass by each other scales as ArtR^ ~ ~ Z~^. As the spin-orbit interaction becomes stronger with increasing Z, it becomes more appropriate to first couple the spin and angular momentum quantum numbers

for each individual electron {j = s + I) and then take into account the electron-electron

interaction by subsequently coupling the electrons together (7 = ji+ j2 + • • •)• The resulting

natural transition from to jj coupling as Z is increased (isoelectronically) is evidenced by

the smooth rearrangement of the energy eigenvalues plotted in figure 6. Standard spectroscopic

notation (Cowan 1981, Heckmann and Trabert 1989) is used to label this figure.

2.6. Selection rules

The change from L5 to jj coupling in HCIs is accompanied by a loss of familiar quantum

mechanical selection rules. For example, electric dipole (El) transitions can then take place even with a change of the total spin quantum number (AS ^ 0). Furthermore, the requirement

NIST-76
Topical review R105

LS Jj

Atomic number, Z

Figure 6. Variation of energy eigenvalues with Z, showing the crossover from LS to jj coupling

in the case of Be-like ions, from Martinson and Curtis (1989).

that the total orbital angular momentum change by AL = 0, ±1, with no L = 0 to L = 1 is relaxed. These and other changes in the selection rules are tabulated in Beyer et al (1997). A

detailed discussion of the origin of the changes to the selection rules in terms of Racah tensors can be found in Heckmann and Trabert (1989).

2. 7. Some issues on the theoreticalfoundations

Although the natural periodic table stops at Z = 92, and researchers have only been able to extend it by another 30% or less so far, it is of interest to consider what happens in principle

at higher Z. In particular, what happens along the hydrogen-like isoelectronic sequence as Z

rises above 137 (the reciprocal of the fine structure constant)? The most precisely tested theory

in all of physics (quantum electrodynamics) predicts that at such high Z, the vacuum of space

will become unstable and undergo a fundamentally new type of phase transition (Greiner et al

1985). One consequence is the spontaneous production of matter. This bizarre scenario is discussed further in section 4.4 below.

The phase transition at ultra-high Z is due to the existence of negative energy states. The

effect of these states must be considered even at more moderate values of Z. A case in point is the 'continuum dissolution' problem. The Hamiltonian generalization of the Dirac equation to the case of two or more electrons becomes a problem once the interaction between the electrons

is taken into account. If even a slight interaction term is allowed in the Hamiltonian, it can be shown that no bound state eigenfunctions exist. This problem is known as the 'BrownRavenhall disease', after the authors of the seminal paper entitled 'on the interaction of two

electrons' that was published 50 years ago (Brown and Ravenhall 1951). Outside of a full field-theoretic treatment, the problem has still not gone away, although clever methods of

effectively ignoring it in computations have been devised (Brown 1987). The ad hoc insertion

of projection operators to forbid certain classes of transitions, for example, is one method of

dealing with the problem (Sapirstein 1998). In a review paper on quantum electrodynamical

NIST-77
R106 Topical review

z"°[Za]^ Z-°[Zaf Z-°[Zaf ... Z-^[Za]^"

Z"^[Zaf Z ' [Zaf

Z"^[Za]^ Z"^[Zaf

Z"°tZaf Z-"[Za]^"

Figure 7, Terms in the Za and 1 / Z expansion. The number of rows and columns extends to infinity

(n, TO oo). Each box contains an infinite number of terms in the mass expansion (visualize along

a 2-axis extending out of the page). Each of the boxes in this three-dimensional matrix contains a

set of Feynman diagrams. Figure after De Sousa Zacaris (1990).

effects in helium, Douglas and Kroll recount a bit of the history of how, before the work of

Brown and Ravenhall, terms in the leading theoretical formulation (the Breit equation) were

simply discarded in order to obtain agreement with experiments (Douglas and Kroll 1974).

Perhaps the most intriguing issue in the theoretical description of HCIs is the abundance of

infinities that appear in the most rigorous calculations of the energy levels. Methods have been

developed of handling these infinities so they will sum to finite values that, in the end, agree

well with experimental measurements. The question of whether this procedure will continue

to hold up under increasing experimental precision and widening breadth of appUcation, is a

topic of active research.

A more concrete look at the issues mentioned in the previous paragraph reveals several

layers of infinities that can arise in the prediction of energy levels of HCIs. First, in one of

the standard formalisms there is an expansion in the dimensionless parameter Za (Z times the

fine structure constant) that results in a sum of an infinite number of terms to calculate. For the

realistic case of a finite mass nucleus, each of the terms in this expansion is itself represented

by an infinite sum known as the 'mass expansion' (an expansion in the small dimensionless

parameter given by the ratio of the mass of the electron to the mass of the proton). Each of the

terms in the mass expansion is given, in turn, by the sum of one or more (sometimes a great

many) Feynman diagrams, each of which itself can be, and frequently is, infinite in value (until

renormalized). For multi-electron HCIs there is an additional expansion in a, which appears

as an expansion in 1 /Z when combined with the Za expansion. Most current effort is focused on calculations represented by high-order Feynman diagrams, such as the two-loop binding

correction (Hides and Shelyuto 1995, Pachucki 1994).

A useful way of organizing the Za and the 1/Z expansion for a two-electron ion is shown in

figure 7. Here, the first column gives the non-relativistic energy and the first row gives the sum

of one-electron Dirac energies. Each box corresponds to a collection of Feynman diagrams.

The well-known Breit interaction is located in the (2, 2) box marked Z~^[Za]^. There are serious concems among theorists about whether the most complete theoretical treatments will

hold up when measurements with increased precision become available at high Z.

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Topical review R107

2.8. Interactions with surfaces

The approach of a slow HCI to a surface presents an unusual experimental situation. The

electrostatic pull of the HCI can be so great that it begins removing electrons from the surface even when it is dozens of atomic diameters away. These electrons are removed from a highly

localized region of the surface, estimated to be on the order of 1-10 nm in diameter (Perez

and Olson 1999). Many of the electrons that are removed swing around the ion and then

travel back to the surface, not necessarily landing back at their point of origin because of

the dynamical evolution of the ion-surface system in the meantime. In addition, some of

the electrons are captured temporarily by the ion and become bound in high-lying Rydberg

levels. This forms a so-called 'hollow atom' (Briand et al 2000, Khemliche et al 1998) in which most or all of the electrons are in an excited state. Hollow atoms have their electronic

wavefunctions concentrated in a large shell that surrounds a relatively empty central core. The

population of high-lying levels that results in a hollow atom can subsequently decay to lower

levels, giving off either a photon or a secondary electron. Typically, the decay proceeds most

rapidly through interaction with other bound electrons in an Auger process. Through the Auger

decay, the partially neutralized ion automatically reionizes itself, as it continues to approach

the surface. Part of the Auger-emitted electrons are directed back towards the surface in a

diffuse beam, and part are ejected outwards away from the surface where they can be readily

detected. In addition, there may be a great many low-energy electrons that are pulled from the

surface and ejected outwards without undergoing an intermediate capture process by the ion. In experiments, up to several hundred electrons have been measured to be projected away from

a surface by a single low velocity HCI (Schneider and Briere 1996). The HCI acts, in effect,

like a miniature electron pump to remove a large number of electrons from a nanometre-sized

region of the surface on a time scale that can be in the femtosecond range. The implications

of this novel situation for the field of nanotechnology are discussed in sections 4.6 and 4.10

below.

There have been a number of efforts to develop quantitative models for the interaction of

slow HCIs with surfaces, but these will not be discussed here. Most of the effort has been

focused on predicting the dynamical evolution of the ion, rather than the long-time response

of the surface. References to the former can be found in papers that develop the classical overbarrier model (Burgdorfer et al 1991), the extended classical over-barrier model (Ducree et al

1998), density functional theory (Amau et al 1997), the close-coupling model (Bahrim and

Thumm 2000), and cascade models (Stolterfoht et al 1999). Some models of surface response

to HCI impact can be found in the references given in the last paragraph of section 4.6.2 below.

3. The production of HCIs

There are many possible ways of producing ionized atoms, some of which are quite exotic. In muon-induced ionization, for example, a muon is captured by a neutral atom and the atom

responds by ejecting all of its electrons (Bacher et al 1989). This mechanism can produce

an HCI in less than a femtosecond, and plays an important role in fundamental studies using

exotic atoms (Siems et al 2000). The primary mechanisms for producing HCIs in the natural

universe, however, are photoionization and electron-impact ionization (Giroux and Shapiro

1996). For the sort of precisely controlled laboratory work described in this review, electronimpact ionization is the primary method used.

NIST-79
R108 Topical review

Figure 8. Energy dependence of typical electron-impact ionization (dotted curve) and excitation

(fiill curve) cross sections for an HCI (see text).

5.7. Electron-impact ionization

In an electron-impact ionization process, electrons with high translational energy collide with an atom and remove a bound electron. This process continues in subsequent collisions, like the

peeling of an onion, until the ionization energy becomes greater than the available translational

energy. The projectile electron can be either a free electron, or bound to another atom or ion. Electron-impact ionization proceeds easily for the first few electrons that are removed, but

the process rapidly becomes more difficult as the charge increases. The difficulty is twofold:

the more deeply bound electrons require more energy to remove (ionization cross sections

decrease), and neutralizing collisions with background gas atoms thwart the step-wise progress

towards the desired charge state (charge exchange cross sections increase). In addition, the

effect of the latter is exacerbated by the former because of the increasing time between ionizing

collisions.

The cross section for electron-impact ionization typically rises gradually from zero at a threshold equal to the ionization energy, and reaches a broad maximum at a factor of 2.5 to 3 above the threshold before falling slowly back towards zero. Figure 8 shows the ionization cross section for the removal of an electron from Xe'^'*' (to produce Xe^^"^). In many cases, the

ionization energy for the next-highest charge state will occur at a lower energy than the peak

of the cross section, however, so the optimum electron energy to maximize the equilibrium

amount of a particular charge state may occur at lower values than the peak of the cross section. In comparison, the electron impact excitation cross section typically rises abruptly to its peak

value at threshold, before falling back towards zero at higher energies (figure 8). For excitation,

the threshold is equal to the separation between the energy levels. The magnitudes of the ionization and excitation cross sections for HCIs are typically much

less than the classical elastic scattering cross section for a hydrogen atom (jtoq ^ 10~'^ cm^).

When the vertical scale of figure 8 is multiplied by 10~^' cm^, the ionization cross section

NIST-80
Topical review

0.00 0 20 40Tune [msec]

60 80 100

R109

Figure 9. Time evolution of the charge states of xenon produced by an intense electron beam (see

text).

shown corresponds to that of the Lotz formula (Lotz 1967). The peak value of the cross section

predicted by the Lotz formula occurs at an electron energy equal to a factor of 'e' (~ 2.72)

above the ionization energy Iq, and has a value (in cm^) approximately equal to

for /o measured in eV. Note that this formula is for a single electron, so the total cross section

for ionization of any one of the N electrons in a given shell (all with approximately the same

Iq) is given by this formula multiplied by N.

For dipole-allowed transitions, the order of magnitude of the peak of the excitation cross

section can be estimated from the Van Regemorter (1962) formula, knowing only the transition

energy and the oscillator strength. For more than an order of magnitude estimate, a more

accurate computation may be required (Sampson and Zhang 1992). For a 4 keV transition and

an oscillator strength of unity, this formula predicts that the peak of the excitation cross section

would be roughly equal to that of the plotted ionization cross section. Because the peak of the

excitation cross section scales as the inverse square of the transition energy, however, it should

be reduced by a factor of 100 at the 40 keV energy shown in figure 8. In order to give a feel for how sequential ionization proceeds, figure 9 shows the results of

a numerical simulation which takes into account the dynamic evolution of the adjacent charge

states under the influence of an intense electron beam. In this figure, the electrons are assumed

to have a single energy (8000 eV) and direction, and a current density of 3900 A cm~^. The

physics included in the simulation is based on earlier work (Margolis et al 1997, Penetrante etal \99\).

Because the competing effect of ion neutralization by background gas becomes more

severe as the ion charge increases, high vacuum systems are required for the production and

transport of slow HCIs. The scaling of electron capture cross sections with increasing charge

is not simple (Pal'chikov and Shevelko 1995), but for energies typical of the ions produced in

table-top devices, they increase roughly linearly with charge. A more precise rule of thumb

1.7 X 10-14

Omax (E = elo) = 0)

1 NIST-81
RllO Topical review

leads to the following formula (Muller and Salzbom 1977) for the mean free path in metres

of an ion of charge state Q (dimensionless), assuming a typical background gas like nitrogen

(ionization potential ~ 12 eV) at a pressure p measured in hPa (1 hPa ^ 1 Torr),

10-^ X (2~' '^ X= ^ . (8)

P

For example, this formula predicts that at UHV conditions of p ~ 10~'^ hPa, a Xe"*^ ion will

have a mean free path A ~ 12 000 m and thus live for 60 s if the relative velocity through the gas

is 200 m s~' (corresponding to a room temperature kinetic energy, ^mv^ = kT). Moving at a more typical speed for an HCI (25 km s~', after acceleration through a 10 kV potential), this lifetime is shortened to 0.5 s. The vacuum requirements for HCIs are lessened in high-energy

particle accelerators, because the cross sections decrease rapidly (e.g. ~ v~^) as one goes into

the very high velocity regime (Pal'chikov and Shevelko 1995).

3.2. Overview of laboratory devices

Martinson (1989) has reviewed the types of laboratory sources for HCIs in use prior to 1988. These fall largely into two categories: those that are relatively cheap, yet of limited

ability (sparks, arcs and exploding wires, supplemented by hollow cathode and electrodeless

discharges for the lowest charge states) and those that are very expensive and involve large

national facilities (magnetically confined fusion devices, particle accelerators and multi-joule

terawatt lasers). Fitting into a new class that falls in between these two extremes are the electron

cyclotron resonance (ECR) ion sources (Geller 1996) and electron beam ion sources (EBIS)

(Donets 1998). These two latter types of ion sources can produce rather high charge states, with a modest investment of resources. The newest source of HCIs to date, the electron beam

ion trap (EBIT) (Levine et al 1988), has evolved out of the EBIS concept and is discussed in greater detail in a separate section below.

Recent advances in producing femtosecond tabletop lasers have made it possible to easily

produce TWpower levels, but the limited pulse length has prevented these lasers from becoming

efficient methods of producing HCIs. Figure 10 shows the charge state distribution obtained

from a 2 TW, 100 fs laser, in comparison to that produced by an EBIT (next section) and an

ion accelerator.

3.3. The electron beam ion trap (EBIT)

The EBIT is a compact and relatively inexpensive device capable of producing and

electromagnetically confining HCIs. A typical EBIT is almost 'table top' in size, costs less than a good electron microscope, and yet can remove most of the electrons from any naturally

occurring atom on the periodic table. A somewhat larger EBIT (about twice the size) has been

used to fully strip even the heaviest naturally occurring element (Marrs et al 1994b). Presently,

there are cryogenic EBITs at Livermore (Marrs etal 1994a), NIST (Gillaspy etal 1995), Oxford

(Silver et al 1994), Tokyo (Kuramoto et al 2000), Berlin (Biedermann et al 1997), Freiburg

(Lopez-Urrutia et al 2000), Frankfurt (Zipfel et al 1998) and (very recently) Berkeley. A

cryogenic EBIT was also built in Russia, but is currently out of service. Smaller 'warm'

EBITs have been constructed at Livermore, Troitzk (Antsiferov and Movshev 1991), Dresden

(Ovsyannikov et al 2000) and Paris (Khodja and Briand 1997). An additional cryogenic EBIT

is under construction in Shanghai.

NIST-82
Topical review Rill

62 63 64 65 66 67 68 69 70 71 72 73

Charge State (Q)

Figure 10. Charge state distribution from (a) a 2 TW, 100 fs laser incident on a tantalum target (Foumier et al 2000), {b) an ion accelerator producing 3 GeV gold ions (Chandler et al 1989) and

(c) an EBIT producing uranium ions with 9 keV electron beam (Schneider et al 1991).

3.3.1. Comparison of EBIT to ion accelerators. From the most fundamental point of view, an EBIT functions as a miniature particle accelerator. In both cases, electric fields produce

high-velocity charged particles which are then directed onto a target to cause collision-induced

ionization. The ions are subsequently confined by magnetic and/or electric fields in an ultrahigh vacuum chamber. There is a great difference in scale and technical detail, however, between an EBIT and an ion accelerator. These differences essentially have to do with the relative motion

of the laboratory and centre-of-mass reference frames.

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R112 Topical review

In the case of an EBIT, the electrons are accelerated and the target atom/ions are at rest in the laboratory frame of reference. In the case of an ion accelerator, the roles are reversed, and

the electrons are essentially at rest in the laboratory frame while the heavy ions are accelerated.

In practice, the target electrons for the ion accelerator are those bound to atoms in a thin foil of

solid material through which the ions pass. The fraction of the kinetic energy in the laboratory

frame that is available for ionization is determined by the kinetic energy of the two collision

partners in the centre-of-mass frame. For the case of the EBIT, the laboratory frame is already

essentially in the centre-of-mass frame, so the energy fraction is 100%. For the case of the

accelerator, the centre-of-mass frame is moving at near the speed of light through the laboratory

frame, so most of the energy is unused. For a moving ion of mass M the fraction of laboratory

energy available for ionization is given by.

where m is the mass of the electron at rest in the laboratory. For gold ions, this fraction is 3 X 10~^, so in order to achieve 8 keV of ionization energy, one must accelerate the gold

ions to 3 GeV (about 20% of the speed of light). This requires sophisticated alternating-field

accelerator technology and a facility the size of a large building. In contrast, an EBIT can

accelerate the relatively light electrons to 20% of the speed of light within a few centimetres

using modest electrostatic fields produced by a commercially available voltage source. While

the electron density in an EBIT is much less than the electron density in an ion accelerator

target (beam foil), the interaction time is also much greater and these two factors tend to balance each other somewhat so that similar distributions of charge states are produced in both

cases. Some examples of the equilibrium charge state distributions produced by an EBIT and

an ion accelerator are shown in figure 10. For many high-precision spectroscopy experiments,

the low velocity of the ions produced by an EBIT is advantageous because it renders Doppler

shifts negligible. In other experiments, however, the high velocities of ions produced by an

accelerator can be used to good advantage, as discussed elsewhere (Beyer and Shevelko 1999).

A storage ring is typically used in conjunction with an ion accelerator in order to capture

the fast ions and recirculate them through an observation region. Such rings use large bending

magnets to essentially form a large ion trap (with a typical circumference of 50 m). In an EBIT,

the ions move much more slowly and can therefore be confined with a more modest effort. In fact, the electron beam that produces the ions in an EBIT, also generates a strong electric

field which itself can be used to trap the ions in the radial direction. In effect, the electron

accelerator that produces the ions is located inside the trap itself, so no additional effort is required to load the trap. Two small electrostatic mirrors separated by a few cm serve to reflect the ions back and forth along the axis of the electron beam, forming a 'hall of mirrors' that effectively extends the trap length to infinity as seen from the ion's point of view. While the

EBIT trap is thus something like a ring with infinite radius, in reality the ions are all localized

in a small physical region of space in the laboratory. This makes it relatively efficient to collect

light from the ions in order to study their structure and behaviour using standard spectroscopic

techniques.

3.3.2. EBIT details. Some of the technical details of an actual EBIT are shown in figure 11. The design and operation of the device have been described in detail elsewhere (Gillaspy 1997,

Levine et al 1988), so only a rough overview will be sketched below. An interactive online

tutorial is available on the web'. The most thorough description to date of the physics and

technology that underlies an EBIT has been given by Currell (2001).

' http://physics.nist.gov/MajResFac/EBrT/inain.html

M + m

M

(9)

NIST-84
Topical review R113

Figure 11. Upper, piiotograph of the electron beam ion trap at MIST, shown next to a cross sectional schematic (approximately 1 m in height). The photograph is reduced by a factor of two in order to show the ion extraction apparatus on top of the EBIT. Lower, detail of the drift tubes shown next

to an enlarged image of the cloud of trapped ions (ion cloud approximately 200 fim wide).

There are three primary subsections of an EBIT: (a) an electron gun to produce the beam;

(b) a drift tube assembly to accelerate the beam to a specified energy; and (c) an electron

collector to serve as a beam dump. In the EBIT shown in figure 11, the electron gun is a

small unit, approximately 1 cm in diameter by 2 cm in length, with a 3 mm diameter cathode

that produces about 150 mA of current. Although the original EBIT device was built with

the electron gun permanently grounded, the one shown in figure 1 1 is mounted on a 'floating'

(electrically isolated) high-voltage platform inside the main UHV vacuum chamber so that the

electron gun ground lines can be operated at negative high voltage relative to the laboratory.

This configuration makes it possible to provide a boost in the available beam energy, beyond

the high voltage capability of the drift tube assembly described below. This is the principle

that operates on a larger scale in the so-called 'super-EBIT' device to produce bare uranium

ions (Marrs et al 1994b). In the super-EBIT, the electron gun rests on a large high-voltage

NIST-85
R114 Topical review

platform that is itself as large as an ordinary EBIT.

The primary magnetic field in the EBIT is a 3 T superconducting Helmholtz pair that is centred on the drift tubes. In the super-EBIT in Japan (Kuramoto et al 2000) this magnetic field

is increased to 4.5 T. The magnetic field serves to compress the electron beam to a high density,

approximately 4000 A cm~^ (Marrs et al 1994a) in the case shown. In addition, there is a smaller electromagnet built into the electron collector that runs at liquid nitrogen temperature,

and another electromagnet in the electron gun region that runs at about 100 K above room

temperature. The magnet surrounding the electron gun is used to cancel the stray field from

the superconducting magnet, and then adjust the value of the small but finite residual magnetic

field that is necessary at the cathode in order to efficiently couple the beam into the drift tube

assembly without magnetic mirroring (Takacs et al 1996). High permeability metal in the

electron gun region shapes the net field appropriately. Detail about the general design problem

of coupling an electron beam into a strong magnetic field can be found in Becker (1995).

The drift tubes are cooled by thermal contact with a superconducting magnet assembly

that is filled with liquid helium {T = 4.2 K). Although generally the Wiedemann-Franz law

links thermal and electrical conductivity, the choice of a suitably anomalous material (sapphire)

allows good cooling and electrical isolation to be obtained simultaneously. The drift tubes are operated at 1-35 kV without any detected leakage current (< 10 /xA).

The outer portion of the drift tube assembly is a large shield electrode that is specially

shaped to reduce electrical breakdown to ground. Inside the shield there are three smaller

electrodes which operate at differential voltages of up to 500 V from the shield voltage. These

three electrodes form a variant of a cylindrical Penning trap (Gosh 1995). Raising the upper

and lower electrode voltages with respect to the centre electrode forms a potential well that traps the ions in the axial direction. The electron beam is threaded through the central axis of

the three inner drift tubes and produces an electric field that strongly supplements the radial

magnetic trapping. The electron beam also provides a significant axial trapping potential

because the centre and end drift tubes have different radii and therefore different amounts of

surface charge induced by the space charge of the electron beam (or, equivalently, different

locations of the virtual image charge). Because of the space charge and image charges, the

electron beam energy at the trap centre is offset from that naively determined by the voltage

placed on the drift tubes. In addition, the beam energy varies somewhat as a function of

radius. These issues are discussed in more detail elsewhere (Currell 2001, Porto et al 2000a).

Finally, the electron beam also provides the critical function of ionizing the atoms in the trap,

and exciting them to electronic energy levels above the ground state in order to make them

fluoresce. In contrast to a conventional plasma which has a broad electron velocity distribution,

the EBIT beam is closer to that of a delta-function (figure 12) and is directed along a well

defined axis. This is extremely useful for sorting out differential cross sections for fundamental

processes such as electron-impact ionization, excitation, and recombination. The energy of the

electron beam can also be rapidly swept back and forth through a pattern that will synthesize an arbitrary electron distribution function (Savin et al 2000).

The electron collector functions by providing a retarding field that partially slows down

the electrons and a magnet that spreads out the beam and allows it to be deposited over a large

surface area that is cooled with liquid nitrogen. This action occurs inside a cylinder with end

electrodes (essentially, an electron trap) to suppress the emission of secondary electrons. All of the EBIT electrodes and magnets are annular about the primary electron beam axis so it is possible to look down from the top of the system and see the ions in the trap and,

beyond it, the glow of the heated electron gun cathode. This configuration makes it possible to eject ions out of the top of the trap and direct them onto surfaces or inject them into secondary

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Topical review R115

a

oau

u

COo

oZ

1.0 F

0.8 -

0.6 -

0.4 -

0.2 -

0.0

T T

EBIT Electron Beam (width xlO)

Maxwell-Boltzmann distribution

20 40 60 80 100 120 140 Speed [10 m/s]

Figure 12. Electron beam velocity distribution in an EBIT compared with a Maxwell-Boltzraann

distribution (non-relativistic). The temperature of the distribution and the peak of the beam velocity

correspond to 8 keV. In order to make it visible on the plot, the beam width has been expanded by

an order of magnitude (500 eV shown, 50 eV available in EBIT).

traps. A high efficiency ion beamline equipped with electrostatic and magnetic beam control

components has been constructed to transport the ions to various other experiments in the

laboratory; this system is of comparable complexity to the EBIT itself and is described in detail elsewhere (Pikin et al 1996, Ratliff et al 1997).

4. Applications

4.1. Atomic reference data

The most immediate application for HCIs is as a testbed for our understanding of the electronic

structure of matter. Most of the atomic 'data' that are tabulated for use as input reference

data for the development of new technologies are generated by calculations, not measured in experiments. Instead, benchmark experiments measure a sampling of representative cases in order to test and improve the underlying theory, upon which most of the reference data are then computed. This underscores the practical importance of having a highly robust underlying

theory that can be applied with high confidence to a wide range of individual cases as the need

arises.As discussed in section 2 above, HCIs allow us to produce a subclass of atoms in which the

basic physical parameters take on extreme values. Large deviations from model predictions

that occur at these extreme values give clues to the nature of the deficiencies in our models and

allow us to improve them in fundamental ways. This sort of quantum mechanical 'shake test' can be extremely important in the development of robust theories of atomic structure that will

hold up to a high degree of confidence within the more limited realm of ordinary experience.

As noted by Martinson and Curtis (1989), one can obtain a better understanding of neutral

atoms by studying HCIs.

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4.2. Cosmic chronometers

In the process of testing theories of ordinary matter under extreme conditions on Earth, one also

obtains information that is relevant to a broader understanding of the cosmos. An interesting

example is the effect of bound-state beta decay in the 'cosmic chronometer' provided by

rhenium- 1 87 (Bosch 1999). The time span of nucleosynthesis in our galaxy can be determined

by the concentrations of decay products of rhenium- 187 in meteorites. In the neutral charge

state, rhenium has a half-life of 42 x 10^ yr. During the course of the evolution of the galaxy,

however, rhenium can spend a significant fraction of its life in a highly ionized state. Recent

laboratory experiments at the GSI facility in Germany (Bosch et al 1996) have proven that

the half-life of highly ionized rhenium is nine orders of magnitude shorter than that of the

neutral state (33 yr, rather than 42 x 10^ yr). The physical mechanism underlying the lifetime

shortening is that beta decay can proceed more rapidly if the emitted electron does not escape

the ion but is instead captured into an open electronic shell. If the open-shell is very deep, as

it is in an HCI, the amount of work saved by not having to lift the electron out of the potential

well is considerable.

The rhenium cosmic chronometer is presently being 'reset' to include the accurately

measured value for the lifetime shortening due to the bound-state beta decay effect described

above. Prior to the measurements, the nine orders of magnitude correction was applied using

theoretical estimates. It can now be seen that these estimates were off by about a factor of

two. Preliminary results for the rhenium clock recalibration reduce the estimated age of the

universe by 10^ yr. The implications, not only for cosmic geneology but also for shedding

light on the future fate of our universe and the value of Einstein's cosmological constant, are discussed by Bosch (1999).

4.3. X-ray astrophysics

A new era in deep space astrophysics began in 1999 when two x-ray observatories of

unprecedented size and capability were launched into orbit: NASA's Chandra and the European

Space Agency's XMM. Although previous x-ray missions had determined that the universe was rich in x-ray emission, they lacked the spectral and spatial resolution to clearly make out

the details. The striking clarity of the data now arriving from the new x-ray observatories are

the astrophysical equivalent of going from legally blind to 20-20 vision. Spectroscopically, it

is reminiscent of the beginning of atomic physics in the early part of the 20th century when

the richness of optical spectra from atoms on Earth was first being observed.

The x-ray data from Chandra and XMM contain a wealth of information about violent

astrophysical processes that occur, for example, when stars explode, galaxies collide and

matter falls into black holes. Radiation from the highly ionized matter that is produced in

these circumstances has a complex spectral signature that challenges our understanding of the

atomic physics and collision dynamics of HCIs. Because the Earth's atmosphere is opaque to x-ray radiation, and because HCIs do not occur naturally on Earth, devices like the EBIT are valuable testbeds for developing both the x-ray telescope hardware itself (Silver et al 2000b)

and the theoretical models that will be used to interpret the telescope observations (Gu et al

1999, Laming et al 2000, Silver et al 2000a). Indeed, with temperatures around 10 000 000 K,

the ions produced and confined inside an EBIT rival the hottest and most reactive plasmas in

the universe.

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Topical review R117

4.4. The structure of the vacuum

The idea that even an ideal vacuum is not just empty space has a long history. Descartes

expressed this idea by saying 'It is contrary to reason to say that there is a vacuum or a space in which there is absolutely nothing'. With the advent of quantum electrodynamics, well defined

ways of subjecting the vacuum to quantitative physical experiments became possible. Perhaps

the most remarkable aspect of the vacuum as described by quantum electrodynamics is that in the presence of a sufficiently strong electrostatic field, it is capable of generating real particles

spontaneously. In effect, beyond a critical value, an electric field causes the 'normal' vacuum

of ordinary experience to become unstable. This instability causes the vacuum to 'decay'

in about 10~^^ s (Greiner et al 1985) through a phase transition from a neutral to a charged

state. The charged state is manifested by the spontaneous generation of positron-electron

pairs. The Pauli exclusion principle stabilizes this emission and limits the number of emitted

particles to a well defined finite value. An instructive gedanken experiment is presented by

Greiner et al (1985) to give some physical insight as to what this means: imagine a box in which all particles are pumped out except for a bare HCI that is the source of a supercritical

nuclear field. Within 10~'^ s, two positrons and two electrons will spontaneously be generated

out of the vacuum, the electrons will bind to the nuclear field, and the positrons are pumped

out. The electronic cloud of the new HCI is the new charged vacuum, which is stable. If the two bound electrons are removed, the vacuum will emit two additional electrons (and

two positrons) to re-establish the situation. In effect, nature begins generating a neutralizing

stream of electrons spontaneously to prevent us from producing even higher charged ions.

The supercritical fields at which this phase transition is predicted to occur can be generated in

highly charged ions around Z > 1/a = 137. The results of including the effect of the finite size of the nucleus shifts the onset of the transition to Z > 173 (the permutation of the last two

digits is coincidental). It was previously thought that such a phase transition had already been

observed in high-energy collisions of two heavy ions (temporarily forming a single superheavy

nucleus) but it is now generally agreed that no experiments so far have achieved the necessary

conditions long enough for the vacuum to decay (Schafer 1996). The study of highly charged

ions with Z < 92 is a stepping stone on the way to this holy grail. Subcritical field effects such

as wavelength shifts (Beiersdorfer et al 1998, Bosselmann et al 1999, Chantler et al 2000,

Mohr et al 1998, Stohlker et al 2000) and changes in the ^-factor (Haffner et al 2000) are subjects of active investigation presently.

4.5. Solar physics and the Earth-Sun connection

One of the first applications of the concept of an HCI in astronomy was to solve a longstanding

problem summarized in the title of an article published in 1939 in the French journal Scientia: 'A great enigma in current astronomical spectroscopy: the spectrum of emission lines in the

solar corona' (Swings 1939). In this paper, the author describes the struggle to explain dozens

of unidentified emission lines observed during a solar eclipse. After essentially ruling out

the speculations contained in 15 other papers, he essentially gave up and concluded that the

problem was a grand enigma. Fawcett describes how Edlen resolved this enigma by identifying

the lines as forbidden transitions in HCIs, calling it 'perhaps the finest research in the annals

of atomic spectroscopy' (Fawcett 1981). The application of the spectra of HCIs to resolve

many other problems in solar physics has continued through the years, and to date remains an

important aspect of this field. Curtently, there is considerable interest in understanding and predicting certain aspects

of solar flares because of the adverse effects they can have on earth. Powerful solar flares can

NIST-89
R118 Topical review

destroy the sensitive instruments in earth satellites if they are not powered down in advance. In May of 1 998, it was reported that a solar flare permanently disabled a $ 1 65 million satellite used

by 45 million pagers in the US. Although optical observations of the Sun often provide hours

of advance warning, sometimes the particles race towards the Earth at half the speed of light,

allowing only a few minutes of warning. Even more disruptive than total destruction, perhaps,

are transient problems that solar flares can inflict on commercial and military satellite behaviour.

Energetic particles from flares can produce streaks of light in star-tracking navigation detectors,

and make them point in improper and unstable directions (disrupting navigation) or change

the electronic memory content and trigger unintended commands to be executed by onboard

computer systems.

Even more challenging than protecting satellites is the problem of protecting humans in space. During a solar flare, astronauts can be exposed to lethal doses of radiation, and airline passengers and crews of commercial jets flying at high latitudes can be subject to significantly

elevated doses (equivalent to dozens of chest x-rays per flight).

Finally, solar storms can disrupt the upper atmosphere of the Earth to such an extent

that high-frequency radio transmission is blacked out. In addition, associated changes in the

Earth's magnetic field can be inductively picked up by transcontinental power grids and lead

to electrical blackouts. On March 13, 1989, for example, the entire Hydro Quebec system,

which serves more than 6 million customers, was blacked out for a sustained period of time,

and most of the neighbouring systems in the United States came close to experiencing the same

sort of outage. The National Oceanic and Atmospheric Administration of the US Department

of Commerce routinely issues online space weather alerts, warnings, and forecasts because of

their potential impact on Earth^.

The spectroscopy of HCIs plays an important role in research directed towards

understanding solar flares, and in providing real-time diagnostics to predict their influence on Earth (Drake et al 1999, Laming et al 2000).

4.6. Microelectronics and nanotechnology

The microelectronics industry continues to be a major driver for the development of new

techniques in the area of nanotechnology. Because ion beams are widely used in the

fabrication of microelectronic devices, this area is a natural one to benefit from the unique

characteristics of HCIs. Of particular interest are slow HCIs that can be produced with small

and inexpensive devices. In the US, research to explore these applications began around

1992 (Schmieder and Bastasz 1993), and has been carried out recently at university and

government laboratories that make up the Research Association for Multiply-Charged IonSurface Interaction Studies^ : Lawrence Livermore National Laboratory (Schenkel etal 1999b),

Oak Ridge National Laboratory (Meyer and Morozov 1999), National Insitute of Stanards

and Technology (Gillaspy et al 1998), J R McDonald Laboratory at Kansas State University

(Bahrim and Thumm 2000, Parks et al 1998), University of Florida (Cheng and Gillaspy

1997) and Cornell (Beebe and Kostroun 1992). In Europe, research in this area has been

carried out largely within the Human Capital and Mobility Network of the European Union

(Amau et al 1997). A number of US and international patents have been issued'^ and small

^ http://www.sec.noaa.gov/

^ http://www.phys.ksu.edu/area/jnnyramcisis.html

US patents: 5 327475 (Ruxamlnc, 1994; lithography), 5 625 195 (France Telecom, 1997; implantation), 5 849093

(1998; cleaning and smoothing). French patents: 2 757 881 and 2764 1 10 (Universite Pierre et Marie Curie, 1998 and

1999; etching, cleaning, and nanomanufacturing), 2 793 264 (X-ion, 2001; cleaning and microfabrication). Japanese

patents: 9 199457 (Hitachi, 1997; cleaning), 8 213 363 (Hitachi, 1996; lithography), 7 280755 (Hitachi, 1995; electron spectroscopy) 7 230 985 and 6 326 060 (Hitachi, 1995 and 1 994; etching). See also, French patent application 2 800 477

NIST-90
Topical review R119

companies have been formed in France (Borsoni et al 2000), the US and the UK specifically

to explore the possible commercialization of the new HCI-based technology. Some larger

existing companies in the US and Japan (Itabashi et al 1995, Mochiji et al 1994, 1996) are

also exploring the possibilities to a more limited degree. In the subsections below, several

applications for HCIs produced with small sources are discussed in more detail. Reviews of

related applications using very fast ions produced with large devices such as ion accelerators

have been published previously in a special issue of the MRS Bulletin (Fischer and Metzger

2000), in books (Fleischer 1998) and in review articles (Fischer and Spohr 1983).

4.6.1. Ion implantation. The most common use of ion beams in microelectronics

manufacturing today is ion implantation, a process in which high velocity ion beams are injected

into materials. The ions are typically accelerated to a well defined velocity by traversing an

electrostatic potential formed by high-voltage power supplies. One of the industry trends is to produce ions beams with higher velocities, but eventually the size and cost of the accelerating

structures and power supplies becomes prohibitive. By using an HCI of the same species

instead, one can obtain a ^-times higher ion energy with the same voltage. For example,

Xe"*^"^ is routinely produced using high voltages of only 8 kV, but when the ion is accelerated

through the same 8 kV potential, it reaches an energy of over 350 keV. The advantages in cost, size and safety can be huge, even for relatively modest Q. The development of ion implanters

that use multiply charged ions have already been built and reported in the literature by several

industry laboratories (Amemiya etal 1998, Horsky 1998, Matsuda and Tanjyo 1996). These

researchers used selected ions in the range from Z = 5 to Z = 33.

4.6.2. Ion lithography. The key step in microelectronics fabrication is lithography, an area dominated by optical techniques. Diffraction is currently limiting decreasing feature size, so making a shift to particle beam methods where diffraction is reduced by many orders of

magnitude is being considered. Electron beams can be focused to very small spots, but because

of their relatively small mass, they scatter through large angles when they enter solid materials

(much like ping-pong balls scattering off a bowling ball). This scattering results in features

that are much larger than the electron beam spot size. The heavier mass of ions makes them

preferable, in this regard, to electron beams. Focused ion beam (FIB) systems have been

used to produce 8 nm features (Kubena et al 1991). FIBs are conmiercially available, but

their throughput for lithographic applications is low. High-throughput systems in which largediameter beams are projected through a stencil mask to produce 50 nm resolution features

are in the development stage. The economic and technical advantages of such ion projection

lithography systems have been reviewed recently (Berry 1998, Kaesmaier and Loschner 2000,

Kaesmaier etal 1999).

Beam focusing and rastering can be effectively applied to high-Z and/or high-g ions as

well, as demonstrated in figure 13 (Fischer and Metzger 2000). Unfortunately, high-throughput

methods using masks cannot be effectively used with fast ion beams because of problems with

required mask thickness and edge scattering (Fischer and Metzger 2000). Both of these

problems can be avoided, however, by using the potential energy carried by slow HCIs to pattern surfaces. The mask need only be a small fraction of a micrometre thick to stop the

sort of HCIs produced by an EBIT, for example. Furthermore, the electrons captured by an HCI during edge scattering will tend to neutralize it, thereby 'turning off' its ability to use the

potential energy to modify subsequent surfaces it hits.

(X-ion, 2001; lithography and reticle) and associated Australian and world patent applications. See also footnotes 5 and 8 below.

NIST-91
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Figure 13. A pointillist portrait of William Helmholtz, painted with high-velocity krypton ions. Each dot is formed by the impact of an individual ion. The width of the head is approximately

250 /Am, about twice the width of a human hair. Figure courtesy of B Fischer (Fischer and Metzger

2000).

Masked ion beam lithography using HCIs was first demonstrated in 1998 (Gillaspy et al

1998), but many open questions remain to be studied before its potential value can be accurately

assessed. In special cases, such as thin-film or shallow-junction applications in which uniform

features with circular cross sections in the 1-100 nm range are required, HCIs might be put

to use immediately (see section 4.10 below, for example). Atomic force microscope images

of a lithographic resist material (PMMA) exposed to Xe'^ ions show that single HCIs can

produce 24 nm wide dots with essentially 100% efficiency (Gillaspy et al 1998). Studies on an aluminosilicate compound (mica) have shown that features of a similar size can be

produced, and that the dot size can be varied by adjusting the ion charge state up or down

(Parks et al 1998). Figure 14 shows a pattern produced on a silicon wafer using HCIs and a

sensitive ultra-thin self-assembled monolayer resist (Ratliff et al 1999). Taking into account

the measured sensitivity of this resist to the HCIs and the currently available beam fluence

(up to 3 X 10^ Xd^ls), the portion of the pattern shown in figure 14 should require about

1 min to produce. Next-generation EBIT-type HCI sources are being designed to improve on

this by several orders of magnitude (Marrs 1999). One commercial organization is currently

using HCIs from a high-fluence ECR ion source to produce silicon oxide nanodots for potential

application in high-density memory devices (Borsoni et al 2000).

The features produced on surfaces by conventional ions seem to be rather different from

those produced by HCIs. This is reasonable given that the physical mechanisms appear to be quite different. In the case of HCIs, a tremendous amount of potential energy seems to

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Topical review R121

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rM«Baa«««iiiiflBa»aB«a9«j raaaaaaaaaaaaaaaaaaaaai taaaaaaaaaaaaaaaaaaaaaaaai fvaaaaaaaaaaaaaaaaaaaaaaai laaaaaaaaaaaaaaaaaaaaaaaai faaaaaaaaaa«asa«««»«a««aai

Figure 14. An array of 10 yxm gold squares patterned on a silicon wafer using Xe"*^ ions, viewed

in wide-angle with a light microscope. Close-up images taken with an electron microsope have been published elsewhere (Ratliff et al 1999).

be deposited in the first few atomic layers near the surface, and the kinetic energy seems to be largely irrelevant. Studies in mica in which the kinetic energy of the HCIs was varied by

several orders of magnitude showed no significant change in the surface features, while varying

the potential energy caused the dots to shrink in size and vanish into the detection limit. The

physical picture that is emerging suggests that individual HCIs can produce fundamentally

uniform nanoscale structures even in the limit that the ion energy is reduced towards zero. This may have important implications for lithographic patterning of shallow-junction devices,

and other cases in which deep ion damage to the underlying substrate must be avoided.

Modelling the permanent changes caused to a surface by an HCI is still at a primitive

stage of development. While considerable work has been carried out by atomic physicists to model the neutralization of the ion during its approach to the surface (see section 2.8), this work has stopped short of attempting to predict the response (dynamic motion) of the atoms

which make up the surface. An initial step in the latter direction has been made by using

a supercomputer to compute the motion of over 34 000 atoms surrounding a locally charged

region of a surface which is assumed to be produced by a single incident HCI. Colour-coded

atomic scale images of this molecular dynamics simulation, showing with sub-femtosecond

time resolution the production of 10 nm diameter craters over the course of 10 s to 100 s of

femtoseconds, are published elsewhere (Cheng and Gillaspy 1997). Subsequent refinement

of the simulation (Hedstrom and Cheng 2000), including charge flow, confirms the basic

predictions of the earlier work. These simulations suggest that the cratering of the surface

should be absent in materials that have a very rapid electronic response (metals, for example).

The possibility of exploiting this material-specific response to develop a novel type of method

of surface patterning is being explored commercially (LeRoux et al 2000).

4.6.3. Ion microprobes. The most developed technological application for HCIs at present

is in the area of surface microprobes. The possibility of using HCIs to upgrade conventional

secondary ion mass spectrometry (SIMS) probes was discussed in 1993 through a Cooperative

Research and Development Agreement between the Advanced Lithography Group and the

National Institute of Standards and Technology. Independent work at the Lawrence Livermore

National Laboratory demonstrated the feasibility and advantages of this method through

laboratory experiments (Hamza et al 1999, Schenkel et al 1999a, b, 1998, Schenkel 2000).

In addition, several patents have been issued to companies and universities in Japan^ and an

established US surface analysis company has become involved in developing this application^.

^ US patents: 5 714757 (Hitachi 1998; SIMS), 5528034 (University of Tokyo 1996; SIMS), Japanese patents: 8 166362 (Hitachi 1995; SIMS).

^ Charles Evans and Associates, Sunnyvale CA; research supported in part by NIST SBIR contract http://www.rdc.noaa.gov/~amdyabstractsl999.pdf (identification of this company name is not intended to imply

recommendation or endorsement by the National Institute of Standards and Technology of any product or service).

NIST-93
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An alternative development in the general area of ion-based microprobes involves the use

of HCIs to power a projection x-ray microscope (Marrs et al 1998). In an initial test of this idea, bare and hydrogen-like argon ions from the Livermore EBIT were focused onto a 20 /itm

spot, where they produced a point source of 3 keV x-rays that was used to image a nickel wire

mesh. See also the last paragraph of section 4.9 below for possible medical applications of

this type of imaging device.

It has been proposed that a next-generation EBIT with relatively modest upgrades (Marrs

1999) will result in an HCI-based microscope with x-ray fluxes equal to that used for scanning

x-ray microscopy at the National Synchrotron Light Source and the Cornell High Energy

Synchrotron Light Source. The ultimate resolution of an HCI-based x-ray microscope remains

to be determined, but the possibility of localizing the truly point-source x-ray emission from

individual ion impacts using secondary charged particle imaging correlated with spatially

sensitive x-ray detection is a tantalizing possibility (Bruch et al 1998).

4. 7. Quantum computing

Recently, a great deal of attention has been devoted to the possibility of creating a new type of

computer that is based on the non-intuitive aspects of quantum mechanics that are not contained

in the laws ofclassical physics (Sackett et al 2000). Quantum computing can be highly efficient because it relies on the parallel evolution of multicomponent wavefunctions. Reading out the

results of the computation, however, requires a measurement which collapses the wavefunction

and leads to a fundamental loss of information during that particular computation cycle. There

are a few highly specialized computations, however, for which a major gain in efficiency over conventional computers can, in principle, be realized. Because one of these specialized

applications is of critical importance for national security (decrypting encoded messages) a

large amount of effort has been directed toward the long-range goal of realizing a quantum

computer in a variety of physical systems. One of the most advanced schemes for quantum

computing uses trapped ions (Sackett et al 2000). In all of the work done to date, the ions have

been singly charged. Some advantages of using highly charged ions have been outlined in the

literature previously (Gruber et al 2000, 2001, Wineland et al 1998). It is unclear, at present,

whether the value of the improvements would outweigh the added difficulty of using highly

charged ions. It appears that no direct experimental activity in the area of quantum computing

with highly charged ions has been pursued to date.

4.8. Energy technology: fusion

Harnessing the mechanism of energy production used by the Sun, nuclear fusion, has been

an active area of research for several decades. The power generated by experimental fusion

reactors has increased steadily during the past 20 years, increasing at a rate that exceeds the more well known progress in the semiconductor industry (Moore's law). Today, conditions

are close to those necessary for a commercial fusion power plant, so part of the scientific effort

is shifting towards engineering design issues.

HCIs may play an important role as a diagnostic tool for monitoring the conditions inside

fusion plasmas. Although x-ray spectral lines are often used in these diagnostics, optical

lines have significant advantages over them (Denne and Hinnov 1987). Working together with

scientists from the Princeton Plasma Physics Laboratory, the NIST EBIT group has explored

the possible use of visible light emitted by highly charged titanium-like ions as a monitor of

the temperature (Doppler width) and magnetic field (Zeeman shifts) inside a hot plasma (Adler

et al 1995). In particular, the 7 = 2 to 7 = 3 fine structure transitions within the ground term

NIST-94
Topical review R123

of these ions seems particularly promising because they appear to violate the usual photon

energy scaling laws (see section 2.3.3 above). This allows one to keep the spectral emission

in the visible and near-UV for a wide range of plasma temperatures (Feldman et al 1991,

Porto et al 2000b). More conventional optical diagnostic lines are rather narrowly tailored to specific temperatures and rapidly fade out as the temperature is varied. When this happens on

the titanium-like sequence, the signal can be regained by shifting to one higher or lower value

of Z in the diagnostic species.

One of the advantages of using optical lines in place of x-ray lines for fusion diagnostics

is that the associated measurement technology is much more developed in the optical regime.

Fibre optics can be used, for example, to efficiently collect photons and direct them into

remotely located measuring instruments, thereby removing the need to conduct experiments

near the high-radiation core of a reactor vessel. In addition, the relatively narrow natural widths

make optical lines preferable to x-ray lines for precise determination of the temperature and

the magnetic field. Table-top ion traps are useful devices for carrying out spectroscopic research in support

of the fusion work that takes place at large national plasma facilities, but could the little traps

be used themselves to obtain fusion conditions? The reactivity per unit volume would seem

to be too low, but a method of boosting the density considerably and actually overcoming the

Brillouin limit has been seriously discussed in the literature (Barnes et al 1993). Recently,

experimental data from a table-top trap have been presented which suggest that the Brillouin

limit has been exceeded by one order of magnitude (Zhuang et al 2000).

4.9. Medicine

The results of many research studies indicate that ion beams are a particularly effective method

of treating cancer and other tumours of the body. One of the main reasons is that high doses can be delivered to the tumour with relatively little damage to surrounding healthy tissue. The

physical basis for this is clear:

(a) ion beams can be precisely directed at the tumor target;

(b) the ion beam energy can be precisely adjusted to come to rest inside the tumor target; and

(c) the deposition of kinetic energy of an ion peaks dramatically near the end of its trajectory

(figure 15).

The last two factors, in particular, make it possible to deliver more dose to the tumour with

fewer side effects than is possible using conventional radiation treatments, thereby improving

the clinical outcome.

The first treatment of patients with ion beams took place in 1954, in a physics research

laboratory in Berkeley. From that time until the Bevalac accelerator was shut down in 1992,

approximately 2000 patients were treated at this location. Over 8000 additional patients have

been treated at the Harvard Cyclotron Laboratory, since 1961 . Based on these and several other

research programmes, the first hospital-based ion beam treatment facility was constructed in

1 990 in Loma Linda, California. This facility was designed to treat over 1000 patients per year, using proton beams. As an outgrowth of the Harvard programme, the Massachusetts General

Hospital recently opened a new proton-beam therapy centre that is also designed to treat about

1000 patients per year. Over a dozen other ion beam treatment facilities, mainly operating at existing physics research facilities, are in operation around the world. To date, over 30000

patients worldwide had been treated with ion beams, most of them protons. Reviews and

references to these and other developments, as well as lists of new ion therapy facilities under

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R124 Topical review

5 10 15 Depth Icm)25

Figure 15. Relative dose of energy deposited at various depths in human tissue by nitrogen ions with an initial velocity of 66% of the speed of light (310 MeV u~' ). Figure after Kraft etal (1991).

construction can be found in the literature (Chu et al 1993, Kraft 2000, Miller 1995) or online

from the Proton Therapy Cooperative Group^.

Although most of the past experience is with proton beams, there are clinical advantages

to using higher-Z ion beams instead. A limited amount of heavy-ion beam therapy has already

been carried out at a number of facilities, with very promising results. The first dedicated

treatment facility to use heavy ions was recently constructed in Chiba, Japan; they have

already treated over 700 patients. Construction of new facilities for ion therapy is being

pursued particularly vigorously in Japan where cancer is the leading cause of death (Soga

2000).

The GSI facility in Germany has been treating a small number of patients with heavy

ions since 1997 (Scholz 2000, Stelzer 1998). A precise beam positioning and energy control

system has been developed there in order to more accurately deposit the ion energy into the

tumor volume and minimize the damage to healthy surrounding tissue. The effectiveness of

the GSI work was apparent from the outcome of the first patient treated at the facility. This

patient had a very large brain tumor and a poor prognosis. After treatment, the tumor fully

disappeared and the side effects were minimal. In a recent report on the clinical outcome of

23 patients treated at GSI through September of 1999 (Debus et al 2000), the local tumour

control rate (full remission and non-recurrence at the treated tumor site) was 94% at 1 yr after treatment. Over 70 patients have now been treated at this location.

Currently, limiting factors in the more widespread application of high-Z ion therapy for

tumours is cost and available technical expertise. Research into more efficient and inexpensive

ways of producing the necessary ion beams is needed. HCIs have an important role to play

' http://neurosurgery.mgh.harvard.edu/hcl/ptles.htm

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in this. In Frankfurt, for example, work is underway to produce a 'MEDEBIS' (a medical

electron beam ion source) for this purpose (Kester et al 1996, 1997).

Although high-Z ions for medical therapy are produced in ultra-high vacuum conditions,

they are eventually delivered to the patient by passing through a thin metal window and a considerable column of air, both of which influence the charge state. For high velocities the

ions can maintain a high charge state, or even increase it, while travelling through the air because of the stripping elfect of the energetic ion-atom collisions (Betz 1972).

In addition to the applications of HCIs for cancer therapy, there are also possible

applications for their use in medical imaging. In particular, the areas of breast cancer (mammography) and heart disease (coronary angiography) have been targeted in a recent

patent^ for the HCI-powered x-ray microscope mentioned in section 4.6.3 above.

4.10. Biotechnology

The crossroads of nanotechnology and biology is becoming a fruitful area of interdisciplinary

research. A fundamental length scale for this research is the distance spanned by a single helical turn of a DNA molecule: 3.4 nm. This length scale is large compared with the atomic scale,

yet small compared with current microfabrication technology. Leaders in the biotechnology

industry have predicted that genomic sciences will benefit greatly from the development of

new methods of manipulating matter on these nanometre-length scales (Haseltine 2000).

Protein sequencing is an area of biotechnology that is in need of better tools to fragment

large molecules into smaller units. Preliminary experiments involving the use of beams of

HCIs for DNA fragmentation have been carried out by C Ruehlicke using the EBITs and

atomic force microscope facilities at NIST (Ruehlicke et al 1997) and Livermore (Ruehlicke

etal 1998).

There is a large demand from the biosciences community for new analysis methods. Mass

spectrometry is an established technique that is widely used, but there are significant limitations

associated with the difficulty in producing gaseous ions of organic molecules. Shock-wave

desorption using fast atomic ion beams (Daya etal 1997, Johnson and Sundqvist 1 992, Reimann

1995) or slow HCIs may be a method of overcoming some of these limitations. Preliminary

observations of HCI-induced desorption of the amino acid L-valine have been presented in figure 11 of chapter 17 of Gillaspy (2001).

The production of artificial supported membranes for use in biotechnology experiments

is an active area of research (Plant 1999). One of the key steps is to produce uniform holes

(pores) in the support for the insertion of membrane proteins. The diameters of these holes

are on the order of 10 nm and smaller. At NIST, researchers in the Atomic Physics Division

and the Biotechnology Division have been collaborating on some preliminary experiments to assess the feasibility of using HCIs to produce these small holes in thin materials.

5. Conclusion

The field of HCI research has developed considerably in the past decade. The motivation for

much of this work derives from the scaling rules summarized in table 1 . These rules reflect the fact that as the ion charge is increased, some phenomena are dramatically enhanced while

others are reduced. Some numerical values to set the absolute scale of a few of the scaling

rules are given in table 2.

^ US patent 6 1 15 452 (Regents of the University of California 2000; imaging).

NIST-97
R126 Topical review

Table 1. Summary of scaling rules for various physical processes discussed in this paper. See text for definitions of terms and notes on range of applicability.

Bohr radius Z~'

Electron density at nucleus, neutral atom Z

Electron density at nucleus, HCI

Parity-violation, HCI Z^

Kinetic energy of HCI accelerated through an electrostatic potential Q

Minimum impact kinetic energy under image acceleration Q^^^

Ionization energy (/) Z^

Photon energy. An = 1 transitions Z^

Photon energy. An = 0 (fine structure) transitions Z'^ Orbital angular momentum of electron Z

Orbital velocity of IS electron in hydrogen-like ion caZ

Zeeman shift Zeeman shift, relativistic correction factor 1 — 0.33(o;Z)^

Stark shift, first order (non-degenerate) Z~'

Stark shift, second order (degenerate) Z~*

Linewidth of hyperfine transitions Z^

Linewidth of intercombination transitions Z'°

Linewidth of electric dipole transitions Z'* Relative line blurring, A^/A., of electric dipole transitions Z^

Electric field experienced by bound electron Z^

Electron-electron interaction within ion Z^

Peak location of ionization cross section (Lotz) / ~ Z^

Peak magnitude of ionization cross section (Lotz) ~ Z~^

Peak location of excitation cross section, electric dipole AE ~ Z^

Peak magnitude of excitation cross section, electric dipole AE~^ ~ Z~'^ Mean free path through background gas, slow ions Q

Mean free path through background gas, fast ions Q/v^

Table 2. Summary of numerical values of selected quantities discussed in this paper See text for definitions of terms and notes on range of applicability.

Bohr radius (cf Compton wavelength of electron, kc = 2.43 x 10~'^ ra) ~ 22AcZ~'

Charge of the electron, at distances » kc 1.60 x 10~'^ C

Charge of the electron, at distances <^ kc oo Crossover velocity dividing 'fast' and 'slow' ions vq = 2 x 10^ m s~' Velocity gain from image charge, hydrogenlike 0.3-1 .2% vo Energy gain from image charge acceleration ~ 1 x

Q^^'^ eV

Neutralization energy of U'"*^ 6 eV

Neutralization energy of U^^^ 763 000 eV

Electric field felt by Is electron, hydrogenlike uranium 10'^ V cm"'

Critical Z for spontaneous generation of matter (point nucleus) 137

Critical Z for spontaneous generation of matter (finite nucleus) 173 Diameter of surface region modified by slow HCI impact ~ 10 imi Peak location of ionization cross section (/ = ionization energy) 2.72 x / Peak magnitude of ionization cross section Equation (7) Mean free path through background gas, slow ions Equation (8)

At the 85th Nobel symposium in 1992, Joe Sucher suggested that just as the progression

of technology can be described by 'ages' (stone age, bronze age, etc) so too does science

enter into ages in which new progress is enabled by some underlying development. Sucher

NTST-98
Topical review R127

coined a new term to describe a change that had taken place in atomic physics research: 'We

entered and are still in the midst of a veritable second flowering, which I will call the ion age'

(Sucher 1993). At the same symposium, another speaker (Briand 1993) made reference to the then-new EBIT devices that were beginning to make it relatively easy and inexpensive to produce very highly charged ions. Meanwhile, half way around the world, a new heavy-ion

accelerator dedicated to the treatment of cancer patients was gearing up (Myers 1993). These

and other developments helped foster new connections between researchers in fundamental

atomic physics and other fields such as materials science and the life sciences.

Since the 1992 Nobel symposium the quality and number of tools available to study HCIs

has increased considerably. There are now in operation or under construction, nearly a dozen

EBITs around the world. In addition, these and other methods of producing and studying HCIs,

such as particle accelerators and storage rings, have improved their capabilities dramatically.

A cursory look at the remaining open questions and possibilities for future research makes it clear that the ion age has just begun.

Acknowledgments

I thank Zoltan Berenyi for help with some of the figures, Pam Novosel for help with the

references and figures, Indrek Martinson and Bemd Fischer for providing original copies of

their figures, and Peter Mohr, Endre Takacs, and John Woodward for critical readings of the

manuscript.

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NlST-102
Trapped Highly Charged Ion Plasmas

E. Takacs ' and J. D. Gillaspy ^

National Institute ofStandards and Technology, Gaithersburg. MD 20899

Abstract. Electron Beam Ion Trap (EBIT) devices and their special features are reviewed with

attention to applications in highly charged ion plasma research. EBIT properties are presented

based on information extracted from a variety of experiments reported in the literature. Topics

discussed include typical parameters (Debye length, Wigner-Seitz radius, Coulomb coupling

parameter, density, temperature, etc.), magnetic trapping mode, ion cloud shape, rotation, and

evaporative cooling. We conclude that the quantitative understanding of highly charged ion

plasmas inside an EBIT requires improved modeling and advanced diagnostic techniques.

INTRODUCTION

Heavy ions with dozens of electrons removed, so-called highly charged ions, have a wide range of potential applications. Imagine an atomic system that is stripped to its core, with only a few electrons remaining at most. The electronic binding energy that

must be overcome to further ionize such a system can be over a thousand times higher

than that in ordinary ions or atoms. Even more striking, the potential energy liberated

when such a highly charged ion re-neutralizes itself can be nearly a million times

larger than that of a conventional ion.

Because many atomic properties scale with high powers of the nuclear charge, ions

along an isoelectronic sequence (ions with different nuclear charges, but the same

number of electrons) quickly enter exotic regimes where conventional intuition fails. Forbidden electronic transitions can become stronger than those allowed by electric

dipole selection rules, and energy levels can become strongly affected by the structure

of the vacuum [1,2].

The enormous amount of free potential energy and ultra-compact size that highly

charged ions possess is as unique as their exotic atomic structure. When such ions

approach surfaces, their strong electrostatic pull on bulk electrons can be felt many

atomic diameters away. As a result, highly charged ions are very effective in

modifying or breaking chemical bonds and crystal structures on the nanometer length

scale [3].

The natural curiosity to study such exotic ions in the extreme strong field limit was

one of the driving forces to build a device, such as an Electron Beam Ion Trap (EBIT),

that can be accommodated in a small laboratory. The first EBIT was put into operation

at the Lawrence Livermore National Laboratory [4,5] almost fifteen years ago. Shortly

after this, the National Institute of Standards and Technology (NIST) became the

Also at Massachusetts Institute of Technology, Cambridge, MA 02139 and University of Debrecen, Debrecen, Bern ter 18, Hungary, H-4026, E-mail: endre.takacs@nist.gov ^ E-mail: john.giIlaspy@nist.gov

CP606, Non-Neutral Plasma Physics IV, edited by F. Anderegg et al. 2002 American Institute of Physics 0-7354-0050-4

151

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second place in the world to have a working EBIT [6,7], followed by new

constructions in several other laboratories [8-12].

EBITs are not the only machines that can create ions in high charge states, but

certainly they are one of the most versatile, offering great control over the

experimental conditions in which highly charged ions are produced. The success of the

EBIT is proven by the numerous widely-cited experiments that have been carried out

by since its inception [4,5,13-18].

Although the device is still in the process of rapid evolution, the fact that current

small-scale EBITs can already produce millions of highly charged ions per second

suggests that applications that take advantage of the unusual properties of very highly

charged ions should be pursued. Some of the basic scientific studies which will

underlie applications of such exotic ions are being carried out by the highly charged

ion community, while more immediate applications are being pursued by industrial

entrepreneurs [19] (and references contained in [2]).

As EBIT devices may one day be capable of producing much larger numbers of

highly charged ions, it is interesting to survey the capabilities and properties of the

present machines to help further the development of possible future designs. The basic

mechanisms of ion production and operation of the EBIT is theoretically understood;

the evolution of the charge states and the dynamical ionization and recombination

balance between the neighboring charges can qualitatively be accounted for with

model calculations [20-23]. However, there are indications in several experiments that a quantitative understanding remains to be developed.

Detailed understanding of EBIT properties is hampered by the fact that only a

limited number of experiments have been performed that specifically target the

properties of the highly charged plasma inside an EBIT. There is, however, a large

database of indirect information contained in other types of measurements. In the

present paper, we will attempt to put these pieces of information together in a coherent

manner to provide a general experimental overview of the detailed operation and

features of an EBIT. We will also point out some discrepancies that have accumulated

in the fifteen years of experience with the device that might point to a better

understanding of its operation and that could possibly even be exploited in future

applications. We hope that with this summary we will stimulate people from the

broader scientific community to contribute to a cross-disciplinary attack on some of

the problems, and in particular to put forward advanced diagnostic and modeling ideas

that will ultimately help to realize new applications for highly charged ions.

HIGHLY CHARGED ION BEAIVIS AND CLOUDS

Most of the applications that take advantage of highly charged ions require large

numbers ions in the form of high quality clouds or beams. EBITs, in this sense,

already qualify as one of the best devices to study. The source region is rather simple

and well controllable, and is qualitatively similar to the precision controlled traps that

the non-neutral plasma community uses. Understanding the basic properties of these

traps can serve as a foundation for designing new, innovative highly charged ion

devices. However, as in atomic structure, it might well be that conventional plasma

152

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properties have to be rethought in the highly charged ion regime, in which case

existing EBITs could be used as test-beds for these ideas.

Some members of the conventional non-neutral plasma community have argued

that highly charged ions could offer some attractive possibilities for the study of

strongly coupled systems at relatively high temperatures [24]. Some of the advantages

of using highly charged ions in quantum information schemes have also been

discussed [25]. One of the first steps towards these goals was the experimental

realization of a highly charged ion crystal in a precision trap that was filled with ions

from an EBIT [26,27]. Some of the cooling schemes appHcable to highly charged ions

have already been put into operation in an EBIT [28-30].

Another interesting possibility is related to the quest for crystalline ion beams with

high center-of-mass energies (>1 MeV, but with small relative energy <1 eV). These

beams would provide the unprecedented quality and brightness sought in many

applications [31-33]. The idea of creating crystalline ion beams in storage rings was

proposed more than fifteen years ago [34]. Despite some promising results [35,36],

the experimental realization in high-energy devices appears to be still lacking [32].

An alternative approach to the crystalline ion beam problem [37] was recently

experimentally realized [32] in a small-scale storage ring. The idea is to first create a

crystalline ion cloud at rest using methods and tools that are applied in strongly

coupled ion cloud studies, and then to accelerate it to high energies without destroying

the ordered structure. The experiment, which used singly charged ions and laser

cooling to create the ion crystals at rest, was able to demonstrate acceleration to about

1 eV energy [32]. It is believed that scaled-up versions of this scheme can be used as high-energy storage rings.

Direct laser cooling of highly charged ions is problematic because of the scarcity of

visible transitions. It has recently been demonstrated, however, that sympathetic laser

cooling can be equally effective [38]. This alternative cooling technique has also been

shown to be very effective for highly charged ions [26,27]. In addition, there may be

other, non-laser based cooling schemes that have yet to be fully developed [45,46].

One of the major differences between Penning traps and EBITs is the presence of a monoenergetic, high-energy, high-density electron beam at the center of the trap.

Some groups, however, have modified their Penning traps to include an electron beam

[39-42]. These works are not motivated by the desire to create highly charged ions,

but rather to create better confinement conditions and more dense plasmas for

applications such as alternative nuclear fusion devices. The dense electron beam

creates a steep spatial ion density gradient that helps to locally overcome the density

limitations posed by the Brillouin limit [39,40]. Similar gradients have been observed

in recent EBIT experiments [43,44]. Furthermore, typical ion densities found in

EBITs (see below) are already above the Brillouin limit.

The possibility of creating high ion cloud densities and high brilliance beams using

present EBITs without auxiliary laser cooling may not be that far-fetched. Recent

model calculations [45,46] suggest that evaporative cooling could be significantly

enhanced under certain conditions, and thus even the strongly correlated regime might

be achievable this way.

153

NIST-105
ELECTRON BEAM ION TRAPS

The basic operational principle and the details of the EBIT components can be

found in several recent reviews [47,48]. These papers give a design and modehng

point of view of the operation of the present-day EBITs. In the present work, we

would like to review a few experiments that determined some of the important

operating parameters of the machine. In some cases these results were not even the

main goal of the particular experiment, but rather a byproduct. Since these

experiments were done with different machines, using different types of ions, and

different experimental techniques, their direct comparison might not be justified in all cases. However, a semi-comprehensive review of the data might help to shed some

light on the capabilities of EBITs and to give a general feel for the operating parameter

ranges.

Operation

The creation of highly charged ions in an EBIT is based on the interactions of ions

with a high density electron beam of about 1 keV to 30 keV energy. The electron

beam is produced by a commercial electron gun and is highly compressed by a pair of

superconducting magnets producing homogenous field of about 3 T in the center of

the machine. Neutral atoms or singly charged ions are injected into a three element

Penning trap and then are stripped of most of their bound electrons during consecutive

interactions with the electron beam. The average ionization stage in equilibrium can be

selected by properly choosing the energy of the electron beam. Radially the ions are confined by both the electric field of the electron beam and the high magnetic field. The effect of the magnetic field dominates only for large distances from the electron

beam. Once the ions of the desired charge state are produced, the electron beam can be

used for exciting electronic transitions for spectroscopic studies, or it can be turned off

completely, leaving the axial confinement solely to the magnetic field [49,50]. Details

of the modeling of ion creation [47] and the trapping dynamics [48] are described

elsewhere. The NIST EBIT is also equipped with a highly efficient ion extraction and

transport system, with a built-in charge-to-mass ratio separator. This system can be

used for diagnostic purposes to infer charge state distributions inside the trap, or as a

facility for ion-surface and ion-gas collision experiments.

In the following paragraph we summarize the typical plasma parameters of EBIT

ion clouds. The measurements that provided the input for these determinations will be

presented in later sections.

Typical plasma parameters

Plasma conditions created in EBITs are similar to those of the solar corona. Electron densities are around 10^^ cm'^ and electron energies are approximately equal

to (or above) those found in 10 million Kelvin temperature plasmas. One distinctive

difference is that in the EBIT the electron energy is monoenergetic, with a width of

only about 16 eV [70] to 70 eV [71]. A typical relative energy spread, AE/E, is under

154

NIST- 106
1 %. The narrow energy spread is the key to creating a charge state distribution that

only includes a few ionization stages. In the case of ions with closed shell electronic

configurations, close to 100 % charge state purity can be achieved [77]. In a more

typical case, there would be a distribution of charge states present. The charge-tomass spectrum of extracted ions [72,73] can be used to monitor the degree of charge

state purity.

Typical measured densities of particular charge states in multi-component EBIT ion

clouds range from 5x10^ cm"^ to 1x10^° cm"^ [7,61]. Local ion densities can be higher

than these values because of the steep gradient in the space charge potential of the

electron beam. Typical measured ion temperatures are between 70 eV [65] and 700 eV

[66, 64], increasing as the ion charge increases.

The Debye length for typical ion densities (n) and temperatures (T) inside an EBIT,

0)

is around 20 ^im to 60 \im. This value is relatively small mainly because of the high

charge (q) of the ions. The size of the ion cloud is 2 to 20 times this value.

The average distance between ions of a particular charge state is given by the

Wigner-Seitz radius.

\ Am ^

^

about 3 fim to 8 fim. These Debye lengths and Wigner-Seitz radii are similar to those

for typical singly charged ion plasmas, but other parameters that scale with the ion

charge can be very different. One such parameter that scales quadratically with ion

charge is the average potential energy between neighboring ions:

K=—— P)

For ions of high charge-states this can be 3 or 4 orders of magnitude higher than for

singly charged systems. It can be more than 1 eV in absolute value. This leads to

another strongly scaling quantity, the Coulomb coupling parameter:

r = -^

(4)

47r£QakT

Because of the high ion charge, even at temperatures close to 1 million Kelvin T is more than 0.01. If the cloud could be cooled to room temperature, F would be around

50, which is well into the range where liquid phase behavior is predicted [60].

155

NIST-107
EXPERIMENTAL CHARACTERIZATION OF EBITS

Although there have been over 300 EBIT publications since the first one in 1988,

relatively few experiments have been performed to date that determine the parameters

described in the previous section. In the following subsections, we discuss these few

experiments, and point to corresponding theoretical work.

The spatial distribution of the electron beam has been measured using a variant of

an x-ray pinhole camera [5, 74, 75], and using Thomson scattering [76]. The x-ray

imaging experiments rely on the fact that excited state lifetimes in highly charged ions

are so short that x-rays are emitted essentially instantaneously at the location that they

interact with the electron beam. A map of the x-ray emission from the ions, therefore,

reflects the electron beam density distribution directly. The measurements are in rough

agreement with predictions based on Herrmann theory [51], although there have been

some discrepancies on the order of 20 %. Generally, it has been assumed that the

electron beam spatial distribution is Gaussian, with a radius rH containing 80 % of the

electron beam given by,

where Be is the magnetic field in the electron gun cathode region; Tc is the temperature

of the cathode; ic is the radius of the cathode; B is the magnetic field at the central drift

tube; m is the rest mass of the electron; k is the Boltzmann constant; Tb is the Brillouin

radius:

determined by the electron current, le , and the electron energy, Ee

.

In order to understand the presence of the parameter Be in the above expression, it is useful to consider some further details of the EBIT design. The electrons are emitted

from a heated cathode in a Pierce-type electron gun configuration [4,5,48] located in

the fringing field region of the main trap magnet. The electrons are accelerated

towards a positively biased drift tube region (the trap center) by a series of guiding

electrodes. It has typically been assumed that the maximum current density is achieved

by minimizing the radius in expression (5), implying that the field at the cathode

should be zero. In this case, however, the trap magnet acts as a magnetic mirror, and a

significant number of electrons will be reflected back away from the trap. We have

shown that theoretically the optimum field is actually a few tenths of a mT (a few

gauss) [52]. The control of the magnetic field is aided by a steel plate placed above

Electron beam

(5)

(6)

156

NIST-108
the electron gun, and a coil built into a sealed chamber that surrounds the electron gun

[5,52]. The magnitudes of the various EBIT parameters (rc~1.5 mm, Tc~1000 K, for

example) allow one to simplify expression (5) considerably to,

c c (7)

On its way towards the drift tubes, the electron beam is compressed by the

increasing magnetic field. Model calculations show that the 3 T magnetic field in the

trap region is homogenous over the 3 cm long ion trap region to better than 0.05 %

[44].

Magnetic trapping

The early EBIT papers stated that the ions were trapped in the radial direction by

the electrostatic field provided by the electron beam (see, for example [5]). The

magnetic field was presented as a technical detail needed to adiabatically compress the

electron beam to high densities at the trap center. From the onset of our work [7,50],

however, we have taken a somewhat different view, which emphasizes the direct

effect of the magnetic field on the ion confinement. With this point of view, we first reported an experiment that demonstrated that an EBIT can be used to measure some

properties of highly charged ions (excited state hfetimes, for example) more

efficiently when the electron beam is turned off [50]. This work was stimulated by

some related work on ion cloud diagnostics at Livermore [53-57]. Both the atomic

lifetime measurements and the cloud diagnostic studies showed that after an initial expansion, the ion cloud stabilized after the electron beam was turned off Only trap

losses like cross-field diffusion and charge exchange with the background gas then

lead to a decrease of the ion signal. Similar issues have been studied more thoroughly

outside of the EBIT community e.g. [24, 60]. We suggest that using an EBIT to produce highly charged ions, and then switching to the "magnetic trapping mode" of

operation might be particularly interesting for many advanced plasma studies.

Ion cloud shape

It is only recently that an image of the measured ion cloud shape has been reported

[2]. Most of the experiments have simply used the EBIT as a light source or as an ion source for atomic physics or ion-surface studies, with relatively little effort allocated

to study the macroscopic properties of the ion cloud itself Ironically, much of the

work that has been published rests on assumptions about the spatial distribution of the

ions in the cloud.

The first systematic study of the ion cloud shape was published recently [43]. This

work used a CCD camera and an optical lens system. Data was collapsed along the

axis of the electron beam to produce a 1 -dimensional cross sectional image with

improved signal-to-noise ratio. Complimentary data, taken under rather different EBIT

157

NIST-109
operating conditions, have been reported in the PhD thesis of M. Tarbutt [44]. Both of

these works were predated by some spatial information that was obtained by Serpa et

al. [50] in connection with the measurement of excited-state atomic Hfetimes. In this

work, the entire spectrometer table was mounted on sliding rails and was moved

perpendicular to the EBIT observation direction by piezoelectric translators, allowing

the transverse distribution of light at the entrance slit of the spectrometer to be mapped

out. Some earlier unpublished work using CCD imaging was briefly described in a Livermore annual report [58].

The lack of any work in imaging the ion cloud immediately after the first EBIT came

online can be partly understood by realizing that it was quite a few years before the

first visible-light spectroscopy was done on an EBIT [18]. The reason for this is that

the scaling laws cause most of the transition energies to shift into the x-ray energy

range as the ion charge is increased. There are a few unusual transitions, however, that

stay in the visible range, because of a fortuitous energy-level crossing. One of these

transitions has been predicted [59] and found [18] in the Ti-like isoelectronic

sequence. There are also transitions that during their rapid scaling to shorter

wavelengths fi-om the infrared or microwave range, pass through the visible range at a

particular charge state or narrow range of charge states. Notable examples of the latter are visible hyperfine transitions in high Z elements.

Another requirement for cloud imaging is that the line should have a lifetime that is long compared to the time it takes the ion to complete several cycles of its motion (the

cyclotron fi-equency varies from 1.2x10^ s"' to 1.3x10^ s"' for ions ranging fi^om U

to Ar ). This condition is necessary, because electron impact excitation can only take

place inside the electron beam. As mentioned above, if the emitting transition has a

short lifetime (like in most of the x-ray lines), the image only reflects the product of

the electron and ion densities, not the full ion cloud shape.

The cloud imaging experiments of Porto et al. [43] used Ar'^^, Xe^'^, and Xe^^^

ions and the experiments of Tarbutt [44] used Ar ions for the measurements. Both

experiments concluded that the ion cloud has a density distribution that sharply peaks

in the center of the EBIT. Porto et al. assumed thermal equilibrium for the ion cloud to model the shape of the measured distribution in the self-consi stent field of the electron

beam and the ion clouds. The fits suggest that the ions may indeed be close to thermal

equilibrium. The temperatures obtained from the fits fall within expected values [43].

At lower trapping voltages, the experimentally obtained widths became wider than

what is expected for even the maximum possible temperatures, which was interpreted as a signature that the ion cloud is non-thermal.

Ion cloud rotation

The possible collective rotation of the ion cloud has not been taken into account in

any of the previous work with EBITs. Clouds of highly charged ions in thermal

equilibrium in a pure magnetic field with densities close to the Brillouin limit should

rotate collectively at half the cyclotron angular fi-equency [60]. How this situation is modified by the presence of the electron beam has yet to be studied in detail.

158

NIST-110
Ion number and density

The ion densities can be measured by detecting x-rays originating from processes

with well-known cross sections, assuming the electron density is known. One of these

is the so-called radiative recombination, which can be calculated to better than few

percent accuracy [13]. This method was used by Gillaspy et al. [7] and by Margolis et

al. [61] to determine the number Ba"*^"^ ions in the trap (3.1x10^) or, with fewer

assumptions, the ion density (1.0x10^ cm'^ - 1.1x10^ cm"^). Margohs et al. [61] also

measured densities of 1.3xlO'^ cm'^ and 0.7xl0'^ cm'^ for Ar'^"^ and Ar'^"^ ions

respectively using the same method.

A different technique was used by Schweikard et al. [54] to determine the number

of ions inside the EBIT. In [54] electrical probes were inserted into the EDIT to detect

ion cyclotron resonance (ICR) signals induced by the ions. From the induced currents,

the number of ions is inferred to be 10^ to 10^ for high charge state Kr ions (Kr^"*"*", Kr^^^, and Kr^^"^). Using these numbers and assuming approximately 10"^ cm^ for the

volume of the ion cloud, we infer 10^ cm'^ - lO'' cm"^ for the average ion densities, in

reasonable agreement with the results of the x-ray method.

It should be noted that the estimated densities using the ICR method are averages

over the entire ion cloud, whereas the x-ray method samples only the region where the

electron beam overlaps with the ion cloud. Because of the density gradient, the x-ray

method should yield higher values than the average density. A better determination of

the local ion densities could be performed by combining either of these methods with

imaging techniques.

Charge state distribution

The evolution of the charge state distribution when the electron beam is on is

determined by a set of coupled differential equations including source and loss terms

for each charge state [20]. Once equilibrium is reached, recombination with electrons

to produce lower charge states and re-ionization keep a balance between the

neighboring charge states. Model calculations can account for the qualitative behavior

of the time dependence, however discrepancies have been reported in several cases

[62,63]. One of the critical issues that comes up in many experiments is the overlap

factor between the electron beam and the ion cloud [20,63]. The modeling of this

parameter relies on the knowledge of the properties of the ion cloud. As it has been

shown in the imaging experiments [43,44], this can be very complicated, especially if

non-thermal clouds are present. Further understanding calls for advanced modeling of

the highly charged ion cloud.

Ion temperatures

A routine procedure in spectroscopy to determine ion temperatures is the

measurement of the Doppler broadening of spectral lines. This method assumes that

the emitting ions are in thermal equilibrium, a condition that might not be satisfied in

some cases as suggested by the direct imaging results. Collective ion cloud rotation

159

NIST-Ul
could also cause spectra! line broadenings, which might explain why the experiments

have not indicated very low temperatures, even in cases where the axial potential

barrier was very small. Nevertheless, the broadening of a relatively narrow spectral

line, measured by high-resolution instruments, gives a general idea about the

temperature ranges that highly charged ions are subjected to in EBITs.

The NIST EBIT group [7,64] used a Fabry-Perot spectrometer to measure the width

of a visible magnetic dipole transition in Ba^"*^. The experimental resuhs indicated an

ion temperature between 500 eV and 1000 eV. These values are much less than what

the depth of the potential trap would allow, which for such a high charge-state

amounted to about 1 7 keV (the reason for this reduced temperature is given in the next

section). Similar conclusions were drawn from the x-ray line-width measurements of

Beiersdorfer et al. [65,66] using Ti^^^ ions. In these cases, the measured widths

indicated equilibrium temperatures of 70 eV - 700 eV. Although the temperatures

showed a dependence on the axial trap depth, the measured values were generally

smaller than the possible temperatures Ti ions were allowed to take, similarly to the

observation of the NIST group using Ba^''"^ ions. Neither of the measurements took

into account the possible collective rotation of the ion cloud, so the actual ion

temperatures could have been even lower than those inferred from the measurements.

Evaporative cooling

The interaction of the ions with the dense and energetic elecfron beam continuously

pumps energy into the cloud via inelastic collisions at a rate of a few keV/s [20]. At

this rate of heating, most of the ions would quickly boil out of the trap. However,

evaporative cooling of the higher charge state ions by elastic collisions with lower

charge state ions that preferentially escape the trap strongly modifies this situation.

Evaporative cooling of heavy ions can be even more efficient using lighter ions

purposely injected into the trap. These lighter ions can be rapidly stripped bare, after

which their charge state evolution is truncated.

Evaporative cooling of highly charged gold (Au , Au , Au , etc.) by low

charge state Ti ions (maximum Ti^^^) was successfully demonstrated by Schneider et

al. [67]. Using this method, trapping times of several hours have been observed,

demonstrating the presence of a strong cooling mechanism. Model calculations of the

effect [69] predicted that highly charged heavy ions can be trapped for indefinitely in

this way.

Conventional evaporative cooling in an EBIT [67-69], as described above, differs

from evaporative cooling in neutral atom taps in that there is no time-dependence of

the trap potentials. This cooling is not lossy because it depends on collisions with

lower charge state ions that "see" a different trap depth. There have been two recent

model investigations [45,46], which propose improved evaporative cooling schemes

for an EBIT using time-dependent potentials. Because of the long-range nature of the

strong Coulomb interaction, evaporative cooling works very differently in the case of

highly charged, compared to the neutral atom case. At low temperatures, instead of

getting weaker, the evaporative cooHng mechanism can be accelerated, leading to very

low achievable temperatures.

160

NIST- 11

2
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Brown, J. M. Laming, J. Dubau, and M. K. Inal, Physical Review A 54 (1 996) 1342.

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the International Symposium on Electron Beam Ion Sources and their Applications, ed. A. Herschovitch (AIP

press. New York, 1989), AIP Conference Proceedings 188 (1989)158.

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Gilbody. W.R. Newell. F.H. Read, and A.C.H. Smith (Elsevier Science Publishers B.V.). 1988. [72] A.I. Pikin, C.A. Morgan, E.W. Bell, L.P. Ratliflf, D.A. Church, and J.D. Gillaspy, Review of Scientific Instruments 67 (1996) 2528.

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163

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uItraviolet and visible radiation

0

Volume 74, Number 10 PHYSICAL REVIEW LETTERS 6 March 1995

Observation of Visible and uv Magnetic Dipole Transitions in Highly Charged

Xenon and Barium

C. A. Morgan,* F. G. Serpa,^ E. Takacs,* E. S. Meyer, J. D. Gillaspy, J. Sugar, and J. R. Roberts

Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

C. M. Brown and U. Feldman

E. O. Hulbert Center for Space Research, Naval Research Laboratory, Washington, D.C. 20375-5000

(Received 27 May 1994)

We have observed an unusual transition which is predicted to result in visible and near-uv emission from very highly charged titaniumlike ions spanning the entire upper half of the periodic table. Measurements of the wavelengths of the 'id'^ ^ Dj-^ D^, transitions in Ba*^" and Xe^^^ are m surprisingly poor agreement with ab initio calculations. This work was carried out in an electron beam ion trap and demonstrates that such a device can be an important tool for visible spectroscopy of highly

charged ions.

PACS numbers: 32.70.-n, 31.25.-v, 52.25.-b, 52.70.Kz

Although ions having identical numbers of electrons display similar spectral structure, the scale of many

atomic properties changes dramatically as the nuclear charge (Z) is increased along an isoelectronic sequence. One example is the rapid decrease in transition wavelengths leading to the characteristic emission of light in the x-ray region of the spectrum for very highly charged

ions. It was therefore unusual when Feldman, Indelicate,

and Sugar [1] reported calculations predicting a small set of observable visible and near-uv magnetic dipole (Ml)

lines for highly charged ions in the titaniumlike isoelectronic sequence. The upper levels of these transitions lie within the ground term, and, therefore, in low density

plasmas (<10'^ cm"^), where competition from electron

collisions is negligible, a large fraction of excited levels should decay to them and result in intense forbidden lines. Even more intriguing is the fact that these lines are pre- dicted to remain in the visible or near-uv region of the spectrum for a range of ions covering the entire upper

half of the periodic table (45 < Z < 92). To our knowledge, this is the only candidate sequence of strong lines with visible or near-uv wavelengths persisting over the 1 to 10 keV range of ionization energies. Once measured,

the lines are easily located even with low resolution spectrometers. In this Letter, we report the first observation

and measurement of these unusual lines. Interest in visible and near-uv forbidden transitions in highly charged ion is not only academic; there are important practical applications, particularly in high temperature

plasma diagnostics. Long wavelength forbidden transi- tions have been seen in the solar corona [2,3] and in toka- maks [4-7], and they have been used extensively to probe

a variety of properties for both kinds of plasmas. For

example, local ion temperatures and bulk plasma velocities can be inferred from the substantial Doppler widths

and shifts of these lines [5,6,8]. It is also possible to de- termine the direction and magnitude of internal magnetic

fields from polarization measurements or from Zeeman

shifting or broadening of the line profiles [9-11]. The

1716

visible and near-uv transitions are preferred, since they can usually be measured more accurately and because re- fractive optics and polarizers can be efficiently employed.

Most of the visible and near-uv diagnostic lines used to date come from ions with the ns^np'' {n = 2,3) ground

configurations [12]. Unfortunately, these lines are ex- pected to move to much shorter wavelengths in the high-Z

(high ionization energy) elements of the sequence appropriate to the next generation of high temperature fusion

devices. Thus, such lines will no longer be as easy to measure and will lose much of their value as diagnostic

tools. The high radiation levels expected near these new

devices will make remote diagnostic using fiber optics desirable, thereby increasing the need for visible or near-uv

diagnostic lines. In the present work, ions were created at the joint

National Institute of Standards and Technology and Naval

Research Laboratory Electron Beam Ion Trap (NIST-NRL

EBIT) [13]. The trap is based upon the design of the Lawrence Livermore National Laboratory EBIT [14] and

consists of three cylindrical drift tubes through which

nearly monochromatic (—50 eV spread) and intense (~

2000 Acm"^) electron beam passes. The electron beam

is focused to a small diameter (—60 /im) by a 3 T axial magnetic field, and the electron energy is determined by

the potential applied to the center drift tube. The ions

are trapped axially along a 2 cm length by biasing the end drift tubes 250 V above the center drift tube; they

are confined radially by the space charge potential of

the electron beam and by the magnetic field. Injected atoms are converted to ions via collisions with the electron beam. A specific charge state can be preferentially

produced by tuning the beam energy to just below

its ionization potential. This capability for producing

and trapping very highly charged ions in a small-scale laboratory device has made the EBIT a valuable source

for x-ray spectroscopy [15]. The low effective electron density inside the EBIT (-10'-^ cm~^), however, also makes it ideal for studying weak, forbidden transitions

NIST-117
Volume 74, Number 10 PHYSICAL REVIEW LETTERS 6 March 1995

within the ground terms of highly charged ions. We have now demonstrated this potential by using the EBIT for

visible and near-uv spectroscopy of such transitions. Barium and xenon were studied because they are easily introduced into the EBIT. Barium is emitted contin- uously from the heated cathode of the electron gun, and xenon can be loaded by directing a stream of neutral gas

into the trap region through a side port. The electron beam energy required to maximize production of titani- umlike barium and xenon ions in the EBIT is 2.259 and

2.026 keV [16], respectively. Although the electron beam

energy is determined by the center drift tube potential, the exact energy is somewhat less than this potential due to beam space charge defects. For a 35 mA beam, we es- timate this space charge correction to vary from 72 to 63 eV as the center drift tube potential is varied from

1 880 to 2570 V. For a higher current beam, the space charge correction is proportionately higher.

Light emitted from the trap was imaged by a pair of

lenses onto a 250 /xm entrance slit of a 0.25 m focal

length, //3.5 Ebert scanning monochromator. The magnification factor of the lens system was 0.8. A bluesensitive photomultiplier was mounted behind a 500 fim

exit slit and operated in photon-counting mode. With a 2360 grooves per mm grating, the instrumental width of

the monochromator was 0.8 nm. The Doppler width of

the spectra from the trapped ions is estimated to be less than 0.08 nm, and any Zeeman shifts less than 0.04 nm.

1500-

1000-

3000

2500

2000

500-

-

1 f • l~ ' 1 - 1

(b)

320 340 360 380 400 Wavelength (nm)

420 440

FIG. 1. Broad survey spectral scans with high (10 nm,

monochromator slits removed) bandpass for (a) the lid^^Djtransition in Ba*^* (right peak) and the M^'^Gtii-^G^/j

transition in Ba*" (left peak), and (b) the corresponding

transitions in Xe*" and Xe*^'. The EBIT drift tubes were set

at 2378 and 2277 V for the portions of the spectrum in (a) to the right and left of the dotted line, respectively, and at 2144 V

for the entire spectrum in (b). the electron beam current was

50 mA throughout.

The wavelength range studied here (320 to 443 nm) was

limited at the lower end by the diminishing transmission

of the lenses. Calibration of the monochromator' s wavelength scale was performed by shining light from a number of low pressure discharge lamps, including mercury,

helium, neon, argon, and xenon, through the EBIT from a

port opposite the spectrometer. Argon and krypton were

also injected, ionized, and trapped during the course of

the experiment to observe previously reported [12,17] Ml

lines in situ. In this case, a relatively dense stream of gas was flowed continuously into the EBIT. This broadened

the charge state distribution by providing a significant flux of neutral atoms from which steady state populations of

the lower charge states, specifically Ar"^'"* and Kr*^^, could

be formed. Such low charge state ions could not other- wise be made, since the EBIT electron beam became unstable at energies below 1 keV. Our in situ measurement

of the krypton line yielded 384.08(20) nm, and that of the argon line yielded 441.32(20) nm. These are to be compared with their literature values [12,17] of 384.09(3) nm

and 441.24(2) nm, respectively. We estimate the overall uncertainty in our wavelength calibration to be ±0.2 nm.

Initially, broad survey scans (340-440 nm) with wide

spectral bandpass (10 nm, monochromator slits removed)

were conducted (Fig. 1). These were followed by nar- rower scans in the regions where prominent spectral fea- tures appeared (e.g.. Fig. 2). Finally, the monochromator was adjusted to the peak of the observed lines, and the magnitude of their signal was recorded as a function of

electron beam energy. The well controlled conditions in the EBIT produced only a few lines in the survey spectra

and greatly simplified the task of identifying them.

There are no previously measured and identified lines from titaniumlike, vanadiumlike, and neighboring 3d^

shell ions of barium and xenon. It was not possible, therefore, to confirm the presence of these ions in the trap by observing an experimentally known transition. The

existence of trapped barium and xenon was confirmed.

1000

c

a

oO

200

392 393 394 Wavelength (nm)

395

FIG. 2. 3d'*^D2-^D3 transition m titaniumlike Ba^^". The

profile shown is the sum of 10 individual scans, each of which was accumulated in 5 min with a drift tube potential of 2378 V.

1717

NIST-118
Volume 74, Number 10 PHYSICAL REVIEW LETTERS 6 March 1995

however, by the observation of known x-ray lines from

a higher charge state (Ne-like) in high-resolution spectra

[18] at increased electron beam energies. Furthermore,

the features seen in low-resolution x-ray spectra, taken si- multaneously with our uv data, were consistent with theoretical predictions [we used the grasp' multiconfiguration

Dirac-Fock (MCDF) code] [19] for radiative recombination into, and transitions between, levels in ?>d'^ shell ions of Ba and Xe. These were the only charge states of Ba

and Xe whose predicted features were consistent with the observed low-resolution spectra. In Fig. 3, we show energy level diagrams for the barium

ions produced in our study. The corresponding diagram

for xenon ions, although not shown, is very similar. After a tentative identification of the prominent spectral feature at 393.24 nm (Figs. 1 and 2) as the titaniumlike bar- ium line, whose wavelength was originally predicted [1] to be 377.8 nm, we used the Cowan relativistic Hartree-Fock

(HFR) atomic structure code with electron correlation correction [20] to improve the prediction. For the 3J'^ configuration, the two Slater integrals, F'^(3d3d) and F'*(3d?>d), and the 5d spin-orbit integral (^) determine the energy

level positions. We scaled the parameters to make the code's predicted wavelength match that of the tentatively

identified line exactly. This scaling factor turned out to be 93% of the ab initio HFR values. This is a relatively

modest adjustment since scale factors between 90% and

95% are typically required for highly charged ions [20]. Using the 93% scaling, a calculation of the Sd'^ ^Dj-^D^

transition in titaniumlike Xe"^^^ gives a value of 409.4 nm.

This is within 5 nm of a strong line recorded during Xe

injection, with the electron beam energy tuned to produce

Xe"^^^. As presented in Fig. 3, the 7 = 3 level can de- cay via the 7 = 2 or the 7 = 4 levels. Calculations using

the Cowan code indicate that the A values for the J = 2-3

transition in xenon and barium are approximately 400 s~'. The A value for the J = 3-4 transition, however, is only

2 s"' in xenon and 17 s"' in barium. Therefore, most of

200000

150000

100000-

ts} 50000

7/2

3/2

7/2

5/2

Ti-like V-like FIG. 3. Energy level diagrams of the lowest levels in the ground terms of titaniumlike and vanadiumlike barium. The

level energies were calculated using the Cowan code and

arrows indicate visible and near-uv M1 transitions.

the decays from the 7 = 3 level will proceed via the 7 = 2- 3 channel, and the transition should be easily observable under our plasma conditions. As a separate check on the scaling factor, we calculated

the 3d^ levels of ironlike Sn^^'*, the closest highly charged

ion of the 3d'^ shell (2 < A' :^ 8) in which the energies of

all the levels of the ground configuration are experimentally known. Using the same 93% parameter scaling

for the 3d^ configuration, we find agreement with the pre- viously measured values [21] to within 1%.

After observing the titaniumlike barium line, we

conducted a theoretical search for other observable Ml

transitions arising from the 3d^ ground configurations.

This involved completing calculations for the other charge states of barium and xenon from calciumlike to ironlike ions, using the 93% scaling of the F* parameters.

By examining the calculated wavelengths and branching

ratios, we predicted that only one more line should be

observable, specifically a 3d^ '^GT/j-'^Gg/z transition in vanadiumlike barium and xenon. The wavelength of this line is predicted to decrease rapidly with increasing Z and

to leave the near-uv spectral range 300 nm) for ions with Z ^ 58. These predictions prompted us to take a survey scan at shorter wavelengths, with the electron beam energy tuned

to produce vanadiumlike barium. A single prominent

feature was found, and the wavelength fell very close to the calculation (Table I). Later, when a survey scan was made in xenon, the vanadiumlike line was also found near

the predicted location. Thus the wavelengths of all four prominent features recorded in the survey spectra matched

the scaled theoretical values closely (Table I).

It should be noted that the noble atomic gases (argon,

krypton, xenon) were introduced separately into the EBIT,

with sufficient time between loadings to allow the previ- ous gas to be removed by a combination of cryogenic

and ion pumping. Features associated with ions of any one of these species were not seen in the spectra of the

others. Further evidence for the proper identification of

these lines came from the measurement of their ampliTABLE I. Predicted and measured wavelengths of visible and

near-uv Ml forbidden transitions in titaniumlike Ba*^" and

Xe*^2 and vanadiumlike Ba*" and Xe*^'.

Ion Transition Wavelength^ (nm) A" (s-') Calculated' Measured

Ba+3^ 393.2" 393.24 ± 0.20 433

409.4 413.94 ± 0.20 434

Ba*" 3d^ *Gt/2-*G<)I2 341.4 343.57 ± 0.20 425

Xe*3' 3d^^Gii2-'G^I2 394.3 396.25 i 0.20 311

^Wavelengths in air. ''The F'^ scaling in the Cowan code was adjusted to 93%

so that the calculated and measured wavelengths of this line were equal. 'Calculated with Cowan code using a 93% scaling of the F^ {3d3d) and F'* {3d3d) Slater integrals.

1718

]VJIST-119
Volume 74, Number 10 PHYSICAL REVIEW LETTERS 6 March 1995

tudes as a function of electron beam energy. Because the ionization potentials of the nearby charge states are only

separated by about 100 eV in both barium and xenon, the populations of these various charge states, and hence the signal strengths of the corresponding emission lines, are expected to change quickly with beam energy. This was

seen in our measurements (e.g., Fig. 4) where it was found

that all of the identified lines exhibited the expected be- havior with electron beam energy, following the predicted

[22] population densities of their parent ions. Although our scaled HFR calculations reproduce the experimental data quite well, a state-of-the-art, fully

ab initio, MCDF calculation [23] fails to yield good

agreement [16]. To our knowledge, there is no other case in which a fully relativistic calculation disagrees with

transition energies of highly charged ions (q ^ +30) by as much as 5%. The measurements reported here therefore present an important challenge to theorists seeking to predict accurately atomic structure from first principles. In summary, we have observed a special set of lines from highly charged ions which are expected to persist in the visible and the near-uv range of the spectrum over a large range of nuclear charge states. It is anticipated that

this will have important practical application for remote

diagnosis in the next generation of high temperature

plasma fusion devices. Measurement of these lines also provides a sensitive probe of atomic structure theory

applied to a range of ions where the accuracy has been

limited by a scarcity of experimental data. We would like to thank Yong-Ki Kim for carrying out ab initio calculations of ionization energies and transition wavelengths. We would also like to thank Mort Levine, Ross Marrs, Dave Knapp, Ed Magee, and the rest of the Livermore EBIT team for guidance during construction of

the NIST-NRL EBIT facility. This work was supported

in part by the Office of Fusion Energy of the U.S. Department of Energy.

120

I

80

i 60

L. -m- V-like Xe * -•- Ti-like Xe A. Cr-like Xe Ionization Potential \ V-like Xe Ionization Potential \^ Ti-like Xe Ionization Potential

f 1 1 1 , 1 . J , 1 , 1 1800 1900 2000 2100 2200 2300 2400 2500

Electron Beam Energy (eV)

FIG. 4. Amplitudes of the 3d'^^D2-^D^ observed titaniumlike and 3d^ ^Gi/i-'^Gg/j vanadiumlike transitions in xenon versus

electron beam energy, for 35 mA of electron beam current. The

corresponding ionization energies are shown by vertical lines.

*Pennanent address: Texas A&M University, College

Station, TX 77843. ^Permanent address: University of Notre Dame, Notre Dame, IN 46556. ^Permanent address: Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), H-4001

Debrecen, Pf. 51, Hungary.

[1] U. Feldman, P. Indelicato, and J. Sugar, J. Opt. Soc. Am.

B 8, 3 (1991).

[2] B. Edlen, Z. Astrophys. 22, 30 (1942).

[3] U. Feldman, Phys. Scr. 24, 681 (1981).

[4] G. A. Doschek and U. Feldman, J. Appl. Phys. 47, 3083

(1976).

[5] S. Suckewer, Phys. Scr. 23, 71 (1981).

[6] S. Suckewer et al, Nucl. Fusion 24, 815 (1981).

[7] E. Hinnov, S. Suckewer, S. Cohen, and K. Sato, Phys. Rev. A 25, 2293 (1982).

[8] K. Ida and S. Hidekuma, Rev. Sci. Instrum. 60, 867

(1989).

[9] U. Feldman, J.F. Seely, N.R. Sheely, Jr., S. Suckewer, and A.M. Title, J. Appl. Phys. 56, 2512 (1984).

[10] D. Wrdblewski, L.K. Huang, H.W. Moos, and P.E.

Phillips, Phys. Rev. Lett. 61, 1724 (1988).

[11] D. Wrdblewski, L.K. Huang, H.W. Moos, and P.E.

Phillips, Rev. Sci. Instrum. 59, 1632 (1988).

[12] V. Kaufman and J. Sugar. J. Phys. Chem. Ref. Data 15, 321 (1986).

[13] J.D. Gillaspy, J.R. Roberts, CM. Brown, and U. Feld- man, in Proceedings of the 6th International Conference on the Physics of Highly Charged Ions —7992, edited by

P. Richard, M. Stockli, C. L. Cocke, and C. D. Lin AIP

Conf. Proc. No. 274 (AIP, New York, 1993), p. 682.

[14] M. A. Levine, R. E. Marrs, J.R. Henderson, D. A. Knapp.

and M B. Schneider, Phys. Scr. T22, 157 (1988).

P. Beiersdorfer, in Proceedings of the 15th International Conference on X-Ray and Inner-Shell Processes —1990,

edited by T. A. Carlson, M.O. Krause, and S.T. Manson, AIP Conf. Proc. No. 215 (AIP, New York, 1990), p. 648.

[16] Y.-K. Kim (private communication).

[17] A. Dollfus, C.R. Acad. Sci. Paris 245, 201 1 (1957).

[18] J.D. Gillaspy, in Proceedings of the 6th International Symposium on Electron Beam Ion Sources and Their

Applications, 1994 (to be published).

[19] F. A. Parpia, I. P. Grant, and C. F. Fischer, GRASP^ (private

communication).

[20] R. D. Cowan, The Theory of Atomic Structure and Spectra

(University of California Press, Berkeley, CA, 1981), p. 456.

[21] J. O. Ekberg, U. Feldman, and J. Reader, J. Opt. Soc. Am.

A 5, 1275 (1988).

[22] B.M. Peneu-ante, J.N. Bardsley, D. DeWitt, M. Clark, and D. Schneider, Phys. Rev. A 43, 4861 (1991); H. Margolis (private communication).

[23] Y.-K. Kim, Phys. Scr. T47, 54 (1993).

[15]

1719

]S[IST-120
Nuclear Instruments and Methods in Physics Research B 98 (1995) 581-584

t Beam

mmsInteractions with Materials & Atoms

Fabry-Perot spectroscopy of a visible magnetic dipole transition

inBa''^

H. Adler \ E.S. Meyer F.G. Serpa ^ E. Takacs ^ J.D. Gillaspy ^ CM. Brown

U. Feldman

^ Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ 08543, USA

Atomic Physics Division, National Institute of Standards and Technology, Gaithershurg, MD 20899, USA

^ E.O. Hurlburt Center for Space Research, Naual Research Laboratory, Washington, DC 20375, USA

Abstract

We are using Fabry-Perot interferometry to study visible lines from highly-charged ions created and trapped within an

electron beam ion trap (EBIT). The 3d'*^D2-^D3 titanium-like barium (Ba^"*"^) line at 3932(2) A was recently measured in Ref. [l] (C.A. Morgan et al., Phys. Rev. Lett. 74 (1995) 1716) using a grating monochromator. We present preliminary

Fabry-Perot spectra of this line with significantly improved resolution. The Doppler-broadened 1 A line width is consistent with an expected ion temperature of less than 1 keV. We discuss the possibility of resolving Zeeman splittings, and of using

these visible lines as a diagnostic in high temperature, low density plasmas, like those that exist in tokamaks and the EBIT

itself.

1. Introduction

In 1991, Feldman, Indelicato, and Sugar [2] predicted an uninterrupted isoelectronic sequence of visible and near-UV forbidden lines extending all the way from tita- nium-like silver (Ag^"^), with an ionization potential of

1.3 keV, to tilanium-like uranium (U^°^), with an ionization potential of 8.2 keV. These lines correspond to a magnetic dipole (Ml) transition from the J = 3 to the 7 = 2 level within the 3d''^D ground configuration of the titanium-like ion. For all lines of the sequence, the calculated wavelengths lie in the range from 3200 to 6100 A [2]. Recently, an electron beam ion trap (EBIT) was used to observe these lines in barium (Ba^"*"^) and xenon (Xe^^'^)

at wavelengths of 3932(2) A and 4139(2) A, respectively

[1]. In addition to providing confirmation for the predictions of Feldman et al. (although with a 4% discrepancy in transition energy), the experiment represents the first time

that an EBIT has been used to measure previously un- known visible transitions in highly-charged ions. In Ref.

[1], a grating monochromator was used to measure the

lines, and the resulting widths - roughly 8 A - were

dominated by the low resolving power of the instrument.

Corresponding author.

Higher resolution measurements of these lines, however, should have several applications. Of immediate inter- est is their utility for diagnosing hot plasmas, since a measurement of Doppler broadening gives a determination of local ion temperature, and a measurement of the Zee- man shift or splitting gives a determination of local magnetic field strength. Studies of these lines also have appli- cation in atomic theory. Because the transition energies are small compared with typical binding energies within the

ion, high resolution measurements provide a sensitive test of atomic-structure calculations. In fact, there is currently a very large discrepancy between the observed wavelength

of the Ba^'*'^ line and the predictions of a fully relativistic atomic code [3]. In light of these important applications, we have undertaken a higher resolution study of the Ba^'*"'^ line using a Fabry-Perot interferometer. We have found that this tech- nique is quite feasible for EBIT studies, and that in fact some of the unique properties of the EBIT simplify the

analysis.

2. Doppler broadening

Even though ion temperature plays an important role in the operation and efficiency of the EBIT as an ion source,

until recently there had been no direct measurement of the temperature of ions trapped within an EBIT. The Doppler

0168-583X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved 0168-583X(95)00016-X 5. PRODUCTION/METHODS/APPLICATIONS

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582 H. Adler el al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 581-584

width of a line emitted from ions of mass M at temperature T is given by [4]. AK = \n2/ {Mc'^) , (1)

where is Boltzmann's constant, and c is the speed of

light. Beiersdorfer et al. [5] have just reported a crystal spectrometer measurement of the Doppler broadening in the Is^ 'So-ls2p ^Pj X-ray transition in helium-like tita- nium (Ti^°^). With resolving powers up to A/4A=5

22000, they were able to measure temperatures down to 130 eV. Such a high resolving power is near the state-ofthe-art for X-ray spectroscopy, but easy to achieve in a

visible measurement, especially with a Fabry-Perot instru- ment.

3. Zeeman effects

Ions trapped in an EBIT are irmnersed in a fairly strong

axial magnetic field used for focusing the electron beam.

We typically operate at the limit of our magnet - 3 tesla (although we have successfully created and trapped ions down to 1.5 T), since ion production and excitation rates increase quickly with field strength. Though the effect of a 3-T field on transition wavelengths and lifetimes is negligi- ble for X-ray studies, Zeeman shifts can be a significant fraction of the total transition energy for visible lines. A magnetic dipole transition from a J = 3 level to a 7 = 2 level should contain 15 distinct Zeeman lines: five with Am = — 1; five with Am = 0; and five with Am =

-I- 1. The Zeeman shift for any one of these lines is given

by

4= -fi/io5j=3'n/=3- (-^Mogj=2'nv=2). (2)

where B is the magnetic field, fi^ = 5.7883 X 10"^ eV/T

is the Bohr magneton, and gj^^ and gj=2 the Lande

g-factors for the 7 = 3 and 7 = 2 levels, respectively. The

Cowan atomic structure programs [4] give gj=3 = 1.282 and gj^2 ~ 1-373 for the 3d^^D levels in Ba^+ (see Ref.

[1] for a description of the scaling parameters used with

the code). Thus, the shifts in the barium lines for B = 3 T

are given by

A = -2.226 X 10-*eVOT^=3 -f2.384X 10~''eVm^=2•

(3)

The major splittings, corresponding to the separations of

the centers of gravity of components with Am = — 1, 0 or + 1, display the normal triplet structure with separations of

the order of 2.3 x 10"" eV. This corresponds to a full- width separation in wavelength of A\ = 0.6 A at 3932 A,

giving 4A/A * 1.5 X 10"'*. The finer splittings within each group are much smaller - approximately 0.15 X

10""* eV, or about 8.5% of the larger splittings. Both the polarization and intensity of the emitted line radiation depend upon Am, and the intensity further de20 -

oL_j I 1 1 I 1 . I I 1 1 1 I 1 1 I I I 0 50 100 150 200 Scan Posibon (1 channel = 1.0 sec)

Fig. 1. Fringe data for the Ba''*"'" line, averaged over 16 scans. The scan period was 200 s, and counts were histogrammed into 1

s channels. The distance between peaks is 3.0(1) A, corresponding

to Av = 19.4(5) cm"'.

pends upon mj for the upper state. For observation perpendicular to the field, and assuming uniform level population, the relative intensities are given by the following

standard formulas [6]: /„(4/n = 0) =7^-/72^ = 9 I^iAm = - 1) = (7 + m)(7 - 1 + m)/4 = (m + 3)(m + 2)/4;

I„{Am= +l) = (7-m)(7-l-m)/4

= (3-m)(2-m)/4. (4)

By observing at 90° to the magnetic field direction as we

do, the a-components ( Am = + 1) should be linearlypolarized parallel to the field axis, and the it-components

( Am = 0) should be linearly-polarized perpendicular to the

field axis [4].

4. Experiment

A typical scan of the titanium-like barium line is shown

in Fig. 1. Light from the EBIT is transmitted through a quartz window side port and then collimated onto a plane

Fabry-Perot interferometer. The Fabry-Perot is then fol- lowed by a fringe-imaging lens (24.5 cm focal length) and

a central aperture 4 mm in diameter placed at the focal

point. Behind the aperture, the light is detected by a photomultiplier tube operating in single-photon counting

mode. The plot in Fig. 1 was created by repeatedly scan- ning the Fabry-Perot mirror spacing d with a piezoelectric element while histogramming photon counts versus scan

position. The total acquisition time for the spectrum was

about 40 min. By temperature controlling the Fabry-Perot

plates, we were able to reduce any drift over the time of a scan to a negligible level.

NIST-122
H. Adler et al./Nucl. Jnslr. and Meth. in Phys. Res. B 98 (1995) 581-584 583

Using the 6328-A line of a He-Ne laser as a reference, we found a finesse of F = 22. Taking into account the change in flatness finesse, parallelism finesse, reflectivity, and pinhole finesse with wavelength, we extrapolated a value of F = 15 for the finesse of the interferometer at

o 3932 A. Chromatic aberration in the optics, however,

could have lowered the finesse substantially by causing the image focus to shift away from the pinhole. After doing a ray tracing analysis, we estimate that this effect can give at most a factor of two reduction in the pinhole finesse. This

leads to an overall lower bound of F = 9 for the total finesse. The free spectral range Act = c/2<i = 19.4(5) cm~ ' (3.0(1) A at 3932 A) was made large (by moving the Fabry-Perot plates as close together as possible) because

of the very large Doppler width expected for the barium

line. We determined d and thus Ac by using a standard technique in which the Fabry-Perot is tilted and the distance between fringes in the back reflection of an

incident laser is measured [7]. Later, the accuracy of this measurement was confirmed by measuring a closely-spaced

pair of lines at 4044 A and 4047 A from a potassium

discharge lamp. The uncertainty shown in Act is 1-sigma

systematic error arising from our tilt-technique measurement.The Ba-'^'^ charge state was created by biasing the center drift tube electrode to 2350 V, which conesponds to an electron beam energy of about 2250 eV (the energy

reduction is due to space charge effects). This energy is just below the titanium-like state's ionization potential.

The end cap electrodes were biased at 500 V to give an

axial trap depth of 17 keV. With the electron beam current

fixed at 50 mA, the counting rate achieved for the barium

line is of order 200/s, which is roughly equal to our dark

count background. This counting rate compares favorably with previous precision X-ray measurements we have made

using a Bragg crystal spectrometer.

5. Discussion

In Fig. 2, we compare numerical simulations of the Doppler- and finesse-broadened Zeeman lines for a number of different ion temperatures with our data. The Zee- man components are expected to be bluned by Doppler

broadening due to the substantial temperature of the ions

in the EBIT. Comparison of our measurements with calculations indicate that the ion temperature is less than 1 keV,

in agreement with the predictions of a numerical simulation of EBIT charge-state evolution developed by Penetrante et al. [8]. The scarcity of visible lines in the highly-charged ions produced in EBIT greatly simplifies the analysis of Fabry-Perot spectra. Furthermore, the precise control over

the electron beam energy that one has with the EBIT

allows fine control over emitting charge state density. This

FABRY-PEROT SCAN (F=15)

140

TD-.leV TO-SOOeV

TD-lOOOeV . TD-lSOOev TD-2000eV DATA

80

§

o 60

40

20 -

0 I I i I I I I I I . I I I . 190 200 210 220 230 240 2S0 260

Relative Phase (deg)

Fig. 2. Simulations of the line profile for different ion temperatures TD and for a Fabry-Perot finesse of f = 15. The data is the sum of two separate peaks from Fig. 1. A best fit appears to lie between TD = 1000 eV and TD = 500 eV, indicating that the ion temperature lies between these two values. At a very low temperature (TD = 0.1 eV), the Zeeman-induced triplet structure can be

seen, although broadened somewhat by the finite finesse.

selectivity helps with confirming line identifications without resorting to a wavelength prefilter. For our first measurements, we achieved a resolving

power of 10'', good enough to get a crude measurement of

ion temperature. We expect there is room to improve by

perhaps another factor of ten by either using a higher

finesse instrument or working at a larger minor spacing d. Resolution of Zeeman splittings should be possible by

using a shallower trap in order to enhance evaporative

cooling (reduce T). Studying ions further up the titaniumlike sequence with larger mass will also help to reduce the Doppler width (increase M).

In the future, we plan to extend our measurements to tungsten, uranium, and various rare earth metals along the titanium-like sequence. We also plan to employ polarizers

in order to separate the different Zeeman lines. For example, eliminating the Am — 0 (ir-) component will make it much easier to resolve the Zeeman splitting between the orthogonally-polarized a-component lines. Our discussion of relative Zeeman intensities above

assumed that the different rrij sublevels of the upper state are populated equally. This might not be true in an EBIT,

since the excitation source (the electron beam) is uni-directional and can create preferential populations in certain m

j

sublevels. As the upper level of the visible transition is populated primarily by cascades, it is difficult to calculate exactly how much effect this will have on the polarization of the visible emission. We are pursuing extensive calculations now and hope to include these effects in a later study.

In conclusion, we have used a Fabry-Perot interferometer to study highly-charged ions created and stored inside

5. PRODUCnON/METHODS/APPLICATIONS

NIST-123
584 H. Adler et al. /Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 581-584

an EBIT. Because of the low energy of visible transitions,

it should be possible to resolve Zeeman shifts as well as to measure magnetic field-induced polarization. The inher- ently high resolution of spectroscopic techniques in the

visible makes it straightforward to measure Doppler broad- ening and determine the temperature of the emitting ions, and the line we have measured should have applications in tokamaks as well as EBITs.

Acknowledgements

We thank J. Sugar for calculating the Lande g-factors

for the J = 3 and J = 2 levels in titanium-like barium, and we thank J.R. Roberts for helpful discussions and a careful reading of this manuscript.

References

[1] C.A. Morgan, F.G. Serpa, E. Takacs, E.S. Meyer, J. Sugar,

J.R. Roberts, J.D. Gillaspy, CM. Brown and U. Feldman,

Phys. Rev. Lett. 74 (1995) 1716.

[2] U. Feldman, P. Indelicato and J. Sugar, J. Opt. Sec. Am. B 8

(1991) 3.

[3] Yong-Ki Kim, private communication (1994).

[4] R.D. Cowan, The Theory of Atomic Structure and Spectra

(University of California Press, Berkeley, 1981).

[5] P. Beiersdorfer, V. Decaux, S. Elliott, K. Widmann, K. Wong,

preprint, 1994; also, these Proceedings (7th Int. Conf on the Physics of Highly Charged Ions (HCI-94), Vienna, Austria, 1994) Nucl. Instr. and Meth. B 98 (1995).

[6] l.I. Sobelman, Atomic Spectra and Radiative Transitions

(Springer, Berlin, 1979).

[7] Burleigh Instruments Inc., private communication (1994).

[8] B.M. Penetrante, J.N. Bardsley, M.A. Levine, D.A. Knapp

and R.E. Marrs, Phys. Rev. A 43 (1991) 4873; we thank H.

Margolis for providing a port of the original Fortran source code to C.

NIST-124
PHYSICAL REVIEW A VOLUME 53, NUMBER 4 APRIL 1996

Anomalous Z dependence of a magnetic dipole transition in the Ti I isoelectronic sequence

F. G. Serpa,* E. S. Meyer, C. A. Morgan,^ J. D. Gillaspy, J. Sugar, and J. R. Roberts Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-0001

C. M. Brown and U. Feldman

E. O. Hulburt Center for Space Research, Naval Research Laboratory. Washington, D.C. 20375-5000

(Received 20 October 1995)

In isoelectronic sequences, transition wavelengths ordinarily move rapidly to shorter wavelengths as Z

increases. However, it has been predicted that a particular sequence of magnetic dipole (A/1) transition wavelengths for the Ti-like ions, Xe"*"^^ through U"*"TM, remain relatively constant in the 320-400-nm range. In the present paper we extend the experimental identifications of the Ti-like M 1 transitions from Ba and Xe to Nd (Nd*'*) and Gd (Gd*^^) to verify this behavior Using the newly acquired wavelengths to adjust atomic

parameters, we have also calculated improved wavelengths for all such Ml transitions between Xe (Z=54)

and Os (Z=76).

PACS number(s): 32.30.Jc. 32.10.-f, 52.70.-m, 31.25.-v

L INTRODUCTION

Since it is common for the wavelengths in isoelectronic sequences of ionic transitions to move rapidly to shorter wavelengths as Z increases, wavelengths are typically within

the range of optical instruments for only a few members of a sequence. They shift into the vacuum ultraviolet and soft x-ray spectral regions, becoming much more difficult to use

as plasma diagnostic probes. Previously, Feldman, Indelicate, and Desclaux [1] reported results from a survey of atomic transitions that violate this general wavelength scal- ing behavior and that would be suitable for plasma diagnostics in future high-energy Tokamak fusion devices. This

search was aimed at finding transitions that fulfill the following requirements: (a) the wavelengths should be ^250 nm so

that transmission optics can be employed; (b) the lines should be reasonably intense and arise from the lowest term

of the ground conSguration, and (c) the emitting ion should have an ionization potential in the 5-10-keV range. The sur- vey included all promising isoelectronic sequences and was

based on calculations using a Dirac-Fock code of Desclaux

and co-workers [2,3], including all refinements [4,5]. Feld- man, Indelicato, and Desclaux [1] discovered only one set of

transitions in only one isoelectronic sequence that fulfilled the stated requirements. They found that the wavelengths of

the Afl, "id D^-^D^ transitions in the Ti-like ions Xe

(Xe""") through U are in the 320-400-nm range, sat- isfying the first requirement. Since die is the lowest term

of the Ti-like ground configuration, 3 J'', and since the spontaneous decay rates of the transition are sufficiently high, the second requirement is also fulfilled. The last requirement is fulfilled for all ions with atomic numbers Z^75.

However, ab initio calculations often are not sufficiently accurate to unambiguously identify the transition wave-

*Permanent address: University of Notre Dame, Notre Dame, IN

46556. ^Permanent address: Texas A&M University, College Station, TX

77843.

I050-2947/96/53(4)/2220(5)/$ 10.00 53

lengths when compared with experimental observations, es- pecially where there exists more than one ion stage in the experiment. Therefore, it is necessary to make experimental

observations along an isoelectronic sequence in incremental

steps to compare with theoretical calculations to assure that further predictions are as accurate as possible. In a recent paper [6], spectra of highly ionized Xe and Ba

in the 320-400-nm range were produced with an electron beam ion trap (EBIT). Each spectrum (see Fig. 1 [6]) contained only two lines, one of which was identified as the predicted Ti-like M 1 transition, thus providing the first ex- perimental verification of the prediction. The wavelength

agreement between the calculations using the Dirac-Fock

code of Desclaux and co-workers [2,3] and the experimental

values was within 5%, and within 4 nm or 1% with the predictions of the Hartree-Fock code of Cowan [7] when the

elecU-ostatic integrals were scaled to 93% of their ab initio values. For highly charged ions, the comparison of Cowan's

code calculations with experimental values typically requires

that the electrostatic integrals be scaled from 90% to 95% of

their ab initio values [7]. This scaling arises from the inter- action of the configuration under investigation with energetically distant perturbers.

In this paper we extend the identification of the Ti-like Ml transitions to Nd"*"^* and Gd"*^'*^. Using the experimentally acquired wavelengths, we established scaling factors for

the electrostatic integrals in the Cowan code for elements Z=54-76. From these scaling factors, improved predictions of wavelengths for Ti-like Ml transitions within the 3d'' ground configuration between Xe (Z=54) and Os (Z=76)

have been obtained.

n. EXPERIMENT

The EBIT at the National Institute of Standards and Tech- nology (NIST), a joint project between NIST and the Naval

Research Laboratory (NRL), was used as the source of excited highly charged ions in this work [8,9]. The desired ion

stage is produced by successive electron impact ionization from an accelerated electron beam. This beam is produced by

2220 © 1996 The American Physical Society

NIST- 125
53 ANOMALOUS Z DEPENDENCE OF A MAGNETIC DIPOLE . .

.

2221

FIG. 1 . X-ray spectra in keV of

(a) Gd and (b) Nd taken with a

Si(Li) detector during injection at 10 kV accelerating potential. The

tick marks along the abscissa cor- respond to calculated transition energies between excited levels and the ground state. The calculations were done with the GRASP^

code [14].

8 9 10 11 Energy (keV)

a electron gun that provides currents up to —150 mA. A pair

of superconducting magnets in a Helmholtz configuration,

producing a uniform axial magnetic field of 3 T, compresses

the electron beam to a diameter of ~60 /tm resulting in a

current density of —5000 A/cm^. Together the electron beam

and the magnetic field produce a radial trap for the ions. Axial trapping of the ions along the electron beam axis is provided by raising the two ends of three collinear, insulated

drift tubes to a positive potential with respect to the center

drift tube bias potential. Together this results in a cylindrical

trap —30 mm long and —200 fim in diameter [10] in the direction of the electron beam. A variable voltage between 2 and 20 kV applied to the drift tubes defines an accelerating

potential for the electron beam. A correction of approximately — 100 eV must be applied to this energy due to the net space charge of the electrons and the positive ions in the

trap. The precise energy of the electron beam (—50-eV

width) is determined from the variable accelerating potential, the center drift tube bias voltage, and the space-charge correction. By adjusting the accelerating voltage slightly below

the ionization energy of the desired ion, the population of a

specific charge state can be optimized.

Desired elements are introduced into the trap region in various ways. Due to the composition of the electron gun

cathode, Ba is naturally present and is injected slowly in our EBIT. Other elements must be introduced externally either by gas injection or by means of a metal vapor vacuum arc (MEVVA) source. The MEVVA attaches to the top of the EBIT vacuum chamber along the electron beam axis, ap- proximately 1.5 m above the trap region. The operation of

the MEVVA has been described in detail previously [11,12]. In brief, the MEVVA consists of a central trigger electrode, a

cylindrical cathode, an anode, and extractor plates and is modeled after the Livermore MEVVA design [13]. The composition of the cathode determines the species to be injected.

The trigger, cathode, and anode all operate at or slightly

above the voltage of the drift tubes to assure that the injected ions will have enough energy to reach the trap. A pulse of

several kilovolts and a few microseconds in duration is applied to the trigger electrode. The resulting discharge generates a smeill amount of plasma between the cathode and trig- ger. This plasma, containing low charge states of the species

being injected, is accelerated towards the trap. An einzel lens between the MEVVA and the trap provides additional focus- ing to increase the number of ions arriving at the trap. A set of eight, 25-mm-high by 2.5-mm-wide, radially positioned slots in the central drift tube provide openings for radiation emanating radially from the trap region. Four 120- iran-diam and four 38-nmi-diam knife-edge flanges in the vacuum vessel allow for radial viewing access to the trap region. One of the ports is blocked by a device to suppress

the buildup of electrons [8]. A Si(Li) detector, with an energy

resolution of —190 eV, is used to monitor the x-ray spectra

through one of the radial ports. This port has a 0.13-mmthick beryllium window to act as the vacuum interface. Wavelengths above 300 nm were also observed radially

through a quartz window port. Wavelength dispersion of this spectra was achieved with a 0.3-m focal length, //5.3

Czeray-Tumer scanning monochromator, with a 1200- groove/mm grating resulting in a reciprocal linear dispersion

of 2.6 nmymm. A low noise, 12-stage, 50-mm-diam, end-on

photomultiplier was placed at the monochromator exit slit. A

commercial cooler was used to reduce the temperature of the photomultiplier to about -25 °C. This cooling and a magnetic defocuser surrounding the photomultiplier to reduce the

active cathode area decreased the detector noise to about 3

counts/s. Because of the detector sensitivity to stray magnetic fields from the EBIT superconducting magnet, two 50- mm-diam (150- and 175-mm focal length) fused silica plano- convex lenses were used to image light from the trap to a new position about 850 mm away from the EBIT. This combination of lenses produced a demagnification of —0.8 and

transmitted —30% of the light incident on the first lens. The

monochromator was attached to an optical table, which was

fully adjustable in the horizontal plane, and moved independently from the two lenses. Two translation stages with a

NIST-126
2222 F. G. SERPA et al. 53

100

80 a 60 o 40

20

100

80

J

40

20

J I L_i_J I I I I I I I 300 310 320 330 340 350 360 370 380 390 400 no. 2. Survey of emission

spectra in the 300-400-nni range

obtained with a scanning monochromator for (a) Gd and (b) Nd.

300 310 320 330 340 350 360 370 380 390 400 Wavelength (nm)

resolution ^10 jjm were used to translate the monochromator and optical table assembly and placed the monochromator's entrance slit at the trap image formed by the lenses. For the present experiment, Gd and Nd where injected

separately into the EBIT using the MEVVA. The main accelerating voltage and the MEVVA anode voltage were set to 10

keV, appropriate to produce Ne-like Nd and Gd. The upper, middle, and lower drift tubes were at +500, +100, and

+500 V, with respect to the main accelerating voltage. To

load the trap with ions, first the upper drift tube offset volt- age was reduced to zero, which combined with the offset of + 100 V on the middle drift tube, forced any residual ions out of the trap. After 2 ms, a pulse was transmitted through a

fiber optic to trigger the MEVVA, and 15 /iS later the upper

drift tube offset voltage was raised again to +500 V to trap

the injected ions. This cycle was repeated about every 2 s and trapping was confirmed by observing the corresponding

x-ray spectra.

ionization energy. The total duration of each cycle was about 2 s. Wavelength survey scans were made between 300 and

400 nm for Nd and Gd (see Fig. 2). The Gd scan was taken

at a 3.34-keV drift tube voltage, a 63-niA electron beam

current, and with a 400-/im entrance and exit slit widths. It showed a prominent peak at 371.3 nm [Fig. 2(a)] and a weaker peak at —314.4 nm [Fig. 2(a)]. The Nd scan was

taken at a drift tube voltage of 2.77 keV, 49-mA electron beam current, and 600-/im entrance and exit slits. It showed

a main peak at —375.3 nm and a weaker feature at —391.0 nm [Fig. 2(b)]. We identified the stronger peaks as the 3rf'*^D2-^D3 Ml transition. By setting the monochromator to the wavelength of the Gd Ml peak at 371.3 nm and varying the accelerating potential, we observed the dependance of the line intensity on

drift tube potential. A plot of this line intensity is shown in

50

ni. DATA

X-ray spectra of Gd obtained with the Si(Li) detector are shown in Fig. 1(a). The transition energies to the ground

state from the 2/7^(2p,/2)3d(l/2,3/2)i,2/7^(^^3/2)35(3/2,

1/2), and 2p^(2p3/2)3rf(3/2,5/2), levels (in j-j coupling) of Ne-like Gd were obtained from calculations using the

raulticonfigurational Dirac-Fock (MCDF) code grasp^ [14]

and yielded 6.11, 6.63, and 7.29 keV, respectively. This

agrees with the main peaks in Fig. 1(a) at 6.2, 6.7, and 7.4

keV. Similar calculations for Ne-like Nd give 5.31, 5.75, and

6.24 keV for the same transitions and are also in agreement

with our measured values of 5.34, 5.78, and 6.27 keV [see

Fig. 1(b)]. In order to obtain the desired Ti-like charge state, a similar timing sequence of events described above was

followed. Ions were injected with a 10-kV accelerating potential, and 3 ms after the upper drift tube bias was raised, the total center drift tube voltage was lowered to the Ti-like

0

'•—

-'—''' ' '' 3J 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 DT Voltage (kV)

FIG. 3. Detector signal from the M\ line of Ti-like Gd as a function of the voltage of the middle drift tube. The dotted line indicates the voltage corresponding to maximum intensity and the dashed line indicates the voltage corresponding to the dielectronic recombination feature.

]SfIST-127
53 ANOMALOUS Z DEPENDENCE OF A MAGNETIC DIPOLE . 2223

1 2 3 4 5 6 Energy (keV)

FIG. 4. X-ray spectra in keV of highly ionized Nd observed

during the M 1 line scan. The peaks labeled a and b are consistent with radiative recombination into the n=5 and 4 shells, respectively, for Ti-like Nd. The peaks labeled c and d are due to dielec- tronic recombination into the Ti-like charge state.

Fig. 3, where the horizontal axis has been corrected for the 100-V offset applied to the middle drift tube but has not been

adjusted for the space-charge effects. The vertical axis shows

the detector signal, which has been normalized for temporal

variations in the number of trapped ions over the time of

acquiring this data. The normalization is derived from the

ratio of the peak signals (at 3.52 kV) at the beginning and

end of the data acquisition. The data exhibit the expected

behavior for the Ti-like charge state, as derived from computer simulations of the charge-state balance in the trap as a function of beam energy [15]. The experimental maximum

found at a central drift tube voltage of 3.52 keV is about 100

V higher than the beam energy of the simulation. This is in agreement with an expected space-charge correction of about — 100 V. The plot also shows a small peak at higher drift tube

voltage. We attribute this to an enhancement of the Ti-like population due to dielectronic recombination (DR) from the Sc-like charge state; the resonant capture of an electron into the 3d shell with the excitation of a 2pi/2 electron to the 3d

shell would require an electron energy of about 3.69 keV,

according to our MCDF calculations. Again this is 100 V

lower than the 3.78-keV energy observed and is consistent with our estimate of the space-charge correction. As a further confirmation of the charge-state identification, an x-ray spectrum taken with the Si(Li) detector during

the Nd scan is shown in Fig. 4. The peaks labeled a and b

are due to radiative recombination into Ti-like Nd with the captured electron in the m=5 and 4 shell, respectively. Our

MCDF calculations predict 3.6 and 4.2 keV for peaks a and

b, respectively. The peaks labeled c and d are due to DR into

the Ti-like charge state, where an electron is captured into the 3d shell and a 2py2 electron is excited to the 3d shell. The observed energy of peaks c and d agrees with our MCDF calculations for die decay of a 3s (4.8 keV) or 3d

(5.3 keV) electron to fill the hole left in the 2p^,2 shell by the DR process. Higher-resolution wavelength scans were made using nar- rower slit widths to better determine the M1 transition wavelengths, as well as to observe other features present at shorter wavelength in Gd. The monochromator wavelength scale

TABLE I. Scaling factors and calculated and observed wavelengths for the 3d^{^D2-^D^) Ti-like Ml transitions. All wavelengths are in nm and the uncertainties are listed in parentheses.

z Element Scale factor A-U J^calc ^expt

54 Xe 0.922 413.0 413.9(2)^ 56 Ba 0.930 393.2 393.2(2)" - 60 Nd 0.946 374.9 375.3(2)" 64 Gd 0.962 368.6 371.3(2)'' 68 Er 0.978 363.6 72 Hf 0.994 356.2 74 W 1.000 352.4 76 Os 1.000 350.7

"Reference [6]. ''This work.

was calibrated by viewing a low-pressure He discharge lamp

through the radial viewport located on the side of the EBIT

opposite the monochromator. The wavelength range used was 296.7-435.8 nm. Also, by injecting small amounts of Ar

and Kr gas into the EBIT, in situ calibration lines could be

observed. The Ai'^'^hs^Ip^Pia-^Pzn and the Ki'^^^3s^3p^ ^P\-^P2 M\ transitions were observed at 441.24(2) and 384.09(2) nm, respectively [17]. The wavelength uncertainties determined from these two wavelength

calibrations, along with a possible systematic wavelength

shift from the data acquisition, were added in quadrature to give an overall uncertainty of ^±0.2 nm.

rv. DISCUSSION AND CONCLUSION

For the Gd 3d* ^Dj-^D-^ Ml transition, the experimental

value differed from its predicted value by about 10 nm when

fixed scaling was used for the electrostatic integrals in the Cowan code [7]. To improve the agreement, we used the measured Gd Ml line and the previously observed lines (Ref. [6]), and adjusted the electrostatic integrals in the Cowan code by a linear variation of the scale factor=0.922

-1-0.004(2 -54) (see Table I). This gave the best fit to the known wavelengths over the range from Z=54 (0.922) to Z = 74 (1.000). For Z>74, this factor is set equal to one, i.e., no scaling, due to the far configuration-interaction effects approaching zero at high ionization. This variation of the

scale factors not only fit our observed wavelengths but also correctly predicted the wavelength of the Ml line for Nd,

which we subsequently observed. Such variation of the scale

factor for the electrostatic integrals is a conunonly observed

behavior along an isoelectronic sequence. For example, in the investigation of the Zn sequence by Litzen and Reader

[16], similar increases with Z of the scale factor for the electrostatic integrals, F^, were found. At W, the Cowan code

wavelength equals that obtained in Ref. [1] from the Dirac- Fock code of Desclaux and co-workers.

In Gd we also found a weaker peak at 314.4 ran. This

peak has a similar beam energy dependence as the M1 line, which suggests that it also belongs to this Ti-like charge

state. A candidate for this is the 3d'^ ^D^ — ^D^ Ml transition, which competes with the 3d'* ^D2-^D^ for line strength

at high Z [1]. An observation of other members of this iso- electronic sequence is needed to confirm this identification.

NIST-128
2224 F. G. SERPA et al. 53

50 55 60 65 70 75 80 Atomic Number

FIG. 5. Z dependence of Ti-like A/1 lines as a function of

atomic number. The flatter curve represents the 3^/" ^Dj-^Dt, transitions and the steeper curve represents the 'id^ ^D^-^D^ transitions. Open circles correspond to the wavelengths calculated with

the scaled Hartree-Fock code of Cowan [7] and the filled diamonds

represent experimental observations.

At lower central drift tube potentials, a second weak peak

appeared at 320.8 nm, suggesting that it belongs to a lower

charge state, e.g., V-like Gd.

Figure 5 is a plot of the calculated wavelengths (open

circles) of the M\ transitions, 3d* ^D2-^D^ and

3d'* ^D^-^Dj obtained with the Hartree-Fock code of Cowan using the scaling in Table I. This figure illustrates the normal behavior of the A/ 1 lines as a function of atomic

number, i.e., a very rapid change in the wavelengths of the 3d* ^D^-^D^ transitions, while the anomalous Z dependence

is portrayed by the small change in the wavelengths of the 3d* ^D2-^Dj transitions. This figure also includes the observed wavelengths (filled diamonds). Table I lists the data

for the 3d* ^02-^0^ transitions, together with the experimental scale factors. Another observation at higher 2 (such

as W) would help extend and confirm the present isoelec- tronic sequence of Ml lines for the 3d* configuration, and

would likely be of great importance for future tokamak di- agnostics.

In conclusion, we have measured and identified the 3d* ^Dj-^Dj Ml transitions for Nd+^^ and Gd^*^. These measurements extend our previous work for Z=54 and 56,

and confirm the suppressed Z dependance for these M 1 transition wavelengths along the Ti-like isoelectronic sequence. To date, we know of no ab initio calculations that can predict these wavelengths with an accuracy comparable to the scaled Hartree-Fock code of Cowan.

ACKNOWLEDGMENT

We would like to thank C. Boyer and G. Holland for their technical assistance in constructing and implementing the MEVVA.

[1] U. Feldman, P. Indehcato, and J. Sugar, J. Opt. Soc. Am. B 8, 3 (1991).

[2] J. P. Desclaux, Comput. Phys. Commun. 9, 31 (1975).

[3] R Indelicato, O. Gorceix, and J. P. Desclaux, J. Phys. B 20, 651 (1987).

[4] O. Gorceix, P. Indelicato, and J. R Desclaux, J. Phys. B 20, 639 (1987).

[5] R Indelicato, J. Phys. (Paris) CoUoq. 150, C-239 (1989).

[6] C. A. Morgan et al, Phys. Rev. Lett. 74, 1716 (1995).

[7] R. D. Cowan, The Theory of Atomic Structure and Spectra

(Univ. California Press, Berkeley, California, 1981).

[8] J. D. Gillaspy et al, in Vlth International Conference on Physics of Highly Charged Ions, edited by P. Richard et al, AIR

Conf. Proc. No. 247 (AIR, New York, 1993), p. 682.

[9] R. E. Marrs, P. Beiersdorfer, and D. Schneider, Phys. Today 47

(10). 27 (1994).

[10] J. D. Gillaspy et al, Phys. Sen, T59, 392 (1995).

[11] I. G. Brown et al, Appl. Rhys. Lett. 49, 1019 (1986). [12] I. G. Brown et al. Rev. Sci. Instrum. 57, 1069 (1986).

[13] A. M. Levine et al, Phys. Scr. T22, 157 (1988). [14] F. A. Parpia, I. P. Grant, and C. F. Fischer, grasp^ (private communication).

[15] D. Knapp (private communication).

[16] U. Litzen and J. Reader, Phys. Rev. A 36, 5159 (1987). [17] V. Kaufman and J. Sugar, J. Rhys. Chem. Ref. Data 15, 321

(1986).

NIST-129
PHYSICAL REVIEW A VOLUME 55, NUMBER 3 MARCH 1997 Kr spectra from an electron-beam ion trap: 300 nm to 460 nm

F. G. Serpa,* E. W. Bell,^ E. S. Meyer,* J. D. Gillaspy, and J. R. Roberts

Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

(Received 9 October 1996)

Kr spectra from 300 nm to 460 nm produced in an electron-beam ion trap (EBIT) are reported in this work. The spectra include magnetic dipole (Ml) transitions from Krxxill and Krxxil, as well as electric dipole

(£1) lines of Kr ll and Kriii. Two new capabilities of the EBIT at the National Institute of Standards and

Technology, time resolved data acquisition and extracted ion analysis, are used to aid in the charge state

identification. [S 1 050-2947(97)03503-8]

PACS number(s): 32.30.Jc, 32.10.-f, 31.25.Eb, 31.25.Jf

I. INTRODUCTION

Recent developments in electron beam ion trap (EBIT)

spectroscopy have shown the importance of the EBIT as a source of magnetic dipole (Ml ) transitions in the visible or near-uv [1,2]. Magnetic dipole transitions arising between

levels of the ground configuration of highly charged ions are not only useful as diagnostic tools for tokamaks [3-5] and in the study of the solar corona [6,7], but also have academic

interest since ab initio calculations fail to account for the observed wavelengths [5,8]. Many such transitions remain

unobserved, particularly for charge states with ionization energies below 2 keV [9]. Measurements of siich lines are important because they make significant impact on the elaboration of spectral [10] and energy level [11] databases. They

are also important for improving predictions along isoelectronic sequences [9] and provide a challenge for ab initio calculations.

In the present work we report spectra from 300 nm to 460 nm obtained with the EBIT during injection of Kr gas. Our

data include two M 1 transitions, one of which has not been

previously observed. These Ml lines arise from transitions between the levels of the ground configuration of Kr xxill (3s^3p^) and Kr XXII (3j^3/j^). A new ion extraction,

transport, and charge-to-mass analysis system at the National

Institute of Standards and Technology (NIST) [12] was used

to obtain information about the distribution of ion charge

states within the EBIT. Data from the extraction system to- gether with time resolved data were used to aid in charge

state identification. Lines of singly and doubly ionized Kr

were also present in our spectra and were used for in situ wavelength calibration.

II. EXPERIMENTAL SETUP

The EBIT at NIST was used as the source of excited highly charged ions in this work [13,14]. The desired ion

Permanent address: Physics Department, University of Notre Dame, Noue Dame, IN 46556.

Present address: Diamond Semiconductor Group, 14 Blackburn

Center, Gloucester. MA 01930. Permanent address: D.E. Shaw Co., 120 West 45th, New York,

NY 10036.

1050-2947/97/55(3)/1832(4)/$10.00 55

stage is produced by successive electron-impact ionization from an accelerated electron beam, produced by a electron gun which provides currents up to ~ 150 mA. A pair of

superconducting magnets in a Helmholtz configuration pro- duces an axial magnetic field of 3 T in the trap region and

compresses the electron beam to a diameter of —60 fim,

resulting in a current density of —5000 A/cm^. Together the electron beam and the magnetic field produce a radial trap for the ions. Axial trapping of the ions along the electron beam axis is provided by raising the two ends of three collinear, insulated drift tubes to a positive potential with re- spect to the center drift tube bias potential. Together this traps a cylindrical ion cloud —30 mm long and —200 fim in diameter [15], oriented along the direction of the electron beam. A variable bias voltage from 2 kV to 20 kV is applied

to the drift tubes to define an accelerating potential for the electron beam. A correction of approximately - 100 eV must

be applied to this energy in order to account for the net space charge of the electrons and the positive ions in the trap in

this particular experiment. The precise energy of the electron beam ( — 50 eV width) is determined from the accelerating bias potential, the center drift tube floating voltage, and the space charge correction. By adjusting the accelerating volt- age slightly below the ionization energy of the desired ion, the relative population of a specific charge state can be optimized. A gas injection system connected to a lateral port, that looks directly into the trap, is used to introduce krypton at- oms into the EBIT from a direction perpendicular to the trap. The gas injection system consists of a tunable gas leak, two pump chambers, and three collinear apertures. The apertures

are used to define the differential pumping of the chambers

and for rough alignment of the gas stream. The first aperture

(0.318 cm in diameter) is located 67.7 cm from the trap and

separates the gas leak from the first chamber. The second

aperture (1.27 cm diameter) is located 44.5 cm from the trap and separates the first and second chambers. The last aperture (0.318 cm diameter) is located 32.1 cm from the trap and separates the second chamber and the injection port to the EBIT. The gas leak allows for tuning of the amount of

gas injected, which is monitored by an ion gauge located in the chamber between the first and second apertures. Typical gas injection pressure values for this experiment were -5 X 10""* Pa to - 2 X 10"^ Pa. Each chamber is pumped by

a turbo pump with a pumping speed of about 50 1/sec.

1832 © 1997 The American Physical Society

NIST- 130
55 Kr SPECTRA FROM AN ELECTRON-BEAM ION TRAP: 1833

The detection system is described in detail elsewhere

[1,2]. Briefly, a cooled, low noise photomultiplier was used

for photon detection, and a scanning monochromator was

used for wavelength selection. Radiation emanating from the

trap was imaged onto the entrance slits of the scanning

monochromator using a set of two plano-convex lenses; this was necessary to keep the photomultiplier away from any

stray magnetic field from the superconducting magnet and to improve the signal. The entire optical table which supports

the monochromator was attached to two perpendicular translation stages which can be positioned with a resolution better than 10 /im. These translation stages were used to place the monochromator' s entrance slit at the image of the trap pro- duced by the lens system.

The trap was emptied of ions at preset time intervals by

sending a fast pulse to the middle drift tube. This pulse

raised the middle drift tube floating voltage above the upper

and lower drift tube floating voltages in less than 1 ms, kept

it constant for —0.1 ms, and lowered it back to its initial value again in less than 1 ms, so that the total duration of the pulse was less than 2 ms. Time resolved data were acquired

by setting the monochromator to a particular wavelength and

accumulating data over thousands of such cycles. Photons

arriving at the photomultiplier were time-stamped with a

resolution better than 0.1 ms. The timing electronics were

designed using CAMAC modules controlled by a list processor interfaced to a computer.

The ion extraction and transport system [12] was used to obtain information about the distribution of ion charge states within the EBIT. Briefly, the trap was emptied on-axis, al- lowing the ions to be extracted from the EBIT along the magnetic field lines, after which a series of electrostatic ele- ments focused and aligned the ion beam before sending it through a charge to mass ratio analyzing magnet. The charge

states were separated using the magnet and a detector counted the number of ions transported through the magnet

for a particular value of the magnetic field. By successively

stepping the magnetic field and emptying the trap, a plot of

the relative populations of the charge states present in the

trap was obtained.

in. DATA

Figure 1 shows a Kr spectrum taken at a drift tube volt- age of 10 kV. The upper, middle, and lower drift tubes float- ing voltages were set at 450 V, 0 V, and 500 V, respectively.

The entrance and exit slits of the monochromator were set to 500 fim. The data shown took about 2 hours to accumulate

and are the sum of seven spectral scans, each one with a

different time between unloading the trap, ranging from 5 ms

to 60 ms. We identify the peak at 384 nm to be the 3j^3p^^P,<— transition in Kr XXIII and the peak at 346.47 nm to be the 35^3p^ ^D3/2<— ^Dj/j transition in Kr XXII. Most of the remaining spectrum, except the peak at 402.58 nm, can be attributed to Kr ll and Kr III. The 384-nm

line was predicted to be at 383.2(40) by Kaufman and Sugar

[9] and observed first by Roberts et al. [20] in a tokamak.

The line at 346.47 nm has not been previously observed but was predicted [9] to be at 344.6(30) nm. Only one other

Af 1 line is predicted in Ref. [9] for the range of 300 nm to 400 nm, a hne at 313.4 nm which was not observed.

2000

n

'—\—'—

r

300 320 340 360 380 400 420 440 460 Wavelength (run)

FIG. 1. Survey spectrum from 300 nm to 460 nm taken during Kr gas injection at an electron beam energy of ~ 10 keV, and 500- ixm slits.

Extracted ion data are shown in Fig. 2, where ion beam

intensity is plotted as a function of analyzing magnetic field. The plot shows the different charge states present in the trap as well as their relative populations. The peaks corresponding to Kr XXIII and Kr xxil are indicated with arrows in the

plot. Weaker peaks near the main peaks indicate the presence

of less abundant Kr isotopes in the trap. The electron beam

energy used during extraction (10 keV) did not maximize the

relative populations of these charge states but allowed us to increase the electron beam current and thus the absolute sig- nal of these charge states compared to that obtained at lower

currents. This particular plot was obtained while emptying

the trap every 40 ms. Another way of aiding charge state identification is to compare the temporal buildup of the individual line intensities. Time resolved data (Fig. 3) were taken for the 384-nm

(Kr XXIII), 346.47-nm (Kr XXIl), 402.58-nm, and 429.3-nm

(Kr ll) lines with an electron beam energy of approximately 5 kV, a current of 84 mA, and a pressure of 2 X 10~^ Pa in the gas injection system. Data corresponding to different transi- tions have been normalized so that all the plots reach the same equilibrium value for long times. The plots show the

trap being emptied near the 20-ms mark. The 346.47-nm line shows a time evolution consistent with our charge state idenKrXXm KrXXn

0.120 0.130 0.140 0.150 0.160 0.170 Magnetic Field (Tesia)

0.180

FIG. 2. Extracted ion beam intensity vs magnetic field. The two

arrows show the peaks corresponding to Kr xxili and Kr xxii.

NIST-131
1834 SERPA, BELL, MEYER, GILLASPY, AND ROBERTS 55

I

' ' ' '

i ' ' ' '

I ' ' ' 'I

' ' ' '

I

' ' ' '

I

' ' ' '

I

' ' ' '

I

' ' ' ' I 10 15 20 25 30 35 40 45 50 time (ms)

FIG. 3. Time resolved spectral intensity of various lines. The

trap is being emptied near the 20-ms mark.

tification (Kr XXll), i.e., it rises just before the Kr XXlll line, as would be expected for a sequential ionization process such

as electron impact inside the EBIT. The data also suggest

that the unidentified line at 402.58 nm belongs to a charge

state lower than Kr XXII. Using the tabulated energy levels of

Kr [1 1] we searched for possible M1 transitions arising from

the ground state term and found no suitable candidate for this unidentified line. The lack of energy levels for the 3p^3d" (n = 2 to 8) charge states of Kr makes the identifi- cation difficult. A possible candidate for this line was pro- posed in a similar spectroscopic study [16] carried out in a

partially overlapping spectral region by the Livermore group

using their EBIT. The identification proposed by that group

is the magnetic dipole (Ml) 2p^2>d^P\^^P2 transition in Kr XIX. The upper level of tliis transition (^^2) is low

enough to be easily populated by direct electron impact excitation or through cascading, and because of its angular mo- mentum cannot decay to the ground state (2p^) via . Calculated energy levels [17] as well as solar coronal

identification of lines [18] arising from the first excited configuration of the Ar isoelectronic sequence exist in the litera- ture but only up to Ar-Iike Ni. The extrapolated energy dif- ference to Kr XIX is in reasonable agreement with our measurement but further observations along the isoelectronic sequence are required for a definite identification. The time

behavior of the Kr 11 line is consistent with a fast rise of this low charge state right after the middle drift tube voltage is raised and lowered. Scans of the lines at 346.47 nm and 402.58 nm were taken

with an electron beam energy of ~ 10 kV, a current of 141 mA, and 100-/im entrance and exit slits. Figure 4 shows two

such scans, both of which also show smaller features at lower wavelengths that we identify as Kr II and Kr ill. We

use these in situ lines for wavelength calibrations instead of

lines from a conventional stationary light source to eliminate

the uncertainty introduced by the wavelength shifts due to the placement of the stationary source outside the EBIT. In order to use these lower charge state lines for wavelength

calibration, the influence of the magnetic and electric field on

their transition wavelength should be considered. The effect of the magnetic field (Zeeman effect) is to remove the m

« 0 —'—I—'—'—'—I—I—'—'—I—'—I—I—I—I—'—

r

345 350 355 360

400 405 410 415 420 Wavelength (nm)

FIG. 4. Spectral scans taken with 100-/x,m entrance and exit

slits. Peaks are labeled with their respective charge state identification.

degeneracy. Considering that only transitions with |Am| = I are allowed for E 1 and M1 transitions, we estimate that the magnetic components of the lines shift by approximately 0.027 nm and 0.075 nm (3 cm

~

' ) for lines at 300 imi and

500 nm, respectively. These shifts are synmietric so there is no net shift but an overall broadening of the line [19]. We

estimate this broadening due to the Zeeman effect to be

about 6 cm~'. To consider the effects of the electric field (Stark effect) inside the EBIT we estimate the electron density to be ~4X10'2 cm"^ (7X10~' C/m^) and conclude

that the maximum macroscopic electric field from the elec- tron beam is ~ I X 10^ kV/m. The actual total electric field will be smaller than this, depending on the degree of neutral- ization of the trap. Assuming 20% neutralization of the trap, we obtain a value of 8 X 10^ kV/m. Another possible contri- bution to the electric field comes from the voltage differential between the central drift tube and the endcaps. Assuming

a potential difference of about 400 V in a distance of about

one-tenth of the trap length (0.2 cm), we obtain an electric

field of about 200 kV/m in the small region of the voltage

drop. This is smaller than the electron beam contribution. The Stark effect does not produce a shift in first order (ex- cept for hydrogen), so in general the shifts are proportional

to the square of the electric field. The Kr lines used

for calibration were sV'*('D)55*-sV('£>)5p and

s''-p\'^P)5s^s'^p\^P)5p for Kr ll, and s^p^{^D)4d

-i^D)5p and s^p^CS)5s-(*S)5p for Kr III. Di Rocco

et al. [21] have measured line shifts due to the microscopic

Stark effect in similar lines of Kr II and Kr III in a pinched

TABLE I. Different contributions to the uncertainties. All val- ues quoted in nanometers. The final uncertainties (l-o- level) are computed by combining the individual uncertainties in quadrature.

Line (nm) Calibration/Fitting Reproducibility Uncertainty

346.47 0.05 0.04 0.06 , 402.58 0.02 0.04 0.05

lSnST-132
55 Kr SPECTRA FROM AN ELECTRON-BEAM ION TRAP: 1835

discharge. They estimate the electric field to be 5 X 10'* kV/m. All the shifts are smaller than 1 cm~'. Using this value and the fact that the shifts are proportional to the square of the electric field, we estimate negligible shifts due

to the Stark effect in the EBIT for low charge states of Kr. By using the low charge states of Kr as wavelength cahbration lines, we obtain 346.47(6) nm for the Kr XXII line and 402.58(5) nm for the unidentified line. The quoted wavelengths are measured in air and the uncertainties are the result of adding in quadrature the uncertainties from the fits and reproducibility at the one standard deviation uncertainty

level. Table I shows the different contributions to the final uncertainties.

In conclusion, we have shown how the EBIT is an important source not only of A/ 1 lines for highly charged ions, but

also for lines in moderately charged ions as well, even when

the electron beam energy is well beyond that required to optimize the production of the moderately charged ions. A

previously unobserved Ml line in Kr XXll was measured. The usefulness of the ion extraction system and time re- solved data as charge state identification tools was also shown.

ACKNOWLEDGMENTS

We would like to thank E. Trabert for helpful discussions.

[1] C. A. Morgan, F. G. Serpa, E. Takacs, E. S. Meyer, J. D.

Gillaspy, J. Sugar, J. R. Roberts, C. M. Brown, and U. Feld- man, Phys. Rev. Lett. 74, 1716 (1995).

[2] F. G. Serpa, E. S. Meyer, C. A. Morgan, J. D. Gillaspy, J. Sugar, J. R. Roberts, C. M. Brown, and U. Feldman, Phys. Rev. A 53, 2220 (1996).

[3] S. Suckewer and E. Hinnov, Phys. Rev. Lett. 41, 756 (1978).

[4] R. U. Datla, J. R. Roberts, W. L. Rowan, and J. B. Mann,

Phys. Rev. A 34, 4751 (1986).

[5] K. H. Burrell, R. J. Groebner, N. H. Brooks, and L. Rottler, Phys. Rev. A 29, 1343 (1984).

[6] B. Edlen, Ark. Mat. Astr. Fys. 28B, 1 (1941).

[7] B. Edlen, Z. Astrophys. 22, 30 (1942).

[8] U. Feldman, P. Indelicato, and J. Sugar, J. Opt. See. Am. B 8, 3 (1991).

[9] V. Kaufman and J. Sugar, J. Phys. Chem. Ref. Data 15, 1 (1986).

[10] T. Shirai, K. Okazaki, and J. Sugar, J. Phys. Chem. Ref. Data

24, 1577 (1995).

[1 1] J. Sugar and A. Musgrove, J. Phys. Chem. Ref. Data 20, 859

(1991). [12] A. I. Pikin, C. A. Morgan, E. W. Bell, L. P. Ratliff, D. A.

Church, and J. D. Gillaspy, Rev. Sci. Insmim. 67, 2528 (1996).

[13] J. D. Gillaspy, J. R. Roberts, C. M. Brown, and U. Feldman, in

Vlth International Conference on the Physics of Highly

Charged Ions, edited by P. Richard, M. Stockli, C. L. Cocke, and C. D. Lin, AIP Conf. Proc. No. 274 (AIP, New York,

1992), p. 682.

[14] R. E. Marts, P. Beiersdorfer, and D. Schneider, Phys. Today,

47(10), 27 (1994). [15] J. D. Gillaspy, Y. Aghtskiy, E. W. Bell, C. M. Brown, C. T. Chantler, R. D. Deslattes, U. Feldman, L. T. Hudson, J. M.

Laming, E. S. Meyer, C. A. Morgan, A. I. Pikin, J. R. Roberts,

L. P. Ratliff, F. G. Serpa, and E. Takacs, Phys. Scr. T59, 392

(1995). [16] J. R. Crespo Lopez-Urrutia, P. Beiersdorfer, K. Widmann, and

V. Decaux, in lOth APS Topical Conference on Atomic Processes in Plasmas, Program and Abstracts, A5 (Lawrence Liv- ermore National Laboratory, San Francisco, 1996).

[17] W. J. Wagner and L. L. House, Astrophys. J. 166, 683 (1971). [18] L. A. Svensson, J. O. Ekberg, and B. Edlen, Solar Phys. 34, 173 (1974).

[19] I. L Sobelman, Atomic Spectra and Radiative Transitions, 2nd

ed. (Springer Verlag, Berlin, 1992). [20] J. R. Roberts, T. L. Pittman, J. Sugar, V. Kaufman, and W. L. Rowan, Phys. Rev. A 35, 2591 (1987). [21] H. O. Di Rocco, G. Bertuccelli, J. Reyna Almandos, F. Bredice, and M. Gallardo, J. Quant. Spectrosc. Radiat. Trans. 41, 161 (1989).

ISnST-133
PHYSICAL REVIEW A VOLUME 55, NUMBER 6 JUNE 1997

Measurement of a magnetic-dipole transition probability in Xe

using an electron-beam ion trap

F. G. Serpa,* C. A. Morgan, E. S. Meyer,^ and J. D. Gillaspy

Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-0001

E. Trabert

Experimentalphysik III, Ruhr-Universitdt Bochum, D-44780 Bochum, Germany

D. A. Church

Physics Department, Texas A&M University, College Station, Texas 77843-4242

E. Takacs

Experimental Physics Department, Kossuth University, Bern ter 18/a, Debrecen H-4026, Hungary

and Institute for Nuclear Research of the Hungarian Academy of Sciencies (ATOMKI), Bern ter 18/c, Debrecen, H-4026, Hungary

(Received 21 January 1996)

The transition probability for the 3J*^D2*— magnetic-dipole transition in Ti-like Xe (Xe^^^) has been measured using an electron-beam ion trap. The unusually weak dependence of the transition energy on nuclear charge Z, and the fact that the ttansition wavelength remains in the 320- to 400-nm range for 54<Z<92,

makes this transition promising as a plasma diagnostic tool. Our measurement of the transition probability

yields 465(30) s"', corresponding to a lifetime of 2.15(14) ms, in good agreement with the theoretical value of 2.4 ms. [81050-2947(97)07506-9]

PACS number(s): 32.30.Jc, 32.10.-f, 52.70.-m, 31.25.-v

I. INTRODUCTION

The transition probability between atomic levels is needed

in order to determine particle densities from absolute emission or absorption measurements. In plasma diagnostics to avoid the complications of self-absorption, optically thin

lines are preferred, that is, regular lines of low transition probability, like spin-forbidden intercombination or electric- dipole-forbidden magnetic-dipole (Ml) transitions. It is also preferred that the lines be in the visible or near-uv range of

the spectrum, so that high-resolution interferometric tech- niques can be employed in order to derive a maximum of

information, including line shapes and widths. Such Ml transitions, without faster competing decays, oc- cur in the ground configurations of many ions. These then

require relatively low excitation energies and thus are easily excited in plasmas. Many Ml transitions have been observed

in Tokamak spectra where their long wavelengths have been

used for the determination of quantities, such as ion temperature [1,2]. Unfortunately, however, the wavelength of most Ml transitions varies strongly with the nuclear charge of the ion along a given isoelectronic sequence, as the associated

level intervals re.sult from the relativistic fine-structure split- ting with its Z"* scaling in simple systems. This makes the search for, and identification of, such Ml transitions difficult for ions beyond those rather simple cases for which the identification of solar corona lines was possible [3-6]. However,

*Permanent address: University of Notre Dame, Notre Dame, IN

46556. ^Permanent address: D. E. Shaw Co., 120 West 45th, New York,

NY 10036.

1 050-2947/97/55 (6)/4 1 96(5)/S 1 0.00 55

in a lengthy search of computed data Feldman, Indelicato,

and Sugar found a particular system in which a single Ml

transition had an almost constant wavelength and transition probability over a wide range of nuclear charges [7]. This

transition, 3d'^^D2^^Di in Ti-like ions (Fig. 1), was subse- quently identified in spectra recorded at the National Institute of Standards and Technology (NIST) electron-beam ion trap (EBIT) facility [8,9] at Gaithersburg. In this work we report a measurement of the associated transition probability, pre- dicted to be about 420 s "

' [10] that corresponds to a lifetime of about 2.4 ms. This prediction was obtained with the Cowan code using the scaled parameters for the radial inte- grals obtained in Ref. [9]. A similar calculation using the multiconfigurational Dirac-Fock GRASP^ code [11] without any scaling of radial integrals yields 1 .97 ms for the lifetime.

J

3

4 —i 2 3,4 S^^

1

0 3d ground state

FIG. 1. Lowest levels of a Ti-like ion, indicating the

3d'* 'D2«— Ml transition of interest at 413.94 nm

4196 © 1997 The American Physical Society

NIST- 134
55 MEASUREMENT OF A MAGNETIC-DIPOLE TRANSITION 4197

The standard method for lifetime measurements in highly

charged ions, beam-foil spectroscopy, does not permit the measurement of such long lifetimes because it would require

extremely long flight paths [12]. The technique of slow ion beams [13] needs to be developed further before it can be used for the ms time range. Hence, the present options are

the use of an ion storage ring [14-16], ion retrapping from

an ion source [17,18], or an EBIT. The last approach was

chosen here. For lifetime measurements using an EBIT, two ap- proaches have been developed. The first relies on trapping

highly charged ions by the electric and magnetic fields pro- duced by the electron beam, drift tube potentials, and the superconducting magnet ("electron trapping mode" [19]). The ion excitation process is modulated for sufficiently separated levels by tuning the electron-beam energy above or below the excitation threshold. Lifetime measurements on

the l52j^5i level in several He-like ions have been per- formed at Lawrence Livermore National Laboratory (LLNL)

by exploiting this technique [20]. However, this technique does not work for transitions between levels of the ground

configuration, because here the level excitation threshold can

be insignificant compared to that for production of the ion

itself. A second approach, discussed in Ref. [21], has been de- veloped in parallel at LLNL and at NIST. In this technique,

the electron beam current is temporarily switched off so that the ions are confined radially only by the magnetic field, as

in a Penning trap, while longitudinal confinement is provided

by the drift tube potentials (the EBIT "magnetic trapping

mode" [22,19]). The LLNL group has done extensive sys- tematic tests on this confinement mode. In particular, they

have shown, using an ion-cyclotron resonance probe and

x-ray spectroscopy [22-25], that most of the ions remain in the trap immediately after the beam is switched off, but

eventually leave the trap with a time constant of greater than

1.5 s. They also confirmed the validity of the lifetime measurement technique on N^"*" by comparing the result obtained in an EBIT [3.92(13) ms at the one standard deviation

level] to the accurate value previously obtained in a storage

ring experiment [3.90(5) ms also at the one standard deviation level] [19,14].

n. EXPERIMENTAL SETUP

The EBIT at NIST [26,27] was used as the source of

excited highly charged ions in this work. In this source, the

desired ion is produced by successive electron-impact ionization from an accelerated electron beam, produced by a electron gun which provides currents up to —150 mA. A pair of

superconducting magnets in a Helmholtz configuration pro- duces an axial magnetic field of 3 T in the trap region and

compresses the electron beam to a diameter of —60 /xm,

resulting in a current density of —5000 A/cm^. Together the electron beam and the magnetic field produce a radial trap

for the ions. Axial trapping of the ions along the electron- beam axis is provided by raising the two ends of three collinear, insulated drift tubes to a positive potential with re- spect to the center drift tube bias potential. This traps a

cylindrical ion cloud —30 mm long and —200 /j.m in diameter [28], oriented along the direction of the electron beam.

A variable bias voltage of 2 to 20 kV is applied to the drift tubes to define an accelerating potential for the electron beam. A correction on the order of 100 eV must be applied

to this energy in order to account for the net space charge of

the electrons and the positive ions in the trap. The precise energy of the electron beam ( — 50 eV width) is therefore determined from the accelerating bias potential, the center

drift tube floating voltage and the space-charge correction. By adjusting the accelerating voltage slightly below the ionization energy of the desired ion, we can optimize the rela- tive population of a specific charge state. A gas injection system connected to a lateral port that looks directly into the trap was used to introduce xenon at- oms into the EBIT from a direction perpendicular to the trap. The gas injection system consists of a tunable gas leak, two pump chambers, and three collinear apertures. The apertures

are used to define the differential pumping of the chambers

and for rough alignment of the gas stream. The first aperture

(0.318 cm diameter) is located 67.7 cm from the trap and

separates the gas leak from the first chamber. The second

aperture (1.27 cm diameter) is located 44.5 cm from the trap and separates the first and second chambers. The last aperture (0.318 cm diameter) is located 32.1 cm from the trap and separates the second chamber and the injection port to the EBIT. The gas leak allows for tuning of the amount of gas injected, which is monitored by an ion gauge located in the chamber between the first and second apertures. Each

chamber is pumped by a turbo pump with a pumping speed

of about 50 1/s. The detection system is described in detail elsewhere

[8,9]. Briefly, a cooled, low noise photomultiplier was used

for photon detection, and a scanning monochromator was

used for wavelength selection. Radiation from the trap was

imaged onto the entrance slits of the monochromator by two

plano-convex lenses; this was necessary to keep the photomultiplier away from the stray magnetic field from the su- perconducting magnet. The two lens system produced a de- magnification of —0.8. The entire optical table which

supported the monochromator was attached to two perpendicular translation stages which could be positioned with a precision better than 10 /xm. These translation stages were

used to place the monochromator' s entrance slit at the image

of the trap produced by the lens system. A Si(Li) detector with an energy resolution of — 190 eV, was used to monitor

the X rays through one of the radial ports.

III. LIFETIME MEASUREMENT

For the lifetime measurements, we set the drift tube volt- age that defines the bulk of the electron-beam energy to 2.172 keV in order to obtain a strong signal from the Ti-like Xe transition. The gas injection pressure of Xe in the first gas

injection chamber was changed from run to run. This pres- sure was varied between 2XlO~^Pa( — 2X10"^ Torr) and

4X 10""^ Pa (~4X 10"^ Torr) of Xe, but did not signifi- cantly affect the pressure in the EBIT outside of the trap region, which remained lower than — 5X10~* Pa

( — 5 X 10" '° Torr). A typical electron-beam current for this experiment was about 60 mA. The anode voltage that defines

the electron-beam current was driven by a function generator

that switched the current off in about 50 /zs, kept it at zero

]^ST-135
4198 F. G. SERPA et al. 55

ltll.^A>ilLll.Ltd..

beam off

... 1

beam on Mi

, ,

I

.

1 1 ] 1 1 r 1 ^1 1 1 [ ^ .

i,...^,.|,,,.,,ji.M...|..,^.,..fW

,

, , ,

1 20 30 40 50 60 70

Time (ms)

FIG. 2. (a) Time evolution of the visible light signal from Tilike Xe. The electron-beam current was switched off and on where

indicated by the arrows, (b) X-ray time dependence.

for about 20 ms to allow the transition to decay, and finally

switched back on in about 50 /u.s. The current was then held

constant for about 200 ms to allow the signal to recover. Time-resolved data were acquired by setting the monochromator to the transition wavelength and accumulating syn- chronized data over many thousands of cycles. The entrance and exit slits of the monochromator were set at 500 ixm. Photons arriving at the photomultiplier or x rays detected by

the Si(Li) detector were time stamped with a resolution better than 50 /xs. The timing electronics were constructed from

computer automated measurement of control (CAMAC)

modules controlled by a list processor interfaced to a computer.

IV. DATA

Data were recorded in both the visible and x-ray spectral ranges. While the visible light, detected in narrow band, of- fered information on the long-lived level of interest, the x

rays, detected in broadband, reflected the total ion population

in the trap while the beam was on rather than the charge state and level of interest. For a sample of both the x rays and

visible light, see Fig. 2. The short lifetimes associated with

the X rays allow us to confirm that the electron beam was

switched off rapidly. In an EBIT, a multitude of processes

build up to a steady state which yields the extended flat part of our signal curve. When the electron-beam current is sud- denly switched off, so is the ion excitation. Consequently,

the signal drops, with a time constant corresponding to the

lifetime of the observed level. In addition, there may be an

initial rapid drop due to ions moving out of the observation

region. If all the ion cloud dynamics were reversible in 50

/xs, the signal recovery after suddenly switching the electron- beam current back on would be much more rapid than the observations. For extraction of the atomic lifetime, we evaluate only the decay part of the signal curve; we discuss the recovery part of the curve below.

V. DECAY CURVE EVALUATION

The decay curve of the visible signal shows an initial steep fall that extends over about 0. 1 ms. We note that this signal loss is over an order of magnitude less than that seen

100

60

20

120 ^80

§o40

U

80

6040

20

0

bkli nil ki Ik fm 1.78 ms

1

1

1 b

1

1 2.11 ms

1 1 #4^f*l 2.03 ms

^

5 10 15 20 25 30 Time (ms)

FIG. 3. Time evolution of the visible signal for three different positions of the entrance slit ( 200 /xm wide), (a) Centered on

electron beam. Integration time ~ 60 min. (b) Shifted by 200 /xm

from center. Integration time ~ 108 min. (c) Shifted by 400 /tm

from center. Integration time ~ 124 min. The dotted line indicates the time at which the electron beam was switched off.

in the LLNL experiments on nitrogen [22,19]. This steep fall can be interpreted either as an actual loss of ions from the

trap as the beam is switched off or as a loss of ions from the viewing region of the detection system. The latter would

arise from an ion cloud expansion when the electron-beam component of the trapping potential is switched off. Our ex- perimental setup permits access to a viewing region of up to ~ 2 mm in width. The 500-^m slit setting used for most of

the measurements restricts the viewing region to ~625

fxm. In order to observe the ion cloud expansion and quantify the effect of a restricted viewing region on the lifetimes we narrowed the slits to 200 ^m and translated the spec- trometer in a direction perpendicular to the viewing axis. Figure 3 shows three sets of data at different entrance slit positions and their repective fitted lifetimes. The expected

ion expansion, when the electron beam is switched off, can

be seen directly in Fig. 3(c). From the decay constants obtained from the data in these tests and the one from a run with 1-mm slits (2.01 ms), we obtain a standard deviation of 0.14 ms for the decay constant. This uncertainty of about 7%

includes the uncertainty due to reproducibility. The final quoted value for the lifetime is the average from five differ- ent data sets taken with 500-yum slits centered on the elec- tron beam. The standard deviation from these measurements

is also a measurement of the uncertainty due to reproducibility and agrees with the 0.14-ms uncertainty obtained above

from the different slit positions.

The data were fitted to a single exponential function plus

a background representing the detector dark rate. Successive

initial data points were omitted to study the influence of the rapid switching of the electron beam on the data evaluation;

after very few points were removed (corresponding to ~ 0.1 ms) the fit results became stable. Figure 4 shows a typical fit to a data set and the corresponding residuals. Similarly the

tail of the decay was truncated to find out about a hypothetical slow decay component (perhaps relating to ion loss from

NIST-136
55 MEASUREMENT OF A MAGNETIC-DIPOLE TRANSITION .

.

4199

FIG. 4. (a) Fit of one data set to a single exponential plus a constant background (;^'^=0.95). Same time scale as Fig. 2. (b) Normalized residuals {y fi,-y)/\fy.

the trap), but no second decay component could be extracted. By comparing fits from different runs we found no signifi- cant dependence for pressures below 4x10"'' Pa (~4X 10"^ Torr) at the gas injection chamber.

An extensive computation was made to assure that the

lifetime was not influenced by slow cascading from higher

levels. Since El transitions are much faster than Ml transitions, we restricted the simulation to configurations that could contribute to slow Ml transitions. Only levels of the

3d'^ configuration contribute to such transitions, since

the levels of 3p^ 3d^4s can decay to 3p^ 3d'^ through two

El transitions via the 3p^ 3d^ configuration. The GRASP^

code [1 1] was used to calculate the energy levels and transi- tion probabilities among the 34 levels belonging to 3p^ 3d'*. We assumed a range of initial populations [equally

populated, statistically populated (27+1), and proportional

to 7] for the levels and found that the predicted contributions

to the lifetime from cascading were smaller than 1%.

Based on the statistics, reproducibility, viewing region, and various truncation exercises discussed above we obtained a lifetime of 2.15(14) ms, at the one standard deviation level. This is considered to be in good agreement with a

theoretical value of 2.4 ms [10]. The lack of symmetry of the decay and the recovery part

of the signal upon switching the electron-beam current off and on indicates that the ions do not reassemble quickly into the electron beam where they can be reexcited. A variation between 5 and 40 ms of the time interval during which the electron beam is switched off yielded no recognizable differ- ences in this behavior. Apparently the ion cloud needs a few

hundred ms to reestablish the steady state. Such an effect suggests that there are irreversible processes at work, which

might involve geometry and ion cloud dynamics.

VI. CONCLUSION

The present study completes the quest [7] for, the identi- fication and wavelength measurement [8,9], and now the

transition rate measurement of a line particularly suited for plasma diagnostics of future high-temperature tokamaks. As

the theoretical expression for the probability of such Ml

transitions depends mostly on angular-momentum factors and the transition energy, this dependence has now been cor- roborated and a suitable benchmark been provided for similar tjansitions. It needs to be stressed, however, that ab initio calculations do not have enough accuracy to predict these

fine-structure intervals, and thus the Ml transition energies,

with anything close to spectroscopic accuracy. The magnetic trapping mode of EBIT operation has demonstrated its usefulness for lifetime measurements in the ms

range. Improvements that are anticipated for future work in- clude faster electron beam-current switching to measure

shorter lifetimes, online diagnostics to monitor the spatial distribution of the ion cloud in the trap, and further investi- gation and modeling of the ion cloud dynamics and its per- turbation by the electron-beam switching.

ACKNOWLEDGMENTS

We thank the LLNL EBIT group for communicating their findings prior to publication. This work has been partially

supported by a NATO Collaborative Research Grant No. CRG 940181. D.A. Church was given financial support from Texas A&M University. E. Takacs acknowledges the Foundation for Hungarian Higher Education and Scientific Research and the Hungarian Science Foundation (OTKA, Contract No. F 019344) and NIST for support.

[1] E. Hinnov, S. Suckewer, S. Cohen, and K. Sato, Phys. Rev. A

25,2293 (1994).

[2] B. Edlen, Phys. Scr. T8, 5 (1984).

[3] B. Edlen, Ark. Mat. Astron. Fys. 28B, 1 (1941).

[4] B. Edlen, Z, Astrophys. 22, 30 (1942).

[5] V. Kaufman and B. Edlen, J. Phys. Chem. Ref. Data 3, 825

(1974).

[6] V. Kaufman and J. Sugar, J. Phys. Chem, Ref. Data 15, 321

(1986).

[7] U. Feldman, P. Indelicate, and J. Sugar, J. Opt. Soc. Am. B 8, 3 (1991).

[8] C.A. Morgan, E.G. Serpa, E. Takacs, E.S. Meyer, J.D.

Gillaspy, J. Sugar, J.R. Roberts, CM. Brown, and U. Feldman,

Phys. Rev. Lett. 74, 1716 (1995).

[9] F.G. Serpa, E.S. Meyer, C.A. Morgan, J.D. Gillaspy, J. Sugar,

J.R. Roberts, CM. Brown, and U. Feldman, Phys. Rev. A 53, 2220 (1996). [10] J. Sugar (private communication).

[11] F.A. Parpia, I.P. Grant, and CF. Fischer, GRASP^ (private communication).

[12] E. Trabert, Nucl. Instrum. Methods B 98, 10 (1995). [13] G. Hubricht and E. Trabert, Z. Phys. D 7, 243 (1987). [14] H.T. Schmidt, P. Forck, M. Grieser, D. Habs, J. Kenntner, G.

Miersch, R. Repnow, U. Schramm, T. Schiissler, D. Schwalm,

and A. Wolf, Phys. Rev. Lett. 72, 1616 (1994). [15] J. Doerfert, E. Trabert, J. Granzow, A. Wolf, J. Kenntner, D. Habs, M. Grieser, T. Schii^ler, U. Schramm, and P. Forck, Nucl. Instrum. Methods B 98, 53 (1995). [16] J. Doerfert, E. Trabert, and A. Wolf, Hyperfine Int. 99, 155

(1996).

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[17] L. Yang and D.A. Church, Phys. Rev. Lett. 70, 3860 (1993). [18] L. Yang, D.A. Church, S. Tu, and J. Jin, Phys. Rev. A 50, 177

(1994) . [19] P. Beiersdorfer, L. Schweikhard, J. Crespo Lopez-Umitia, and

K. Widmann, Rev. Sci. Instrum. 67, 3818 (1996).

[20] B.J. Wargelin, P. Beiersdorfer, and S.M. Kahn, Phys. Rev.

Lett. 71, 2196 (1993). [21] J.D. Gillaspy, Phys. Scr. T65, 168 (1996).

[22] P. Beiersdorfer, B. Beck, St. Becker, and L. Schweikhard, Int.

J. Mass Spectrom. Ion Processes 157-158, 149 (1996).

[23] P. Beiersdorfer, St. Becker, B. Beck, S. Elliott. K. Widmann,

and L. Schweikhard, Nucl. Instrum. Methods B 98, 558

(1995) . [24] L. Schweikhard, J. Ziegler, P. Beiersdorfer, B. Beck, St. Becker, and S. Elliott, Rev. Sci. Instrum. 66, 448 (1995).

[25] P. Beiersdorfer, B. Beck, R.E. Marrs. S.R. Elliott, and L. Sch- weikhard, Mass Spectrom. 8, 141 (1994). [26] J.D. Gillaspy, J.R. Roberts, CM. Brown, and U. Feldman, in Vlth International Conference on the Physics of Highly

Charged Ions, edited by P. Richard, M. Stockli, C.L. Cocke, and CD. Lins, AIP Conf. Proc. No. 274 (AIP, New York,

1993), p. 682.

[27] R.E. Marrs, P. Beiersdorfer, and D. Schneider, Phys. Today

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Chantler, R.D. Deslattes, U. Feldman, L.T. Hudson, J.M. Laming, E.S. Meyer, CA. Morgan, A.I. Pikin, J.R. Roberts, L.P.

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NIST-138
J. Phys. B: At. Mol. Opt. Phys 31 (1998) 3345-3352. Printed in the UK PII: 50953-4075(98)91396-7

Lifetime measurements in the ground configuration of

Ar^^"^ and Kr^^^ using an electron beam ion trap

F G Serpat§, J D Gillaspyf and E Trabertf

t Atomic Physics Division. National Institute of Standards and Technology, Gaithersburg, MD

20899-0001, USA

I Experimentalphysik 111, Ruhr-Universitat Bochum, D-44780 Bochum, Germany

Received 6 February 1998

Abstract. We have measured the radiative lifetimes for the 2s^2p ^P3/2 level of Ar""*" and the 3s^3p^ ^P2 level of Kr^^"*^. These measurements were performed by monitoring the temporal behaviour of their associated radiative decays during magnetic trapping mode in an electron beam ion trap (EBIT). Our lifetime results. 8.7(5) ms for Ar'^"*" and 5.7(5) ms for Kr^^"*", are compared with theory.

1. Introduction

Radiation originating from the ground configuration of highly charged ions is an important

source of information for astrophysical and laboratory plasmas. In low-density plasmas, such

as those encountered in tokamaks, the solar corona and other astronomical objects, the main

decay channel for excited levels of the ground state configuration is through magnetic dipole

(Ml) transitions [1, 2]. The lifetimes associated with these electric-dipole (El) forbidden

transitions are much longer than those for levels with El -allowed transitions, and can be as long as many milliseconds. The traditional method for lifetime measurements, beam-foil, cannot be employed due to the extremely long flight paths which would be needed [3], while

the technique of slow ion beams [4] needs to be developed further before it can be used

for the millisecond time range. Heavy-ion storage rings, a tool recently utilized for lifetime

measurements in the millisecond range [5], may be severely limited by the available ion

beam currents for highly charged species and by geometrical constraints which cause a low

detection efficiency. Consequently, calculations of these transition probabilities have been

the main source for such Ml transition rate data until quite recently. Many such calculations,

involving different approaches, have appeared in the literature [6-12], but the dearth of

measurements has made comparison with experiment difficult. Thus, the measurement of

these transition probabilities is crucial not only in providing the experimental data needed

for the understanding and modelling of plasma sources such as fusion devices or the solar corona, but also in testing sophisticated calculations. Such measurements have become

possible only quite recently [13-15]. In a previous paper [15] (paper I), we reported one

such measurement in an electron beam ion trap (EBIT) that relied on the trapping of ions

in the 'magnetic trapping mode' [16-18]. In this paper we report two new measurements

in highly charged ions that determine the lifetime of the 2s^2p ^Py2 level in the B-like ion

Ar'-^+ and of the 3s23p2 ^Pj level in the Si-like ion Kr22+.

ij Permanent address: University of Notre Dame, Notre Dame, IN 46556. USA.

0953-407.5/98/153345+08$19.50 © 1998 lOP Publishing Ltd 3345

lSfIST-139
3346

320 340 360 380 400 420 440 460 480

Wavelength (nm)

Figure 1. Survey scan from 320 to 480 nm. The scan was taken at an electron beam energy of

3.7 keV and at an Ar pressure of 1.3 x lO"' Pa (1 x 10"^ Torr) in the gas injection chamber.

The ground state configuration of Ar' ^''' 2s^2p consists of two levels, ^Py2 and ^Pi/2.

These levels are degenerate in the non-relativistic limit, but split due to relativistic effects

(fine structure). Since these states have the same parity, the upper level cannot decay

radiatively through an El transition so it decays to the level either via an Ml or an electric quadrupole (E2) transition. The corresponding spectral line at 441.24 nm was

first observed by Lyot et al [19] in the solar corona and subsequently identified by Edlen

[20]. Since the Ml decay is about five orders of magnitude faster than the E2 decay, the

lifetime of the ^P3/2 state is essentially the inverse of the transition probability of the Ml

branch. This transition probability is predicted [7] to be ~103.97 s~', which translates into a lifetime of ~ 9.62 ms. We expect the transition to be prominent since higher excited states can only decay into these two states. Another advantage of the simplicity of this system is that there are no other long-lived low-lying levels. This precludes the possibility of slow

cascades from within the ground term and therefore facilitates the direct measurement of the associated lifetime. A survey scan from 320 to 480 nm during Ar gas injection is shown in figure 1. The scan shows the prominence of the -P3/2 —> ^Pi/2 transition and some weaker

Aril and Ar ill lines. The ground state configuration of Kr^-"*" 3s^3p^ consists of five levels. Again, all these

levels have the same parity, so that the levels can only decay via Ml or E2 transitions. Figure 2 shows a level diagram of these states, together with their main decay channels

and associated transition probabilities, taken from Biemont et al [8]. The ^P2 level decays

primarily to the ''Pi level via an Ml transition (150 s~') as well as a slower (4.83 s~')

E2 decay component to the •'Pq level. These two decay branches translate into a predicted

lifetime of 6.46 ms for the ^P2 state. The ''P2-^Pi transition was first observed and identified by Roberts et a/ [21] in a tokamak at 384.09(3) nm. This transition is particularly interesting

because it is the only one in the 3s-3p- ground state configuration that remains above

~90 nm as nuclear charge (Z) is increased along the isoelectronic sequence (284.1 nm at Mo28+; ~130 nm at W^°+) [8]. This makes it a candidate for high-resolution observation for plasma diagnostics since visible and near-UV transitions can usually be measured accurately

NIST-140
Lifetime measurements ofAr''^'*' and Kr^^'*' 3347

35i

J O 1 2

Figure 2. Diagram showing the five levels of the ground configuration of Kr^^+, their decay

type and calculated [8] transition probabilities (in s~'). In the case where a level can decay

through Ml and E2 the fastest branch is indicated.

and because refractive optics and polarizers can be employed efficiently. This transition has

also been shown to be prominent in EBIT [22]. Figure 2 also shows possible cascade effects

into the state from the 'So and 'D2 states. The 'D2 level decays primarily to the •^P2 and

^Pi levels with a transition probability of about 8000 s~' for each branch; this is significantly

faster than the transition probability associated with the ^P2 (~150 s~') decay. The 'So level has two main decay channels, to -'P2 and ""Pi, the latter being much faster (~ 51 100 s~'), and

therefore should not affect the measurement significantly. Nevertheless, in order to assess

the influence of these cascades in our measurement, we modelled the system including all five levels and the possible transitions connecting them. We used different models for the

initial populations of the levels (equally populated, statistically populated and populated

proportional to 7 4- 1) and found no significant effects concerning the decay of interest at the 1% level.

2. Experimental set-up and procedure

The experimental procedure and set-up was the same as the one described previously [23].

The EBIT at NIST [24, 25] was used as the source of excited highly charged ions. In

this source, ions in the desired charge state are produced from neutral atoms by successive

electron-impact ionization effected by collisions with the accelerated electrons from an

electron gun which provides currents of up to ~ 150 mA. A pair of superconducting magnets

NIST-141
3348 F G Serpa et al

in a Helmholtz configuration produce an axial magnetic field of 3 T in the trap region and

compress the electron beam to a diameter of ~ 60 /i.m, resulting in a current density of

~5000 A cm~^. Together, the electron beam (by its attractive Coulomb potential and the

partial space-charge compensation) and the magnetic field (via Lorentz forces) provide a

radial trap for the ions. Axial trapping of the ions along the electron beam axis is provided

by raising the two end tubes of three collinear, insulated drift tubes to a positive potential

with respect to the centre drift tube bias potential. This traps a cylindrical ion cloud

~30 mm long and ~200 /zm in diameter [26], which is oriented along the direction of

the electron beam. A variable bias voltage of ~1.5 kV to ~20 kV is applied to the drift tubes to define an accelerating potential for the electron beam. A correction of the order

of 100 eV must be applied to this energy in order to account for the net space charge of

the electrons and the positive ions in the trap. The precise energy of the electron beam

(~50 eV width) is therefore determined from the accelerating bias potential, the centre

drift tube floating voltage and the space-charge correction. By adjusting the accelerating

voltage slightly below the ionization energy of the desired ion, we can optimize the relative population of a specific charge state. A gas injection system was connected to a lateral port

that looks directly into the trap and is used to introduce neutral krypton and argon atoms

into the EBIT from a direction perpendicular to the trap axis. The detection system has also been described in detail elsewhere [27]. In brief, a cooled,

low-noise photomultiplier was used for photon detection, and a scanning monochromator was used for wavelength selection. Radiation from the trap was imaged onto the entrance

slit of the monochromator by two plano-convex lenses; this was necessary in order to keep

the photomultiplier away from the stray magnetic field of the superconducting magnet.

The two-lens system produced a demagnification of ~0.8. The entire optical table which

supported the monochromator was attached to two perpendicular translation stages which

could be positioned with a precision of better than 10 fxm. These translation stages were

used to place the monochromator's entrance slit at the image of the trap produced by the

lens system. A Si(Li) detector with an energy resolution of ~190 eV was used to monitor

the x-rays from the same trap volume through one of the radial ports.

3. Lifetime measurement

The ionization potential of Ar'-*"*" is 756 eV. Trying to maximize the relative population of

this charge state by setting the electron beam energy slightly below this ionization potential

will force the EBIT to run at low electron beam current. We opted to set the electron beam

energy to much higher values in order to increase the current and therefore to obtain a

stronger overall signal from the transition. Different electron beam energies were used for Ar in order to search for possible systematic effects. Typical values for the electron beam

energy and collector current were 1.5 keV at 30 mA, 8 keV at 118.6 mA and 5 keV at 70.2 mA. In the case of Kr^^"*", the ionization potential is 1 .05 keV. Therefore, it was possible

to run at electron energies close to the energy at which the relative population of Kr^^+

is maximized. A typical electron beam energy and collector current for Kr was 1.4 keV

at 30 mA. The gas injection pressure of Ar and Kr in the first chamber of the injection

system was changed from run to run. This pressure was varied between 6.6 x 10"^ Pa

(5 X 10-' Torr) and 4 x 10"'* Pa (3 x 10"^ Torr), but did not significantly affect the pressure

in the EBIT above the trap region, which remained lower than 6.6 x 10"* Pa (5 x 10-'° Torr).

The anode voltage that defines the electron beam current was driven by a function generator

that switched the current off in about 50 /xs, kept it at zero for about 80 ms to allow the

level population to decay and finally switched it back on within about 50 pts. The current

lSfIST-142
Lifetime measurements of Ar'-^'^ and Kt^^* 3349

was then held constant for about 280 ms to allow the signal to recover. Time-resolved data were acquired by setting the monochromator to the transition wavelength and accumulating

synchronized data over many thousands of cycles using an 'event mode' data acquisition

system [15]. The entrance and exit slits of the monochromator were set to 500 /zm, which

translates into a bandpass of 1.3 nm. Photons arriving at the photomultiplier or x-rays detected by the Si(Li) detector were time-stamped with a resolution of better than 50 /zs.

4. Data analysis

Data sets (see figure 3) show the temporal behaviour of the signal observed, in this case on the 2s^2p ^P3/2 -> ^Pi/2 transition in Ar'-*"^. The figure shows an initial fast decay

immediately after the electron beam is switched off at time r ~ 19 ms. Then the signal

decays exponentially for about 50 ms and begins to recover at r ~ 72 ms when the beam is switched back on. The observed decay curves where fitted by either one or two exponentials

plus a constant background and covered a time interval of about six lifetimes of the level of

interest. Both single- and double-exponential fits gave similar results for the primary decay

component within the quoted uncertainties. X-ray data from the SiLi detector were used to determine the exact time at which the switching occurs by observing the sudden decay of

the prompt x-rays. In order to take into account the initial drop in signal, the data were

truncated stepwise until the fitted lifetime value stabilized. Typically, after about less than

1 ms the fit was stable and produced reasonable reduced chi-squared values near unity. In order to check for collisional quenching and other effects, results from the different runs were compared and showed no dependence on beam energy or on pressure over the range

studied. Figure 4 shows the lifetimes values obtained from different runs as a function of

pressure for the 2s^2p ^P3/2 level in Ar'^"*". Figure 5 shows the spread for the fitted lifetimes for the 3s^3p^ ''P2 level in Kr^^"*". The results of the measurements are summarized in table 1, where calculated values have been included for compjirison. Since the transition probabilities depend on the third power of the energy separation, a second column has

been added to the calculations where the transition probabilities have been modified by

300

0

0 10 20 30 40 50 60 70 80 90 100

Time (ms)

Figure 3. Temporal dependence of the light emitted at 441.24 nm from the -P3/2 ^P|/2

transition in Ar'^"*^.

NIST-143
3350 F G Serpa et al

9.5

^

9.0

E 8.5-1

8.0

7.5

, 1

100 1000

Pressure (10^ Pa

)

Figure 4. Lifetime as a function of gas injection pressure for the ^P3/2 state of Ar'^"''. Pressure shown on a logarithmic scale.

7.5

7.0

6.5-1

6.0

5.5

5.0

e5*

4.5

2 3 4

Run Number

Figure 5. Fitted lifetimes for the 3s-3p^ state of Kr^^"*" obtained from different runs. The

error bars shown for each point are the uncertainties from the fitting procedure.

using the accepted experimental value for the transition energies, in the cases where a

discrepancy between the calculations and experiment would make a difference. Our value

for the radiative lifetime of the 2s-2p ^?y/2 level in Ar'-*"^ is about one and a half standard

uncertainties [28] lower than the available calculations. Results are quoted to one standard

uncertainty, obtained from the standard deviation from the weighted average of the lifetimes obtained from different runs, so that fitting and reproducibility effects are included. The

experimental radiative lifetime for the 3s^3p- 'P2 level in Kr^^"*" is lower than the result of

NIST-144
Lifetime measurements of Ar'-^'^ and Kr^^'^ 3351

Table 1. Radiative lifetimes for the 3s^3p- -^P: level of Kr^^+ and the 2s^2p ^Pj/: level of

Ar'-'+. The final column shows the modified calculated radiated lifetimes obtained from the

transition probabilities, where the experimental values for the energy separations have been

used instead of the calculated ones.

Ion Level Experiment (ms) Calculation (ms) Mod.

Ar'-'- Zs^Zp 2p.v2 8.7(5)^ 9.62'' 9.5 ^

9.41'' 9.5%'^

Kr22+ 3s23p^- 5.7(5)" 5.83=8

6.46fs

6.(}9''i 6.78fs

" This work.

^ Verhey [7]. Wavelength not available in the paper. Froese Fischer [6]. Modification did not affect the result. Cheng [12]. Huang [10].

' Biemont [8],

!^ Radiative lifetimes shown are the inverse of the sum of the transition probabilities to the ^P| and the •'?» states.

relativistic calculations of Biemont [8] by about one and a half standard uncertainties, but

it agrees well with a more recent calculation by Huang [10] that includes relativistic effects as well as the Breit interaction and Lamb shift contributions. Unfortunately Huang's energy

separations disagree with experiment by as much as 5%, so that the modified calculated

transition probability is higher than our measurement by about one and a half standard

uncertainties.

5. Conclusion

We have reported two new lifetime measurements of excited states within the ground

configuration of Ar'-'"'" and Kr^^"*" using an electron beam ion trap. These results, with

uncertainties at the ~ 8% level, provide data in a region where measurements are scarce. We find that our results for the total radiative lifetimes are about one and a half standard

uncertainties below the theoretical values in both cases.

Acknowledgment

This work has been partially supported by a NATO Collaborative Research grant no CRG

940181.

References

[I] Hinnov E, Suckewer S, Cohen S and Sato K 1994 Phys. Rev. A 25 2293

[2] Edien B 1984 Phys. Scr. T 8 5

[3] Trabert E 1995 Nucl. lustrum. Methods B 98 10

[4] Hubricht G and Trabert E 1987 Z Phys. D 7 243

[5] Doerfen J, Trabert E. Wolf A, Schwalm D and Uwira O 1977 Phys. Rev. A 78 4355

[6] Froese Fischer C 1983 J. Phys. B: At. Mol. Phy.s. 16 157

[7] Verhey T R, Das B P and Perger W F 1987 J. Phys. B: At. Mol. Phys. 20 3639

[8] Biemont E and Bromage G E 1983 Mon. Not. R. A.stron. Soc. 205 1085

[9] Sugar J and Kaufman V 1984 / Opt. Soc. Aw. B 1 218

NIST-145
3352 F C Serpa et al

[10] Huang K-N 1985 At. Data Niicl. Data Tables 32 503

[II] Curtis L J 1989 Phyx. Rev. A 40 6958

[12] Cheng K T. Kim Y-K and Desclaux J P 1979 At. Data Nucl. Data Tables 24 1 1

1

[13] Yang L and Church D A 1993 Phy.'!. Rev. Lett. 70 3860

[14] Yang L. Church D A. Tu S and Jin J 1994 Phys. Rev. A 50 177

[15] Serpa F G, Morgan C A. Meyer E S. Gillaspy J D, Triibert E, Church D A and Takacs E 1997 Phys. Rev. A

55 4196

[16] Beiersdorfer P. Beck B, Becker St and Schweikhard L 1996 Int. J. Mass Spectrom. Ion Processes 157-158

149

[17] Beiersdorfer P, Schweikhard L, Crespo Lopez- Urrutia J and Widmann K 1996 Rev. Sci. Instrum. 67 3818

[18] Gillaspy J D 1996 Phys. Scr. T 65 168

[19] Lyot B and Dollfus A 1953 C. R. Acad. Sci.. Paris 237 855

[20] Edl^n B 1954 Mon. Not. R. A.uron. Sac. 114 700

[21] Roberts J R, Pittman T L, Sugar J, Kaufman V and Rowan W L 1987 Phy.\. Rev. A 35 2591

[22] Serpa F G. Bell E W, Meyer E S. Gillaspy J D and Roberts J R 1997 Phy.i. Rev. A 55 1832

[23] Serpa F G. Meyer E S, Morgan C A. Gillaspy J D. Sugar J, Roberts J R, Brown C M and Feldman U 1996

Phy.i. Rev. A 53 2220

[24] Gillaspy J D 1997 Phy.t. Scr. T 71 99

[25] Marrs R E. Beiersdorfer P and Schneider D 1994 Phys. Today 47 27

[26] Gillaspy J D et al 1995 Phy.i. Scr. T 59 392

[27] Morgan C A. Serpa F G, Takacs E, Meyer E S, Gillaspy J D, Sugar J. Roberts J R, Brown C M and

Feldman U 1995 Phys. Rev. Lett. 74 1716

[28] Taylor B N and Kuyatt C E 1994 Guidelines for evaluating and expressing the uncertainly of NIST

measurement results NIST Technical Note 1 297

NIST- 146
PHYSICAL REVIEW A, VOLUME 61, 054501 UV light from the ground term of Ti-like ytterbium, tungsten, and bismuth

J. V. Porto, L Kink, and J. D. Gillaspy

National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8421

(Received 24 September 1999; published 7 April 2000)

We have used an electron-beam ion trap to measure the wavelength of the J = 2-3 magnetic dipole transi- tion in the Sd" ground term of Ti-like ytterbium, tungsten, and bismuth. This fills in a gap in previous measurements along this isoelectronic sequence, as well as extending previous work to the highest Z yet. With

the addition of our results, measurements of this line now cover a sufficient range of Z to allow an interpolation of reliable wavelength estimates for the unmeasured members of the isoelectronic sequence from Z= 52 to 83. We provide a table of these wavelengths for each member of the sequence in this range, and compare the measured wavelengths to recent calculations. Our results show that a long-standing discrepancy between

prediction and experiment disappears in the high-Z limit. PACS number(s): 32.30.Jc, 32.10.-f, 52.70-m, 31.25.-v

Since Feldman, Indelicato, and Sugar made the surprising

prediction [1] that a visible-near-uv transition in the ground

term of the Ti-like isoelectronic sequence was nearly Z inde- pendent over the upper half of the Periodic Table, several experimental measurements were made to test this calculation [2-4]. The predicted line arises from an Ml transition between the lowest 7 = 2 and 3 levels of the 3d'* ground

configuration, and the radiative decay rates are high enough

to make the Une reasonably strong under most plasma conditions. In addition to being a curiosity (transitions of this

sort typically scale as Z'*), there is significant interest in these lines because their unexpected weak dependence on Z

makes them potentially important for plasma diagnostics

[1-5]. The unusual nature of these lines also makes them

useful for testing relatively uncharted aspects of many-body

atomic calculations. The original Dirac-Fock calculations, which included correlations among all Af-shell electrons, disagree with experi- ment by as much as 5% for Z—60 [3]. This is a surprisingly

large discrepancy for such a relativistic calculation of a

highly charged ion, the error presumably arising from missing correlation effects. Subsequent calculations [6] for 54

^Z^64, which included correlations via excitations to vir- tual subshells, reduced the discrepancy in this range of Z to = 1.5%. In recent years relativistic many-body atomic calculations have improved dramatically [7,8] and it is essential to have high-quality experimental data over as wide a range of Z as possible to compare with the calculations. Systems like the Ti-Uke sequence, where many-body effects are very important and the visible transition can be measured accurately,

are particularly well suited as tests of the theory. In addition,

since the 7=2-3 optical transition (A£=3 eV) arises from

a small difference between relatively large energies {E = 80 eV for tungsten), the Ti-like system serves as a good

test of effects which influence the 7=2 and 3 states differently. Very recent multiconfiguration Dirac-Foch (MCDF)

calculations of this line in the very high-Z ions tungsten and

bismuth [9] provide strong motivation for measuring them. Electron-beam ion traps (EBIT's) are versatile tools for measuring atomic properties over an extremely wide range

of charge states. There have been measurements of the 7 = 2-3 line previously reported for five different Ti-Uke ions:

1 050-2947/2000/6 1 (5)/054501 (4)/$ 1 5 .00

four in the range from Xe (Z= 52) to Gd (Z = 64) [2,3] and

a single measurement at Au (Z = 79) [4]. Some of these measurements were confirmed in subsequent experiments in in- dependent labs [10,11]. Reference [11] also provided an in- teresting qualitative explanation for the unusually weak Z

dependence of this line. The goal of this study was to measure wavelengths near Z=70 and above Z = 80 in order to provide data over the entire region of the isoelectronic se- quence where the line exists. The experimental arrangement

is similar to that described in previous measurements [3]. CyUndrical high-voltage drift tube electrodes surrounding

the trap region provide the accelerating potential for a nar- row, intense electron beam which is confined by a 37" magnetic field. A metal vapor vacuum arc (MEWA) source pro- duces low-charged Yb, W, or Bi ions, which are injected into the EBIT, trapped, and further ionized by the electron beam.

The spectrometer we used is a 0.3-m focal length CzemyTumer scanning monochrometer with a linear dispersion of

2.6 nm/mm, coupled to a cooled, blue-sensitive photomultipUer [3]. At the operating temperature of - 25 °C, the back- ground count rate for the photomultiplier was 3 or 4 Hz. The

charge state of interest was optimized by adjusting the elec- tron energy to be just below the ionization potentials of the

ion, which were calculated to be 4246 eV for Yb*^"^, 4920 eV for W^^"^, and 6662 for Bi^'"" [12]. For the ytter- bium and timgsten measurements, nitrogen was introduced

into the trap to provide low-mass, low-charged ions for evaporative cooling of the heavy, highly charged ions [13]. During the bismuth measurement, we found that weak lines

in some charge state of nitrogen were blended wdth the Ti- hke bismuth hne, so argon was used as a cooling gas. In all cases, the gas was expanded into the trap region through a 3.2-mm-diameter hole 16 cm from the trap, from a chamber

at -1.3X10"'' Pa.

In these measurements, the identification of the 7 = 2-3 Une was aided by the fact that the line disappeared when the MEWA ion loading was stopped, and that the threshold for observation of the measured Unes was at the ionization en- ergy for the production of the ion. (See Fig. 1 for the energy dependence of the tungsten line, and Fig. 2 for the spectra.) Due to the presence of the electron beam, there is a space- charge correction which decreases the actual energy of the

61 054501-1 ©2000 The American Physical Society

NIST-147
BRIEF REPORTS PHYSICAL REVIEW A 61 054501

CScoU

coo

a.

4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 Corrected Drift Tube Voltage (kV)

FIG. 1. Plot of the intensity of the observed 7=2-3 M\ tran- sition in W^^"^ as a function of electron-beam energy. The drift tube

voltage has been corrected for an estimated 220-V space charge

shift. The dashed line represents the ionization potential for W^'"^

(i.e., the threshold for production of W"^), and the dotted bne

represents the ionization potential for the Ti-like state, W^^"^

.

electrons in the trap region, and this shift in the accelerating

potential has been included in the data of Fig. 1 . At operating

conditions of 90 mA and ~5kV, the space-charge offset is calculated to be - 220 V. This estimate does not include the

i

cm

oU

346 348 362 364 366 368 Wavelength (nm)

382 384 386

FIG. 2. Selected parts of the observed spectra during the ytter- bium, tungsten, and krypton measurements. The spectrometer slits in these measurements were 300 fj,in, and each peak represents an

average of four scans.

partial neutralization of the electron beam by the ions, and

therefore the shifted electron energies serve as lower bounds. We note that while the addition of the cooling gas increases

the total number of metal ions (Yb, W, or Bi) in the trap, it usually lowers the average charge state at a given beam en- ergy (due to charge exchange with the neutral gas). With the cooling gas, the electron-beam energy which maximizes any

given charge state is raised to higher values. This fact can be

seen in Fig. 1 , where the maximum signal intensity from the

W^^"*" occurs at a beam energy above the ionization potential

for the ion. We note that the bismuth line had the correct threshold, and disappeared when the MEWA ion loading

stopped, but the intensity did not decrease even when the beam energy was as high as 10 kV, which is significandy

above the ionization potential for Ti-Uke bismuth. We at- tribute this behavior to the increased charge exchange cross section for the highly charged Bi*'"*^, which pushes the charge-state balance to lower charges.

During the ytterbium and tungsten measurements, the

calibration of the spectrometer was checked in situ against two previously measured lines in Kr which are on either side of the two Ti-like lines that we measured: the 3s^3p^ ^Djri-^Dsn line in Kr^'^ at 346.47(6) nm [14], and

the 3s^3p^ ^Pi-^Pj line in Kr^^^ at 384.09(3) nm [15]. All four lines were measured with 300-/im entrance and exit

slits on the spectrometer, but the Yb and the 384-im3 Kr lines were also measured with 150 and lOO-fj-m slits in order to measure sht-dependent systematics. During the tungsten run, the calibration lines indicated a 0.2-nm systematic offset in wavelengths, possibly due to misahgimient of the spectrometer. The tungsten data were therefore shifted to correct for

this offset. In the bismuth measurement, nearby transitions in N^^ were also used for calibration: the three lines arising

from the ^S-^P multiple! of the 3^-3^ transition array at 347.87, 348.30, and 348.49 nm [16]. The bismuth and cali- bration spectra were taken with 250-/im slits in order to be

able resolve the closely spaced nitrogen hnes. One interesting aspect of the Ti-like sequence is that the J=4 level, which is the highest energy level of the ground

term at low Z, crosses both the 7 = 3 and 2 levels as Z is increased. Unfortunately, at high Z the imobserved UV J = 4-3 transition competes with the J -2-3 transition we observe, and at Z=74 (tungsten) the branching ratio to the J — 2 level drops to only 16% [17]. With entrance and exit sUts of 300 fLTa, the maximimi count rate we observed for the y= 2-3 line in W^^"^ was only 4 Hz, which is more than four times smaller than the rate measured on the same line in

Gd'*^"'^ under similar conditions [3]. For bismuth (Z=83)

there is a further factor of 2 decrease in signal, making measurements of this line in such a high charge state even more

difficult. The combined standard uncertainty ( 1 — cr level) arising from the caUbration, peak fitting and reproducibility considerations are 0.15 run for Yb"^^ andBi^'+ and 0.2 run

for W"+.

The measured wavelength for the 3t/'* 7=2-3 line in Yb48+^ W""^, and Bi^^"^, along with previously measured

wavelengths for the same transition at different Z's, are shown in Fig. 3. We had previously foimd [3], that a scaling of the electrostatic integrals in the relativistic Cowan code

054501-2

NIST-148
BRIEF REPORTS PHYSICAL REVIEW A 61 054501

FIG. 3. Plot of the measured wavelengths of the J =2-3 A/1

transitions in the 3 J* configuration of the Ti-like isoelectronic se- quence (circles), including the new measurements of Yb'""^,W^^'^, and Bi^' ^ , which are indicated by arrows. (The measurements for Z«64 are from Refs. [2,3], and the measurement at Z = 79 is from

Ref. [4].) The calculations of Refs. [1,17] (triangles) and [6,9] (squares) are also shown. The soUd curve through the open circles

is a cubic spline of the calculations, and the solid line through the measured points is the estimate described in the text.

[18], which was linear in Z, described the measured wavelengths for Z between 52 and 64. Extending this scaling to higher Z but with the scale factor capped at 1.0, however,

failed to provide good agreement, since errors quickly ex- ceeded 5%. In contrast, MCDF calculations improve at higher Z, as can be seen in Fig. 4, where we have plotted the

relative error of the calculations of Indelicato and co-workers

[1,17] and of Beck [6,9]. Although the discrepancy between

the measurements and theory is not expected to have a

simple functional dependence on Z, it is fairly smooth and

the calculations approach experiment at higher Z. The recent

calculation of Beck at Z= 83 [9] is in very good agreement

with our measurement. With the addition of the three measurements that we re- port here, the relatively smooth discrepancy between measurement and calculation can be used as an estimate of the unmeasured wavelengths for 52«Z^83. In the absence of a

clear functional dependence for the wavelengths, we used a

simple cubic spline interpolation to estimate the difference between the data and the calculated curve shown in Fig. 3; the result is shown in Fig. 4. Using this interpolation, the wavelengths of the other Ti-like ions in this range of Z were

estimated, and the resulting values are given in Table I and

shown in Fig. 3. We include two points for Z«53 which

involve extrapolations beyond the measured values. It is difficult to estimate robust errors for the interpolation and extrapolation, but we made consistency checks by removing

each data point individually from the ensemble and reinterFIG. 4. Plot of the difference between calculated data and measurements (see Fig 3). The circles are from the calculations of Refs.

[1,17], and the squares are from the calculations of Refs. [6,9] The

line through the circles is a cubic interpolation constrained to pass through all the measured points.

polating to predict the removed point. With the exception of

the measured end points at Z=54 and 83 (which become

extrapolations once the end data points are removed) and the point of maximum curvature at Z = 60, the estimated values

are within 0.1% (<0.4 nm) of the measured values. In all cases (including the extrapolation from Z=79 to 83) the fits to subsets of the data predict the eliminated points to within 0.2% (<0.8 nm). We expect the estimates to be more acTABLE I. Table of measured and interpolated values for the wavelengths of the 7= 2-3 transition within the ground term of the

Ti-like isoelectronic sequence. A conservative estimate for the uncertainty in the interpolated values is 0.8 nm. (See the discussion in the text.) Measured wavelengths are indicated as (a) Ref. [2], (b) Ref. [3], (c) this work, and (d) Ref. [4].

z X (nm) Z \ (nm) Z X. (nm)

52 449.4 63 371.9 74(c) 362.6(2) 53 429.6 64(b) 371.3(2) 75 361.0

54(a) 413.9(2) 65 370.8 76 359.2 55 402.2 66 370.3 77 357.4

56(a) 393.2(2) 67 369.8 78 355.3 57 386.5 68 369.2 79(d) 353.2(2) 58 381.4 69 368.5 80 351.1 59 377.8 70(c) 367.64(15) 81 348.9

60(b) 375.3(2) 71 366.5 82 346.6

61 373.6 72 365.3 83(c) 344.29(15) 62 372.6 73 364.0

054501-3

NIST-149
BRIEF REPORTS PHYSICAL REVIEW A 61 054501

curate than this when all the measured points are used (particularly for 63<Z<83), and the uncertainty for most points

is likely to be limited by the 0.2-nm uncertainty in the measured wavelengths. Nonetheless, we take 0.8 nm as a conservative estimate for the uncertainty of the entire range studied. Thus the interpolation provides a 0.2% estimate of the lowest Ti-like 3d^ 7 = 2-3 transitions for 52^Z^83.

We would like to thank Y. -K. Kim for helpful discus- sions and for performing atomic structure calculations that aided this experiment. We thank D.J. Alderson for assistance

in operating the EBIT and F.G. Serpa for assistance with the spectrometer. I.K. acknowledges support from the Swedish

Foundation for International Cooperation in Research and

Higher Education (STINT).

[1] U. Feldman, P. Indelicato, and J. Sugar, J. Opt. Soc. Am. B 8, 3 (1991).

[2] C.A. Morgan, F.G. Serpa, E. Takacs, E.S. Meyer, J.D. Gillaspy, J. Sugar, J.R. Roberts, CM. Brown, and U. Feldman,

Phys. Rev. Lett. 74, 1719 (1995).

[3] F.G. Serpa, E.S. Meyer, C.A. Morgan, J.D. Gillaspy, J. Sugar,

J.R. Roberts, CM. Brown, and U. Feldman, Phys. Rev. A 53, 2220 (1996).

[4] E. Trabert, P. Beiersdorfer, S.B. Utter, and J.R. Crespo LopezUrruda, Phys. Scr. 58, 599 (1998).

[5] F. Parente, J.P. Marques, and P. Indelicato, Europhys. Lett. 26, 437 (1994).

[6] D.R. Beck, Phys. Rev. A 56, 2428 (1997).

[7] P. Indelicato, S. Boucard, and E. Lindroth, Eur. Phys. J. D 3, 29 (1998).

[8] J. Saperstein, Rev. Mod. Phys. 70, 55 (1998).

[9] D.R. Beck, Phys. Rev. A 60, 3304 (1999). [10] D.J. Bieber, H.S. MargoUs, P.K. Oxley, and J.D. Silver, Phys.

Scr. 73, 64 (1997).

[11] D. Kato, C. Yamada, T. Fukami, 1. Ikuta, H. Watanabe, K. Okazaki, S. Tsurubuchi, K. Motohashi, and S. Ohtani, Phys.

Scr. 80, 446 (1999).

[12] Y. K. Kim, (private communicadon).

[13] M. B. Schneider, M. A. Levine, C. L. Bennett, J. R. Henderson, and R. E. Marrs, in International Symposium on Electron Beam Ion Sources and Their Applications, edited by A. Hershcovitch, AIP Conf. Proc. No. 188 (AIP, New York, 1989), p. 158.

[14] F.G. Serpa, E.W. Bell, E.S. Meyer, J.D. Gillaspy, and J.R. Roberts, Phys. Rev. A 56, 2428 (1997). [15] V. Kaufman and J. Sugar, J. Phys. Chem. Ref. Data 15, 321

(1986).

[16] R. Hallin, Ark Fys. 32, 201 (1966).

[17] P. Indelicato, Phys. Scr. T65, 57 (1996). [18] R. D. Cowan, The Theory of Atomic Structure and Spectra

(University California Press, Berkeley, CA, 1981).

054501-4

]SfIST-150
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 71, NUMBER 8 AUGUST 2000

Direct imaging of hiiglily charged ions in an electron beam ion trap

J. V. Porto,^^ I. Kink, and J. D. Gillaspy

National Institute of Standards and Technology, 100 Bureau Dr. Stop 842, Gaithersburg,

Maryland 20899-8421

(Received 29 March 2000; accepted for publication 10 May 2000)

We have directly observed the ion cloud distribution in an electron beam ion trap using visible and

ultraviolet fluorescence from lines in the ground term of Ar'^"*", Xe^'"*^ and Xe^^^ ions. Using a

gated intensified charge coupled device camera, we have the capability to measure both static and

dynamic ion cloud distributions. The images provide information about the trapped highly charged

ions which is difficult to obtain by other methods. To demonstrate the usefulness of the technique, we took images of static ion clouds under different conditions and compared the distributions to a

simple model. We also recorded time resolved images which show that we can monitor

the relaxation of the ion cloud toward equilibrium when the trapping conditions are suddenly

changed. The information provided by such measurements can be used to improve models of ion

cloud dynamics and, combined with modeling, these techniques can help improve measurements of

atomic data using electron beam ion traps. [50034-6748(00)04208-8]

I. INTRODUCTION

Electron beam ion traps (EBITs) have proven to be valuable devices for research involving x-ray spectroscopy of

highly charged ions.^"^ There are currently at least eight major EBIT facilities in operation around the world, and more

are being planned. Because EBITs have been used largely

(and very successfully) as static light sources, however, ex- perimental studies of the geometry and motion of the ion cloud within the traps have been scarce. This has been one of

the key limitations which has prevented EBITs from being

used to measure absolute (without recourse to normalization)

cross sections of atomic collision processes. X-ray fluores- cence of ions has been successfully used to image the elec- tron distribution in the electron beam of an EBIT,'* but the

short lifetime of x-ray transitions precludes the possibility of

measuring the entire ion cloud distribution. The extension of

spectroscopic studies with an EBIT from the x-ray into the

visible regime,^ where long-lived excited states can be

probed, has opened up the possibility of directly imaging the

full ion cloud. With direct knowledge of the ion cloud dis- tribution absolute cross sections can in principle be obtained.

In addition, EBITs have begun to be used in a transient mode

of operation to either form pulsed beams of highly charged

ions^"' or measure excited state atomic lifetimes.^"^° In hght

of these developments, the lack of experimental knowledge

about the geometry and dynamics of the ion cloud inside an EBIT has become apparent. In this article, we present the

results of a study in which the ion cloud is directly imaged in

visible and UV light with a charge coupled device (CCD)

camera, and the dynamical motion of the cloud is studied under a variety of conditions.

'Author to whom conespondence should be addressed; electronic mail: trey@nisLgov

II. EXPERIMENT

The National Institute of Standards and Technology

(NIST) EBIT has been described elsewhere," and we describe here only the aspects relevant for the current study.

The EBIT consists of a narrow electron beam (radius a = 35 ^tm) confined by a 3 T magnetic field which is used to ionize and trap ions at high charge states, producing ions with charges up to +73e. In the experiments described here, the electron beam was subject to a constant flux of neutral gas which, when ionized, provides the source of ions in the

trap. The space charge of the electron beam provides a trap- ping potential in the radial direction p, and three cylindrically symmetric electrodes surrounding the beam provide a

trapping potential in the axial direction z (see Fig. 1). A

positive voltage applied to the middle of the three elec- trodes relative to the electron gun determines the energy of

the electron beam. The voltage of the two end cap elec- trodes relative to the middle electrode is used to control the

axial trap depth. The combined trap potential V(p,z) is nearly independent of z in the 3 cm space between the two

end cap electrodes. In the radial direction the potential is logarithmic in p for p>a (see Fig. 2). Inside the electron beam the potential depends on the density distribution of

electrons, as we discuss in more detail below. The trapping

potential depends on the ion cloud distribution as well since the ions will partly neutralize the electron beam space charge

at high ion densities. In addition to the electrostatic potential, the constant magnetic field also plays a role in confining the

ions. The end cap electrode voltages can be switched rapidly,

allowing for fast axial dumping of ions. Both the radial and

axial trapping potentials have typical depths of up to a couple of hundred volts per charge. The resulting trapped ion cloud is long (30 mm) and narrow (typically between 0.07 and 1 mm in diameter).

The imaging system consists of two coaxial lenses oper- ated at nearly infinite conjugation, with the electron beam

3050

NIST- 151
Rev. Sci. Instrum., Vol. 71, No. 8, August 2000 Imaging of ion in a trap 3051

TABLE I. Transitions used to image trapped ions.

To CQnerc

FIG. 1. Cross section of the cylindrical EBIT trapping electrodes. The trap consists of an electron beam threading three drift tubes. The upper and lower

drift tubes are held at a potential of V£+ Vz , while the middle drift tube is held at Vj . The resulting axial potential is indicated schematically. The

dashed hne represents the image potential formed by the electron beam and

the internal geometry of the electrodes. The solid line represents the total potential including the voltage apphed to the end caps. The resulting

radial potential is shown in Fig. 2.

near the focus of the first lens (focal length /=20cni) and

the camera imaging plane near the focus of the second (/ = 35 cm). The lenses were placed 1 cm from each other and

the total distance between the electron beam and camera

plane varied between 55 and 65 cm, depending on the choice of magnification. The lenses were standard cemented achro- mats, chosen to minimize spherical aberrations on the blue

side of the visible spectrum. The magnification depended on

the placement of the camera and electron beam relative to the

/ (a)

/ /

\ /

/''^^ ""^ W ///

^^^'^^^

1 V 1

P (Mni)

FIG. 2, Examples of the EBIT radial space charge trapping potential. The

plots are for the conditions of the four static measurements described in the

text and listed in Table II.

Ion Transition X (nm) Lifetime (ms)

Ar'3* 441.2^ 8.7(5)" Xe^'+ 396.3"^ -3"

Xe''" 413.9' 2.15(14)"^

^Reference 19. •Keference 20. 'Reference 21. "Reference 22.

''Reference 23.

lenses and was measured independently. Images were taken

at magnifications of 1 .75 and 2.65. At these magnifications, a

single pixel corresponded to 15.3 and 10.1 ^tm, respectively.

The spatial resolution was determined to be about twice the pixel size by imaging a 75 /Mm pinhole under various conditions. In addition, two in situ tests of the magnification were made in order to roughly check the calibration of the magnification. An image of the entrance slit of a previously cali- brated optical spectrometer'^ was projected into the EBIT

and subsequently re-imaged onto the CCD. In addition, the sht in the drift tube electrode was imaged. Both measure- ments confirmed the overall magnification of the system. Narrow band interference filters (AX.= 10 nm) were used

to isolate the particular line of the charge state of interest. Simultaneous measurements with a spectrometer indicate

that contamination from Unes from low charged states that

fall within the 10 nm width of the filter were small but measurable, particularly for Ar'^^. However, their influence will be largely ignored throughout this article. The camera used was a commercial intensified CCD which could be gated on and off within a few nanoseconds. We took advantage of the

vertical symmetry of the ion cloud distributions by operating

the camera in ' 'full vertical binning' ' mode. In this mode the two-dimensional image is projected along the symmetry axis of the EBIT into the last row of the CCD before being digitized, capturing the radial distribution while averaging over

the homogeneous axial distribution. Full two-dimensional

images were taken in order to check the camera and EBIT

alignment and assure that the full vertical binning mode did not distort the radial image significantly. For the data pre- sented here, the misalignment was less than 2 pixels out of

580, which corresponds to an angle of less than 0.2°. The

total collection time required to produce a single vertically

biimed image was typically about 10 min. In addition to the imaging system, a visibleAJV spectrometer and solid state x-ray detector are also attached to the EBIT on different ports."

III. ION CLOUD DISTRIBUTIONS

We studied ion cloud distributions by observing fine structure transitions in the ground term of three different

ions: Ar'^"*^, Xe^'"" and Xe^^"" (see Table I). The transitions are excited by the electron beam and the lifetimes are all more than 1 ms. By contrast, ion motion in the azimuthal

direction (due to the magnetic field and the radial electric

field) has orbital periods that range from several nanosecNIST-152
ANALYSIS OF BROADBAND X-RAY SPECTRA OF

.

PHYSICAL REVIEW E 63 046409

lemadc in solar and astrophysical spectra, are harder for us to check experimentally because of the problems of charge ex- change in our experiment enhancing the 2p^ — 2p^3s intensities. These effects of charge exchange may however allow the measurement of state selective charge-exchange cross sections, relative to those for electron-impact excitation or ionization. As well as applications of neutral beam heating in tokamaks, we expect such data to be very useful in interpretation of cometary x-ray spectra [46-48]. Here, neutral material outgassing from the comet nucleus encounters highly

charged ions in the solar wind. The ensuing charge-exchange

collisions populate highly excited states of the solar wind

ions, followed by x-ray emission as these states radiatively

decay. An astrophysically relevant experiment would require

trapped ions of C, N, O, or Ne, and the admission to the EBIT chamber of the appropriate neutral gas, i.e., H2O, CO2,

etc.Among our line identifications are several lines with potential for electron-density diagnostics. The density sensitivity arises because the relative populations among the differ- ent levels of the groimd configurations varies with density. If

this results in higher population accumulating in a level of

higher angular momentum (i.e., the 2s^2p ^P-nj in Kr

XXXn, the 2j^2p2 in Kr XXXI. or the 2s^2p^ ^D^a in Kr XXX) than the ground level, then excitations to levels among the n = 3 configurations with higher angular momentum than are possible from the ground state alone, result. Hence new lines from these high angular momentum level

states become visible at higher densities.

I In Hg. 3(a) we show two line intensity ratios in Kr XXXII

i that vary as a result of n = 3 , y = 5/2 levels becoming excited, calculated for an electron temperature of 2 X lO' K. The

electron densities over which this ratio varies is lO'^ cm , which is the relevant density range for high Z

underdense plasma radiators. This should be very useful,

. since the density diagnostic available in the Kr A'-shell spec-

|l tram, the ratio of the ^Sq— ls2p^P2 to either of the

Is^ ^Sq—Is2p^ ^Pi lines only becomes sensitive at densities greater than 10^' cm"^, i.e., higher than is usually

I I achieved in such experiments. One solution to this is to sus-

!f pend pellets of material of slightly lower Z in the Xe filled enclosure, so that the corresponding He-like transitions are

sensitive in the right range. This would be most appropriate

in the hottest regions of the Xe enclosure, since Kr and simij| lar ions will be ionized beyond the B-like charge state, but

! elsewhere the B-Uke density diagnostic we discuss might be

preferable. We note that previously discussed L shell diagi nostics in Xe [6,49] for the electron temperature are still

I dependent on the electron density, due to the density sensiII tivity of the dielectronic recombination process and the ionI ization balance itself in such experiments. The density diagnostic we identify is much less dependent on the electron temperature, and would go some way towards resolving such

ambiguities. Further density sensitive line ratios at the same electron temperature in Kr XXXI and Kr XXX are plotted in Fig.

3(b), which may have appUcations to tokamak plasmas with

density in the range 10*^- lO''* cm"^

I

- Kr XXXII 2183.7/2126.1

17 18 19 20 21 22

(a) Lo9(Electron density in cm"^

100

1 1

1

1

' 1 1 1 1 1 1 1 1

0

0

IE£• 10

*wc0

1 Kr XXX 2047.8/2048.9- ^^-ic;^

- Kr XXX 2047.8/1 998.r "V, Kr XXXI 21.13.6/2p6p.1

12 13 14 15 16 17

(b) Log(Electron density in cm"^

FIG. 3. Electron-density (logion^) dependence of line intensity

ratios at electron temperature 7'^ = 2X10' K calculated with a model described in Sec. IV for selected transitions in Kr XXXXXXn (identified in Table U).

VI. CONCLUSION

We have demonstrated the value of using an EBIT plasma

model for the analysis of complex and only partially resolved

x-ray spectra obtained with a broadband high-resolution x-ray detector. This approach can be apphed in the determination of numerous physical characteristics of the EBIT

plasma, such as measurements of important reaction rates and the validation of diagnostic Une ratios. In this paper we

have identified many new lines in the spectra of L-shell Kr

ions, some with diagnostic potential applicable in other fields of plasma physics, and have made progress towards understanding conditions in the EBIT trap itself.

ACKNOWLEDGMENTS

The authors thank D. J. Alderson for his technical support. I.K. acknowledges support from the Swedish Foundation for Cooperation in Research and Higher Education

(STINT). J.M.L. was supported by NASA Contract W

19353 (Applied Information Systems Research Program) and

ONR/NRL 6.1 Research Option "Solar Magnetism and the Earth's Environment."

046409-9

NIST-153
Rev. Sci. Instrum., Vol. 71, No. 8, August 2000 Imaging of ion in a trap 3053

ments presented here.) The radius po is typically taken to be

the radius of the electron beam a. The offset Vq can be

chosen in order to satisfy boundary conditions appropriate

for thermal distributions, namely that V(p = 0) = 0. In this case Vq represents the potential at the radius of the electron beam relative to the center of the trap, and it is relatively insensitive to the electron distribution n^ip). For example,

for a constant electron density distribution Vq is given by

(2)

For a Gaussian beam distribution of the same width and total linear charge density, we calculate Vq

1.08 47reo

(3)

and we expect any reasonable distribution to have a similar

offset Vq . The total radial potential can therefore be approximated as

V{p) 21n(p/a)+l p>a

(p/af p^a (4)

where Vo = XMireg- This is equivalent to modeling the elec- tron distribution as a constant density XMircg for p<a. The

total trap depth in the radial direction is given by Eq. (4) evaluated at the inner radius R of the trap electrodes, AVp

= V(R).

In the axial direction the trap electrodes provide a radially independent, nearly square well potential. The walls of

this axial potential be outside the field of view of our camera, and do not directly affect the visible ion distribution, apart from determining the overall temperature. Given the very fast axial motion of an individual ion, any ion with enough velocity in the z direction to escape the trap will do

so within between 1 and 100 /is, so the trap depth AV^

determines the largest possible axial velocity in the trap. In addition to the applied end cap voltage V^, the axial trap depth A has an additional contribution V^^ arising from

the geometry of the trap. The additional potential is an image

charge effect arising from the electron beam passing through

the constriction of the radial opening in the trap electrodes from the inner diameter of the trap (/? = 5 mm) to the inner diameter of the end cap electrodes (r=1.5mm): AV^^V^

+ Vim (see Fig. 1). This offset is determined by the logarithm

of the ratio of inner electrode radii. It also scales with

A./47reo , and is given by

^im / 5 mm

Vo ~ "\ 1.5 mm

= 2.4. (5)

The axial image charge potential is smaller than the radial space charge contribution, and is typically less than 30 or 40

V.

If the ion densities are not small, then the image potential from the ions themselves must also be added to the trap. The distributions of all the ion charge states «,(p) are not

necessarily known, but under the assumption that the ions

are in thermal equilibrium, the shape of the individual ion

distributions n,(p) can be deteimined self-consistendy by

FIG. 5. The self-consistent solution for the total electric potential arising from the electron beam and an ion cloud in thermal equilibrium. The dashed

line represents the potential from the electrons only (i.e., no neutralization). The solid lines represent the potential including an ion cloud containing 65% of the total charge of the electron beam, with different scaled temperatures as indicated. The scaled temperature is given by kTIQV^.

requiring that they obey Boltzmann distributions. At low

temperatures where the ions are concentrated near the elec- tron beam, we expect the total potential depth to scale ap- proximately with the total linear charge density Xtot^^^e

+A.jon. At higher temperatures the degree of neutralization

will be less since the ions do not overlap well with the elec- tron beam. A simple model which shows this effect assumes

that there is only one charge state Qi in the trap. Figure 5 shows the total combined potential from the electron beam

and a self-consistentiy determined thermal distribution of

ions which has 65% of the total charge of the electron beam. At low ion temperature, the total potenual depth is neutral- ized by roughly 65%, but at the highest temperatures the

potential compensation is only 10%. We also find that due to this weak neutralization, the shape of the ion cloud at higher

temperature has a surprisingly weak dependence on the number of ions in the trap. Regardless of the exact form of each nj(p), trapped ions always weaken the radial confinement and lower the total radial trap depth AVp at a given temperature. Since AVp is entirely due to the electron space charge but only a portion of A is, the addition of the ion space charge decreases A Vp

relative to A V^, . Given the right conditions, changes in the value of AVp/AV^ as the trap is increasingly neutralized should be observable, providing a rough measure of the elec- tron beam neutralization by ions. Due to the insensitivity of

the trap potential to high temperature ion clouds (as argued

above and demonstrated in Fig. 5) a decrease in AVp is likely to be more easily observable for cold ion clouds. In- deed, for the ion clouds described here, we do not see a weakening of A Vp

.

NIST-154
3054 Rev. Sci. Instrum., Vol. 71, No. 8, August 2000 Porto, Kink, and Gillaspy

0 100 200 300 400 soo Trap Voltage (V)

FIG. 6. The widih of the ion cloud image as a function of static trapping

voltage Vj . The EBIT parameters for the different data labeled as shown are given in Table II.

•

-

Argon ,

• *

-

•

c o ° S o o

° 0 o a o

° Xenon

0 ° . 0°°

(a) • o • (b)

o ° ••

1 1

o

1

(d)

1 0 S 10 15 20 25 30

FIG. 7. The widths from Fig. 6, but plotted as a function of the scaled trapping voltage, V^/I^q. The vertical line indicates the total radial depth of

the trap in the low ion density limit, Vj- Vi„ = 9.6Vo.

B. Static ion cloud widths

In order to quantify the thermal expansion of the cloud,

for each image we calculated a width for the imaged distri- bution. Given the shape of the distributions, which are not

well described by Gaussian or Lorentzian functions under all conditions, the full width at half maximum is not very in- dicative of die average width of the cloud. We therefore

calculated the weighted average width by integrating over

the image

5l{x)x^dx TM* (6) il{x)dx

Here, I{x) is the intensity of the image at a given position, and X is measured from the position of peak intensity. (The

largest integration range is limited by the slits in the end cap

electrodes, which mask the image at ±1200 ^tm.) The result- ing widths as a function of end cap trapping voltage are shown in Fig. 6 for four different measurements. The differ- ent electrostatic trapping conditions in each of the measurements are given in Table EI. We expect the ion cloud's dependence on to separate

into three regimes corresponding to (i) an open trap at negative Vj(A Vj<0), (ii) a closed trap at intermediate which

is dominated by axial losses (0<AV^<AVp), and (iii) an

axially closed trap at large which has saturated due to the

increased prominence of radial losses at the inner surface of

the electrodes ( A V^>A Vp). Based on the preceding discussions it seems appropriate to scale all trapping potentials by

Vo=X./4ireo; and the widths of the ion clouds plotted

against the scaled trap voltage is shown in Fig. 7. The cross- over from the axial loss dominated to the radial loss dominated trap can be seen at = 1 1 Vg for both the argon and

xenon measurements.

The axial trap depth A = 4- Vj^, is equal to the radial trap depth when V^z^AV^-Vj^. Equation (4) gives AV^

= V{R)=^\\Vq. Without substantial trap neutralization the

transition from -dependent to -independent behavior

should occur roughly at V^ = 11 Vo-2.4Vo = 9.6Vo. The data

in Fig. 7 become independent of the axial potential at a rea- sonably well defined value of which is somewhat above

this value. A direct comparison between AVp and AV^ can

provide information about the trap neutralization, and the

relative sharpness of the transition indicates that it might be

possible to use ion images to roughly measure the neutral- ization of the beam. In particular, for the xenon measure- ments where the beam neutralization would be largest the

transition occurs at the same large value of V^IVq as for argon. Since neutralization of the radial potential should lower the saturation point, this indicates that the beam is not very neutralized. This analysis is complicated by the fact that hot ion clouds are not effective at neutralizing the radial

potential, as discussed above and shown in Fig. 5. The tech- nique should be more effective for ions that are cooled by

simultaneous injection of lower charged ions. This simple

argument also ignores the possible differences in radial and

axial escape rates, which are determined by the different velocity diffusion rates in the two directions due to the pres- ence of the magnetic field. One might expect that a substantial difference in diffusion rates would smear out the

transition to the radial loss regime, but a detailed description of the diffusive behavior is needed to fully interpret the images.The transition to the open trap regime when A = 0 is difficult to observe using the ion cloud widths because the

signal vanishes as the trap is made shallower. The time scale

for highly charged ion production is much larger than a

single axial transit time, and ions leave the open trap before

NIST-155
Rev. Sci. Instaim., Vol. 71, No. 8, August 2000

3 -2 -1 0

FIG. 8. The x-ray count rate fiom neon-like xenon (Xe***) as a function of

the scaled end cap voltage V^/Vq. The vertical line marks the calculated values of V^/Vq for an open trap, Vi^/Vo=-2A. The data were taken at electron beam currents of 120 mA (open circles), 100 mA (filled circles), and 70 mA (squares). The beam energy was 8 kV.

they can be sequentially ionized to high charge state. We

can, however, use the vanishing of the total collected hght as a function of to measure the point V^= — Vj^ at which the

trap is "open." Since we are measuring the intensity and not

the actual spatial distribution of ions, the measurement can be made using any fluorescence proportional to the number

of ions in the trap, e.g., x-ray fluorescence. (X-ray lines are

easier to measure because most of the emission of highly

charged ions occurs in the x-ray regime, and x-ray count

rates are typically much larger than visible or UV rates.) As

an example we show in Fig. 8 the total x-ray fluorescence of

the neon-like lines in Xe'"'^ as a function of V^/Vq. The x

rays were collected in the 3-5 keV range with a solid state detector. Since the ion density is necessarily small when the

trap is nearly emptied, we can safely use Eq. (5). The three data sets were taken at conditions where Vq differed by a

factor of 1.7, but they clearly indicate that the trap empties at V^ = -2AVq, in agreement with the simple calculation of Eq. (5). In the intermediate regime, where the cloud width is approximately linear in , the cloud size can be used as a temperature diagnostic. Since the ion cloud shape is deter- mined by the ratio of the trap depth to the temperature, it is useful to scale the temperature by Q. Vq. In the absence of

interactions between different charge states, the scaled temperature would be roughly the same for all charge states at a given V^/Vq. This is because without cooling from lower

charge states of the ion the temperature would increase until the ion cloud filled the trap. Highly charged ions would see a much deeper potential, but the temperature would be proportionally higher, leaving the spatial distribution approximately

the same. Interactions among the different charge states tend

Imaging of ion in a trap 3055

-too -200 0 200 400 600

Radia] Distance (pm)

FIG. 9. A plot showing the effect of low charged nitrogen cooling ions on

the spatial distribution of Xe'^* ions. The narrower image was taken with

4.1 X 10~* Torr of nitrogen in the gas injection chamber, while no nitrogen was added for the wider image. All other trap conditions were identical. The

pressure of xenon gas in the injection chamber was 2.7X 10"' Toir and the end cap voltages were set to 300 V. The dashed lines are fits to a truncated Boltzmann distribution, with temperatures of 0.77gVo and 0.9QVq. The

inset shows the width of Xe^'"^ ion cloud as a function of nitrogen gas

pressure in the injection chamber.

to drive the ions toward equilibrium, so that ions of higher

charge j2i have a smaller spatial distribution and smaller

scaled temperature kTIQiV^ than ions of lower charge state. This can clearly be seen in Fig. 7. The slope of width versus V^/Vq for xenon ions (Q = 32e) is roughly half that for the argon ions (Q=l6e) under similar scaled trap conditions. Using a separate low charged cooling gas, we can see

the effects of reduced temperature direcdy on a single charge

state without having to compare different ions. Figure 9

shows two Xe^^"^ images taken with and without the simul- taneous injection of nitrogen, which acts as a cooling gas. The intensity from the cooled ion cloud was a factor of 3

larger than from the uncoolcd cloud, so the data have been

scaled in order to compare the shapes. The cloud width as a

function of injection gas pressure is plotted in the inset of the

figure. Qearly the cloud width provides a qualitative measure of the ion cloud temperature.

In order to determine more quantitative measures of the

temperature, more detailed modeling of the distribution is required. The simplest estimate assumes a thermal distribu- tion in the low ion density limit. For a classical distribution

in the static thermal Umit, the effects of the magnetic field can be ignored,*^ so V(p) should represent only the electrostatic potential. The logarithmic potential in Eq. (4) results in a Boltzmann distribution exp-(QiV{p)/kT) that has a power

law dependence on p. A fit to this distribution is shown as the dashed lines in Figs. 3, 4, and 9. Despite the reasonable agreement with the data, the temperature determined from

NIST-156
3056 Rev. Sci. Instmm., Vol. 71. No. 8, August 2000 Porto, Kink, and Gillaspy

lOOxJo'

-30O -200 -100 0 100 200 300

Distance (^m)

FIG. 10. A representative ion cloud image at low trapping voltage . The

ion is Xe^'* a: Vj=OV, which corresponds to a scaled trap depth of roughly 2.4 Vq. The vertical lines represeni the maximum possible radial extent of ions with less than this energy. The dashed line is an image of a 75 /mi pinhole taken with the imaging system.

such a fit must be interpreted with care. One limitation of the model is the finite lifetime of the visible lines we observe. Despite being longer than any of the dynamical scales for individual ions, fluorescence from an ion requires that the ion has been excited within the last few milliseconds. The

predominant source of excitation is the electron beam, and

for very hot distributions there are likely to be some ions whose trajectories are completely outside the beam and will remain dark. (Excitation rates from collisional processes in- cluding charge exchange with background gas are expected

to be much lower.'^) The effect this has on the shape of the

visible distribution must be determined from more complicated distribution modeling.

Another interesting limitation of the simple Boltzmann

distribution is that it is not normalizeable at temperatures

kT^kT^=QiVo. A logarithmic potential is simply too weak to support a bound, high temperature ion cloud and

very energetic ion clouds are necessarily nonthermal. In the EBIT the Boltzmann distribution is necessarily truncated at p—R. At low temperature this does not significantly alter the

distribution. At high temperature, however, the cooling rate increases as T approaches r^a, from below, since the trun- cation of the distribution at R cuts out an increasing fraction of the ions. The increased flux of ions from small to large p

implies that the role of the magnetic field must be considered

in determining the shape of the distribution. In addition, the ion temperature (to the extent that it can be defined) is not expected to be much larger than . The temperatures de- termined from the fits behave qualitatively similar to the widths shown in Fig. 6. Using the truncated Boltzmann dis- tribution, the largest fit temperatures kT^ were less than = 1.0j2Vo. The fits were relatively poor at large radius for

0 100 200 300 'too Delay Time (ms)

FIG. 1 1 . The width of Ar"* ion clouds as a function of time after raising the end cap voltage to 300 V. The widths were determined from images like those shown in the inset. (The images shown were collected in a 5-ms-wide

time window at increasing delay times of 0, 4, 20, 60, and 95 ms, in order of

increasing width.) The different symbols represent data taken under similar conditions but on separate occasions several months apart. The electron beam energy and current was 7.25 kV and 89 mA, respectively. The sohd

line is a fit to a simple exponential, which gives a time constant of 210 ms.

the high temperature argon data, as can be seen in Fig. 3. Understanding the conditions for nonthermd distribu- tions is important for determining the ion cloud distribution

in a given experiment. A nonthermal ion cloud could be

manifest as a nonthermal form for the distribution in the p, z, Vp , and components, or as an inequilibriimi between dif- ferent phase space directions p-z and Vp-v^. Directly im- aging the ions' radial distribution can help provide information about the p and Vp dependence. These effects should be

largest at low density, where the ion-ion collision rate is too low relative to the escape rate to establish equilibrium. The

cylindrical symmetry implies that the radial and axial motions are not well mixed without collisions. We can look for indications of nonthermal distributions at low trap depths

where the ion density is necessarily small. In all weakly trapped ion clouds we've observed, the width of the cloud appeared larger than expected based on

estimates'^ '* of the cloud temperature under equilibrium

conditions. For example. Fig. 10 shows a Xe^^"*" ion cloud at V^ = 0\, which corresponds to a trap depth of about 2.4 Vq.

Given reasonable estimates of the ion cloud temperature'* under equilibriimi conditions, the cloud width should not

substantially exceed that of electron beam. In fact, the maxi- mum energy possible for any ion in a roughly thermalized cloud (i.e., a truncated Boltzmann distribution) would be

g,A . The radius at which the radial potential equals this energy, V{pj^)=AV^, is only about 70 fim. The ion cloud appears considerably wider than even this maximum width. This could indicate a nonequilibrium distribution and a

smaller than thermal overlap with the electron beam. AnNIST-157
Rev. Sci. Instrum., Vol. 71 , No. 8, August 2000 Imaging of Ion in a trap 3057

Other possible explanation could arise from instabilities in the electron beam that time average to give a wider electron

distribution. Given the significant implication of this finding

for numerous EBIT experiments, we carefully tested the camera imaging system to determine its resolution in order to

rule out instrumental sources for the excess widths. The

dashed line in Fig. 10 is a reference image of an illuminated 75 fim pinhole taken with exactly the same imaging arrange- ment as the EBIT images. The pinhole is roughly the diameter of the electron beam, and indicates a total instrument

resolution of about 25 ^tm.

C. Dynamic ion cloud widths

The speed at which the intensified CCD can be gated

provides us with the possibility of measuring ion cloud dy- namics. As an example, we quickly raised the end cap volt- ages on an originally empty trap and watched the ion cloud develop toward equilibrium. This process requires first the production of ions and then the subsequent heating and dif- fusion of the ions to their steady state distribution. Several images taken while viewing the Ar^^"^ line are shown in the

inset of Fig. 11. The images were created by repeatedly

dumping the trap, then raising the end caps to 300 V at t = 0. The signal was summed in a delayed, 5-ms-wide time window. The widths as a function of the time after raising the end cap voltages arc shown in Fig. 11, and the total integrated intensity is shown in Fig. 12. The intensity of the

line was also monitored simultaneously with a time resolved spectrometer whose input and output shts were opened to 1 mm, and the spectrometer signal is included in the inset to Fig. 12 for comparison. The experiment was performed at a beam energy of 7.25 kV, which is much larger than the ion200 Delay Time (ms)

FIG. 12. The total intensity of Ar"* ion cloud image as a function of time

for the conditions descritjed in Fig. 1 1 . The inset shows an expanded view of

the early times. The solid line of the inset is simultaneous data taken with a

spectrometer observing the same Une in Ar"*

.

Delay Time (ms)

FIG. 13. The width of Xe'^"^ ion clouds as a function of time after quickly lowering the trapping voltage from 300 to 230 V at f=0. The dashed line represents the static equilibrium cloud width established for times i<0. For

the solid circles, the end caps were held at 230 V. For the open circles, the end caps were subsequenUy raised back to 300 V after 2.5 ms.

ization potential for Ar'^"*" (=750 eV). In this case, at very

early times when the ions have not been substantially heated,

the production and loss rates of a given charge state are determined largely by electron impact ionization. In this particular measurement, it turns out that the time scale for ion production is sufficientiy smaller than the time scale for ion heating that the two time scales can be distinguished, as indicated by the arrow in Fig. 12. Qearly, a large fraction of

the Ar'^"*^ ions are produced within 10 ms, a time scale de- termined by electron impact ionization cross sections. As the

ion cloud heats up, other charge changing and loss processes become important (e.g., charge exchange and ion evaporation) and the net rate of ion production slows down.

The initial production time scale is consistent with esti- mates made from the electron current density and the ionization cross sections for argon. The characteristic time to pro- duce ions depends on the sum of the characteristic times of

all the lower charge states, T=2r, , where the individual times depend on the current density and the electron impact

ionization cross section o-f, T, = e/(7crf'). This is dominated by the cross sections of the last few charge states, which are on the order of 10~^°- 10" "cm^ for Ar'^"^ through Ar'^.'* With a current density of J -2000 Acm"^(y/e= 12X 10^' s~' cm"^) the estimated time

for Ar'^"*^ production is approximately 5-10 ms. The slower

evolution associated with the increase of ion cloud widths

occurs on a time scale in excess of 200 ms. Systematic studies of the time scale dependence on the ion charge state, ion

density, electron energy, and electron density should allow an experimental determination of heating rates. As another example of the usefulness of the dynamic

imaging, we performed preliminary measurements designed

NIST-158
3058 Rev. Sci. Instrum., Vol. 71, No. 8, August 2000 Porto, Kink, and Gillaspy

1.0 p

01 -

oob. \ I I L

0 10 20 30 40 Delay Time (ms)

FIG. 14. The intensity of Xe'^* ion clouds as a function of time for the same conditions described in Fig. 13. The intensities have been scaled by

the static equilibrium intensity. The solid lines aie fits to simple exponentials.

to observe velocity diffusion. When the axial trap depth is rapidly lowered, ions with axial velocity large enough to

exit the trap will leave within one axial transit time, which

for hot ions is less than 10 ^ts. Ions which have enough total energy to leave the trap but which have large radial and

azimuthal velocities Vp and and small will not leave

the trap until the velocity components have been partially mixed. Velocity mixing can occur via coUisions or trap imperfections. We can use the camera to watch the mixing

process. We performed two similar experiments while viewing the Xe^^""" hne. A nearly steady state ion cloud was pre- pared by leaving the end cap voltages at 300 V for 300 ms.

In the first measurement, we lowered the end cap voltages to 230 V at f=0 for 2.5 ms, and then raised the end caps back

to 300 V. The rise and fall times for the end cap switching were =0.7 ms. We followed the ion cloud evolution by tak- ing 5-ms-long images at different delay times after the trap depth was lowered. In the second measurement, we simply lowered the end cap voltages to 230 V without raising them

again. The widths and intensity of the resulting cloud dynamics are shown in Figs. 13 and 14. The images show a drop in intensity of about 25% at r = 0, corresponding to the

loss of ions with large axial velocities. The relaxation of the

remaining ions occur^ on a time scale of approximately

10-20 ms after the t = 0 trap switch. This technique clearly shows potential for studying coUisonal dynamics which are extremely difficult to observe using other techniques, and the dynamic capabilities of the imaging system can provide in- formation about the heating, cooling, and collision rates within the ion cloud. ACKNOWLEDGMENTS

The authors would like to thank D. J. Alderson for technical assistance in the lab, E. Takacs and M. Laming for helpful discussions, and E. Trabert for helping initiate this experiment. I. K. acknowledges support from the Swedish

Foundation for International Cooperation in Research and Higher Education (STINT).

'J. D. Gillaspy, Phys. Scr., T T6S, 168 (1996). ^P. Beiersdorfer, G. V. Brown, J. C. LopezUrrutia, V. Decaux, S. R. Elliott, D. W. Savin, A. J. Smith, G. S. StefanelU, K. Widmann, and K. L. Wong,

Hyperfine Interact. 99, 203 (1996). 'D. Schneider, Phys. Scr., T T59, 189 (1995). "S. B. Utter, P. Beiersdorfer, J. C. Lopez Urrutia, and K. Widmann, Nucl.

Instrum. Methods Phys. Res. A 428, 276 (1999). ^C. A. Morgan, F. G. Serpa, E. Takacs, E. S. Meyer, J. D. Gillaspy, J. Sugar, J. R. Roberts, C. M. Brown, and U. Feldman, Phys. Rev. Lett. 74, 1716 (1995). ^D. Schneider, D. Dewitt, M. W. Qark, R. Schuch, C. L. Cocke, R. Schmieder, K. J. Reed, M. H. Chen, R. E. Marrs, M. Levine, and R. Former, Phys. Rev. A 42, 3889 (1990). 'L. p. Ratliff, E. W. BeU, D. C. Parks, A. I. Pikin, and J. D. GUlaspy, Rev.

Sci. Instrum. 68, 1998 (1997).

*F. G. Serpa, J. D. Gillaspy, and E. Traebert, J. Phys. B 31, 3345 (1998). 'E. Traebert, P. Beiersdorfer, G. V. Brown, A. J. Smith, S. B. Utter, M. F. Gu, and D, W. Savin, Phys. Rev. A 60, 2034 (1999). '"D. a. Church, Phys. Scr., T T59, 216 (1995). "J. D. Gillaspy, Phys. Scr., T T71, 99 (1997).

'^C. A. Morgan, F. G. Serpa, E. Takacs, E. S. Meyer, J. D. GiUaspy, J. Sugar, J. R. Roberts, C. M. Brown, and U. Feldman, Phys. Rev. Lett. 74, 1716 (1995). "R. p. Feynman, R. B. Leighton, and M. Sands, Lectures on Physics (Addison-Wesley, Reading, MA, 1966).

'"p. Beiersdorfer, L. Schweikhard, J. C. LopezUrrutia, and K. Widmann,

Rev. Sci. Instrum. 67, 3818 (1996).

'^B. M. Penentrante, J. N. Bardsley, D. DeWitt, M. Clark, and D. Schneider, Phys. Rev. A 43, 4861 (1991).

'*P. Beiersdorfer, A. L. Osterheld, V. Decaux, and K. Widmann, Phys. Rev. Utt. 77, 5353 (1996). "E. Sokell, F. J. Currell, H. Shimizu, and S. Ohtani, Phys. Scr., T T80B,

289 (1999). '*M. A. Lennon, K. L. BeU, H. B. Gilbody, J. G. Hughes, A. E. Kingston, M. J. Murray, and F. J. Smith, J. Phys. Chem. Ref. Data 17, 1285 (1988). "B. Edlen, Mon. Not. R. Astron. Soc. 114, 700 (1954).

^"F. G. Serpa, J. D. Gillaspy, and E. Trabert, J. Phys. B 31, 3345 (1998). ^'C. A. Morgan, F. G. Serpa, E. Takacs, E. S. Meyer, J. D. Gillaspy, J. Sugar, J. R. Roberts, C. M. Brown, and U. Feldman, Phys. Rev. Lett. 74, 1716 (1995). Calculated using the relativistic Cowan code.

^^F. G. Serpa, C. A. Morgan, E. S. Meyer, J. D. Gillaspy, E. Trabert, D. A. Church, and E. Takacs, Phys. Rev. A 55, 4196 (1997).

NIST-159



PHYSICAL REVIEW A VOLUME 54, NUMBER 2 AUGUST 1996

Polarization measurements on a magnetic quadnipole line in Ne-like barium

E. Takacs,* E. S. Meyer, J. D. Gillaspy, and J. R. Roberts

Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

C. T. Chantler,^ L. T. Hudson, and R. D. Deslattes Quantum Metrology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

C. M. Brown and J. M. Laming*

E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington. D.C. 20375

J. Dubau

Observatoire de Paris, CNRS UPR No. 176, 92195 Meudon Cedex, France

M. K. Inal

Institut des Sciences Exactes, Departement de Physique, Boite Postale 119, 13000 Tlemcen, Algeria

(Received 12 December 1995) We have measured the absolute polarization of the 2p^ ^ So-2p^CP^/2)3s[3/2]2 magnetic quadnipole transition in Ne-like barium, excited in an electron-beam ion trap at a variety of energies. We find strong evidence

for the existence of resonant excitation processes that are not explained by our collisional-radiative calculations even when the polarization arising from impact excitation is included. At energies well away from where the resonances occur, the agreement between experiment and theory is good. [81050-2947(96)06208-7]

PACS number(s): 32.10.-f, 32.30.Rj, 34.80.Kw

L INTRODUCTION

In the absence of strong external electric or magnetic

fields, atomic states differing in magnetic quantum number,

but otherwise having identical principal and angular momentum quantum numbers, are degenerate in energy. Since the magnetic quantum numbers describe the spatial orientation of the atom's electron charge cloud, there may still be observable differences between such degenerate states if some

type of spatial asymmetry is present. For example, if colli- sional excitation occurs by impact in a preferred direction, the magnetic sublevels of the excited state can be populated

with nonstatistical probabilities. When the state decays, the emitted electromagnetic radiation will be spatially anisotropic and partially polarized [1]. Anisotropic excitation mechanisms are quite common in astrophysical plasmas and are readily reproduced in a laboratory environment. In solar flares, ions and atoms can be

excited by electrons moving along fixed magnetic field lines, which give rise to a preferred direction in space [2]. A similar situation occurs in supernova shock waves [3] and also in polar aurorae and possibly in jets in active galactic nuclei. On earth, there have been many crossed-beam or beam-foil

studies whereby atoms and ions are excited in a spatially

asymmetric way [4]. In the experiment described in this paper, we study the

*Also at Experimental Physics Department, Kossuth University,

Debrecen, Bern t. 18/a., Hungary.

Present address: School of Physics, University of Melbourne,

Parkville, Victoria 3052, Australia. *Also at SFA Inc., Landover, MD 20785.

1050-2947/96/54(2)71 342(9)/$ 10.00 54

polarization of radiation emitted from ions that have been

excited by impact with a unidirectional monoenergetic elec- tron beam inside an electron-beam ion trap (EBIT). The

EDIT is a powerful tool for creating very highly charged ions

for atomic structure and electron-ion interaction studies. Techniques for measuring electron-impact ionization [5], excitation [6], and recombination [7] cross sections using an EBIT have been demonstrated; however, these measure- ments have aU been adjusted to take into account polarization effects. Inside an EBIT, the ions interact with a narrow (about 60- /im-diam) beam of electrons. This well-collimated electron beam acts as a quantization axis, creating a cyhndrically

symmetric environment for the ions. Care must be taken then

in interpreting emission hne intensities when they are used

for obtaining electron-ion interaction cross sections. Polarization of the emitted radiation is especially important when

measurements are made with spectrometers in which the en- ergy disperser is polarization selective (e.g., Bragg crystal x-ray spectrometers). Even when a polarization-insensitive energy detector is used [e.g., solid-state Si(Li) detector], polarization is important because the detector is generally positioned normal to the electron beam rather than at the "magic angle" of 55° (i.e., the angle 8 at which polarization

for dipole radiation disappears, given by cos^6=l/3). Depending upon the experiment, however, polarization can be a

tool rather than a complication in the analysis. The measure- ment of the polarization or the angular distribution of photon

emission gives information about the magnetic sublevels in- volved in electron-ion collisions that would normally remain

hidden in a simple energy dispersive measurement. As we

will illustrate below, polarization-sensitive measurements may also be used to detect resonance processes that would

1342 © 1996 The American Physical Society

NIST-161
54 POLARIZATION MEASUREMENTS ON A MAGNETIC 1343

Otherwise be too weak to be observed directly. In what fol- lows, the polarization P is defined as

/||(90°)-/,(90°)

/||(90°) + /^(90°)' ^ '

where /||(90°) and /j^(90°) are the intensities of the parallel and perpendicularly polarized radiation measured at 90° with

respect to the axis of symmetry (the electron-beam direction

in our experiment).

II. OBSERVED Ne-LIKE Ml TRANSITION

In the standard spectroscopic notation for 7]/ (pair) coupling [8] used for noble-gas spectra, the spectral line we report on in this paper arises from the 2p^'5o-2/7^(^P^/2)35[3/2]2 magnetic quadrupole transi- tion in Ne-like barium (hereinafter referred to as the M2

transition or the A/ 2 line). This transition was originally ob- served in Ne-like iron created in the solar corona [9]. It has

since been observed in several other Ne-like systems in a

laboratory Tokamak source [10], as well as in an EBIT [11]. In barium, the energy of this transition is calculated to be

4.563 keV, corresponding to a wavelength of 2.717 A. The

upper level is the lowest-energy excited state of the Ne-like

systems. Because of its high angular momentum, many of

the high angular momentum excited states preferentially de- cay to it. This results in a complicated cascade feeding

scheme. Because the transition is magnetic quadrupole, it has a

relatively small Einstein-A coefficient for an x-ray transition: about 3.0X10* s~' in Ne-like barium [13]. However, in low-density plasmas such as that in an EBIT, the relative intensity of the line is comparable to that of the large Einstein-A coefficient lines. This is because of the strong cascade feeding and the fact that the number of decays per

unit time is bottlenecked by the electron-impact excitation

rates, rather than the decay rates. In previous EBIT work,- the Ml line was studied by

Beiersdorfer et al. [II] using a flat crystal spectrometer. The

electron-beam energy dependence of the intensity relative to a reference line made it possible to separately measure the

effects of different indirect line formation mechanisms.

These included resonance excitation of the line, dielectronic recombination onto F-like barium, and inner-shell ionization of Na-hke barium. As the authors pointed out in their paper,

their diffractive crystal was oriented to preferably reflect x

rays with polarization vector parallel to the electron beam

direction. They could not orient it for x rays of complementary polarization and therefore had to use theoretical esti- mates of the polarization to compare their observed hne in- tensities with predictions based on the various line formation

mechanisms (both direct and indirect). They estimated the polarization P, defined in Eq. (I) above, to be — 0.05 ±0.10 over the entire range of electron-beam energies

used. That is, they assumed that the polarization had no de- pendence upon electron-beam energy and justified this as- sumption with calculations of the impact energy dependence

(or, rather, relative independence) of the polarization of the 2p^'5o-2p5(2p«^2)345/2]t electnc-dipole line [11] using

the method of Zhang, Sampson, and Clark [12]. However, as

we remarked above, the formation of the upper level of the M2 line is dominated by cascade decays from high angular

momentum states. The relative contribution of the different cascade paths dramatically changes with electron-beam energy. Some of the paths are completely excluded once the energy of the electron beam falls below the excitation thresh- old for the root state of the path (i.e., the highest-energy state

in the path). In turn, the feeding of the different magnetic

sublevels of the M2 line also changes with the electron-beam

energy. Thus, even if the direct electron-impact excitation cross sections for the magnetic sublevels do not change rela- tive to one another with impact energy, there may still be a

significant change in the polarization because of the (energydependent) cascade feeding process. Studying the importance of this effect on the energy dependence of the polarization was the primary motivation for the present work.

III. CALCULATIONS

To gain a better understanding of the cascade feeding of

the upper level of the M2 line we carried out calculations with a coUisional-radiative model. An analysis using the HULLAC computer code [13] was done at a number of

electron-impact energies between 5.2 and 7.8 keV, which

coincided with the energy range in our experiment. The out- put of the code gives the relative populations of the levels, the decay rates, electron-impact excitation rates, and energies

for the different transitions involved. The calculation in- cluded all of the « = 3 euid n = A energy levels of Ne-like barium. Because the excitation threshold energies vary a

great deal over the « = 3 and n = A levels, the number of

levels involved in the cascade process increases quickly with electron-beam energy. At 5.2 keV, only 23 levels can be

excited, whereas at 7.8 keV, all 89 « = 3 and n = A levels can

play some role. If we consider that the population transfer between the different magnetic sublevels strongly depends on the angular momentum values of the upper and lower

levels of the cascading transition, it is clear that the change

in the relative population of the magnetic sublevels of the M2 line can be significant. Figure 1 shows the Grotrian dia- gram of the levels involved in the population of the upper

level of the M2 line at 7.8 keV beam energy. It can be seen from this figure that the upper level is preferentially populated from upper states with total angular momentum differ- ent from zero. The various excitation and cascade fractions were calculated using the HULLAC code. The cross section and collisional-radiative programs in the HULLAC code do not treat sublevels of different M for a given J. In order to model the polarization expected follow- ing cascades through a number of levels, we wrote a

collisional-radiative program that explicitly considers each M sublevel. In order to simplify the problem somewhat, we

only considered collisional excitations and deexcitations be- tween the ground state and the various excited levels, since x-ray radiative transitions are much faster than collisional transitions in the EBIT. Using angular momentum relation- ships, the Einstein-A coefficient between magnetic sublevels

in terms of the M averaged A coefficient A{Ji—^Jj) is [14]

NIST-162
1344 E. TAKACS et al. 54

Is

1/2

groBiid state 2p* J=0

odd

n = 4

n = 3

55

SO

45

40

35

- 30

25

20

HG. 1. Partial Grotian diagram showing all the levels involved in the cascade feeding of the upper level of the Ml transition at 7.81 keV

beam energy. The dominant levels are shown

with long bars and connected with arrows, while

the levels that individually contribute less than a few percent to the population are shown by short

bars.

X{2Ji+\)A{Ji-^Jj),

— Mj m Af,

(2)

where the quantity ("^j^. ^ ) is the usual Wigner 3-y symbol. In this symbol, q is the multipolarity of the transition and m = Mj — Mj, by the properties of the 3-j symbol. In our

modeling, we take the decay rates AUi~*Jj) from the pre- viously described HULLAC runs. We take collisional excitation cross sections from the same source, but split these up

into the Af -dependent quantities according to a variety of

approximations. The coUisional-radiative matrix inversion is run to establish the populations of each M sublevel. Following this, the polarization in each emission line is calculated according to Eq. (19) of Inal and Dubau [15]. As an initial estimate for the polarization fraction of each

level, we took the limits given by Percival and Seaton [1]. We assumed that only the orbital angular momentum sublevel Af, = 0 is populated in the electron impact and then coupled the orbital angular momentum with the electron

spins to form J. As such, this approximation assumes exact

1,5 coupling and therefore a polarization fraction that can

only be realized for impact excitation of neutral atoms at threshold. This approximation turns out to overestimate the measured polarization by a large factor, but the dependence on electron-beam energy is quaUtatively correct. A similar jj coupling approximation (i.e., assuming sublevels with

Mi = 0 only are populated) gives an even larger polarization

(leading to a larger discrepancy), jj couphng might be ex- pected to be a better approximation for Ne-like barium, so it

is probable that the cause of the discrepancy is our neglect of

the depolarization that occurs when the incident electron

scatters through a large angle in the Coulomb field of the

target ion. The direction of the scattered electron sets the quantization axis for the excited ion, so the stronger the in- teraction (and thus the larger the scattering angle), the greater

the depolarization will be.

Accordingly, we decided to perform detailed calculations of the impact-excitation cross sections from the ground state to the magnetic sublevels of each of the participating excited

levels at an impact energy of 410 Ry (5.578 keV). These were undertaken by two of us (J.D. and M.K.I.) essentially

following the formalism in Inal and Dubau [15]. The important simplifications are that the collisions are treated nonrelativistically and that we have only one energy point at which

the polarization is calculated. Note that the total cross sec- tions we use still retain their energy dependence (which is actually quite weak over the energy range we consider) since these are taken from our earlier HULLAC computations. Eventually a total of 37 levels were included in the calculation,

allowing us to employ the cascade model for electron-beam

energies up to 5.98 keV; above this, higher excited states are involved for which we have no polarization fraction calculations. Putting these results into our modified collisional- radiative model gives the polarizations that are shown in Table I. These results are about a factor of 4 smaller than

those predicted by the simple model described in the preceding paragraph.

We should emphasize that only excitation by direct elec- tron impact followed by radiative decay has been included in our model. Since the polarization fractions were calculated at one impact energy only, we are implicitly assuming that the polarization fractions for a given excited state do not change

substantially with beam energy over the range of interest. Energy scaling is included for the total cross sections, but not

for the fractions going to each M sublevel. Overall, the only

substantial omission in our calculation is that resonant pro- cesses appearing at energies near 5.2 and 5.8 keV have been

neglected. We believe that the effect of resonance excitation

is quite evident in the data presented below and can explain

the regions in which there is a significant discrepancy be- tween our calculations and our experiment.

rV. INSTRUMENTS

Descriptions of the history and operating principle of the EBIT have been published elsewhere [16,17]. Our machine

]SfIST-163
54 POLARIZATION MEASUREMENTS ON A MAGNETIC . 1345

TABLE I. Polarizations of the A/2 and E\ lines. Polarizations of

the Af2 and £1 lines are calculated using the coUisional-radiative model and realistic starting polarization fractions. From left to right the columns are beam energy in keV, A/2 hne polarization, E\ line polarization, and the number of levels included in the calculation.

Energy t, I \KC V ) V'o) \'0) No. or levels

5.04 -12.5 4J1 19 5.10 - 12.5 4.77 19 — 12.2 3.77 21 5.27 -12.0 3.71 23

5.40 -12.1 3.57 25

5.50 - 12.4 2.63 33

5.58 - 12.4 2.63 33

5.70 -12.4 2.63 33

5.80 -14.8 2.64 37

5.90 -15.0 3.07 37

5.98 -15.0 3.07 37

[18] is similar in design to the Lawrence Livermore National Laboratory EBIT, which is described in detail in Ref. [19]. The highly charged ions are created, excited, and trapped

radially by a 60-/im-diam, 3500-A cm ~ ^ electron beam. A

series of three cylindrical drift tubes—two end cap tubes biased at 250 V positive with respect to a center drift tube

provide axial trapping for the ions. The voltage applied to the center drift tube Vq determines the electron-beam energy

(5.0-8.0 keV in our experiment). The electron-beam energy

is not precisely equal to eVf,, however, because the space charge of the electron beam itself depresses the on-axis potential. We corrected for this effect using a simple calculation based on Gauss's law. Our electron-beam energy scale has perhaps a ± 50-eV absolute uncertainty due to the space- charge correction; the relative uncertainty is on the order of

only a few eV, though. Observation of x rays emitted by ions

in the trap is made at 90° with respect to the electron-beam

direction through a series of two berylhum windows that have a total thickness of 0. 1 75 mm. Since bariimi, which is a dopant in the electron gun cathode, boils off of the cathode and fills the trap automatically, it is one of the easiest ele- ments to study in an EBIT. Just by turning on the electron beam and tuning to an energy above the 3.3-keV ionization

potential of Na-like barium, an abundant sample of Ne-like barium is created. About 81% of naturally occurring barium

has zero nuclear spin, so that the effect of the hyperfine

interaction on line polarizations is neghgible [20]. The measurement was carried out using two identical Johann-type, bent crystal x-ray spectrometers operating si- multaneously. A description of the spectrometers and detectors can be found in Ref. [21]. For the wavelength range

studied (approximately 2.72 A), we used a Ge(220)

(.2d — 4.00 A) crystal. During the polarization measurement,

the two spectrometers were installed so that their respective

reflection planes were perpendicular to each other and at 90° to the electron-beam direction. The Bragg angle is near 45° (42.8°), so the spectrometers function as near-perfect x- ray polarizers. The polarization of the emitted radiation is obtained by inserting the observed normalized intensities in Eq. (1) and then dividing the result by a correction factor

Q [22], where Q=l-Yl-Y2, with K's representing the amount of parallel-polarized radiation that leaks through into the perpendicular-polarized measurement (and vice versa). The Q factor depends on crystal quality, curvature, alignment, and angle and is very close to unity for high-quality

crystals and a Bragg angle close to 45°. A very thin perfect

crystal (or a thick or thin mosaic crystal) would have

Y = X/{ 1 + X), where X= cos^(2^ or about 0.6% for the exact angle used in our experiment. For the case of a thick

perfect crystal, the exponent on the cosine would be 1 and

the value of X would be 7.7%. Our crystal has been previously quantified and is intermediate in thickness between the

limiting forms, leading to Y values that are about 1.9% and

an overall value of 2 = 0.963(5).

V. DETERMINATION

OF THE ABSOLUTE POLARIZATION

In order to determine the absolute line polarization, the orthogonal spectrometers had to be intensity cross calibrated. This can be done by observing an unpolarized line if one can

be found near the same energy as the line under study. This

method of intensity calibration takes account of the different geometrical and detector efficiency factors at the same time. The ideal candidate isotropic line is one that has a J=0

upper level, since in that case there is only one magnetic

sublevel and therefore P = 0. For intensity cross calibration, we used the 2p^ ^So-2p^i^Pl^)3s[3/2]° electric dipole tran- sition in Ne-Iike barium (hereinafter referred to as the £1

transition or the £1 line). This line appears at 4.568 keV,

which is much closer to the M2 line than a similar electric dipole transition that was used for the same purpose by

Beiersdorfer et al. [11]. The proximity of our El line to the M2 line allowed us to simultaneously observe them in high

resolution with our spectrometers. Calculations using the coUisional-radiative model and the HULLAC code (including all n = 3 and n = 4 states) predict

that at an electron-beam energy of 7.8 keV, the upper level of the £1 line is primarily (>80%) populated by cascades from states with J=0 angular momentum. As a conse- quence, even though the upper level of the £ 1 hne has nonzero angular momentum, it is mostly unpolarized and isotropic. Figure 2 shows the partial Grotrian diagram of the levels involved in the population of the upper level of the £ 1 line, which we constructed from the output of the HULLAC code. Using the HULLAC data, we estimate that the absolute value of the polarization for the £1 line is less than 3% over the

entire range of beam energies in our experiment. For calibra- tion purposes, we assumed complete isotropy for the £1 line and took account of the small polarization due to 7 =7^ 0 cas- cades in the error bars on our final results for the M2 line polarization.

It is interesting to compare the ratio of the efficiencies of

the two spectrometers based upon observation of the mostly

unpolarized £1 line with a sophisticated computer model. The two spectrometers are very nearly identical, but because

the x-ray source is in the shape of a thin cylinder (only ions within the 60 pum diameter of the electron beam and within

the 2 cm length of the center drift mbe can be excited within

the EBIT), there is a rather large geometrical effect; that is, their relative efficiencies depend strongly upon their orienta-

]SnST-164
1346 E. TAKACS et al. 54

FIG. 2. Partial Grotrian diagram showing all the levels involved in the calculation of the cas- cade feeding of the upper level of the £1 transi- tion for electron-beam energy 7.81 keV. The notation is explained in the caption to Fig. 1.

ground iUte

lion. The computer model [23] predicts that the spectrometer

[Hirpendicular to the electron beam has a factor of 1.9±0.20 greater efficiency than the one oriented parallel to ihc electron beam. Using the unpolarized El line as a refer- ence, we observe experimentally an efficiency factor of 1.83.

VI. DATA ACQUISITION AND EVALUATION

Data were taken at 15 different electron impact energies between 4.97 and 7.81 keV. The energies were chosen to fall below the excitation thresholds of certain n = 4 and n = 3

levels so as to exclude them completely from the cascade

feeding routes to the upper level of the £1 line. Each data

point represents between 6 and 12 h of collection time. Longer times were required for the lowest energies because

fewer levels are excited that can potentially feed the A/ 2 or

li\ lines, thus making the lines appear weaker. Figure 3

.shows typical spectra at a beam energy of 6.00 keV for each

spectrometer. The M2 and £1 lines are clearly resolved and

no other strong features are present in the spectrum. For all of the data points, electron-beam currents ranged between

n.'j and 150 mA, depending on the energy we had set. The

current and energy were held fixed during the entire collection time and a small amount of N2 was injected to improve

evaporative cooling of the trapped barium ions [24]. After

the data were taken, the six best spectra from each spectrometer were summed to generate two spectra with the highest

signal-to-noise ratio. These were used to determine the line centers as accurately as possible and to extract the individual spectrometer response functions. The line centers and re- sponse functions were then held fixed in subsequent fits used

10 extract the polarization-dependent line intensities. The

data evaluation was done using a spectrum fitting program

1 25]. Initially, peaks were fit to Voigt line shapes, but it was

ijuickly discovered that the peaks had a predominantly

(luussian shape. Accordingly, fits to Gaussian line shapes

with linear background subtraction were performed for all of

Ihe spectra. From fits to each of the 15 different spectra

(representing 15 different electron-beam energies), the intensily of both lines (7 = 2 and 7=1) was determined. Comparing the intensity ratio of the two lines for each spectrometer gave a measure of the absolute polarization in accordance

with Eq. (1).

Vn. SYSTEMATIC ERRORS

We have already mentioned some minor corrections that must be applied to the measured polarization, arising from a small polarization dependence in the crystal reflectivity, as well as a small deviation in the Bragg angle from 45°. Ac400 r

300

g 200 oU

100

200

150

4560 4570 4580 4590

X-ray Energy (eV)

FIG. 3. Simultaneous data from the spectrometers that measure

perpendicular (top) and parallel (bottom) polarization. The beam

energy was 6.00 keV. The M2 and El lines are clearly resolved in both spectra. X-ray energy increases to the right, so that the M2 line

lies to the left (lower energy) of the £1 line.

NIST-165
54 POLARIZATION MEASUREMENTS ON A MAGNETIC . 1347

cording to our model calculations [23] of the crystal reflectivity (accurate to 1-2%), the corresponding correction to the polarization is no more than 1.6%, with a far smaller

uncertainty. These model calculations also allow us to apply

a theoretical normalization to the two spectrometers so we

can experimentally confirm that the reference line is indeed

unpolarized, P = 0.0(1). In addition, the model calculations imply that the x-ray signal emerging from our EBIT forms

an effective source height of 13(7) mm.

Another correction and accompanying source of error

arises from the transverse motion of electrons in the beam.

The presence of the strong magnetic field Bq in the EBIT

allows rigid-rotor motion of the electron beam at a maximum

angular frequency approximately equal to the electron cyclotron frequency (0^^ = eBQ/m^ [26]. This rotation can develop

when the beam moves through a magnetic-field gradient. In the EBIT, the electrons move from a region of near-zero magnetic field at the cathode to a region of 3-T magnetic

field in the center of the trap [27]. The incident electron

velocity vectors therefore lie on the surface of an inverted cone and this leads to some depolarization from the case of a

perfectly aligned beam.

The total velocity of the electron beam is determined by

the potential Vq applied to the center drift tube. We estimate

the transverse component of this total velocity using several independent methods. First, we note that under our conditions of 3-T field, the angular frequency is 5xlO" Hz,

which means a maximum transverse kinetic energy of 700

eV for electron orbits inside a 60-/im-diam beam. This trans- verse energy is consistent with the fact that we have been

unable to operate our EBIT below 700 eV total beam energy

without picking up significant stray currents in the electrodes (such as the electron gun anode) that surround the electron beam at various points along its trajectory. The maximum

possible rotation velocity estimated in this way gives an

angle of incidence with respect to normal ("pitch angle")

ranging from 21° to 17° for total beam energies ranging

from 5.0 to 7.8 keV, respectively. The final angular velocity of the beam could be less than the maximum, however. It is essentially determined by the magnetic field at the cathode

since v]^/B is an adiabatic invariant for a charged particle

traveling through a fixed magnetic field. Thus we make our second estimate as follows. We note that at the cathode, i;|

is of order 2kT^lm^ , where 7^= 1500 K is the temperature of the cathode. The cathode magnetic field B^ is not known,

but it is believed to be of order a few hundred microtesla (a few gauss) since that is the level of control that the EBIT

bucking coil (which is used to null out the field at the cath- ode) gives. As discussed in the Appendix, a theoretical esti- mate of the cathode magnetic field gives values 5^=240-210 /i,T (2.4-2.1 G), which give pitch angles of 24° -20° for beam energies in the range 5.0-7.8 keV, re- spectively. This second estimate should be considered as a rough estimate for the "typical" pitch angles, but since it is somewhat larger than the more rigorously obtained pitch

angles obtained in the first estimate, we use the first estimate

for analysis of our data. With the incident electron velocity vectors lying on the surface of an inverted cone, there are two geometric effects

that will cause some depolarization. The first is that the angle

of observation with respect to the incident electron velocity

vector will deviate from 90°. The second and more serious problem is that for electron velocity vectors with nonzero components lying in the plane normal to the observation di- rection, the axes for the polarization measurement will be

rotated. The first case was accounted for by running polarization calculations as described above, but for an average

angular deviation from observation at right angles. The cosine of this angle is given by

(cos;3) = (2/77) I

sin( <^p)cos( d)de=2s\n{ t/),,)/ 77=0.2,

(3)

where is the typical electron pitch angle to the beam axis, given by arctan(i;j^ /d||), taken here to be about 17°. In the

limit of small angles, the observed polarization is reduced by

a factor 1 -{cos/3)^ [15], or about 0.96. The second effect reduces the observed polarization by a

factor

ir/2

(cos(20)) = (2/7r) tan^((^p)sin^(^)

r ^alio T+

dd

= \-\an\<l>p)

==0.90. (4)

This expression is easily obtained from the general form of

Eq. (1) rotated through an angle (f> rather than evaluated at 90°.The combined result of the two geometric effects is the product of the two, so the measured polarization is approximately 87% of the true value. This percentage will change

slightly (about 5%) with beam energy in the range we consider, but not sufficiently to alter the energy dependence of our observed polarization.

We do not have direct measurements of the magnitude of

the transverse kinetic energy of electrons in the EBIT or of

the cathode magnetic field (from which the final transverse

kinetic energy can be estimated). We note, however, that a polarization measurement of a suitable line might in fact be a good way of measuring this. A strong line with a large polarization could be monitored as the cathode magnetic field Be is changed through tuning of the bucking coil current. Another potential source of systematic error arises from

anisotropy in the emitted radiation from the E\ line used for normalization. In the analysis, we determined the cascade

feeding scheme with the hullac code as in the case of the Ml line. Our estimate for the polarization of the £1 line, which is probably an overestimate for energies well above

the excitation threshold, is P = - 1 .6% at high energies. Over

the entire range of our energies, the £1 polarization is esti- mated to remain below 3% and the effect on the polarization of the M2 line, in turn, is less than 0.01. Note that in esti- mating the polarization of the £ 1 line we have neglected the

effect of resonances, but these should further drive the polarization towards the assumed value of zero, just as they do

for the M2 line. Finally, there is an uncertainty in the absolute beam energy. As discussed above, there is a depression of the energy due to the negative space charge of the electron beam, which

tends to reduce the on-axis potential with respect to the drift

NIST-166
1348 E. TAKACS et al. 54

10 r

4J 5.0 5.5 6.0 6.5 7.0 7J 8.0 Electron Energy (keV)

FIG. 4. Polarization P of the M2 line as a function of beam

energy. The 15 beam energies were chosen to fall below the excitation thresholds of certain cascading levels, thus completely excluding those levels from the cascade scheme. Both the simple

theory (which neglects levels with a nonzero orbital angular momentum, as appropriate near the excitation threshold) and the best

presently available theoretical estimate (v/hich includes collisional

radiative effects but not resonant excitation processes) are also shown with a dash-dotted line and a dashed line, respectively.

tube electrode voltage Vq. This space-charge correction can

be calculated; however, there is an additional complication

because trapped positive ions (both barium ions and back- ground gas ions) partially neutralize the electron-beam space

charge. This neutralization is difficult to determine theoretically, but it is possible to estimate it experimentally by observing the wavelength of a radiative recombination line (which changes with electron-beam energy) or by observing

the change in intensity of a dielectronic recombination reso- nance line widi beam energy. We determined that the overall space-charge correction in our situation was 250 eV, with an

uncertainty of ± 50 eV. The uncertainty, however, represents an unknown but constant offset that must be apphed to all of

the data; the relative beam energies are known to within ±5 eV. Thus, in the results that follow, the energy scale has

an offset uncertainty of about ± 50 eV, but the shape of the curve of polarization versus energy is certain within the onestandard-uncertainty error bars shown.

Vra. RESULTS

Our measurements are shown in Fig. 4 as a plot of the

polarization P of the Ml line as a function of electron- impact energy. The beam energies were chosen to exclude

certain levels from the cascade scheme, thereby simplifying

the theoretical calculations and maximizing the significance of each data point. For example, all « = 4 levels were ex- cluded in the measurements below 6.3 keV. Also, at these electron-impact energies, neither the resonance excitation to

the F-like charge state nor the inner-shell ionization of the Na-like charge state can play a role since both are energetically excluded. The theoretical estimates are also shown in Fig. 4. The

dash-dotted Une corresponds to an approximation where only

the sublevels with zero orbital angular momentum are populated. This is a very crude approximation and supposed to be

valid only near the excitation threshold. The fairly strong decrease in the absolute value of the polarization around 5.5 keV beam energy coincides with the

intensity increase seen by Beiersdorfer et al. [1 1] in the parallel polarized component of the Ml line. This intensity in- crease was interpreted by Beiersdorfer et al. [11] to be the

result of a resonance excitation of the upper level of the Ml line. In this two-step process, a dielectronic resonance

transition to one of the autoionizing Na-like levels takes

place. The dielectronic process is followed by an autoionization, whereby the final state includes the upper level of the Ml line. If the interpretation is correct, the 25% decrease in the line polarization could be accounted for by the resonance

excitation process itself. However, since polarization calculations already show a similar tendency, we have to conclude

that at least part of the polarization change is due to the change in the cascade feeding scheme for the upper level of

the Ml line. The assumption made by Beiersdorfer et al. in Ref. [ll]ofa-5± 10% polarization that is independent of

electron-impact energy holds over a good part of the energy

range of our measurement, but not in those places where

resonances can occur. In our case, we measured a negative

line polarization with an average value of — 0.12±0.10%.

The absolute value of the polarization increases close to the

direct excitation threshold energy and decreases strongly near 5.2 and 5.8 keV, where LNO and LOO dielectronic resonance (DR) excitations are allowed (in Auger notation,

LOO denotes an L-shell electron promoted to the O shell while an incident unbound electron is captured in the O

shell). These results do not change the basic conclusions of

Ref. [11] and we believe that these deviations of the polar- ization from its average value are in fact due to the indirect processes reported by Beiersdorfer et al. [11]. In fact, we

attribute the region of strong deviation near 5.8 keV to an LOO resonance process that was not observed in Ref. [11].

It appears that the resonant excitation process (DR fol- lowed by autoionization) creates unpolarized radiation. In a simple picture, this might be expected, since even if the initial DR process populates the m sublevels of the doubly excited intermediate state nonuniformly, the subsequent elec- tron emission in the autoionization process will scramble the quantization axis of the final-state singly excited ion.

IX. CONCLUSION

We have measured the absolute polarization of the

'

2p^ 'So-2/7^(^P3/2)3s[3/2]2 magnetic quadrupole transi- tion in Ne-like barium. We find that for electron-impact excitation in the energy range from 5.0 to 7.8 keV, this line shows a strong negative polarization. The polarization typically falls between —4% and -18%, but changes sharply near 5.0 keV, as well as near 5.1-5.3 and 5.8 keV.

We have also found a steady decrease in the polarization

between 5.5 and 6.0 keV, which should be due at least in part

to the change in the cascade feeding routes. It appears that the quantitative agreement between models and experiment

is greatly improved when realistically calculated polarization

fractions are included. The precise degree of agreement at certain energies is probably fortuitous, however, since important resonant processes have not been included. In fact, it seems that polarization measurement may represent a sensi-

]SnST-167
54 POLARIZATION MEASUREMENTS ON A MAGNETIC 1349

live method for confirming the presence of such resonant

excitation processes. In our data, for example, a decrease in the absolute value of the polarization is quite prominent near

5.8 keV, which we interpret as being due to an LOO dielectronic resonance excitation. This resonance is difficult to isolate in a plot of total line intensity versus beam energy and was not observable in Ref. [11]. Finally, we note that polarization measurements might

also be used to determine the magnitude of rigid-rotor motion in the electron beam of an EBIT. This rotation might be

controlled through tuning of the cathode magnetic field, and

it would be interesting to see if such effects are indeed important in EBIT. ACKNOWLEDGMENTS

We thank E. Bell and F.G. Serpa for assistance during the course of this experiment and V. Jacobs for helpful discussions. One of the authors (E.T.) acknowledges the Foundation for Hungarian Higher Education and Research for support.

APPENDIX

Here we discuss the intensity of the magnetic field at the EBIT cathode . While the minimum electron-beam radius

is obtained for zero field at the cathode, under this condition

the electrons carmot enter the trap. By maximizing an expression for the current density in the ion trap j.

(Al)

in the trap with respect to B^ , where Uj^, is the perpendicular

velocity in the trap (as opposed to at the cathode, denoted by

v^c)' is the electron density, and is the number of

electrons per unit length in the beam (assimied constant), we

can derive a theoretical estimate for the optimum cathode magnetic field. The beam radius R is given by Herrmann's

theory [28]

1 1 1+4 „2d2 4 +

V B,r^

1/2 1/2

(A2)

where is the Brillouin radius, is the cathode radius,

B, is the trap magnetic field, T^. is the cathode temperature,

is the electron mass, and 77 is the charge to mass ratio q/m^ of the electron. A consideration of adiabatic invariant quantities leads to the following expression for the perpendicular velocity in the trap:

1/2 (A3)

Substituting Eqs. (A2) and (A3) into (Al), differentiating

with respect to B^ , setting the result equal to zero, and rear- ranging gives

Bl = B', (A4)

which can be simplified for v>v^, and R — rf, to give

4i;

R' 1/3 (A5)

Taking r^=1500 K, which gives 1.5X 10^ ms~',

/? = 3X10~^ m, r^= 1.5X10"^ m, and B,=3 T, gives a value for Be of (2.1 — 2.4) X 10"'* T for electron-beam energies of 5.0-7.8 keV, in reasonable agreement with (and completely independent from) the value inferred from the sensitivity of the electron beam to the bucking coil current.

[1] I. C. Percival and M. J. Seaton, Philos. Trans. R. Soc. London

Set. A 251, 113 (1958).

[2] See, e.g., J. M. Laming, Astrophys. J. 357, 275 (1990).

[3] J. M. Laming, Astrophys. J. 362, 219 (1990).

[4] J. M. Laming, Nucl. Instrum. Methods Phys. Res. Sect. B 43, 359 (1989).

[5] D. Vogel, P. Beiersdorfer, R. Marts, K. Wong, and R. Zasadzinski, Z. Phys. D 21, S193 (1991); K. L. Wong, P. Beiersdorfer, D. Vogel, R. Marrs, and M. Levine, ibid. 21, S197 (1991).

[6] R. E. Marrs, M. A. Levine, D. A. Knapp, and J. R. Henderson,

Phys. Rev. Len. 60, 1715 (1988); C. M. Brown, U. Feldman,

G. A. Doschek, J. F. Seely, R. E. LaVilla, V. L. Jacobs, J. R. Henderson, D. A. Knapp, R. E. Marrs, P. Beiersdorfer, and M. A. Levine, Phys. Rev. A 40, 4089 (1989).

[7] D. A. Knapp, R. E. Marrs, M. A. Levine, C. L. Bennett, M. H. Chen, J. R. Henderson, M. B. Schneider, and J. H. Scofield, Phys. Rev. Lett. 62, 2104 (1989); D. A. Knapp, Z. Phys. D 21, S143 (1991).

[8] W. C. Martin, R. Zalubas, and L. Hagan. Atomic Energy Levels—The Rare-Earth Elements, Natl. Bur. Stand. (U.S.)

Circ. No. 60 (U.S. GPO, Washington, DC, 1978).

[9] M. Loulergue and H. Nussbaumer, Astron. Astrophys. 24, 209

(1973); J. H. Parkinson, ibid. 24, 215 (1973). [10] M. Klapisch et al, Phys. Lett. 69A, 34 (1978). [11] P. Beiersdorfer, A. L. Osterheld, M. H. Chen, J. R. Henderson,

D. A. Knapp, M. A. Levine, R. E. Marrs, K. J. Reed, M. B. Schneider, and D. A. Vogel, Phys. Rev. Lett. 65, 1995 (1990).

[12] H. L. Zhang, D. H. Sampson, and R. E. H. Clark, Phys. Rev. A

41, 198 (1990). [13] M. Klapisch, A. Bar Shalom, W. H. Goldstein, E. Meroz, A. Chon, and M. Cohen, hullac code for atomic physics, 1988

(unpubUshed).

[14] This is derived from the Wigner-Eckart theorem and properties of the 3-y symbols.

[15] M. K. Inal and J. Dubau, J. Phys. B 20, 4221 (1987); 22, 3329

(1989) (this paper corrects some typographical errors in the relevant equation). [16] R. E. Marrs, P. Beiersdorfer, and D. Schneider, Phys. Today

47 (10), 27 (1994). [17] J. D. Gillaspy, Proceedings of the Sixth International Sympol^JIST-168
1350

sium on Electron Beam Ion Sources [Phys. Scr. (to be published)]. [18] J. D. Gillaspy, J. R. Roberts, C. M. Brown, and U. Feldman, in Vlth International Conference on the Physics of Highly

Charged Ions, edited by P. Richard, M. Stockli, C. L. Cocke,

and C. D. Lin, AIP Conf. Proc. No. 274 (AlP, New York,

1993), p. 682.

[19] M. A. Levine, R. E. Maris, J. R. Henderson, D. A. Knapp, and M. B. Schneider, Phys. Scr. T22, 157 (1988).

[20] J. R. Henderson, P. Beiersdorfer, C. L. Bennett, S. Chantrenne, D. A. Knapp, R. E. Marrs, M. B. Schneider, K. L. Wong, G'. A. Doschek, J. F. Seely, C. M. Brown, R. E. LaVilla, J. Dubau,

and M. A. Levine, Phys. Rev. Lett. 65, 705 (1990).

[21] B. P. Duval, J. Barth, R. D. Deslattes, A. Henins, and G. G.

Luther, Nucl. Instrum. Methods 222, 274 (1984); A. Henins, Rev. Sci. Instrum. 58, 1173 (1987).

54

[22] B. Cleff, Acta Phys. Pol. A 61, 285 (1982). [23] C. T. Chantler (private communication) [using the computer

code described in C. T. Chantler, Ph.D. thesis, Oxford University, 1990 (unpublished)]; J. Appl. Crystallogr. 25, 674 (1992);

25, 694 (1992)]. [24] B. M. Penetrante, J. N. Bardsley, M. A. Levine, D. A. Knapp,

and R. E. Marrs, Phys. Rev. A 43, 4873 (1991). [25] J. Vegh, Candidate's thesis, Institute of Nuclear Research of

the Hungarian Academy of Sciences (ATOMKI), 1991 (un- published).

[26] R. C. Davidson, Theory of Nonneutral Plasmas (Benjamin,

New York, 1974). [27] J. D. Jackson, Classical Electrodynamics (Wiley, New York,

1975).

[28] G. Herrmann, J. Appl. Phys. 29, 127 (1958).

E. TAKACS et al.

NIST-169
PHYSICAL REVIEW A VOLUME 54, NUMBER 3 SEPTEMBER 1996

Radiative and inner-shell dielectronic recombination in a highly charged barium ion

D. J. McLaughlin

Department of Physics, University of Hartford, West Hartford, Connecticut 06117

Y. Hahn

Department of Physics, University of Connecticut, Storrs, Connecticut 06269

E. Takacs, E. S. Meyer, and J. D. Gillaspy

Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

(Received 28 September 1995)

The direct radiative recombination (RR) to /i = 3, 4, and 5 levels and the resonant dielectronic recombination (DR) cross section involving 2p^3d excitation of Sc-like barium Ba^^"^ and Ti-like barium Ba^"*"*" are calculated and compared with a recent electron beam ion trap experiment at the National Institute of Standards and Technology. Assuming a fractional population of Ba^^^ in the trap of about 30%, we obtain good

agreement between theory and experiment for the cross section ratio t7(DR)/(T^RR)(«=4), as well as for (7(RR)(/j=5)/tT(RR)(«=4). The result confirms again that the simple angular momentum averaged procedure can be effective in treating DR for heavy open-shell ions when the energy resolution is not high. A large, broad peak below 2 keV in the x-ray spectrum is being theoretically examined. [SI 050-2947(96)04008-5]

PACS number(s): 34.80.Kw, 32.80.Hd

I. INTRODUCTION

The availability of the electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory (LLNL), Oxford,

and the National Institute of Standards and Technology

(NIST) has opened up a rich field of experiments with highly

charged ions at low velocities. There have been several re- cent measurements of x-ray spectra emitted by highly

charged ions produced by EBIT [1,2] and merged beams [3] that involve electron capture with inner-shell excitations. In a recent investigation carried out with the EBIT at NIST, scandiumlike barium Ba^'^([Ar]3c?^) and titaniumlike barium Ba^+([Ar]3t/^) ions were created, trapped, and excited using an electron beam of approximately 2.3 keV energy. A strong x-ray emission peak was observed at 4.6 keV,

and several smaller peaks were measured at lower energies,

down to approximately 2 keV where the spectrum is gradually cut off due to detector limitations (Fig. 1). This experi- ment is particularly interesting because x-ray peaks arising

from both dielectronic recombination (DR), labeled a in Fig.

1, and radiative recombination (RR), labeled b in Fig. 1, can

be studied simultaneously. Since the binding energy of the 3d orbital is about 2.3 keV and noting the energy difference between the 2p and 3i/ orbitals is 4.6 keV, we attributed the principal x-ray peak to a DR process in which a continuum

electron collisionally excites alp electron in the trapped ion and is in turn captured into the 3d orbital. One of the M-shell electrons {3d) of the recombined ion subsequently

decays radiatively to the 2p vacancy, thereby emitting a 4.6 keV x-ray, i.e., an x ray of energy twice the incident kinetic energy of the projectile electron. The peak due to this DR

process is denoted in Fig. 1 as ab because it also contains

contributions from RR corresponding to radiative capture

into states with principal quantum number «=3. The smaller DR peak labeled a ' at 4.0 keV corresponds to radiative de- cay of an M-shell 3s electron into the 2p vacancy. The x ray

1 050-2947/96/54(3)72040(10)/$ 10.00 54

peaks at 3.7 keV (Z74) and at 3.3 keV {b5) and 3.4 keV {bA)

correspond to direct radiative capture into states with n=A

and 5, respectively, while the broad peak {b) at 2.9 keV

corresponds to RR into states with n>6. Finally, a very large

and broad bump (c) was observed, stretching over the en- ergy range 1-2 keV. This peak is not treated here. The de- pendence of this spectrum on electron-beam energy was

studied by varying the beam energy by as much as 100 eV

on both sides of the DR resonance. These spectra are shown

in Fig. 2. We note that as the kinetic energy of the electron beam moves away from the DR resonance energy the RR

peak in ab shifts to higher energy while the resonances, a and a', gradually disappear. Peaks bA and ZjS (as well as bump c) remain more or less the same in magnitude but

40x1

0'

: E-Beam Energy = 2350 eV ab 1

L_,.t^.t-.r-rt-i-rr1l 1 1 1 1 1 .1 1 1 1 1

b

, , , 1 , . .Vi . . .Vt'rill

0 1 2 3 4 5 Energy (keV)

FIG. 1. Sc-like barium spectrum at an electron-beam energy of approximately 2350 eV and a beam current of 65 mA. The detector

resolution is about 200 eV, and the spread in electron-beam energy

is about 30 eV. The spectrum was acquired over six hours.

2040 © 1996 The American Physical Society

NIST- 170
54 RADIATIVE AND INNER-SHELL DIELECTRONIC . 2041

1000 r

600 r

1000 r

800 -

§ 600 EE-Beam Energy from 2270 eV to 2340 eV

Steps of 10 eV, offset of 50 counts each.

2 3 Energy (keV)

E-Beam Energy from 2440 eV to 2350 eV

Steps of 10 eV, offset of 50 counts each.

FIG. 2. Sc-like barium spectra taken at electron-beam energies ranging from 2270 eV to 2440 eV in steps of 10 eV. The top graph shows

the spectra for energies above the main DR reso- nance, with the beam energy decreasing with offset from the x axis. The bottom graph shows the spectra for energies below the main DR reso- nance, with the beam energy increasing with offset from the x axis.

2 3 Energy (keV)

slowly shift to higher energies with increasing electron energy. The main peak ab is reduced in size off resonance, but

there is a small recognizable peak which persists at about 4.6 keV. This important information, together with approximate

energy levels of the ground and excited states of Ba^^^ cal- culated by Kim [4], is sufficient to assign tentatively the peaks b4 and b5 and the residual peak in ab to RR processes, and the main portion of the peak ab and a' to DR

processes. We present here the experimental data of DR and RR for

the Ba^"*"^ and Ba^^"^ target ions and details of an estimation of their cross sections. Our calculations indicate that the 4.6 keV peak ab and the peak a' at 4 keV do indeed arise from

DR and estimates of the relative magnitude of the accompanying RR cross sections are found to be in favorable agree- ment with the data.

n. EXPERIMENTAL DISCUSSION

The operation of the EBIT has been described in several reviews [5]. Briefly, the EBIT consists of a 10-150 mA electron beam focused to less than 100 fim diameter by a strong

magnetic field. The electron beam is accelerated to between

1 keV and 30 keV by a series of positively biased cylindrical electrodes or drift tubes. Atoms and low-charged ions that

are injected into the center of the drift tubes are stripped to higher-charge states by the electron beam and trapped radially by the space charge of the electron beam as well as the action of the magnetic field. The trap along the electron beam is formed by a collection of three drift tubes, a center

drift tube and two end caps. The end caps are biased 100-

500 V above the center one to provide axial trapping for the

ions. The electron-ion interaction energy is controlled by the absolute voltage applied to the center drift tube. Barium is perhaps the easiest element to study in the EBIT because it is a primary dopant in the electron gun

cathode, and sufficient barium evaporates from the heated cathode to provide an abundant source for the trap. To make

a specific charge state, the electron-beam energy must be

tuned appropriately (to an energy above the ionization potential of the next lowest-charge state, but slightly below the ionization potential of the desired charge state). The energy

of the electron beam is controlled by the voltage Vq applied

to the center drift tube, but it is not exactly equal to e Vq. The

space charge of the electron beam depresses the on-axis potential somewhat, and therefore lowers the electron-beam en- ergy below this. It is a simple matter to calculate the effect of

the electron space charge on the energy [6], but inside the EBIT there is a complication due to the partial neutralization

NIST-171
2042 McLaughlin, hahn, takacs, meyer, and gillaspy 54

of the negative electron space charge by the positive charge

of trapped ions. It is difficult to determine the total number

of trapped ions of all charge states (including those of any background gas trapped), but through a combination of cal- culation and measurement of the position of certain radiative recombination (RR) lines, it is possible to get a good esti- mate. We estimate that the overall space-charge correction to the energy is 150 eV, with an uncertainty of ±50 eV. In order to produce Ba^^*, it is necessary for the electron-beam energy to be greater than the ionization potential of Ti-like barium (2.259 keV). In order to prevent the conversion of

Ba^^"^ into Ba^^""", it is necessary to tune the beam energy to

just below the ionization potential of the Ba^^""" state itself (about 2.355 keV). We adjusted the drift tube potential so as

to give a space-charge corrected value of the beam energy of

about 2.35 keV. We confirmed that the beam energy was in the correct place by monitoring a previously observed [7] visible line in Ti-like barium using a monochromator and

phototube. By maximizing the strength of this line and then

increasing the center drift tube voltage by 100 V, we could be confident that the beam was optinuzed for producing Sc- Uke barium. The EBIT was designed with several side ports at 90° to the electron beam which look directly into the ion trap. Certain ports are covered by thin (0.125 mm) berylhum windows, so as to hold the vacuum without appreciably attenuating X rays of energy greater than 2.5 keV. We took a number of Sc-like barium spectra using a solid-state Si(Li)

detector. The spectrum shown in Fig. 1 was integrated over

six hours at an estimated beam energy of 2.35 keV and a beam current of 56 mA. Although the monochromaticity of

the electron beam has not been measured directly, we expect

that the energy spread in the beam is less than 50 eV and

probably less than 30 eV full width at half maximum

(FWHM). The detector has an energy resolution of approximately 200 eV, and the combined detector-window system

has about 45% efficiency at 2.5 keV, which increases to about 75% at 3.5 keV and 90% at 4.5 keV.

Using a computer simulation of the evolution of the charge state balance within the trap [8], which neglects the

effect of DR, we estimate that over 90% of the trapped

barium ions are either in the Ba^'*'*^ or Ba^^"^ states, with a

ratio of 1:1.4 between diem at 2.35 keV electron-beam energy. Including the effect of the expected DR process in the

calculation, the ratio of Ba^^^ to Ba^* may be reduced significantly. In the analysis diat follows, we account for this reduction by allowing P, the ratio of Ba''^"^ ion density to the sum of the Ba'^"^ and Ba^'** ion densities, to be reduced by a

factor of p (0<^<1) which we track throughout the analysis

to insure that the final comparison with theory is not circular. To confirm the line identifications described above, we measured spectra at a number of different beam energies in 10 eV steps. The precision with which we can change the center electron-beam energy is better than 2 eV, although the spread in electron-beam energy might be several tens of eV. The spectra taken at steps in beam energy are shown in Fig.

2. The main peak {ab) is reduced in magnitude as the beam

energy is varied and at the same time its centroid shifts with

energy. This is because the peak {ab) is composed of one DR peak and one RR peak. The resonant DR peak does not move, but its intensity is reduced when the RR peak moves.

Therefore the overall shift is more apparant off resonance. The radiative recombination (RR) lines all shift in the direc- tion of change in the beam energy.

ra. DR CROSS SECTION

The DR process proceeds as

^-+A2+(/)^/i(^-" + **(rf)-A(^-')+*(/ ) + hcj. (1)

The initial capture is a resonant process with conservation of energy and momentum leading to a doubly excited autoionizing resonance state ^4**. As this state relaxes by x-ray emission (t/—*/ ), the x-ray energies hoi=E^—Ef provide

the distinct signature of the process. The DR cross section o^\i^d—>f ) calculated in the distorted-wave-projection

operator method [9] and in the isolated resonance approximation, is given explicitly by

4tt

(^c^o)

2 ToV,id-*i,e,)<oid-^f )

XS{Ej-^E, + eJiTTal), (2)

where k^. is the wave number of the continuum electron, is the Bohr radius, and Tq is the atomic unit of time. Vg(i—*d)

is the inverse of the autoionization rate i4„(<i—>/) of the in- termediate state, and they are related by V^ = gj/(2gj)Aa,

where and g, are the statistical weights of the intermediate and initial states, respectively. Also introduced in (2) is the fluorescence yield

Trid) r,{d) ^

^^^^=r.(j)+r„(j) = T(d)- = t

where T,(d) = l,fA,{d^f ) and r„(J) = S,,/l„(£/->i') are

the total radiative and autoionization widths, respectively, of

the d state of full width rid) = r,(d) + r^id). The Lorentzian factor is defined as

S{Ej-*Ei + ec)-= r{d)

2tt iE, + e,-Ej)'

with

I

Sde,= l. (4)

All possible states /" that are connected to d, and which are allowed by energy conservation and angular momentum and

parity rules, are summed over in r^id).

The bound-state orbitals used in the calculation of Aa(d-^i) and the radiative decay rate A^id-^f ) are computed numerically with the nonrelativistic, singleconfiguration Hartree-Fock code [10]. The continuum-wave

function is calculated with the Hartree-Fock direct and exphcit nonlocal exchange potentials. Because of the strong

configuration mixing required for the ions of interest here, and the large number of angular momentum coupled states involved in the intermediate states (d), A^ and A^ may be

conveniently evaluated in an angular momentum average (AMA) scheme in which all the couplings are averaged. This

scheme only requires specification of the orbitals directly

NIST-172
54 RADIATIVE AND INNER-SHELL DIELECTRONIC .

.

2043

involved and their statistical factors [11]. Spectator electrons play no direct role in AMA, except to screen the active elec- trons (thereby altering the radial wave functions) and to modify some of the statistical factors. An additional justifi- cation for the choice of the AMA is that the experimental

width of the electron beam is ABe,p~30 to 50 eV, which

averages out contributions from many term levels. Our past experience shows that the AMA and the resulting o*** are

quite reasonable, particularly when dominant states d are in- volved, as is the case here [12]. [For a given <i-state configuration there is only one AMA state but, in general, many LS

terms, so the calculation of DR cross sections is conveniently

done in the AMA scheme. In fact, in the limit that rr§>rQ,

tij(J)~l, and the total DR cross sections obtained in differ- ent coupling schemes are identical to that of the AMA be- cause of the sum over all terms involved.] The AMA scheme

tends to overestimate the cross section by roughly

20%~40%, however.

Since the full width T^d) of the DR peak described by S

is very narrow, on the order of ~0. 1 eV or less, it is convenient for comparison with experiment to define an energy

averaged cross section [11]

^\i,e,^d-^f)=^^

I

a^\i,e,^d^f )de[

-DR

(5)

where is the kinetic energy of the recombining electron. Obviously, depends on the choice of Ae^. while

5'^'^=Ae,o*'^for the area under the DR peak is independent

of Ae,. . The energy-averaged form is especially useful when

experimental energy resolution of the electron beam is much

larger than V{d), and/or in describing many resonances which are present in a small energy interval. The choice for the energy bin Ae^ is completely arbitrary, so long as ^e^>Y{d). Two convenient choices are I^e^=W=\h&

detector-window resolution, or Ae<. = AB=the electron- beam width. We choose in this paper

(6)

because the DR data are presented with the width W. As will become clear below, however, the final comparison is inde- pendent of this choice, especially when W>AB.

rV. RR CROSS SECTION

We estimate the direct radiative recombination cross section for the processes

e ~

-I- [ Ar] 3 d" -> [ Ar] 3 4 / -h ft oj

,

(7)

(8)

where n=A for the Ba-*^^ ion and n = 3 for the Ba^'"^ ion. The direct recombination cross section defined for capture

(8) into a completely empty subshell nl is given by [13,14]

JO)RR_.

x(<5'")'](^«^). (9)

where a=e Ihc and is the energy of the continuum elece l'' tron. The radial integrals in (9) are defined as 7?*^ = f^R^ iiRnir^dr, where /?„/ and 7?^ /< are the bound state and continuum radial wave functions, respectively, and

R^ ii is here momentum normahzed [13]. For capture (7) into a partially filled shell, the RR cross section is reduced

simply as

2(2/^+1)

JO)RR

(10)

where /ly is the number of holes in the subshell n Ij before the

radiative recombination transition and /y^ is the orbital angular momentum quantum number. The bound-state and

continuum-wave functions are calculated as in the DR case

described above. Alternatively, we may also obtain the o-^f using the scaled Coulomb formula given in Ref. [9], with Zeff=(Zc+Z/)/2~45, where Zq is the nuclear core charge of

the target ion and Z/ is the degree of ionization of the target ion before capture. For sufficiently large n, the RR cross- section scales as Mr?, as can be seen from Kramer's formula

[15],

.Kramer. ^'^ *0 'eff

^eff 1^

(11)

where v is the average effective principal quantum number

given by v = n — jx, and /it is the quantum defect. In general,

the relativistic and multipole contributions are known [13] to approximately cancel each other, so that the simple nonrelativistic dipole approximation (9) seems to provide a reliable estimate. While the DR cross section described above is peaked at a

particular resonance energy, the nonresonant RR cross section is continuous and slowly varying with the beam energy. Therefore for comparison with experiment the RR cross section is integrated over an energy bin width equal to the width

of the experimental electron-beam AB as

5"^=

f <T^''de,^^B^\

JAB

(12)

where it is assumed that A.B<W. In the opposite case of W<^B, the AS in (12) is to be replaced by W.

V. RESULTS

The spectrum shown in Fig. 1 contains all the x rays emitted by both the RR and DR processes for an electron- beam energy of 2.35 keV (with strong attenuation below 2 keV due to absorption in the beryllium windows). Since the absolute x ray intensities depend on, among other factors, the trapped ion density and the overlap between the electron beam and the ions, which are not known, we focus on only

lSfIST-173
2044 McLaughlin, hahn, takacs, meyer, and gillaspy 54

TABLE I. The autoionization and radiative rates for the resonance state ls^2s^2p^3s^3p^3d^ of the

Ba^'"*" ion are presented in units of (1/sec) in the AMA coupHng scheme. Brackets indicate powers of 10.

1 A.id^iJ,) / A,(d-.f ) ftw(Ry)

[Ai]3d^ 172 1 0.310[+14] [AT]3d'^ 0.259[+15] 340

3 0.418[+15] [AT]3s3d^ 0.136[+14] 315

5 0.567[+13]

[Ne]3p^3d^ 131 1 0.547[+13]

[J^e]3s3p^3d^ 141 0

2

0.126[ + 15] 0.341[+13]

[Ne]3s3p^3d'* 156 130.146[+13]

0.126[+14]

[hie]3s^3p'^3d^ 151 130.528[+15]

0.127[+14]

[_Nc]3s^3p^3d'^ 166 0

2.4

0.468[+14]

0.493[+15]

0.401 [+14]

their relative magnitudes. In this case, we are comparing

resonant and nonresonant total cross sections, each with a unique energy dependence. Therefore 5'^'^ of (5) and 5*^ of

(12) are the basic theoretical quantities for comparison with

experiment, since these are the quantities that the experimentalist directly measures. (It is also assumed that the x-ray

detector for the RR and DR at 90° does not affect the relative

ratios.)

A. Theoretical result The results of the calculation are presented in Tables I and

n. The autoionization and radiative rates calculated in the AMA scheme are presented in units of 1/sec. Reemphasizing

that the choice of the energy averaging bin size Ae^ is completely arbitrary [i.e., for Ae^>r{d)], the DR cross section

is averaged over a bin size equal to the resolution of the x-ray detector W where Aej= W=(15±3) Ry. This dependence will drop out in the cross-section ratios. The AMA

procedure generally overestimates o^^ by approximately

20%-40%, so that the corrected theoretical ratio should be

reduced [11,12] by a factor of 1.3. Although the AMA theory

is relatively simple, the adjustment factor in fact contains the

details of state coupling; it is the result of many detailed calculations performed previously. For the Ba^^"*" system, we find that the autoionization width of the resonance state d=ls^2s^2p^3s^3p^3d^ is r^(i/) =0.172X10'^ sec~', the radiative width is rr(i/)=0.273X lO'^ sec"', and thus the fluorescence yield is &)(d) =0.137. The energy averaged total DR cross section for the 2p—*3d excitation capture is then

o^j5+ = 2.7x 10~2i cm2/1.3=2.lX 10"2' cm (13)

>2p radiative decay contri- the Ba34+ where both the 3 J—••2p and 3s

butions are included. For d=ls^2s^2p^

sec"', r^=0.318X10'^ sec"', w=0.136, yielding

case with

15^2522^53^23^63^6 fj^^j r„=0.203xl0'*

^f34+=1.9XlO~2' cm2/1.3=1.4X 10"^' cm^. (14)

The 2Pj/2— 2/^3/2 splitting is on the order of 500 eV, so only

the 2P3/2 excitations satisfy the resonance condition. This is

TABLE II. The autoionization and radiative rates for the resonance state Is 2s 2p 3s 3p 3d of the Ba^"^ ion are presented in units of (1/sec) in the AMA coupHng scheme. Brackets indicate powers of 10.

1 ec(Ry) Ic Aaid-^iJc)

[Ar]3d^ 178 1 0.454[+14]

3 0.615[+15]

5 0.821[+13]

[Ne]3p^3d^ 135 1 0.538[+13]

[Ne]3s3p^3d^ 145 0 0.126[+15l

2 0.325[+13]

[Ne]3s3p^3d^ 161 1 0.177[+13]

3 0.147[+14]

[•Nc]3s^3p'*3d^ 155 1 0.517[+15]

3 0.126[+14]

[Ne]3s^3p^3d^ 172 0 0.542[+14]

2 0.576[-l-15] 4 0.471[+14]

/ Arid-^f) Aaj(Ry)

[Ar]3d^

[fiiT]3s3d^

0.305[ + 15] 0.133[+14]

339.0

312.0

NIST-174
54 RADIATIVE AND INNER-SHELL DIELECTRONIC 2045

TABLE ni. The direct radiative capture cross section a^^ is given for capture into states [Ar]3 J^n/ for the cases n =3, 4, and 5

for the Ba^^"*" ion in (cm^) corresponding to the experimental elec- tron beam energy e^=173 Ry. Brackets indicate powers of 10.

TABLE IV. The direct radiative capture cross section a^^ is given for capture into states [Ar]3 for the cases n=3 and 4 for the Ba^''^ ion in (cm^) corresponding to the experimental electron beam energy e^.= 173 Ry. Brackets indicate powers of 10.

^w(Ry) CT^ (cm2) / ftaj(Ry)

[Ai]3d^4s

[Ai]3dUp

[Ai]3dUd

[Ai]3dUf

[Ar]3^f^5j

[Ar]3^f^5p

[Ai]3d^5d

[Ar]3J^5/

[Ai]3d^5g

333.0

263.0

259.0

253.0

247.0

226.0

224.0

221.0

218.0

215.0

1.62[- 3.37[-

1.05[-

1.04[- 2.68[-

I.63[- 5.10[- 5.58[-

2.03[-

2.03[-

22]

23]

22] 22]

23] 23]

23]

23]

23] 24]

accounted for by further reducing the DR cross sections obtained in the AMA scheme by a factor of 1/1.5, as has been

done above. Tables lU and FV show the direct radiative recombination

(RR) cross sections along with the photon energies corre- sponding to the electron-beam energy of 172 Ry=2.35 keV.

Since the energy of the emitted x ray is the same in both

cases, we first note that the peak ab is a combination of DR

{2p + l,-*3d3d-^2p3d+y) and RR (n = 3). From Tables

I-IV, the theoretical cross-section ratios for the two ions are

-DR

<^Ba35-' in = 3)

-~13, (15)

and

-DR

a^34+(n = 3)

11.

[AT]3d^

[AT]3dUs

[AT]3dUp

[Ar]3 dUd

[Ar]3c^''4/

326.0 259.0

255.0 249.0

243.0

1.40[-22}

3.24[-23]

1.01[-22]

L02[-22]

2.55[-23]

Therefore the contribution of about 8% to the ab peak from

direct radiative recombination to the 3 level, (RR, n-3),

must be subtracted before comparing with the theory. Next, we examine the relative size of the two DR peaks

ab and a'. They arise from the initial radiationless capture

2p + l^^3d3d followed by radiative decays; either 3d—>2p

(with a photon energy 4.6 keV for Ba^^^); or 3s—>2p (with a photon energy 4 keV for Ba^'"^). Similar decays occur for

the Ba^'*'^ ion. From Tables I and II, the relative probability

for these two branches is

A,{3d-^2p)fA,i3s-^2p)=^l9. (16)

This is consistent with the ratio seen in Fig. 1 , where the two DR peaks, a = ab — b3 and a', have an intensity ratio of about 16. In order to compare the resonant DR process with the nonresonant RR process, we adopt the energy integrated cross-section S. For the DR case, we have

(17)

which is independent of W, the detector resolution. For the RR case, assuming W>AB,

(18)

FIG. 3. Sc-like barium spectra of the DR peak

ab taken at electron-beam energies ranging from 2270 eV to 2440 eV in steps of 10 eV. The graph

clearly shows the Ba

35 -t -Ba^''* DR separation.

—I1 22B0 2300 2320 2340 2360 2380 2400 2420 2440

Electron Beam Energy [eV]

NIST-175
2046 Mclaughlin, hahn, takacs, meyer, and gillaspy 54

TABLE V. The ratio of the DR and RR cross sections 77 are given corresponding to capture into 4/ levels for the Ba^^"*" and

Ba^''"'' systems.

Ba^

,72,^^^(3t/-2p)

Ba^

14

0.7 7±1

0.4±0.1

11

0.6 6±1

0.3±0.1

As noted earlier, is nearly independent of AB, so that

is proportional to AS. For the present experiment, we

take AB=(4±1) Ry. (See Sec. V D and Fig. 3.) To facilitate comparison of the DR and RR cross sections we use the n=4 RR capture cross section to normalize the

data. This is necessary partly because of the difficulty of determining absolute ion densities and ion-electron spatial

overlap inside the EBIT. The cross-section ratios are inde- pendent of these parameters, such as the ion and electron

densities. For convenience, we thus define Tf^a^^(^e^)/c^, with Ae^ given by (6). Treating the

Ba^^"*" system first, we find, for tfie full participation of 6 2p

electrons.

B. Experimental result

Since experimentally H'>AB, the RR cross section is ef- fectively measured in terms of Afi. Therefore, following

(12), it is more convenient for comparison with experiment

to define

-DR (W) w

AB

W

Obviously, the W dependence of 77 drops out in 77', since a^\W) W is independent of W.

We should make two further points before the comparison

is made. First, for Ba^"*"*^ and Ba^^*, the RR x rays are only

separated in energy by about 80 eV, which is well within W,

so that X rays from both Ba^^ and Ba^^"*^ are collected. Second, the experimentally derived DR cross sections are very much dependent on the relative abundances of the Ba^^"*" and

Ba^''*. The DR resonance energy difference for the two

charge states is also —80 eV, which is outside the electron beam width AB. Therefore, when is fixed at the DR reso- nance for the Ba^'^ ion, no DR is allowed for the Ba^''*

target ion (see Sec.V D.) The theoretical 77' should be further multiphed by P, the ratio of the Ba^'"^ ion density to the sum

of the Ba^* and Ba^^^ ion densities before comparing with

experiment. Thus

V = 5^ \^B (22)

For P—l/3, obtained by combining the results of the numerical simulation [8] for the charge state distribution and an

intermediate value of /3=0.6, we have

5^^{3d-*2p) 3.9X10-2' cm^

57 2.7X10" 22 cm^

and

^2p) =

d^^i3s-^2p) 2.0X10-22 j,ni2 2.7X10-22 cm2

-14,

(19)

-0.7.

The theory must contain the AMA correction factor of 1/1.3 and the py2 correction of 1/1.5. Thus for the Ba case we

finally have the theoretical result

'7Srr5'"^(3rf-2p)- 77Z^(3rf-2;,) ——-7± 1 „AMA 1 1

'/"''(3^-2p)-77ro;(3.-2p)— —~0.4±0.1. -AMA 1 1

(20)

W 200 eV

AB^'-irW'^'-''^''^ (23)

and thus ^' ~ 77 in the present experiment. (For comments on

the 45 eV beam width see Sec. V D.) As our assumption for

/S varies over the full range of physically allowed values, corresponding to the DR cross sections varying from the extremes of zero to infinity, the value listed in (23) remains

between 2.6 and 0, respectively. Our estimates of uncertainty on (23) reflect a more reasonable range of allowance for the DR correction. The experimental DR (3 J—>2p) cross section is extracted from the data by first subtracting the RR(/j=3) contribution. To eUminate the dependence on the ionic and electronic densities in this subtraction, we set

DR,adjusled^ exp iid^2p)

^cr[g(3J->2p)- ,r crZ.^(n = 3). (24)

"theoryV " —

Applying the same procedure to the Ba^"^

obtain the result summarized in Table V

we find rj^%(3d-^2p) = n,

Ceor''(3d-2p)=6±l, and 77=^~(3^-.2p)=0.3±0.1

AMA / T

.VtheoryV ^ J

system we

For this case 2p)=0.5,

The 3s—*2p transition corresponding to peak a ' need not be

adjusted. From the fit of the experimental data, we can di- rectly obtain only 77'. The relative magnitudes of the DR and

RR peaks {ab and b4) and (a' and b4) are then given by

NIST-176
54 RADIATIVE AND INNER-SHELL DIELECTRONIC . 2047

77;,p(3d-^2;»)-ll±3.

v:,p(3^--2p)-0.8±0.2. (25)

Hence, to compare with (20) the quantities in (25) are converted to 77 using (21) as

(26)

Comparing (26) with (20) we find for the Ba^^"*" case

97exp(3^/^2/j) = 7±3,

77exp(35-2p) = 0.5±2 (27)

and the theoretical peak ratios are found to be in excellent agreement with the experimentally observed values.

C. RR ratios We also compare the relative magnitudes of the RR peaks

f The theory predicts (see Table m) for Ba^^"" ^RR

l(4,5)a.eoTy=4E = 0.54 (0.51),

_RR (28)

^(4,6)a,ecry=-RE = 0.31 (0.30),

0-4

and

.RR

^(5,6)a,eory = ^= 0.58 (0.58),

where the numbers in parentheses are the ratios expected

from the scaling behavior of a^\n). The fs for Ba^""^ (from Table TV) are similar to the above, (28). Clearly, the calculated ratios (which were done in the distorted-wave ap- proximation) are consistent with the scaling. This is also

the case in (11) when e^t>{Z\f^2v^). The experiment gives

^(4,5)exp~0.56 (29)

which is quite close to the theoretical value of 0.52 obtained from (26), after averaging the weighted contributions from

the two charge states.

D. Electron energy dependence of the DR peak

The DR part of peak ab [which contains both the DR and

RR (n =3) contributions] may further be analyzed by varying

the electron beam energy. Figure 3 shows two peaks corre- sponding to the DR contribution from Ba^^* and Ba^^, re- spectively. The RR (a!=3) contribution should be an ap- proximately constant background on the order of 1/20 of the

large peak value, as shown in (15). Thus, in Fig. 3, the RR

count should be on the order of 35. The figure shows an

upper bound on the electron-beam width of AB=5 Ry=65

eV (FWHM). The total width of the line is made up by the

fine-structure sphtting of the 3 level into 7 = 3/2 and 7=5/2

levels and the electron-beam energy width. The 3rf fine- structure splitting can be calculated to be around 20 eV

0.7

0.6

.1 0.5 B

3

Q.0 0.4

w 0.3 o

1 0.2

, . , .1

. ,. , .

' .* [ !

1

1

^ —-I- i -

j Sc-liki — 1

!

i

•

:

^ :

!

i -.,|., i. - j-. . -j .. ,

.

/\ '

i !

;

i

!

i i

;

1 , j . 1 . ; , .

!

i ;

i . 1 . 2.28 2.30 2.32 2 34 2.36 2 38 2.40 2.42 2.44 Beam Energy (k»V)

FIG. 4. Relative charge state populations of scandiumlike Ba^'^ and titaniumlike Ba^'*^ ions in the EBIT as a function of the kinetic energy of the electron beam.

which gives a value of about 45 eV for the beam energy

width. This is in accordance with our earlier estimate for the width to be between 30 eV and 50 eV. We used the above

determined 45 eV beam energy width throughout our analysis. The large peak in Fig. 3 centered around e^—2330 eV

corresponds to the DR process for the Ba^^""" target ion, while

the small peak around 2400 eV should be the contribution from Ba^"*". The reason for the reduced count rate for the second peak is the change of the charge state balance by

changing the beam energy. Figure 4 shows the dependence

of the Ba^^"^ and Ba^*"^ charge state populations on the electron-beam energy using the results of our model calculations (Ref. [8]). To be able to compare the experimental DR

ratios with theory, one has to normalize the peak intensities to the same number of ions in the particular charge state the resonance corresponds to. This can be done using the populations of the corresponding charge states as shown in Fig. 4. The ratio of the relative population of the Ba'^"*" ions at 2.33 keV and the Ba^"*"" ions at 2.39 keV is 44%/26%=1.69.

Since this ratio does not include the effect of DR itself on the charge state balance the value given here may be biased by

the fact that the DR process removes ions from the particular

charge state in which they are resonant. Multiplying the

theoretical DR cross-section ratio with this number our esti- mate for the intensity ratio is ^^35+ ) -bR,.. . . 1.69= 1.3X 1.69=2.2. *(34+)

(30)

This is very good agreement with the experimentally observed peak intensities shown on Fig. 3. We note that the changing charge state balance complicates the analysis of the

results in all the cases when the experiment requires the change of the electron-beam energy. This is the situation,

e.g., when one studies resonant processes just as in our case. Since DR removes ions from the charge state in which they

are resonant, there are fewer ions in the appropriate charge

state at the peak centroid than in the tails. Thus, the estimate of the DR cross section obtained by using the integral of the peak is biased low, and the estimate of the width is biased

high. These biases are not as important in obtaining the ratios of the two DR resonances, however. To resolve this problem

in future experiments we plan to apply a transient operation

NIST-177
2048 Mclaughlin, hahn, takacs, meyer, and gillaspy 54

technique. In this method one optimizes for a certain charge

state balance at a given beam energy then transiently changes

the beam energy to a different value where the observation is made. By choosing the off-steady-state time sufficiently short one can make sure that the number of ions in a given

charge state is always the same independently of the transient energy. Alternatively, rather than the beam flipping

procedure described above, the beam ramping technique may

also be used.

VI. DISCUSSION

We have presented recent experimental data on RR and

DR processes in Ba^^"^ and Ba ions obtained with the EBIT at NIST, as well as a theoretical analysis. The x-ray peak {ab), corresponding to the DR 2/7—>3 J excitation fol- lowed by the 3i/— radiative decay, has been compared to the n=4 RR x-ray peak b4. Note that the energy of the DR

X ray is twice that of the incident electron kinetic energy. In

addition, the smaller peak o ' at 4 keV has been shown to be

consistent with the ab peak and with b4. In all cases, good

agreement is obtained to within ±20%, after slight but justifiable adjustments are made to both the theoretical and ex- perimental values. The present analysis confirms our earlier experience with the AMA procedure that, for complex openshell ions such as Ba^^"^" , and for W and AS large, the method works well in describing the dominant transitions. As the experimental resolution improves, more refined treat- ment should be necessary. Comparisons of the RR peaks

relative to each other was not possible for n>5 due to the uncertainty in isolating their contributions from the data. However, the plateau around 2.6 keV is probably due to the RR contribution from n>5. Thus all the significant features of the x-ray spectrum above 2.4 keV have been satisfactorily

explained.

The x-ray energies for the DR processes differ by

-20-30 eV for the Ba^'"" and Ba^''^ target ions, while for

the two ions differ roughly by 80 eV. Therefore, within the

resolution of the electron beam energy AB«50 eV, it is impossible for both ions to be involved in the DR process at the same time. On the other hand, the RR peaks can have contributions from both charge states, as the detector resolution

is W—200 eV, much larger than the 80 eV difference in the X rays from two different ions, for each n. Therefore part of

the broadening of the peaks may be due to the presence of two or more charge states in the neighborhood of Ba^^"^. We

emphasize that under the experimental conditions W>A.B>T(d) the RR x-ray counts were detected from both

ions Ba^^"''^''^ with a band width of Afi, while the DR

counts were from the Ba^^"*" only. Therefore, we except the

spectral shapes to change as W and AB, and their ratio, are

altered. The final broad peak at the x-ray energies hu)<2 keV is currently being examined theoretically [16]. This peak is produced mainly by collisional excitation fluorescence along

with the cascade contribution from the primary RR process discussed above, and by Bremsstrahlung. In addition, the DR

processes contribute which involve the A/ -shell electron excitations to high Rydberg levels (n >6) accompanied by capture to these levels. Preliminary estimates indicate that the individual cross sections for such M-shell DR processes are roughly 100 times smaller than die 2/7 excitation DR consid- ered above. This is to be expected from the approximate

scaling. There are very many available intermediate reso- nance states to be included, however, with the angular momenta of the levels extending to large values /^lO. The ex- pUcit calculation of the M-shell DR is difficult and time

consuming, even in the AMA procedure. A direct attempt is being made [16], employing the same procedure as in the L-shell DR, while a simpler alternate theoretical procedure is being developed. We also conjecture that at least some part of this low-energy x-ray peak is the result of a recombination

effect [Radiative DR(RDR)] proposed recently [17], in which the excitation capture takes place simultaneously with an x-ray emission. Such a process is DR-like, but nonresonant, and should be important when DR is weak or forbid- den. Previous estimates suggest that the RDR contribution can be a few tens of a percent of DR, and can compete with RR processes as well. Further experimental study of the DR lines in Ba^'*"^ and

Ba^^"*" is being planned, employing a transient technique

which prevents tfie DR resonance from altering the charge

state balance. By repeatedly switching the electron-beam en- ergy on to resonance for approximately 10 ms and then off resonance for approximately 50 ms, the charge balance may

remain approximately steady at its off-resonance value. This

would increase the Ba^^"*" ion fraction P decrease its uncertainty, and simplify the analysis.

This relatively simple EBIT experiment has yielded a sur- prisingly diverse amount of information, as demonstrated in

this report. In addition, it would be very interesting to extend

the x-ray measurement below the 2 keV limit, where a rich variety of physical processes are expected.

ACKNOWLEDGMENTS

The theoretical part of the work reported here was sup- ported in part by a DOE Grant.

[1] D. DeWitt, D. Schneider, M. H. Chen, M. B. Schneider, D.

Church, G, Weinberg, and M. Sakurai, Phys. Rev. A 47, R1597 (1993).

[2] D. Knapp, P. Beiersdorfer, M. H. Chen, J. Scofield, and D.

Schneider, Phys. Rev. Lett. 74, 54 (1995).

[3] G. Kilgus, D. Habs, D. Schwalm, A. Wolf, N. R. Badnell, and

A. Miiller, Phys. Rev. A 46, 5730 (1992); and J. Linkemann, J. Kenntner, A. Muller, A. Wolf, D. Habs, D. Schwalm, W.

Spies, A. Frank, A. Liedtke, G. Hoffmann, E. Salzbom, N. R.

Badnell, and M. S. Pindzola, Nucl. Instrum. Methods Phys. Res. B 98, 154 (1995).

[4] Y.-K. Kim (private communication).

[5] A. Levine, R. E. Marrs, J. R. Henderson, D. A. Knapp, and M.

Schneider, Phys. Scr. T22, 157 (1988); R. E. Marrs, C. Bennet, M. H. Chen, T. Cowan, D. Dieuich, J. R. Henderson, D. A. Knapp, M. A. Levine, K. J. Reed, M. B. Schneider, and J. H.

NIST-178
54 RADIATIVE AND INNER-SHELL DIELECTRONIC . 2049

Scofield, J. Phys. (Paris) 50, CI -445 (1989); R. E. Mans,

Conunents At. Mol. Phys. 27, 57 (1991); J. D. Gillaspy, J. R.

Roberts, C. M. Brown, and U. Feldman, in Vlth International Conference on the Physics of Highly Charged Ions, edited by

P. Richard, M. Stockli, C. L. Cocke, and C. D. Lin (AIP, New

York, 1993), p. 682.

[6] Assuming a uniform charge density within the beam and then applying Gauss's law gives the correct answer to within 5%.

[7] C. A. Morgan, F. G. Serpa, E. Takacs, E. S. Meyer, J. D.

Gillaspy, J. Sugar, J. R. Roberts, C. M. Brown, and U. Feld- man, Phys. Rev. Len. 74, 1716 (1995).

[8] B. M. Penetrante, J. N. Bardsley, D. deWitt, M. W. Clark, and

D. Schneider, Phys. Rev. A 43, 4861 (1991); We thank H. Margolis for an improved C language version of B. M. Penetrante' s original Fortran code, and D. Knapp for a similar but

simpler independently written code.

[9] J. Gau and Y. Hahn, J. Quant. Spectrosc. Radiat. Transfer 23, 121 (1980); Y. Hahn, Phys. Rev. A 12, 895 (1975). [10] C. Froese-Fisher, The Hartree-Fock Methodfor Atoms (Wiley,

New York, 1977).

[11] Y. Hahn, Advances in Atomic and Molecular Physics, Vol. 21, edited by D. R. Bates and B. Bederson (Academic, New York,

1985), p. 123; Y. Hahn and K. J. LaGattuta, Phys. Rep. 166(4), 195 (1988), particularly p. 212.

[12] D. J. McLaughlin and Y. Hahn, J. Quant. Spectrosc. Radiat. Transfer 28, 343 (1982). [13] Y. Hahn and D. W. Rule, J. Phys. B 10, 2689 (1977). [14] D. J. McLaughhn and Y. Hahn, Phys. Rev. A 43, 1313 (1991). [15] H. A. Kramer, Philos. Mag. 46, 836 (1923). [16] D. McLaughlin and Y. Hahn (unpublished).

[17] Y. Hahn, Z. Phys. D 33, 247 (1995).

NIST-179
Physica Scripta. Vol. 58, 178-181, 1998 The use of a Spherically Curved Crystal Spectrometer for X-ray

Measurements on Electron Beam Ion Trap

Y. Aglitskiy,** F. G. Serpa,*t E. S. Meyer,^ J. D. Gillaspy,' C. M. Brown,^ A. Ya. Faenov^ and T. A. Pikuz^

' National Institute of Standards and Technology Gaitheisburg, MD 20899, USA

^ Naval Research Laboratory, Washington D. C. 20375, USA

' Multicharged Ions Spectra Data Center of VNIIFTRI, Mendeleevo, Moscow region, 141570, Russia

Received March 3, 1997; accepted in revisedform December 22, 1997 PACS Ref: 61.10-i, 61.50Ah, 52.25Nz

Abstract

Spherically curved crystal spectrometers arc demonstrated for use on an

Electron Beam Ion Trap (EBIT) for the first time. Such spectrometers are characterized by high light collection efficiency and relative insensitrvity to source position, simultaneously, giving them an advantage that no X-ray

spectrometer previously used on an EBIT has had. One of the spectrometer

crystals tested is composed of mica, giving it the additional advantage that

it can be used with reasonable efficiency up to very high order, allowing

spectra to span a broad wavelength range &om 0.5 Angstroms to 20

Angstroms. Spectra &om Ne-like barium and He-Uke argon are presented.

1. Introduction The Electron Beam Ion Trap (EBIT) is well known as a unique source for high precision measurements of multi- charged ion spectra [1-8]. When compared to the com- monly used high temperature and often high density

plasmas, the EBIT plasma line shapes are not distorted by

Doppler and Stark broadenings and shifts. The inevitable presence of dielectronic satellites in the vicinity of almost

every line of multicharged ions mimics the line shift in conventional ion sources. In the case of EBIT the right choice of an electron beam energy can significantly diminish this compUcation. The spatial stability and the very small diameter of the ion trap make it possible to use slitless spectro- meters throughout wide wavelength regions. Even a simple

flat crystal spectrometer can provide quite high resolution in

this case. Von Hamos X-ray crystal spectrometers with focusing in the direction perpendicular to the dispersion plane were successfully used for spectral recording of highly ionized atoms

excited in the Lawrence Livermore National Laboratory

EBIT [3]. Since this method is spectroscopically equivalent

to the flat crystal scheme, it limits the possible reference

lines to internal ones. The use of spectrographs with focus- ing in the dispersive plane (Johann or Johannson type)

[4-7] is insensitive to source position which gives one the

possibility to use reference lines from an external source Le. a conventional X-ray tube. The somewhat low EBIT count

rates and resultant collecting times require the use of lownoise detectors with high quantum efEciency. Position sensitive proportional counters (PSPC) meet these requirements

* Science Applications International Corporation, McLean, Virginia, 22102, USA.

t Physics Department, University of Notre Dame, Notre Dame, IN 46556, USA.

but they have relatively poor spatial resolution (200 pm to 300 Jim). Therefore, if the goal is to have high spectral

resolution, one has to increase the size of the crystal spectro- meter i.e. the radius of the focusing crystaL This leads in turn to lower intensity and makes the detector noise requirements even more difficult The total number of ions

in the trap has been estimated to be close to 10* [2]. This

number is sufficient for some observations but it is worthwhile to use more efficient and sophisticated techniques of

spectral recording to reduce the time required to collect statistically good data.

It has already been shown that the use of spherically curved crystals can significantly improve the efficiency of X-ray spectrometers [9-18]. The most spectacular results were obtained for point source devices such as laser produced plasmas and low inductance vacuum discharges of differ- ent types. Actually such crystals can produce an ima ge of an

X-ray source [19-27]. In this work we present the first appUcation of spherically curved crystals of mica and quartz

for recording X-ray spectra of multicharged ions excited in an EBIT. The quartz crystal with the 1011 orientation has high

resolution and high reflectivity in the wavelength region

below 6.67 A and is very useful for recording weak spectra from the EBIT. A spherically curved mica crystal spectro- meter has several useful properties. The first available order

of reflection from mica corresponds to the lattice spacing

2d « 19.9 A. It can record X-ray spectra with a low energy

limit of about 700 eV. The EBIT has demonstrated the abihty to produce the most highly ionized atoms in the laboratory, and spectra with very short wavelengths. It is also opportune to seek new and worthwhile objectives at lower energies and lower stages of ionization as well More

efficient spectrometers will allow studies that formerly had

to be bypassed because of the low brightness of the EBIT. For instance in the trade-off between the magnitude of the Lamb-shift, which increases with Z, and the accuracy with which it can be measured, the optimum does not necessarily

fall in the region of extremely high energy transitions. There

are also many other interesting spectra in the soft X-ray

region to study. For example, sodium-like as well as other

ions with open M-shells belonging to elements in the middle

of the ;>eriodic table, have their resonance lines in this soft X-ray spectral region. Also transitions without change of

principal quantimi number (An = 0) of extremely heavy

multicharged ions are found in this region as well. An = 1 transitions in multicharged ions of rare earth elements with

Physica Scripta 58

NIST-180
The use of a Spherically Curved Crystal Spectrometer for X-ray Measurements on Electron Beam Ion Trap

an open 3d shell are also found in this region. Thus

developing sensitive spectrometers for soft X-ray spectra could be quite fruitful when apphed to EBIT sources. In the case of mica, it is possible to use more than one

order of reflection. In fact one can use orders n = 1, 2, 3, 4,

5, 7, 8, 10, 11, 12 and 13 with quite reasonable efBciency

[9-18, 21-29]. This means that while maintaining optimal

geometric conditions, we can apply mica as a high

resolution X-ray analyzer from 0.5 A to ahnost 20 A. The

possibility of having spectra recorded in multiple orders of

reflections is also useful for some applications of X-ray spec- troscopy to high precision atomic physics.

A difi"erential masurement of the ground state Lamb-shift

in H-like ions is an example of such techniques [30]. This

idea of the measurement is based on the approximate 4 :

1

ratio in the wavelengths of Balmer P and Lyman a lines of

all H-like ions. That means these transitions can be observed at the same Bragg angle simultaneously for mica

crystals in the first and fourth, second and eighth, or third and twelfth orders of reflection, respectively.

The Bahner P transitions (n = 4 -» n = 2) are much less sensitive to the quantum electrodynamics effects than the Lyman a {n = 2-*n = 1). This means that precisely calcu- lated Balmer P lines can be used as the reference for the Lyman a wavelength measurements, and hence for the extraction of the ground state Lamb shift. In the case of an EBIT, the intensity of Balmer transitions could be optimized by choosing the appropriate energy of the electron beam. This idea is attractive for EBIT measurements since it does not require an external calibration source. (Spherically curved crystals in principle aflow the use of an external source of reference lines because they have the advantages of

the X-ray optical schemes with vertical and horizontal

focusing).

It should be noted that using any crystal for high preci- sion measurements with reference lines, and having the lines under investigation appear in different orders, requires information on refraction in the crystal This effectively changes the Bragg angle and should be taken into account

as a small difference in the 2d spacing between the different orders. Quite reliable calculations of refraction are available

for many crystals [31]. As for mica, independent experimental work must be peformed since its properties vary sUghtly

for samples of different origin.

2. The geometry of a spherically curved crystal X-ray

spectrometer

The main properties of a spherically bent crystal can be

explained using two independent mutually perpendicular

planes: the horizontal or meridional or dispersive plane and

the vertical or sagittal or focusing plane. Since the sphere

itself does not have any specific geometrically determined

planes, the first plane can be defined by the position of the

source, detector and vertex of the crystal The second plane

is perpendicular to the first Let us consider the rays in the meridional plane shown in

Fig. 1, where a is the distance from the source to the crystal

vertex, b is the distance from the crystal vertex to the image,

H is the length of the crystal in the meridional plane and R

is the crystal bending radius. The glancing angle 0 of the radiation at the crystal vertex is related to the wavelength A

Fig. 1. Optical scheme of the foimation of spectra spatially resolved in one

dimension with a spheiically curved crystal

by the Bragg condition

:

2<i, sin 0 = nX, (1) where n is the reflection order and d„ is the interplanar dis- tance for the n-th reflection order. After reflection from the crystal, spectrally selected beams

are formed in the meridional plane. Due to astigmatism,

these beams focus at different points in the meridional and

sagittal planes. For the positions of these foci, we have in the sagittal plane

l/a + 1/b, = 2 sin 0/R (2) and in the meridional plane

l/a + l/ij„ = 2/R sin 0, (3) where and are the distances from the crystal vertex to the sagittal and meridional focL To achieve the best possible spectral resolution and the

greatest intensity in the image, we must place the detector at the distance from the crystal. For example, if distance a = Rsm0 (a. source on the Rowland circle), then

h = h, = R sin 0/(2 sin^ 0 - 1). (4)

In such a scheme the linear magnification in the sagittal plane is bja = [sin 0(2 sin^ 0 - 1)] " (5) The best spatial resolution achieved thus far is about

2 ^m to 4 (mi [22, 24, 27].

3. Experimental set up

Observations have been performed with spherically curved

crystals manufactured by the National Institute for the

Physical, Technical and Radiotechnical Measurements

(Russia). Mica (2d = 19.9 A) and quartz (1011, 2d = 6.67 A)

are available with radii of curvature of 100, 186, 250 and

443 mm. We have used both mica and quartz crystals with a

radius of 250 mm since that best fits the dimensions of our EBIT vacuimi chamber. For the present study, the spherical crystal was mounted

in the vacuum chamber of the tunable Naval Research Laboratory X-ray spectrometer [19] which is attached to an EBIT viewport at NIST. This vacuum chamber has a vari- able angle between the entrance and exit arms (see Fig. 2). The actual range of the available Bragg angles was 23° to

67°. The spectrometer is normally tuned by varying the

crystal radius while the lengths of the arms are constant. The use of this chamber is not optimal since we are unable

to change the distance between the crystal and the detector

Physica Scripta 58

NIST-181
The use of a Spherically Curved Crystal Spectrometer for X-ray Measurements on Electron Beam Ion Trap 181

400

CM

'«>• c

Q 200

0

Ne-Iike Ba

A

2.6 2.7 2.8 2.9

Wavelengths (A)

Fig. 5. Spectrum of Ne-like Ba recorded with a quartz 1011 spherically bent crystal in the first order of reflection.

the most intense line in the Ba Ne-like spectra appeared at the very edge of the wavelength region available for this specific crystal and was not properly recorded. He-like Ar spectrum (see Fig. 6) is interesting for our purpose because of its proximity to H-like Ar, which is a good candidate for differential Lamb-shift measurements. There are several He-like transitions including resonsince,

intercombination and satellite lines detected in the present

experimental spectrum. The intensity ratio of the recorded

lines, however, can not be attributed to a single electron energy since the energy of the electron beam was changed

during the collection time. There is a practical difference between using the spectro- meter with (two dimensional) photographic detection when

compared to one dimensional position sensitive detection. To take full advantage of the spherically curved crystal in the case of a point source, we have to put all elements in the

right place according to Bragg's law and the lens formulas

(2) and (3). If we are not concerned about the spatial resolution of the X-ray image in the vertical direction, we

are less restricted concerning the source being at the exact

distance given by the formulas. In our EBIT case, we have a

long but very narrow vertical X-ray source, so defocusing of a spectral line in the vertical direction does not change the

flux density in the image plane very much. We can still take advantage of the reflected beam compression in the vertical direction, especially if our detector is one-dimensional. All

70 He-like Ar ls2p ^Pi

cn

«-•B

3

O

u

3.9 4.0 4.1

Wavelengths (A)

Fig. 6. Spectrum of He-like Ar recorded with a spherically bent crystal of quartz 1011 in the first order of reflection.

we require is for the radiation reflected from the crystal to

fall completely in the input window of the detector. Also, we

are no longer limited by the minimum Bragg angle, since we

do not reqxiire the best vertical focus to be exactly in the position of best spectral focusing Le. on the Rowland circle.

5. Conclusion and future developments

The EBIT port available for X-ray spectroscopy and the PSPC detector were not equipped yet with a window trans- parent to the lowest energy X-rays. A direct test in the wavelength region longer than 6 A (limit presently set by the 160 pm Be windows on the EBIT and detector) is in the plan

for future development.

Two improvements are foreseen that could increase the efBciency of the X-ray spectral detection on EBIT. The first

is to use a detector with higher spatial resolution. This step

will permit us to decrease the crystal radius to a more rea- sonable value and consequently increase the cotmt rate. The

second, demonstrated here, is using efficient spectrometers

with spherically bent crystals. That also improves the count

rate and makes the signal to noise ratio more acceptable for X-ray detection. Technical parameters of contemporary CCD detectors are quite close to those needed for the successful registration of relatively weak EBIT spectra. Combining spherically

curved crystals and CCD detectors can provide a powerful

high resolution device for X-ray spectroscopy in a wide

wavelength range.

References

1. z

3.4.

5.

6.

7.

8.9. 10.

11. IZ

13.

14.

15.

16.

17. 18.

19. 20.

21.

22.

23. 24.

25.

26.

27.

28. 29.

30.

31. 3Z

Levine, M. A., Mans, R. E., Henderson, J. R., Knapp, D. A. and Schneider, M. B., Physica Scripta T22, 157 (1988). Gillaspy, J. D. et al., Physica Scripta T59, 392 (1995). Beiersdorfer, P. et al.. Rev. ScL Instrum. 61, 2338 (1990). Marrs, R. E. rt a/, J. Physique, ColL CI, suppL 1, 50, a-455 (1989). Brown, C. M. et al., Phys. Rev. A40, 4089 (1989). Takacs, E. et al., Phys. Rev. A54, 1342 (1996). Marrs, R. E., Exp. Meth. Phys. Sciences 29A, 391 (1995). Serpa, F. G. et al., Phys. Rev. 53, 2220 (1996). Faenov, A. Ya. et al., Physica Scripta 50, 333 (1994). Nilsen, J. et al., Phys. Rev. A50, 2143 (1994). Pikuz, T. A. et a/, J. X-ray Science Tech. S, 323 (1995). Pikuz, S. A. et al., Physica Scripta 51, 517 (1995). Faenov, A. Ya. et a/, Phys. Rev. A51, 3529 (1995). Faenov, A. et ai, Phys. Rev. A52, 2644 (1995). Skobelev, L Yu. et oi, J. Exp. Theor. Phys. 81, 692 (1995). Magunov, A. L et a/, J. Exp. Theor. Phys. 83, 267 (1996). Osterheld, A. L. et a/, Phys. Rev. A54, 3971 (1996). Skobelev, L Yu. et o/., Phys. Rev. E. (1997). Brennan, S. et aL, Rev. ScL Instrum. 60, 2243 (1989). Boiko. V. A. et oL, J. Sov. Laser Res. 6, 85 (1985). Bryunetkin, B. A. et al.. Quant ElecUon. 24 356 (1994). Pikuz, S. A. et al., J. Exp. Theor. Phys. 61. 638 (1995). Faenov, A. Ya. et al.. Physica Scripta 53, 591 (1996). Pikuz, S. A. et al.. Rev. ScL Instrum. 68, 740 (1997). Dyakin, V. M. et ai. Quant Electron. 26, 75 (1997). Aglitskiy. Y. et al., Phys. Plasmas 3, 3438 (1996). Brovra, C. et aJ, Phys. Plasmas 4, 1397 (1997). Pikuz, S. A. et al., Kvantovaya Electronika 22, 21 (1995). Pikuz, T. A. et al., "Measurements and Calculations of Flat and

Spherically Bent Mica Crystals Reflectivity and Using Them for Dif- ferent Applications in the Spectral Range 1-19 A." Proceedings of SPIE-95, San Diego 9-11 July (1995), vol 2515, p. 468-486.

Silver, J. D. et a/, Phys. Rev. A36, 1515 (1987). Brennan, S. and Cowan, P. L., Rev. ScL Instrum. 63, 850 (1992). Duval, B. P. et a/, NucL Instrum. Meth. 222, 274 (1984).

Physica Scripta 58 NIST-182
180 Y. Aglitskiy et al.

Fig. 2. Schematic of the ezpaimental appaiatos set up. The spherically carved crystal spectrometer was in a vacuum chamber with independent

pumping. A beryDhim filter between the two vacuum diambets limited the wavelength available for registration to /I < 6 A. A position sensitive proportional counter was attached directly to the spectrometer vacuum

chamber at a fixed distance of the crystal

to accommodate the fixed radius of the crystal. In the hori- zontal direction, however, a spherically curved crystal

spectrometer is quite similar to a conventional Johann type

spectrometer. This means that the depth of focus depends on the size of crystal illuminated by X-rays of a given wavelength. The EBTT source is about 70 pm in diameter and is located about 200 mm to 300 mm outside the Rowland

circle (depending on the Bragg angle). In this position we

are able to obtain good spectral resolution (A/AA about 1000

to 2000 depending on die crystal and Bragg angle) even though we cotild not put the detector at exactly the right

position (Le. on the Rowland circle).

CuKa

1 n=ll CuIQ 0=10 A

AO,

Co Kb

_«1 I III CuKs * " " 11/

.

, . , > 15.5 16.0 16.5 17.0 Wavelengths (A)

(a)

Vaaadium Ka

Ql 1 1 , 11 2J027 2.5066 Wavelenetbs (A)

(b)

Fig. 3. (a) The densitometer tracing of CuK. and spectrum recorded in

several orders of refection on X-ray film with a mica {R = 10cm) crystaL The inset shows the high degree of resolution achieved in the lltb order of

reflection, (b) The spectrum of V K. doublet recorded with a PSPC

detector.

Physiea Scripta 58

A backgammon type position sensitive proportional cotmter has been .used as a detector [32]. The spatial resolution of this detector (about 200 \an to 300 |im) is suffi- cient for the large radius cylindrically curved crystal spectrometer with high spectral dispersion, but seriously

limits the resolution that could be achieved with our spherical crystals. The spectral resolution in all experiments pre- sented here was limited by the spatial resolution of the

detector. Before being coupled with the EBIT as an X-ray source, the 100 mm mica X-ray spectrometer underwent a test with a conventional X-ray tube. X-ray film was used as a detector. This gave us information about the inherent spectral resolution of the spectrometer in the correct geometry inde- pendent of the limiting properties and location of our posi- tion sensitive detector. A typical densitometer trace is presented in Fig. 3(a), where we resolve the Cu K, doublet in the 11th order of reflection. This result demonstrates the good optical quality of the crystal bending. It also illustrates the convenient technique of tuning a mica spectrometer for the soft X-ray region by using relatively hard X-ray radi- ation outside the vacuimi chamber. Of course, the actual resolutions for different wavelength regions reflected at the same Bragg angle in difl'erent orders would be different The

spectrum of the V K„ doublet was recorded in the seventh order of reflection from the mica crystal (R = 250 mm), see

Fig. 3(b). A position sensitive proportional coimter was used

as a detector. This picture demonstrates the good resolution obtained in a geometry similar to the main EBIT experiment

4. Test results and discnssion

Spectra of Ne-like Ba and He-like Ar have been chosen for study in our experiment The identifications of the spectral

lines have been made on the basis of similarity of the recorded spectra to the well known previously studied ones. Precise measurements of the wavelengths and relative intensities were not among the main goals of the experiment

described. Two regions of the Ne-like Ba spectnmi were

recorded. The first was recorded with the R = 250 mm mica

crystal in the seventh order of reflection and shows three spectral lines (see fig. 4). The second was recorded with the R = 250 mm quartz crystal (first order of reflection) in the wavelength region close to the 2p^-2p^3s transition in Ba

46"^ as is shown in Fig. 5. With the present vacuum

chamber we cannot set the Bragg angle smaller than 23°, so

" 2.40 2.45 2.50 2.SS 2.60 Wavelengths (A)

Fig. 4. Spectrum of Ne-like Ba recorded with a mica spherically bent

crystal in the 7th order of reflection.

NIST-183
Physica Scripta. Vol. T73, 87-89, 1997 Progress Towards Absolute X-ray Spectroscopy on the NIST

Electron-Beam Ion Trap: Current Status and Results

C. T. Chantler,' D. Paterson,' L. T. Hudson,^ F. G. Serpa,^ J. D. Gillaspy^ and R. D. Deslattes^

' School of Physics, University of Melbouroe, Partville, Victoria 305Z Australia

' Atomic Physics Ehvision. NIST, Gaithetsburg. MD20899, USA

ReceiBed September 23, 1996: accepted in revisedfonn March 21. 1997

Abstnct

Recent obtervation of Hydrogen-like ions of Vanadium at the NIST

Electron-Beam Ion Trap and measuiements of Helium-Iike resonance line are presented. One particular feature of the current secies of experiments is the poasibility of absolute calibration in a near Doppler-&ce enviroiunent. The current approach to the pcsiible measurement of QED effects in such

a lyitem is reviewed. There is great potential for precision measurements in the near fiiture.

1. Introdnctioa The development of EBIT devices arouod the world since

the first EBIT at Livennore [1] is symptomatic of their

interest and utility for investigating questions of fundamental and applied significance. The NIST [2] and Oxford

EBITs began operational status at similar times and hence

rank as equal third operational devices in this short history. In this context the contributions to this issue from the Oxford EBIT [3], the Tokyo EBIT [4] and the BerUn EBIT

[5] indicate the development of tools and techniques to address important problems.

The NIST EBIT has had several priorities including the development of a tool for precision X-ray spectroscopy and

QED (Quantiun Electro-Dynamics) measurement with

minimal Doppler broadening and Doppler shifts that have

characterized fast-beam experiments and limited their overall precision [6]. All EBIT facilities share this advantage with regard to Doppler shifts, and all share the diffi- culty of relatively low fiuxes. The latter difficulty can be

addressed by compromising the data collection technique lo generate data quickly, or by slow and careful development

of techniques appropriate for EBIT investigations and

allowing absolute measurements of significant quantities.

We propose the latter approach as being more likely to produce important and meaningful new results.

2. Absolute polarization measurements The X-ray investigations at NIST have recently reported

successful and precision measurements of absolute polarization of a magnetic quadrupole line in Ne-like barium [7]. The polarization is not measured simply relative to sur- rounding Imes or an assumed unpolarized structure (although this is an important tool for confirming

polarizations). Instead, a polarization-sensitive method of

detection is used in two configurations, with calibration of

the detection in both an experimental and theoretical sense

at the required (0.1%-!%) level. This provided an absolute calibration, rather than a relative measure of degree of polarization. This minimises

uncertainty in the results, and (in these complex systems) allows detailed analysis of theory in the application to the systems involved. Complex multi-level cascades and population mechanisms are responsible for the final polarizations

observed, and without an absolute calibration this is diffi- cult to ar.alyse. Our results, with an accurate absolute cali- bration, observed and investigated dramatic variation of

cascade-feeding contributions to polarization across a wide

range of electron-beam energy. This general approach provides more accurate and

probing results than alternate techniques.

3. X-ray measnremeots

This general principle can be applied to the spectroscopic measurement of energy. In brief, any series of spectral lines should be interpretable in terms of absolute energy and

absolute intensity. The two issues are sometimes incompatible in a system of low flux like the EBIT, or for particular

high-efliciency but low resolution detectors (e.g. proportional counters). The two requirements are that an appropriate

absolute energy cahbration sequence be used in investigations, and that this be able to be confirmed by theory. In particular, the use of reference fines of unknown wavelength

and unknown systematics to "calibrate" secondary fines is able to provide useful observations, but other results may be

questionable. An exception to this is the Lyman oc-Balmer P

intercomparison technique used in accelerator observations,

for reasons which are explained in several places [8]. In the context of high-resolution X-ray measurements, the standard approach is to use diffracting crystals, so that dynamical diffraction theory will be the theoretical basis for the confirmation of cahbration and results. This is made

more difficult by the low-flux regime, so that curved crystals are a necessary tool to provide adequate statistics. Further

details and progress on this issue are provided in another

paper of this issue [9]. Our programme in this direction has pursued precision measurements of neon-like barium in the NIST EBIT, in part as a testing ground for the techniques and approaches

used, since barium is ubiquitous in EBITs and neon-fike

levels are convenient and accessible. It is also important in

its own right, as a study of closed shell spectra and possible

astrophysical and laboratory population mechanisms as indicated earlier [6]. Figure 1 presents data taken from a single run on neonlike banum, taken with a precision high-resolution spectro- meter. This data was collected over 6.2 h of EBIT operation, with simultaneous observation by a Si(Li) detector which

Physica Scripta T73

NIST- 184
88 C. T. Chantler, et al.

160

140

- 80

oU 4020

tL

^

Na-like barium

L (|2p^,l-'3s3d)„-»g.s.

- ((2p,^J '3s-1d)^^, -> g.s

1

kibe

4.900 4.950 Energy (keV)

5.000

Fig. 1. Spectrum of Neoo-like Bwiuzn recorded over 62b oo the NIST

EBIT wilb a high-predsion curved crysul spectrometer. StaUstics are dear ud contributions from Na-like barium charge states in this run are indi- cated. The spectrum has been truncated as adjacent regions show no spectral features.

300 350 400 450 500 550 600 650 Channel

Fig. 2. Spectrum of Helium-like resonance lines of Vanadium taken on the NIST EBIT. The x-axis is channel luunber, and linear in energy (see text). Lithium-like contamination is present but unlikely to impair the final meanuement, given the data quahty and resohition obtained.

50

45

-1—r—I—[ I I—rVLya,

300 400 500 600 700 800

Channel

Fig 3. Obscrvalion of Hydrogen-Uke Vanadium Ly a, (2pj,2-ls) and Ly

(2p,,2-ls). Siausiics are limited but the peaks are clear and resolved. The

channel number indicates the integrated and peak counts for a given tran- sition

aids identificatioD of charge state and species contanunation. Absolute calibration is aided by the overlap in energy of this spectrum with the robust calibration line of Ti K^i 3 . This observation does not overlap with the polarization measurement referred to earlier, but reveals details of a

similar interest and complexity. This work is proceeding.

4. QED measureoMots Some of the most fundamental questions can be addressed by investigating few-electron systems. In particular QED

and electron correlation in systems of high effective coupling

strengths may be tested effectively in hydrogenic and

helium-like levels [10, 11]. For the medium-Z regime,

Lyman a wavelength precision of 30 ppm or so is required

to contribute usefully to the current comparison of theory

and experiment.

There has been significant interest in heUum-Uke transitions in the region Z = 19 to Z = 26 given recent speculation of errors in theory at the 40 ppm level [12]. Helium-like

Vanadium is therefore ideally situated to investigate such

concerns. Our investigations of the resonance lines are proceeding, with the quahty of the data and statistics indicated

in Fig. 2. This represents data collected over 24 h. Possible

drifts between individual scans are allowed for by use of cUnometry readings [9]. Na-like barium contamination is present but clearly separated. The energy calibration must

be further refined before a serious estimate of imprecision can be made. The necessary involvement of theory to confirm experimental calibration is discussed in [9] and is the basis of the principle of research which we are pursuing.

Recently, the first observations of hydrogen-like Vana- dium have been made at the NIST EBIT in a lower

resolution geometry. The 13eV separation between V Ly a, and V Ly is clearly resolved despite the lower resolution

(higher flux) arrangement. A current spectnmi from a single run is given in Fig. 3. Currently statistics is quite Umited. The ionization and recombination balance in EBITs

makes hydrogenic systems more challenging than closed

shell systems, and in particular leads to much lower fluxes. The population and emission rates also vary greatly between systems and spectral components. In each of these systems, operating conditions on the MEWA source and

EBIT have been optimised. Nonetheless, our results confirm

that for individual sp>ectral lines the Ne-like barium spectra

obtained fluxes of 4 times the He-like Vanadium flux and 13 times the hydrogenic ion flux. This reveals the operational status of the MEWA injector, demonstrates one of the specifications of the NIST

EBIT, and highlights the potential for precision measurement. Currently statistics is limited for high-resolution

spectra and hydrogenic systems - but it appears that further

analysis will allow a QED measurement at the significant

level.

5. Summary

The NIST EBIT is fully operational, and has successfully

acquired high-resolution data with a curved crystal spectro- meter suited to EBIT conditions. We have collected data penaining to three important physical systems: neon-like

Physica Scnpta T73

NIST- 185
«

Progress Towards Absolute X-ray Spectroscopy on the NIST Electron-Beam Ion Trap: Current Status and Results 89

barium, helium-like vanadium and bydrogenic vanadium.

Further work will pursue application of these principles to few-electron nickel and other systems. Analysis of systematic

shifts and hence of the absolute calibration of the spectra is proceeding. They are expected to produce robust and reli- able estimates of energies of these lines soon, and may be compared to alternate observations which reference secondary calibration lines of low intrinsic accuracy.

References

1. Levinc M. A., Mans, R. E, HendeRon. J. R., Knapp, D A. and

Schneider. M. B., Physica ScripU TZ2. 157 (1988). 1 Gtllaspy, J D., Roberts, J. R, Brown, C. M. and Feldman. U., "The NIST EBIT: A Progress Report", Proc. Vlth International Conference on the Physics of Highly Charged Ions (Edited by P. Richard, M. Stoddi and C. D. Lin) 274. 682 (AIP press. New York 1993).

3. Margolis. H. S, Groves, P. D, OxUy, P. K, Silver, J. D. and Vamey.

A. J., Physica Scripta. this issue.

4. Asada, J. et al., Physica Scripta, this issue.

5. Biedennann, C. Forster. A., Fuflmann, G. and Radtke, R.. Physica

Scripta, this issue. 6 Gillaspy, J. D. et al., Physica Scnpta TS9. 392 (1995). 7 Takacs, E. ei al., Phys. Rev. A54. 1342 (1996).

8. Laming, J. M. et al^ Nucl. Instr. Meth. B31. 21 (1988).

9. Paterson. D. et al., Physica Scripta, this issue.

10. Brodsky, S. J. and Mohi, P. J., "Quantum Electrodynamics of high-Z

few-electron atoms", in: "Physics of Highly-ionized Atoms" (Edited by

R. Mamis) NATO ASI Series B201 (Plenum, New York 1989) 111.

1 1 . Deslattes, R. D.. Physica ScripU T46. 9 ( 1993). IZ Beiersdorfer, P, Bitter. M, von Goeler. S. and Hill. K, Phys. Rev. A40. 150(1989).

NIST- 186

Physica Scripta T73
Physica Scripta.VoL T80, 440-442, 1999

Precision X-ray Spectroscopy at the NIST Electron-Beam Ion Trap:

Resolution of Major Systematic Error

C T. Chantler,' D. Paterson,' L T. Hudson,^ F. G. Serpa,^ J. D. Gillaspy^ and E. Takacs^

' School of Physics, University of Melbourne, Parfcville, Victoria 3052, Australia

^ Atomic Physics Division, NISX Gaithersburg, MD20899, USA

^ Experimental Physics Dept, Kossuth University, Debrecen, Bern L 18/a., Hungary H-4026 and Institute of Nuclear Research of the Hungarian Academy of

Sciences (ATOMKI), Debrecen, Bern ter 18/c Hungary

Received September 14, 1998: revised November 30. 1998: accepted December 3, 1998

PACS Ref: 3230.Rj, 34.80.Kw, 31.30.Jv

Abstract

Some of the most critical tests of the QED Lamb shifts are currently being pursued in the X-ray regime by investigating the core levels of medium Z

atoms such as vanadium. The current approach to the measurement of

the Lamb shift in such a system is outlined. We present major progress in understanding and reducing the systematic errors within our system. The

error budget has been reduced to a level where critical tests of QED can

be performed. Recent observations of Lyman a in hydrogenic vanadium

at the NIST Electron-Beani Ion Tiap are presented.

1. Introduction

One of the goals of much current research is to test Quantum

Electro -Dynamics (QED) critically in new and important

regimes. Parallel investigations include muonium [1], positronium [2] and g-2 experiments in different systems. Most effort has been directed to Lamb shift measurements

in hydrogenic and hehum-like systems.

In our current research we target medium Z hydrogenic and

helium-like ions where transition energies are in the X-ray

region. To critically test current theory a precision of 10-30 ppm is required. In such measurements absolute calibration

is essential Using methods outlined below we have achieved

cahbration at the desired precision. This work continues

the efforts towards precision spectroscopy on an Electron-Beam Ion Trap [3] and the progress made earUer with

respect to calibration and systematics for X-ray energies [4].

2. X-ray spectroscopy and QEID measurement

Investigation of pure QED effects (rather than correlation) often involves the hydrogenic Lyman a transitions. These have been best studied in the X-ray regime, because they look

directly at the full Is Lamb shift and are resonant transitions. These experiments have usually been performed at accelerators. Some of the best measiuements have reached 30-40 ppm, depending on Z [5]. Diffracting crystals are used in these medium Z investi- gations of ls-2p transitions. However, systematic effects relat- ing to diffraction including refractive index corrections,

differential depth penetration and focussing, and source size or geometric effects are often simplified or neglected. This

is particularly true for curved crystal geometries, which are often used to obtain increased signal and lower statistical error. These critical issues led to the development of the first theory to combine curvature and mosaicity in a dynamical

diffraction theory [6].

Physica Scripta T80

3. QED measurements on the Electron-Beam Ion Irap

(EBIT)

An EBIT involves an electron beam, focussed by sup- erconducting electromagnets, which attract and ionise positive ions. By selecting the beam potential you can select the charge state of the positive ions, so it is a sensitive probe

for high Z ionised atoms [7]. EBITs avoid the concern with Doppler broadening, shifts and limiting uncertainties that dominate in conventional accelerator experiments. EBIT

experimental approaches are complementary with acceler- ator approaches: two experiments using different techniques

and with different limiting systematics are a sensitive probe

of the vahdity of experimental results. EBITexperiments hold promise for a significant improvement in QED

measurement. There are only two major issues with EBIT-based QED

measurements:

1) Flux and statistics, and

2) Calibration and systematics

In our first experiment hehum-Uke vanadium was studied and the uncahbrated spectra have been published previously

[8]. The spectra are clean, resolution is good, and the statistics after a long accumulation of data are adequate. Hence the hehvun-like vanadium measurement is competitive on the first issue with other sources and other QED measurements. However, the optimised population of hydrogenic vanadium

in the same experimental run was much poorer, and produced

inadequate statistics for a 30 ppm QED measurement

We have made a number of improvements in Melbourne

and at NIST, which has led to higher resolution spectra, increased fluxes, and good statistics for a QED measurement

in hydrogenic vanadium (see Lyman a spectra, Rg. 1). At this stage calibration and systematics are the limiting errors.

4. Resolution of systematics in EBIT measurement

Investigation of the systematic issue has encouraged us in our

current experimental plan. Often, results have emphasised

a high-flux collection mode with relative calibration to less well-defined lines. We have concentrated on absolute cali- bration of lines by direct reference to standard sources used

as the second link in an X-ray metrological programme.

The wavelength of calibration lines are determined by correcting Bearden's values [9,10,11]. Recent work in this area has reduced the errors on Bearden's values, in general by

© Physica Scripta 1999

NIST- 187
Precision X-ray Spectroscopy at the NIST Electron-Beam Ion Trap: Resolution of Major Systematic Error 441

Hydrogenic Vanadium - NIST 1998. Flux tripled

Cleaner spectrum, higher resolution 120 p-' ' ' '

I

' ' '

'

I

200

VLya^

2p

5431 eV

1/2

" ^ ^1/2

VLya,

5444 eV

300 400 500 Channel

600 700

51

0

' 1 ' ' ' 1

' 1 1 '

'——1——

1

-50

-1- wsdge in

+

+ ^

*

100

we<lgg,-<Jut + wedge in 150 wedge out

200

CrKff N*iKa VKBCrKo VKa TiK3 ; ^ - - - 23 30 32 34 36 38

Spectrometer diffiacting angle (degrees)

fig. 2. Experimenul results for wedge posidoo, uncorrected for the plate

function. Several data points are plotted for each calibration line and wedge

position and the variation between these points indicates the shifts due to other experimental parameters such as detector position and source size. Discrepancies due to wedge position can exceed 150 arcseconds.

fig. L New hydrogenic vanadium spectrum showing good statistics.

25%-30% [12] from 12 ppm to 8-10 ppm.We also do not rely on a single absolute calibration energy, but require a series

to calibrate the dispersion function of the spectrometer.

Our curved-crystal spectrometer is not immune to systematics, but their effect is at a much lower order than

other techniques. Our observations two years ago were loosely limited at 100 ppm (or a rather weak QED meastirement) as a consequence of these systematics. It was clear then that the problem was due to an inadequate application of diffraction theory and the computation of effective refractive index

centroid shifts. Other researchers have ignored this problem,

which compromises results at around the 100 ppm level. A critical insight related to the effect ofthe Seemann wedge.

The "wedge" is a knife-edge limiting the region of the crystal across which effective diffraction can proceed to an image on the detector. The location "out" means that the ftdl crystal surface may be used; "in" will restrict the diffraction to regions near the optimum axis of the crystal spectrometer.

Both approaches give common results when the spectrometer

is tuned to give a focused profile on the Rowland circle (i.e. when the detector is centered on the Rowland circle). As

we go further from this region, the two results for wedge "in" and "out" follow quite different trends and systematics as illus- trated in Fig. 2. This is a plot of experimental calibration data

across the entire range of calibration lines measured. The dis- crepancy angle is the difference between the fitted dispersion function of the spectrometer (for wedge "in" data only)

and the data. Several data points are shown for each individ- ual calibration line and wedge position. The difference between measured experimental values and the predicted

theoretical position shows the magnitude of shifts associated with wedge position (as indicated by the two arrows diverging

with decreasing diffraction angle). The spread of values at a

given angle indicates other sources ofexperimental error, such

as detector alignment.

We have proven that the wedge "in-out" discrepancy is due

to asymmetric partial focusing on the detector plate. Extrapolated shifts at the Rowland circle do not agree with shifts at our flat, extended detector, and must be corrected. The conse- ® Physica Scripta 1999

Profiles & Diffraction Shifts for CrKa

0.6

CO 0.5

'

1

1 ' '

1 1

1 —1—CrKOi inn.it CrKa, inji

- -+- • CrKa outn CrKa outa

- - X CrKa^ out.n CrKa out, film, 7t CrKa out, film.ci V -^^\

X

iiiiiiiriTTi-i 1 1 1 1 > ii ii -12000 -8000 -4000 0 4000 8000 12000

position (0.1 |im, = 1.8 arcsec)

Fig. 3. Theoretical modelling of diffiaction profiles for various wedge positions and detector types. Profiles labeled "film" are those imaged by a detector

that always hes along the Rowland circle. The polarisation of the incident X-rays are labeled as "rc" or "a", denoting electric field vector perpendicular

or parallel to plane of incident radiatioii, reqwctively. The qualitative efifects of changing the wedge position iioni 'out' to 'in' arc dramatic.

quence is illustrated in Hg. 3 which shows theoretically cal- culated diffraction profiles for the chromium Ka

calibration line. Plotted are two sets of calculations for our

experimental situation with various wedge positions. The first set of profiles, labeled "film", are for a detector which always

remains on the Rowland circle (traditionally photographic

film is used). The second set is for a fiat detector located

at a fixed distance from the crystal The profiles change to a more complex shape with significant offsets of the centroid when the detector is flat and not located on the Rowland

circle. These profiles are calculated using dynamical theory

of diffraction for curved crystals, observed at either a curved

detector on the Rowland circle or a defocused location on

a flat plate. The profiles caimot be predicted from flat crystal

profiles and include significant additional broadening and

asymmetry.

Rgure 4 illustrates how the theoretical modelling accurately predicts the real systematics as shown in Hg. 2 and

as a result we have resolved this major systematic error. The variation with diffracting angle is highly non-linear with

Physica Scripta T80

NIST- 188
442 C T. Chantler et aL

e. o

-50

-100

-150

-200

•

-

•

-

wedge out wedge m

CrKB WiKa

J , ' ^

VKpCrKa

1 ... 1

VKaTiKp '.

38

Theoretical prediction +R1 correction (degrees)

Fig. 4. Theoretical modelling of shifts due to wedge position for one set of experimental parameters (somT:e size and detector positionX There is very good qualitative and quantitative agreement between experimental results (see

Fig. 2) and the theoretical modelling.

This represents a 3% - 5% Lamb shift measurement, which is comparable to the results obtained using accelerator and other

techniques. The main sources of uncertainty are ail accessible

to fiirther investigation, and the error could certainly halve under appropriate conditions. The hardest limit in the above

list is the statistics, but several approaches are being pursued

which could halve this in subsequent investigations.

5. Conclusion Measurement of the Lamb shift in hydrogenic medium Z

atoms such as vanadium remains one of the most critical tests of QED. Major progress in imderslanding and developing

absolute X-ray spectroscopy for an EBIT source has been

outlined. We are pursuing developments to fiirther increase

the precision of current approaches in the X-ray regime.

wavelength. With the inclusion ofthe plate function systematic

shift the dispersion function of the spectrometer can now be

determined with errors of less than one arcsecond. This cor- responds to a precision of 7-8 ppm, with the final error on a measured transition being fiirther reduced. Hence the systematics in the calibration has now dropped by about a

factor of 5-10, or down to circa 10-20 ppm, and no longer

hmit the measurement.

So the error budget has dramatically reduced to the

following:

Spectrometer Systematics (dispersion fimction uncertainties which were 100+ppm in 1996 for any curved crystal EBIT

investigation) reduced to 10-20 ppm

EBIT Statistics (100 ppm for H-like V in 1996); now

15-20 ppm

Bearden (reference wavelengths) 10-12 ppm

Calibration profile fitting, Temperature, Doppler shifts< 5ppm

Total: 20ppm — 30ppm

References

1. Chu, S. era/., Phys. Rev. LetL 60, 101 (1998); Maas, F. E. era/., Phys. Lett A187, 247 (1994).

2. Chu, S., Mills Jr., A. R and Hall, J. L, Phys. Rev. Lett 52, 1689 (1984); Fee, M. S. et al., Phys. Rev. Lett 70, 1397 (1993).

3. Gillaspy, J. D., er a/., Physica Scripta T59, 392 (1995).

4. Paterson, D., et al., Physica Scripta T73, 400 (1997).

5. Deslattes, R. D., Physica Scripta T46, 9 (1993).

6. Chantler, C T, J. Appl. Cryst. 25, 674 (1992); Chantler, C T., J. AppL

Cryst 35, 694 (1992). 7 GiUaspy, J. a, er al., Physica Scripta T59, 392 (1995).

8. Chantler, C T., er al., Physica Scripta T73, 87 (1997).

9. Bearden, J. A., Rev. Mod. Phys. 39, 78 (1976).

10. Kessler, E. G. et al., Phys. Rev. A26, 2696 (1982).

11. Schweppe, J., Deslattes, R. D., Mooney, T. and Pbwell, C J., J. Electron

Spectrosc. RelaL Phenom. 67, 463 (1994).

12. Deslattes, R. D., Kessler, E. G., Indelicato, P. and Lindroth, E., section

4.2.2 InL lables for Crystallography, (Kluwer Academic Press, Stockholm,

1999).

Physica Scripta T80

NIST-189

© Physica Scripta 1999
PHYSICAL REVIEW A. VOLUME 62, 042501

Absolute measurement of the resonance lines in heliumlike vanadium on an electron-beam ion trap

C. T. Chantler,' D. Paterson,' L. T. Hudson,^ F. G. Serpa,^ J. D. Gillaspy,^ and E. Takacs^-*

^School of Physics, University of Melbourne, 3010, Australia ^National Institute of Standards and Technology, Gaithersburg, Maryland 20899

(Received 14 February 2000; published 11 September 2000)

Absolute measurements of the energies of resonance lines in heliumlike vanadium on an electron-beam ion

trap are reported. The \s2p '/',-l^^, \s2p ^Pi-'ls^ (ls2p ^Pj-^Ij^ and ls2p ^Pq-*Is^) blend and lj2i^5|-»li^ transitions are 5205.10 ± 0.14 eV. 5189.12 ± 0.21 eV. 5180.22 ± 0.17 eV, and 5153.82 ± 0.14eV, respectively. This agrees with recent theoretical calculadons and the experimental precision lies at the same level as the current uncertainty in theory (0.1 eV). These measurements represent a 5.7-8% deter- mination of the QED contribution to the transition energies and are the most precise measurements of heli- umlike resonance lines in the Z= 19-31 range. The measurement of the \s2s ^Si-*\s^ transition is also

sensitive to the ls2s 'S, QED contribution at the 40% level. The intensity of the strong ls2p 'P,—*l5^ component of the blend compared to the total blend intensity is 94% ±12%. This is in accord with current

theoretical predictions but disagrees with an earlier reported ratio. PACS number(s): 31.30.Jv, 32.30.Rj, 12.20.Fv, 34.80.Kw

I. INTRODUCTION AND BACKGROUND

The calculation of two-electron ion energy levels has been

a topic of much research since the discovery of quantum

mechanics. The contribution of relativistic and quantum

electrodynamic (QED) effects has been intensely studied in the last three decades [1,2]. Two-electron systems provide a

test bed for not only quantum electrodynamics and relativistic effects calculations, but also for many-body formalisms

[3].Theoretical calculations of energy levels in heliumlike

ions using variational techniques, relativistic corrections, and

a Za expansion in the unified approach were pioneered by

Drake in 1979 and 1988 [4,5]. Since then there has been

active research in the calculation of energy levels in heliumlike ions using new techniques. Major progress in theoretical calculations of QED contributions has occurred in the last decade, particularly in developments avoiding the Za expansion by using all-orders techniques [6-9]. Plante et at. [6] provide a recent review of calculations of the n=\ and n = 2 states of heUumlike ions and present relativistic all-order many-body calculations. To critically test QED contributions at the 10% or better

level in medium Z ions requires an experimental precision of 10-30 ppm. In many cases experiments of sufficient preci- sion have not been performed which could resolve the dif- ferences between theories [6,9,10]. Repeated claims in the

literature have suggested that across the Z= 19-32 range

experimental determinations of the ls2p 'P]—Ij^ transi- tion energy are greater than theory [11-13]. We have di- rected our research at an absolute precision measurement of heliumlike resonance lines in vanadium (Z=23) to investi- gate these claims in the center of the region of interest (he- liumlike resonance lines refer to n = 2 to n = 1 transitions in two-electron ions where one electron remains in the ground

{Is) state as illustrated in Fig. 1). No prior high precision

*Permanent address: Institute of Experimental Physics, University of Debrecen, Debrecen, Bemter 18/a, H-4026 Hungary.

'P..

Is2p "P2.

Pi:

^PoEl Ml El

El

hyperfine inducedMl

2So

ls2s

2E1

FIG. 1 . Radiative decay scheme of medium Z heliumlike ions, indicating the nature of decay: El electric dipole. Ml magnetic

dipole, M2 magnetic quadrupole radiation, and 2£1 two-photon

£1 process. The hyperfine-induced ls2p ^Pq—^Is ^ transition is only allowed if the nuclear magnetic moment is finite (vanadium

7/2 ft). The notation of Gabriel [14] is indicated for each resonance

line.

absolute measurements in vanadium have been performed,

which was an additional motivation. For the lj2p 'Pi —» Is^ transition (w line, notation of Gabriel [14]) it has been

claimed that the heliumlike resonance line calculated by

Drake is too large by about 60 ppm [12] based upon a series of measurements of the w line (Z= 19-26) [1 1]. The QED

contributions to the heliumlike resonance lines in vanadium

are 480-550 ppm of the transition energies [5] and therefore precise measurements (better than 30 ppm) of these transi- tions are sensitive to, and can test, calculations of the QED

component.

The uncertainty of theoretical determinations of energy

states in two-electron ions has been a focus of recent attention [10]. Earlier theoretical work [5] claimed an uncertainty

1050-2947/2000/62(4)/042501 ( 1 3)/$ 1 5.00 62 042501-1 ©2000 The American Physical Society

lSnST-190
C. T. CHANTLER et al. PHYSICAL REVIEW A 62 042501

/

y/

/

/Rowland circle

Detector

EBrr

source Vacuum \\

Calibration source

k Crystal

FIG. 2. Spectrometer configuration at the NIST EBIT; note that

the EBIT source is located well inside the Rowland circle. The

spectrometer is oriented such that the axis of the spectrometer is perpendicular to the long axis of the EDIT source and the detector arm moves vertically with changes in diffraction angle.

of 1 ppm due to uncalculated higher-order terms. However,

the discrepancy between current theories approaches 30 ppm

in the Z=23 region. Recent work of Persson et al. [10] es- timates missing correlation effects in QED terms to be 20

ppm in the Z=23 region (absolute value of order 0.1 eV

across medium and high-Z elements). Precise experimental

determination of the heliumlike resonance lines can probe

these discrepancies and uncertainties. Most precision spectroscopy of medium-Z ions has been

conducted at accelerators or tokamak plasmas. The recent development of the electron-beam ion trap (EBIT) has of- fered another spectroscopic source to experimenters. Our experimental method takes advantage of the Doppler-free and

relatively clean spectra produced by an EBIT coupled to an

external calibration source to allow absolute measurements

on highly charged ions. We have made absolute measure- ments of the heUumlike resonance lines in vanadium with an

uncertainty of 27 ppm for the \slp 'Pi—»1^^ transition. Our results for vanadium do not support evidence of a sys- tematic discrepancy between theory and experiment [11-13].

These are the first precision x-ray measurements conducted

at the NIST EBIT facihty.

n. EXPERIMENT

The experimental configuration is shown in Fig. 2. Other

experiments have been conducted on the NIST EBIT using

this spectrometer including polarization studies by Takacs

etal [15]. The EBIT uses an intense and monoenergetic electron beam which is magnetically confined to trap and ionize charged ions [16,17]. Metal ion species such as vanadium are created by a metal vapor vacuum arc (MEWA) ion source and are sequentially ionized as they enter the trap region of

the electron beam. The electron-beam energy distribution of

the EBIT is narrow and the energy can be tuned. The heli- umlike vanadium* resonance lines are excited by setting the electron-beam energy to 6.9 keV (including space-charge

correction), 1.75 keV above the direct electron-impact excitation energy. The electron-beam current was typically 140

mA.

We employ a Johann spectrometer with a Ge(220) crystal (2£/= 4.0007 A) which has a useful wavelength range be- tween 1.9 and 2.8 A in first order. The radius of crystal curvature is 1.846 m and the resolving power (X/AX.) of the spectrometer in the wavelength region of investigation is 2000-2500. The spectrometer has a Seemann wedge that can

be lowered towards the crystal pole, reducing the diffracting

crystal area used and the x-ray throughput but improving the

resolution [18]. The detector is a two-dimensional positionsensitive proportional counter that employs capacitive charge

division from a backganmion design cathode to determine

position [19]. The spectrometer has a fixed crystal radius so

the detector does not always remain on the Rowland circle. This has substantial systematic effects on measurements

[20,21].

The spectrometer is aligned with respect to the EBIT so

that the plane of crystal dispersion is parallel to the electron beam axis (see Fig. 2). The range of Bragg diffraction angles

accessible with the spectrometer is 25.0-60.9° and the heli- umlike resonance lines of vanadium are observed at approximately 39°. The crystal acts as a polarizer at Bragg angles

near 45° and therefore radiation polarized perpendicular to the electron beam axis is the dominant diffracted component.

The detector has a window wider than the full heliumlike vanadium spectrum of the EBIT. Hence several detector angles centering on various features of the spectrum were

able to characterize detector linearity. Our work is concerned with the measurement of heliumlike resonance lines. Dielectronic satellites can affect the pre- cision of the measurements. By tuning the electron-beam en- ergy well above the dielectronic resonance energies they are not excited, resulting in a spectrum clean of dielectronic satellite lines. Transitions from the lithiumlike states of vana- dium of type XslllV lie very close to the lines of interest. Efforts must be made to minimize or eliminate these satellites from the spectra observed. The lithiumlike resonance

lines that directly concern this research—due to their close

location to the heliumlike resonance lines and their relatively

large electron-impact cross section for inner-shell excitation—are the q and r [14] or Xslslp ^P-ia -^1^^2^ ^5i/2 and \s2s2p ^ P ifi-* I s'^ls ^5,/2 transitions. Our treatment of the lithiumlike resonance lines is discussed

in detail in Sec. IV.

A. Calibration

Calibration is a detailed and extensive procedure because we map the dispersion function of the spectrometer across a broad range of wavelengths around the region of interest. Using a large number of calibration lines ensures the disper- sion function is well determined and a* wide range of wavelengths ensures that any higher-order wavelength-dependent

effects are included. We also systematically investigate the

042501-2

NIST- 191
ABSOLUTE MEASUREMENT OF THE RESONANCE LINES

avin4

PHYSICAL REVIEW A 62 042501

500 ^"120 540 560 channel number

FIG. 3. Curve fitting to vanadium Ka calibration lines. The

fitted functions are Lorentzian convolved with a slit function. Four

individual profiles are fitted (see text) and indicated by dotted lines. +represents data points and the solid line indicates the sum of fitted components. Residuals to the fit are shown by a dashed line underneath the main plot.

effects of variations of source size and the Seemann wedge

position during calibration. Thus calibration is a time con- suming but necessary procedure for an absolute precision measurement. The calibration source consists of a 20-keV electron gun

and a series of metal targets (Mn, Cr, V, Ti) located on the opposite port of the EBIT to the spectrometer as indicated in Fig. 2. We locate the calibration source and the EBIT source

inside the Rowland circle by design to increase flux, reduce

systematic uncertainties, and increase the spectral band pass

to the point that the entire vanadium spectrum can be ac- quired in parallel [21]. Calibration spectra are collected for a range of Ka and KfB characteristic wavelengths (1.9-2.8 A)

about the heliumlike resonance lines of vanadium. Bragg dif- fraction angles of cahbration lines are in the range 29°-45°

while the heliumlike resonance lines of vanadium are ob- served at approximately 39°. Ka^ and Kai are well resolved

in our system and so the Ka doublet provides two reference wavelengths at one detector location [20,21]. This is useful

for absolute calibration of the detector channel number in micrometers. A typical vanadium Ka cahbration spectrum is shown in Fig. 3. Approximately 100 individual calibration measurements were made with some 20 independent inte- grated observations of heliumlike resonance lines in vana- dium.

B. Energy dispersive measurement A sohd-state Si (Li) detector is used when optimizing the EBIT conditions to maximize the flux of heliumlike vana- dium transitions and to minimize contamination firom other charge states and dielectronic recombination transitions. An

example of a typical Si(Li) spectrum is shown in Fig. 4. The

resolution is sufficient to identify hydrogenic and heliumlike vanadium as well as various barium charge states (barium,

emanating from the electron gun, is common in EBIT spec1

2.0xltf

'-

I 1.0x1* - 3

O

S.Oxltf -

0

energy (keV)

FIG. 4. General features of the Si(Li) spectrum observed on the NIST EBIT: The highest peak at 5.2 keV is composed of a blend of

the heliumlike vanadium resonance lines, and the broad peak be- tween 4.6-4.9 keV is predominantly neonlike barium and other barium charge states.

tra). A small amount of pure nitrogen gas is leaked into the

trap to increase the proportion of higher charge states via evaporative cooling.

In the EBIT an axial trapping potential is created by a

series of three drift tubes. The voltage applied to the middle

drift tube determines the energy of the electron beam and

sets the position of the trap with respect to the ground potential. The drift tube voltages can be switched rapidly in a time sequence. This is necessary in order to catch the ions

injected from the MEWA ion source and to empty the trap

after a certain amount of time to avoid the accumulation of unwanted long lifetime charge states and background barium

ions. The time sequencing was optimized using the signal

from the Si(Li) detector. Once the optimum EBIT operating conditions are chosen,

crystal spectrometer data are collected while continuing to monitor the performance with the Si(Li) detector. A single

spectrum is collected and saved for each period, approximately 2 h, between liquid nitrogen fills of the outer cryogenic jacket of the EBIT. Spectra are collected continuously

for several days with a 24-hour-a-day EBIT operation. Spectra collected under the same conditions are then summed

during the analysis of data. Figure 5 shows a heliumlike vanadium spectrum from 24 h of observation with the crystal

spectrometer. The position and relative intensity of the q

lithiumhke resonance line is also indicated in Fig. 5. Contamination from the r line is negligible.

C. Clinometry

During the relatively long data acquisition periods, care must be taken to monitor and minimize any shifts in the

diffraction angle or detector position. Clinometers continually monitor the detector and crystal angles with arcsecond

resolution and can be used to determine any changes in angular position during EBIT observations. The clinometers

also record certain large amplitude vibrations of the detector arm and crystal mount. Such vibrations may be caused by

faults in vacuum pumps or rapid surging of liquid nitrogen

through the feed lines to the EBIT. Data obtained displaying

these vibrations are rejected because the diffraction profiles

042501-3

]^nST-192
C. T. CHANTLER et al. PHYSICAL REVIEW A 62 042501

150

St 100

50

28 H 0

00^ -20

A

^v

400 channel number

600

FIG. 5. Heliumlike vanadium spectra observed at the NIST

EBIT. Resonance lines and lithiumlike satellite lines are labeled according to Gabriel's scheme. Summed observations represent some 24 of observation. The q{\s2s2p P^n-^ I s^2s ^5 1/2) lithi- umlike satellite contamination is at a level which does not impair

the final measurement. Spectra are fitted with a Lorentzian con- volved with a slit profile and a background. The dotted lines are the

fitted profiles to each resonance line; the solid line is the sum of all fitted profiles and background.

will be broadened, compromising resolution and precision.

Rejected data comprised a small subset of the total measurements. Most observations were made with the Seemann

wedge removed to maximize flux. A post-measurement calibration was conducted to insure

that no changes in the spectroscopy system had occurred

during the measurement process. The postrun calibration, when subsequently analyzed in conjunction with prerun cali- bration data, was used to ensure against physical shifts and

changes in alignment.

m. CURVED CRYSTAL THEORY AND SYSTEMATIC

SHIFTS

Curved crystal geometries are often used in precision x-ray spectroscopy to obtain increased signal and lower statistical error, or to focus an extended source. All systematic

shifts down to the level of 10-30 ppm must be accounted for

to perform useful tests of QED in medium Z highly charged

ions. Dynamical x-ray diffraction plays a crucial role in any wavelength dispersive spectroscopy such as ours. A systematic shift which is commonly accounted for in x-ray diffrac- tion is the refractive index (RI) correction which is due to the mean change in refractive index in the crystal changing the angle of incidence relative to the air or vacuum. The angle of

diffraction Og is shifted causing Bragg's law of diffraction

to become

nX = 2d sin

\-RI

n\ = 2dsin Ogl 1

(1)

sin dgj

(2)

where RI is the index of refraction (specific to \ and the

crystal). The magtytude of the refractive index correction is 100-300 ppm [22,23], depending upon the wavelength, crystal, and geometry, and so must be determined to a few percent.Most curved and flat crystal spectroscopy QED investigations have addressed systematic corrections based upon raytracing methods, incorporating the simple refractive index

correction [24-26]. Some of these QED measurements were

calibrated using wavelengths observed in different orders of

diffraction [11]. These measurements, however, did not use a

full dynamical diffi-action theory to calculate systematic

shifts and therefore did not consider significant effects relat- ing to depth penetration, lateral shifts, source size, and detector location. Any of these effects can cause systematic

shifts comparable to the refractive index correction.

A. Depth penetration

Depth penetration refers to the mean depth that x-rays

penetrate into the crystal and the effect that this penetration has upon the observed difft°action angle. For curved crystals

depth penetration at angles other than normal to the surface

will result in a variation of the angle of incidence in successively deeper crystal layers. The depth penetration of x-rays

into a crystal can have effects larger than refractive index

shifts [23]. It is therefore the dominant systematic in many

curved crystal measurements, and is independent of refractive index and ray-tracing (geometrical) corrections. The

magnitude of the depth penetration is dependent upon dif- fraction order and angle of incidence. The resulting shift in angle for curved crystals (radius 2^^ where is the Rowland circle radius) can be estimated using

^^in/out=arccos -ieB±a.),

(3)

where 3 is the mean depth of penetration and is the mean

angle of diffracting planes to the crystal surface [23]. Simple

estimates of d can be made but a rigorous calculation with

1% precision requires the dynamical diffraction theory. The

strong dependence upon diffiraction order, due to the dependence of attenuation {/x) upon energy, implies that any x-ray QED measurement employing diffraction in different orders should address this effect explicitly.

B. Lateral shifts

Lateral shifts are also due to depth penetration and refer to a shift in the exit location of an x-ray at the crystal surface. For flat crystals this results in a transverse shift at the detector and is dependent on jj, and extinction. For curved crystals the lateral shift causes a shift at the detector (if positioned on

the Rowland circle) of A i'ia,erai(>l .S) =7?,arccos[ $] - 2R^B , (4)

where

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ABSOLUTE MEASUREMENT OF THE RESONANCE LINES PHYSICAL REVIEW A 62 042501

<1> = [(2 cosB — cosA)cos/4 — Vsin'^ A ( sin^ A + cos"^ B - cos fi cos A ) ] (5)

and B= Og + ap, A-B + 9a, and 6^ = Adin+ A. d^^^ as given

by Eq. (3). Simpler, less precise estimates can be formulated

for specific situations but ultimately any calculation relies on

a precise determination of depth penetration. For low angle

and low order diffraction, lateral shifts will be 10-100% of

the refractive index corrections [23]. Equation (4) will deter- mine the lateral shift on the Rowland circle but not the shift

at a detector located off the Rowland circle.

C. Source and detector shape and position

Geometrical effects such as source size and position can

cause shifts of a similar magnitude to refraction [23]. While

ray tracing can adequately describe these situations outside

the crystal, the effect of dynamical diffraction, associated depth penetration, and lateral shifts modifies these estimates. Estimates of shifts or errors presented in curved crystal

geometries are often calculated for an ideal detector located on the Rowland circle. However, the detection surface is usually flat and therefore does not conform to the Rowland

circle. Detectors located on a fixed length detector arm will also travel off the Rowland circle with a variation in the detection angle unless the crystal curvature is also varied

precisely.

Mosaicity and crystal curvature effects are also often dis- regarded but these effects must be considered for the preci- sion which has been the aim of this research. These issues

led to the development of the theory that combines curvature and mosaicity in a dynamical diffraction theory by Chantler

[27,28]. One systematic shift is caused by the calibration source not being in the same location as the EBIT source. Theoretical modeling determines the shifts associated with

this mislocation and the correction is 3-5 ppm depending upon wavelength.

Detailed theoretical modeling of our spectroscopy system

has revealed a wavelength-depjendent systematic error associated with detector shape and location due to asymmetric

partial focusing on the detector plate. Conventional shifts calculated for detection on the Rowland circle do not agree with shifts computed for flat extended detectors. This sys- tematic error can be 100-200 ppm and would be present in any Johann curved crystal spectrometer with fixed length

detector arm and flat detection surface.

In later work [20,21] this theory was expanded to incor- porate flat surface detectors located off the Rowland circle. We have therefore developed the computational techniques

to deal with all of the above-mentioned systematics in a rig- orous and integrated manner.

rV. DATA ANALYSIS AND ERROR SOURCES

Calibration and EBIT spectra are fitted with Lorentzian Une shapes convolved with slit profiles. The width of the Lorentzian and common slit components are free parameters

in the fit. The background is negligible and is not fitted. For

calibration lines, two components are fitted to each line following Deutsch et at. [29] and Hartwig et al. [30] to repro- duce the asymmetry of real profiles (see Fig. 3). The heliumlike resonance lines and satellite contamination are fitted by

a single component with a Lorentzian convolved with a slit profile in addition to a quadratic background. Figure 5 shows

the result of profile fitting to a heliumlike vanadium spec- trum.The location of the calibration lines with two fitted components is determined from the peak position of the sum of

the two components. Peak values are used to correspond to the reference values of Bearden [22] (this point is discussed

further in Sec. IVB). For the heliumlike resonance lines the centroid position and peak position correspond since the line shapes are symmetrical and only one component is fitted to each line.

A. Dispersion function determination Bearden 's tabulation of characteristic x-ray lines [22], al- though published in 1967, is the most comprehensive data

set available. Reference wavelengths for calibration lines are corrected from Bearden' s values using the recent CODATA

determination of lattice spacings [31]. The adjustment re- quired is relatively constant at —15 ppm across the wavelengths we use for calibration. The dispersion function is fitted to the 10 calibration wavelengths: Ti Kfi^^ , V Ka^ , V

Ka2, V A-ySi j, Cr Ka^, Cr Ka2, Cr Kfii^^, Mn ATa,, Mn

Ka2 , and Mn Kfiy^ . Prerun and postrun calibration data can

be compared for consistency and systematic shifts. The dis- persion function relates the wavelength of a spectral feature located at the detector center to the angle of diffraction which is measured by clinometers attached to crystal and

detector arms of the spectrometer.

Clinometer values for the crystal and detector are continually recorded with any spectrum and the centroid of each

clinometer distribution is the measure of the angle for crystal

(5g) and the detector (Idg). The dispersion function is de- termined by a bivariate nonlinear least-squares fit to the function

0 = arcsin DC-Ail)

A(2) — arcsin CC-A(3)

A(4) +A(0),

(6)

where DC is the centroid of the detector clinometer, CC the centroid of the crystal clinometer, 0 the reference wavelength of the calibration line (adjusted for refractive index

correction and other systematic shifts), and i4(0)-A(4) the parameters of the fit relating to offsets and scaling of the clinometer response function, which has been demonstrated

to be highly linear in sin d. We observe all the resonance lines of heUimilike vana- dium at one spectrometer setting. Thus the position-sensitive detector scale (in channels//im or channels/arcsecond) is an

important input into the dispersion function. Any systematic shifts must be accounted for when deter- mining the dispersion function. As described in the preceding section, we account for refi'active index corrections,

depth penetration of x rays in the crystal, shifts associated with source size and location, and shifts due to detector

042501-5

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C. T. CHANTLER et al. PHYSICAL REVIEW A 62 042501 TABLE I. Error budget for heliumlike vanadium measurement.

Magnitude of error (ppm) Resonance line

Error source

Statistical uncertainty of centroid position 9.5 31.0 22.3 12.9 Reference wavelengths (Bearden [22]) 12 Adjustment of Bearden wavelengths <0.5

Diffraction theory* 6

Dispersion function determination* 20

Temperature and diffraction crystal 2d spacing variation <5

Doppler shifts <4

Total excluding statistics'' 24.9

Total 26.7 39.8 33.4 28.1

^Summation of related uncertainties (see text). •Total excluding statistics: Sum of all errors except statistical uncertainty of heliumlike vanadium observations.

shape and location. The dispersion function is not a simple

relationship between angle and wavelength but a complex

(but smooth) function of reference wavelengths, clinometer

values, detector scale, and systematic shifts.

B. Error budget

The error budget is laid out in Table I. The statistical uncertainty varies between 9.5 ppm (w,ls2p ^ P^—^ls^) and

31.0 ppm (x,ls2p •'Pj—* with the relatively low flux of

the EBIT prohibiting trivial improvement. The uncertainty is determined from the curve fitting routine and is the standard

deviation of the centroid position for each resonance. This

corresponds to a determination varying between 1/27 and

1/77 of the widths. Recent work by Deslattes et al. [32] on reference wavelengths has resulted in lower estimates of uncertainties than

those on Bearden's values. Bearden's uncertainties in general

have been reduced by 25-30% from 12 ppm to 8-10 ppm.

In most cases, including all the wavelengths used in our in- vestigations, the further refinement yielded the same value

for the wavelengths within uncertainties. Our values (ad- justed from Bearden) and those of Deslattes et al. have a small discrepancy of 0.5 ppm. This discrepancy is insignifi- cant compared to the uncertainty of the wavelengths and

does not compromise our final result. The peak locations in the spectra and the derived centroid determinations for V Kai , V Kaj differ by 50-80 ppm. The choice of peak and centroid values in our calibration system is an important systematic effect. We have found that the shift does not create an error in first order if theory and

experimental results are computed consistently for peak or centroid values. Peak values for single-vacancy spectra re- main the preferred measure for reporting or determining

characteristic energies despite advanced methods for deter- mining centroids [33]. In the final analysis peak values are

therefore used and determined from curve fitting to experi- mental profiles as discussed in Sec. IV.

Diffraction theory uncertainty refers to the diffraction cal- culations that are used to determine systematic shifts. The

reliability and self-consistency of these calculations when

compared to calibration lines resulted in a variance of less than 6 ppm. The magnitude of each of the component shifts are of order 100 ppm (and vary quite dramatically for the range of angles and energies of interest), but the uncertainty on the net result is 6 ppm. We emphasize that 6 ppm is not

the net shift which varies for our experiment from about 70

to 300 ppm. The uncertainty is low for the total calculated

shift because of relative insensitivity of the result to experi- mental uncertainties in source location, source size, and detector location. To detail one example, the shift associated with the mislocation of the calibration and EBIT sources

results in a small shift (3-5 ppm); the separation of these sources is 0.25 m. It follows that uncertainty in the longitudinal position of the sources at the millimeter level is not

critical. The uncertainty of the dispersion function determination shown in Table I is currently the dominant systematic correction. The determination of the dispersion function in- volves interrelated variables, and the absolute uncertainties of these input variables do not add linearly to the final uncertainty of the function. For example, the calibration source

size uncertainty is 10% but in the Johann geometry this contributes less than 5 ppm to wavelength determination. Longitudinal source location is very uncertain (±1 mm) but in our geometry this has a negligible effect on the final accuracy. The overall contribution of calibration source size and

aligrunent uncertainty is 5 ppm. The statistical error associ- ated with the calibration lines is 2—3 ppm and the error as- sociated with the calibration profile fitting is 3-4 ppm.

The major contribution to the dispersion function uncertainty is the detector scale uncertainty. This uncertainty relates to the absolute calibration of the detector position scale

in channels per micrometer and the linearity of that scale. The heliumlike resonance lines are widely spaced on our

detector ("w"-"z" separation «=190 channels or ='8.5 mm)

and therefore any uncertainty in the detector scale is critical. The absolute detector scale is determined at less than 0.5%

which results in an average contribution to the dispersion

042501-6

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ABSOLUTE MEASUREMENT OF THE RESONANCE LINES . . . PHYSICAL REVIEW A 62 042501 TABLE II. Energies of heliumlike resonance lines in vanadium, comparison to theory.

NIST EBIT and Univ. Melbourne (this work)

Theory'

Energy QED cont.*" Energy ^E^ Expt test of QED

Notation Transition (eV) (ppm) (eV) (ppm) (%) w \slp 'P,-»1j^ '^0 5205.15 471.1 5205.10(14) -10(27) 5.7% . X \s2p ^P^-*\s^ '^0 5188.72 478.1 5189.12(21) 80(40) 8.4% y \s2p ^P^^\s^ '^0 5180.31 482.5 5180.22(17)** -19(33) 6.9%

z \s2s ^5,^1j^ '^0 5153.88 415.5 5153.82(14) -11(28) 6.7%

"Drake (Ref. [5]) ""QED cont.: QED contribution to transition energy using QED definitions of Drake [5]. 'A£: Energy (expt.-theory)/theory. **Blend of y and \s2p ^Pq-^\s^ transitions.

function imprecision of 15 ppm.

The contribution of satellite lines has also been included

in the dispersion function uncertainty. The q{\s2s2p ^Pjn

-^Is^ls ^5i/2) lithiumhke satellite is explicitly fitted (see

Fig. 5) and has a peak intensity of 13% of the w intensity.

Any dielectronic satellites will be at least an order of magnitude less intense than the lithiumlike satellites or less than 1% of the intensity of the heliumlike resonance hues [with

fiill widths at half maximums (FWHMs) corresponding to 600-800 ppm of the resonance energy]. Thus any unresolved

satellite, even located at a half-width from the centroid of a heliumlike resonance line, will cause a shift of no more than 3-4 ppm.

Dispersion function uncertainty has been reduced to 20

ppm through the careful determination of systematic shifts. This has been a major achievement of this work. We have

done explicit analysis to determine the error associated with

the omission of the systematic shift caused by a flat detector shape and location off the Rowland circle. This revealed a poor determination of the dispersion function and errors of 100 ppm, which was corrected for as described above. Further improvement with future experiments is expected.

Doppler shifts due to the low thermal velocities of the highly charged ions in the EBIT are not significant sources of uncertainty. On average the Doppler shifts will be zero as

there is no preferred direction of motion. Doppler broadening

is 1.8 eV for 1 keV ions and we allow for a possible 1%

asymmetry of velocity distribution resulting in a maximum

Doppler shift of less than 0.02 eV or less than 4 ppm. Temperature effects, particularly on the diffraction crystal 2d

spacing, are also insignificant sources of uncertainty. The

linear expansion coefficient of the germanium crystal we use

is 5.95X10~*K~' [34] and the crystal is in moderate vacuum (10~^Torr) with monitored external temperature

variations of =2 °C. Also any direct lattice spacing errors have only a second-order effect on the measurement accuracy (because of the calibration) so that the effect on wavelength determination will be less than 5 ppm.

Summing all systematics errors in quadrature results in a 24.9 ppm systematic uncertainty and a total uncertainty for

the resonance fines of 27-40 ppm. The main sources of uncertainty are therefore statistical, reference wavelengths, and

dispersion function determination. All may be regarded as

soft limits at present (subject to improvement with additional work in the future). Methods of reducing statistical uncertainty by improving spectrometer efficiency are being investigated and improved flux from the NIST EBIT has been

achieved in subsequent studies. Reference wavelength uncertainty is limited at present to Bearden's values but some of

the metals we employ for calibration have been measured

with improved precision recently [35]. Future measurements

of the characteristic wavelengths on, for example, a vacuum

double-crystal spectrometer are very likely to reduce uncertainty.

V. RESULTS AND DISCUSSION

Our results are the first absolute measurements of all the resonance lines in heliumlike vanadium using an EBIT. We

do not rely on a single calibration energy, but require a series of calibration lines to determine the dispersion function of

the spectrometer. These measurements represent a 27-40

ppm determination of the heliumlike resonance lines in va- nadium. Results are summarized in Table n and the notation of Gabriel [14] for each transition is indicated.

A. Comparison to Theory

Our results for all of the heliumlike vanadium resonance

lines are compared to the theory of Drake [5] in Table H. The

values for the w{ls2s ^Pi-*ls^) and z(ls2s ^Si-*\s'^)

transitions lie just below theory but well within experimental

uncertainty. The x(ls2p ^P2—^ls^) transition is the least intense of the heliumlike resonance lines, so the statistical uncertainty is larger. However, the result for the x line is less than 2(r from theory [5] which is within reasonable statistical variation. The comparison of the y (ls2p ^Pi—+1j^) and

ls2p ^Pq—* Is^ blend to theory is discussed in detail in Sec. VF.

The uncertainty of theoretical calculations including the estimation of missing or uncalculated terms has been receiv- ing increasing scrutiny as techniques have advanced. One of

the most recent two-electron Lamb shift calculations by Per- sson et al. [10] estimates missing correlation effects in QED

contributions at 0. 1 eV for all elements or 20 ppm of transi- tion energies in medium Z ions. In earlier work, Drake [5]

042501-7

MST-196
C. T. CHANTLER et al.

TABLE m. Selected w{\s2p 'P,—

PHYSICAL REVIEW A 62 042501

1 s^) transition energies (eV) for vanadium and surrounding medium Z ions.

Theoretical transition energies

ATh*

(ppm)

QED''

(eV)

2e QED" z Experiment Ref. Unified [5] AO [6] CI [7] MCDF [9] (eV)

18 3139.553(38) [47] 3139.577 3139.582 3139.617 3139.65 23 1.055 0.09 22 4749.74(17) [11] 4749.63 4749.64 4749.71 17 23 5205.10(14) 5205.15 5205.16 2.474 0.16 24 5682.32(40) [11] 5682.05 5682.06 5682.15 18 26 6700.08(24) [27] 6700.40 6700.43 6700.54 6700.60 30

32 10 280.70(22) [13] 10 280.14 10280.19 10280.39 24 7.674 0.40

'ATh: Maximum discrepancy between theories. •QED: QED contribution to the ground state (1j^ 'Jq) [5]. '^e QED: Two-electron QED contribution extrapolated (see text) [10]. *rhis work.

claimed uncertainty for Z= 23 was less tlian 0.(X)5 eV or 1 ppm of heliumlike resonance lines due to uncalculated

higher-order terms. Some of the latest theoretical calculations for the w transition in medium Z ions are summarized

in Table HI. The discrepancy between theories is indicated and the maximum discrepancy ranges from 23 to 30 ppm for Z= 18-26 and differences are consistent. Our measurements

are at this level of uncertainty.

Table n also shows the QED contributions to heliumlike resonance lines in vanadium as determined by Drake. The

QED contributions are also expressed as a proportion of the relevant transition in ppm. The level at which our measurements test these contributions is between 5.7% and 8%. The

theoretical QED contributions include mass polarization and

nuclear size effects but these contribute less than 1% to the

total. If the QED contributions to the 21 states are assumed to be correct, then the 1 jQED contribution is measured to 6%.

Our result for the w line in vanadium is within our experi- mental uncertainty of the theory of Drake [5] and Plante

et al. [6]. We therefore find no evidence of the earlier re- ported trend that experimental values are greater than theory

[11]. Figure 6 shows our result for the w transition compared

to theory [5] and other measurements for heliumlike vana- dium and surrounding medium Z heliumlike ions. Our measurements are compared with other experiments in the context of medium Z x-ray measurements in the following

discussion.

B. Two-electron Lamb shifts The two-electron Lamb shift has been a topic of considerable interest recently following significant variation in the recent calculations of two-electron contributions to the ground state of medium and high Z heliumlike ions [10,6,5]. The "two-electron Lamb shift" can be defined as the differ- ence between the hydrogenic \S\a Lamb shift and the QED

contributions to the heliumlike ground state {\s^ ^Sq). The

two-electron Lamb shift relates to the two-electron radiative QED Feynman diagrams (vacuum polarization and self- energy) due to interactions between the electrons [10]. Nonradiative two-electron QED (ladder and crossed-photon dia- grams) are negligible for Z^32. We have calculated the twoelectron Lamb shift by subtracting the results of Johnson and

Soff [37] for the hydrogenic \sy2 Lamb shift firora the QED

contributions to the heliumlike ground state from Drake [5]. For Z=32 (germanium) we obtain 0.31 eV which can be

directly compared to the 0.4 eV result of Persson et al. [10]. For vanadiimi, the two-electron Lamb shift calculated in this manner is 0.15 eV or 29 ppm of the 1^25 '/*,—> 1j^ transi- tion energy. Two-electron QED calculations for Z<32 are not available ft^om Persson et al, so no direct comparison

can be made with Drake's calculation of vanadium.

As a simple alternative calculation, we have estimated the two-electron Lamb shift for heliumhke ions of Z less than 32

by extrapolating the results of Persson et al. for Z= 32-92

using a power-law fit (2e QED=aZ''). The Z power dependence is interestingly found to be fe«2.5. Derived results for Z= 18 and 23 are tabulated with Persson 's result for Z=32

in Table III. Theory might expect a power-law dependence

of /7 = 3 as the two-electron contributions scale as Z^. The

result {b'^l.S) from the power-law fit to the sum of all computed contributions is in reasonable agreement with the ex- pected theoretical value. For vanadium the two-electron Lamb shift is 0.16 eV or 31 ppm, shghtly larger than the

UJ ^ 50

lU

'a. X«

UJ

-SO

100

<» (I

18 20 22 24 26 z

28 30 32

FIG. 6. Measurements of the w{\slp 'fj—^Ij^) transition in medium Z ions: comparison to theoretical work of Drake [5]. £(exp): experimental u-ansition energy; ^(theory): theoretical transition energy. 0, this work; O, Beiersdorfer et al. (1989), Ref.

[11], Z=32, MacLaren et al. (1992), Ref. [13]; •, other sources

(see Table III references).

042501-8

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ABSOLUTE MEASUREMENT OF THE RESONANCE LINES . . . PHYSICAL REVIEW A 62 042501

TABLE IV. Experimentally determined energies (eV) of heliumlike vanadium 1^2/—

»

Is^ resonance lines. The source of highly charged

ions is indicated.

Key Transition PLT tokamak

Ref. [11]

LIVS plasma

Ref. [39] Energy

LLNL EBIT [12] E^-E" dE^

TFR tokamak [38] E„—E dE

w ls2p 'P,->1j^ 5205.27(21) 5205.58(55) 5205.33' X ls2p ^Pj-^ls^ 5188.86(22) 16.47 0.04 17.0 0.56 y ls2p ^Pi^\s^ 5180.30(22) 25.03 0.19 25.7 0.82 ^0 \s2p ^Pq^Is'^ 5180.30(22) 25.03 -1.28 25.7 -0.65

z \s2s ^5,-1^2 5153.90(30) 51.43 0.16 51.9 0.66

'^Ey^ — E: difference between energy of w transition and energy of relevant transition. ''«/£=(£„-£),henry ~ ( ~ ^) expl • ^Reference line set to semiempirical value of 5205.33 eV.

experimental uncertainty of 0.14 eV of our measurements. The agreement between this result obtained from extrapolation of the results of Persson et al. and the value obtained by

direct calculation from Johnson and Soff and Drake is excellent (0.01 eV difference). For Z=32, where the comparison

can be made directly without extrapolation, the difference is 0.09 eV, remarkably consistent with the uncertainty estimate

(0.1 eV) of Persson et al. for contributions fi^om missing correlation effects in QED. As was the case with the experiment

of Marrs et al. [36], a significant improvement in experimental precision would provide a critical test of two-electron QED.

In terms of basic physical effects included, the calculations of Drake and of Persson et al. are equivalent up to all terms of order (assuming that the many-body perturbation

theory expansion has converged sufficiently well), and also terms of order a'*Z^ and a'*Z^. Any difference between the two calculations should therefore scale as a^Z'*, at least through the intermediate range of Z. Persson et al. states that the missing correlation effects in their two-electron QED calculations is estimated to be of the order of 0.1 eV for all elements. Formally this should only be

applied to the range of elements 32<Z<92. The associated

uncertainties for Z<32 are unknown, but could be expected

to increase in this regime. In the calculations of Drake, the uncertainty due to relativisdc correlation effects in QED

scales as a;'*Z'*. The sources of the uncertainty are quite dif- ferent in the calculations of Drake and of Persson et al. The

lowest-order Lamb shift is of order ac^Z*, and so the leading two-electron correction is of order smaller by a

factor of 1/Z. Higher-order correlation effects contribute fur- ther terms of order . In the calculations of Per- sson et al., it is the uncertainty in these correlation corrections to the lowest-order (in a) Lamb shift that accounts for

their estimate of 0.1 eV for all elements. In contrast, the calculation of Drake accurately sums this entire series of

terms, but there are relativistic correlation corrections to the QED shift of order a'*Z'*,ar^Z^,..., not included in this cal- culation. This accounts for Drake's error estimate of

1.2(Z/10)'* cm"' [5], scaling as Z*. It has been suggested

that Drake's error estimate should be multiplied by 8 for the ground state, to take into account a l/n^ scaling.

In summary, the calculation of Drake accurately treats the 1/Z expansion of the nonrelativistic two-electron QED shift, but not the aZ expansion of relativistic and correlation corrections. The calculation of Persson et al. is improved for the aZ expansion, but not the 1/Z expansion of the nonrelativistic correlation terms. For larger Z the calculations of Pers- son et al. may be preferred, and for smaller Z the calculations of Drake may be preferable.

C. Excited-state QED

Measuring the hehumlike n = 2 to /i = 1 transitions can be

sensitive to the excited-state QED contributions if the preci- sion is sufficient. The QED contribution to the ls2s ^5] is 0.333 eV from Drake [5] or 65 ppm of the transition energy. Our measurement of the 2 (1^25 ^5]—»1j^) transition is sensitive to the 1^2^ ^5] QED contribution at the 40% level. Most previous experiments in the medium Z region have

only measured the w transition, while others have only measured the w and close-lying x and y lines. Ours is one of the most precise measurements of the z transition in medium Z

ions.Of course, the result is a direct measure of the ls2s ^5] — 1 QED contribution. Most such measurements are in- sensitive to the excited-state {2s or 2p) QED contribution. However, our measurement is indeed sensitive to the excited-state QED contribution at the 40% level. Our result

is comparable to the 51 -ppm measurement for Z=32 [13]

which tests the ls2s ^Sy QED contribution at the 48% level.

D. Previous heliumllke vanadium observations

Four other observations of heliumUke vanadium spectra have been reported (see Table FV). Two of these [38,12] were relative measurements to the w line and, as such, cannot be compared to absolute measurements of the w line. These

relative observations include a study of heliumlike vanadium

at an EBIT [12] and are the only prior observations of the heliumlike resonance lines other than the w line (the x, y, and

z lines).

042501-9

NIST-198
C. T. CHANTLER et al. PHYSICAL REVIEW A 62 042501 Two measurements [39,11] were conducted at a lowinductance vacuum spark plasma and a tokamak plasma, re- spectively. In both cases only the w line was reported. The

first study by Aglitsky et al. [39] in 1988 was part of a broad

survey of resonance transitions in heliumlike ions (Z = 16-39). A double Johann spectrograph was used to record

spectra. Characteristic K lines were used for calibration based upon reference wavelengths of Cauclois [40]. The precision reported for the w transition in vanadium was 105 ppm.The second study by Beiersdorfer and co-workers [11] was part of a survey of medium Z elements in the heliumlike

isoelectronic sequence in tokamak plasmas at the Princeton Large Torus (PLT). The uncertainty reported for vanadium

was 40 ppm. Close-lying Lyman series lines were used for

calibration. Shorter-wavelength calibration lines and heliumhke resonance lines were observed in second-order diffraction. As discussed in Sec. HI, significant systematic shifts can occur in measurements that utilize several diffraction orders unless properly accounted for. The theoretical values

of Mohr [41] were used in that analysis for the hydrogenic

transitions and therefore QED tiieory was assumed to be correct [11]. A review of hydrogenic medium Z measurements

demonstrates that the experimental evidence is incomplete in

this region of Z. While several high precision absolute measurements of hydrogenic transitions have been reported for argon (Z = 18, e.g., Marmar et al. [42] and Beyer et al. [43]) and iron (Z=26, Chantler [27]), the elements with Z=19 to Z=25

have not been measured precisely and absolutely and in some cases have not been measured at all. For example, the Lyman a transitions in vanadium have only been investi- gated recentiy [44]. The practice of using internal reference hnes based upon QED theory, while having spectroscopic

advantages, cannot reliably be used to infer conclusions

about QED contributions in two-electron systems.

Dielectronic satellite lines are often associated with heli- umlike transitions particularly in plasmas where a wide

range of electron energies are present. Satellite contamination in hot plasmas is often the major limiting uncertainty on

the precision of energy measurements [45]. Bulk motions

within the plasma can also be very fast and vary between

charge states [46]. This may be contrasted to the situation in an EBIT where very low satellite contamination can be

achieved and Doppler shifts are negligible.

The measurements of the w resonance line in heliumlike vanadium [39,11] are shown in Table IV and plotted in Fig. 6 where they can be compared to current theory and other experiments including this work. The results of Aglitsky

et al. [39] lie higher in energy than theory but within experi- mental error. The measurements of Beiersdorfer et al. [11]

across the range of Z observed were consistenUy higher than

theoretical energies but most, including vanadium, were in- dividually within experimental errors of theory. It was

claimed that the comparison shows that there are systematic

differences between data and predictions, which indicate a need to include additional corrections in the calculations

[11]. The implication of this series of measurements was that there was a systematic offset between theory and experiment.

and that the uncertainty in the offset was smaller by roughly

a factor of \[n than that of the individual measurements

[more precisely, the mean offset of the six measurements is 34(15) ppm]. Our result for the w line in vanadium indicates there is no systematic offset from theory; our value for the

offset, -10(27) ppm, differs fi-om that inferred from the measurements of Beiersdorfer et al. by 44(27) ppm.

E. Other absolute medium Z measurements A survey of experimental measurements across medium Z

heliumlike resonance lines is shown in Table in and Fig. 6 and compared with recent theory. The most precise absolute measurement of medium Z heliumlike ions is attributed to Deslattes and co-workers [47] with a 12 ppm measurement

of the w transition in argon. The x and y transitions were also measured in this work to similar accuracy, but the z transi- tion was not observed. Our methodology is similar to that of

Deslattes et al. in the use of an external x-ray calibration standard lying close to the wavelength of interest. The recoil- ion experimental method used therein also eliminates the need for Doppler corrections and uncertainties in that work

as opposed to other experiments but is comparable to our work on the EBIT. A novel experimental arrangement pro- duced Ar'*'*' by collisions of MeV U^^"*" ions with an argongas target [43]. A Johann curved crystal spectrometer was employed with a position-sensitive proportional counter of

the backgammon type (both the spectrometer and detector

are similar to those used in this research). This spectroscopy

method may have systematic errors associated with the ge- ometry and detector type and position as discussed above

(Sec. HI). Three-electron satellite contamination in these observations was considerably reduced by adjusting the gas tar- get pressure. The contribution of the residual satellite contamination to the overall precision of wavelength

determination was estimated to be 3 ppm. However, as with

all spectra with strong satellites, the limiting uncertainty is difficult to quantify. Argon is at the lower end of the medium

Z elements where QED effects are smaller relative to the

transition energies. This very precise measurement of argon

[47] is within experimental uncertainty of the unified [5] and

all orders (AO) [6] calculations, but excludes the configuration-interaction (CI) [7] and multiconfiguration

Dirac-Fock (MCDF) [9] calculations, as shown in Table HI.

F. Intensities of \s2p ^Pi—*\s^ and \s2p ^Pa—*\s^ transitions The probability of the \s2p ^Pq—^Xs^ transition reveals information about the dominant processes in an electron- beam ion trap. In helium, the \s2p ^Pq level can only decay

to the ground state by two or three photon processes. El Ml

or 3£1, respectively, but the probabihty is very low for all Z

[48]. Vanadium has a finite nuclear moment (7/2^ [49]), so

the transition from the ls2p ^Pq state to the ground state (1^2 '5o) can also occur via the hyperfine interaction [50]. Mohr has calculated the ls2p 'Pq—>1j^ transition rate to be

10.5X 10^ s"' in vanadium [50].

042501-10

]SfIST-199
ABSOLUTE MEASUREMENT OF THE RESONANCE LINES . . . PHYSICAL REVIEW A 62 042501

TABLE V. Experimental determinations of the relative intensity of the strong y{ls2p ^P,—»1j^) component compared to the total intensity of the blend (y and \s2p 'Po— \s^), using theoretical energies of Drake, Ref. [5]. y% of total blend intensity TFR tokamak [38] LLNLEBIT[I2] NisT EBrr

Normalized to experimental w line 99% ± 15% 94% ±12%

Normalized to theoretical w line 44%'' 87% ±15% 97% ±12%

"NIST EBIT: This work.

"Tlelative measurement, no errors were published.

For vanadium, earlier theoretical work has claimed a ratio of 4:1 for the intensity of the y (Islp ^Pi—*ls^) transition

relative to the ls2p^PQ—^ls^ transition based upon

electron-impact excitation cross-section calculations [12]. The calculation of Mohr combined with the y transition rate (1.638X 10'-' s~') of Drake [51], and assuming equal initial- state populations, results in a ratio of 1560:1. The competing

ls2p ^Po-*ls2s rate is only 2% of the Is2p^Pq — 1j^ transition rate so it does not influence the result. The hyperfine-induced decay of the ^Pq level in vana- dium cannot be resolved from \he y CPi) transition in any

available experimental method [12,38]. The relative intensities of the ^Pq and y transitions therefore shift the observed

centroid. The energy separation between ^Pq and y lines is 1.47 eV in vanadium which is equivalent to 290 ppm of the energy of these transitions. We determine the relative intensity of the y component

compared to the blend to be 94 ± 12 % (and consequently the ^Pq component is 6± 12%) based on the theoretical energies of Drake [5] and the experimentally determined centroid

position of the blend. The width of each component is also

consistent: w = 3.82(29) eV, x = 4.3(9)eV, y-^Po = 4.1(7) eV, and z = 3.27(38) eV. There is no direct evi- dence for individual high-n or recombination satelhte contamination. The width of the y — ^Po blend is within 1 cr of

that of the other resonances and narrower than the (weak) x

line. This is consistent with our findings for the ratio of intensities based upon the centroid position and suggests an

almost complete absence of the ^Pq transition. The results for position and width of the y~^Po blend from a second

study of vanadium at the NIST EBIT agree with those of the

first study, although the statistical uncertainties are larger. Our absolute experimental determination of the relative intensity of the y component is compared to relative measurements [12,38] of the heliumlike resonance lines in vana- dium in Table V. Achard et al. [38] results are substantially

different from recent theory (see dE values in Table FV) and

were conducted at a tokamak plasma where satellite contamination was intense. A comparison between EBIT experiments and their re- spective measurements is more revealing. The results from

the Livermore EBIT [12] for the relative intensity of the y component compared to the total blend intensity are pre- sented in Table V. We have presented the relative intensity,

firstly, based upon the absolute experimental results compared with theory and secondly based upon relative differ- ences from the w line and compared with differences as cal- culated in theory (results are normalized to the theoretical

position of the w line). Relative intensities of the y component derived from the EBIT measurements range from 87%

to 99% (with 15% uncertainty) for the Livermore EBIT and

from 94% to 97% for the NIST EBIT, depending upon nor- malization. Drake [51] has suggested that differences between EBIT

results for the ratio may be due to pressure differences be- cause low-pressures and steady-state conditions could lead to a build up of the ^Pq population relative to the ^Pj state. The experimental evidence from both EBIT studies supports an almost complete absence of the ^Pq transition and the

results from the NIST EBIT are within experimental uncertainty of Drake's prediction (1560:1) [51] but exclude the

earlier theoretical calculation of 4:1 [12]. The differences between EBIT results are within experimental errors. We

conclude that the differences in EBIT experimental conditions have had a negligible effect on wavelength determination. This supports the value of absolute measurements from

an EBIT source for precision QED investigations.

VI. CONCLUSIONS

The use of weU-determined reference wavelengths has been crucial to this research, and has led to the first absolute determination of heliumlike resonance lines in vanadium us- ing an EBIT. The uncertainties of the w {\slp 'Pj—Ij^)

and z {\s2s ^5]—Ij^) lines are 27 and 28 ppm, respectively, and the measurements of all the transitions are a 5.7- 8% test of QED contributions. The results are in accord

with, but do not discriminate between, current theories of Drake [5] and Plante et al. [6] which differ by 0.01 eV. The

measurement of the z (1^2^ ^S\—*\s^) transition is sensitive to the 1 J 25 '5 1 QED contribution at the 40% level. The absolute calibration is achieved by using a spread of

characteristic wavelengths {Ka and KfS) to map out and rig- orously determine the dispersion function of the spectrometer. We have developed the dynamical diffraction theory necessary to evaluate systematic shifts at the precision level required to test QED. Systematic shifts associated with the shape and location of the detector used in the Johann geometry have been evaluated and reduced to a level that is comparable to the uncertainty of the reference wavelengths used

for calibration. Our measurement of the w line in vanadium is not in accord with previous measurements of medium-Z heliumlike

ions (Beiersdorfer et al. [11]) conducted at a tokamak plasma

042501-11

NIST-200
C. T. CHANTLER et al. PHYSICAL REVrew A 62 042501

and the EBIT at LLNL [13] which reported a trend of ex- perimental energies above theory. The location and width of

the y — ^Pn blend implies a relative intensity of the y component of 94% compared to the total blend intensity. This is in accord with recent predictions based upon transition rates but in disagreement wiUi an earlier claim. The unique spectroscopic advantages of the EBIT have

been crucial in the success of these QED investigations, al- lowing Doppler-free and low-satellite contamination spectra

to be measured. The benefits of absolute calibration combined with rigorous diffraction theory in precision tests of QED have been demonstrated.

ACKNOWLEDGMENTS

We would like to acknowledge the assistance of A.

Henins, R. D. Deslattes, and L. Ratliff during the experi- ments at NIST. We thank G. W. Drake for helpful discus- sions and comments. This research was supported by the Australian Research Council.

G. W. Series, The Spectrum of Atomic Hydrogen: Advances

(Utopia, Singapore, 1988).

P. J. Mohr, in Atomic, Molecular, & Optical Physics Handbook (AIP. Woodbury, NY, 1996), Chap. 28. R. Mamis Physics of Highly-Ionized Atoms (Plenum, New

Yoric 1989). G. W. F. Drake, Phys. Rev. A 19, 1387 (1979). G. W. Drake, Can. J. Phys. 66, 586 (1988). D. R. Plante, W. R. Johnson, and J. Sapirstein, Phys. Rev. A

49. 3519 (1994). K. T. Cheng, M. H. Chen, W. R. Johnson, and J. Sapirstein, Phys. Rev. A 50, 247 (1994). M. H. Chen, K. T. Cheng, and W. R. Johnson, Phys. Rev. A

47, 3692 (1993).

P. Indelicate, F. Parente, and R. Marrus, Phys. Rev. A 40, 3505 (1989); P. Indelicato, Nucl. Instnim. Methods Phys. Res. B 31, 14 (1988); P. Indelicato, O. Gorceix, and J. P. Desclaux,

J. Phys. B 20, 651 (1987). H. Persson. S. Salomonson, P. Sunnergren, and I. Lindgren,

Phys. Rev. Lett. 76, 204 (1996).

P. Beiersdorfer, M. Bitter, S. von Goeler, and K. W. Hill, Phys. Rev. A 40, 150 (1989).

P. Beiersdorfer, M. H. Chen, R. E. Marrs, M. B. Schneider, and R. S. Walling, Phys. Rev. A 44, 396 (1991).

S. MacLaren, P. Beiersdorfer, D. A. Vogel, D. Knapp, R. E. Marrs. K. Wong, and R. Zasadzinki, Phys. Rev. A 45, 329

(1992). A. H. Gabriel, Mon. Not. R. Astron. Soc. 160, 99 (1972). E. Takacs, E. S. Meyer, J. D. Gillaspy, J. R. Roberts, C. T. Chantler, L. T. Hudson, R. D. Deslattes, C. M. Brown, J. M.

Laming, J. Dubau, and M. K. Inal, Phys. Rev. A 54, 1342

(1996).

J. D. Gillaspy, et al., Phys. Scr. T59, 392 (1995).

J. D. Gillaspy, Phys. Scr. T65, 168 (1996). A. Henins, Rev. Sci. Instrum. 58, 1173 (1987). B. P. Duval, J. Barth, R. D. Deslattes, A. Henins, and G. G.

Luther, Nucl. Instnim. Methods Phys. Res. A 222, 274 (1984). D. J. Paterson, C. T. Chander, C. Tran, L. T. Hudson, F. G.

Serpa, and R. D. Deslattes, Phys. Scr. T73. 400 (1997). C. T. Chander, D. J. Paterson, L. T. Hudson, F. G. Serpa, J. D.

Gillaspy, and E. Takacs. Phys. Scr. T80, 440 (1999).

J. A. Bearden, Rev. Mod. Phys. 39. 78 (1967). C. T. Chantler and R. D. Deslattes, Rev. Sci. Instrum. 66, 5123

(1995).

[24] J. P. Briand, M. Tavemier. R. Marrus, and J. P. Desclaux,

Phys. Rev. A 29, 3143 (1984).

[25] H. F. Beyer and D. Liesen, Nucl. Instrum. Methods Phys. Res. A 272, 895 (1988). [26] P. Beiersdorfer. R. E. Marrs, J. R. Henderson, D. A. Knapp, M. A. Levine, D. B. Piatt, M. B. Schneider, D. A. Vogel, and

K. L. Wong. Rev. Sci. Instrum. 61, 2338 (1990).

[27] C. T. Chander, D. Phil, thesis. Oxford University, 1990 (un- published).

[28] C. T. Chantler, J. Appl. Crystallogr. 25, 674 (1992); 25. 698

(1992). [29] M. Deutsch and M. Hart, Phys. Rev. B 26. 5558 (1982).

[30] J. Hartwig, G. Holzer, J. Wolf, and E. Forster. J. Appl. Crystallogr. 26, 539 (1993).

[31] J. Schweppe, R. D. Deslattes, T. Mooney, and C. J. Powell, J. Electron Spectrosc. Relat. Phenom. 67, 463 (1994).

[32] R. D. Deslattes, E. G. Kessler. P. Indelicato. and E. Lindrodi.

International Tables for Crystallography (Kluwer Academic

Press, Stockholm, 1999), Vol. C, Sec. 4.2.2. [33] R. D. Deslattes, Phys. Scr. T46, (1993).

[34] R. D. Deslattes, E. G. Kessler, W. C. Sauder, and A. Henins, Ann. Phys. (N.Y.) 129, 378 (1980).

[35] G. Holzer, M. Fritsch, M. Deutsch, J. Hartwig, and E. Forster, Phys. Rev. A 56, 4554 (1997).

[36] R. E. Marrs, S. R. Elliot, and T. Stohlker, Phys. Rev. A 52, 3577 (1995).

[37] W. R. Johnson and G. Soff, At. Data Nucl. Data Tables 33, 405 (1985); P. J. Mohr, Phys. Rev. A 46, 4421 (1992).

[38] M. H. Achard et al, M. Comille, J. Dubau, and M. Loulergue

TFR Group, Phys. Rev. A 32, 3000 (1985).

[39] E. V. Aglitsky, P. S. Antsiferov, S. L. Mandelstam, A. M.

Panin, U. I. Safronova, S. A. Ulidn, and L. A. Vainshtein, Phys. Scr. 38, 136 (1988).

[40] Y. Cauclois and C. Senemaud, Wavelength of X-ray Emission

Lines and Absorption Edges (Pergamon, London, 1988).

[41] P. J. Mohr, At. Data Nucl. Data Tables 29, 454 (1983).

[42] E. S. Marmar, J. E. Rice, E. Kallne, J. Kallne, and R. E. LaVilla. Phys. Rev. A 33. 774 (1986).

[43] H. F. Beyer, R. D. Deslattes. F. Folkmann, and R. E. La Villa,

J. Phys. B 18. 207 (1985).

[44] D. J. Paterson, Ph.D. thesis. University of Melbourne. 1999

(unpublished).

[45] E. Trabert, The Spectrum ofAtomic Hydrogen: Advances (Utopia, Singapore, 1988). Chap. 6.

042501-12

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ABSOLUTE MEASUREMENT OF THE RESONANCE LINES . .

.

[46] E. Kiillne, J. Kallne, P. Richard, and M. Stockli, J. Phys. B 17, LI 15 (1984).

[47] R. D. Deslattes, H. F. Beyer, and F. Folkmann, J. Phys. B 17, L687 (1984).

[48] H. F. Beyer, H. J. Kluge, and V. P. Shevelko, X-Ray Radiation of Highly Charged Ions (Springer, Berlin, 1997).

PHYSICAL REVIEW A 62 042501

[49] R. C. Weast, Handbook of Physics and Chemistry (CRC Press, Boca Raton, Fl, 1985).

[50] P. J. Mohr, in Beam-Foil Spectroscopy, edited by I. A. Sellin and D. J. Pegg (Plenum, New York. 1976). [51] G. W. Drake (personal communication).

042501-13

NIST-202
ELSEVIER Nuclear Instruments and Methods in Physics Research A 444 (2000) 156-160

NUCLEAR

INSTRUMENTS

& METHODS

IN PHYSICS

RESEARCH

Section A

www.elsevier.nl/locate/nima

Laboratory astrophysics and microanalysis with

NTD-germanium-based X-ray microcalorimeters

E. Silver"'*, H. Schnopper% S. Bandle^^ S. Murray^ N. Madden^ D. Landis^

J. Beeman^ E. Haller^ M. Barbera^ G. Tucker'*, J. Gillaspy^ E. Takacs^ J. Porto'

^Harvard-Smithsonian Centre For Astrophysics, 60 Garden Street. Cambridge. MA 02138, USA

^Lawrence Berkeley National Laboratory. I Cyclotron Road, Berkeley, CA 94720, USA

'Brown University, Providence, Box 1843, RI 02912, USA

"^Osservatorio Astronomico di Palermo, Palermo, Italy 'National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD, USA

Abstract

With the ability to create cosmic plasma conditions in the laboratory it is possible to investigate the dependencies of

key diagnostic X-ray lines on density, temperature, and excitation conditions that exist in astrophysical sources with

X-ray optics and a high-resolution X-ray microcalorimeter. The same instrumentation can be coupled to scanning

electron microscopes or X-ray fluorescence probes to analyze the elemental and chemical composition of electronic,

biological, geological and particulate materials. We describe how our microcalorimeter and X-ray optics provide

significantly improved capabilities for laboratory astrophysics and microanalysis. © 2000 Elsevier Science B.V. All

rights reserved.

1. Introduction

High resolving power, quantum efficiency

approaching 100% and a bandwidth that spans the

X-ray energies from 0.1-10 keV are properties of

the microcalorimeter that make it the ideal spectrometer for laboratory astrophysics and microanalysis studies. An X-ray optic [1,2] was used to couple an X-ray microcalorimeter [3,4] to an Electron Beam Ion Trap (EBIT) at NIST [5]. Broadband X-ray spectra from astrophysically relevent

ionic species were obtained with an energy resolution approaching that of a Bragg crystal. The

* Corresponding author.

unique plasma conditions in the EBIT allow

these microcalorimeter experiments to demonstrate

the limitless potential to acquire a database of

atomic structure and cross sections in the isolatedion limit (atomic energy levels, oscillator strengths

and transition cross-sections). The microcalorimeter can obtain significant quantities of as- trophysically relevent spectral data in relatively

short times. Examples of newly obtained spectral

data are presented in Section 3. The same X-ray optic has been incorporated

into a microcalorimeter-based X-ray microanalysis system that can be attached to a scanning

electron microscope (SEM) or an X-ray fluores- cence probe. The microcalorimeter allows X-ray

microanalysis at low SEM beam energies. This

0168-9002/00/$ -see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: 50168-9002(99)01 349-2

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E. Silver et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 156-160 157

improves significantly its spatial resolution, sensitivity to minor elemental constitutents, and ability to unambiguously identify X-ray signatures from

a mixture of light and heavy elements. Microcalorimeters have made it possible to distinguish

K lines of low Z elements from L lines of transition

elements or the M lines of very high Z materials.

Representative measurements are shown in Section

4 and those from other investigators may be found

in the literature [6,7].

2, The microcalorimeter and X-ray optic

A 2 X 2 array of NTD germanium-based microcalorimeters [8] views the laboratory plasma or the

X-ray microanalysis sample through an X-ray optic

developed at SAO. The X-ray optic [1,2] is a cyhndrical spiral that focuses X-rays in a point-to-point

geometry. The design can satisfy an extremely wide

variety of focal lengths and energy ranges.

For SEM-based microanalysis,the eventual goal

is to replace the hquid hehum in the microcalorimeter cryostat with a mechanical cryocooler.

Estimates of the vibrational and acoustical effects of a cryocooler operating in close proximity to an SEM show a need to decouple the microcalorimeter-cryocooler combination from the

SEM. An X-ray optic was designed to focus X-rays on a detector located 1.5 m from the SEM. The

solid angle for X-ray collection is equivalent to that

expected without the X-ray optic when the same

detector is located 3-5 cm away from the sample.

For laboratory plasma experiments on the EBIT,

it is extremely difficult to locate the microcalorimeter in close proximity to the source vol- ume. Access to a viewing port at this distance is restricted by magnet coils, cryogenic lines and

other instruments. One meter is the nominal distance of closet approach for a microcalorimeter

cryostat. This hmits the sohd angle to ~2x

10~^sr and makes the measurements extremely

time consuming.

For our measurements at the NIST EBIT, the

optic was equidistant (762 mm) from the source and

detector. Fig. 1 shows the performance of the optic

for this geometry as a function of X-ray energy

when viewed by a 1 mm^ detector 1.5 m from the

• •

\m 2

1 mm detector 762 mm focal length

1

EHIclenc

nisslon

1

P microcalorimeter' " • ~ " (0.32 (im polyimide +

Win* 0.44 jun aluminum) +

0.01 0 EBIT

° (0.5 nm polyimide)

0 0.5 1 0 1.5 2.0 2.5 3 Energy (eV) 01 23456789 10 Energy (keV)

Fig. 1. Relative counting rate with and without the X-ray optic

for a 1 mm^ detector 1524 mm from the X-ray source. The inset shows the transmission efficiency of the detector and EBIT

windows.

source. The data (dark circles) are the ratios of the

counting rates obtained with the X-ray optic to

the rates measured without the X-ray optic. The

solid curve is a spline fit to the data. The figure inset

shows the calculated transmission efficiency of the

detector and EBIT windows. The relative counting

rate should be interpreted differently depending

upon whether the apphcation is laboratory astro- physics or microanalysis. In the plasma experiments where the distance of closest approach is limited, the X-ray optic can increase the X-ray

intensity by as much as 250. If decouphng the

microcalorimeter and cryocooler from the SEM is the objective, then the optic provides the same sohd

angle with the microcalorimeter 15 times further

away from the SEM than it would be for a closely

coupled detector.

3. Laboratory astrophysics

The EBIT is a device to produce and study hot

laboratory plasmas in an experimentally well-controlled environment. Details of its operation can be

found in the literature [5].

Previous EBIT-based laboratory astrophysics

programs made use of crystal spectrometers. These

SECTION IV.

NIST-204
158 E. Silver et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 156-160

experiments are complex and require, for example,

a large number of runs to establish the parametric

dependences in collisional excitation dominated

and recombination dominated regimes. The analysis of the data requires high-resolution spectra of

the He and H-like lines and the recombination

continua. An additional complication is the polarization dependence induced by the strong anisotropy of the EBIT electron beam. The effect of

polarization needs to be accounted for in the evaluation of intensities for hnes excited by dielectronic

electron capture and by electron impact. Precise

values for crystal reflectivities as a function of polarization are required to correct the raw data.

The microcalorimeter is the ideal spectrometer

for the EBIT-type plasma. The efficiency of the

microcalorimeter shortens the measurement time

by several orders of magnitude when compared

with that needed by crystal spectrometers. The

microcalorimeter obtains data simultaneously over

the entire energy range of interest for all of the

astrophysically relevant spectral components. In

addition, the microcalorimeter is insensitive to the

polarization of the EBIT plasma X-ray emission.

OVIl Beam Current = 35.9 ma

400 450 500 550 600 650 700 750 800

Energy (eV)

Fig. 3. Microcalorimeter X-ray spectra of ionized nitrogen and oxygen from the NIST EBIT. The collection time was ~ 20 mins.

Fig. 2 is the first measurement of helium-like

FeXXV with'a microcalorimeter and was obtained

without the aid of an X-ray optic. About three

hours were needed to acquire the ~ 100 counts

shown. Figs. 3 and 4 are representative of the significant results that can be obtained with much

greater efficiency using the X-ray optic. For

c

3

Ou

6500 6550 6600 6650 6700 Energy (eV)6750 6800

Fig. 2. The first laboratory plasma spectrum of helium-like FeXXV made with a microcalorimeter on the LLNL EBIT [4].

NIST-205
E. Silver et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 156-160 159

400

300

200

L emission lines

200 300 400 500 600 700 800 900

80

F IR ^

1500 2000

Energy (eV)

3500

Fig. 4. MicrocaloriiTieter X-ray spectra of ionized argon from

the NIST EBIT. The collection time was ~ 20 mins.

80

60

40

20 H

0 -'^

5 key electron

excitation OKa

TlLccP NKa.TLTi./

CKa

0.2 0.3 0.4 0.5 0.6 70

60

50

40

30

20

10

SIKa WMa

1 5 keV electron

excitation

WMp,SiK|3

1700 1750 1800 1850 1900

Energy (eV)

Fig. 6. Top: X-ray spectrum obtained from titanium target. Bottom: Si K and W M lines from SEM microanalysis of

a W-coated silicon wafer. Energy is 6 eV without accounting for the natural line widths.

example, it took about 20 mins to acquire the ~ 12 000 counts from argon X-rays shown in

Fig. 4.

4. SEM-based microanalysis

180

160

140 ^ 120

100

80 ^ 60

40

20

0

=^ CO.

0

Energy (keV)

Fig. 5. Broad band spectrum from a multielement sample.

A spectrum from a multielement sample in an SEM demonstrating the broad energy band of the

microcalorimeter-optic combination is shown in

Fig. 5. The counting rate was 25 Hz. An expanded

view of the low energy part of the spectrum is shown in the inset. Fig. 6 shows a spectrum obtained when sihcon (Si) coated with > 3000 A of

tungsten (W) is irradiated by electrons in an SEM.

The W Ma emission is shown resolved from Si Ka.

Fig. 6 also shows the O K hne well resolved from

the Ti L emission for a sample of TiO.

References

[1] E. Silver et al., US Patent application. Serial No. 09064476,

1998.

SECTION IV.

NIST-206
160 E. Silver et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 156-160

[2] H. Schnopper et al., SPIE 3766 (1999).

[3] E. Silver et al., manuscript in preparation, 1999.

[4] E. Silver et al., unpublished data, 1995.

[5] J. Gillaspy et al., Physica Scripta T59 (1995) 392.

[6] D. Wollman et al., Nucl. Instr. and Meth. A 444 (2000) 145. These Proceedings.

[7] S. Friedrich et a!., Nucl. Instr. and Meth. A 444 (2000) 151. These Proceedings.

[8] S. Handler et al., Nucl. Instr. and Meth. A 444 (2000) 273. These Proceedings.

NIST-207
The Astrophysical Journal, 541 :495-500, 2000 September 20 © 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

LABORATORY ASTROPHYSICS SURVEY OF KEY X-RAY DIAGNOSTIC LINES USING A

MICROCALORIMETER ON AN ELECTRON BEAM ION TRAP

E. Silver, H. Schnopper, S. Bandler, N. Brickhouse and S. Murray

Harvard-Smithsonian Center For Astrophysics, 60 Garden Street, Cambridge, MA 02138 M. Barbera

Osservatorio Astronomico G.S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy E. Takacs'

Massachusetts Institute of Technology, 77 Massachusetts Avenue, 26-239, Cambridge, MA 02139-4307

J. D. GiLLASPY, J. V. Porto, and I. Kink

National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899

J. M. Laming

Naval Research Laboratory, Code 7674L, Washington, DC 20375 AND

N. Madden, D. Landis, J. Beeman, and E. E. Haller

Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720

2000 February 9 ; 2000 April 25 ABSTRACT

Cosmic plasma conditions created in an electron beam ion trap (EBIT) make it possible to simulate

the dependencies of key diagnostic X-ray lines on density, temperature, and excitation conditions that

exist in astrophysical sources. We used a microcalorimeter for such laboratory astrophysics studies because it has a resolving power !^ 1000, quantum efficiency approaching 100%, and a bandwidth that spans the X-ray energies from 0.2 keV to 10 keV. Our microcalorimeter, coupled with an X-ray optic to increase the effective solid angle, provides a significant new capabifity for laboratory astrophysics measurements. Broadband spectra obtained from the National Institute of Standards and Technology EBIT

with an energy resolution approaching that of a Bragg crystal spectrometer are presented for nitrogen, oxygen, neon, argon, and krypton in various stages of ionization. We have compared the measured line intensities to theoretical predictions for an EBIT plasma.

Subject headings: atomic data — atomic processes — line: identification — methods: laboratory —

plasmas — X-rays: general

L INTRODUCTION

X-ray spectroscopic measurements are used to determine

the temperature distribution, density, ionization state, and

elemental composition of hot cosmic plasmas. Knowledge

of these basic parameters provides an understanding of

physical processes in the hot universe such as atmospheric

heating, transport, shock waves, and accretion. Plasmas of

special interest are found in the magnetically heated coronae of late-type stars, the accretion disks surrounding

collapsed stellar objects, the bubbles of gas heated by super- novae and the tunnels throughout interstellar space they

create, the halos of elliptical galaxies, the nuclei of active

galaxies, and the vast regions in galaxy clusters. The deter- mination of the physical parameters that define the plasma

relies on complex models of the continuum and line emissions. Experimental data are unavailable, except in rare

cases, and the atomic data that are needed for these models

are largely derived from theoretical work.

The current understanding of the fundamental processes occurring in highly ionized plasmas is not commensurate

with the quality of data that will be obtained by the new

X-ray telescopes and spectrometers flown on ASCA,

Chandra, and XMM. With a nominal goal of a 1% cali- bration, the Chandra X-ray Observatory, launched in 1999

July, had a 6 month ground-based cahbration effort, which was followed by a comprehensive set of in-flight measure-

' Permanent address: Kossuth University, Debrecen, Hungary.

ments. By contrast, few individual atomic rates are known

with an accuracy less than 20%, and emission lines that depend on many processes (e.g., both direct excitation and

cascades from recombination) are less accurately known.

Dielectronic recombination rate coefficients and uncertain

ionization state fractions derived from different ionization

balance calculations may differ by factors of 2 or more. To achieve the best scientific interpretation of the data

from Chandra, XMM, and ASCA, theoretical calculations must be verified or modified by the results obtained from

spectroscopic measurements in the laboratory. An excellent way to study the behavior of highly charged plasma ions is to confine them in an electron beam ion trap (EBIT). The

EBIT produces customized, well-characterized, homogeneous plasmas well suited to a wide variety of precision measurements. The manipulation of the plasma conditions

in the EBIT will generate a comprehensive database for comparison with theoretical atomic physics calculations. The EBIT is also uniquely capable of simulating specific

slices through astrophysical plasma conditions (ions at rest, electrons with nearly constant energy) to allow the systematic examination of how the atomic structure and dynamics

of plasma ions influence the energy release in cosmic X-ray

sources. The literature contains numerous examples of EBIT applications (Gillaspy 1996; Beiersdorfer et al. 1996).

Measurements of impact excitation rates, excited state life- times, ionization cross sections, resonant excitation and die- lectronic recombination cross sections, and of course, 495

NIST-208
496 SILVER ET AL. Vol. 541

precision wavelength measurements providing tests of quantimi electrodynamics have all been demonstrated.

High-resolution X-ray crystal spectrometers and broadband semiconductor ionization detectors have been the tra- ditional tools for these investigations. Although they have

yielded many excellent results (Beiersdorfer et al. 1992; Brown et al. 1998), these instruments are limited either by a poor energy resolution (solid state detector) or by low

efficiency caused by a narrow bandwidth (crystal

spectrometer). An additional complication is the polarization dependence induced by the strong anisotropy of the EBIT electron beam (Henderson et al. 1990; Takacs et al. 1996). The effect of polarization must be included in the

analysis of crystal spectrometer-derived intensities for lines excited by dielectronic electron capture and by electron impact. In the experiments discussed here and in subsequent

papers related to this work, the solid state ionization detector and the crystal spectrometer are replaced with a cryogenic X-ray microcalorimeter. The microcalorimeter

combines the broadband capability of the semiconductor

ionization detector with the high resolving power of a Bragg crystal. X-ray photons absorbed in the microcalorimeter are converted into heat, causing a temperature

rise proportional to the X-ray energy. It is the ideal instru- ment for EBIT plasma studies and this was first demonstrated in a pilot experiment we carried out in 1995 (Silver

et al. 1999). During the intervening time, an X-ray optic has

been developed to increase the solid angle subtended by the microcalorimeter. This paper describes the first application

of the microcalorimeter/optic combination to the spectroscopic study of astrophysically relevant plasma ions in an EBIT at the National Institute of Standards and Technology (NIST).

In § 2 we review the operation of the EBIT. Section 3 is a

description of the experimental setup and the adaptation of

the microcalorimeter and X-ray optic to the EBIT. In § 4 we

present the X-ray spectra of highly ionized nitrogen, oxygen, neon, argon, and krypton. In § 5 we compare the measured intensities of the helium-hke lines to the theoretical predictions for the EBIT plasma.

2. THE ELECTRON BEAM ION TRAP (EBIT) In traditional laboratory plasma X-ray sources such as tokamaks, mirror machines, and those produced with

lasers, the physical processes responsible for X-ray line for- mation cannot be isolated easily. The interpretation of the spectra obtained from these sources requires modeling of

the charge balance together with a mix of line formation

physics. In 1988 a new plasma source was developed jointly

at the Lawrence Berkeley National Laboratory (LBNL)

and Lawrence Livermore National Laboratory (LLNL),

called the Electron Beam Ion Trap (EBIT) (Levine et al. 1988). This source operates at electron densities of wlO*^ cm ~ ^ . It simplifies the plasma diagnostics by eliminating the transport issues and line-of-sight integrations that make

tokamak spectra difficult to interpret. The only other US

EBIT faciHty is located at NIST (Gillaspy 1996). In an

EBIT, the plasma is eliminated as the perturbing medium

and is replaced by a narrow, well-defined, electron beam. At

NIST, an energy-tunable, 0-150 mA, electron beam can be

accelerated to moderate energies (700 eV to 30 keV) by a

series of cylindrical electrodes (drift tubes). The beam is focused to a small diameter (70 fim) by a superconducting.

Helmholtz pair magnet that provides a maximum axial field of 3 T. Atoms or slightly charged ions are introduced into the center of the'middle drift tube, where the electron beam

has its highest kinetic energy and tightest focus, are stripped of their outer electrons by impact ionization, and reach

successively higher charge states. The limiting charge state

is determined by the voltage applied to the center drift tube.

It is reached when the ionization potential of the ion is greater than the energy of the colliding electrons. An axial trap for the ions in the center is formed by applying a small

positive bias (30-500 V) on the end cap drift tubes located on either side of the center drift tube. Radial trapping is provided by the space charge of the electron beam itself, and

also by the axial magnetic field which restricts the motion of

the ions to modified cyclotron orbits. In addition to cre- ating the highly charged ions and trapping them, the elec- tron beam plays a third important role; it creates excited

states that can decay by photon emission.

When compared with a conventional plasma that has a

broader electron distribution, i.e., a Maxwell-Boltzmann

distribution, the electron beam is nearly monoenergetic. It

is precisely tunable and rapidly switchable in energy (at rates of several keV ms~^). A wide range of species to be

studied can be loaded into the trap either as gases or metal

ions. A gas at room temperature, or a volatile liquid or

solid, can be introduced as neutral atoms through a series of

collimating holes through a side port. Metals can be loaded

vertically (along the magnetic field lines) as slightly charged

ions ( -f 1 to +4) using a metal vapor vacuum arc (MEWA)

ion source. The NIST EBIT is equipped with an " event mode " data

acquisition system that is used to measure transient processes and atomic lifetimes. This system is capable of

switching EBIT operating parameters (beam energy, beam

current, trapping voltage, etc.) at high speed and data can

be time stamped to correlate it with the EBIT configuration.

3. EBIT SPECTROSCOPY WITH A MICROCALORIMETER

AND X-RAY OPTIC

Operating at 65 mK, our microcalorimeters combine

excellent energy resolution with relatively high count rate performance. The spectrometer has 95% quantum efficiency at 6 keV and a large collecting area in the 0.2-10 keV energy

band. In the calorimeter. X-ray photons are absorbed in a

foil of superconducting tin. The temperature rise is proportional to the X-ray energy and is measured with a neutron transmutation-doped (NTD) germanium thermistor that is attached to the underside of the absorber

(Haller 1997). The small heat capacity of the composite

calorimeter produces a relatively large temperature change

( « 5 mK). The NTD thermistor is impedance-matched to a

junction field effect transistor negative voltage feedback

circuit (Silver et al. 1989). The detector used at NIST has an

energy resolution of 5.9 eV at 1.5 keV and 7 eV at 6 keV.

The microcalorimeter eliminates the need for multiple, inefficient crystal spectrometers to measure X-ray emission

lines that span the 0.2-10 keV band. Consequently, the microcalorimeter shortens the total measurement time by

several orders of magnitude. It is also insensitive to the polarization of X-ray emission.

The minimum distance from the plasma center to the vacuum wall is 1 10 mm in the NIST EBIT. The close proximity of other ongoing experiments mounted on adjacent

viewing ports of the EBIT, however, made it difficult to

NIST-209

m
No. 1, 2000 SURVEY OF KEY X-RAY DIAGNOSTIC LINES 497

locate the microcalorimeter closer than 1 meter. This would

have limited the solid angle to 2 x 10~^ sr, comparable to that of a crystal spectrometer, and would have made measurements extremely time consuming. A greater solid angle was obtained by viewing the EBIT plasma through an

X-ray optic made from thin gold coated plastic foil (Silver, Schnopper, & Ingram 1998; Schnopper et al. 1999). The

design can satisfy a wide variety of focal lengths and energy

ranges. The focused image has a half-power diameter that is < 1 mm (Schnopper et al. 1999). For the EBIT plasma

experiments, where the distance of closest approach is limited, the optic was placed equidistant (762 mm) from the source and detector. Since the EBIT creates a plasma in the shape of a narrow line, the X-ray optic can increase the X-ray intensity by at least 30 times, helping to compensate

for the X-ray absorption of the microcalorimeter windows

at low energies (Silver et al. 1999).

4. LABORATORY ASTROPHYSICS MEASUREMENTS

The capability to selectively study ions of individual

species is one of the major strengths of the EBIT/

microcalorimeter combination. In the real case of a cosmic

plasma, temperature, and density variations exist that lead

to severe line blending. An accurate diagnosis will be very

difficult without having access to a database that can be

estabhshed with measurements of the kind we describe here. We have surveyed the K X-ray emission from various

charge states of nitrogen, oxygen, and neon; L and K emission from argon; and L emission from krypton. For nitro- gen, oxygen, neon and argon, in particular, the He-like resonance (w) Is^ ^S~ls2p ^P, intercombination (x, y) Is^ ^ S-ls2p ^P, and forbidden (z) Is^ *S-ls2s transitions are observable. The relative strengths of these are useful as density and temperature diagnostics (Gabriel & Jordan

1969). For the measurements presented here, we have deter- mined the density dependent ratio, R = Z/{X + Y) and the temperature dependent ratio G = (X + Y + Z)/W for nitrogen, oxygen, neon, and argon and compare these

valui^s to those predicted for a model of the non- Maxwellian EBIT plasma.

Beam Currenl = 35,9 mA

Electron Energy = 2 keV

550 600 650 Energy (eV)800

Fig. 1.—Microcalorimeter X-ray spectra from collisionally excited ions

of nitrogen and oxygen from the MIST EBIT. The contributions to He-like

nitrogen (N vr) and He-like oxygen (O vn) from the forbidden line (Z; dark

blue line), intercombination hne (Y, X; red line) and resonance line {W;

dark green line) are shown. The collection time was ~ 30 minutes.

4.1. Nitrogen and Oxygen

Oxygen and nitrogen were injected into the EBIT and for an electron beam current of 35.9 mA the spectrum in Figure

1 was obtained in 30 minutes. The He-like and H-like

(Lyman a) transitions, of nitrogen (N vi and N vn) and

oxygen (O vii and O viii) are shown along with some higher

transitions (Is^-lsnp; n > 2). The relative contributions of the resonance {W; dark green), intercombination {X, Y; red) and forbidden (Z; dark blue) lines to the He-hke emission of nitrogen and oxygen are determined from a least-squares fit {solid black line) to the data {blue histogram) and for a nominal detector energy resolution of 6 eV. The values for R are listed in Table 1 for both nitrogen and oxygen

.

4.2. Neon

Neon was injected into the EBIT and X-ray spectra were

obtained for various electron beam energies, electron beam

currents, and trap confinement (or "dumping") times. A

spectral survey of three different machine conditions is pre- sented in Figure 2. Each spectrum was obtained in 20

minutes. A higher resolution spectrum follows in Figure 3 that shows the line structure more clearly.

The first and second panels in Figure 2 show mixtures of H-like and He-like neon species. The data in the second

panel were obtained at a higher electron beam energy, but

the same electron beam current as for the data in the first panel. Since different electron beam velocities compress or

stretch out the electrons along a line for the same beam

Electron energy « 2.5 keV

Currsnt = 37 itiA Trap dumping time = 200 ms Ne X Ne IX 1S-2P is3p

800 850 900 950 1000 1050 1100 1150 1200

120

1 100 B 80 c

60 oO 40 -

20

NelX

2

1s - 1s2p,s NeX

1s-2p

Electron energy = 6.9 l<eV Current a 37 mA

Trap dumping time = 800 ms

Ne IX ls^-ls3p NelX

..2 ... A.

800 850 900 950 1000 1050 1100 1150 1200

60

5040

30

20

10

Kntrom intennediato

I y Ne cliarge slates Ne VII, Ne VIII, NelX

Electron energy - 1 .26 keV

Cur-ent « 1 5 mA

Trap dumping lime «= 125 ma

iM

800 850 900 950 1000 1050

Energy (eV)

1100 1150 1200

Fig. 2.—First two panels are spectra of H- and He-like spectra of neon (Ne x and Ne ix). The data in the second panel were obtained at the same

electron beam current but higher beam energy. The intensity of the H-likc

Is 2p line increases relative to the He-like lines, Within the He-like

structure, the contribution from the intercombination line decreases at lower densities and higher beam energies (see text). The beam current and beam energy in the third panel are lower than in the second. Fewer H-and

He-like ions and more ions of lower charge states (Li-likc (Ne vni) and

Be-like (Nevn)) are produced, The spectrum in each panel was obtained in 20 minutes.

NIST-210
498 SILVER ET AL. Vol. 541 TABLE 1 Measured Values for R and G and Inferred Electron Density

•Rmeasurtd Elcctron DcHsity Gn,„5„„j Element Z,\X + Y') (cm"') (X + Y + Z)/W

Nvi 0.25 ± 0.11 +3.5 X 10" 1.05 ± 0.17

4.5 X 10" - 1.5 X 10" Ovn 0.55 ± 0.11 +1.7x10" 0.97 ± 0.10

6.0 X 10" - 1.1 X 10" NeDC 1.6 ± 0.2 +4.0 x 10" 0.95 + 0.06

1.3 X 10'^ - 3.0 X 10" Arxvn 1.1 ±0.3 2 x 10"" 0.86 + 0.12

Note.— Uncertainties shown represent one combined standard devi- ation

" From the machine parameters plus an estimate of the beam radius.

current, the electron density is inversely proportional to the square root of the electron beam energy. Therefore, the elec- tron density in the second panel is approximately one half of that m the first panel. By increasing the electron beam

energy and keeping the ions in the trap longer (greater trap

dumping time), the probability for reaching higher charge

states improves. Thus, the intensity of the H-hke (Ne x)

Is -> 2p transition increases relative to the He-like transition. Within the He-like structure, the contribution from

the intercombination lines decreases at lower densities and

higher electron excitation energies as expected. As the

density decreases, the collisional transfer to the upper

state from the metastable level decreases. Also, the cross

section for excitation from the ^Sj level to the ^P^ decreases

with increasing electron energy. This is evident by the deeper valley between the peaks in the second panel. In the

third panel, the electron density is the same as in the second

c

3

OO

Electron energy = 2.2 keV Ne IX Current = 39.5 mA

120 W

100 J

80

•

Y,X 1 z 4

•A* / •

60 R = Z/(X+Y) = 1.6 G = (X+Y+Z) / W = 0.95 -

1 M /

•1 1* r

•I i*ir\

la l^W*

•1 n 1 20 nk 0860 880 900 920 940 Energy (eV)

960 980

Fig. 3.—Fit to the resonance (W; blue line), intercombination (X,Y; red

line) and forbidden (Z; dark green line) lines yields a line ratio R = Z/

(X + Y) = \.6 and G = (X + Y + Z)/W = 0.95. The collection time was

29 minutes.

panel but the electron beam energy is only slightly above

the He-like ionization potential (1.12 keV). In addition, the frequency for dumping the ions from the trap is 6.4 times higher than in the previous panel. Consequently, fewer Hand He-like ions are created. We have used the HULLAC

350

300

250

150

100

so

200 300 400 500 600 700 800 900

Ai^on XVII and Aigon XVI Satellite Unas

G=(X+Y+ZVW = 0.8e

1000 1500 2000 Energy (eV)

2500 3000 3500

Fig. 4.—Simultaneously observed K and L spectra from Argon. The inset at the upper left shows the richness of the L emission features from argon (blue

line). The inset at the right is an expanded view of the He-like complex of Ar xvn (red line) and the additional contributions of the Ar xvi satellite lines. The

collection time was 40 minutes.

NIST-211
No. 1, 2000 SURVEY OF KEY X-RAY DIAGNOSTIC LINES 499

180 ^ Kr L emission lines : 6 kV excitation 160 - [Ne]

1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 Energy (eV)

Fig. 5.L emission lines of highly charged krypton ions measured at 6 keV electron beam energy. Energies of the marked lines ([Ne] means

Ne-like Kr, etc.) are taken from Burkhalter et al. (1979).

(Klapisch et al. 1988; Goldstein et al. 1988) suite of codes to

identify the lines between 891 and 933 eV as Ne viii (Li-like)

lines of the type \s2s2p, ls2s^, ls2p'^ to ls^2s or ls^2p. The

next peak down in energy at 870 eV is due to similar

transitions in Be-like Ne vii, and the smallest peak at x 850

eV is neutral neon plus any low charge state of neon that

drifts into the trap from the plasma edge.

In another series of measurements the gain of the pulsr - height analysis system was increased to provide higher precision in the measurement of the helium-hke complex in Ne IX. The data are shown in Figure 3, where the contributions from the resonance, intercombination and forbidden

lines are clearly seen. The values for R and G are shown and

hsted Table I.

4.3. Argon

A broadband spectrum of argon that includes the L emission lines between 400 and 700 eV and the K emission lines

at approximately 3100 eV is shown in Figure 4. Simulta- neous high-resolution measurements of the L and K emission from the same portion of the plasma are not possible

with any X-ray spectrometer other than a microcalorimeter.

The argon L emission spectrum is very rich, but has

received comparatively little theoretical work. Consequently a definitive identification of these hnes will have to await

further analysis. The argon K spectrum, on the other hand,

has been studied more carefully and the K spectrum is fitted with the known contributions from the Ar xvii He-like hnes

and the satelhte lines from inner shell excitation in Li-like Ar XVI whose transitions have the form \s^nl-ls2pnl with

n>2. (PhilHps et al. 1993). The values for R and G are shown and also hsted in Table 1.

4.4. Krypton

Krypton gas was introduced into the EBIT and X-ray

spectra were obtained for different gas pressures and elec- tron excitation energies. A sample spectrum obtained for an

electron beam energy of 6 keV is displayed in Figure 5. The

lines include contributions from Li- like to Mg-like ions and

will be discussed in detail in a separate paper (Kink et al. 2000).

5. DISCUSSION

The broad bandwidth capability of the microcalorimeter,

coupled to an EBIT with an X-ray optic, made it possible to measure simultaneously spectra from highly ionized ele- ments of astrophysical importance. The high resolving

power of the detector allowed a determination of the relative contributions to the helium-like complex from the reso- nance, intercombination and forbidden hnes. In principle, their relative intensities provide information about the density and temperature of the non-Maxwellian EBIT

plasma. These values are compared to the predictions of a model for the EBIT plasma. Levels are excited by electron impacts, and de-excited either by radiative decay or by elec- tron impact. The energy levels, radiative decay rates and

electron impact excitation cross sections are calculated using the HULLAC suite of codes. They compute bound

state energy levels and wave functions using a parametric

potential method, and excitation cross sections in the dis- torted wave approximation. Since these techniques omit

resonances in excitation cross sections we were free to choose EBIT beam energies that avoid such complications

and to isolate particular processes of interest. Theoretical values for R = Z/(X + Y) for N vi, O vii, Ne IX and Ar xvii are shown in Figure 6. The observed

ratios for R and the densities inferred for these ratios are shown by the open data points with error bars and are also given in Table 1. Nitrogen and oxygen were in the EBIT

trap at the same time and the measured i?-values for the He-like emission lines predict the same electron density for them to within the 1 a errors. The inferred electron density of 5.8 X 10" cm"^, together with the electron beam energy

of 2 keV and beam current of 35.9 mA, provides a measure

of the electron beam radius of 69 /im. This is consistent with

the expectation that the radius of the electron beam ( 35

at 6 keV) will increase as the beam energy is reduced. (Takacs et al. 1996; Levine et al. 1988) The neon measure6 -t——TMT

'-'-^—^--^— —

T

4

-

N VI OVIl Ne IX Ar XVII 0 data N VI a data 0 VII & data Ne IX data Ar XVII >

+

><

is]

II

2

-

10^ 10^° 10" 10^2 1013 lo^"* i

Electron Density (cm'^)

Fig. 6.—Calculated values for R = Z/(X + 7) for N vi (black), O vn

(blue line), Ne rx (red line) and Ar xvu (green line). The measured values and

their 1 a uncertainties are shown by the open data points.

NIST-212
500 SILVER ET AL.

ments were made under similar machine conditions and the R-values for Ne rx are consistent with those for N vi and

O vn. These resuhs may also be affected by the gyration of

the ions in and out of the electron beam. The inferred density would be lower than the actual value. The /?-value

for the At xvn spectrum, obtained with a beam energy of 6.7 keV is consistent at the 1 a level with density estimates of

K, 10' cm~^ to 10 cm~^. However, the theory shows that

the J?-value for Ar xvn is not a sensitive indicator of the

density for such high electron energies. The measured value

is therefore plotted in Figure 6 at the nominal electron

density that can be estimated from the beam current and

beam energy. The observed ratios for G do not agree well with our

theory; all experimental values indicate too much population in the 2 and 2 states compared with 2 (The

predicted G-value is 50 times larger than the experimental

values for N vi, 30 times for O vn, 10 times for Ne ix, and it is 4 times larger for Ar xvn). This is most likely the result of

charge exchange into excited states of the He-Uke ion from

the H-like ion. The ionization balance for these observations is determined mainly by the electron impact ionization caused by the EBIT beam (whose energy is much

greater than the ionization potential for the H-like ions of

N, O, and Ne) and by charge exchange recombination when

highly charged ions in the trap coUide with the neutral atoms that provide the gas loading. UnUke radiative recombination, where the primary electron capture is in the ground state of the recombining ion, charge exchange

mainly populates excited states. These subsequently decay

by a radiative cascade. The relative excitation rates to the

levels of the 2 and 2 terms are determined by the

statistical weights of the levels, and, therefore, are very

similar to the electron impact excitation rates for N vi, O vn, and Ne rx. In principle, the R ratio should be essentially unchanged while the G ratio will be altered. Since the G ratio involves the 2 ^P intensity, it behaves quite differ- ently under charge exchange than under impact excitation. The charge exchange process is not pursued further in this paper. Note, however, that the EBIT offers the potential to

measure charge exchange rates into excited states. This may

be important for interpreting cometary X-ray spectra or perhaps spectra from other plasmas where neutrals can

coexist with highly charged ions, such as in the shock wave

of supernova remnants.

6. SUMMARY

A microcalorimeter coupled to an EBIT with an X-ray

optic provides a significant new capabihty for laboratory

astrophysics measurements. In this pilot survey, broadband

spectra obtained with an energy resolution approaching

that of a Bragg crystal spectrometer have been obtained for nitrogen, oxygen, neon, argon, and krypton in various

stages of ionization. The relative intensities of the He-hke

emission lines, in particular, were measured and compared

to a theoretical model of the non-MaxwelUan EBIT plasma.

The electron densities that are inferred from the ratio of

forbidden to intercombination lines of nitrogen, oxygen, and neon spectra agree well with theory. Charge exchange

in the EBIT appears to influence the intensity of the He-like resonance Hne relative to the forbidden and inter- combination hnes. Further measurements are necessary to model properly the effects of charge exchange.

E. Silver, H. Schnopper, and S. Bandler acknowledge

support in part by NASA grant NAG 5-5104, I. Kink

acknowledges the Swedish Foundation for Cooperation in Research and Higher Education (STINT), and M. Laming

acknowledges support from NASA contract W19539

(Apphed Information Systems Research Program) and by

the NRL/ONR Solar Magnetism and the Earth's Environment 6.1 Research Option. The authors wish to thank

Richard Deslattes for helpful discussions and for encouraging them to make these measurements, John Raymond

for suggesting measurement objectives with important

imphcations for astrophysical spectral observations, Kate

Kirby for critically reviewing the manuscript, and Frank

DeFreze and Douglas J. Alderson for their excellent technical support.

Beiersdorfer P., et al. 1996, Hyperfine Interactions, 99, 203

Beiersdorfer, P., Phillips, T. W., Wong, K. L., Marrs, R. E., & Vogel, D. A.

1992, Phys. Rev. A 46, 3812 Brown, G.V., Beiersdorfer, P., Liedahl, D .A. Widman, K., & Kahn, S. M.

1998, ApJ, 502,1015

Burkhalter, P. G., Shiloh, J., Fisher, A., & Cowan, R. D. 1979, J. Appl.

Phys., 50, 4532

Gabriel, A, & Jordan, C. 1969, MNRAS, 145, 241

Gillaspy, J. D. 1996, Phys. Scr., T65, 168

Goldstein, W. H., Oreg, J., Zigler, A, Bar-Shalom, A., & Klapisch, M. 1988, Phys. Rev. A, 38, 1797

Haller, E. E. 1997, Proc. Fourth International Symposium on Low Temperature Electronics and High Temperature Superconductors, ed. C. L. Qaeys, S. I. Raider, M. J. Deen, W. D. Brown, & R. K. Kiischman

(Pennington: Electrochemical Society), 298

Henderson, J. R, et al. 1990, Phys. Rev. Lett., 65, 705

Kink, I., et al. 2000, in preparation

Klapisch, M., Bar-Shalom, A., Goldstein, W. H., Meroz, E., Chon, A., &

Cohen, M. 1988, HULLAC Code for Atomic Physics, unpublished

Levine, M. A., Marrs, R. E., Henderson, J. R., Knapp, D. A., & Schneider, M. B. 1988, Phys. Scr., 22, 157

Phillips, K. J. H., Harra, L. K., Keenan, F. P., Zarro, D. M., & Wilson, M.

1993, ApJ, 419, 426

Schnopper, H. W., et al. 1999, Proc. SPIE, 3766, 350

Silver, E., Schnopper, H., & Ingram, R. 1998, US Patent Application, Serial No. 09064476

Silver, E., et al. 1999, in Proc. Eighth International Workshop on Low

Temperature Detectors, ed. P. de Korte & T. Peacock (Amsterdam:

Elsevier), 2000

.1989, Proc. SPIE, 1159, 423

Takacs, E., et al. 1996, Phys. Rev. A, 54, 1342

NIST-213
PHYSICAL ElEVIEW E, VOLUME 63, 046409

Analysis of broadband x-ray spectra of highly charged krypton from a microcalorimeter detector

of an electron-beam ion trap

L Kink,' J. M. Laming,^ E. Takacs,^ J. V. Porto,' J. D. Gillaspy,' E. Silver,'* H. Schnopper,'' S. R. Handler,'* M. Barbera,' N. Brickhouse,'* S. Murray,"^ N. Madden,^ D. Landis,^ J. Beeman,^ and E. E. Haller^

^National Institute of Standards and Technology, Gaithersburg, Maryland 20899

^Naval Research Laboratory, Code 7674L, Washington, DC 20375

^Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

^Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138

^Osservatorio Astronomico G. S. Vaiana, Palermo, Italy ^Lawrence Berkeley National Laboratory, Berkeley, California 94720

(Received 13 June 2000; published 28 March 2001)

Spectra of highly charged Kr ions, produced in an electron-beam ion trap (EBIT), have been recorded in a broad x-ray energy band (0.3 keV to 4 keV) with a microcalorimeter detector. Most of the spectral lines have

been identified as transitions of B- to Al-like Kr. The transition energies have been determined with 0.2%

uncertainty. A semi-empirical EBIT plasma model has been created to calculate a synthetic spectrum of highly

charged Kr and to determine a charge state distribution of Kr ions inside the EBIT.

DOI: 10.1103/PhysRevE.63.046409 PACS number(s): 52.70.-m, 32.30.Rj, 34.50.Fa, 34.80.Dp

I. INTRODUCTION

Laboratory plasmas produced from high Z gases have at- tracted interest in a number of applications. Laser-irradiated gaseous targets are routinely used for producing calibrated x-ray sources. The gas target remains below the critical density of the laser, improving the efficiency with which laser

light is absorbed, and converts much more of this energy to radiation, losing less to kinetic energy and conduction as in the cases with solid targets [1,2]. They also have advantages

compared to solid targets in some potential technological

applications (e.g., x-ray lithography [3]). High-Z gases are also useful as "radiating divertors" in tokamaks. These create a buffer between the hot tokamak plasma and the toka- mak chamber, thereby reducing particle fluxes and heat

losses to the chamber walls (c.f. Ref. [4]). The investigation

and application of plasma diagnostic potential of many of

these applicable elements have largely been focused on

Af-shell emission (e.g., see Refs. [5,1]). However in principle the L shells of these elements offer significant additional diagnostic potential [6]. In this paper we investigate line energies and relative intensities in ions of Kr between B- and

Al-Uke. Electron-beam ion traps (EBIT's) have proven to be versatile tools for investigating properties of very highly

charged ions (see e.g , Ref. [7]). These properties are important for testing different theoretical atomic models as well as

in a number of applications where high-temperature plasmas

are involved. It is typical for these ions that a large fraction of the photon emission occurs in the x-ray spectral region because of the fact that electron excitation energy increases with nuclear charge Z [8]. Therefore spectroscopic investigations in the x-ray region are important for determining the properties of highly charged ions. The majority of the experimental spectroscopic data in the x-ray region have so far been obtained with various types of crystal spectrometers.

Although these instruments can provide the best resolving

1063-65 1 X/200 1/63(4)/046409( 1 0)/$20.00

power necessary for precision studies, there are several limi- tations in their application for investigations of highly ion- ized plasmas. Their low transmission efficiency, which typically decreases with increasing spectral resolution, limits the resolving power typically to around thousands to tens of

thousands. The narrow x-ray energy range that can be cov- ered with one particular crystal sets limits to the studies where different parts of the spectra have to be compared, a

situation that often occurs in the analysis of complex spectra

of laboratory and astrophysical heavy ion plasmas, where

fines from different charge states may overlap. Semiconductor detectors [Si(Li), Ge, etc.] that can detect x rays in a broad energy band, on the other hand, have low-energy reso- lution (several hundred eV), which is usually not sufficient for detailed spectroscopic investigations of highly charged

ions.An x-ray microcalorimeter [9,10] is a recently developed

type of x-ray detector that has several unique properties that significantly improve spectroscopic measurements involving x-ray photon detection. These instruments can cover a broad energy range (0.1 to 10 keV) with relatively high-energy

resolution (about 6 eV) and close to 100% quantum effi- ciency. These properties make data acquisition more efficient and can provide information that is difficult or sometimes

impossible to obtain by conventional methods. For example,

since the spectra contain accurate fine intensity information

recorded over a broad spectral range, the comparison with a

synthetic spectrum provided by plasma modeling calcula- tions can yield information about various atomic and plasma

parameters, e.g., excitation cross sections, electron and ion

densities, etc. In this paper we demonstrate the microcalorimeter-EBIT combination in an investigation of highly

ionized Kr.

Until now, the experimental spectroscopic data in the x-ray region for highly charged Kr (^^=23 + ) ions have re- mained rather Umited. In addition to piu-e atomic physics

interest, these transitions have applications in radiative cool- ing experiments with tokamak plasmas by puffer gases

63 046409-1 ©2001 The American Physical Society

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I. KINK et al. PHYSICAL REVIEW E 63 046409

[11,12]. The A/i = 0 transitions within n = 2 and n = 3 configtirations that occur in the extreme ultraviolet (EUV) re- gions have been studied by many groups with different tech- niques (e.g., Ref. [13-15]). However, the x-ray transitions from the n = 3 levels to the ground state have been observed

only by Burkhalter et al. [16] and Gordon et al. [17] who

studied these transitions in the F-, Ne- and Na-like Kr ions produced in a z-pinch plasma. We determine line energies in

the ions Krxxrv-xxxn, and identify some line ratios that have appUcations as electron density diagnostics for tokamak

(lO'^-lO"* cm~^) or laser-plasma (10^^- 10^' cm~^)

sources. In the following paragraphs details of the instrumentation,

the plasma ionization balance modeling and spectral analysis

will be summarized. The results of the Une identifications of

the observed x-ray lines of B- to Al-like Kr as well as the charge state distribution of the krypton plasma will be presented, and finally some discussion of the electron-density

sensitivity some of these lines are expected to exhibit is given.

n. EXPERIMENTAL SETUP

The experimental setup is thoroughly described in Refs. [18-20]. Here only a general outline is given. The Kr ions

are radiaUy confined inside the EBIT trap region by the combination of an electric field due to an intense beam of electrons and a homogeneous strong magnetic field parallel to the electron beam. Axially the ions are trapped by an electric

field created by a set of cylindrical drift tube electrodes. The

electron beam is compressed by a 3 T magnetic field that is generated with a superconducting coil to achieve maximum

current density. In our measurements the beam current was

typically 115 mA with an estimated radius of 35 fim (current density «=3 kA/cm^). The beam energy (typically in the few

keV range) was one of the adjustable experimental parameters. The krypton gas was let into the trap using a gas in- jection system attached to the EBIT [21]. By varying the

neutral gas pressure inside the EBIT the charge state distri- bution and dynamics can be manipulated [22] due to shifts in

the balance between ionization and recombination processes. The trap was dumped every 1.35 s to prevent a slow

buildup of contaminating heavy ions, small amoimts of which are present inside the EBIT despite the ultrahigh vacuum conditions (typically ==10~' Pa). Collisions of Kr atoms and ions with beam electrons lead

to gradual ionization and excitation of the Kr ions. The

X rays emitted in the region of 0.3 keV to 10 keV were

collected with a focusing x-ray optic [23,24] and detected by

a neutron transmutation-doped germanium-based microcalorimeter [10,20], where the increase of the temperature of the

detector, that is proportional to the energy of the absorbed

photon, is recorded. The x-ray optic and detector were separated from the EBIT with a thin (20 nm) mylar window to prevent contamination of the vacuum inside the EBIT. The

transmission characteristics of the x-ray optic and a description of the detector can be found in Ref. [20]. Over the spectral region where the analyzed Kr hnes appear, the combination of the total transmission of the optical components and

TABLE I. Experimental conditions for Kr spectra.

Set Beam energy (keV)

Gas injection pressure

^ (X 10-* Pa)

I 5.77 1.33

II 12.44 1.33 m 6.06 11.7 rv 6.06 0.5

^Measured in the gas injection chamber outside the EBIT before expansion into the low-pressure region at the trap center [21].

the efficiency of the microccdorimeter is a smooth and flat function of the photon energy, and therefore no corrections

to the Une intensities have been made. Four recordings with

Kr gas were taken (Table I), varying the gas pressiu-e and

electron-beam energy. The data acquisition time for each

spectrum was about 20 min. Spectra of highly charged N, Ar,

and O were also recorded under identical experimental conditions for detector energy caUbration purposes.

m. DATA ANALYSIS

Experimental spectra were caUbrated using He-like N

[25,26] and Ar [27], H-like N and O [28], and Ne- and F-like Kr [16] lines. Altogether, 15 reference hnes were used that spread over the large energy interval of 300 eV to 4000 eV. Three groups of lines are well distinguished in the spectra; N

and O lines at the low-energy side, Kr lines in the center, and

Ar lines at the high-energy side. Polynomial functions were

used for energy calibration and the best fit was achieved with a third degree polynomial. The weighted average scattering

of reference lines was 1.4 eV, where inverse squares of the

uncertainties of the reference lines (mainly due to statistical uncertainty in the line positions) were used as the weights.

The spectral lines were first fitted with a sum of Gaussian

profiles to get accurate Une positions using the computer

code GFIT [29]. The typical fiiU width at half maximum was

2.5 data channels, corresponding to approximately 6 eV. The

statistical uncertainty (one standard deviation) of the Une position was typically 0.5 eV to 1.0 eV which, taking into account the acciuacy of the caUbration, gives an uncertainty

of 1 .7 eV for identified single Unes. Since most of the lines are blended with close-lying Unes we quote 3.5 eV as an

estimate of the combined standard uncertainty of transition energies. This value includes the statistical uncertainty (uniform distribution) in the Une position of 3.2 eV and the caUbration uncertainty of 1.4 eV. The final energies that are presented in Table n are the averages from the four different measurements. The quoted uncertainty agrees well with the

statistical deviation of transition energies from four different measurements (average == 2.5 eV) indicating that there are no

significant systematic deviations between different measurements. In Fig. 1 , the part of the spectrum where the Kr Unes

appear is shown. This spectral structure contains Unes of Alto Be-Uke Kr ions. Most of the strong lines come from the Am = 1 transitions between n = 2 and « = 3 configurations (L

transitions). The electronic structure of ions with many elec046409-2

NIST-215
ANALYSIS OF BROADBAND X-RAY SPECTRA OF . . . PHYSICAL REVIEW E 63 046409

TABLE II. Measured and calculated energies of El, E2, and A/1 transitions in B-Iike to Al-like Kr Ions.

Lower level Upper level Leading

eigenfunction

contribution ^

B-Iike {2s^lp^?\a)

7 2230.3 1.9 2231.9 2po

1/2 2.s2.p3p (l.3/2)tr, \ ^/ 3/2 89%

1.1 2229.5 n 3/2), 90%

1 2223.9 0.7 2217.9 (0,3/2)3/2 93%

22 2183.7 7.1 2187.8 3d 'D3/2 100%

1 'y^Of^ \ 1 ZiZO. 1 "JOT 7 2s2p^ 4p

3/2 2s2p3d (0,5/2)°5/2

1.1 2126.6 2p 'P°3/2 3d 'D3/2 100%

30 2089.1 2.4 2090.1 2s2p^ 2s2p3d (L5/2)°5/2 72%

1.4 2079.2 'P./2 2s2p3d (L5/2)°3/2 49%

18 2010.7 1.5 2010.1 'D3,2 2s2p3s (Ll/2)°,/2 81%

31 1995.7 0.8 2001.0 2d

Pl/2

C-like {Is^lp^ ^Po) 53 2113.6 12.5 2118.6 2p' 'Po 2p3d ( 1/2,3/2)° 1 99%

10 2060.1 0.8 2058.5 •D2 (1/2.5/2)° 3 98%

18 2010.7 0.6 2010.4 2p3p (1/2,1/2)0 99%

22 1875.6 0.8 1879.9 2s2p^ ^D°,

N-like (2s^2p^ ''S°3/2) 10 2060.1 2.7 2055.1 2p^ 3/2 2p^CP)3d ( 2.3/2) m 48%

44 2047.8 4.2 2053.0 2/?^('D)3d (V^'-"'*'/3/2 2.3/2) ir. 43%

2.9 2051.3 2n^(^P)3d (2 3/2)cn 26%

1 2035.8 1.1 2033.8 (1.3/2)<:o 66%

0.7 2031.8 94%

18 2010.7 1.3 2012.0 ^ 3/2 2p\^D)3d (2,5/2)5/2 34%

31 1995.7 1.3 1995.2 2p\^P)3d (1,5/2)5/2 81%

4.0 IQQ/1 Q 4qo

3/2 (0,5/2)5/2 /J/0 0.6 1992.0 2p^0D)3d (2,3/2)3/2 43%

1 1986.4 1.6 1990.1 2pH^P)3d (2,3/2)5/2 26%

0.8 1985.6 (0,3/2)3/2 79%

1 1941.1 0.5 1942.7 2pyD)3s (2,1/2)5/2 51%

17 1788.2 0.8 1790.3 2s2p* 'P3/2 2pH^P)3p (2,l/2)°3/2 30%

O-Iike {Is'^lp* ^Pj) 31 1995.7 0.7 1992.0 3P,*^2 ^1/ ^ f Jtt 3oo

1 63%

2.8 1991.0 7„3/2po\'5 J ^D°. 47%

1 1986.4 0.9 1987.2 3po 50%

2.9 1987.2 U 2 59%

0.7 1986.6 2ty'l^P°\3d ^D°, u 3 44%

0.7 1985.6 Ipo

3 49%

46 1981.5 7.5 1984.0 \ VJ )D(i U 3 50%

0.6 1979.3 3po^ 30%

1 1973.5 1.4 1968.6 42%

1 n \Qftfi 1 dA<7n 'I't 10

i.l 1964.5 41%

1 1941.1 0.7 1939.9 2p^(^P°)3d 3F°3 51%

32 1932.4 0.7 1935.4 'P. 2p\^D°)3d 3po^ 50%

0.9 1935.4 59%

6.7 1934.2 'P2 2p^{*S'')3d 'D°3 26%

046409-3

NIST-216
I. KINK et al. PHYSICAL REVffiW E 63 046409

TABLE II. (Continued).

£7 /«\7\ b £'«p(eV) hh £^fA(eV) Lower level Upper level Leading

eigenfunction

contribution

1.2 1930.6 63%

38 1925.2 1.1 1927.3 'Pi 3po^ 30%

0.8 1926.7 63%

2.2 1924.9 'P2 2p^(*S°)3</ 24%

0.9 1924.3 'F4 100%

42 1917.5 0.5 1918.3 2p^{*S°)2d 51%

0.8 1917.1 39%

2.2 1917.1 'Po 45%

22 1875.6 0.6 1877.9 'Pi 2p'(^P"')3j 59%

24 1861.5 1.1 1864.4 'P2 2p^^D°)3j 100%

0.6 1864.4 3do" 2 59%

44 1854.1 1.3 1851.0 'P2 2pVS°)3j 40%

50 1803.9 1.3 1809.7 40%

2.1 1803.9 5S»2 48%

u.o loUZ.o 3p1^1 2/7^(^D°)3s Dl /O

1.1 loUZ.O ir»L>2 'D°2 OOyo 36 Xj U.6 ^D°3 lUUvo

F-like (2j 20 2025.3 0.7 2020.2 Is^lp' ^P°3/2 2s2p 3p (1,3/2)5/2 82%

18' 2010.7 0.6 2009.8 (2,3/2) 1/2 50%

0.6 2007.5 (1,1/2)3/2 74%

31 1995.7 0.8 1999.7 (2,3/2)5/2 99%

0.6 1996.8 (2,3/2)3/2 82%

46f 1981.5 0.6 1983.4 (2,1/2)5/2 99%

42' 1917.5 10.0 1919.5 2s^2p'*0D)3d (2,3/2)5/2 64%

5.6 1918.0 (2,3/2)3/2 36%

0.6 1914.8 (2,5/2)5/2 80%

2.8 1914.8 (2,3/2) 1/2 60%

1' 1900.7 2.1 1903.6 2s^2p*CV)2>d (1,5/2)5/2 77%

2.3 1901.9 (1,3/2)5/2 63%

22' 1875.6 0.6 1876.2 'P°l/2 2s^2p'*0D)3d ( 2,5/2) ,/2 51%

1.4 1873.4 (2,3/2)3/2 30%

4.2 1873.1 2po" 3/2 2s^2p*CP)3d (0,5/2)5/2 47%

33 f 1869.7 2.1 1869.4 (0,3/2)3/2 41%

24' 1861.5 0.9 1863.0 ^P°l/2 2s^2p*0D)3d (2,3/2)3/2 36%

6.8 1861.9 2po" 3/2 2s^2p*CP)3d (2,5/2)5/2 68%

3.4 1858.2 (2,5/2)3/2 57%

44' 1854.1 1.4 1853.8 ( 2,5/2) ,/2 56%

56' 1847.0 0.7 1842.0 252p^ 'S,/2 2s2p^3d (1,3/2)° 3/2 51%

16' 1783.1 1.7 1785.5 25^2^^ 2p°3/2 2s^2p*CD)3s (2,1/2)5/2 78%

12' 1771.5 1.0 1771.4 2s^2p*CP)3s (1,1/2)3/2 99%

0.9 1769.9 2s^2p\^P)3p (2, 1/2)° 5/2 76%

1' 1735.6 0.6 1742.3 2j^2/(^P)3j (0,1/2),/, 51%

0.5 1732.1 'P°I/2 2j^2/('D)35 (2,1/2)3/2 74%

21

1722.6 2.1 1724.1 2s^2/(^P)3i (2,1/2)3/2 75%

17' 1716.1 3.0 1719.1 (2,1/2)5/2 79%

1.3 1718.9 2j2p« 'S,/2 2s2p^3s (l,l/2)°3/2 75%

"

1 1697.2 1.2 1699.3 (2,l/2)°3/2 93%

7 1615.8 2.7 1611.2 2s^2pyD)3p (2,l/2)°3/2 32%

046409^

NIST-217
ANALYSIS OF BROADBAND X-RAY SPECTRA OF

.

PHYSICAL REVIEW E 63 046409

TABLE n. (Continued).

/

^

* exp £:^,„(eV) /' th £,/,(eV) Lower level Upper level Leading

eigenfunction

contribution *

Ne-like (2^ '2p^ 'So)

1 2035.8 1.4 2034.8 2p^ 'So 97%

12' 1961.9 3.1 1960.5 76%

i' 1941.1 1.1 1945.4 76%

44 f 1854.1 28.0 1852.7 Ip^Zd 'P", 53%

1803.9 23.0 1803.4 46%

III i.j U. / VIIZ.I 2p 3/> 4/%

21 1722.6 0.7 1720.1 67%

31' 1705.5 6.7 1705.9 Ip^Zs 63%

1.3 1703.7 Ip'-ip 53%

44' 1651.5 11.8 1652.4 2p^3s 'p°, 63%

5.9 1649.1 3p°2 100%

Na-like (2s^2p^2,s ^S,^) 56 1847.0 14.4 1841.1 2p*3s 2p^2,s-id (l,3/2)°3/2 43%

13' 1834.0 9.6 1836.8 ( 1,3/2)°,/2 95%

17 1788.2 15.0 1790.3 (2,5/2)°3/2 47%

7.3 1789.1 (2.5/2)'' 88%

Mg-like (2j^2p^3s- 'Sq)

1 1822.5 25.0 1827.6 2^*3*^ *So 2p^3s^7,d (1/2,3/2)°, 97%

16 1783.1 21.4 1778.5 (3/2,5/2)°, 80%

Al-like (2j^2p*3j^3/7 ^P°,;2) 50 1803.9 6.8 1802.8 3s^3p ^P°,/2 2p^3s^3p3d {l,3/2ha 90%

^Taken from data set IV (see Table I) in arbitrary units proportional to the number of photons.

''Average of data sets I-FV (see Table I). Estimated combined standard uncertainty is 3.5 eV.

'^Calculated with model (Sec. IV) for data set II. Intensities are in arbitrary units proportional to the number of photons/s.

""Calculated with relativistic parametric potential method (huluvc code). ^Calculated with superposition-of-configurations method (cowan code).

^Identified first by Burkhalter [16] or Gordon [17].

irons in an open 2p shell is relatively complicated and results in numerous closely spaced lines that we were not able

to resolve experimentally. Because of the EBIT excitation mechanism, lines that have a strong electric dipole (El)

transition probability to the ground state dominate the spectrum. However, in the low-density enviroimient (negligible

collisional deexcitation) inside the EBIT E2 (electric quadrupole) and Ml (magnetic dipole) transitions are also observable. The complexity of the spectra (for example, a spec- trum of B-like Kr consists of more than one hundred

2p^-2p^3l lines that are calculated to be stronger than 1/200

of the strongest line) demands additional tools for correct

line identifications. We used two different theoretical calculations as an additional aid. The transition energies were cal- culated using the superposition-of-configurations method de- veloped by R. D. Cowan [30]. For energy-level structiu-e calculations we included all n — 2 and « = 3 configurations

except the configurations with an open Is subshell, and

M =4 configurations excluding all innershell excited configu- radons. The electrostatic and configuration interaction pa- rameters were scaled to 90% of their ab initio values whereas spin-orbit parameters were kept at 95% of their ab

initio values. It has been shown previously [30] that these estimates predict level energies closer to the experimental

ones. Line intensities were calculated using a plasma model

(see next section) that is based on a relativistic parametric

potential calculation using the HULLAC code [31,32]. The

qualitative behavior of the spectral lines under different ex- perimental conditions was used to support the identifications.

rv. KR IONIZATION BALANCE

A. Basic Procedures The ionization fraction of the Kr ions with charge q is given by

046409-5

NIST-218
I. KINK et al. PHYSICAL REVIEW E 63 046409

1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 Energy (eV)

FIG. 1. Comparison of the experimental (set I, Table I) and

fitted synthetic spectra of Kr calculated with the model described in Sec. IV. (a) equal charge state distribution, (b) nonequal (fitted) charge state distribution including charge-exchange processes. Ro- man numbers mark lines belonging to the corresponding Kr spec- tnim (e.g., XXVn marks Ne-like Kr etc.)

dfa

+ 0,)/,] + «^,(X,+ i/,+ ,-X/,), (1)

where lq,Rq,Dq, and Xq are the rates for electron-impact

ionization, radiative recombination, dielectronic recombination, and charge-exchange recombination, respectively, for

the charge state q, is, the electron density and n^^ is the

density of neutral Kr atoms in the trap. The calculation of the various rates is discussed in more detail below. Equation 1 neglects charge exchange between ions and three-body re- combination, the inverse of electron collisional ionization. The latter would scale as «^ and, although insignificant for

the EBIT beam, becomes important as the electron density

increases. The EBIT trap was emptied every 1.35 s. As we will describe below, the relevant ionization and recombination

rates are = 100 s~ ' , allowing us to considerably simplify our model by assuming a steady-state ionization balance, i.e., putting the time derivatives in Eq. (1) equal to zero. Another

important simplification comes from the observation that the

ion-ion collision time in the trap, given by Ref. [33], is

T-= —p — 10 ^s (2) A^TT riiq e logA

for an ion density n,==6X lO'"* m~^ (this is about 2% of the

ion density required to fully compensate the space charge of

the electron beam), where w,, T,, and «, are the ion mass,

temperature, and density, e is the elementary charge, and

A = (47r/3)npXo with Xo= \[kT^€^ln^q^ being the Debye

length for electron temperature Tg and density . For typi- cal conditions inside the EBIT, logA^'^lS. This collision time

is about an order of magnitude less than the ionization and

recombination times, allowing us to put all ion temperatures

at the same value. Note that when a light gas (e.g., oxygen)

with significantly lower charge q is introduced to provide

evaporative cooling [22], this inequality is not met. The coolant gas may then have a different temperature than the trapped ions, as indeed it must have in order to cool the trap. A relevant objection to the foregoing is that the Debye

length is approximately an order of magnitude larger than the EBIT electron-beam radius, and the plasma parameter should more reaUstically be given by A = (4';r/3)«g\or^ . The value of logA is then =14, and our conclusions still hold.

B. Ionization, Recombination, and Loss Rates Cross sections for electron-impact ionization are taken from Ref. [34] for levels up to the « = 3 . The cross sections

are essentially identical to those coining from the Lotz for- mula (see e.g., Ref. [35]). For the monoenergetic EBIT beam

the rates are formed by simply multiplying the cross section by the electron velocity. Radiative recombination cross sections are taken in the Kramers' approximation (see e.g., Ref.

[36]). We sum the radiative recombination cross section to each of the n = 2 and « = 3 configurations to arrive at the

total cross section. Other workers [35] have used a formula

due to [37] that sums the radiative recombination to « = <». This sum, however, includes only recombination to s states. In our model we neglect the dielectronic recombination

because for the beam energies at which our spectra were

taken (around 6 keV and 12 keV), it is unlikely that any

dielectronic resonances will exist in ions with ionization potentials in the range 1 keV to 4 keV.

Charge-exchange recombination with neutral Kr atoms in- side the trap are treated in a simple level crossing model. The

potential of the ion-neutral system is written (in atomic units, where e=mg = h=\) as V= —/^+(9— l)/r= — /q, where

Iq and Iq are the ionization potentials of the ion (after the charge-exchange process) and neutral (before charge ex- change), respectively, and r is the ion-neutral distance. Hence at a pseudocrossing r„=(9— 1 )/(/, — /q), and the cross section is given by (Tcx~ '""'"L ' subject to the constraint

that r,^^{2q^'^+ 1 (see Ref. [38] for details). During the EBIT runs described in this paper, it was not possible to get an accurate measurement of the neutral Kr density near the trapped ions (assuming that the charge exchimge occurs be- tween highly charged trapped ions and neighboring Kr neutrals) and so the above formulas were used to model the

relative charge-exchange rates for the various Kr ions, with

the absolute normalization left as a free parameter to vary when optimizing the model prediction to the observations. The rates for ion reactions with electrons (impact ionization, radiative recombination) need to be modified to account

for the time that the ions spend outside the electron beam in radial excursions. We neglect the azimuthal drift imposed on

the ion orbits by the magnetic field. The potential in which

the ions move is given by the solution to Laplace's equation

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PHYSICAL REVIEW E 63 046409

in cylindrical symmetry, subject to the boundary condition

that at a radius equal to the drift tube radius (r^,), the potential is 300 V (the voltage applied to form the electrostatic

trap). Then the fraction of time that an ion spends inside the beam is given by

f'^bdr ( fri, dr rr„ dr]

f= _/ _+ _ , (3) Jo y [Jo y Jrfc y

J where /-^ and r„ are the electron-beam radius and maximum

ion radius, and v = v{r) is the ion velocity given by ntiV^/l = 3kTif2—qeV. Evaluating the integrals (using r — rh^rf, in the second) we arrive at the following expression for f.

80

arcsm

iacsinyfr}+ sinh V4+I77

(4)

where r}={qe\Qll\)l6-neQkTi and r„ = r^exp[3^r,7r^

^{qe\Qll\)—\^ with Qll being the charge per unit length of

the electron beam. This fraction is implemented in our model

neglecting the compensation of the trapped positive ions, consistent with an ion density of 6X10''*ra~^ assumed

above. The remaining process to be considered is that of ion esc^)e from the trap. We follow the treatment of Ref. [35],

assuming only radial escape to be important. They give an

expression for the ion loss rate dN^

dt

(5)

where A^^ is the number of ions of charge q and AAT^ is the

fraction of them with energy E>qeVi,, and Vj, is the trap- ping potential, i.e., the number of ions energetically allowed

to esc£^ from the trap. The escape rate v^^^ape is given by

"""" r,{r,-ry) l+{T,qeB/mi) = 10^ (6)

which is derived from standard results for cross-field diffusion. For ions of charge q from 26 to 32, AiV^/A^^< 10~^

giving a maximum loss rate of order 10 s~ ' . This is an order

70 -

60

m 50 J ^ 40 -

CD>

n 30- o

tr 20 -I

Experiinental data model w/o CE

model with CE

2p'-2p*3s

1600

I I I

I I I I II 1650 1700 1900

Energy (eV)

FIG. 2. Comparison of Ne-like Kr 2p^-2p^3s and Ip^-lp^ld

lines (set IV, Table I). Charge exchange. Synthetic spectra are nor- malized using 2p^-2p^3d lines.

of magnitude lower than the collisional ionization and re- combination rates and is neglected.

C. Modeling and Fitting of Charge State Distributions A charge state distribution was fitted to the experimental

data by the following procedure. Model spectra for each Kr

iotiization state (Al- to Be-like) were computed for the rel- evant EBIT beam parameters using the hullac suite of

codes (Table ID). These calculate the energy levels and ra- diative decay rates using a parametric potential, and the elec- tron impact excitation cross sections in the distorted-wave

approximation. The total synthetic spectrum is a sum of

spectra of the ten (Al- to Be-like) different Kr ions where

individual spectra are scaled with the relative charge state fraction. This total spectrum was fitted to the data using the charge state fractions as the fitting parameters on the as- sumption that electron-impact excitation was the sole excita- tion process, and a charge state distribution was obtained. Generally the agreement between experimental and fitted synthetic spectra was not satisfactory [Fig. 1(a)]. In particular, we noted a large discrepancy for the Ne-like and F-like TABLE III. Configurations included for Kr ions in HULLAC calculations.

Ion Configurations

UBe- BC-NOF-Ne- Na- Mg2s 2p 3s 3p 3d

2s^2s2p2p^2s3s2s3p2s3d

2s^2p 2s2p^ 2p^ 2s'3s 2s^3p 2s^3d 2s2p3s 2s2p3p 2s2p3d

2s^2p^ 2s2p^ 2p* 2s^2p3s 2s^2p3p 2s^2p3d

2s^2p^ 2s2p* 2p^ 2s^2p^3s 2s-2p^3p 2s^2p^3d

2s^2p* 2s2p^ 2/7* 2s^2p^3s 2s^2p^3p 2s^2p^3d

2s-2p^ 2s2p^ 2s^2/3s 2j^2/3p 2s^2p'*3d 2s2p^3s 2s2p^3p

2s^2p^ 2s^2p^3s 2s^2p^3p 2s^2p^3d 2s2p^3s 2s2p^3p 2s2p^3d

2s^2p^3s 2s^2p^3p 2s-2p^3d 2s^2p^3s^ 2s^2p^3s3p 2s^2p^3s3d 2s2p^3s^ 2s2p^3s3p 2s2p^3s3d

2s^2p^3s^ 2s^2p^3s3p 2s^2p^3s3d 2s^2p^3p^ 2s-2p^3p3d 2s^2p^3d^ 2s^2p^3s'3p 2s^2p^3s^3d

2s2p^3s^3p 2s2p^3s^3d

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TABLE IV. 3 s/3d intensity ratios for some transitions in the Ne- and F-like Kr.

Ion LeveP Model Model Exp."

w/o CE with CE "

Ne-Iike 3s ^P°2 0.11 0.38 0.5 3j 'P°, 0.44 0.60 0.9 3s V, 0.24 0.34 0.4

F-like 2pVD)3s (2,1/2)5^2 0.1

1

0.25 0.3 2pVP)3s {2,m)sa 0.12 0.43 0.4 2p\'D)3s (2,1/2)3^2 0.18 0.26 0.3

intensity ratios have been calculated for transitions from these levels to the ground level relative to 3<f 'P, ground level and 2p''('D)3d (2,3/2)5/2—* ground level transitions in Ne- and F-like Kr, respectively. ''Charge exchange (CE), see discussion in text. TBoth experimental and theoretical data have been taken for 6 keV, low pressure (set IV, Table I) conditions. Other spectra showed qualitatively similar behavior.

charge states, where substantially more intensity was observed in the 2>s-^2p lines relative to the 3d—*2p Unes of

these ions (Fig. 2). The observation that these intensity ratios deviated more from the theory for the 6 keV Kr spectrum at high pressure than for the low pressure led us to suspect that charge exchange might play a role in exciting these lines. The fitted ionization balance was then matched with a model with suitable choices for the normalization of the charge exchange recombination rates and the ion temperatiu-e

in the trap. The emission spectrum from each ion was recal- culated now including the effects of charge exchange, ap- proximated by assuming that each level considered in the

target ion was populated according to its statistical weight, followed by radiative cascading down. In this way the 35

population in Ne- and F-like ions is boosted, since significant extra population cascades through these levels. In Fig. 2 the

strongest 3 j and 3d Unes are shown together with the calculated spectra; the numerical values are presented in Table IV. The spectra from each ion iterated in this way were refit- ted to the data treating the charge exchange rates and the

temperature as additional fitting parameters. The final fitted charge state distribution is presented in Table V.

V. DISCUSSION

From Figs. 1 and 2 it can be seen that our modeling ap- proach gives a satisfactory account of the relative line intensities, at least for the strong well detected lines, over a wide

range of charge states. We draw particular attention to the

relative intensities in the Ne-like charge state Kr XXVII.

From Fig. 2 it can be seen that irrespective of the inclusion of charge exchange in our model, the intensity ratio between

the 2p^ 'So-2p53rf^Di and the 2p^ ^So-2p^3d 'P, lines agrees with our calculations. This is quite different from the

isoelecttonic system Fe XVII, where discrepancies between

theory and observations are well documented in the solar physics literature [39-44], and are becoming apparent in x-ray observations of stellar coronae as well ( [45] and ref- erences therein). The ratios of 2p^—2p^3s Unes to those from the 2p^—2p^3d configurations, which are also probTABLE V. Fitted and modeled charge state distributions.

Ion 12 keV 6 keV (low pressure) 6 keV (high pressure)

Fit^ Model'' Fit Model Fit Model

Li- 0.010 0.021 0.024 0.011 0.003 0.0009

Be- 0.097 0.070 0.10 0.051 0.060 0.008 B- 0.17 0.14 0.16 0.13 0.091 0.032 C- 0.19 0.20 0.17 0.21 0.096 0.085 N- 0.16 0.20 0.13 0.23 0.14 0.15 O- 0.14 0.16 0.14 0.18 0.11 0.20

F- 0.087 0.010 0.10 0.11 0.17 0.21 Ne- 0.066 0.060 0.085 0.055 0.17 0.17 Na- 0.026 0.025 0.040 0.018 0.081 0.093 Mg- 0.019 0.008 0.022 0.004 0.044 0.037

"Fitted ionization balances are determined by least-squares fits to experimental data, using theoretical line intensities for each ion, iterated as described in the text to include the charge-exchange contribution. Vodel ionization balances are calculated by using the ionization and recombination rates described in the text, with the magnitude of the charge-exchange rates adjusted to give the best match to the "fitted" ionization balance.

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PHYSICAL REVIEW E 63 046409

lematic in solar and astrophysical spectra, are harder for us to check experimentally because of the problems of charge ex- change in our experiment enhancing the 2p^ — 2p^3s intensities. These effects of charge exchange may however allow the measurement of state selective charge-exchange cross sections, relative to those for electron-impact excitation or ionization. As well as applications of neutral beam heating in tokamaks, we expect such data to be very useful in interpretation of cometary x-ray spectra [46-48]. Here, neutral material outgassing from the comet nucleus encounters highly

charged ions in the solar wind. The ensuing charge-exchange

collisions populate highly excited states of the solar wind

ions, followed by x-ray emission as these states radiatively

decay. An astrophysically relevant experiment would require

trapped ions of C, N, O, or Ne, and the admission to the EBIT chamber of the appropriate neutral gas, i.e., H2O, CO2,

etc.Among our line identifications are several lines with potential for electron-density diagnostics. The density sensitivity arises because the relative populations among the differ- ent levels of the ground configurations varies with density. If this results in higher population accumulating in a level of

higher angular momentum (i.e., the 2s^2p ^Pyi in Kr

XXXn, the 25^2/72 in Kr XXXI, or the 25^2^^ ^D^a

Kr XXX) than the ground level, then excitations to levels among the n = 3 configurations with higher angular momentum than are possible from the ground state alone, result. Hence new lines from these high angular momentum level

states become visible at higher densities. In Fig. 3(a) we show two line intensity ratios in Kr XXXII

that vary as a result of n = 3 , y = 5/2 levels becoming excited, calculated for an electron temperature of 2 X 10^ K. The

electron densities over which this ratio varies is lO'^ — 10^' cm~^, which is the relevant density range for high Z

underdense plasma radiators. This should be very useful, since the density diagnostic available in the Kr AT-shell spec- trum, the ratio of the ^Sq—\s2p^P2 to either of the ^Sq—\s2p^ ^Pj lines only becomes sensitive at densities greater than 10^' cm~^, i.e., higher than is usually

achieved in such experiments. One solution to this is to sus- pend pellets of material of slightly lower Z in the Xe filled enclosure, so that the corresponding He-like transitions are

sensitive in the right range. This would be most appropriate

in the hottest regions of the Xe enclosure, since Kr and similar ions will be ionized beyond the B-Uke charge state, but elsewhere the B-like density diagnostic we discuss might be

preferable. We note that previously discussed L shell diagnostics in Xe [6,49] for the electron temperature are still dependent on the electron density, due to the density sensitivity of the dielectronic recombination process and the ionization balance itself in such experiments. The density diagnostic we identify is much less dependent on the electron temperature, and would go some way towards resolving such

ambiguities. Further density sensitive line ratios at the same electron temperature in Kr XXXI and Kr XXX are plotted in Fig.

3(b), which may have applications to tokamak plasmas with

density in the range iO'^- lO''* cm"^.

I I

10 r

1 r

- Kr XXXII 2183.7/2126.1

' 17 18 19 20 21 22

(a) Lo9(Electron density in cm"^

0

100

1

1 1 1

*« 10 c

Kr XXX 2047.8/2048.9- ^^^s^::,::^^

1 Kr XXX 2047.8/1 998.r Kr_ XXXI 2113.6/2p60,1

12 13 14 15 16 17

(b) Loq(Electron density in cm"^

FIG. 3. Electron-density (logK/i^) dependence of line intensity

ratios at electron temperature = 2 X 10^ K calculated with a model described in Sec. IV for selected transitions in Kr XXXXXXn (identified in Table H).

VI. CONCLUSION

We have demonstrated the value of using an EBIT plasma

model for the analysis of complex and only partially resolved

x-ray spectra obtained with a broadband high-resolution x-ray detector. This approach can be applied in the determination of numerous physical characteristics of the EBIT

plasma, such as measurements of important reaction rates and the validation of diagnostic line ratios. In this paper we

have identified many new lines in the spectra of L-shell Kr

ions, some with diagnostic potential applicable in other fields of plasma physics, and have made progress towards understanding conditions in the EBIT trap itself.

ACKNOWLEDGMENTS

The authors thank D. J. Alderson for his technical support. I.K. acknowledges support from the Swedish Foundation for Cooperation in Research and Higher Education

(STINT). J.M.L. was supported by NASA Contract W

19353 (Applied Information Systems Research Program) and

ONR/NRL 6.1 Research Option "Solar Magnetism and the Earth's Environment."

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046409-10

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The Astrophysical Journal, 545:L161-L164, 2000 December 20 © 2000. The Ameiican Asimmimicil Siiticiy. All nglus reserved. Pnnied in U.S.A.

EMISSION-LINE INTENSITY RATIOS IN Fe xvii OBSERVED WITH A MICROCALORIMETER

ON AN ELECTRON BEAM ION TRAP

J. M. Laming' I. Kink,^ E. Takacs,' J. V Porto,^ J. D. Gillaspy/ E. H. Silver," H. W. Schnopper," S. R. Bandler,*

N. S. Brickhouse," S. S. Murray," M. Barbera,' A. K. Bhatia,'* G. A. Doschek,' N. Madden,^

D. Landis,^ J. Beeman,' and E. E. Haller''

Received 2000 June 16: accepted 2000 October 11; published 2000 December 6 ABSTRACT

We report new observations of emission line intensity ratios of Fe xvii under controlled experimental conditions,

using the National Institute of Standards and Technology electron beam ion trap (EBIT) with a microcalorimeter

detector. We compare our observations with coUisional-radiative models using atomic data computed in distorted wave and /?-matrix approximations, which follow the transfer of the polarization of level populations through

radiative cascades. Our results for the intensity ratio of the '5o-2/7'3ii '/? 15.014 A line to the 2/?* '5o- 2p^3d^D, 15.265 A line are 2.94 ± 0.18 and 2.50 ± 0.13 at beam energies of 900 and 1250 eV, respectively.

These results are not consistent with collisional-radiative models and support conclusions from earlier EBIT work

at the Lawrence Livermore National Laboratory that the degree of resonance scattering in the solar 15.014 A line has been overestimated in previous analyses. Further observations assess the intensity ratio of the three lines between

the 2p^-2p^3s configurations to the three lines between the2p''-2p^3d configurations. Both i?-matrix and distorted wave approximations agree with each other and our experimental results much better than most solar and stellar observations, suggesting that other processes not present in our experiment must play a role in forming the Fe xvii spectrum in solar and astrophysical plasmas.

Subject headings: atomic data —methods: laboratory —stars: individual (Capella) —Sun: corona —

techniques: spectroscopic —X-rays: general

The high elemental abundance of Fe and the closed shell structure of Ne-like ions cause Fe*"* to be one of the dominant

ions in forming emission-line spectra from plasmas with temperatures ~5 X 10** K. Fe xvn lines will be the dominant lines

in most spectra obtained by gratings on Chandra and XMMNewton from a wide variety of objects. For example, in the Capella spectra acquired by Chandra (Brinkman et al. 2000; Canizares et al. 2000), Fe xvii contributes four or five of the six strongest lines observed. In conditions of ionization equilibrium, Fe xvn is unique in providing an electron temperature diagnostic

between two sets of these strong lines (Raymond & Smith 1 986). Solar flare observations have long pointed to discrepancies be- tween the observed intensity ratios among the strong emission

lines of Fe xvn in the 15-17 A region. The Capella spectra referred to above hint that similar problems may exist for astro- physical sources as well. Among the six strong lines arising from

transitions between the ground state 2p* '5o excited states 2p^3s '/^, '/^, 'Pj, and 2p'3t/ 'D,, and the strongest line from 2p'3^f '/^ at 15.014 A often appears diminished in intensity relative to other features. This has led to suggestions that it may

be affected by resonance scattering that removes photons pre-

' E. O. Hulburt Center for Space Research, US Naval Research Laboratory, Washington, DC 20375.

' National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899.

' Massachusetts Institute of Technology. 77 Massachusetts Avenue, 25-239, Cambridge, MA 02139-4307; and University of Debrecen, Debrecen, Bem ter 18/A. H-4026, Hungary.

* Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge. MA 02138.

' Osservatorio Astronomico G. S. Vaiana, Piazza del Parlamento 1, 90134

Palermo, Italy.

' Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771.

' Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA

94720.

dominantly from the line of sight in this transition (Schmelz,

Saba, & Strong 1992; PhiUips et al. 1996, 1997; Bhatia & Kastaer

1999; Saba et al. 1999). Furthermore, the overall intensity of the

lines from the 2p'3s configuration often appears enhanced rel- ative to the intensity theory would predict when compared with

the lines from the 2

p^ 3d configuration. This has led some authors

(U. Feldman 2000, private communication; see Sampson &

Zhang 1987) to suggest that an inner shell ionization of a 2p

electron from the Na-like Fe ion plays a role in forming the spectrum, producing Ne-like Fe in the excited configuration

2p^3s. Other explanations suggest that strong dielectric recombination occurs (Liedahl et al. 1990) or simply that the theoretical atomic models do not include a sufficient number of excited

levels to adequately treat all the radiative cascades. In this Letter we describe initial results of experiments designed to test some

of these ideas. In a study of tokamak spectra of Fe xvii, Phillips et al. (1997) found good agreement between observations and synthetic spectra calculated from models including the lowest 37 fine-structure

levels of Fe xvn (i.e., all configurations up to 2s2p^3d), in- cluding dielectronic and inner shell satellites of Fe xvi. The

collisional data used in Phillips et al. (1997) are similar to some

of those used here. They are computed in the distorted wave

approximation and tabulated in Bhatia & Doschek (1992). To- kamak plasma sources do not allow the individual atomic processes to be experimentally isolated and studied in detail. More

control is possible with the use of an electron beam ion trap (EBIT). Positive ions are trapped in the space charge of an

essentially monoenergetic electron beam that also ionizes and

excites the ions. The ion charge state is selected by tuning the electron beam energy. In the case of Ne-bke ions, almost complete selectivity is possible. Taking advantage of this. Brown et

al. (1998) made detailed observations of Fe xvn at beam energies of 850, 1 150, and 1300 eV. They observed lines originating from

levels with principal quantum number n up to 11. Their mea- L161

NIST-224
LI 62 EMISSION-LINE INTENSITY RATIOS IN Fe xvii Vol. 545

fiOO 700 800 900 1000 1100 1200 1300 1400 1500 1600 Energy (aV)

Fig. 1.—Section of a raicrocalorimeter spectrum of Fe L shell lines recorded

at an EBIT beam energy of 2.54 IceV. Lines from Fe xvii to Fe xxni are prominent.

surement of the intensity ratio of the strongest 15.014 A line to the 15.265 A line originating from the 2p^3d^D^ level was

3.04 ± 0.12, which is in between theoretical values (generally

closer to 4) and ratios observed in solar and stellar coronae

(usually in the range 2-2.5). Table 2 of Brown et al. (1998) gives

a useful summary of the various calculations and observations

of this intensity ratio. However, Brown et al. (1998) in their paper report no results for the intensities of the 3s-3p transitions. Our experimental technique couples the National Institute of

Standards and Technology (NIST) EBIT with a microcalorimeter

detector and is discussed in detail in Silver et al. (2000). This

offers a combination of high throughput, wide bandpass, and

sufficient spectral resolution to allow most lines to be resolved

(see Kelley et al. 2000; Silver et al. 2000). One modification from previous experimental runs was to move the microcalorimeter to a new observation port on the EBIT, which allowed much

closer access to the trapped ions in the electron beam. This further improved the high-energy bandpass and allowed us to dispense

with the X-ray optic. Another important feature is that the microcalorimeter when used in this configuration is polarization

blind. Thus, the only correction required for this effect is for the degree of spatial anisotropy in the emission of polarized light; the measured intensity will depend on the degree of polarization

through this anisotropy, which is different for lines of different

polarization. As we discuss below, such effects are rather small and can be adequately modeled. A part of a spectrum taken at a beam energy of 2500 eV is shown in Figure 1 to illustrate the

capabilities of the detector. These data were collected in only

15 minutes. The complete spectrum extends to much higher

energies and would include the Fe K line complex if the beam

energy was sufficiently high. Of course, in this case the Fe L

spectrum would not be so strong, and it is these lines, and specifically Fe xvn, that are the main focus of this Letter. We observed Fe xvn transitions at a variety of electron beam

energies between 900 and 4000 eV. Comparing spectra at beam

energies of 900 and 1250 eV allows us to study what effect (if any) enhanced radiative cascades at the higher energy might

have. At 900 eV, we are also well below the thresholds for

excitation by inner shell ionization of Fe xvi (Sampson & Zhang

1987) should any be present in our trap (see below). For these two beam energies, experimental intensity ratios were obtained by fitting a sum of Gaussian functions to the experimental data.

650 TOO 7S0 800 850 900 Energy (eV)

Fig. 2.—Fit to the six strong Fe xvn lines recorded at an electron beam

energy of 900 eV. The line wavelengths are 15.014, 15.265, 15.456. 16.780. 17.055, and 17.100 A. The last two transitions are blended together at the resolution of the microcalorimeter and are fitted as a single component. The

upper panel gives the residuals from the fit.

and the area underneath a particular Gaussian was treated as a

line intensity. The Gaussian functions slightly underestimate the "wing" of the spectral lines, and better resemblance is achieved

with Voigt functions. However, the results from these different

fits remained well within quoted error bars, determined from the

statistical quality of the fit (dominant uncertainty) and systematic

uncertainties (much smaller contributions) due to the detector/ window efficiency, and therefore the results from Gaussian fits are presented. The data, fit, and residuals for the 900 eV beam

energy spectrum are shown in Figure 2. At these beam energies, there is no evidence of inner shell transitions in Fe xvi. The

strongest of these lines, the 2s^2p''3s ^Su-j-2s'^2p^3s^ ^P^^, cal- culated to be at a wavelength between 17.29 and 17.31 A

(Phillips et al. 1997; Bautista 2000), would be easily visible in our spectra, resolved from the Fe xvii 17.055 and 17.100 A

lines. Using higher resolution crystal spectrometers observing one of the Lawrence Livermore National Laboratory EBITs under similar conditions. Brown et al. (1998) also saw no evidence

for these transitions. The spectra were corrected for the transmission efficiencies of the microcalorimeter windows. (The quantum efficiency of

the detector is 100% for energies below 5 keV.) The window

transmission has been well defined for the entire microcalor- imeter bandpass of 0.1-10 keV. The three windows are made

of polymide (800 A) and aluminum (1 100 A), and their thick- nesses have been measured to an accuracy of 0.5% (Powell et

al. 1997). The efficiency curve is smooth and featureless in the energy band that includes the Fe L emission. We have taken

great care to minimize potential contamination of these windows, but over long periods of time (days) it is possible that minute amounts of hydrocarbons could collect on the outer window (the most susceptible since it views the vacuum connection to the EBIT). Our calculations show that only several monolayers of nitrogen or oxygen would freeze out on the window given the cleanliness of the EBIT vacuum connection and the extremely low pressures at the initiation of the cooling

cycle. Since the Fe xvn lines were measured over a period of

NIST-225
No. 2. 2000 LAMING ET AL. LI 63

TABLE 1 Observational and Theoretical Fe xvii Intensities Relative to the 15.014 A Line

, ,,, ^ Theoretical (0.9 keV)" Theoretical (1.25 keV)' Line Wavelength Energy Upper i '__

(A) (eV) Level Data (0.9 keV) Distorted Wave ^-Matrix Data (1.25 keV) Distorted Wave y?-Matrix

15.265 812.5 2p'3J'0, 0.34 ± 0.02" 0.25 0.25 0.40 ± 0.02 0.26 0.26

15.456 802.5 2p'3J'P, o.lO ± 0.01 0.047-0.036 0.059-0.046 0.09 + 0.01 0.035-0.027 0.034-0.027

16.780 739.1 2p'2s-P,' 0.45 ± 0.04 0.46-0.41 0.47-0.41 0.40 ± 0.02 0.51-0.44 0.47-0.42

17.055 727.1 2p'3s'P,' 0.54-0.47 0.60-0.53 0.58-0.50 0.55-0.47

17.100 725.0 2p-2s'Pj 0.34-0.30 0.43-0.38 0.41-0.35 0.39-0.33 17.055+ n.lOC 0.88 ± 0.09 0.88-0.77 1.03-0.91 0.92 + 0.12 0.99-0.85 0.94-0.80

Theoretical range goes from unpolarized to maximum polarization case, /{-matrix cross sections are from Mohan et al. 1997 substituted for first 26 excited

levels.

' Uncertainties correspond to 1 <t.

' Some authors swap the LS coupling notation for these levels, with 16.780 A originating from 'f, and 17.100 A from

'' Experimental result is for the sum of 17.055 and 17.100 intensities; theoretical results are for each line separately and their sum.

1 hr (as opposed to several days), the chances of a contamination buildup occurring is unlikely. Furthermore, we point out

that the Fe xvii line ratios measured by our instrument are

relatively insensitive to even a large amount of contamination. For example, 3000 A of nitrogen or oxygen ice formed on the outer cryostat window could reduce transmission by 25% but only alter the ratios for the 3s/3d lines by 9%; the 2p-3d line ratios would remain unchanged. This scenario is highly unlikely since 3000 A of N or O ice is 2 times thicker than the window

itself and would undoubtedly break it. We compare our observations with theoretical results computed using a model Fe xvii ion comprising the configurations up to Islp^Ad, i.e., 73 levels, with extra configurations

2s^2p^ns, Is^lp^nd, and Islp^np for n = 5 and 6 included

for their radiative cascades. This brings the total up to 113 levels. Excitation rates are calculated for monoenergetic electron beams

between 0.9 and 4.0 keV energy, including only those levels that are energetically accessible in each case. The calculations use impact excitation cross sections computed in a distorted wave

approximation as described in Bhatia & Doschek (1992) for all transitions among the lowest 73 levels. As an alternative, Rmatrix cross sections from the ground state to the configurations up to 2p^3d (i.e., the first 26 excited levels) can be substituted from Mohan, Sharma, & Eissner (1997). Energy levels and ra- diative decay rates are taken from the distorted wave target cal- culation, supplemented with those for higher lying configurations taken from a computation with the HULLAC (Hebrew University Lawrence Livermore) Code for Atomic Physics (M. Klapisch, A. Bar Shalom, W. H. Goldstein, E. Meroz, A. Chon, &

M. Cohen 1988, unpublished; Goldstein et al. 1988). Collisional excitation rates to these higher levels are scaled from those for the lower lying ones. For the purposes of modeling emission from the EBIT, we

follow the analysis in Takacs et al. (1996) and expand our 113-

level model to the 457 magnetic sublevels present. This is done

in order to account for the polarization arising from the excitation by the electron beam. For the upper levels of the strongest transitions (i.e., those with j = 1), excitation cross sections to in- dividual magnetic sublevels are given in the relativistic distorted wave approximation by Zhang, Sampson, & Clark (1990). For

transitions where a nonrelativistic approach is valid, these data

are supplemented with simple results from a Coulomb-Bethe

approximation.

All levels with j >3 are assumed to be unpolarized. More

accurate nij dependent cross sections will be substituted as they

become available, but the current procedure should model the transmission of polarization through radiative cascades sufficiently accurately to allow us to evaluate the anisotropy of the emission from the EBIT. More precise work would be necessary with crystal spectrometers.

In Table 1 we compare our experimental Fe xvii line intensity

ratios with those calculated by the methods outlined above for beam energies of 0.9 and 1 .25 keV. The theoretical results come

from the distorted wave cross sections calculated herein and from

the same distorted wave results but with the cross sections for excitation from the ground state to the first 26 excited levels replaced by the /?-matrix results of Mohan et al. (1997). The

range of theoretical ratios extends from results calculated in the

limit of zero polarization to those in the limit of maximum polarization. The finite distribution of pitch angles in the electron beam causes the true theoretical result to lie somewhere between

these two limits. This depolarization can be estimated from re- sults given in Gu, Savin, & Beiersdorfer (1999). Even in the

limit of no depolarization, the polarization correction required

for the microcalorimeter is rather small, less than would be the case for a crystal spectrometer, but remains the dominant sys- tematic uncertainty in our experiment at these two beam energies.

Brown et al. (1998) measured intensity ratios for the 15.014/ 15.265 A lines of 2.93 ± 0.16, 3.15 ± 0.17, 2.77 ± 0.19, and

3.00 ± 0.20 from observations at beam energies of 1 150, 1 150, 850, and 1300 eV, respectively. They then average these values

to give a mean result of 3.04 ± 0.12, compared with theoretical predictions of around 4. Explicit in this argument is an assumption that the polarizations of these two lines are the same, since

the observations were made with crystal spectrometers at Bragg

angles where they are polarization sensitive. Such an assumption

is supported by theory (Zhang et al. 1990). However, the result obtained using this assumption demonstrates that this same the- ory gives an intensity ratio between these two lines inconsistent with experiment. While it is quite possible that the calculations get the polarizations right and the intensity ratio wrong, it is certainly not guaranteed. The microcalorimeter employed in the present work is not polarization sensitive, and our experimental

values for the intensity ratio—2.94 ±0.18 at 0.9 keV and

2.50 ± 0.13 at 1.25 keV—are more robust, being sensitive only

to the possible difference in the anisotropy of the emitted ra- diation. The first value is consistent with the average result of Brown et al. (1998), while the second is slightly lower. These

results support the suggestion of Brown et al. (1998) that the

effect of resonance scattering in the 15.014 A resonance line in solar spectra has hitherto been overestimated.

Intensity ratios from our data for the three lines originating

from 2p 3s to those from 2p'3d are shown in Figure 3. For

energies greater than 1.25 keV, they are obtained by summing

NIST-226
L164 EMISSION-LINE INTENSITY RATIOS IN Fe xvn Vol. 545

4v

6-

.

, H-matrix / ,DW

ixperimental

3'

5

'_

ao 0-5 1.0 1.5 2.Q 2.5 3.0 3.5 4.0 4.5 5.0 Energy (KeV)

Fig. 3.—Variation of the intensity ratio (/hth, + + /,7.ioo) / Unoit +

Ajjm + ^isisi) witli electron beam energy. The first three lines originate in the 2f^s configtjranon and the second three in the 2p'3d. For comparison, results from model calculations using the distorted wave results (see Bhatia & Doschek

1992) are shown, and where appropriate the /{-matrix results of Mohan et al. (1997) are overlaid. The range given for each calculation corresponds to the

limits of unpolarized (upper curve) and completely polarized (lower curve)

emission. The observational ratios from various solar and stellar observations

are shown for reference on the same plot. For these, the energy scale on the X-axis should be disregarded. Fe xvii is generally formed in plasmas at a temperature of 5 x 10' K, and electrons with energies close to threshold, i.e., less than 1 keV, dominate the excitation rate. The references are (1) Blake et

al. (1965); (2) Parkinson (1975); (3) Hutcheon, Pye, & Evans (1976);

(4) McKenzie et al. (1980); (5) Phillips et al. (1982); (6) Acton et al. (1985);

(7) Brinkman et al. (2000); and (8) Canizares et al. (2000).

the number of detected photons over the spectral regions where

the Ip^-lp^lis and 2p^-2p'3d lines appear, respectively,

correcting for the presence of the Fe xix lines at 15.114 and

15.210 A (820 and 815 eV, respectively), by measuring the strong Fe XIX 2p'-2p^3d lines at 13.465, 13J07, and 13.521 A (920

eV) and subtracting off the appropriate ratio determined from

calculations with HULLAC. Since Fe xix has a ground-state,

polarization corrections are negligible. The calculated ratios, in the range 0.25-0.33 for beam energies 2-4 keV, also agree well with higher resolution observations of solar active regions (re- cendy reassessed by Phillips et al. 1999). The error bars are estimated from the Poisson counting statistics and uncertainties

arising from background subtraction (i.e., the Fe xix lines) and

in the detector/window quantum efficiency but are dominated

by a ±20% error in the measured intensity of the Fe xix blend

at 920 eV. Also shown in Figure 3 are observational ratios of

the three 3s lines to the three 3cl lines taken from various solar and stellar observations. The experimental ratios for the electron beam energy of 0.9 keV suggest that the models including the /?-matrix cross sections

are less preferable than those using only distorted wave cross

sections. The former cross sections appear to overestimate

slightly the emission in the 17.055 and 17.100 A lines. Elsewhere we have a small preference for the /?-mauix results, altiiough we caution here that a small contribution to the Fe xvii emission may arise by recombination from Fe*'^ into excited states of Fe xvii, and until this effect is adequately modeled firm conclusions might be premature. Our experience with similar spectra of Kr xxvii (Kink et al. 2000) suggests that die inclusion of recombination would increase the theoretical 3s/3d ratios. More

importantly, the difference between the two theoretical ap- proaches and our experiment is significantly smaller at all energies than the discrepancies between the solar and astrophysical observations summarized in Figure 3 (with the exception of

Phillips et al. 1982) and either theory. The basic electron impact

excitation theory for these lines in Fe xvii appears to be correct,

in contrast with that for the 15.265/15.014 ratio (see Table 1), and one must look to other processes to model the solar and

astrophysical observations satisfactorily. These may invalidate the electron temperature diagnostic of Raymond & Smith (1986). These measurements are a subset of a larger survey of spec- troscopy experiments performed with the microcalorimeter on

the NIST EBIT. Future work will concentrate on acquiring

spectra with significantly higher statistical quality, simply by

increasing the integration time at each beam energy. This might

also allow the simultaneous observation of radiative recombination features from the EBIT.

A. K. B., G. A. D., and J. M. L. were supported by NASA

contract W19539 (Applied Information Systems Research Pro- gram); G. A. D. and I. M. L. were also supported by the NRL/

ONR Solar Magnetism and the Earth's Environment 6.1 Research Option. E. H. S., H. W. S., and S. R. B. acknowledge

support in part by NASA grant NAG5-5104, and I. K. ac- knowledges support from the Swedish Foundation for Cooperation in Research and Higher Education (STINT). We also acknowledge the comments of an anonymous referee, partic- ularly in regard to the Fe xix hnes.

REFERENCES

Acton, L. W., et al. 1985. ApJ, 291, 865

Bautista, M. A. 2000, J. Phys. B, 33, 71 (erratum 33. 2833)

Bhatia, A. K.. & Doschek, G. A. 1992. At. Data Nucl. Data Tables, 52. 1 Bhatia, A. K.. & Kasmer. S. O. 1999. ApJ. 516. 482

Blake. R. L.. Chubb, T A., Friedman. H.. & Unizickei; A. E. 1965. ApJ. 142,

1 Brinkman, A. C. et al. 2000. ApJ, 530. LI 1

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Brown, G. V.. Beiersdorfer, P.. Liedahl. D. A.. Widman. K., & Kahn, S. M.

1998, ApJ, 502, 1015 (erratum 532, 1245 [2000])

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Goldstein. W. H., Oreg. J., Zigler, A., Bar Shalom. A., & Klapisch, M. 1988.

Phys. Rev. A, 38. 1797

Gu. M. F.. Savin. D. W. & Beiersdorfer. P. 1999. J. Phys. B, 32. 5371

Hutcheon, R. H., Pye, J. P., & Evans, K. D. 1976, MNRAS. 175. 489

Kelley, R. L., et al. 2000. Nucl. Instrum. Methods Phys. Res.. 444. 170

Kink, I., et al. 2000, Phys. Rev. E., submitted Uedahl, D. A.. Kahn, S. M., Osterheld, A. L., & Goldstein, W. H. 1990. ApJ.

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R. M.. Feldraan, U., & Doschek, G. A. 1980. ApJ, 241, 409

Mohan, M., Sharma, R., & Eissner, W. 1997. ApJS. 108. 389

Paricinson. J. H. 1975, Sol. Phys., 42, 183

Phillips, K. J. H.. et al. 1982, ApJ, 256. 774

Phillips, K. J. H., Greer, C. J.. Bhatia, A. K.. Coffey. I. H.. & Keenan, F. P. 1997, A&A. 324, 381

Phillips, K. J. H., Greer. C. J., Bhatia, A. K., & Keenan, F. P. 1996. ApJ, 469. L57

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Powell. F. R.. et al. 1997, Proc. SPIE. 31 13. 432 Raymond. J. C, & Smith. B. W. 1986. ApJ. 306. 762

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Schmelz. J. T. Saba. J. L. R.. & Strong. K. T 1992. ApJ. 398. LI 15

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NIST-227
Physica Scripta. T92, 454-456, 2001

Microcalorimeter/EBIT Measurements of X-ray Spectra of Highly

Charged Ions

I. Kink', J. M. Laming^ E. Takacs^ J. V. Porto', J. D. Gillaspy', E. Silver'*, H. Schnopper'*, S. R. Bandler"*, M. Ba^bera^

N. Brickhouse^ S. Murray^ N. Madden^ D. Landis^ J. Beeman^ and E. E. Haller*

' National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

^ Naval Research Laboratory, Code 7674L, Washington, DC 20375, USA

' Massachusetts Institute of Technology, Cambridge, MA 20139, USA

* Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA

' Osservatorio Astronomico G. S. Vaiana, Palermo, Italy

' Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

Received July 31. 2000: accepted October 18. 2000

PACS Ref: 32.30.Rj, 34.50.Fa, 34.80.Dp, 32.70.Fw

Abstract

Spectra of highly charged Ar, Kr, Xe and Fe ions, produced in an Electron Beam Ion Trap (EBIT), have been recorded in a broad X-ray energy band

(0.2 keV to 10 keV) with a microcalorimeter detector. The first analysis of

the Kr spectra has been completed and most of the spectral lines have been

identified as transitions of B- to Al-like Kr. Line intensity ratios of Fe XVII

have been measured and compared with theoretical models.

1. Introduction and experimental setup

Electron Beam Ion Traps (EBITs) have successfully been

used for investigating properties of very highly charged ions

for more than a decade (e.g. [1]). They have several advantages over traditional laboratory high temperature plasma

sources such as tokamaks, mirror machines and those pro- duced with lasers, where the physical processes responsible

for X-ray line formation cannot be isolated easily. In an EBIT, the plasma is eliminated and replaced by a narrow,

well-defined, electron beam. At the National Institute of

Standards and Technology, USA (NIST), an energy-tunable

electron beam of up to 150 mA can be accelerated to moderate energies (700 eV to 30 keV) by a series of cylin- drical electrodes (drift tubes). The beam is compressed to a small (70 nm) diameter by a superconducting Helmholtz

pair magnet that provides a maximum axial magnetic field of 3 T. Atoms or singly charged ions are introduced into

the center of the middle drift tube where the electron beam

has its highest kinetic energy and tightest focus, and are

stripped of their outer electrons by impact ionization. Nearly

every charge state of every stable element can be produced

and their properties investigated [1], providing important

tests of different theoretical atomic models, as well as data

for a number of applications where high temperature

plasmas are involved. Typically, a large fraction of the photon emission from these ions occurs in the X-ray spectral

region.

The experimental high resolution spectroscopic data in the X-ray region have up to now been obtained largely with vari- ous types of crystal spectrometers. Although these instru- ments can provide the best resolving power necessary for precision studies, there are several limitations in their application for investigations of highly ionized plasmas. They

have low efficiency, which typically decreases with increasing

spectral resolution, and a particular crystal cover a narrow

Physica Scripta T92

energy range, which makes comparisons of different spectral

regions difficult. Semiconductor detectors (Si(Li), Ge, etc.) that can detect X-rays in a broad energy band, on the other hand, have low energy resolution (several hundred eV),

which is usually not sufficient for detailed spectroscopic

investigations of highly charged ions. The microcalorimeter

used in this experiment combines high energy resolution, very broad bandwidth, and close to 100% quantum efficiency

[2]. In the calorimeter, single X-ray photons are absorbed in a foil of superconducting tin that is kept at 65 mK. The

temperature rise of the absorber is proportional to the X-ray energy and is measured with a neutron transmutation-doped

(NTD) germanium thermistor which is attached to the underside of the absorber. The NTD thermistor is impedance-matched to a JFET negative-voltage feedback circuit

[3]. The detector used in the present experiment has an energy resolution of 5.9 eV at 1.5 keV and 7 eV at 6 keV

[2]. The observed X-ray emission lines span from 0.2 keV

to 10 keV. A special X-ray lens [2] can be placed between

the EBIT and microcalorimeter to focus the X-rays onto

the detector.

2. Results

Spectra of Ar, Kr, Xe and Fe have been recorded from

200 eV to 10 keV under diflFerent conditions (electron beam

energy and current, ion /atom density, trap dumping time).

Examples of the spectra are presented in Figures 1-4. Observed lines are mainly due to « = 2 to « = 3 transitions in Li-like to Mg-like ions except, for Xe where most of

the lines are due to « = 3 to « = 4 transitions in Ni-hke

and neighboring ions. The first analysis of the Kr spectra

is completed [4]. Most of the spectral lines have been identified with 0.2% transition energy uncertainty as transitions of B- to Al-like Kr. Synthetic spectra for each Kr ionization

state (Al- to Be-like) were created using a model that includes collisional radiative calculations implemented in the HULLAC suite of codes [5,6] and charge-exchange

processes. The latter plays an important role since the experimental intensity ratios deviate from the theory more

at high gas injection pressures than for the low pressure. The intensities of the synthetic spectral lines were fitted to experimental ones using the charge state fractions,

temperature, and relative charge exchange rates as fitting © Physica Scripta 2001

NIST-228
Microcalorimeter/EBIT Measurements of X-ray Spectra of Highly Charged Ions 455

Energy (eV) Energy (eV)

Fig. 1. Spectrum of highly charged Ar at electron beam energy £b = 6 69 keV, beam current h = 120 mA, trap dumping time — 200 ms, gas pressure Fig. 4. Spectrum of highly charged Fe at E\, = 0.9 keV, 4 = 14.2 mA, and

Pf^ = 1.33 X 10^Pa[4]. Mostofthelinesaredueton = 3ton = 2tiansitions E\, = 2.Q keV, /b = 33.0 mA; ti = 1.0 s. in Be-like and neighboring Ar ions.

experimental spectrum E=12.4keV, 1=115 mA

synttietic spectrum

1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500

Energy (eV)

Fig. 2. Spectrum of highly charged Kr at £b = 12.4 keV, /b = 115 mA,

ti = 1.35 s, pg = 1.33 X 10^ Pa [4]. Synthetic spectrum is calculated and

fitted using a model thoroughly described in Ref. [4].

1.75 keV

1 .48 keV

Energy (eV) Fig 3. Spectrum of highly charged Xe at ^b = 1.48 keV, 4 = 42 mA, and

£b = 1.75 key /b = 51.6 mA. No trap dumping, pg = 1.25 x IQ-" Pa [4]. Most of the lines are due to n=4to n = 3 transitions in Ni-like and

neighboring Xe ions. © Physica Scrota 2001

parameters and thus establishing semi-experimental ioniz- ation balance inside the trap. Details of the model, fitting

procedure, and results can be found in Ref. [4]. Due to the high elemental abundance of Fe and the closed

shell structure of Ne-Uke ions, Fe'*+ is one of the dominant

emitter of line spectra from astrophysical plasmas with temperatures around 5 x 10* K. However, among the six strong

hnes between the 2p* 'So ground state and the excited states 2p53s 'Pi, 'Pi, ^P2 and Ip^M 'Pi, 'Di, 'Pi, the strongest line at 15.012 A from the 2p^3d 'Pi, often appears diminished in intensity relative to other spectral features [7]. One suggested

explanation is that it may be affected by resonance

scattering, predominantly removing photons from the line of sight in this transition (e.g. [8,9]). Furthermore, the overall intensity of the lines from the 2p^3s configuration often appears enhanced relative to what theory would predict

when compared to the lines from the 2p^3d configuration.

This has been explained by inner shell ionization of a 2p elec- tron of the Na-Hke Fe ion [10], strong dielectric recombination [11], and also by incomplete theoretical atomic

models that do not include a sufficient number of excited

levels to adequately treat all the radiative cascades. We compared our observations with theoretical results computed

including up to 113 levels (457 magnetic sublevels for the polarization correction) using both distorted wave [12] and /^-matrix [13] excitation cross sections (Figs 5,6). Our

values for the intensity ratio of the 15.260/15.012 A lines are consistent with those of [14], supporting the suggestion

that the effect of resonance scattering in the 15.012 A

resonance line in solar spectra has hitherto been overestimated.

Acknowledgements

The authors thank D. J. Alderson for his technical support. IK acknowledges

support from the Swedish Foundation for Cooperation in Research and

Higher Education (STINT). JML was sup-ported by NASA Contract W 19539 (Applied Information Systems Research Program) and by the NRL/ONR

Solar Magnetism and the Earth's Enviroiunent 6.1 Research Option. ES, HS, and SB acknowledge support in part by NASA Grant NAG5-5104.

Physica Scripta T92 ]^ST-229
456 /. Kink et al.

15

650 700 750 800 850 900 Energy (eV)

Fig. 5 Fit to the six strong Fe XVII lines recorded at £b = 900 eV. The line wavelengths are 15.012 A, 15.260 A, 15.449 A, 16.776 A, 17.051 A, and 17.100 A. The last two transitions are blended together at the resolution of the micTOcalorimeter, and are fitted as a single component. The upper panel gives the residuals from the fit. Error bars come from Poisson statistics.

1.2

O

0.2 -I 1 1 1 1 1 1 1 1 1 0.0 0.5 1.0 1 5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Energy (keV)

Fig. 6. The variation of the intensity ratio (/i«.776+/i7.05i+^i7.ioo)/('i5.oi2+

/1S.260+/1 3.449) with electron beam energy. The first three lines originate in the 2p^3s configuration and the second three in the 2p^3d. For comparison,

results from model calculations using the DW (distorted wave) results [12] and, where appropriate, the J{-matrix results [13] are overlaid. The range given

for each calculation corresponds to the limits of impolarized and completely

polarized emission. The experimental error bars correspond to Poisson

statistics.

References

1. Gillaspy, J. D., Physica Scripta T65, 168 (1996).

2. Silver, E. et al, Nucl. Instr. Meth. Phys. Research A 444, 156 (2000).

3. Handler, S. et al, Nucl. Instr. Meth. Phys. Research A 444, 273 (2000).

4. Kink, I. et al, Phys. Rev. E (submitted, 2000).

5. Bar-Shalom, A. and Klapisch, M., Computer Phys. Comm. 50, 375

(1988).

6. Bar-Shalom, A., Klapisch, J. D. and Oreg, J., Phys. Rev. A 38, 1773

(1988).

7. Laming, J. M. et al, Astrophys. J. Lett (accepted, 2000).

8. Phillips, K. J. H. et al, Astron. Astrophys. 324, 381 (1997).

9. Bhatia, A. K. and Kastner, S. O., Astrophys. J. 516, 482 (1999).

10. Feldman, U., private communication.

11. Liedahl, D. A. et al, Astrophys. J. 350, L37 (1990).

12. Bhatia, A. K. and Doschek, G. A., At. Data Nucl. Data Tables 52, 1 (1992).

13. Mohan, M., Sbarma, R. and Eissner, W., Astrophys. J. Suppl. 108, 389

(1997).

14. Brown, G. V., Beiersdorfer, P., Liedahl, D. A., Widman, K. and Kahn,

S. M., Astrophys. J. 502, 1015 (1998).

NIST-230
on-surface interactions

A beam line for highly charged ions

A. I Pikin,^' C. A. Morgan,''^ E. W. Bell,^' L. P. Ratliff, D. A. Church,^' and J. D. Gillaspy

Atomic Physics Division. National Institute of Standards and Technology, Gaithersburg Maryland 20899

(Received 13 March 1996; accepted for publication 8 April 1996)

The design and operation of a beam line for transporting and charge-to-mass selecting highly

charged ions extracted from the National Institute of Standards and Technology electron beam ion

trap (EBIT) are described. This beam line greatly extends the range of experiments possible at this

facility. Using die transport system, pure beams of low-energy, highly charged ions up to Xt^'^

have been produced with substantially higher fluxes than previously reported from an EBIT source. Design choices and computer modeling for the various components of the beam line are explained

in detail. © 1996 American Institute of Physics. [S0034-6748(96)03907-X]

I. INTRODUCTION

Many areas of science, including controlled nuclear magnetic fusion research, x-ray lasers, and astrophysics, have driven the need to study plasmas of highly charged ions

as well as plasma-surface interactions. The description and

modeling of natural and manmade plasmas require detailed information on the processes occurring within the plasma. In recent years, the growing need for atomic physics data for highly charged ions has fostered innovation in the ways that such data are gathered. The advent of the electron beam ion

trap (EBIT) at Lawrence Livermore National Laboratory

(LLNL) in 1986' provided a versatile new instrument for the study of trapped highly charged ions. A successor to the electron beam ion source,^ the EBIT is a smaller device whose performance in producing the highest charged ions

(up io q—92+)^ has greatly exceeded its predecessor.

The original EBIT design is well suited for experiments

that study processes that can be measured in situ. Data for experiments in electron-ion collisions that are readily measurable by this device include cross sections for dielectronic recombination,"* electron-impact excitation,^ and ionization.^ A great number of other experiments, however, require the

ions to be extracted from the trap. One such class of experi- ments is the study of ion-surface interactions, including the investigation of surface damage, electron emission, and x-ray

emission.*'^ Ion-gas interactions such as charge exchange

and recoil-ion experiments may be studied as well.* Another

important class of experiments involves recapturing ions into secondary ion traps for atomic physics experiments, which

cannot be done in the presence of the EBIT electron beam

(long excited-state lifetime measurements,^ cold ion dynamics studies,*''" and precision mass measurements," for ex- ample). For many of these types of experiments, data for highly charged ions are very sparse or nonexistent. Finally, extracted ions may also be used to reveal important information about the conditions inside the EBIT'^ and, therefore, provide complementary information for spectroscopic stud-

'•Pennanent address: Univenity of Maryland, College Park, MD 20742.

""Permanent address: Physics Department, Texas A&M University, College

Station, TX 77843-4242.

'^'Permanent address: JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309-0440.

2528 Rev. Sci. Instrum. 67 (7), July 1996

ies being performed on ions trapped in the EBIT. Clearly, a system to extract and transport ions created in an EBIT

broadens the scope of the work that can be done with the device. The first EBIT ion beam line was constructed and demonstrated at LLNL over five years ago,'^ however, the published descriptions of this system are not sufficiently detailed

to allow it to be reproduced. The EBIT beam line at the National Institute of Standards and Technology (NIST) is the second such system to come into operation worldwide, and

the present article describes it in full detail. The design of the system described here was based on extensive computer

modeling and therefore embodies different design details for

the individual ion-optical elements, as well as a different overall layout of the positions of the components with re- spect to each other. The main general requirements were to charge-to-mass select the ion beam and to maximize the amount of the beam transported to the experimental chamber

while keeping distortions in the beam to a minimum. The

resulting beam line is highly efficient and transports essentially all the ions extracted from the EBIT either continu- ously or in a pulsed mode, over a wide range of pulse

lengdis. Detailed descriptions of the operating principles of our EBIT have been provided elsewhere,''* so only a brief over- view as it relates to operation as an ion source is presented

here. The EBIT operates by trapping and ionizing any species of atomic ion in the space charge of an intense electron beam and a high magnetic field. The operating parameters of

the EBIT can be tuned to maximize the number of trapped

ions in any desired charge state or range of charge states. Ions can be extracted from the trap into the beam line continuously or in pulses, typically 50 /iS to 100 ms wide. Extracted ions with kinetic energies in the range 3 to 12 kV per charge have been studied, but both limits can be extended.

II. BEAM LINE

A. Overview

A schematic of the beam line for the NIST EBIT is depicted in Fig. 1 and some of the typical operating parameters for the various elements are listed in Table I. Figure 2 shows the configuration of the ion-optical elements in the beam line. Constrained by the geometry of the EBIT and the

0034-674a/96/67(7)/252a/6/$1 0.00 © 1996 American Institute of Physics

NIST-231
0 12 3 4

Distance from the Exit of the EBIT (m)

FIG. 1. Schematic view of the exterior of the beam line. Locations of ion

optical elements are indicated by numbers (see Table I).

layout of the laboratory, the beam line extends up to the

ceiling, across the laboratory, and back down into a second

experimental area 3.2 m from the EBIT. The ions emerge

vertically from the EBIT near the ceiling of the laboratory, so the first element in the beam line is a 90° electrostatic bender. Two pairs of electrostatic deflectors used for steering

and alignment of the beam are located just after the electrostatic bender. Two einzel lenses are employed both to pre- vent the beam from diverging as it travels across the room to the experimental area and to focus the beam at the object

plane of the charge-to-mass analyzing magnet. At this focus, a four-jaw slit defines the entrance of the magnet. A second

four-jaw slit is placed at the image plane of the magnet,

defining the exit of the analyzer. Two Faraday cups mounted

on motion feedthroughs are used to monitor the beam current

FIG. 2. Schematic view of the interior of the beam line.

Rev. Sci. Instrum., Vol. 67, No. 7, July 1996

FIG. 3. Example of transport modeling showing the radius of the beam as

a function of distance from the exit of the EBIT (computed widths are indicated by points). The solid line connecting the points represents the beam half width in the vertical direction and the dashed line represents the beam half width in the horizontal direction. Locations of ion optical ele- ments are indicated by numbers (see Table I).

at positions after the first electrostatic deflector and after the magnet entrance slit. The entire system is fabricated from ultrahigh vacuum

compatible materials. Vacuum pumps maintain a pressure in the beam line of about 10"^ Pa (10~* Torr) or better, after baking the system to 100 °C. This level of vacuum is suffi- cient to prevent significant losses to the ion beam from

charge-exchange collisions with residual gas in the beam

line.

B. Computer modeling

The beam line was designed with the aid of two computer modeling programs, transport' ' is a program that uses matrix multiplication to determine the size and diver- gence of an ion beam as it travels through various drift spaces and ion-optical elements in the beam line. It is useful

for studying the beam line as a whole, but does not take aberrations such as fringing fields into account. The rough

positioning of the elements was determined with TRANSPORT. Figure 3 shows the results of a calculation for the beam line design. The points represent the calculated radius of the ion beam in the two transverse directions taken at several positions along the beam line (the boxed numbers in the figure correspond to the positions of the optical elements

listed in Table I). The points are connected by a solid line for the radius of the beam in the vertical direction and by a dashed line for the radius in the horizontal direction. The

lines represent interpolated (not calculated) beam radii. This

particular example predicts a beam with a spot size approximately 2 mm in diameter at the exit slit of the magnet

The second modeling program, SIMION,'* is a commonly

used ray tracing program that allows the description of indi- vidual elements in either cylindrically symmetric or planar

geometries. The electrodes are user defined on a grid, the potentials are calculated using numerical relaxation methods,

Highly charged ions 2529

NIST-232
TABLE I. Typical operating parameters for the beam line for 7.6 keV per charge Xe***. The position of each element in Fig. 1 is indicated in the first column.

Element No. Description Typical value

1 Electrostatic bender 1.3 kV

2 First electrostatic deflector -100 to +100 V

3 Second electrostatic deflector -100 to +100 V

4 First einzel lens 1.9 kV

5 Second einzel lens 2.4 kV

6 Magnet entrance slit 2 mm

7 Analyzing magnet 0.11 T

8 Magnet exit slit 3 mm

and voltage contours and ion trajectories are computed. Because each element is defined by the user in detail, the full effect of distortions introduced in the ion beam by the elec- trodes is taken into account. SEvnON is a two dimensional program, so it is capable of modeling elements that do not

contain a mixture of planar and cylindrical geometries, such

as the einzel lenses and the electrostatic bender. However,

the electrostatic deflectors cannot be described using such

simple geometries, thus, they cannot be modeled adequately

by SIMION. Using SIMION to model the entire beam line would be impractical; however, it was extremely useful for

the optimization of the design for individual elements.

C. Beam line elements

1. Electrostatic bender

The electrostatic bender is used to deflect the ion beam

through 90° and cannot be used for charge-to-mass selection

in the present beam line. The bender is shown in Fig. 4 and

consists simply of two parallel plates that are bent through

90° and operated at equal and opposite voltages. Located just

above the EBIT, the bender employs relatively large elec- trodes to increase the acceptance of ions from the EBIT. The

plates are 10 cm wide and have radii of curvature of 22 cm

for the inner electrode and 26 cm for the outer electrode. This bender, which is housed in an elbow with an inner diameter (i.d.) of of 15 cm, is the only element of the beam

line that is not in a 10-cm-i.d. chamber. A 1.3-cm-diam aperture in the outer electrode allows direct transmission of

ions when the bender is held at ground potential. This aperture is covered by a 90% transmission mesh to obviate disMesh

FIG. 4. Schematic view of the 90° electrostatic bender.

2530 Rev. Sci. Instrum., Vol. 67, No. 7, July 1996

FIG. 5. Schematic view of one of the electrostatic deflectors. The deflector consists of two pairs of electrodes that steer the beam in the vertical and

horizontal directions.

tortions in the electric field of the bender caused by the break

in the electrode. The outer electrode can also be used as a Faraday cup to monitor the ion flux from the EBIT.

2. Electrostatic deflectors For small-angle steering of the beam, electrostatic deflectors made from pairs of obliquely cut cylindrical elec- trodes are used. Figure 5 demonstrates the general design of one such deflector. Each electrostatic deflector consists of two pairs of electrodes rotated in the azimuthal direction by

90° with respect to each other, in order to provide steering of

the ion beam in the horizontal and vertical directions. In each

electrostatic deflector, the horizontal and vertical deflectors are separated by about 5 mm, but this is not a critical dimension. The cut-electrode pairs are each 8.1 cm long, have an

inner diameter of 3.8 cm, and are cut at a 32° angle. In each

pair, the electrodes are separated by approximately 2 mm

and are operated with voltages of opposite polarities. Each

electrode is operated with an independent power supply. In

the beam line, two deflectors (eight electrodes total) are placed in series in order to provide the capability for arbi- trary offset of the ion beam in both position and angle. Electrodes of this basic design are widely used for measuring the transverse shift of beams in accelerators and beam

lines. To our knowledge, the reversed application described

here (in which the electrodes are used for precise and rela- tively distortion-free deflection of the ion beam) is not widely known or reported in the literature, although it has been used for beam line control at least once before.'^ The

most significant advantage of this design over deflectors with

planar symmetry is the minimization of the effects of fringing fields on the ion beam, so the ion beam is not adversely

affected by traveling through the deflector. Furthermore, the

cylindrical nature of the design uses most of the internal volume of vacuum chamber, allowing the largest possible inner diameter for the electrodes. A deflector of this design can also be used as a focusing element by applying a voltage

offset to both elements.

3. Einzel lenses

To aid in the focusing and transport of the ions, two

identical einzel lenses are situated along the beam line. The

design of these lenses was determined by a compromise be- tween minimizing distortion to the ion beam as is passes through the lens (this would demand a large inner diameter

of the lens) and a requirement that the voltage on the central

Highly charged ions

NIST-233
17.7 cm 1 .25cm

Shield Suppressor

/

End

yElectrode

Centrol

I Electrode

FIG. 6. Schematic view of one of the einzel lenses.

electrode not exceed 5 kV for the desired focusing properties

(this would favor a lens with a small inner diameter). Figure

6 shows the geometry of the lenses. Each of these lenses

consists of three cylindrical elements with inner diameters of

3.8 cm and central electrodes 3.8 cm long. There are shields on the ends of the outer electrodes that prevent the insulating material used to mount the lenses from charging due to stray

ions. The shields are also used as Faraday cups during the

initial tuning of the ion beam.

Each einzel lens is operated with positive voltage on the

central electrode and the end electrodes grounded. Negative

voltage will also focus the ion beam, but to get the same

focusing, the voltage must be significantly higher.'^

4. Analyzing magnet

Before the analyzing magnet, the beam line transports an

ion beam containing all charge states of all ions extracted from the EBIT. The analyzing magnet is used to separate out

ions with the desired charge-to-mass ratio and transport them

to the experimental chamber. The magnet is used as the sec- ond 90° bend in the beam line, rather than the first, so that the beam can be properly aligned and focused for optimal

performance of the magnet. The magnet for the present beam

line is of a double focusing dipole design^" with a nominal

bending radius of 20.3 cm. It accepts a beam that is focused

to a point by the einzel lens before the magnet (the object

plane) and then focuses it in two dimensions to a point after the magnet (the image plane). Under typical operating conditions, for example, 7.8 kV per charge Xe''^^, a current of 55 A is used, yielding a magnetic field of 0. 1 1 T. At the maximum current of our power supply, 240 A, the magnetic

field in the gap of the magnet is 0.32 T, which is enough to deflect U^"^ ions with 8.0 kV per charge of kinetic energy

through 90°. In the magnet, the vacuum chamber has a gap

of 3 cm in the direction parallel to the field lines. The magnet

is operated under computer control and can be used to measure a charge-to-mass spectrum by recording the count rate from a detector, which is located after the image plane, as the

field is ramped. The magnetic field can be fixed at a single

peak of such a spectrum for experiments involving a single

charge-to-mass selected state.

FIG. 7. Schematic view of one of the Faraday cups and suppressor electrode.

At the object and image planes of the magnet are four- jaw slits, adjustable apertures that consist of four knife-edged copper plates that can be independently adjusted with linear motion feedthroughs. This configuration allows the definition of a rectangular aperture with arbitrary dimensions and position. Typically, the entrance slit is set to about 2 mm in the direction perpendicular to the field lines of the analyzing

magnet. With proper tuning of the ion beam, this width is sufficient to allow over 90% of the incident ion beam

through the slit.

5. Ion detectors

There are two identical Faraday cups in the horizontal

part of the beam line, shown schematically in Fig. 7. The

first is located after the first electrostatic 90° bender, between

the two sets of electrostatic deflectors, and the other is lo- cated in the object plane of the magnet after the four-jaw slit. The Faraday cups have apertures 1.25 cm in diameter and

have electrodes that suppress secondary electrons created during ion collection. A -30 V bias applied to these elec- trodes is sufficient to suppress the secondary electrons. Both

Faraday cups are mounted on linear motion feedthroughs and

can be inserted and removed from the ion path. Analog sig- nals from the Faraday cups are observed using an oscillo- scope after amplifying the signal with a current amplifier

using a typical gain of 10^ V/A. These Faraday cups are mainly used for rough alignment of the ion beam, but they

are also very useful for diagnosing the ion extraction and

transport efficiency. After the exit slit of the analyzing magnet, an electron multiplier detector^' with a 2-cm-diam active area is mounted on a linear motion feedthrough. The detector is usually operated in pulse counting mode, but it can also be used

as a Faraday cup to monitor the ion current under high flux conditions. A pulse height distribution from the multiplier

for a beam of Xe''^^ ions is presented in Fig. 8. The inset in the figure shows the shape of a single pulse from the detecRev. Sci. Instrum., Vol. 67, No. 7, July 1996 Highly charged ions 2531

NIST-234
20 40 60 80

Pulse Height (mV)

100

FIG. 8. Example of a pulse height distribution of signal pulses from the

electron multiplier used to characterize the beam line performance. The

discriminator setting for the detector was 15 mV (shown by the solid line). The inset shows the shape of a single pulse from the detector.

tor. By setting the discriminator level to 15 mV (shown by

the vertical line in the figure), most of the background is cut

off while most of the signal is retained.

D. Ion beam tuning and voltage scaling

When ions are extracted from their trapped position in- side the core of the EBIT (which is held at a potential U),

they gain a kinetic energy qU. The velocity of any ion extracted into a field-free region will, therefore, be proportional

to the square root of the charge-to-mass ratio {q/m) and the square root of U, resulting in a beam of several different

velocities corresponding to the various q/m values. Electrostatic elements will transmit all of these ions because the

effect of the increased force on more highly-charged ions

will be balanced by the effect of the higher velocities of

these ions. Thus, ions with differing velocities will follow

the same trajectories through electrostatic elements. Changing the value of U requires linear scaling of all electrostatic elements. These properties are illustrated by the equation de- termining the potentials ±V that must be applied to the electrostatic bender plates in order to deflect an ion by 90°,

V=U{s/R), where s is the plate separation and R is the

radius of curvature of the trajectories.

The situation for an ion in a magnetic field is different. The magnetic fiel d required to deflect an ion by 90° is B

= (l/R)\j2Um/q. Thus, for constant U, all values of q/m

will be separated by a magnetic field. Furthermore, if an ion experiences a charge-changing collision with background

gas in the horizontal section of the beam line, the ion will be

transported through the magnet at a different magnetic field and will not contaminate the parent line in the mass spectrum. When changing the accelerating potential, the magnetic field must be scaled by the square root of U.

III. BEAM LINE PERFORMANCE

Ion beams of several highly charged ion species have

been transported through the beam line and into the experi- mental chamber after the analyzing magnet. Measurements

0.108 0.112 0.116 0.120

Analyzing Magnet Field (T)

FIG. 9. Mass spectrum of xenon ions with a magnet entrance slit of 2 mm.

comparing the signal detected at the two Faraday cups and on the electrostatic bender indicate that essentially the entire beam is transported to the analyzing magnet.

Figure 9 shows a charge-to-mass scan for 7.8 kV per charge Xe ions with the EBIT operated in a continuousextraction mode. The entrance slit for this scan was 2 mm

and the spot size on the detector was about 3 mm in diameter. The number of ions in the peak with the most counts is about 250 000 ions per second, assuming a detector effi- ciency of 100%. The total integrated ion flux for the 19 charge-to-mass peaks in the spectrum is 2 500 000 ions per

second. For ions extracted under the same conditions except

in pulsed mode with a repetition rate of 3 Hz and a pulse

length of 10 ms, the largest peak has about 70 000 counts per

pulse, corresponding to a peak flux of over 200 000 ions per second of a single charge-to-mass selected value. We observe, therefore, high and approximately equal time- averaged extraction fluxes in both pulsed and continuous

modes of operation, in contrast to the behavior reported by

LLNL'^ in which the continuous mode of operation was ob- served to be ten times less efficient than the pulsed mode.

We note also that our overall flux values are substantially

higher than reported in the earlier studies,^ '^ even when we

assume a detection efficiency of 100% rather than the 50%-

60% assumed in the previous studies.'^ Note that while most of the different charge states are separated in Fig. 9, the signal from several isotopes of the same charge state is not resolved. Xenon has several stable isotopes (three of which have natural abundances greater than 20%) and, since we use xenon with a natural abundance

of isotopes, the spectrum is a convolution of a range of

charge states and all the stable isotopes. The highest peak in Fig. 9 actually results from a mixture of two ions, '^^Xe"*^"*" and '^^Xe^^"*". The highest continuous count rate for a pure peak ('^^Xe'""') is about 180 000 per second. Figure 10 displays part of another charge-to-mass scan

for 7.8 kV per charge Xe ions taken with similar operating

conditions. For this scan, the entrance slit was reduced to 0.5 mm in order to improve the resolution. There is a small shift in the positions of the peaks compared with Fig. 9 because of

2532 Rev. Sci. Instrum., Vol. 67, No. 7, July 1996 Highly charged Ions

NIST-235
ACKNOWLEDGMENTS

1— \/. r->~^x . . .'s^i . xy . I . r. . ] . . , ,>

i. . . i 0.108 0.109 0.110 0.111

Analyzing Magnet Field (T)

FIG. 10. Mass spectnun of Xe ions with a magnet entrance slit of 0.5 mm.

slightly different beam tuning. In Fig. 10 many isotopes are

clearly separated, even for the highest charge states. The sig- nals from some isotopes of ions are still overlapping, however. For example, '^^Xe"^"^ and '^'Xe''^"^ have virtually the same charge-to-mass ratio {q/m = ~ 1/3), and, thus, cannot be

resolved with a conventional mass-analyzer. Clearly, in the case of xenon, it would be advantageous to use isotopically enriched gas for the production of the ion beam. This would

increase the signal for a particular line and greatly simplify

the structure of the mass spectrum.

IV. CONCLUSION

An efficient beam line for the transport and mass selec- tion of highly charged ions from the NIST EBIT has been

constructed and is now in use. This addition greatiy increases

the range of experiments now possible at the NIST EBIT

facility. The beam line incorporates design features never

before used with an EBIT, in order to transport ions virtually

without loss to an analyzing magnet that is capable of resolv- ing isotopes of most ions and charge states. The beam line can presently deliver »nto a spot 3 mm in diameter a continu- ous beam of over 200 000 charge-to-mass selected highly

charged ions (Xe"*^"*^) per second, a value that exceeds previously reported continuous values^ '^ by several orders of magnitude. The system has also been operated with pulsed

beams (70 000 ions per pulse, 10 ms pulse length, 3 Hz

repetition rates) to boost the instantaneous flux by another

factor of 35 while maintaining a similar time-averaged value.

The authors would like to thank Professor Robert

Tribble for assisting us with the TRANSPORT code. We also thank D. Alderson for his technical assistance.

'M. a. Levine, R. E. Marrs, J. R. Henderson, D. A. Knapp, and M. B.

Schneider, Phys. Scr. T 22, 157 (1988). ^E. D. Donets, V. I. Ilyuschenko, and V. A. Alpert, Preprint, Joint Institute for Nuclear Research, R7-4124, Dubna (1968); E. D. Donets, Phys. Scr. T

3, 11 (1983). 'R. E. Marrs. S. R. Elliot, and D. A. Knapp, Phys. Rev. Utt. 72, 4082

(1994).

"P. Beiersdorfer. T. W. Phillips, K. L. Wong, R. E. Marrs, and D. A. Vogel.

Phys. Rev. A 46, 3812 (1992).

'S. Chantrenne, P. Beiersdorfer, R. Cauble, and M. B. Schneider, Phys. Rev. Lett. 69, 265 (1992). *D. H. Schneider, M. A. Briere, J. McDonald, and J. Biersack, Radiat. Eff. Defects Solids 127, 113 (1993). 'D. C. Parks, R. Bastasz, R. W. Schmieder, and M. Stockli, J. Vac. Sci. Technol. B 13, 941 (1995). *D. A. Church, Nucl. Instrum. Methods B S3, 504 (1991). 'D. A. Church, Phys. Rep. 228, 253 (1993). '"Joseph N. Tan, J. J. Bollinger, B. Jelenkovic, and D. J. Wineland, Phys. Rev. Lett, (in press). "C. Carlberg, I. Bergstrom, G. Bollen, H. Borgenstrand, R. Jertz, H.-J. Kluge, G. Rouleau, R. Schuch, T. Schwartz, L. Schweikhard, P. Senne, and F. Soderberg, IEEE Trans. Instrum. Meas. 44, 553 (1995). '^D. R. DeWitt, D. Schneider, M. W. Clark, M. H. Chen, and D. Church.

Phys. Rev. A 44, 7185 (1991). "D. Schneider, D. DeWitt, M. W. Clark, R. Schuch, C. L. Cocke, R. Schmieder. K. J. Reed. M. H. Chen. R. E. Marrs. M. Levine, and R. Former, Phys. Rev. A 42, 3889 (1990)

'^J. D. Gillaspy, Proceedings of the 6th International Symposium on Electron Beam Ion Sources and Their Applications, Stockholm, 1994 [Phys.

Scr. (submitted)]. '*K. L. Brown, F. Rothacker, D. C. Carey, and Ch. Iselin. "transport—^A Computer Program for Designing Charged Particle Beam Transport Sys- tems" (CERN 80-04, European Organization for Nuclear Research, Geneva, 1980); D. C. Carey "New Features in transport" (Fermilab, TM-1064, Batavia, IL 1981). "D. a. Dahl and J. E. Delmore, EGG Publication No. EGG-CS-7233, Rev.

2. April 1988, Idaho National Engineering Laboratory. EG&G Idaho. P.O. Box 1625. Idaho Falls, ID 83415.

'^L. L. Gol'din and A. A. Kuz'min, Instrum. Exp. Tech. (USSR) 10, 251

(1967). '*K. K. Onosovskiy (private communication).

'^For a discussion of the focusing properties of einzel lenses, see E. Harting and F. H. Read, Electrostatic Lenses (Elsevier, New York, 1976). ^R. W. Schmieder (private communicaion).

^' Hamamatsu Model R2362 electron multiplier. Certain commercial equip- ment, instruments, or materials are identified in this article in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Insti- tute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.

Rev. ScL Instrum., Vol. 67, No. 7, July 1996 Highly charged ions 2533

NIST-236
Continuous highly charged ion beams from the National Institute

of Standards and Technology electron-beam ion trap

L. P. Ratliff, E. W. Bell,^' D. C. Parks, A. I. Pikin,^' and J. D. Gillaspy

Atomic Physics Division, National Institute of Standards and Technology. Gaithersburg, Maryland 20899 (Received 17 December 1996; accepted for publication 20 January 1997)

We describe our newly modified beam line and present its performance in conjunction with the National Institute of Standards and Technology electron-beam ion trap. We find that, contrary to previously reported results from similar ion sources, the highest intensity time-averaged ion fluxes are achieved by letting the ions boil out of the trap in a continuous stream rather than periodically

dumping the trap to produce a pulsed beam. We produced continuous beams of

3.0(6) X 10^ Xe**"^ ions per second and lower flux beams of charge states up to Xe''^'*". Also, in pulsed mode, we created beams with very high peak flux, over 10'" Xe'^^ ions per second. [50034-6748(97)01005-8]

I. INTRODUCTION

The electron-beam ion trap (EBTT)' is a versatile device

that has transformed the study of highly charged ions. Initially, most EBIT experiments involved detecting photons

(visible and x ray) emitted from excited, highly charged

trapped ions.^'^ More recently, the EBIT device has also proven to be useful as a source of highly charged ion beams

for use in experiments that study collisions between ions and

external solid targets'''^ and those that study ions in external

traps.

The EBIT and its predecessor, the electron-beam ion source (EBIS),^ have been used primarily to produce pulsed

beams of very highly charged ions. For some collision ex- periments, however, the ions in a pulse can arrive undesir- ably close together. For instance, in a coincidence experi- ment where two collision products are to be detected and

related to a single event, the events must be temporally separated to avoid confusion about which products resulted from

a particular event. For such applications, extraction of the ions in a continuous stream is ideal, allowing both the maximum signal and the maximum separation between events. Several groups in the EBIT/EBIS community have made

continuous beams of multiply charged light ions: Becker,

Kleinod, and Klein* have made continuous beams of Ne*^

with a flux of 10^ ions per second and of Ne"*"^ with a flux of 3000 ions per second. Other groups have seen low current continuous beams of higher charge states: Tawara et al.^ have made continuous beams of I"*'"^ with a flux of 10^ ions per second and Schneider et al.'* have made a continuous

beams of Ar'^"" and Xe'^'*' with a flux of ~2X 10^ ions/s. Stockli'° has made quasicontinuous beams of 3x 10^ Ar'^""" ions per second (time averaged count rate) with a 50% duty

cycle. In this article we report the first demonstration of high

continuous fluxes of very highly charged ions using an EBIT/EBIS type source (3X 10* ions/s of Xe'^"'). In addition to the high flux offered by the continuous mode of op-

''Permanent address: Diamond Semiconductor Group, Gloucester, MA

01930.

""'Permanent address: Brookhaven National Laboratory, Upton, NY 11973.

1998 Rev. Sci. Instrum. 68 (5), May 1997

eration, we also find that this mode is simpler to optimize

and more stable than the pulsed mode.

Other experiments require fast pulses of ions that are microseconds long. A prominent example is an experiment

in which the ions are produced in an EBIT and then trans- ferred into a secondary penning ion trap specially designed

to carry out more detailed studies.^ As the pulse length of the ions extracted from the EBIT is decreased, the energy spread of the extracted ions can be expected to increase, thereby

making the transported beam more sensitive to chromatic

(energy) aberrations in the ion optics and increasing losses. When our system is operated in pulsed mode, however, we

find that the losses are significantly less than previously

reported.*

II. THE ELECTRON BEAM ION TRAP

The details of the National Institute Standards and Technology (NIST) EBIT can be found elsewhere."-'^ The trap consists essentially of three cylindrical drift tubes through

which a monoenergetic, magnetically confined electron beam

travels. The electron beam ionizes gas atoms (xenon in the present work) which are injected into the drift tubes. These

ions are then trapped in the radial direction by the space charge of the electron beam and by the magnetic field. To

trap the ions in the axial direction, the upper and lower drift tubes are held at a higher potential than the center drift tube, forming an electrostatic well. The base pressure is 10~* Pa

(10~'° Torr). Highly charged ions are created by successive

electron impact of the lower charged ions. The charge state distribution in the trap can be adjusted by changing the in- jected gas flow, the electron-beam energy, and other voltages

and magnetic fields that control the EBIT. The electron beam is created by a commercial three el- ement electron gun located 16 cm below the center of the

trap. The gun has a barium oxide cathode, and focus and

anode electrodes whose potentials are adjusted to optimize

the performance of the EBIT. The electron beam leaves the gun, travels through a transition electrode, through the drift tubes, and is stopped inside an electron collector electrode. The transition electrode helps to guide the electron beam

through a small hole in the grounded liquid-nitrogen shield.

NIST-237
TABLE I. Typical operating parameters for the NIST EBIT optimized for the production of continuous Xe'" '^ ion beams.

Parameter Value

Electron-beam current Center drift tube voltage Upper drift tube voltage Lower drift tube voltage

Transition voltage Suppressor voltage

Extractor voltage

Collector voltage Bucking magnet current (and field) Focus voltage of electron gun

Count rate of Xe"""^

135 mA

7.8 keV

7.925 kV

7.935 kV

6.20 kV

580 V

3.0 kV

1.40 kV

17.9 A (12.5 mT)

6 V

3X 10* counts/s

The combination of a positive potential on the electron col- lector electrode and a magnetic field that cancels the fringing

field from the primary trap magnet forces essentially all of

the electron beam to hit the collector. A suppresser electrode

is located just below the collector to prevent secondary elec- trons in the collector from traveling back toward the electron gun. Inside the collector, a negatively biased cone called the

extractor helps to guide the ions out of the EBIT, while pre- venting secondary electrons from leaving the collector re- gion. The ions leaving the extractor are focused by an einzel

lens before leaving the EBIT. Table I shows the typical EBIT

parameters for maximizing the extraction of Xe'*^'*" ions. Upon escape over the upper drift tube of the trap, the ions are accelerated to a kinetic energy of qVo-> where q is the ion charge and is the potential on the upper drift tube. They then travel through a beam line consisting of electrostatic elements that focus and align the beam and a magnet

used for charge-to-mass analysis of the beam. The beam line was described in detail in Ref. 13, but has since been modified (see Fig. 1); we have installed three more einzel lenses, a third set of deflectors, and a second electrostatic bender in the vertical part of the beam line (after the analyzing magnet's exit slit). The lenses are of a different design from those

reported earlier.'^ The edges between adjacent elements in the new einzel lenses are rounded (0.8 cm radius of curvature) to minimize spherical aberrations to the beam. In all three lenses, the end electrodes are 1 .4 cm long and the center electrode is 2.5 cm long. The lens located below the sec- ond electrostatic bender has an inner diameter of 2.5 cm, and

the other two lenses have inner diameters of 3.8 cm. The

deflectors are identical to those described in Ref. 13. The

bender is the same as that described in Ref. 13 except that the radii of the inner and outer plates are 11.5 and 14 cm,

respectively, and the plates are 6.1 cm wide. The outer bender plate has a mesh covered hole to allow ion passage when the bender is off, allowing us to switch the beam be- tween two different beam lines. In passing through this mesh, the beam is calculated to be attenuated by approximately 15%. Directly below the bender is a target chamber

which is used for ion-surface interaction studies. The data presented here were taken with an electron multiplier located

in that chamber after some focusing elements, and with a Faraday cup that can be inserted into the beam directly above

FIG. 1 . Schematic of the ion-beam line.

the multiplier. By comparing the electron multiplier to the Faraday cup we determine the multiplier's efficiency to be 50% for Xe''^"^ ions. The composition (charge-to-mass ratio) of the beam is measured by ramping the analyzing magnet

and obsen/ing the transported ion signal on the electron multiplier. Once the beam line voltages are optimized, they typically require very little adjustment when changing the ion species or charge state. The voltages on the ion optical ele- ments must be scaled with the ion energy, however, when

the EBIT is readjusted to emit ions with a different

velocity.'^

Small external magnetic field coils located near the center drift tube of the trap (perpendicular to the primary magnet

coils) are used to deflect the electron beam slightly. The

electrodes inside the EBIT were aligned during assembly to be coaxial to an accuracy of approximately 50 /zm, except

for the extractor which may be off axis by several times that amount. The steering coils might make it possible to partially

compensate for the misalignment in the extractor. It is ob- served that adjusting the steering coils does greatly increase

the transmitted ion-beam current. Another parameter that is important is the magnetic field produced by the bucking coil that surrounds the electron gun. This field is used to adjust the residual magnetic field near

the electron gun, so that the electron beam is formed at the optimum field, believed to be a few hundred ^tT (a few G).''* Very small changes in this field (<0.5%) can cause a change in the transported beam flux of a factor of 10 or more.

III. PULSED EXTRACTION

The standard way to extract beams from EBIT and EBIS

devices is a pulsed mode, whereby all of the ions in the trap

Rev. Sci. Instrum., Vol. 68, No. 5, May 1997 Highly charged ion beams 1999

NIST-238
TABLE 11. Pulse widths, peaks currents, and time-averaged currents for pulses extracted with different ramp times.

Ramp time FWHM" Peak current Time-averaged

Ion (nA) current (pA)

Ar"* 5 12 15 0.26 Ar"* 20 13 26 0.7 Ar'^^ 120 20 16 0.6 Ar-^^ 5000 1000 1.1 2.2 Ar'^* 20 000 4000 0.3 2.4

11 12 75 1.8 22 18 130 4.6 Xe-"" 120 28 92 5.2 Xe-^^ 10 000 2800 1.7 9.6

"Full width at half-maximum.

are periodically released into the beam line by raising the

potential of the center drift tube above that of the upper drift tube. This causes the ions to be ejected from the trap axially

into the beam line. The upper drift tube is held at a lower

potential than the lower drift tube, so that all of the ions escape over the upper drift tube into the beam line. After the

trap is dumped and the center drift tube is lowered, the charge state distribution is rebuilt. For maximum current, the center drift tube is ramped to its maximum voltage in about 20 ms and the ions are ejected in a pulse a few milliseconds

wide. For xenon extraction, the trap is usually dumped every

0.3 s to optimize the time-averaged current of the higher

charge states. In experiments where the ions are retrapped into a sec- ondary trap, a fast ramp time (on the order of microseconds)

is essential for efficient loading the secondary trap. In practice, the efficiency of the ion-beam extraction and transport

are reduced as the pulse length is reduced, perhaps due to an

increased energy spread in the extracted ions causing in- creased aberrations in the ion optics. Table II gives the pulse

widths, peak currents and time averaged currents for pulses

extracted with different ramp times. These data were taken by first adjusting the beam line and EBIT parameters to optimize the current in continuous mode, then changing to pulsed mode without adjusting anything except the time for ramping the center drift tube to its maximum value. The ion

current on the Faraday cup is displayed on an oscilloscope

after being amplified with a gain of 10^ to 10^ V/A. The

amplifier's minimum rise time is 10 /xs which increases the measured width of the shortest pulses, making the measured

peak current less than the actual value. Even with this problem, we observe less than a factor of 10 loss in our beam

current when the ramp time is reduced from 4 ms to 10 fjis. This is compared to a loss of a factor of 100 when decreasing

the extraction time from 400 to 5 /as in a similar ion source.^ Combining our high overall flux with our efficient transport

at short extraction times, we have achieved peak currents of

over 100 nA of Xe"*^"^ for 10 /as rarnp times. We consider the ion trap to be full when the sum of all of the positive ion charges is equal to the negative charge of

the electron beam in the trap region. With our operating conditions, there are approximately 5X10^ electrons in the trap. Comparing the ion current on a Faraday cup before the ana2000 Rev. Sci. Instrum., Vol. 68, No. 5, May 1997

lyzing magnet to the current in the Xe''^^ peak after the magnet, and assuming no loss of Xe"*^^ in between, we de- termine that 6% of the charge is in the Xe"*^* peak. This

implies that if the ion trap is full, we should observe

10^ Xe"^"" ions per pulse, which is identical to that obtained from our Faraday cup measurement of 5 pC (10^ Xe"*^^ ions) per pulse. This agreement is somewhat surprising because

the calculation is quite crude, however, it illustrates the fact

that the pulsed mode has a practical limit in total ion flux per pulse and that, based on trap capacity, we might be near that limit now. In a previous spectroscopic study of the trapped

ions m the NIST EBIT, "it was determined that the electron beam was approximately 50% space-charge neutralized.

IV. CONTINUOUS EXTRACTION

In the continuous mode of operation, also referred to as leaky mode'*'"' or dc mode,* '^ ions escape the trap by gaining enough energy to pass over the electrostatic barrier at the upper drift tube which is held at a constant potential. The

ions gain the required energy through collisions with electrons. Extracted ions are replaced continuously by injected gas atoms. We have observed a higher time-averaged current of

ions in continuous extraction mode than in pulsed mode,

even for charge states as high as Xe"*^"^. The highest single

isotope count rate that we have measured is 3.0(6) X lO^C'^^Xe''^'*") ions/s which corresponds to a current of over 20 pA. The uncertainty in these measurements is limited by the accuracy of the Faraday cup. The beam is considerably easier to tune when extracting ions using the continuous mode, because the count rate is more stable than

in the pulsed mode. In fact, to get the maximum flux in pulsed mode, we optimize the beam for continuous mode

and then switch to pulsed mode without adjusting any of the electrodes. For some experiments the trap must be dumped periodically, even during continuous extraction, to avoid the buildup of heavy ions in the trap which tend to displace the

lighter atoms. The source of the heavy ions can be residual gas atoms, or barium atoms which are continuously given off by the electron gun. This is not important for xenon, but can

be significant for lighter atoms, such as neon or argon.

V. MASS SCANS

A mass scan showing the number of transported ions versus the field of the analyzing magnet for continuously

extracted '^^Xe ions with an accelerating potential of 7.8 kV

is shown in Fig. 2. The electron-beam current in the EBIT

for this scan was 135 mA. The highest peak in the scan is '^^Xe^^^ at 1130 G. There appear to be very few low-charge contaminant

ions in our beam compared with other beams from similar sources." '" This may be because our detector efficiency

drops from 50% for Xe**"" to 1% for N^"". Figure 3 shows a higher-resolution scan for xenon ions obtained by narrowing

the slits before and after the analyzing magnet. The double

peaks in the scan arise from the 10% '^''Xe present in the isotopically enriched '^^Xe seed gas.

Highly charged ion beams

NIST-239
1

-

1

136„ 44+ xe

1

i

36„ 40+ Xe

, I

-

136 „ 36+ Xe

1

1100 1150 1200 1250

Magnetic Field (lO"* T)

FIG. 2. Mass scan of '^'Xe ions from a continuous beam.

There is very little difference in the distribution of

charge states in pulsed and continuous operation. This is somewhat surprising because the dynamics of the charge

state build up and ion extraction are quite different in the two

modes. With the electron-beam energy at approximately 8 keV, the spectrum is peaked at Q = 44. In contrast to our

results, other groups observed a large preference toward

lower-charge states in continuous extraction.'' In the case of

light ions (neon and argon), this preference might be due to a buildup a heavy ions in the EBIT/EBIS suppressing the for- mation of highly charged lighter ions. This does not explain,

however why this effect is seen in the case of heavy ions such as xenon and iodine. In a simple picture, the lowercharge-state ions see a lower potential barrier and are ex- pected to boil out more readily than those with higher charge

states."' " Another factor is the upper drift tube potential that we use in continuous mode, >100 V, is much higher than

that ( ~ 20 V) used by some of the other workers.'^ '' While

the optimal upper drift tube potential must depend on the

details of the trapping and extraction dynamics of each machine, the deeper axial well should allow our ions to remain

in the trap, interacting with the electron beam, for a longer

time.By increasing the EBIT drift tube potential, charge states

1100 1150 1200 1250

Magnetic Field (10^ T)

FIG. 3. High-resolution scan of xenon ions. The resolution was increased over that in Fig. 2 by narrowing the analyzing magnet's entrance and exit

slits. The .small peaks are due to the ~ 10% "^Xe in our enriched '-"Xe

.sample.

Rev. Sci. Instrum., Vol. 68, No. 5, May 1997

FIG. 4. Mass scan of xenon ions at an electron energy of 1 1 .6 keV. The

largest peak is "'Xe"^'^, and charge states up to Q = 49 can now be seen.

up to Xe"'^ have been produced in significant quantities.

Figure 4 shows a mass scan of xenon ions extracted continu- ously from the EBIT at 11.5 keV/q (the drift tube voltage is 11.5 kV). The Xe'*^^ peak is still nearly the largest, but we now measure almost 10 000 counts per second of Xe'**''". Presumably, we could increase our production of higher

charge states by increasing the drift tube voltage, but we

would have to lower it just before extraction since some of

the power supplies on our beam line are at their present

limits with an 11.5 keW/q beam.

VI. DISCUSSION

We have demonstrated that it is possible to maximize the production of highly charged ion beams from an EBIT

source using a continuous extraction technique. In this way, we have created the highest-flux Xe''^"^ beam reported from

thiis type of ion source. Contrary to previous results and predictions, our continuous extraction mode produces a similar

distribution of charge states to that produced in pulsed mode.

Finally, we have shown that we can extract the ions in fast pulses without the heavy losses previously seen by other groups. The improved performance of our system compared

to other systems was verified with an independent set of ion

detectors and electronics in a collaborative experiment car- ried out with scientists from the Livermore EBIT group. We attribute the performance of our system to ion optics that are less subject to aberrations than those in use elsewhere.

/

' M. A. Levine, R. E. Marrs, J. R. Henderson, D. A. Knapp, and M. B. Schneider, Phys. Scr. T22, 157 (1988). ^P. Beirsdorfer, G. V. Brown, J. Cre.spo Lopez-Urrutia. V. Decaux, S. R.

Elliott, D. W, Savin, A. J. Smith, G. S. StefaneUi, K. Widmann, and K. L. Wong, Hyperfine Interact. 99, 203 (1996).

'j. D. Gillaspy, Phys. Scr. T6S, 168 (1996). *D. Schneider, D. Dewitt, M. W. Clark, R. Schuch, C. L. Cocke, R. Schmieder, K. J. Reed, M. H. Chen, R. E. Marrs, M. Levine, and R.

Fortner, Phys. Rev. A 42, 3889 (1990). ^D. H. G. Schneider and M. A. Briere, Phys. Scr. 53, 228 (1996). 'D. Schneider, D. A. Church, G. Weinberg, J. Steiger, B. Beck, J. McDonald, E. Magee, and D. Knapp, Rev. Sci. Instrum. 65, 3472 (1994).

^ E. D. Donets, in The Physics and Technology of Ion Sources, edited by I. G. Brown (Wiley, New York, 1989), p. 245. 'R. Becker, M. Kleinod, and H. Klein. Nucl. Instrum. Methods Phys. Res. B 24/25, 838 (1987). 'H. Tarawa, T. Iwai. Y. Kaneko, M. Kimura, N. Kobayashi, A. Matsumoto,

Highly charged ion beams 2001

NIST-240
S. Ohtani, K. Okuno, S. Takagi, and S. Tsurubuchi, J. Phys. B 18, 337

(1985). '"M. P. Stockli, Rev. Sci. Instrum. 67, 892 (1996). "J. D. Gillaspy, J. R. Robens, C. M. Brown, and U. Feldman, in Vlth

International Conference on the Physics of Highly Charged Ions, AIP

Conf. Proc. 274, edited by P. Richard, M. Stockli, C. L. Cocke, and C. D. Lin (AIP, New York, 1993), p. 682.

'^J. D. Gillaspy, Phys. Scr. (to be published).

'^A. I. Pikin, C. A. Morgan, E. W. Bell, L. P. Ralliff, D. A. Church, and J. D. Gillaspy, Rev. Sci. Instrum, 67, 2528 (1996).

'"E. Takacs, E. S. Meyer, J. D. Gillaspy, J. R. Roberts, C. T. Chancier, L. T. Hudson, R. D. Deslattes, C. M. Brown, J. M. Laming, J. Dubau, and M. K.

Inal, Phys. Rev. A 54, 1342 (1996).

'^C. A. Morgan, F. G. Serpa, E. Takacs, E. S. Meyer, J. D. Gillaspy, J. Sugar, J. R. Roberts, C. M. Brown, and V. Feldman, Phys. Rev, Lett. 74,

1716 (1996). The electron beam energy shown in Fig. 4 of this reference includes a 70 eV space-charge shift at 2.026 keV beam energy, obtamed

by assuming a 50% neutralization of the full .space-charge correction pre- dicted by a simple model. This shift is required in order to make the peak

positions agree with accurate calculations of the atomic ionization energies. "M. Kleinod, R. Becker, J. Dietrich, G. Isaia, H. Klein, A. Lakatos, B.

PHsterer, and H. Thomae, in Proceedings of the 5th International Symposium on Electron Beam Ion Sources and their Applications, edited by E. Donets and 1. P. Yudin (Joint Institute for Nuclear Research, Dubna, Russia, 1992), p. 64. "R. W. Schmieder, in Physics of Highly Ionized Atoms, edited by Richard Marrus (Plenum, New York, 1989).

'"j. McDonald and J. Steiger (unpublished).

2002 Rev. Sci. Instrum., VoL 68, No. 5, May 1997

NIST-241

Highly charged ion beams
ELSEVIER

Nuclear Instruments and Methods in Physics Research B 124(1997) 431-434

Beam

mm

Interactions

\^ with Materials & Atoms

X-ray emission for 3-137 keV Ar'^"^ impacting Si02

E. Takacs ^ ^ Z. Onodi-Szucs \ L.P. Ratliff \ J.D. Gillaspy ^ J. Palinkas

^ Experimental Physics Department, Kossuth University, Bern ter 18 / a. Debrecen, Hungary

^ Atomic Physics Division. National Institute of Standards and Technology. Gaithersburg, MD 20899, USA

Abstract

We report X-ray spectra from Ar'^"^ impacting a SiOj surface at 30° from normal incidence, for projectile energies

ranging from 3 to 137 keV. At the highest energy we see only X-rays from the filling of the argon K shell. As the projectile energy is reduced, these argon X-rays are gradually reduced to zero while a silicon K„ peak appears. We interpret these data

to be the result of the onset of a direct transfer of a silicon K-shell electron to the K-shell vacancy in the argon projectile. We

suggest that this transfer occurs through an argon-silicon Auger process which takes place during a close collision in the

solid. The velocity dependence of this process is not yet fully understood.

1. Introduction

The interaction of slow, highly-charged ions with solid surfaces has become a subject of intense research in recent

years. The studies are facilitated by the availability of

powerful, compact sources of highly-charged ions: the Electron Cyclotron Resonance Ion Source (ECRIS) [1], the Electron Beam Ion Source (EBIS) [2], and the Electron Beam Ion Trap (EBIT) [3]. The study of these interactions

is important for understanding basic collision processes, plasma-wall interaction phenomena, and novel methods of modifying surfaces at nanometer length scales. Despite considerable progress in characterizing and

modeling the interaction of highly charged ions with sur- faces during the past few years, there are still a number of

critical issues under debate, and the overall understanding

is still rather incomplete. Several recent reviews give extensive accounts of the present understanding [4-8].

Although the surfaces and projectiles differ from one

experiment to another, the following picture is believed to be quite general. While the ion approaches the surface, still at a large distance (several, or even tens of nanometers,

depending on the charge), electrons are transferred from

the surface to the high lying levels of the ion until the ion

is completely neutralized ("hollow atom" formation). The

highly excited states of the projectile relax by various mechanisms such as Auger processes, radiative decay, and

perhaps even surface plasmon formation. The stage of

relaxation in which the projectile reaches the surface de- pends not only on its speed and impact angle but also on

Corresponding author. Fax -1-36 52 416 181, e-mail

etakacs@tigris.klte.hu.

the random statistics of the various decay mechanisms. It

is widely accepted, however, that even at the lowest impact velocities achievable in the laboratory, the potential energy stored in the ion has not yet been fully transferred onto the surface prior to impact. Specifically, if the projectile originally had inner shell (K, L) vacancies, most of

these vacancies are usually carried into the surface. Note

that there is a lower limit for the impact velocity due to the

interaction of the projectile with its image charge in the case of metals [9]. The situation for insulators is not so

clear. In the case of an idealized insulator with no electron flow, localized charging of the surface from a single incident ion can be sufficient to slow down and repel the ion backwards [10]. In an experiment where a normally

insulating LiF target was heated to prevent charging, the

results were interpreted as evidence that the dielectric response of an insulator can accelerate ions into the sur- face [11], as in the case of metals. When the ion reaches

the surface, the electrons which orbit the nucleus of the

projectile at distances larger than the screening length of

the solid peel off and are replaced by electrons which, due

to the proximity of the surface, can now be captured

directly into lower lying energy levels. There are several

studies however, which show that even with this process

available, the inner shell vacancies of the ions can be

preserved in this phase (e.g. Refs. [12-20]). The filling of

the L- and K-shell vacancies usually happens at the time of

interaction of the ion with the bulk or in some cases after the ion has reflected from the bulk and leaves the solid

[20]. It was also pointed out earlier [20] that the slow ion does not penetrate deep into the solid. There is a shallow

region below the surface in which the ion can suffer one or more large angle deflections in close encounters with the atoms of the solid and scatter out of the target. In these

0168-583X/97/$ 17.00 © 1997 Elsevier Science B.V. All rights reserved

PII S0168-583X(96)01054-3 II. CONTRIBUTED PAPERS

]SfIST-242
432 E. Takdcs et al./Nucl. Instr. and Meth. in Phys. Res. B 124 (1997) 431-434

close collisions, electrons can transfer from the tighter

bound levels of the target to the unfilled core of the

projectile. In a follow-up work Stolterfoht et al. [27] presented a multiple-cascade model for a more complete

theoretical analysis of the filling process. The hydrogen-like argon ion has played an important

role in these studies. It was used, for example, in the first experiment where hollow atom formation was shown [12]. Several other milestone exp>eriments used the same ion to shed light on the basic interactions and processes in ionsolid collisions [12-18]. Ar'^"^ ions have a K inner shell vacancy which energetically lies well below the empty L

shell (by more than 3 keV) and therefore possesses a large amount of potential energy. If the K hole is filled by a

radiative transition from a higher-lying level, a characteristic X-ray is observed. In earlier studies, the mean energy

and the structure of the K„ line were investigated to determine the dynamics of the filling of the K, L, and M

shells [12-18].

Most of the experiments so far have been carried out

with projectile charge states below Q = 20, leaving the very highly charged regime {Q > 30 + ) relatively unexplored [7]. Also, as fjointed out recently by Winecki et al.

[21], most previous experiments have been done with

heavy solid targets and light ion projectiles (Z^on < ^soiid^- a situation which may favor certain processes over others. A complete understanding will undoubtedly need to be

guided and tested by a variety of experiments carried out

for different projectiles, charge states, substrates, energies, and angles. In the present work, we present data from a new study which helps fill out the full range of possible

regimes. We present X-ray data for highly charged but

relatively light ions (Ar'^*) impinging on a substrate containing lighter (Si and O) atoms. Unlike the experiment

of Winecki et al. [21], however, we use a substrate containing atoms which are not too light to be able to produce

inner shell X-rays that can be readily detected through

conventional window materials. This allows us to directly

monitor X-rays produced by the substrate as well as those produced by the projectile. We also vary the total impact

velocity directly, at a fixed angle, rather than using the method widely employed previously of varying only the incident angle to adjust the normal component of velocity.

2. Experiment

The experiments were carried out at the EBIT facility

at the National Institute of Standards and Technology

(Gaithersburg) [22]. Ar'^^ ions were created in the trap and extracted into a continuous beam which was subsequently charge-to-mass analyzed and transferred to our

experimental chamber by a new, highly efficient beam line

[23]. The vacuum in the beam line and experimental

chamber was approximately 10~^ Pa (10"* Torr), so the chance of charge exchange collisions with background gas

was insignificant. Upon extraction from the EBIT, the ions were accelerated to the energy of the upper drift tube of

the EBIT. The collision velocity was adjusted by changing

a voltage applied simultaneously to the sample and to an

aperture located just above the sample. A modification of

the energy due to the space charge of the electron beam in the EBIT was taken into account. Image charge acceleration, which is important for very low velocity and for metal targets, was not accounted for here. We used a 50 nm thick SiOj sample (prepared on a

silicon substrate) as a target. Because we had no in situ surface cleaning capabilities, we expect that there was a

thin layer of hydrocarbons on the surface of the target. As

in related studies by others, however, (Refs. [17,20], etc.) we believe that the presence of this layer did not prevent

us from extracting some general conclusions which are of

considerable significance. In fact, we speculate that the very large internal energies stored in the highly charged

ions may make experiments with them less sensitive to surface contamination than corresponding experiments carried out with low charge ions. The ion beam hit the target at 30° with respect to the normal to the target surface. The X-rays were detected at 90° with respect to the ion beam by a Peltier-cooled silicon PIN diode detector. The detector has an instrumental width

of 250 eV at 5.9 keV X-ray energy. Since we detected X-rays with energy below 2 keV, we had to correct for the transmission through the 25 /xm thick beryllium window

in front of the detector. The solid angle of detection was 4X 10"'* sr. With the present setup, our typical X-ray

count rate is 5 to 10 counts per s. A typical spectrum took approximately 40 min to collect. The energy calibration of

the detector was done with the K„ and lines of Sc, Ti, and Ca from a commercial calibration source.

3. Analysis

In general, the spectra had three main components: two

characteristic X-ray peaks and a broad background (see

Fig. 1). The two peaks, at about 1.75 and 3.1 keV are

identified as K„ X-ray emission lines of Si and Ar respectively (to differentiate between the energy levels and transitions in silicon and argon, we will use the superscripts

K^', K'^', K^', K^'). Note that while we can readily detect K„ X-rays from the silicon, the use of a much lower mass

substrate (carbon) by Winecki et al. [21] positioned the C

K„ X-rays well below 1 keV, into the range at which

conventional X-ray windows are opaque. We attribute the broad background structure in our data

to an instrumental effect consisting of a combination of window transmission and X-rays not coming from the

target (possibly, bremsstrahlung radiation from charged

particles accelerating into the ion deceleration plate). This

background is present even when the target is removed and

changes with the deceleration voltage. A fourth-order reNIST-243
E. Takdcs et al./Nucl. Instr. and Meth. in Phy.s. Res. B 124 {1997) 431-434 433

X-ray energy (keV) X-ray energy (keV)

Fig. I . X-ray spectra at various projectile energies. The spectra are

fitted with a broad background and characteristic X-ray peaks.

The top graph shows the Ar K lines at the highest beam energy. It exhibits the same structure as in earlier observations [12-17].

gression was used to fit the broad structure, excluding the regions of the two sharper peaks. Improved modeling and

correction for the broad structure might improve the accuracy of the final results, but for the present level of

analysis, the simple correction we have made appears to be

sufficient (as evidenced by the quality of the fit described

below).

The two X-ray lines are fit using a Gaussian

function. As seen in the raw data shown in Fig. 1, the

relative intensities of the two K„ X-ray lines vary with

projectile velocity. In particular, the K^' line is the

strongest at low velocity while the K^"^ line is the strongest

at high velocity.

4. Interpretation of the silicon and argon X-ray lines

The energy of the K^' line indicates that the X-rays

originate from neutral or low-charge-state silicon. A simple interpretation of the data is that at low energies, where

highly charged ions are known to be most efficient in removing electrons from surfaces [7], a locally charged

region of the surface builds up and then explodes under its own Coulomb repulsion, producing a high temperature

microplasma which emits X-ray radiation. Although the 10 keV internal potential energy stored in the sum of the

Ar"^ vacancies is more than enough energy to produce

the observed X-rays, large scale molecular dynamics simulations of such Coulomb explosions in silicon [24] show

that even in an idealized situation, the temperature inside the exploding region falls about 100 X short of that re- quired to produce the observed silicon K„ X-rays. Further- more, the simultaneous decrease in intensity of the Ar K„

X-ray peak observed in our data suggests that a more

likely interpretation of our spectra is that a silicon K-shell

electron is being directly transferred to the Ar projectile in a close collision at or beneath the surface. The possibility that this electron transfer from the

silicon K shell to the argon K shell occurs via a radiative coupling can be ruled out empirically by the lack of

observed X-rays at the requisite energy of 2.2 keV. We

attribute the appearance of the silicon K vacancy, therefore, to an interatomic Auger decay involving the transfer of a K-shell electron from the silicon to the K shell of the argon, ejecting a second electron in the process. This

process happens in a close encounter of the argon projectile with a silicon atom in the solid. Since the Bom-Oppenheimer approximation is satisfied for inner shell elec- trons (the ion moves slower than the bound electron) a

transient, quasi-molecule is formed between the projectile ions and the target atom of the solid. In this complex, a vacancy transfer can take place in which the K'^'^ inner

shell vacancy is filled by a K^' electron and a second

electron is ejected.

The most likely candidate for the second electron in the Auger process would seem to be the other remaining

K-shell electron in the silicon, since this electron would be

highly correlated with the first (in the same shell). Since

the energy required to transfer a K-shell electron from

neutral silicon to the K shell of hydrogenic argon is sufficient (barely) to remove the remaining K-shell elec- tron in silicon, it is possible to create this double-vacancy

silicon ion. In this case, however, half of the silicon X-rays

should be emitted with energies that are shifted, due to missing screening from the second K-shell electron, by

approximately 140 eV [25] from the value that we measure. Higher integrity experimental data, with improved

counting statistics for example, would be useful to definitively rule out this possibility. In the absence of evidence for the Auger process with two final vacancies in the K shell, the next likely candi- dates (those with the next highest electron-electron correlation with the K shell) involve the ejection of an L-shell electron from either the silicon or the argon. Since intra- atomic Auger rates decrease significantly with increasing energy of the emitted electrons, the alternative with the most deeply bound L-shell electron is favored. We hypothesize, therefore, that the process we observe is dominated

by a (K'^' K^' L'^') Auger decay.

The observation of the emitted K^'K^'L'^' Auger elec- tron would be the direct evidence for the process, but, in the present experiment, we did not have the capability for

11. CONTRIBUTED PAPERS

NIST-244
434 E. Takacs el al./Nucl. Instr. and Melh. in Phys. Res. B 124 (1997) 431-434

such a measurement. If such a vacancy transfer takes place

with a fairly large cross section it is a competitive way of

filling the inner shell vacancies as pointed out by Briand et

al. in a different context [15]. They considered the quasimolecule Auger transition as a way of filling the M-, N-, and 0-shell vacancies to form hollow atoms within the

solid. Stolterfoht in Ref. [26] provided evidence for autoexcitation producing inner shell vacancies in slow Ar"*" on SiH^ collisions. The author pointed out that the process dominates at very low collision energies. This fact also supports the interpretation of our present findings.

5. Summary

X-ray observations of highly charged Ar""^ impinging on SiOj at total energies of 137 keV and below show that a large signal from the silicon appears when the ion

velocity is decreased, rather than increased. This is inter- preted as a potential energy effect, namely, a K'^'^K^'L'^'^ Auger transfer which leads to the subsequent emission of a

hard Si K„ X-ray. Due to the deep binding energy of the

silicon K shell, one can infer that this process must take

place during close collisions (at or beneath the surface). This direct filling of the vacancy proceeds fast enough to quench the radiative processes which would otherwise allow the projectile to decay by emitting its own characteristic K„ radiation. More work is needed in order to understand the velocity dependence in the context of

presently available models of the interaction of highly

charged ions with solids, and to extend these measure- ments to higher charge state projectiles where the situation may be qualitatively different [7].

Acknowledgements

We thank Dr. Yong-Ki Kim for helpful discussions and

calculations. Part of the work was done with the support of

the Hungarian Science Foundation (OTKA, contract number F 019344). One of the authors (E.T.) acknowledges

the Foundation for the Hungarian Higher Education and

Research for the support.

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NIST-245
PHYSICAL REVIEW B VOLUME 55, NUMBER 4 15 JANUARY 1997-11

Nanoscale modification of silicon surfaces via Coulomb explosion

Hai-Ping Cheng

Department of Physics & QTP, University of Florida, Gainesville, Florida 32611

J. D. Gillaspy

Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

(Received 12 March 1996) Coulomb explosions on silicon surfaces are studied using large-scale molecular-dynamics simulations. Pro- cesses under investigation begin by embedding a region consisting of 265-365 singly charged Si"^ ions on a Si [111] surface. The repulsive electrostatic energy, initially stored in the charged region, leads to a local state with ultrahigh pressure and stress. During the relaxation process, part of the potential energy propagates into the surrounding region while the remainder is converted to kinetic energy, resulting in a Coulomb explosion. Within less than 1.0 ps, a nanometer-sized hole on the surface is formed. A full analysis of the density, temperature, pressure, and energy distribution as functions of time reveals the time evolution of physical

properties of the systems related to the violent explosive event. A shock wave that propagates in the substrate

is formed during the first stage of the explosion, 0</<100 fs. The speed of the shock wave is twice the average speed of sound. After the initial shock the extreme nonequilibrium conditions leads to ultrarapid evaporation of Si atoms from the surface. Qualitatively similar features are observed on a smaller scale when

the number of initial surface charges is reduced to 100. Our simulations demonstrate the details of a process

that can lead to permanent structure on a semiconductor surface at the nanoscale level. The work reported here provides physical insights for experimental investigations of the effects of slow, highly charged ions {Q

>40, e.g.) on semiconductor materials. [50163-1829(97)02104-8]

I. INTRODUCTION

Fabricating structures at the nanometer length scale has become an increasingly active area for physical, chemical,

and material sciences.'"^ Various experimental techniques,

from the assembly of nanosized clusters and chemical etch- ing to ion beam lithography and surface sputtering, have

been developed to construct nanostructured materials.'"''* These and other advances in nanotechnology have created

opportunities for contemporary scientific investigations of

the physical properties of novel materials and the transient

states and processes of physical systems. Many physical phe- nomena that occur in the nanometer regime and femtosecond

to picosecond time domain exist under extreme physical conditions. Much research effort has been focused on the physical mechanisms underlying material properties and

nanomachining.'^"'^ Understanding the physical origins of

the observed phenomena, the time evolution of thermodynamical properties of systems far away from equilibrium,

and the relationships among physical properties, intermediate

states in the physical processes, and the ultrafine structure of

materials is of basic and technological importance.

Accompanying the technological achievements, scientists have been seeking new theories to explain the results observed in nanoscale experiments. Much attention has been

given to the development of new methodology for theoretical

studies. The necessity for new methods arises for several reasons. First, traditional condensed matter approaches can

not be applied directly to nanostructured systems because of

surface or boundary effects. Second, traditional statistical theories are not suited for solving problems associated with

states far from equilibrium. Third, traditional atomic and mo0163-1829/97/55(4)/2628(9)/$10.00 55

lecular theories fail to predict the behavior of nanoscale sys- tems due to the large number of degrees of freedom in these systems.

Advances in computer science and technology have exerted a significant influence on scientific research in the area of nanoscale science. Large-scale computer simulations are becoming powerful tools for investigations of complex

physical and chemical processes.

^'^"^^ By deriving physical

quantities such as kinetic energy, potential energy, pressure, and temperature from the trajectories of atoms in phase

phase, simulations can correlate microscopic pictures with macroscopic phenomena. Time-dependent simulations of molecular dynamics allow studies of physical and chemical

properties as functions of time. Results from the simulations can often be compared directly to the experimental measurements, and can also be used as input for analytical theoretical modeling.'^ Consequently, innovative ideas such as directing

chemical reaction pathways,^'*'^^ controlling cluster-surface

collision outcomes,'^ preparing, size selecting, and identifying nanocrystals' become well understood realities in the laboratory after iterations in the cycle of simulation, modeling, and experimentation.

n. EXPERIMENTAL BACKGROUND

The goals of our investigations are to search for new

methods for nanofabricating of semiconductor surfaces and

to study characteristic features of the phenomena involved in these processes. We are especially encouraged by the recent progress in the electron beam ion trap''*'^^"^'^ (EBIT) experi- ments. In these experiments, the projectile atoms are highly

ionized (Q>40+ typically), and impinge on the target sur2628 © 1997 The American Physical Society

NIST-246
55 NANOSCALE MODIHCATION OF SILICON SURFACES .

.

2629

face with low kinetic energy. Unlike high kinetic energy

particle-surface sputtering experiments, the key issue in the EBIT experiments is the internal electrostatic potential en- ergy of the ions. This internal energy can be over 750 keV,

five orders of magnitude higher than that for a conventional

(singly charged) ion. When a single such highly charged ion

neutralizes itself by impacting a solid, it can create a surface

structure of order 10 nm in dimension. The size of the structure can be varied by adjusting the charge of the incident

ion,^^'^' and the efficiency of the feature formation is essentially 100% (one feature for each incident ion). Speculations

emerging from experimental evidence suggest that there may

be technological applications of the highly charged ions as a new method for modifying or etching semiconductor or in- sulator surfaces.^^'^°~^^ However, there is very little information about the Coulomb explosion process, which is pro- posed to be the main cause for surface damage.^^'^^ In fact, it

is only recently that clear evidence for the existence of sur- face Coulomb explosions induced by slow highly charged

ions has been obtained.^^'^^ In this paper, we report the results of computer simulations of Coulomb explosions on sili- con surfaces. These simulations are performed in conjunction with ongoing experimental studies of highly charged

high Z ion-surface bombardment at NIST.^"

ni. MODELING AND SIMULATIONS

The basic physical picture leading to a Coulomb explosion in a surface is as follows. As a highly charged ion ap- proaches a surface, its intense Coulomb field is sufficient to rapidly pull electrons from the solid, even when the ion is

still many atomic diameters away from the surface (e.g., up

to 540 Bohr radii, in one modeP^). Electrons are captured

into high-lying Rydberg levels, producing a superexcited

"hollow atom," which may be fully neutralized.^-"^-'^ The

atom can decay (collapse) towards its ground state via Auger

cascade, ejecting electrons in the process,^^'^"* or by other mechanisms such as radiative decay or surface plasmon

formation.''^ Only if electrons are ejected fast enough and the

ion approaches the surface slowly enough can electrons continue to be removed from the solid during the ion's approach.

A fully stripped incident ion of atomic number Z can there- fore remove at least Z electrons, perhaps quite a few more,

during its approach. If the ion is not fully neutralized and

collapsed prior to impact, more electrons can be removed

during and after the collision with the surface. Observations

exist for incident charges up to Th^^"^ (Z = 90) where ap- proximately 300 ejected (free) electrons per ion were

detected,^*-'*^ not including the additional Q = 15 electrons

that were removed simply to neutralize the incident ion. Theories predict that by increasing the charge of the incident

ion even further it should be possible to remove over 1000

electrons per ion.'*^ Earlier scaling-law extrapolations from

low-charge data had indicated that this number might be several times larger still.'** If even a fraction of these electrons

are removed sufficiently fast that the solid cannot replenish

them, a localized region of the surface will become charged. Data on GaAs indeed show that the replenishment of electrons from the surrounding solid to the locally charged re- gion does not proceed fast enough to circumvent the effect

that we discuss here.-'* As we will show, the localized

charged region overcomes the cohesive forces that hold the

solid together, and the region explodes under its mutual electrostatic repulsion. Subsequent shock waves and thermal pro- cesses ensue, leaving a crater-like feature in the surface. To

our knowledge, the work reported here is the first full-scale three-dimensional molecular dynamics calculation for the Coulomb explosion of a surface. Earlier work on the problem was restricted to two dimensions, and carried out for 50

times fewer atoms than we consider here.'' The simulations begin by preparing a Si [111] surface in

its ground state. The sample consists of 34 560 atoms distrib- uted over 24 layers. Thus, the thickness of the surface is approximately 40 A (70 a.u.), and the width and length are about 140 A. Periodic boundary conditions are applied only

to the substrate in the x and y directions but not to the ions. This ensures that there are no anificial long-range Coulomb

interactions existing between neighboring unit cells. Our re- sults are checked for artifacts by performing additional simulations with different sample sizes. In order to stabilize the surface, 6 layers of static atoms are placed at the bottom. The

remaining 18 layers, above the static layers, are dynamical.

Temperature control is applied only to the deepest dynamical

layer. This method of applying temperature control leads to a

realistic simulation of atomic motion during a dynamic

event. At the same time, it allows energy exchange between

the system and a constant temperature heat bath. At f=0, 265-365 atoms in a hemispherical region in the center of the surface are singly charged. This initial conditions is chosen to imitate the consequences of bringing a slow, high-charged ion, such as Xe"^'*^ or U"^'^ into the neighborhood of a nonmetal surface. Such an initial condition has been put forth by a number of researchers active in

this field,^^'^^'^^'^* but to our knowledge this is the first time

that the subsequent Coulomb explosion has been investigated

at the atomic level by detailed simulations and calculations

rather than simply depicted as an artist's conception. Since

there is some evidence that the time scale for the electron emission from a surface can be very short,^*'^'*'*"'''*'''' we

separate the dynamics of the surface explosion from the pro- cess of multielectron capture and Auger cascade, which leads

to the initial conditions assumed here. This simplified model

is supported by the evidence of fast Coster-Kronig transi- tions in ion-silicon surface interactions.^'* Here, we focus on

the dynamics of the surface atoms after a huge amount of

repulsive electrostatic energy is suddenly deposited. The

number of ions is chosen to be either 365 or 265. This is consistent with experimental results,^* in which the number

of electrons removed from the surface is found to be more

than 4 times the charge number of the slow ( u, is of order

10^ m/s) incoming ion. Each silicon atom interacts with the other atoms via a three-body Tersoff potential function,'*^ which has been

tested for both crystalline and amorphous silicon. The Si"^ ions interact simply via pairwise Coulomb repulsion. In or- der to describe interactions between Si and Si"*^, we combine

information from experimental measurements and first- principles calculations^" on Si-Si and Si- Si"^ dimers. These

data are then fitted to a function.

V(/-,;) = e (1)

NIST-247
2630 HAI-PING CHENG AND J. D. GILLASPY 55

The term Mr is chosen due to the interaction of the charge

(on Si*) and the induced dipole (on Si). The parameters cr and e are chosen to be 3.69 Oq 9.17 eV, respectively. It should be pointed out that the Si-Si and Si'^-Si"^ potentials used in our simulations are not extensively optimized. However, for the events of interest here, they capture the basic physical features of the important atomic interactions, and

provide sufficient numerical accuracy as well. The equations of motion for each particle in the dynamical substrate are then integrated using Gear's predictor- corrector algorithm.^' The time step of A/ = 0.4 fs is chosen

to maintain energy conservation. The temperature of the bot- tom dynamical layer is controlled to be 300 K. The systems

evolve in time according to classical Newtonian dynamics.

Positions, velocities, and accelerations of all the atoms and

ions are obtained via numerical solution of the classical equations of motion at each time step.

IV. RESULTS

Figure 1 shows several snapshots of a system with 365

ions, at r = 0.0, 40.0, 80.0, and 360.0 fs. We see that the region filled with ions expands significantly at /=40 fs, and a hole is formed by 80 fs. At this point, the size of the hole is similar to the size of the initially charged region. The last snapshot displays a much larger hole with many Si atoms, as well as Si"*" ions, exploding from the surface. More atoms and ions leave the surface as the simulation continues. We

separate the process into two stages. First is the initial, ex- tremely violent explosive stage, which ends at 80-100 fs. This is followed by a period of rapid, high-temperature

evaporation. The detailed dynamics of these stages can be

analyzed by studying various physical properties, such as pressure, energy, density, and temperature.

Figure 2 depicts the temperature of several subdivisions in the substrate as a function of time. These subdivisions are three-dimensional concentric shells that coincide with the center of the hemispherical charged region. The thickness of

each shell is chosen to be 2.6 A (5 a.u.). The innermost shell

is just outside the initially charged region. Panel (a) shows

the results for a 365-ion system: the temperature of each

shell increases sharply at a rate of over lO'* K/s, peaks at over 10^ K, and then begins to relax back towards ambient

temperature. Two striking phenomena can be observed in

this picture. First is the time delay for two adjacent shells to undergo the sudden temperature jump. This is a signature of

shock-wave propagation. The speed of the wave is estimated

to be 2.0X 10"* m/s, or a little more than 2 times the average speed of sound in silicon.^^ The rapid dissipation of the shock wave at about 80-100 fs coincides with the time re- quired for the charged region to expand significantly and lose

its geometric symmetry. Second, we note that the rate of

temperature increase observed here is extremely fast. Compared with other methods used for rapid heating, for ex- ample, by laser heating^^'^'* or cluster-surface collision,'^ lO'* K/s is several orders of magnitude faster. Even if we

factor out the initial impact of the shock wave, the temperature at 100 fs is above 6000 K, corresponding to an average

rate of about lO'^ K/s. After 100 fs the temperature begins to decrease, as Si atoms are ejected from the surface. Panel (b) shows the same quantities for a system starting with 265

ions. Again, we observe a pattern very similar to that in panel (a). The time for the shock wave to collapse also coincides with that in (a). The only noticeable difference is the temperature. The order of the magnitude of the heating rate, however, remains the same. The initial energy in the charged region is calculated to be

87.3 keV (3.21X 10^ a.u.) due to the Coulomb repulsive in- teraction of 365 ions. For 265 ions, the initial energy is 50.8 keV (1.87X 10^ a.u.). Figure 3 shows the three curves rep- resenting the kinetic energy (solid line), potential energy

(dashed line), and energy propagating to the substrate (dotted

line). For r<100 fs, the amount of potential energy converted

to the kinetic energy of the ions is greater than the energy

transfer to the substrate. The two curves cross each other at 95 fs in both panels (a) and (b). At the crossing point, the

total kinetic energy of the ions is slightly less than 10^ a.u. for 365 ions, and 580 a.u. for 265 ions. We estimate tliat these numbers correspond to 31% of the initial potential en- ergy. After 100 fs the rate of increase of the kinetic energy of

the ions slows significantly. The crossing time of the two

energy curves coincides with the ending of the propagation

of the shock wave. Somewhat later, at 110-120 fs, the potential energy drops below the other two energy curves.

In Fig. 4, snapshots of the pressure and potential energy

distributions (for 365 ions) at several times are shown. At t = 0, the large amount of repulsive Coulomb energy stored in the charged region causes an extremely high pressure in the hemispherical region. The highest pressure in the system cor- responds to 1.4 X 10^ GPa of pressure. However, the value of

highest pressure quickly drops to 4.3 X 10^ GPa at ? = 40 fs, and then to 1.8X10^ GPa at r = 80 fs. At ? = 360 fs, the pressure is still very high, about 9.2 GPa, even though it is much smaller than its initial value. The potential energy dis- tribution undergoes a similar evolution, where the highest- energy value for f=0, 8, 16, 40, and 80 fs is 11.0, 10.7, 9.9, 7.3, and 5.2 a.u., respectively. This value further decreases to

1.5 a.u. at 360 fs. Note that the pressure decreases faster than

the energy during the expansion, due to the relation P

^{dEldV).

Density distributions (for the 365 ions system) at different time instants are displayed in Fig. 5. At t=0, the distribution

is uniform throughout the substrate as expected (not shown).

The density of the charged region decreases rapidly, in contrast with the formation of a high density ring near the center

region. The highest density found in the explosion, which

occurs between 40 and 55 fs, reaches more than 50% of the

initial density, thus indicating an extremely nonequilibrium

environment. A hole of 35.3 A in diameter and 13.8 A in depth is quickly formed at the center of the surface. This

initial phase (at r<100 fs) of the hole formation is mainly

due to Coulomb explosion. A total number of 141 particles,

including 115 ions and 26 atoms, are ejected from the sur- face. At t< 100 fs, the large amount of energy that is deliv- ered into the substrate melts the silicon crystal. Eventually, atoms with sufficient kinetic energy evaporate from the surface. At r=360 fs, the size of the hole is about 62.5 A in diameter and 19.6 A in depth. A total number of 768 particles, including 598 atoms and 170 ions, have left the sur- face. The diameter of the hole further increases to more than 10 nm at about 600 fs.

NIST-248
55 NANOSCALE MODIFICATION OF SILICON SURFACES 2631

FIG. 1. (Color) Snapshot of the time evolution of the Coulomb explosion process. Red and green spheres are used to indicate Si^ ions

and Si atoms, respectively. This particular system consists of 365 ions. The initial Coulomb repulsive energy stored in the hemispherical

region is about 87.3 keV. Between f = 0 and 40 fs, the charged region expands significantly. At r = 80 fs, over 100 ions are ejected from the

surface, forming a pronounced hole. By 360 fs, the hole is much larger, and about 800 atoms and ions are driven from the surface.

Figure 5 also shows the temperature distributions at several times, The temperature of hottest spot in the system is 7.1X10^ 2.5X10\ 9,2X10\andl.6Xl()''Kforf = 8, 16,

40, and 80 fs, respectively. We note that the temperature is derived from the total kinetic energy ol" the ions. As can be

seen in the figures, the system is far from equilibrium. Even

after the initial shock wave passes, the temperature gradient

is very large in the substrate. This extremely nonequilibrium

situation is directly responsible for the rapid evaporation of

the atoms.

NIST-249
2632 HAI-PING CHENG AND J. D. GILLASPY 55

o

CD

Q.

E

|2

Subdivisions

20 40 60 80 100 120

time (fs)

FIG. 2. Temperature of subregions of the substrate as functions of time. Panel (a) is the plot of a system consisting initially of 365

Si* ions, and panel (b) is for 265 Si"^ ions. The subregions corre- spond to several concentric shells with a thickness of 5 a.u. The

jumps in temperature seen in each shell indicate a shock wave

propagating through the substrate. The patterns in panels (a) and (b) are very similar. After 100 fs, the temperatures in different shells converge to the same value. Numbers are given in a.u. where

9.50X 10"'' a.u. =300 K.

V. DISCUSSION

The results of our simulations are in rough agreement

with experiments. The fact that the crater does not preserve

the hemispherical shape of the initial conditions but instead becomes more oblate may be partly an artifact due to the

finite number of layers used in the simulation. Results from

simulations with only 100 ions in the initially charged region

show crater formation of a similar type, but with a more

spherical shape. If the crater shape shown in Fig. 5 is con- volved with a typical atomic force microscope (AFM) tip radius of 30 nm the apparent (measured) depth would be

only 0.4 nm for a 10-run-diameter crater. A more spherical

crater of the same diameter would appear to have roughly the same apparent depth, so the distinction between actual depths would not show up in existing experiments. Craters in mica have been observed to have apparent depths of 0.3 run

3500

3000

2500 3 2000

(d 1500 C 500 —o

2000

O

>k 1500

0)

\

UJ

1000

500

1 1 1 1 1 1

1 1 1 1 11 1 1

1

1

I I I I II

(a) j

iri i | iiii|iii i| iii i |

i iii|ii n| ii i i|i m

1 1 . I . . . I . • . I . . . I . . . . I . • I . . .

,

100 200 300 400

Time (fs)

FIG. 3. Kinetic energy (solid line) and potential energy (dashed

line) of the ions during the Coulomb explosion. The dotted line is the energy transferred from the ions to the substrate. Panel (a) is for

the 365-ion system and panel (b) the 265-ion system. Notice that

after the initial shock, which ends at SO- 100 fs, most of the energy

originally stored in the ions is transferred to the substrate. After this time, the potential energy decreases more slowly than at the beginning. At the same time, the gain in kinetic energy of the ions slows down significantly compared to that during the initial 80 fs. Panels

(a) and (b) display similar patterns. The energies are plotted in a.u. where 1 a.u. =27.2 eV.

and diameters of 7 nm,^^ both of which scale up with in- creasing charge of incident the ion.^*''^ As of yet, there are no published data showing crater topography in silicon under

the influence of highly charged ion bombardment. Such data would be of great theoretical and practical interest. The question of whether there is a critical range of electrical conductivity above which Coulomb explosions are quenched by the

ability of free carriers to flow quickly into the region being

charged by the incident ion might be addressed by varying

the initial temperature and doping of the silicon. By using

cryogenic temperatures and highly doped silicon, the full range of possibilities from perfect insulator to semimetals can be probed in one system.

We emphasized that this simulation does not address the question of whether or not surface Coulomb explosions exist (or can be made to) in the laboratory. It only helps elucidate how the process would unfold and what the aftereffects would be if the assumed initial condition could be realized. Together with the explosion time scales determined in this work, a number of atomic physics and materials time scales must also be considered in order to predict what conditions need to be achieved in the laboratory in order to produce the Coulomb explosion.^^"^ '*^'^^ This will be the topic of a more

NIST-250
55 NANOSCALE MODIHCATION OF SILICON SURFACES 2633

I

0.02

0.00

-0.02

U

10

8

6

4

1

(c) (C)

(d)

(e)

4 U

2:

10 I

FIG. 4. (Color) Spatial distributions of pressures (left column) and potential energy (right column) for the system with 365 ions, at several time instants. The times represented in panels (a)-(e) and (a')-(e') are 0, 8, 16, 40, and 80 fs, respectively. All units are in a.u., where 1 a.u.= 2.94X lO'- Pa or 2.94X lO" GPa in pressure. The same color scales are used in (a)-(c) and (a')-(d') to demonstrate the dissipation between 0 and 16 fs. Different color scales for (d), (e), and (e') are used to focus on the detail of the patterns in the plots.

complete analysis once both upper and lower bounds on all the relevant time .scales become better known. Recent experi- ments have yielded evidence that the necessary conditions may already have been achieved in two instances.

^^"^^

We would also like to stress the fact that we consider this simulation to be only a first step towards a more detailed understanding of the Coulomb explosion process. The

present simulations focus mainly on effects derived from

NIST-251

ij
2634 HAI-PING CHENG AND J. D. GILLASPY 55

0.010

0.006

:5? 0.002 t

0.25

0.15

0.012

0.008

0.004

0.00

FIG. 5. (Color) Spatial distributions density (left column) and temperature (right column) for the system with 365 ions, shown at several time instants. The times represented in panels (a)-(e) and (a')-(e') are 8, 16, 40, 80, and 360 fs. All units are in a.u., where 1 a. u. = 6.75 XIO^ ^ or 6.75 X lO^"* cm~^ in density. The same color scales are used in (a) and (b) and (c) and (d) to demonstrate the density changes between 8 and 40 fs. A different color scale is used for (e) to display the hole on the surface at 360 fs. For the right column, each picture has a different color scale due to the rapid changes in kinetic energy. Panels (a')-(d') show the hot leading edge of the ions during the explosion. For (e') two color scales are used to show both the extremely hot region and the details of the temperature distribution in the

substrate. Deep blue to deep red describes 0<7"<0.1 a.u. (or, 0<7"<3xlO'' K), and deep red-pink-white descnbes 0.1<r<1.4 a.u., where 1.4 a.u.=4.4x 10^ K.

NIST-252
55 NANOSCALE MODIHCATION OF SILICON SURFACES . 2635

Stored potential energy. In the case of ion- surface bombardments in which the kinetic energy of the incident ion is comparable to or greater than the potential energy, the effects of

the ion colliding on the surface should be considered. Experimental evidence with highly charged ions on mica, however, shows that crater topography is essentially independent

of the kinetic-energy -potential-energy ratio (R) for R = 0.08-18 (kinetic energy from 4.4-440 keV.^^ Studies on GaAs with singly charged ions having kinetic energy comparable to that of the slowest highly charged ions have

shown craters that consist of only one or a few ejected atoms.^ These craters are so small that they have only been

observed with UHV-STM (ultrahigh vacuum scanning tun- neling microscope) techniques, and they would therefore be

invisible in all of the highly charged ion studies referenced in

this paper. It is reasonable, therefore, to neglect the kinetic energy effects as a first-order approximation, as we have

done in this paper. We note, however, that material- dependent variations (either in the solid or the projectile)

may be important. Molecular dynamics simulations of the impact of a single neutral argon atom with a kinetic energy

of 1 keV have shown that hundreds of atoms may be dis- lodged from volatile (weakly bound) surfaces consisting of a

cryogenically condensed gas.^^ Simulations of experiments

with massive energetic projectiles such as bucky balls^^ impinging on silicon, or multiply charged fast proteins^ impinging on biomolecular surfaces can also cause massive dis- ruptions. Very recent studies of nnica surfaces suggest that very high kinetic energy (80 meV) low-charge ions may produce surface craters similar to those observed with slow, very highly charged ions.^^ Another aspect of the ion-surface interaction that can in- fluence the degree to which our assumed initial conditions

are realistic is the fraction of the ion's internal energy that is neutralized (a) during its approach to the surface, (b) during

impact, and (c) subsequent to impact. Much work is still required to fully understand each of these three phases and,

in particular, the relevant time scales that govern processes

within each of them.

In addition to the limitations mentioned above, this work

does not include heat conduction due to the electronic degree

of freedom. We consider the amount of energy (heat) carried by the electrons as secondary compared to the overall energy

redistribution. Also, the charge transfer between Si and Si"^ has not been considered because the time scale for charge

transfer in semiconductors is much greater than the time

scale of the explosion. More complete simulations of the ion-surfaced interactions are in progress.

ACKNOWLEDGMENTS

The authors acknowledge valuable discussions with Dr.

E. Meyer, as well as his efforts of stimulating collaborations between theoretical and experimental work. The simulations

are performed using computer facilities at Quantum Theory

Project, University of Rorida and at the NIST computing

center.

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(1995) .

NIST-254
Large Scale Molecular Dynamics Simulation of a

Surface Coulomb Explosion.

Hai-Ping Cheng

Department of Physics & QTP

University of Florida

Gainesville Fl 32611

and

J. D. Gillaspy

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

Highly charged ions colliding with a solid have been predicted to produce localized Coulomb

explosions on the surface. We have modeled the explosion using a large scale molecular

dynamics simulation. Our results show the temporal evolution of three different types of craters which are formed when an incident highly charged ion produces 100 singly-charged Si atoms in various initial distributions on the surface. The total number of ejected particles ranges from

245 to 317 and appears to be determined by the initial shape of the ionized region rather than simply by the initial repulsive energy restored in the charged region. Gontraiy to intuition, a long and thin cylindrical distribution is the most efficient pattern for ejecting particles. In all three cases, the number of ejected neutral particles is much greater then the number of ejected

ions (6-10 times as many atoms as ions). The angular distribution of ejected particles is also analyzed.

INTRODUCTION

Recent developments in highly charged ion- surface bombardment experiments (reviewed in [1]) have generated a major need for theoretical modeling

and understanding at the atomic level. In some of the experiments, when a slow, highly charged ion (HCI)

collides with a non-metallic surface, a hillock or crater of nanometer-size can be generated and directly imaged

with atomic force microscopy [1-2]. Several goups have

speculated that research with HCIs may lead to important practical applications in the fabrication of

nanostructures on surfaces [1-7]. The experimental investigations have shown

that the creation of surface features is sustained down to very low kinetic energies, at which point most of the energy is deposited in the form of internal potential energy of the ion [2]. A Coulomb explosion was

postulated as a mechanism for the formation of such

craters as early as 1979 [8]. Since then, considerable

effort by a number of groups [1-15] has been put into studying the electronic transitions and ionization processes during HCI-surface bombardment.

Explicit descriptions and depictions of the

surface Coulomb explosion dynamics [4, 5, 7, 9, 15] have been largely phenomenological in character,

however . Since the lattice dynamics governs the

formation of craters, understanding the Coulomb

explosion at the atomic level is a first step towards a more complete and fundamental understanding of the

overall process. Our previous paper [16], involving

approximately 300 surface charges, was the first that provided a microscopic description of Coulomb

explosion based on a large-scale molecular dynamics

simulation. We demonstrated the explosion process on a

silicon surface with specified model initial conditions. Shockwave propagation, ultra-rapid evaporation of

surface atoms, and the formation of craters were

analyzed in detail, and atomic-scale visualizations of

the explosion process were presented.

In this work, we carry out additional simulations involving fewer initial surface charges and we focus on a comparative study of the influence of the

initial shapes of the charged region on the final crater shape, as well as on the angular distribution of emitted

particles.

SIMULATIONS

Numerical solutions of the Newtonian

equations of motion are obtained via Gear's predict- corrector algorithm [17] for an N particle system. Each

CP392, Application of Accelerators in Research and Industry, edited by J. L. Duggan and I. L. Morgan AIP Press, New York © 1997 197

NIST-255
of the 34560 particles interacts with each of the other

particles. We use the Tersoff potential [18] for Si-Si

interactions, 1/r Coulomb potential for Si*-Si* interactions and a 12-4 potential of the following form

for Si-Si* interactions:

V(i;,)=£

12

(1)

The parameters e and o are obtained as described

previously [16] by combining first-principle calculations

and experimental data. The substrate is represented by a slab that is periodically repeated in x and y directions. In the

positive z direction, we use free boundary condition for the surface. We use a few static layers at the bottom of

the substrate to stabilize the systems. By controlling the temperature of the layer(s) between dynamical layer

and static layers, we simulate a infinite substrate in -z

direction. To properly treat the system we are interested

in, the periodic boundary conditions in x, y directions

are not applied to ions. At t=0, 100 Si ions are embedded in the Si

surface. The shape of the three initial charge

distributions we choose to study are hemispherical, flat disc, and long thin cylindrical. The initial temperature

of the substrate in all three cases is very low, about 10

K. The temperature of the static layers is fixed during

the explosion processes to provide a heat bath for the dynamical layers above. We use time interval. At = 0.4

fs, to achieve energy conservation during the processes. The simulations are carried out for 1 .6 ps.

(b)

(c)

RESULTS

Three initial shapes of the charged region is shown in figure 1. The positions of the atoms and ions at 160 fsec after the inditial condition is illustrated in figure 2. At 1.6 ps, we collect particles that are more

than 0.6 nm (12 a.u.) above the surface layer for each of

the three simulations. The total numbers of ejected

particles are 245, 245, and 317 for (a) hemisphere, (b)

flat disc, and (c) long cylinder. The number of ejected

ions of these three situations are 31, 35, and 24

respectively. At a first glance, the relative numbers of

ejected ions is intuititive. Since (b) has the most ions on the surface layer, it is relatively easier for the ions to be pushed out. For the same reason, case (c) has the

least number of ejected ions. Nevertheless, the total numbers of ejected particles are not what we would

expect since they do not increase or decrease

monotonically as a function of total initial energies. The

repulsive Coulomb energies stored in the charged region

for the three shapes (a)-(c) at t=0 are, 9.61 keV, 8.95 keV and 9.33 keV respectively. By summing up the

FIGURE 1. Molecular dynamics simulation of a Coulomb

explosion in silicon at t=0.0 for initial charged region shaped as (a) hemisphere, (b) disc, and (c) cylinder. Ions are indicated with a daker shade than the atoms.

amount of energy dissipated into the substrate, we find

that case (c) is higher than the other two cases. The

higher efficiency of this geometry for depositing energy

into the lattice explains the increased evaporation of surrounding lattice atoms.

From the simulation data, angular distribution of the total ejected particles can be obtained. Fig.3

displays the distribution of azimuthal angles of three

simulations. All of the three histograms have a similar broad distribution over 0-100 degrees. On the other hand,

the distribution patterns in the ejected ions of the three

situations are quite different from each other (see Fig.4).

The cylindrical shape has a relatively uniform

distribution.

198

NIST-256
9

(a)

(b)

(c)

FIGURE 2. Molecular dynamics simulation of a Coulomb

explosion in silicon at t=0.160 for initial charged region shaped as (a) hemisphere, (b) disc, and (c) cylinder.

-' I

'

I

' I'

I

'

I 'I

' uu ' I '

I 'I

' I'

I

'

I

' I'

I

' I'

I

'

I '-I

Angle (degree)

FIGURE 3. Distribution of azimuthal angle of all the particles

(Si* and Si) ejected from the surface. Panel (a)-(c) represent a 100 ion system with a initial charge distribution of hemisphere,

flat disc, and cylinder respectively.

CONCLUSIONS

Our simulation of Coulomb explosions on a Si surface suggest that t=100 fs is an important time scale. In comparing this work to our previous work, we find

that for all the systems under investigation, such as systems with 100, 265, and 365 ions, 10^ fs is the time

for Shockwaves to collapse and for potential energies to drop to about e

' of their initial values. The numbers of

ejected particles depend on the energy that is dissipated

into the substrate. This is the mechanism for the initial

199

NlST-257
10 I

' I ' I ' I' I ' I ' I

' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I

Angle (degree)

3. R.W. Schmieder and R.J. Bastasz, Proc. Vlth Intl. Conf. Physics Highly Charged Ions, AIP Conf. Proc. 274, 675

(1992).

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5. 11. R. Morgenstem and J. Das, Europhysics News, 25, no. 1, 3 (1994)

6. J.D. Gillaspy, Y. Agilitskiy, E.W. Bel!, CM. Brown. C.T. Chantler, R.D. Daslettes, U. Feldman, L.T. Hudson, J.M. Laming,

E. S. Meyer, C.A. Morgan, A.L. Pikin, J.R. Roberts, L.P. Ratliff, F. G. Serpa, J. Surga, and E. Takas, Physica Scripta, T59, 392

(1995).

7. I. Hughes, Physics World, 8, no. 4, 43 (1995);

8. I.S. Bitenskii, M.N. Murakhmetov and E.S. Parilis, Sov. Phys.

Tech. Phys. 24, 618(1979).

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11. F. Aumayr, H.. Kurz, D. Schneider, M.A. Briere, J.W. McDonald, C.E. Cunningham, and H.P. Winter, Phys. Rev. Lett., 71, 1943 (1993); P. Varga, Comments At. Mol. Phys., 23, 111 (1989).

12. J.P. Briand, B. d'Etat-Ban, D. Schneider, M.A. Briere, V. Decaux, J.W. McDonald, and S. Bardin, Phys. Rev. A, S3, 2194

(1996) . 13. J. Burgdorfer, C. Reinhold, and F. Meyer, Nucl. Inslrum. and

Methods in Phys. Res., B, 98. 415 (1995). 14 M. Grether, A. Spielder. R. Kohrbruck, and N. Stolterfoht, Phys. Rev. A, 52, 426 (1995).

15. E. Parilis, Caltech preprint.

16. Hai-Ping Cheng and J.D. Gillaspy, Phys. Rev. B, Jan. 15

(1996).

17. M.P. Allen and D.J. Tiidesley, Computer Simulations of Liquids (Clarendon, Oxford, United Kingdom, 1987).

18. J. Tersoff, Phys. Rev. B 39, 5566 (1988).

FIGURE 4. Distribution of azimuthal angle of all the ions

(Si* only) ejected from the surface. Panel (a)-(c) represent a 100

ion system with a initial charge distribution of hemisphere, flat disc, and cylinder respectively. Notice the stronger shape

dependence in fig.4 than in flg.3

shape dependence of the total ejected particles. Finally, we conclude that the fraction of energy that goes into the substrate decreases as the number of ions increases

from 100 to 365.

REFERENCES

1. D.H. Schneider, M.A. Briere, Physica Scripta, S3, 228

(1996).

2. D.C. Parks, R. Bastasz, R.W. Schmieder, and M. Stockli, J. Vac.

Sci. Technology. B13, 941(1995).

200

NIST-258
VOLUME 9, Nos. 3-4

JANUARY 1 998

COMA^AT 9 (3-4) 285-434 (1998)

http://www.elsevier.nl

COMPUTATIONAL

MATERIALS

SCIENCE

Editors;

U. LANDMAN and

R.NIEMINEN

lAST ISSUE OF THIS VOLUME

ELSEVIER

ISSN 0927-0256

NIST-259
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Coven Distributions of porticle, velocity ond temperoture during Coiomb explosion. For more detoils see the poper by H.-P. Cheng ond J.D. Gillospy on page 285 of this issue.

NIST-260
ELSEVIER Computational Materials Science 9 (1998) 285-294

COMPUTATIONAL

MATERIALS

SCIENCE

Surface Coulomb explosions: The influence of initial charge

distributions

Hai-Ping Cheng ' *, J.D. Gillaspy ^

' Department of Physics and QTP, Uniuersity of Florida, Gainesville, FL 32611. USA

** Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Received 14 March 1997; accepted 10 May 1997

Abstract

Using molecular dynamics simulations, we have studied the Coulomb explosion processes on silicon surfaces. Three

different initial shapes of ion distributions are used to model the possible localized charge distributions generated in highly

charged ion (HCI)-surface impact. The three distinct distributions include a hemispherical, a flat disk, and a long and thin cyhndrical geometrical. At ; = 0, 100 singly-charged Si ions are embedded in the Si(lll) surfaces. In about 100 fs, the strong repulsive electrostatic forces in these three systems cause Coulomb explosions and thus create three different shapes

of craters on the surfaces. All simulations are carried to 1.6 ps, at which point the size and shape of the craters are nearly

stabilized. The detailed analysis of the ejected ions, atoms, and the substrates reveals the dynamical consequences of the

different initial conditions. For these 100 ions, the differences in the total number of the ejected particles, ranging from 245

to 317 particles, appear to be determined by the initial shape of the ionized region and not by the initial repulsive energy

restored in the charged region. Contrary to intuition, a long and thin cylindrical distribution is the most efficient pattern for ejecting particles. The underlying mechanism is that ions with this initial configuration transfer more energy to the surrounding atoms. In all three cases, the number of ejected neutral particles are much greater then the number of ions (6-10

times as many atoms as ions). Among the ejected particles, a small percent of particles are found to return the surface at a

later time. The angular distribution of ejected particles are also analyzed. While the differences in the distributions of polar

angle of the Si atoms of the three configurations is small, the differences in distributions of the ions portray a strong shape

dependence in the polar angle. © 1998 Published by Elsevier Science B.V.

1. Introduction

Recent developments in highly charged ionsurface bombardment experiments [1-5] have generated a major need for theoretical modeling and un- derstanding of the HCI-surface interactions and the

Corresponding author. Tel.: 1-352-3921597; fax: +1-352-

3928722; e-mail: cheng@qtp.ufl.edu.

dynamical consequences of the interactions at the atomic level. The basic physical picture of HCIs

interacting with a surface is as follows (Fig. 1): when

a HCI approaches a surface, its intense Coulomb

field rapidly pulls electrons from the solid, even when the ion is many atomic diameters away from

the surface (up to 80 bohr radii or more in some

models [6,7]). Some of the emitted electrons are captured into high-lying Rydberg levels of the ion,

producing a super-excited 'hollow atom' [8-10],

0927-0256/98/S 19.00 © 1998 Published by Elsevier Science B.V. All rights reserved. PI} 50927-0256(97)00048-7

NIST-261
286 H. P. Cheng. J.D. Gillaspy / Computational Materials Science 9 ( 1998) 285-294

while Others simply escape. The hollow atom can

rapidly decay (collapse) via Auger cascade, ejecting

electrons in the process [11,12]. The ion can then

continue to remove electrons from the solid, eventually removing many more electrons than would be

required for a simple neutralization of the ion. Fur- thermore, highly excited surface atoms can also un- dergo processes that contribute to the massive elec- tron emission. Emissions of up to approximately 300

electrons per ion have been observed [11,12], and some theories predict that by further increasing the charge of the incident ion it should be possible to remove over 1000 electrons per ion [6]. In insulators or semiconductors, experiments on

multiply charged ion (MCI)-LiF [13], MCI-GaAs

[14,15], HCI-Si02, and HCI-mica surface bombardment have shown that at a given kinetic energy,

the sputtering yield and/or secondary ion yield

and/or size of induced surface defect increases as

the charge of the projectile ions increases. The ef- fects are much larger for HCI bombardment compared with conventional ion-surface bombardment.

Nanometer size surface features have been observed

via atomic force microscopy (AFM) in some cases. We note that ordinary surface sputtering relies on

direct contact between the projectile ion and the

surface and is dominated by the kinetic energy. The

experimental investigations have also shown that the

creation of surface features is sustained down to very

low kinetic energies at which point most of the

energy is deposited in the form of internal potential

energy of the ion [2]. This phenomenon suggests the

potential application of HCI beams in the fabrication

of shallow nano- structures on surfaces. A Coulomb explosion was postulated as the

mechanism of the formation of the craters in the

Fig. 1. Schematic picture of HCl-surface interactions, (a)-(c)

illustrate three of the possible Auger cascade processes after t^, the lifetime of a particular excited state A ' of the projectile ion.

HCI-surface interactions in the early 70' s [16]. Since

then numerous efforts have been made in studying

the electronic transitions and ionization processes

during the HCI-surface bombardment [17-24]. Most

of the descriptions of the surface dynamics have

remained primarily phenomenological until very re- cently, when we published the first full-scale molecular dynamics simulations [25] that provide a microscopic description of Coulomb explosions on Si [1 1 1] surfaces. In our previous paper, we have demonstrated the explosion processes with simple model

initial conditions. Shockwave propagation, ultra-rapid

evaporation of surface atoms, and the formation of

craters were analyzed in detail and the first atomicscale visualizations of the explosion process were

presented. Previous depictions of surface Coulomb

explosions have been essentially artist's conceptions

[3,26-31].

Since the lattice dynamics govern the formation

of craters, continuing of our investigations of the Coulomb explosion at the atomic level is necessary

and an important step towards a more complete and

fundamental understanding of the overall process. Our previous study is based on a simplified theoretical model that assumes an instant ionization of atoms

in a hemispherical region in the surface. Realty,

however, can be more complicated. The detailed

dynamics of charge transfer from the surface to the incoming ion and the charge flow from the substrate

to the surface remains unknown. While our initial results capture the fundamental physical picture, there are still many dimensions in the parameter space

which can be explored. A modification of the simple

model will allow the investigation of the effects of

the initial density distribution of the charged region.

This focus will allow for the possibility of multiplycharged surface ions, ions spreading over a larger area with neutral atoms in between, and a variety of

geometrical shapes. A large range of physical situations, including a partially quenched Coulomb explosion, can then be modeled. To date, very little experimental information is available on the exact distri- bution of the positive charge. Investigations of the correlation between the initial density distribution

and/or geometry of the charged region and the topology of the craters will provide valuable input

for theoretical modeling of electron emission and the resulting crater topography. Further improvement of

NIST-262
H.-P. Cheng. J.D. Gillaspy / Computational Materials Science 9 1 1998) 285-294 287

the model will include the rate of electron hopping

from substrate to the surface ions, the initial kinetic energy of the incident HCI, the time duration needed

for a complete neutralization of the HCI, and differ- ences among different surface materials. All of these processes are involved in crater formation. It is necessary to isolate and test the importance of each

individual factor. In this work, simulations were performed with a focus on the comparative study of the influence of

the initial shapes of charged region on the final crater shape, as well as on angular distribution of

emitted particles. We investigated the Coulomb ex- plosion on Si(lll) surfaces caused by the repulsive

interactions among the 100 ions. From the simulation

results, we analyzed the differences in the physical

properties such as number of ejected atoms and ions,

angular distributions, and energetics as consequence

of the different shapes of the initial charged region.

Dynamics of substrate atoms were studied via a detailed analysis of their phase-space trajectories.

This work has demonstrated the dynamic properties

of the surface for a longer time period than our

previous work, and along all directions of Si(lll)

NIST-263
288 H.-P. Chenii. J.D. Gillaspy / Compuuitional Materials Science 9(1998) 285-294

Subdivisions

0.3

0:2 CO

0.1

. ' ' '

1 1

1 I 1 1

1 1 11 1 11 1 1 . ' ' 1 ' • ' 1 ' ' ' 1

'

.

Am ^

0 200 400 600 800 1000 1200 0 50 100 150 200 Time (fs)

Fig. 3. Temperature of sub-regions of the substrate as a function of time. The subregions correspond to several concentric shells with a

thickness of 0.255 nm (5 a.u.). The jumps in temperature seen in each shell indicate a shock wave propagating through the substrate. After 120 fs, the temperatures in different shells converge to the same value. Numbers are given in a.u. where 9.50X 10""* a.u. = 300 K.

surface crystalline. Shockwave propagation and dis- sipation are demonstrated using visualization. Pre- liminary results from part of this work were pre- sented earlier in a short conference paper [32].

2. Simulations

Classical MD simulations based on parameterized

interatomic potentials are used to study Coulomb

explosion processes. The classical equation of motion m,-^=-^U {R„R„ ...,R„), (1)

for each atom (ion) in the dynamical substrate is integrated using Gear's predictor-corrector algorithm [33]. Positions, velocities, and accelerations of

all the atoms and ions are obtained via numerical

solution of the classical equations of motion at each

time step, and stored for later analysis. From the

phase-space trajectories, physical properties such as distributions of temperature, pressure, stress, and

potential energy are derived [25,34,35].

The model interaction potential functions are cho- sen to properly describe the material properties and

the dynamical processes. In our simulations we use Tersoff potential [36] for Si-Si interactions, 1 /r

Coulomb potential for Si^-Si"^ interactions and a 12-4 potential for Si-Si interactions. The 12-4

potential has the following form:

12 4" -

(2)

The parameters e and a are obtained as described

previously [25] by combining first-principle calcula- tions and experimental data. It should be pointed out

that the Si-Si and Si"^-Si"*" potentials have not been optimized extensively. However, for the events of interest in this study, this potential captures the basic physical features of the important atomic inter- actions.

Fig. 4. Top view of a Si surface initially with a (a) hemispherical, (b) flat disc, and (c) long cylindrical charged region. The snapshots are taken at r = 400 fs. The colors indicate the value of the square root of the speed of the atoms. The numbers are given in a.u. where I.95X 10"^ a.u. = 1.0 km/s. Hexagonal Shockwave fronts for all three cases can be observed.

Fig. 5. Top view of a Si surface initially with a long cylindrical charged region. The snapshots are taken at (a) / = 500 fs and (b) 600 fs. Panel (c) is a top view of 1.02 nm (20 a^) under the surface at 400 fs. The colors indicate the value of the square root of the speed of the atoms. The numbers are given in the same units as defined in Fig. 4. The Shockwave fronts are again visible with a hexagonal geometry in panels (a)-(c).

NIST-264
NIST-265
290 H. -P. Cheni^. J D. Gillaspy / Computational Materials Science 9 {I99H} 285-294

The substrate is represented by a slab that is periodically repeated in xy directions. The total number of atoms (ions) in the unit cell is chosen to be 45,360. The size of the slab is about 13 X 13 X 5.6

nm"\ The direction of the (111) surface is chosen

along positive z, in which a free boundary condition

is applied. The total number of surface layers is 17, out of which three layers at the bottom of the

substrate are held static to stabilize the systems.

During the explosion, the temperature control is applied to a layer right above the three static layers

to allow heat exchange between the system and a

heat bath. This arrangement allows simulations of a

substrate which is infinite in negative z direction. To

properly treat the system for our dynamic studies, the

periodic boundary conditions in x, y directions are not applied to ions. As we already introduced in the previous paper

[25], a simple model is used to describe the initial conditions. In this simple model, the ionization of a

local region in the surface occurs much rapidly

compared to lattice motion. This simplification is made to simulate the consequences of bringing a slow, highly-charged ion, such as Xe'""*^ or U^^^ to the vicinity of a non-metallic surface. Based on the simple model, at / = 0, 100 Si ions are embedded in the Si surface. The shape of the three initial charge

distributions we choose to study are: (a) hemispherical, (b) flat disc, and (c) long thin cylindrical. The

initial temperature of the substrate in all three cases

is sufficiently low, about 10 K, so that it is a very

poor electrical conductor under equilibrium conditions. The temperature control to the deepest dynamical

layer is applied during the explosion processes. Initially, we used a time interval, Af = 0.4 fs, to achieve

energy conservation during the processes. After about

1.0 ps, the time interval is increased to 0.8 fs to maximize efficiency. All three simulations are carried out for 1 .6 ps.

3. Results

The positions of the atoms and ions at r = 0 and

1.6 ps are illustrated in Fig. 2 (cross sections). The

radius of the hemisphere is 0.92 nm (18 a^). The

height and radius of the flat disc is 0.37 nm (7.3 a^).

and 0.97 nm (19 Qq), respectively. For the cylindrical shape, the height and radius are 1.53 nm (30 Oq),

and 0.577 nm (1 1.3 Oq), respectively.

The temperature analysis in the subdivisions adja- cent to the explosion center indicates strong shock wave propagation in the substrates for the first 100 fs (Fig. 3). This phenomenon has been described in our previous paper for systems with 265-365 ions. The

speed of the Shockwave in case (a) is 15 km/s. This

is slightly lower than the value in the systems with more ions [25] since the energy that causes the Shockwave is lower in the 100 ion case compared to the 265 or 365 ion cases. Further analysis on the kinetic energy reveals that the shock waves can last much longer than 100 fs, which is the time scale for extremely strong shock we presented in our previous

paper.

In Fig. 4 we illustrate the distribution of kinetic energy of each individual particle in a system. The

color bar is calibrated according to the square root of

speed of the atoms in order to increase the contrast of the wave front. At 400 fs, we can see the shock wave moving away from the center of the explosion.

All three simulations display a hexagonal shape of

the wave front which reflects the symmetry of sur- face crystalline structure. The cylindrical shape, case (c), at 400 fs displays

the most clear picture of shock wave propagation.

The hexagonal patterns are similar to the one in Fig.

4 upon viewing the lower surface layers (Fig. 5). The

patterns are blurred at later times (Fig. 5), indicating further dissipation of the shock. At 600 fs, the wave

fronts reach the boundary of the system, and our

simulation with the chosen unit box size cannot

properly follow the propagation any further. The disc shape, case (b), shows a different picture. It does not have a clear shock wave ring as seen

in case (c). On the other hand, in some directions, the energy propagation is faster than most of the other directions. The colors indicate that some parti- cles ahead of the hexagonal wave front carry higher

kinetic energy than particles at the hexagonal shock wave front. This fast-modes propagation is quasi-one

dimensional as shown in Fig. 4. The characteristic pattern quickly diminishes as the depth into the surface layer increases.

The hemispherical shape, case (a), falls in be- tween (b) and (c). It should be mentioned that the

]NnST-266
H.-P. Cheng. J.D. Gillaspy / Computational Materials Science 9 { 1998) 285-294 29]

'square root of velocity' color scale makes the Shockwaves considerably more easy to visualize. Although this choice is still somewhat ad hoc, it suggests a way to detect Shockwave fronts in a complicated system. Further development is underway to process the images.

At 1.6 ps, we collect particles that are more than

12 a.u. above the surface layer for each of the three simulations. The total numbers of ejected particles are 245, 245, and 317 for (a) hemisphere, (b) flat disc, and (c) long cylinder. The numbers of ejected

ions corresponding to the three situations are 31, 35,

and 24, respectively. At first glance, the relative numbers of ejected ions is intuitive. Since (b) has the most ions on the surface layer, it is relatively easier

for ions to be pushed out. For the same reason, case

(c) has the least number of ejected ions. Nevertheless, the total numbers of ejected particles are not what we would expect since they do not increase or decrease monotonically as a function of total initial energies. The repulsive Coulomb energies stored in

(A

0)

(0

0)

E

3

100 Angle (degree)

Fig. 7. Distribution of the polar angle of all the particles (Si"^ and

Si) ejected from the surface. Panels (a)-(c) represent a 100 ion system with a initial charge distribution of hemisphere, flat disc, and cylinder, respectively.

>

0

Ui

0)

"5>

C

u

0 400 800 1200 1600 Time (fs)

l ig. 6. Energies as a function of time. The solid line is the

ptiiential energy of the ions, the dashed line is the kinetic energy

ol the ions, and the dotted-dashed line is the energy dissipated

nilo the substrate. Panels (a)-(c) represent a 100 ion system with

an initial charge distribution of hemisphere, flat disc, and cylinder,

respectively. Notice the curve crossing at 100 fs.

(0

o

n

E

3

0 20 40 60 80 100 Angle (degree)

Fig. 8. Distribution of the polar angle of all the ions (Si"^ only)

ejected from the surface. Panels (a)-(c) represent a 100 ion system with a initial charge distribution of hemisphere, flat disc, and cylinder, respectively. Notice the stronger shape dependence

in Fig. 3 than in Fig. 2.

NIST-267
292 H.-P. Cheng, J.D. Gillaspy / Computational Materials Science 9 (J998) 285-294

the charged region for the three shapes (a)-(c) at

r = 0 are 9.61, 8.95 and 9.33 keV, respectively.

To explain this phenomenon, we study the partitioning of energy during the explosions. Fig. 6 displays the time dependence of three energies: the

potential energy of the ions, the kinetic energy of the

ions, and the energy dissipated into the substrate. It

is immediately seen that for case (c) the third energy

Fig. 9. Density plots of the three systems at 1.6 ps. Panels (a)-(c) are results from simulations with hemispherical, flat disc and thin cylindrical shape initial charged regions, respectively.

NIST-268
H.'P. Cheng. J.D. Gillaspy / Compiitaiional Maienals Science 9 (1998) 2S5-294 293

(energy dissipated into the substrate) is higher than

the other two. The total increase of kinetic energy of

the ions are, on the other hand, lower than in case (a)

and (b). This explains why case (c) has only 24 ions

ejected from the surface while the total number of

ejected particles is the highest among the three. The

higher efficiency of the geometry of case (c) for depositing energy into the lattice also explains the

clear shock wave pattern at 400 fs as we discussed

earlier. There is another interestmg feature seen in Fig. 6. At ? = 100 fs, the three curves cross over for (a) and

(b). For (c) two of the three curves cross each other

while the third energy does not increase enough to meet the other two. From our previous paper, 100 fs

is the time duration for the shock wave to collapse

(or weaken its strength substantially). The energy

curves demonstrate the same crossover pattern for 345 ions and 256 ions (both have hemispherical

initial configurations).

We also notice that systems with fewer numbers

of ions deliver energy more efficiently to the substrate compared to systems with higher numbers of

ions. At r = 400 fs, the electrostatic energy that is transferred to the substrate in systems (a)-(c) is 58%, 65% and 70%, respectively. Meanwhile, only

49% of energy is transferred in a system with 265

ions and 46% in a system with 365 ions. Here, a

direct comparison gives a ratio of 58%:49%:46% for

the systems with 100, 265, and 365 ions which are

initially distributed in a hemisphere.

From the simulation data, angular distributions of

the total number of ejected particles are obtained

from the direction of the velocity of the particles.

Fig. 7 displays the computed distributions of polar

angles, i.e. the angles between the velocity of ith atom and z coordinate, from three simulations. All of the three histograms have a similar broad distribution over 0-100 degree. Note that there is finite population between 90-100° that indicates a small

percentage of particles may eventually return to the surface. On the other hand, the distribution patterns

of the same angle in the ejected ions calculated from

the three situations are quite different from each

other (see Fig. 8). The cylindrical shape has a rela- tively uniform distribution compared to the other two

cases. Finally, the shapes of the craters generated by the

three initial charge configurations is depicted in Fig.

9. The number densities of atoms (ions) in the

substrate are obtained by averaging over the az- imuthal angles. The distributions are then displayed

in a two dimensional plane. The hemispherical ge- ometry is preserved after the explosion in case (a). In

case (b), the crater has the shape of frustum of a cone, and in case (c), a cone-shape crater (with a

blunted tip) is formed after the explosions. These

shapes suggest that the shapes of the initially charged

region, which have been shown from our studies to be important in energy partitioning, may be traced

back from the topology of the final shape of the

craters.

4. Conclusions

Our studies of Coulomb explosion on a Si surface

suggest that r = 100 fs is a characteristic time-scale.

For all the systems under investigation, such as systems with 100, 265 and 365 ions, 100 fs is the time for ultra strong Shockwaves to dissipate dramatically and for potential energies to drop to about e~'

of their initial values. At a later time, a few hundred

femtoseconds, Shockwaves continue to propagate.

The wave fronts preserve approximately the surface

symmetry, i.e. hexagonal shape. The intensities and

speeds of the waves decrease as they propagate away

from the explosion centers. In addition to the normal

Shockwave modes, we have also observed a special one dimensional Shockwave mode which propagates

at a much faster speed than the two dimensional

waves. The total number of ejected particles depends on

the energy that is dissipated into the substrate rather than the total initial potential energy of the ions. This

is the mechanism for the initial shape dependence of

the total ejected particles. From the energy analysis we also conclude that the fraction of energy that goes into the substrate decreases as the number of

ions increases from 100 to 365. This fact suggest that there will be saturation of the yields as a function of

the initial charge number in potential energy driven

sputtering processes. Contrary to the simple picture in which a surface

Coulomb explosion is presumed to eject primarily

ions, our simulation predicts that 90% or more of the

NIST-269
294 H P. Cheng. J.D. Gillcispy / Computational Materials Science 9(1998) 2S5-294

ejected material may be neutral. Previously, a qualitative application of the simple picture to experimental data (which showed that most of the ejected

material is neutral) led some investigators [37] to conclude that the Coulomb explosion is a subdominant process. Our results indicate that even a simple

model of the Coulomb explosion can lead to the same qualitative results, and therefore a more quantitative analysis is required before forming conclusions about the dominance of various mechanisms.

We believe that the simulations should be refined

further and the experiments extended to higher charge

states (where the Coulomb explosion model may be more applicable) before such a comparison is made,

however. A model that includes change of the elec- tronic states of the substrate atoms (ions) would

allow the investigations of effects of electronic excitation and charge transfer processes in the substrates

during the explosion, for example. We are presently

working to achieve this goal by implementing the electronic-atomic hybrid molecular dynamics

method. Meanwhile, the simple model presented here

has already yielded significant insights into the basic nature of the surface Coulomb explosion scenario.

Acknowledgement

Acknowledgement is made by the authors to the Department of Energy for partial support of this work, and to the NIST supercomputer center and

QTP computer center.

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;i7] P. Apell, J. Phys. B 21 (1988) 2665.

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;20] F. Aumayr, H. Kurz, D. Schneider, M A. Briere, J.W. McDonald, C.E. Cunningham, H P. Winter, Phys. Rev. Lett 71 (1993) 1943.

[21] J.P. Briand, B. d'Etat, D. Schneider. M. Clark, V. Decaux,

Nucl. Instrum. Methods B 87 (1994) 138.

[22] J.P. Briand, B. d'Etat-Ban, D. Schneider. M A. Briere, V. Decaux, J.W. McDonald. S. Bardin. Phys. Rev. A 53 (1996)

2194.

[23] J.P. Briand, L. de Billy. P Charies, S. Essabaa, P. Briand, R.

Geller, J.P. Desclaux, S Bliman, C. Ristori. Phys. Rev Lett. 65 (1990) 159.

[24] B. d'Etat, J.P. Briand, G. Ban, L. de Billy, J.P. Desclaux. P. Briand, Phys. Rev. A 48 (1993) 1098.

[25] H.-P. Cheng. J.D. Gillaspy, Phys. Rev. B .55 (1997) 2628.

[26] R. Morgenstem, J. Das, Europhys. News 25 (1) (1994) 3.

[27] R. Morgenstem, J. Das, in: J.W. Rabalais (Ed.), Low Energy

Ion-Surface Interactions, Wiley, Chichester, 1994, p. 190.

[28] D. Schneider, M A Briere, T. Schenkle, EBIT, N-Division,

Livermore Annual Report, 1994.

[29] I. Hughes, Phys. Worid 8 (4) (1995) 43.

[30] E. Parilis, Caltech preprint.

[31] K Mochiji, N. Itabashi, S. Yamamoto, H. Shimizu, S. Ohtani. Y. Kakto. H Tanuma, K. Okuno, N. Kobayashi,

Surf. Sci 357-358 (1996) 673.

[32] H.-P. Cheng, J.D. Gillaspy, Proc. of the Fourteenth Int Conf. on the Application of Accelerators In Research and Industry. Denton, TX, 1997, in press. [33] M.P. Allen, D.J. Tildesley, Computer Simulations of Liquids,

Clarendon, Oxford, United Kingdom, 1987.

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NIST-270
mm

ELSEVIER

Beam Interactions

with Materials ft Atoms

Nuclear Instruments and Methods in Physics Research B 134 (1998) 46-52

Non-kinetic damage on insulating materials by highly charged ion

bombardment

D.C. Parks M.P. Stockli ^ E.W. Bell L.P. Ratliff ^ R.W. Schmieder ^

F.G. Serpa J.D. Gillaspy ^

* Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

^ J. R. Macdonald Laboratory, Department of Physics, Kansas Slate University, Manhattan, KS 66506-2604, USA

Diamond Semiconductor Group, Gloucester, MA 01930, USA

^ Sandia National Laboratories, Livermore. CA 94551, USA

Received 19 May 1997; received in revised form 26 August 1997

Abstract

We have measured the damage caused by the impact of low velocity, highly charged ions on insulating surfaces. Atomic force microscopy allows us to observe directly the surface topography with nanometer resolution. Using constant velocity (100 keV) Xe*"^ ions (25 ^ ^ ^ 50) impinging on mica, we observe damage caused by single ion impacts.

Impact sites typically are circular hillocks. Within the range and accuracy of the data, the height and volume of the damaged regions are well approximated by a linear function of ion potential energy. © 1998 Elsevier Science B.V.

PACS: 34.50.Dy; 61.16.Ch; 61.72.Qq; 61.80.Jh

Keywords: Highly charged ion; Surface damage; AFM; Coulomb explosion; EBIT; EBIS

1. Introduction

While the effects of ion bombardment on solid

materials due to the kinetic energy carried by ions

is well known, the role of the potential energy carried by the ions is much less understood. Using

highly charged ions of a single atomic species

and holding the velocity at a low constant value,

'Corresponding author. Tel.: 301 975 3131; fax: 301 975

3038; e-mail: daniel.parks@nist.gov.

' Permanent address: Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA.

it is possible to observe ion-surface interactions

and surface damage where the potential energy is the independent variable.

The interaction of ions and solids is understood

in the context of deposition of ionic kinetic energy

and the associated lattice damage [1]. Slow ions

{E < 100 keV for Xe on Si targets) interact principally by means of a few direct collisions with target

atoms (nuclear stopping). Relatively few atoms are sputtered or displaced. Somewhat swifter ions

(100 keV < £: < 10 MeV for Xe on Si targets) are initially slowed mainly through interaction

with target electrons (electronic stopping). Several

0168-583X/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. /'//S0168-583X(97)00504-1

NIST-271
D C. Parks et al. I Nucl. Insli: and Meth. in Phys. Res. B 134 (1998) 46-52 47

atoms may be displaced or sputtered. Ions with even higher velocities {£ > 10 MeV for Xe on Si targets) can create extensive tracks of displaced at- oms including additional lattice damage along the

tracks of secondary showers. In this regime many

atoms and ions may be sputtered.

The interaction of highly charged ions with sol- ids involves some very different additional mechanisms for energy deposition and lattice damage

formation. The potential energy carried by highly

charged ions is released upon their neutralization

and decay to the ground state. If this neutralization can be effected very quickly, as it probably is in the case of a highly charged ion impacting on a solid

surface, the potential energy carried by the ion will be deposited into a relatively small volume of the

target material. The amount of potential energy

available is enough to cause lattice damage and particle sputtering. In comparison to the much studied

kinetic processes, the processes for damage formation and ionic neutralization that involve highly

charged ions are much less well understood.

Ion sources capable of delivering sufficient

quantities of the required slow, but highly charged

ions in a well-collimated beam have become available only recently. The Electron Beam Ion Source

(EBIS) and the related Electron Beam Ion Trap

(EBIT) are now capable of providing slow beams

of ions with charge states previously obtainable

only by means of passing high velocity ions through

stripper foils. The EBIS/EBIT sources can provide

beams of ions whose potential energies are comparable to or greater than their kinetic energies.

In the present experiments, we measure the

physical damage on insulating solid surfaces after

irradiation by a beam of relatively slow, but highly

charged ions. While the mechanisms for conversion of internal energy into solid material damage

have yet to be elucidated, the experimental results do show unambiguously that the amount of near

surface damage increases nearly linearly with the

amount of potential energy carried by the ions.

Neutrahzation of highly charged ions in solid

materials is a topic of much recent discussion

and study [2-13]. Here we postulate a model for in- teractions between highly charged ions and solid

surfaces. As a highly charged ion approaches a sol- id surface, the surface potential is eventually lowered enough that electrons may traverse the gap

to the ion (classical over-the-barrier model) [8,14].

Such electrons are principally extracted from the

valence states of the surface and end in high lying

Rydberg states of the approaching ion [15]. A so- called "hollow atom" [16] configuration results. Auger and radiative processes within the atom begin filling the inner vacancies. These cascades re- lease potential energy in the form of energetic

free electrons and photons. Electrons are continuously extracted from the surface until the ion is neutralized or enters the solid material. After impact it becomes possible for electrons to transfer

directly from core states of the target atoms to low core states in the projectile [17]. The ion quickly comes to a charge equilibrium determined by its velocity and the particulars of the target material.

During the neutralization process, electrons re- quired for neutralizing the ion are, of course, ex- tracted from the surrounding material. This local

depletion of target electrons could result in a "coulomb explosion" [18]. Briefly, a coulomb explosion can be described as follows. The neutralization of

the highly charged ion principally removes electrons

from a relatively small volume near the point of entry into the solid. If the electronic relaxation times

within the target material are slow compared to the

ionic neutrahzation time, there is formed a volume

with a net positive charge near the surface. The re- maining positively charged constituent ions are then

mutually repulsed. The subsequent dynamics result

in ejection of ions and neutrals as well as significant

lattice damage due to atomic displacement. It is not

yet clear if this coulomb explosion process is responsible for the material damage reported here.

2. Experiment

We irradiated freshly cleaved single crystals of

mica (ruby mica, Goodfellow ^) at normal inci-

^ Certain commercial materials or products are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Insti- tute of Standards and Technology, nor does it imply that the materials or products identified are necessarily the best avail- able for the purpose.

NIST-272
48 D C. Parks el al. I Nucl. Jnstr. and Melh. in Phys. Res. B 134 (J 998) 46-52

dence. Mica was chosen for a variety of beneficial

characteristics. It is known to be an extremely

good insulator that can also easily be prepared

to give a pristine, very flat surface. Furthermore,

mica is a commonly imaged substrate for Atomic

Force Microscopy (AFM) studies and is known

to be stable under contact mode microscopy.

AFM imaging of mica shows that atomically flat terraces up to 10 jim in width are commonly observed.

Two diff"erent ion sources were used; the Kansas

State University Cryogenic Electron Beam Ion

Source (KSU-CryEBlS) and the NIST EBIT. Be- cause of limitations of the electron beam energy,

the highest charge state for which we achieved sufficient ion flux in the KSU-CryEBIS was q = 41.

At the NIST EBIT we were able to push the upper

limit to ^ = 50. Samples were irradiated by q = 44

ions from both sources with quantitatively similar

results. The principle of operation of the two ion sources is similar. Since it is well described in the litera- ture [19], only a short description of the EBIT will be given here. An electron beam is launched axially along the bore of a superconducting magnet.

The electron beam is squeezed to a very small di- ameter (~100 jim) by the magnetic field {B=3

T). The electrons are accelerated to energies suffi- cient to ionize tightly bound electrons by applying

a large positive voltage to a series of drift tubes coaxial with the electron beam. Background gas at- oms ionized by electron impact become radially

trapped by the magnetic field and by the space

charge of the electron beam. Ions are trapped axially by voltages applied to the drift tubes. This is accomplished by setting the end drift tube seg- ments at positive potentials slightly higher than

the central drift tube sections. The drift tube potentials and the space charge of the electrons and

ions inside the trap determine the energy of ex- tracted ion beams.

The highest charge state for which a sufficient population is attained is hmited primarily by the

electron beam energy and the time spent inside

the trap. Ions are stepwise ionized by successive

collisions with high velocity electrons. Since the cross section for ionization by electron impact

vanishes at threshold, the electron beam energy is

commonly set to be several kV above threshold

for the desired charge state. Lifetime of ions inside

the trap is limited by the escape of fast moving ions

and charge exchanging colhsions with lower

charge ions or background gas atoms.

Ions are extracted from the trap either in a pulsed mode or in a continuous mode. In the

pulsed mode the center drift tube potential is periodically ramped up to the level of the end drift tube, expelling the ions into the beam line. The

end drift tube voltage in the desired extraction di- rection is set slightly lower than the opposite end

drift tube voltage. In continuous mode, ions continuously spill over the end drift tube, when they

have gained sufficient energy from collisional heating with electrons or other ions. Ion beam currents

typically were 0.5-10 pA in each charge state delivered to the target.

Charge state selection of ions is accomplished

by means of a 90° bending magnet. The magnet

is capable of resolving adjacent charge states even

for Xe^°+. To enhance the yield and to reduce

beam contamination from other charge states composed of differing isotopes, we used 92%

(NIST) and 99% (KSU) isotopically enriched

'^^Xe. In the worst case of '^^Xe^°+, contamination

from '^''Xe'*^+ is estimated to be less than 10%.

There is less than 3% '^^Xe(''-')+ in the ^^^Xe''+ beam for q in the range 45 ^ ^ ^ 47. All other

charge states have contamination less than 1%.

The ion beam was decelerated and focused on

the target by means of an electrostatic lens assembly. The potential at the target was adjusted to set the final ion beam kinetic energy to 100 keV for

each charge state. Mica targets were exposed

through a mask with a 3 mm diameter circular aperture. The masked region provided an easily ref- erenced control region for each irradiated

surface. The beam current was measured with a suppressed Faraday cup immediately downstream

from the target position.

Adjusting the potential energy carried by the

ions was accomplished by varying the charge q in the range 25 ^ ^ ^ 50. In this way we could isolate

those effects due to Coulomb potential energy

from those due to kinetic and chemical processes. To avoid contamination of the beam by other

atomic species we did not use Xe charge states with

NIST-273
D C. Parks et al. I Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 46-52 49

charge to mass ratios the same as those derivable

from background gases (i.e., ''He, '^C, '''N and

'^O). All irradiations were done at room temperature in UHV with typical background pressures

approximately 1 x 10^ Pa. The ion bombarded surfaces were imaged in air using contact mode AFM (Nanoscope II Considering previously noted mixing of lateral forces

into topographic signals [20], care was taken to minimize this effect. We used pyramidal Sx^'Hn tips

with a nominal cantilever stiffness of 0.58 N/m.

These relatively short, stiff cantilevers are also tor- sionally rigid, reducing the lateral force component mixed into the topographic signal. All

images were taken with the scan size set to

1 \m\ X 1 \im.

Optimally, all samples would be imaged on the same day with the same probe. The relatively short

mean time of probe use before becoming contaminated and thus changing its imaging and adhesion

characteristics make this goal unattainable. In- stead, samples were imaged using several probes.

For all images, the force between tip and sample was reduced to the minimum stable value to minimize torsional bending of the cantilever and to re- duce scanning damage to the features of interest. The adhesion between tip and sample as estimated

from pull-off forces was 10'^ N. Most of this adhesion is due to surface tension on the water film that

is ubiquitous in ambient conditions. However, not

knowing the microscopic radius of the imaging

section of the probe prevents calculation of the more important parameter, the local pressure ex- erted by the probe.

As previously reported [20], the damage created

by single ions in mica is susceptible to modification

by contact mode AFM imaging. Almost universally, the observed topography was erased by repeated

scanning over any given region. This process was

usually completed in fewer than 10 scans. We did

not, however, observe a peehng open of the blisters as was noted by other researchers [21]. This may be

due to our effort to keep contact forces low, as the

transformation of blisters into pits may be a result of repeated scanning at high forces. To reduce the

effect of scanning induced modification of the fea- tures of interest, data were only collected from the

initial scan over any region.

3. Results

AFM images show the irradiated sections of the

mica surfaces to be randomly marked with circular

hillocks (Fig. 1). They are best described as right

circular cones, with height to width -aspect ratios

of about 1 : 20. Typical dimensions of the hillocks are 10-20 nm diameter and 0.5-1 nm height. These

hillocks occur only in the irradiated regions. The

areal density of hillocks corresponds well to the ex- pected ion density as calculated from the irradiat- ed area, measured ion flux and exposure time.

Typically the ion fluence was 10-50 ions per square

micron. The calculations are consistent with the id- entification of the observed hillocks as damage

caused by individual ion impacts.

The measured peak height and the integrated

volume of the hillocks are shown in Figs. 2 and

3. In Fig. 2 the data is plotted as a function of

ion charge q. Fig. 3 shows the same data plotted

as a function of ion potential energy. One anomalous point at ^ = 48 (i; = 40 nm^ /? = 0.43 nm)

which may have been contaminated by a defective

AFM tip is not shown in the plot. (All other samples were imaged using multiple probes ensuring

against this problem.) The error bars represent

the standard deviation of all individual hillock

Fig. 1. Atomic force microscope image of two Xe'""*" ion impact

sites on mica. Damage is seen as conical hillocks that are typi- cally 20 nm in diameter and 1 nm high. (Note exaggerated vertical scale.)

NIST- 274
50 D.C. Parks el al. I Nud. Instr. and Meth. in Phys. Res. B 134 (1998) 46-52

200

150 -

E

c

E

E

Ion Charge q

Fig. 2. (a) Volume of damage hillocks on mica as measured by

AFM as a function of ion charge for 100 keV Xe''"^ ions. The

solid line is a linear fit to the data. The broken line is a linear

fit constrained to pass through the origin. Error bars represent

the standard deviation of the measurements, (b) AFM measure- ment of peak height of damage hillocks on mica as a function of

ion charge. The solid line is a linear fit to the data. The broken

line is a linear fit constrained to pass through the origin. All lin- ear fits are unweighted.

measurements for samples irradiated with like charged ions. We observe the height, volume,

and area to all increase with increasing q. A lower

threshold for observable damage is noted at q ^ 30. The average of 0.15 nm height of the hill- ocks measured for g = 30 is only slightly greater

than the noise in the surrounding background.

We do not observe damage by 100 keV Xe ions

for q < 30. The observational threshold value we observe is roughly consistent with that found by other re- searchers [22]. We do observe damage for 100

200

120

0 I *"

I I I I I I I I I I I I I t I I 1 I 0 20 40 60 80 100 120

Potential Energy (keV)

Fig. 3. The same data as shown in Fig. 2, but plotted as a function of potential energy instead of ion charge, (a) Volume of damage hillocks on mica as measured by AFM for 100 keV

Xe'"*^ ions. The solid line is a linear fit to the data. The broken

line is a linear fit constrained to pass through the origin. Error

bars represent the standard deviation of the measurements. Data points are labeled with the ion charge, (b) AFM measure- ment of peak height of damage hillocks on mica. The solid

line is a linear fit to the data. The broken line is a linear fit constrained to pass through the origin. Assignment of ion charge is the same as in (a).

keV Xe^^ ions for q < 35, which is somewhat less than the threshold estimated in [22]. Although

not specified, the kinetic energy for those ions is stated to be between 200 and 500 keV.

4. Discussion

As seen in the graphs, the measured damage in- creases proportional to the potential energy carried by the ions. This agrees with the increase in

NIST-275
D.C. Parks et al. I Nucl. Instr. and Math, in Phys. Res. B 134 (1998) 46-52

the sputtering rate as a function of potential ener- gy observed with Ar^+ ions on GaAs targets [23]. Much of the damage increase is observed to occur above q = 44. This agrees well with the greater in- creases in the potential energy per electron with re- moval of L shell electrons. The calculated ionization

energy per electron for L shell electrons is approximately twice the value calculated for M shell electrons.

Previously we have measured the damage

caused on mica by irradiation with Xe'*^''" (potential energy = 50 keV) ions at several differ- ent kinetic energies. Those measurements do not

show any significant change in hillock dimensions

for kinetic energies in the range 20-800 keV [20]. While it is true that 100 keV ions are expected to do some lattice damage, that damage is distributed

along the ion path (~1 ^m). This is in contrast to the damage from the deposition of ion potential

energy that we expect to be deposited within a small volume near the surface. So even though

the ions have 100 keV kinetic energy, only a small

amount of that energy is transferred to the lattice in the near surface region.

Although we do not see damage from lower

charge ions, some undetected kinetically derived

damage is surely present. High velocity ions in mica are shown to leave tracks that are detectable by

AFM as mechanically softer regions [24]. High velocity ions are also known to produce hillock type

defects in mica [25] and in the organic crystal L-valine [26]. The physical processes by which hillocks are created instead of the perhaps more expected craters are not understood. We speculate that the ef- fect is closely related to the layered structure of

mica. This structure may make it especially sensitive to damage by highly charged ion impact. Mica consists of charged alumino-silicate sheets

weakly bonded by counter ions. Coulomb repulsion as the result of local removal of electrons

may easily force adjacent sheets apart at the rela- tively weakly bonded ionic layers. Locally this may be seen as a raised hillock at the surface.

Raised surface regions may be generated by a

large inter-layer rupture immediately below the

surface, forming a void. Alternatively, the collective effect of small expansions between each of

51

several layers might also result in a measurable

protrusion at the surface.

5. Conclusions

We observe damage features on insulating sur- faces that are not kinetically derived. We have pre- viously seen [20] that Xe'^^ ions (potential

energy = 50 keV) even at slow velocities (kinetic

energy = 20 keV) cause measurable damage on

mica surfaces. In this study using constant velocity

ions (kinetic energy = 100 keV) and varying the

ion charge we see the damage to be a function of

the potential energy. At ^ = 25, a Xe ion carries

8 keV potential energy. We do not observe damage

on mica using these ions. However, at ^ = 30, the

15 keV potential energy carried by a Xe ion is enough to cause observable damage. We observe

the height and volume of the damage to increase

with increasing potential energy through our max- imum value of ^ 50 where the potential energy is 100 keV.

Acknowledgements

Those sections of this research done at the J.R. Macdonald Laboratory were supported by the Division of Chemical Sciences, Office of Basic Energy

Sciences, Office of Energy Research, US Dept. of

Energy.

References

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Range of Ions In Solids, Pergamon Press, New York,

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[2] S. Winecki, C.L. Cocke, D. Fry, M.P. Stockli, Phys. Rev. A 53 (1996) 4228.

[3} H. Winter, Z. Phys. D 21 (1991) SI 29.

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[5] J.P. Biersack, Nucl. Instr. and Meth. B 80/81 (1993) 12.

[6] D.H. Schneider, M.A. Briere, J. McDonald, J. Biersack, Radiat. Eff. and Defects in Sohds 127 (1993) 113.

[7] H. Kurz, K. Toglhofer, HP. Winter, F. Aumayr, Phys. Rev. Lett. 69 (1992) 1140.

NIST-276
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[8] J. Burgdorfer, P. Lemer, F.W. Meyer, Phys. Rev. A 44

(1991) 5674.

[9] F. Aumayr, H. Kurz, D. Schneider, M.A. Briere, J.W. McDonald, C.E. Cunningham, HP. Winter, Phys. Rev.

Lett. 71 (1993) 1943.

[10] J.W. McDonald, D. Schneider, M.W. Clark, D. DeWitt,

Phys. Rev. Lett. 68 (1992) 2297.

[1 1] R.W. Schmieder, R.J. Bastasz, Nucl. Instr. and Meth. B 43

(1989) 318.

[12] G. Schiwietz, D. Schneider, M. Clark, B. Skogvall, D. DeWitt, J. McDonald, Radiat. Eff. and Defects in Solids 127 (1993) 11.

[13] L Bitensky, E. Parilis, S. Della-Negra, Y. Le Beyec, Nucl.

Instr. and Meth. B 72 (1992) 380.

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(1991) 5674.

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[16] E.D. Donets, Nucl. Instr. and Meth. B 9 (1985) 522.

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228.

[19] E.D. Donets, in: I.G. Brown (Ed.), Electron Beam Ion Sources in The Physics and Technology of Ion Sources, Wiley, New York, 1989, p. 245.

[20] D.C. Parks, R. Bastasz, R.W. Schmieder, M. Stockli, J. Vac. Sci. Techno). B 13 (1995) 941.

[21] M.A. Briere, D. Schneider, J. McDonald, M. Reaves, C.

Riihlicke, G. Weinberg, D. Knapp, Nucl. Instr. and Meth. B 90 (1994) 231.

[22] C. Ruehlicke, M.A. Briere, D. Schneider, Nucl. Instr. and

Meth. B 99 (1995) 528.

[23] K. Mochiji, N. Itabashi, S. Yamamoto, H. Shimizu, S. Ohtani, Y. Kato, H. Tanuma, K. Okuno, N. Kobayashi,

Surf. Sci. 357/358 (1996) 673.

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NIST-277
Masked ion beam lithography with highly charged ions

J. D. Gillaspy,^' D. C. Parks, and L. P. Ratliff National Institute of Standards and Technology, Gaithersburg, Maryland 20899

(Received 29 May 1998; accepted 16 September 1998)

Masked ion beam lithography using highly charged ions is demonstrated for the first time by

producing an array of hundreds of ordered micrometer wide dots using Xe'*^'*^ on

poly(methylmethacrylate) resist. At low dose, exposure of the resist is incomplete and isolated single-ion impact sites can be seen within the exposed areas. Atomic force microscope images of the single-ion impact sites show craters with a width of 24 nm. At high dose, the exposure is complete

and the dot morphology is consistent with limitations from the mask. Scanning electron microscope

images indicate that the sidewall slope is steeper than four. © 7995 American Vacuum Society.

[S0734-211X(98) 13006-8]

I. INTRODUCTION

Recent technical advances have made it possible to pro- duce beams of very highly charged ions {g>30+ per ion)

using compact laboratory sources based on electron beam

ion traps (EBITs).''^ This has led to a number of studies which explore the fundamental interaction of ions with sur- faces in an unusual regime in which the potential energy,

rather than kinetic energy, plays the dominant role. The

amount of potential energy that a single highly charged ion

releases during neutralization at a surface can be over 750 000 eV.^ Using an EBIT source, such ions can be pro- duced and easily formed into beams with accelerating potentials of V=0.1-10kV, resulting in ions with kinetic energies, KE=qV, in the range of 1 keV to 1 MeV. The amount

of continuous beam current that can be produced from such

ion sources has increased by more than a factor of 1000

during the past few years,^" and further increases of many

orders of magnitude are expected soon. Presently, it is possible to produce a 100-200 pA continuous beam of xenon

with charge states in the range of 25-1- to 44+}'^ In such a beam, the ^ = 44+ component alone can be optimized to be

over 20 pA. These currents already exceed those reported in useful focused ion beam lithography systems.^ The next generation of highly charged ion sources which are under devel- opment should have a brightness over four orders of magnitude higher than that of present EBITs, and should yield 2

X 10'° ions mm~^ mrad~^ s~' in a single charge state.* Fueled by these advances in instrumentation, a number of

studies involving the interaction of highly charged ions with

surfaces have already been carried out in the two EBIT laboratories in the U.S.'"^ Complementary work has been carried out at other facilities in the U.S.''* and abroad'"'^ where more moderately charged ion beams can be produced with

higher intensity. These studies have already produced a number of intriguing results, such as potential energy mediated

formation of nanoscale surface features on mica,' ''' and dra- matic enhancements in the yield of secondary ions' ''' and

neutral particles. '*'''' These results have led to proposals of

possible applications in the area of nanotechnology,^ '° '^~^' some of which are discussed in more detail below. Highly

'Electronic mail: John.Gillaspy@NIST.gov

charged ions are already being used in practice as a surface

analysis tool in at least one laboratory.''' In most of the work done so far, however, emphasis has been on understanding the basic (and relatively uncharted)

phenomenology of how these exotic ion beams interact with

surfaces. In all of this work, the highly charged ions were formed into relatively large diameter beams (of order 1 mm)

with no microscopic pattern encoding. Microscopic imaging

of surfaces has shown only a random spatial distribution of

features. Although there are applications in which random

distributions of features can be exploited for the manufacture

of useful devices,^^'^^ a much wider variety of applications can be envisioned if micropatteming can be achieved. This

article demonstrates that highly charged ions can be used to produce a regular array of dots in a commercial lithographic

resist using a standard stencil mask technique.

II. EXPERIMENT

The ion beam was produced at the NIST EBIT ion- surface interaction facility.^ A charge/mass selection magnet was used to purify the beam into a single isotopic charge

state, '^^Xe**"^. The beam current, 11 pA through a 3-mmdiam aperture, was measured with a suppressed Faraday

cup.'' The exposure was made for 15 min, for an integrated ion dose of 2.0X 10'° cm~^. The target surface was a section of a silicon wafer, coated with 400 nm of poly(methylmethacrylate) (PMMA) resist (4.5% solution spun on at 3000

rpm for 40 s and baked at 82 °C for 30 min). This target was

screened from above with a commercial nickel stencil mask

composed of a two dimensional square array of nominally 1 ^m square openings arranged on a grid with a 12.5 fim

pitch. Target exposure was carried out under a vacuum of approximately 10" * Pa (I0~* Torr). After exposure, the tar- gets were removed from the vacuum and submerged in an equal parts mixture of standard PMMA developer chemicals

(isopropyl alcohol and methyl isobutyl ketone) at room temperature. After 30-45 s in the developer, the targets were

rinsed in isopropyl alcohol and dried with nitrogen gas. Imaging was subsequently carried out with an optical microscope, a scanning electron microscope (SEM), and a contact mode atomic force microscope (AFM).

3294 J. Vac. Sci. Technol. B 16(6), Nov/Dec 1998 0734-21 1X/98/16(6)/3294/4/$1 5.00 ©1998 American Vacuum Society 3294

NIST-278
3295 Gillaspy, Parks, and Ratliff: Masked ion beam lithography with highly charged ions 3295

(a) c c

c c

(b)

(c)

Fig. 1. (a) SEM image showing four adjacent dots fabricated in PMMA with

highly charged ions. The separation between the dots is approximately 10

fim. Tht figure shown is a small portion of a larger array of over 250 such

dots. Each dot is the result of multiple ion impacts, (b) Closeup SEM image

of a comer of a single dot. The scale is given by the 100 nm white horizontal

bar. (c) SEM image of a portion of a stencil mask of the type used to produce (a).

When observed with the optical microscope, over 250

regularly spaced dots filled the field of view, but were in- completely resolved. The dots were well resolved by the SEM [Fig. 1 (a)]. A closeup of the comer of a single dot [Fig.

1(b)] indicates an edge roughness of less than 100 nm. Some

cracking of the substrate is evident and may have been

caused by overheating the sample in the SEM (operated at energies up to 20 keV). The images were taken with the

electron beam energy at 10 kV [Fig. 1(a)] and 15 keV [Fig.

1(b)]. A portion of a typical mask of the type used in this work is shown in Fig. 1(c). The openings in the masks were

highly nonuniform and evidently the actual region used to produce Fig. 1(a) contained significantly smaller openings

than those shown in Fig. 1(c). Figure 1 suggests that the

quality of the dot shape in this study was limited by the mask.

JVST B • Microelectronics and Nanometer Structures

Fig. 2. Depth profile of a dot like those shown in Fig. 1(a), measured with an atomic force microscope.

The kinetic energy of the ion beam (discussed below) was

determined by the difference in the potential between the

target and the ion source, 4.8 kV in this case. The mask was

pressed in contact with the PMMA surface at the edges, but

the extreme flexibility of the mask prevented us from being

certain that the mask was in contact with the surface over the

entire exposed area. An AFM was used to determine the depth of the dots (Fig.

2). The indicated horizontal distance between the two trian- gular markers on the right hand sidewall of Fig. 2 is 160 nm

and the vertical distance is 172 nm. The slope on the side- walls is Umited by the profile of the AJ^ probe. To actually

determine the sidewall slope with the AFM would require a more sophisticated measuring instrument and technique,

such as that used by Griffith et al.^^ If experimental data for ordinary ions on PMMA^' is extrapolated to Xe and linearly scaled with energy, a range of 172 nm is predicted—in excellent agreement with our observed depth. We note that a

surface roughness which is substantial on the nanometer

length scale appears along the bottom of the dot. When a target was exposed to a low dose of ions and

developed, no macroscopic removal of PMMA was observed, but single-ion impact sites could be imaged directly, as shown in Fig. 3. The apparent width of the impact sites is 24 nm. The AFM could only probe the impact sites up to a depth of 3 nm, limited by the AFM probe tip. This low-dose

target was bombarded using an ion accelerating voltage of

8.0 kV.

On several occasions we searched for dots before devel- oping the PMMA to see if evidence for direct ablation of

material could be obtained. Although no such evidence was

found, the possibility that the ablation craters are smaller than the developed craters, and the limited amount of effort spent on this attempt, make it impossible to rule out the existence of direct ablation. Other groups have recently measured how the direct ablation of various materials increases with ion charge,'""'^ but no work of this sort has been done

on PMMA. Very recent work by the Livermore EBIT group

has shown that a single highly charged ion can directly ablate as many as 1400 particles from GaAs substrates.

III. DISCUSSION

The minimum dose required to expose PMMA with a

conventional Ga'^ ion beam at 100 keV is 6 x 10" cm~^ 30

times higher than our integrated dose.^ Interpolation of re- sults from a previous study^' of the species dependence for

NIST-279
3296 Gillaspy, Parks, and Ratliff: Masked ion beam lithography with highly charged ions 3296

Fig. 3. Atomic force micrograph of nanoscale dots fabricated in PMMA

with highly charged ions. Each dot is formed by exposure to a single Xe'"*

ion. The apparent width of the dot which is bracketed by the horizontal white indicator is 24 nm.

singly charged ions indicates that Xe' should be only about

50% more efficient in exposing PMMA than Ga'"^. We caution that no definitive conclusion concerning the relative dose efficiency for Xe"*^"^ should be made at this time, however, because our beam homogeneity was not particularly well characterized and the local dose may have varied substantially from the integrated value. A beam viewing system

is presently being designed which will allow us to microscopically characterize the beam profile and to more precisely determine the local dose. The emphasis in this article, therefore, is primarily on the topography of the surface features. An enhanced surface sensitivity to ions that are very

highly charged is not unlikely, however, particularly for chemical and physical processes that are based on electron rearrangement. If the number of electrons/ion removed from

the target is equal to the charge of the ion, then a Xe''^"^ ion dose of 2.4 X lO'*' cm~^ results in an electron removal of over 1 X 10'^ cm~^. The actual number of electrons/ion re- moved, however, might be much larger. On other surfaces,

for example, it has been shown by direct measurement that a

single highly charged ion can pump as many as 300 electrons away from the surface' in addition to the number of electrons

that are required to neutralize the ion itself. Explanations for

this electron pumping mechanism can be found in such processes as Auger reionization during collapse of highly excited "hollow atom" states formed during neutralization of

the ion.^ Another contribution to the collapse of hollow

atoms," and the interaction of highly charged ions with sur- faces in general, involves the emission of x rays. It is possible that this x-ray emission contributes to an increased

sensitivity of PMMA to these ions (in fact, a patented sub- micron lithography process has been proposed based on the use of X rays generated locally on a surface by the neutralization of multiply charged ions).^' There is little other experimental data involving the imaging of ion tracks at the surface of PMMA which can be

directly compared to our results. Melngailis^ has pointed out

that conventional ions are often visualized as producing co- lumnar defects (tracks) of approximately 10 nm diameter in PMMA. Kubena^^ has used a focused Ga"^ ion beam to pro- duce dots in PMMA with a width as small as 6 to 7 nm when

observed with a SEM. In Kubena's study the average dot width was 25 nm when exposed with an average of 82 ions/

dot, and 15 nm when the exposure was lowered to 23 ions/

dot; extrapolating these data by the square root of the exposure to 1 ion/dot yields an estimated single ion track width of

6 nm. Dobeli ei al}^ have used an AFM to image PMMA

after exposure to ions produced by a particle accelerator. They found that the craters produced by low charge and high

velocity monatomic ions decreased in width as the ion en- ergy was lowered, and that craters as large as the ones we

observed were not found unless the kinetic energy was much

higher (>10x) than that used in the present work. This,

together with the energy deposition considerations discussed

in the next paragraph, suggests that the extreme potential energy carried by our ions is dominating the interaction near

the surface. From the work completed thus far it is not possible to

fiilly sort out the interplay between kinetic and potential en- ergy in the formation of the surface features in our experiment, but a number of aspects are already quite clear. Other

experiments on thin carbon foils have shown that ions of

similar charge and kinetic energy are fully neutralized within 0.01 /i.m of surface penetration.'^ Electronic binding energies in atoms can be accurately calculated, so it is known

that the total potential energy transferred during neutraliza- tion of a single Xe"*^"^ ion is about 50 000 eV. The energy

loss rate at the surface which results from combining these two numbers is 5000 keV//tim, substantially more than that of ordinary ions.^^ It is clear, therefore, that highly charged

ions should be capable of strongly modifying the surface of

materials even when the kinetic energy is reduced towards

zero. This has been confirmed experimentally in considerable detail in the studies of Parks et al?'^^ on mica. In that work, a xenon ion beam of similar charge state and kinetic energy was shown to induce surface features of similar lat- eral dimension to those observed here. Furthermore, the sur- face topography was shown to scale in dimension with ion charge but remain unchanged when the kinetic energy was

varied over a range which extended nearly two orders of magnitude below the energies used here. The ability of highly charged ions to strongly modify sur- faces, even in the limit of low velocities when the range should only be of order 1 nm, has motivated the speculation

that exotic ion beams of this sort might be used as a new tool for shallow-junction and thin-film semiconductor processing,

where the ability to limit unwanted damage deep in the substrate is desirable. '^'^^ Other potential applications have been

suggested as well, such as using highly charged ions as a potent tool for "soft" (low velocity) sputter cleaning and

probing of surfaces.'"'^' The enhanced x-ray yield which can accompany the interaction of highly charged ions with

NIST-280
3297 Gillaspy, Parks, and Ratliff: Masked ion beam lithography with highly charged ions 3297

surfaces might be considered for possible exploitation in the development of improved forms of particle induced x-ray emission (PIXE) diagnostic tools. As mentioned above,

highly charged ions are already being used in a prototype

SIMS diagnostic tool at Livermore;'^ similar work is under- way at Kansas State University,^^ in collaboration with scientists from Japan. In all of these applications, the determination of the spatial zone of the surface interaction is relevant, and thus our work at NIST has focused on atomic

scale imaging of surface topography. Although rapidly growing, the field is still in its infancy, and a great deal more

remains to be learned about the fundamental processes in- volved in the interaction of highly charged ions with surfaces

before the feasibility of developing any of these proposed

applications can be ascertained.

IV. CONCLUSION

We have demonstrated that beams of highly charged ions can be used to pattern a lithographic resist on the micrometer

length scale. We have explored the surface topography with

atomic force microscopy and scanning electron microscopy,

and have presented the first direct surface images of single- ion tracks in PMMA. The fundamental transverse length

scale, at the potential and kinetic energies used in this work,

appears to be several tens of nanometers, suggesting that the method demonstrated here can be extended deep into the submicrometer range. ACKNOWLEDGMENTS

The authors thank Alexander Shapiro and James Kelly for

the SEM imaging, and Kris Helmerson for the optical imaging. We thank John Melngailis, Jon Orloff, Skip Berry, Stacy

Mogren, and Walt Finkelstein (and numerous members of

the Advanced Lithography Group consortium under his di- rection) for their encouragement, advice, and other assistance

in this and our ongoing related work with highly charged

ions.

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Sci. Instium. 68, 1998 (1997).

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*A. I. Pikin, C. A. Morgan, E. W. Bell. L. P. RaUiff, D. A. Church, and J. D. Gillaspy, Rev. Sci. Instrum. 67, 2528 (1996).

'j. Melngailis, Nucl. Instrum. Methods Phys. Res. B 80, 1271 (1993).

^R. E. Marrs, Nucl. Instrum. Methods Phys. Res. B (to be published).

'D. C. Parks, R. Baslasz, R. W. Schmieder, and M. Stockli, J. Vac. Sci. Technol. B 13, 941 (1995).

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'a. Arnau, F. Aumayr, P. M. Echenique, M. Grether, W. Heiland, J Limburg, R. Morgenstem. P. Roncin, S. Schippers, R. Schuch, N. Stolterfoht,

P. Varga, T. J. M. Zoilros, and H, P. Winter, Surf. Sci. Rep. 27, 113

(1997). '°M. Spom, G. Libiseller, T. Neidhard. M. Schmid, F. Aumayr. H. P. Winter, P. Varga, M. Grether, D. Niemann, and N. Stolterfoht, Phys. Rev.

Lett. 79, 945 (1997).

"j.-P. Briand, S. Thuriez, G. Giardino, G. Borsoni, M. Froment, M. Eddrief, and C. Sebenne, Phys. Rev. Lett. 77, 1452 (1996).

'^N. Itabashi, K. Mochiji, H. Shimizu, S. Ohtani, Y. Kato, H. Tanuma, and

N. Kobayashi, Jpn. J. Appl. Phys., Part 1 34, 6861 (1995).

''D. C. Parks. M. P. Stockli. E. W. Bel!, L. P. Ratliff. R. W. Schmieder. F

G. Serpa, and J. D. Gillaspy, Nucl. Instrum. Methods Phys. Res. B 134, 46 (1998). Schenkel, A. V. Barnes, A, V. Hamza, D. H. Schneider, J. C. Banks, and B. L. Doyle, Phys. Rev. Lett. 80, 4325 (1998). "R. W. Schmieder and R. J. Bastasz, in Vlth International Conference on

the Physics of Highly Charged Ions, Manhattan. Kansas. J 992, edited by

P. Richard, M. Stockli, C. L. Cocke, and C. D. Lin, AIP Conf. Proc. No. 274 (AIP. New York, 1993). p. 675.

'*D. H. Schneider, M. A. Briere, J. McDonald, and J. Biersack, Radial. Eff

Defects Solids 127. 113 (1993).

'^J. Limburg. J. Das, S. Schippers, R. Hoekstra. and R. Morgenstem, Phys. Rev. Lett. 73. 786 (1994). "R. Morgenstem and J. Das, Europhys. News 25. 3 (1994).

"I. Hughes, Phys. World 8. 43 (1995).

^°}. D. Gillaspy. Y. AgliUkiy. E. W. Bell, C. M. Brown, C. T. Chantler. R. D. Deslattes, U. Feldman, L. T. Hudson. J. M. Laming, E. S. Meyer, C. A. Morgan, A. I. Pikin, J. R. Roberts, L. P. Ratliff, F. G. Serpa. J. Sugar. and E. Takacs, Phys. Scr. T59, 392 (1995). ^'N. Kakutani, T. Azuma, Y. Yamazaki, K. Komaki, and K. Kuroki, Nucl.

Instrum. Methods Phys. Res. B 96, 541 (1995).

^^C. R. Musil, D. Jeggle. H. W. Lehmann. L. Scandella, J. Gobrecht, and M. Doebeli, J. Vac. Sci. Technol. B 13. 2781 (1995). ^^H. W. Deckman and J. H. Dunsmuir. J. Vac. Sci. Technol. B 1. 1109

(1983).

^*}. E. Griffith. L. C. Hopkins. C. E. Bryson, A. Berghaus, E. J. Snyder, J.

J. Plombon, L. A. Vasilyev. M. Hecht. and J. B. Bindell. J. Vac. Sci. Technol. B IS, 2189 (1997). "R. G. Wilson. J. Appl. Phys. 73. 2215 (1993).

^*T. Schenkel. A. V. Hamza. A. V. Barnes, D. H. Schneider. J. C. Banks. and B. L. Doyle. Phys. Rev. Lett. 81. 2590 (1998). "S. Matsui. K. Mori. T. Shiokawa. K. Toyoda, and S. Namba, J. Vac. Sci. Technol. B 5, 853 (1987).

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^"S. Winecki, C. L. Cocke, D. Fry. and M. P. Stockli. Phys. Rev. A S3. 4228 (1996). ^'K. S. Golovanivsky and E. M. Omeljanovsky. U.S. Patent No. 5,327.475 (Ruxam. Inc.) (1994). "R. L. Kubena, J. W. Ward. F. P. Stratton. R. J. Joyce, and G. M. Atkinson, J. Vac. Sci. Technol. B 9, 3079 (1991). "M. Dobeli, F. Ames. C. R. Musil. L. Scandella. M. Suter. and H. A.

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NIST-281
APPLIED PHYSICS LETTERS VOLUME 75. NUMBER 4 26 JULY 1999 Exposure of self-assembled monolayers to highly charged ions

and metastable atoms

L P. Ratliff, R. Minniti. A. Bard, E. W. Bell.^' J. D. Gillaspy.''"=' and D. Parks

Atomic Physics Division. National Institute of Standards and Technology, Gaithersburg. Maryland 20899

A. J. Black and G. M. Whitesides"'

Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138

(Received 18 February 1999; accepted for publication 2 June 1999)

The doses of neutral metastable argon atoms (Ar*) and highly charged xenon ions (HCIs) required

to damage self-assembled monolayers (SAMs) of alkanethiolates on gold are compared in a set of experiments carried out concurrently. The extent of damage to the SAM is determined by

developing the samples in a gold etching solution, then measuring the decrease in reflectivity of the

gold; •« 10^ Ar* are required to cause the same amount of damage as 1 HCI, as measured by this assay. We have also demonstrated HCI micropatteming of a surface using a physical mask,

suggesting the application of this system in lithography. ® 1999 American Institute of Physics.

[50003-6951(99)03330-6]

Efforts to improve the techniques of micropatteming

surfaces have involved the use of alternative resists or un- conventional exposure methods. In this letter we do both

simultaneously, using a beam of highly charged ions (HCIs)

and a beam of neutral metastable atoms to expose an ultra- thin self-assembled monolayer (SAM) resist. Besides providing one of the first demonstrations of lithographic patterning

using HCIs, the results allow us to compare and contrast the

relative efficiencies of ion and atom exposure quantitatively.

The internal energy (defined as the energy released in bringing an atom or ion to its neutral ground state) of metastable helium and argon atoms has been used previously to damage SAMs of dodecanethiol (DDT) on gold.'"^ By using

a mask to pattern the exposure, followed by etching, one can form patterns in gold and then in silicon. The highly charged ions used in this work, Xe"*^*, have

a much greater potential energy (51.3 keV) than the atoms

used previously, but they also have greater kinetic energy

(350 keV for ions vs <0.1 eV for atoms). The ions neutralize rapidly (typically in less than =»20 fs"*), however, depositing

the potential energy into a small volume localized in the first few nanometers of the surface.'*"* Because the kinetic energy

is deposited in the solid over a region hundreds of nanometers deep, the energy density deposited on the surface is dominated by the contribution from the potential energy. Previous studies on mica have confirmed that the amount of damage to the surface caused by ions correlates with changes

in internal potential energy (related to the charge of the ion) and not kinetic energy.^'* Figure 1 summarizes the process used to expose SAMs

to the beams. The procedure for exposing the substrates to the beam of Ar* is described in detail elsewhere.^ The flux of Ar* was 1.7X l(f fj.m~^ s~' in a coUimated beam =7.5 mm

in diameter. The ion beam was a continuous stream of =•11.0X10* Xe"*^"^ ions/s from an electron beam ion trap

"Present address: Diamond Semiconductor Group, Gloucester, MA 01930.

"'Authors to whom correspondence should be addressed. "Electronic mail: john.gillaspy@nisLgov

(EBIT),' the ion beam was passed through an aperture 3 mm

in diameter, 5 mm above the sample.

Gold (40 nm thick) was evaporated onto silicon wafers using titanium (2 nm thick) as an adhesion promoter. The

wafers, coated with gold, were stored under ambient conditions prior to the formation of SAMs. SAMs were prepared

by immersing the gold films into a solution of dodecanethiol (DDT) in absolute elhanol (0.01 mol) for at least 16 h. Samples were then rinsed with ethanol, and blown dry in a stream of nitrogen gas before being loaded into the vacuum

system.

In order to compare directly the doses of atoms and ions required to damage SAMs, we exposed samples to each of

the two beams, concurrently, keeping the samples in each chamber under vacuum for the same total time. The pressure

in the chamber in the case of the atom exposure experiment was approximately 1 mPa (=• 10"' Torr) while in the case of

the ion exposure the pressure was approximately 1 /iPa (« 10"* Torr). We performed 12 exposures of HCIs of =»90 min each on 6 separate samples of SAMs of DDT (Table I). The dose given in Table I is calculated from the average of

-SAM of alkanethiol -Au(40 nm) on

Ti(1 nm)

Expose to meiastabte atoms

or highly charged ions

MaskAtoms or ions damage the SAM

Etching removes goM in regions where SAM was damged

Measure ttidoiess d

gold by reAedivity

image pattern

in gold

FIG. 1. Schematic of the experimental procedure.

0003-695 1 /99^5(4)/590/3/$ 1 5 .00 590 © 1999 American Institute of Physics

NIST-282
Appl. Phys. Lett.. Vol. 75, No. 4. 26 July 1999

Ratliff et al. 591

TABLE L Exposure time, dose, and loss of reflectance measured when SAMs of DDT are exposed to HCIs and etched.

Time Dose Loss of Time Dose Loss of

is) , X i u cm ) Reflectance (sec) (X 10 cm ^) reflectance

5400 L35 22% 5100 1.35 48%

5400 L24 12% 5040 1.30 41%

4380 1.33 8% 4980 1.39 42%

4380 1.29 7% 4800 1.36 41%

4500 1.34 16% 4620 1.29 41%

4620 1.34 14% 4860 1.27 35%

Average 1.32(04) 27(15)%

the flux before and after the exposure (the flux of ions can

vary by up to 10% during the exposure). We determine the

flux of ions with an electron multiplier in pulse counting

mode; the ion detection efficiency is 0.50 with an uncertainty of less than 0.25.'^ After exposure to the ion or atom beam, the samples were removed from the vacuum chamber and immediately

etched in an aqueous ferricyanide solution (1 mol// potas- sium hydroxide (KOH), 0.1 moU/ potassium thiosulfate (K2SO4). 0.01 moL// potassium ferricyanide [KjFeCCN)^].

and 0.001 mol// potassium ferrocyanide [K4Fe(CN)6] for ='20 min). After etching, the samples were rinsed with distilled water and blown dry. The exposed regions were visible

as darker areas corresponding in size to the widths of the beams. The degree of etching of the exposed regions relative to the unexposed regions was quantified as a decrease in reflectivity of the surface.^ A HeNe laser (X.=632 nm) was focused

onto the sample at normal incidence and the reflected beam

was separated from the incident beam with a polarizing beam

splitter cube. The beam waist was calculated to be =» 15 fj-m, but the resolution was limited by the point spacing in our

scans to 70 fim. The intensity of the reflected beam was

monitored with a photodiode as the sample was stepped

through the laser beam. The reflectivity was measured at points along several horizontal scans across the surface, each

scan separated vertically by =°0.5 mm, and then averaged.

The fractional loss in reflectivity is defined as the unexposed

value (average signal outside of the exposed region for that particular sample) minus the average signal from the ex- posed region divided by the unexposed value.

In the case of HCI exposure through a mask, some of the parameters discussed above were changed slighdy. Chro- mium rather than titanium were used as an adhesion layer, and SAMs of hexadecanethiol (HDT) rather than DDT were

used.After etching, the regions of the surfaces that were ex- posed to HCIs show a decrease in reflectivity relative to that of unexposed gold. The maximum possible decrease in reflectivity gives a value that corresponds to that of bare sili- con; none of the samples showed this maximum. By averaging the reflectivity across the exposed region for each

sample, the analysis accounts for the fact that the distribution of ions across the beam (unlike the beam of atoms) is nonuniform. The average loss in reflectivity is 0.27(15) caused

by an average dose of ions of 1.32(4) X lO" ions/cm^ The

uncertainty that we quote is the standard deviation of the 12

no. 2. Reflectance of gold vs dose for exposures of SAMs of DDT to Ar*.

individual data points. Since there may be a systematic component this deviation, we have used this conservative esti- mate of the uncertainty and not reduced it by the square root of the number of data points. Given that there are 4X 10'* alkanethiol molecules/cm^, we infer that each ion—and the subsequent etching process—generates a crater in the gold

approximately 35 nm in diameter and encompassing =3000

alkanethiol molecules in the SAM. This is in reasonable agreement with the directly imaged results for randomly dis- persed single ion impact craters obtained subsequent to our work by the Livermore group (50-63 nm diameter),'" given

the fact that the SAMs used were different, and that the

direct images do not appear to be corrected for imaging

probing tip effects (tip radii approximately 70 nm). These

craters are comparable in size to those formed in a more

standard resist, poly (methylmethacrylate), which was ex- posed to Xe"*^ [after etching, single ion impact craters 25 nm

in diameter were observed with atomic force microscopy

(AFM)]."

In order to compare exposure with HCIs to that with Ar*. we chose various doses of Ar* to bracket the exposure

obtained with HCIs and interpolated to the dose which gives an equivalent exposure as measured by the reflectivity. The

results of our reflectivity measurements for exposures of SAMs of DDT to metastable argon atoms is plotted in Fig. 2. The slope of the solid line is 4.1(12)X 10~'^ cm^/atom, in reasonable agreement {<2a) of the previously determined

value.^ The statistical contribution to the overall (combined

standard) uncertainty of the present result is 0.6 X 10"'^ cm^/atom. The overall uncertainty is limited by the detection efficiency (e) of the metastable atoms, as described

previously.^

A loss of reflectivity of 27% (the average loss obtained with HCIs) thus corresponds to a dose of 6.6 X 10'^ atoms/cm^. This value implies that multiple impacts

(~16) of Ar* are required to damage a point on the surface such that etching will remove the underlying gold. Therefore, ~ 10'^ Ar* are required to cause (after etching) the same amount of damage as one Xe**"^ ion, as compared to the ratio of potential energy, 4400. That is, the HCI is much

more efficient at converting its potential energy to surface damage than is the metastable atom. This nonlinear energy dependence is not surprising because a similar effect has been seen in a comparison of damage to SAMs using Ar*

(P.E. = 12 eV) and He* (P.E.=21 eV) projectiles.^ In that case, an increase in the potential energy per atom of less than a factor of 2 resulted in a decrease in die required dose of

NIST-283
592 Appl. Phys. Lett., Vol. 75, No. 4, 26 July 1999 Ratllff ef al.

I • I

lOQnm

I

FIG. 3. SEM of (a) the TEM grid and (b) the gold patterns made by exposing SAMs of HDT (0 HCIs. The insets show that the edge roughness of the

features in gold is comparable to that of the TEM grid.

more than an order of magnitude. It was speculated that the damage was caused by secondary electrons, and comparison

of the metastable atom dose (assuming one secondary electron per Ar*) with the dose used in experiments where elec- tron beams were used to expose SAMs, gave reasonable agreement. Perhaps the increased efficiency of the He*

comes from the decrease in inelastic mean free path'^ as the electron energy goes from 12 to 20 eV, which allows the electron to undergo a higher density of collisions within the SAM. The mean free path increases above 20 eV, but this increase might not be relevant as the damage mechanisms

might be quite different for the two projectiles as their potential energies differ by over three orders of magnitude. Because the HCI deposits a large parcel of potential energy

into a small volume of the surface in a short time, the excitation is concentrated and collective effects might be important. Two models predict cratering of surfaces due to the

large energy density deposited by a single HO. One model

asserts that the HCI will extract many electrons from the surface leading to a charge buildup which will Coulomb

explode,'^ ejecting ions from the surface and causing a shock wave that ejects neutral material as well.''* A second model

predicts that the HCI impact excites many surface electrons from bonding states to antibonding states, rendering the material structurally unstable.'^ The result is the displacement

of a large number of surface atoms.

In order to demonstrate the ability of HCIs to pattern SAMs, we exposed SAMs of HDT to the ion beam through a physical mask—a nickel transmission electron microscopy

(TEM) grid (Fig. 1). Figure 3 shows SEM images of gold

patterned by exposing SAMs on gold to a beam of HCIs

through a mask for "2 h: the flux was measured prior to exposing the samples, and the dose calculated to give a 50%

loss in reflectivity of the gold after etching. Samples were

etched in an aqueous ferricyanide solution to reveal the pattern in gold. The edge roughness of the features in gold

("•lOO nm) is comparable to that of the TEM grid (Fig. 3

inserts).

In summary, we have demonstrated that Xe"*^* damages

SAMs in doses five orders of magnitude lower than doses of Ar*. Using currently available high flux sources of

HCIs,'''*"'* the overall exposure time can be shorter than

that used in previous studies involving metastable atoms. Furthermore, since the production of HCI beams is an area of research still in its infancy and appears to be progressing

rapidly, there is much room for future improvements. The

next generation of EBIT devices, for example, will have a greatiy increased intensity and brightness; the brightness, a

figure of merit that combines intensity and emittance (ability to focus to a small spot), is expected to be improved by four orders of magnitude.'*

We have also used HCIs to pattern gold films by exposing SAMs of HDT on gold to ions through a stencil mask,

and then etching the gold from the exposed regions. Because

this process is significantly more efficient than patterning with metastable atoms, it may be suited to lithography. Fur- thermore, because of the high energy density the HCIs deposit on the surface, thicker SAMs can be used, leading to a more robust process with greater contrast.

This work is supported by ONR/DARPA and NSF Grant No. PHY 9312572. This study made use of the MRSEC

Shared Facilities supponed by the NSF under Award No. DMR-9400396. A.B. was supported by the Humboldt Foundation. We gratefiilly acknowledge Doug Alderson's technical assistance with the EBIT.

' K. K. Berggren. A. Bard. J. L. Wilbur. J. D. Gillaspy. A. G. Helg. J. J. McQelland, S. L Rolston. W. D. Phillips, M. Prentiss, and G. M. Whitesides. Science 269. 1255 (1995). ^A. Bard. K. K. Berggren. J. L. Wilbur. J. D. Gillaspy, S. L. Rolston, J. J. McQelland, W. D. Phillips, M. Prentiss, and G. M. Whitesides, J. Vac.

Sci. Technol. B IS, 1805 (1997).

^S. Nowak, T. Pfau, and J. Mlynek, Appl. Phys. B: Lasers Opt. 63. 203

(1996) . *T. Schenkel, M. A. Briere. A. V. Barnes. A. V. Hamza. H. Schmidt- Bocking. K. Bethge. and D. H. Schneider. Phys. Rev. Lett. 78, 2481

(1997) . 'T. Schenkel. M. A. Briere. A. V. Barnes, A. V. Hamza. H. Schmidt- Bocking. K. Bethge. and D. H. Schneider, Phys. Rev. Lett. 79, 2030

(1997) . 'T. Schenkel, A. V. Hamza, A. V. Barnes, and D. H. Schneider. Phys. Rev. A 56. R1701 (1997). 'D. C. Parks. M. P. Stockli, E. W. Bell, L. P. RaUiff, R. W. Schmieder. F. G. Serpa, and J. D. Gillaspy, Nucl. Instrum. Methods Phys. Res. B 134. 46

(1998) . 'D. C. Parks, R. Bastasz. R. W. Schmieder. and M. P. Stockli. J. Vac. Sci. Technol. B 13, 941 (1995). 'L. P. Radiff. E. W. Beil. D. C. Parks. A. I. Pikin. and J. D. Gillaspy. Rev.

Sci. Instrum. 68. 1998 (1997).

'"T. Schenkel, M. Schneider, M. Hattass, M. W. Newman. A. V. Barnes, A. V. Hamza, D. H. Schneider, R. L Cicero, and C. E. D. Chidsey, J. Vac.

Sci. Technol. B 16, 3298 (1998). "J. D. Gillaspy. D. C. Parks, and L. P. Radiff, J. Vac. Sci. Technol. B 16, 3294 (1998). '^M. P. Seah and W. A. Dench. Surf. Interface Anal. 1. 2 (1979). '^L S. Bttenskii. M. N. Murakhmetov, and E. S. Parilis, Sov. Phys. Tech.

Phys. 24, (1979).

'*H.-P. Cheng and J. D. Gillaspy. Phys. Rev. B 55. 2628 (1997). "T. Schenkel, A. V. Hamza. A. V. Bams, D. H. Schneider. J. C. Banks, and

B. L. Doyle. Phys. Rev. Lett 81, 2590 (1998).

'*E. D. Donets, Rev. ScL Instrum. 69, 614 (1998). "Z. Q. Xie, Rev. Sci. Instrum. 69, 625 (1998). "R. E. Marrs, Nucl. Instrum. Methods Phys. Res. B 149. 182 (1999).

NIST-284
Institute of Physics Publishing Journal of Physics B: Atomic, Molecular and Optical Physics

J. Phys. B: Al Mol. Opt. Phys. 34 (2001) 1277-1287 www.iop.org/Journals/jb PU: 80953-4075(01)19625-2

Separation of inner-shell vacancy transfer

mechanisms in collisions of slow Ar^'^+ ions with Si02

E Takacs' 2 Z Berenyi^, J D Gillaspy^ L P Ratliff, R Minniti',

J PeduUa', R D Deslattes' and N Stolterfoht^

' National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

^ University of Debrecen, Insitute of Experimental Physics, ATOMKI, Bern ter 18, Debrecen

H-4026, Hungary

^ Hahn Meitner Insitut, Glienikerstrasse 100, D- 14 1 09 Berlin, Germany

Received 30 November 2000

Abstract

We have studied the spectrum of x-rays emitted when 1 30 and 200 keV kinetic

energy hydrogen-like argon ions impact sihcon dioxide surfaces. Specifically,

we were interested in the mechanism for creation of K-shell holes in the silicon

target atoms, which can be filled with the release of a 1 .75 keV x-ray. Two

mechanisms have been hypothesized for the vacancy transfer between the Kshells of silicon and argon atoms: 'direct vacancy transfer' and 'projectiledecay-product-mediated vacancy transfer'. To separate these mechanisms, we

used a target with a metallic coating (preventing close collisions between Si

and Ar but allowing x-ray transmission) and a target without such a coating.

We found that x-ray photoionization is the dominant mechanism in both cases and measured an upper limit for the contribution from the 'direct mechanism'

on the uncoated sample. Furthermore, we measured the relative strengths of

the Ka, K;6 and Ky lines of the argon projectile as a function of kinetic energy

and found satisfactory agreement with charge exchange and cascade model

calculations.

1. Introduction

Collisions of ions with surfaces have been studied for many years and the results have been

successfully applied to various technologies including ion implantation [1], secondary ion mass

spectroscopy (SIMS) [2], ion etching [1], focused ion beams for lithographic mask repair [3]

and surface polishing [4]. Although the understanding of these processes is continually

evolving, successful models that describe them have existed for many years [5-7]. These

models allow engineers to vary parameters associated with the ion, such as its speed and mass,

in order to optimize a particular process for a particular surface. One potentially important

parameter that has been largely ignored in this optimization is the ion's charge. With the recent development of compact sources of highly charged ions (HCIs), there is a great potential for

further innovation in ion-related technologies. With this new generation of sources, ions of a

0953-4075/01/071277+11 $30.00 © 2001 lOP Publishing Ltd Printed in the UK 1277

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1278 E Takacs et al

wide variety of masses and a wide variety of charge states can be formed with kinetic energies

comparable to their potential energies, which can be tens or even hundreds of keV. Experiments

with these HCIs have revealed, for instance, that the sputtering yields and secondary ion yields

are quite high due to the localized deposition of the ion's large potential energy, and nanoscale

surface features attributable to individual HCI impacts can be seen on various surfaces [8-10].

Using a crystal microbalance, Spom et al recently showed that highly charged argon

ions produce strong potential sputtering in Si02. This result supports the assertion that an

efficient potential energy transfer takes place on the surface. Furthermore, sputtering with

argon ions was found to be less effective than sputtering with xenon ions possessing about

the same amount of potential energy [11]. The authors attribute this effect to the fact that, in argon, the energy is available in larger quanta because inner-shell vacancies provide most

of its potential energy. This study emphasizes the need to understand the details of how the

potential energy carried by inner-shell vacancies of the projectile is transferred to the solid. The basic understanding of the HCI-surface interaction is based on the classical over-thebarrier model [12]. That is, the ion begins to extract electrons from the surface even before

impact. These electrons become bound in high-lying states of the projectile while the core

states remain vacant, forming a 'hollow atom'. Upon impact, electrons in high-lying states are 'peeled off' and are soon replaced by electrons that become bound in intermediate states, forming a second, smaller 'hollow atom'. The projectile relaxes further as it travels through

the solid. Although at the kinetic energies typically used in these experiments (a few eV to a few hundred keV) the ions may penetrate hundreds of nanometres into the solid, the potential

energy is released within a few nanometres of the surface [13]. In this way, the potential energy of the projectile can be deposited into a very small volume

(nanometre sized) in a very short time (femtoseconds). Therefore, the projectile and the solid are both far from equilibrium and the interaction cannot be treated as a small perturbation

to either the projectile or the surface. This situation makes the theoretical treatment of this interaction quite complicated. A deeper understanding of the effects that come into play at moderate and high charge states could enable one to tailor the projectile ion, including its charge state, to various applications. In this way, one might improve techniques currently in use and find new applications. In order to deepen the current understanding of the effects that are important in HCI-surface interactions, experiments must be designed that probe specific

aspects of the collisions as well as models to explain these phenomena. Such a study is presented in this paper. Among the key issues in the field today are (1) the mechanism and timescale of the

hollow atom formation and decay, and (2) the mechanism, the time evolution and the spatial

distribution of the deposition of the HCI's potential energy into the substrate. These two issues

are intimately related because it is in the process of neutralization and decay that the projectile

deposits its potential energy into the surface. To resolve these questions, the dynamics of the

interaction must be investigated. One fruitful approach is to study the secondary particles

such as electrons [14-17], x-rays [18, 19], atoms, ions and clusters [20,21] that are emitted

from the projectile and from the surface. Investigations where emission from both the target

and the projectile can be observed at the same time are particularly useful because the relative

intensities of the various emission products carry information about the effectiveness of the

energy transfer mechanisms.

X-ray spectroscopy experiments that involve the interaction of one-electron Ar^^"^ ions

with solids are among the workhorses of the field. Many important details about the time scale

and mechanism of the hollow atom formation and decay have been revealed in the investigation

of such systems ([19,22] and references therein). Our group pointed out that in the interaction

of slow Ar'^"^ ions with SiOi surfaces, x-rays emitted by the target, in addition to those emitted

]^ST-286
Collisions of slow Ar'^"^ ions with Si02 1279

by the projectile, can be observed [23]. These x-rays were interpreted differently in previous

investigations (e.g. [24]). The observation of even a weak silicon K x-ray peak (relative to the argon projectile K peak) is important because the silicon fluorescence yield (5.0% [25])

is at least two or three times smaller than that of the argon L shell (depending on the electron

configuration in the argon hollow atom at the time of the K x-ray emission). This means that the fraction of argon ions creating inner-shell vacancies in Si is at least two or three times higher

than the observed target to projectile K line intensity ratio. In our previous study [23], we also

took spectra at several different projectile energies, but the presence of instrumental artifacts in our data and uncertainties about the normalization of the beam flux hitting the target led us to avoid making quantitative conclusions about the velocity dependence of the intensity ratios of

the target and projectile K x-ray lines. Lehnert et al [26] measured the velocity dependence in a later experiment and argued that photoionization of target atoms by K x-rays from the argon

projectile could explain their observed line intensity ratios. In the current paper, we directly

separate the various possible mechanisms for the creation of K vacancies in silicon by Ar'^"^ impact.

The potential energy of the K-shell vacancy in Ar'^"^ ions is 4.12 keV (the binding energy

of a single K-shell electron). While there are different mechanisms through which this energy

can be transferred to the surface, the largest energy quantum that the surface can absorb in a

single step (neglecting less probable higher-order processes) is 1.84 keV, the binding energy

of a Si K-shell electron. This process can occur via a 'direct vacancy transfer' from the argon

to the silicon in a close collision. Because of the large mismatch between the silicon and

argon K energy levels, the probability of a one-electron process is relatively low. However, it was suggested that the process can take place via a dielectronic mechanism, where the excess energy is taken away by an electron from either the target or the projectile itself [23].

A second type of vacancy-transfer mechanism, 'decay-product-mediated vacancy

transfer', is a multi-step process. In the first step, the projectile fills its K-vacancy by emitting

a high-energy x-ray or electron. In the second step (or in several further steps), the decay

product (x-ray or electron) deposits its energy into the solid. We argue in the next section that the electron-mediated mechanism is not important in our experiment. This leaves the direct

transfer mechanism and x-ray-mediated mechanism to be separated experimentally.

In addition to studying the vacancy transfer that results in silicon K emission, we also

look at the structure of the argon K peak. Specifically, we look at the relative intensities of

the Ka, K)6 and Ky peaks, which are determined by the populations of the L, M and N shells,

respectively, at the time of emission. The ratios of these peaks, which are quite different from

the case of a ground state atom with a single K-vacancy, give information about the cascade

of transitions that fills in the hollow atom. These ratios are compared with those calculated by

the charge exchange and cascade model [27]. In the following paragraphs we will describe our method for separating the direct and the

'x-ray-mediated' vacancy-transfer mechanisms by using two different target samples. First we

will present our assumptions and their justifications. This will be followed by the experimental

and data analysis details. Next we will present our results on the Si and Ar K intensity ratio and its consequences. After that we will discuss our results on the ratios of the different

components of the argon K peak. We will close our paper with a conclusion.

2. Separation of the vacancy transfer mechanisms

In the present experiment, we isolate the 'x-ray-mediated vacancy-transfer mechanism' from

the other mechanisms, 'electron-mediated vacancy transfer' and 'direct vacancy transfer',

by covering a silicon sample with a thin metallic layer. This layer prevents the ions from

]NJIST-287
1280 E Takacs et al

penetrating into the silicon before their K vacancies have been filled while allowing highenergy x-rays to penetrate largely unimpeded. The rate of 'x-ray-mediated vacancy transfer', as measured using the coated sample, is compared to the total rate of vacancy transfer, as measured with a sample that is not metallized, to determine the relative importance of the

mechanisms. This technique relies on two key assumptions:

Assumption I. The K-vacancy in the argon ion is filled within the metallic layer.

This assumption is well established by both theory and experiment. In studies where HCIs

are sent through 10 nm foils with kinetic energies comparable to those used in the present

experiment, they were found to emerge from the foil in their equilibrium charge states and

in their ground states [13]. According to calculations, the K-shell of an Ar'^"^ ion with a

kinetic energy of 170 keV is filled in the first 10 nm of a solid [27]. Silicon K-shell vacancies,

therefore, can be created by the 'direct-vacancy-transfer' mechanism (which requires a close

collision) only if the Si atom is located within 10 nm of the surface. Furthermore, there is experimental evidence [28] that the filling can take place at much shorter distances in certain

situations. It was shown that a single monolayer of LiF molecules on Au(lll) completely

shields the interaction of hydrogen-like oxygen projectiles with the gold substrate with respect

to neutralization [28].

Assumption 2. The contribution to silicon K-shell vacancy production from energetic

electrons is negligible.

Among the decay products of the argon hollow atom, the only two that are energetic enough to create Si K vacancies are K x-rays (3.4 keV) and K Auger electrons (KLL: 2.5 keV and KLM:

2.9 keV). The ratio of the number of x-rays emitted per projectile ion to the number of electrons

emitted per projectile ion can be extracted from the K-shell fluorescence yield of the argon

hollow atoms. The relevant fluorescence yield in this dynamic system is an average of the

fluorescence yields of the individual hollow atom electron configurations that exist at the time

of the decay of the K-vacancy. A conservative lower limit is 1 1.8%, which is the fluorescence

yield in neutral argon [25]. This yield means that up to ten times more K Auger electrons than

K x-rays could be produced when hydrogen-like argon ions collide with a surface. Therefore,

it is important to consider the effect of electrons on the K-vacancy creation in silicon. From the

electron spectrum of [29], where the same collision system was investigated, we can estimate

that about 2/3 of the electrons are KLL and 1 /3 are KLM Auger electrons.

Although there may be more electrons than x-rays emitted when the argon K-shells decay,

we will show that their effect on the silicon K-shell ionization is negligible. As they penetrate

the surface, electrons quickly lose energy in collisions with the electrons in the solid. From

the energy distribution of the electrons receding from a colHsion event, we estimate that within

about five collision steps, more than 99% of the argon KLL and KLM Auger electrons would

be below the ionization threshold for the silicon K-shell. Comparing the cross section for the

removal of a valence electron in Si02 (which is the dominant process) to the ionization cross

section of the silicon K-shell [30], it turns out that only about 10""^ of the argon K Auger

electrons create K vacancies in silicon. To compare the silicon K-vacancy production from electron impact to that from x-ray

absorption, we need to look at the efficiency of the latter process. Because the energy of the

argon x-rays lies just above the K absorption edge of silicon (1.84 keV), the x-ray absorption is dominated by K-shell ionization. We estimate the effect of other contributing processes to be

about 10% [31]. Assuming that the angular distribution of x-rays emitted from the projectile

NIST-288
Collisions of slow Ar' ions with Si02 1281

is isotropic, approximately half of the emitted x-rays penetrate the solid, and most of these

x-rays create K-shell vacancies in silicon. Combining the estimates above (the small fraction of fast electrons that create inner-shell

holes and the large fraction of x-rays that create inner-shell holes), we conclude that, although

the fluorescence yield may be small, the contribution from fast electrons to the creation of

silicon K vacancies is about 10^ times less than that from x-rays. This result justifies our second assumption.

We would like to note here that, although fast Auger electrons do not efficiently contribute

to inner-shell vacancy creation, they play an important role in transferring the ion's potential

energy to the substrate. Fast electrons quickly distribute their kinetic energy into a shower

of slow electrons, which come mainly from the valence band of the crystal. It is still to be investigated whether either the electronic defects created in this way or the positive

charge surplus has enough density to induce a local modification of the crystal structure. Understanding the details of the processes caused by secondary electrons is important in determining whether the ion impact can create the necessary conditions for a structural

instability [32] or a Coulomb explosion [33-35] to occur in the solid. The attenuation length in silicon for 3 keV x-rays is 4 /xm [31]; therefore, most of

the vacancies that are created by the x-ray-mediated mechanism are located well below the

surface. Because the energy of the silicon K x-rays lies below the K absorption edge of silicon

(1.84 keV), their attenuation on the way out of the solid is small.

Apart from the above, in the comparison of the different samples we made an implicit

assumption that our method is independent of the electronic structure of the target. This

assumption is made because we compare relative intensities of projectile and target x-rays

rather than absolute yields.

3. Experimental arrangement

The experiments were performed at the NIST electron beam ion trap (EBIT) facility. Hydrogenlike argon ions created inside the EBIT were transported to the sample by a highly efficient ion

extraction and transport system [36]. The sample was kept at ground potential, so the energy of

the ions was determined by the voltage applied to drift tubes in the region where the ions were

created. We made measurements at 8 and 12 kV drift tube voltages; The projectile energies were roughly equal to the charge state times the potential difference between the central drift tube of the EBIT and the target sample. There is a small correction to the energy of the ions

due to the space charge of the electrons inside the EBIT; this was taken into account in the

analysis.

Our experiments were performed with two targets. The first target was a special multilayer

structure of silicon, molybdenum and carbon. The second target consisted of a 100 nm Si02

layer on a silicon substrate. In order to observe silicon target x-rays originating purely from the 'x-ray-mediated

mechanism' we made a special multilayer structure. On top of a silicon substrate, we prepared

a molybdenum layer and a carbon layer using low-pressure ion beam sputtering combined

with simultaneous ion beam polishing. The thicknesses of the layers were measured by highresolution diffractometry. The method of production and characterization is fully described

in [37]. The molybdenum film thickness varied from 24.3 to 25.4 nm across the length of the

sample and the interface between the molybdenum and silicon was about 0.6 nm thick. The

thickness of the carbon layer was 5 nm with an uncertainty similar to that of the molybdenum.

The thick molybdenum film completely prevented die ions from penetrating into the silicon

before their K vacancies were filled. (The molybdenum layer also prevents the penetration of

NIST-289
1282 E Takacs et al

PIN-DIODE

DETECTOR

BERYLLIUM

WINDOW

. TURBO PUMP

Figure 1. Experimental arrangement. X-rays from the projectile and the sample were detected by

a Si PIN diode detector at 90° to the direction of the incoming beam.

energetic electrons although, as we have shown above, their effect is already negligible even

without the coating.) The carbon on the top served as a passivation layer that ensured that there was no unknown heavy element contamination on the top of our sample. The penetration of

the x-rays through the top layers was more than 80% and was accounted for in the analysis.

The x-ray data were taken with a PIN-diode x-ray detector with a 12.5 jxm thick beryllium

window in front of the sensitive area. The detector was energy calibrated and its resolution

measured using the Ka and lines of So, Ti and Ca from a commercial x-ray source. The

250 eV resolution allowed us to separate the target and projectile x-rays, but was not enough

to resolve the different components of the projectile K peak. The detector was placed at 90"

with respect to the ion beam and 30° with respect to the surface normal as shown in figure 1

.

In our earlier experiment [23], we used an ion deceleration system and placed the target

at a biased potential in order to study collisions at very low energy (typically a few hundred

eV). As we pointed out in that work, however, an observed broad bremsstrahlung background

probably signalled the existence of secondary charged particles. Our present bremsstrahlungfree measurements confirm this hypothesis and indicate that most of the previously observed

silicon signal was due to impact ionization by spurious background electrons. To ensure that

this effect can be fully ruled out in the present measurements, the target was kept at ground

potential for the experiments where the target x-rays were studied. For those experiments

where we studied only the argon spectrum, we biased the sample in order to adjust the kinetic

energy.

4. Data analysis

Following a standard procedure that is used for solid-state silicon detectors [38], the spectra were fitted with asymmetric line-shape functions that took into account the response function

t ft t t

TURBO PUMP

BAFFLE

AR'^^ BEAM

TARGET ^ ^ ^ ^

IS[IST-290
1283

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 X-Ray Energy (keV)

Figure 2. X-ray spectrum from the Si02 sample showing the projectile K x-ray peaks at around

3 keV and the silicon target K peaks at around 1.8 keV. The target x-ray peak partially overlaps

the 'escape' components of the projectile peaks. The error bars indicate one-standard-deviation

uncertainties associated with counting statistics.

of our detector. The line-shape consists of a Gaussian with an exponential low-energy tail and

a so-called 'escape peak'. The width of the Gaussian, the slope and intensity of the exponential

tail and the position and intensity of the escape peak with respect to the main peak were treated as instrumental parameters and were held at the same value in all of the spectra. All the spectra were fitted simultaneously to determine these parameters. Only the positions and intensities of the Gaussian peaks were free parameters.

Although the 250 eV resolution of the x-ray detector did not allow us to separate the

Kor, and Ky components of the projectile peak, the shape of the argon K peak made

it necessary to include all three components in the fits. A spectrum from the Si02 sample,

together with the fitted curves, is shown in figure 2. The blending of the silicon K x-rays with the escape peaks of the and Ky lines of argon complicated the determination of the

silicon x-ray intensity. However, the escape peak of the argon Ka transition is fairly well

separated from the silicon line, and the argon Kor, K/S and Ky structure is well separated from

other features in the spectrum. Therefore, the information about the relative intensities of

the argon K lines was used to determine the positions and intensities of the argon K escape

peaks and to extract the intensity of the silicon line. The uncertainties of the results in our

figures and text are one-standard-deviation uncertainties derived from the least-squares fit to the measured spectra, assuming that we have the correct functional form and that our noise

is limited by counting statistics. Specifically, the uncertainties reported are the square root of

the corresponding diagonal element of the covariance matrix, with the points in the spectra

weighted as the square root of their intensities. We calculated the contribution to the silicon K x-ray yield due to 'x-ray-mediated vacancy

transfer' relative to the argon K x-ray yield as described in [26]. The fluorescence and the

attenuation in all the layers were taken into account for both samples (Si02-Si and C-Mo-Si).

In addition to the silicon x-rays, molybdenum L x-rays were present in the C-Mo-Si spectra

around 2.4 keV. The intensity of the Mo Lafi x-ray peak was 0.2% of the Ar Ka peak and its inclusion in the analysis was necessary to fit the spectra.

NIST-291
1284 E Takacs et al

3.5

3.0

w 20

CI 1

S 1.5 -I

K 10

0.5 -h

0.0

Sum of direct vacancy transfer and x-ray photoionization

X-ray photoionization

Direct vacancy transfer (upper limit)

50 100 150 200 250 Projectile energy (keV)

300 350

Figure 3. Contributions of the different silicon K-vacancy production mechanisms. The plot represents the ratios of the Si and Ar K x-ray peak intensities. The solid curve is a calculation of the contribution from x-ray photoionization. The upper limit of the 'direct vacancy transfer' contribution (dashed curve) is estimated using calculations of Stolterfoht et al [27]. The dot-dash

curve is the sum of the other two. Grey diamonds, data obtained using the Si02-Si sample; open

circles, data from [26].

5. Results

5.7. Experimental evidence for the importance of the 'x-ray-mediated vacancy transfer'

As pointed out earlier, in the case of the C-Mo-Si multilayer sample, the sole origin of silicon

x-rays was the photoionization caused by the penetration of energetic argon K x-rays into the

silicon substrate. We found that the intensity of the silicon K x-rays relative to the intensity of

the argon K x-rays was 0.0183(30). This experimental value agrees with the theoretical value

of 0.016 calculated taking into account the attenuation of the incoming and outgoing x-ray

intensities in the thin molybdenum and carbon films. This result is experimental verification

that the x-ray fluorescence is indeed an important process for the K-vacancy transfer between

the projectile argon and the target silicon atoms as suggested by Lehnert et al [26].

5.2. Upper limitfor the 'direct vacancy transfer' contribution

Having shown that x-ray photoionization dominates the vacancy transfer, we wanted to set an upper limit for the 'direct vacancy transfer' contribution. In order to do this, we collected

spectra using the uncoated sample and extracted relative line intensities from the fits. Figure 3

shows the measured ratio of the silicon K x-ray intensity to the argon K x-ray intensity for

the Si02 sample. The experimental values from Lehnert et al [26] are also shown on the

plot. These data are complementary to this paper because they span a wider energy range, even though their larger uncertainties do not allow one to rule out 25% or less 'direct vacancy

transfer' contribution to the peak intensities. The calculated values for the contributions from

the 'x-ray-mediated vacancy transfer' and from 'direct vacancy transfer' (discussed below) are

also shown on the plot.

The calculation of silicon K x-ray yield from 'x-ray-mediated vacancy transfer' takes into

account the effect of the 100 nm Si02 layer on the top of Si, which modifies the results only

slightly. The calculated value, 2.02%, is independent of the projectile energy.

NIST-292
1286 E Takacs et al

dynamics and successfully describes the projectile energy dependence of the structure within

the Ka line measured with high resolution in [24]. In this paper; the measured intensity ratios,

Ko;/K;6 and Ka/Ky, are predicted by the theory to within 25% as shown in figure 4. Using

a microcalorimeter, Legros et al [40] measured the ratio (K;6 + Ky)/KQr to be 31%; the same

ratio extracted from our spectra gives 29% and agrees well with this value. Our experiment

shows essentially no energy dependence in either of the ratios, whereas the theory predicts a

slight change in the case of Ka/K^ and a stronger change (about 40%) of the Ka/Ky ratio over the measured energy range. If there is any change of the experimental ratio at the lowest

projectile energies, it is hidden within the error bars of our measurement. In contrast, a ground

state argon atom with a single K-vacancy emits only Ka and K/8 radiation (the intensity ratio,

Ka/KyS, is 9.5 [41]); Ky radiation is absent because the N shell is empty. In the case of an HCI, however, any of these shells can be filled in a cascade from the high-lying states of the

hollow atom.

6. Conclusions

We investigated the interaction of slow (kinetic energy less than 220 keV) hydrogen-like argon

ions with silicon surfaces. We confirmed our earlier [23] and other [26] observations that target x-rays are indeed produced in this process. We used a novel technique to experimentally

separate the mechanisms for vacancy transfer from the K-shell of the projectile to the K-shell of

the target and found that 'direct vacancy transfer' is negligible compared with photoionization

of the target atoms by x-rays originating from the decaying K-shell hole of the projectile.

Based on this paper, we emphasize that, for a complete picture of the interaction of slow

HCIs with surfaces, the interactions of secondary collision products with the target should

be taken into account. These products (electrons and x-rays) can carry a significant part of

the projectile's potential energy; therefore, they can play a dominant role in the deposition

of potential energy into the solid. As a result, they play a role in setting the timescale of the

interaction and in the spatial distribution of the energy deposition into the target. The timescale

and spatial distribution are important parameters for the further relaxation of the surface, and

can determine whether the interaction causes only transient effects or permanent damage to the surface.

Acknowledgments

We wish to thank Douglas J Alderson for his excellent technical support. ET acknowledges the

support of the Hungarian Science Fund (OTKA, F19336) and the US-Hungarian Joint-Fund

(JFNo 562/96) for support. RM acknowledges support from the National Research Council.

We would like to thank Cedric J Powell and Larry T Hudson for their critical reading of this manuscript.

References

[ 1 ] Van Zant P 1 990 Microchip Fabrication: a Practical Guide to Semiconductor Processing (New York: McGrawHill)

[2] Brundle C R, Evans C A, Wilson S and Fitzpatrick L E 1992 Encyclopedia of Materials Characterization

(Boston: Butterworth-Heinemann)

[3] DeMarco A J and Melngailis J 1999 J. Vac. Sci. Technol. B 17 3154-7

[4] Chason E and Mayer T M 1993 Appl. Phys. Lett. 62 363-5

[5] Ziegler J F, Biersack J P and Littmark U 1985 The Stopping and Range ofIons in Solids (New York: Pergamon)

[6] Young D A 1997 Radial. Meas. 27 575-86

ISraST-293
Collisions of slow Ar' '"^ ions with Si02 1285

20 Ko/Ky

16

.oS^ 12

X 8 Ko/Kp

. 0 I , I 0 10 20 30 40 50 60 Perpendicular projectile energy (keV)

Figure 4. Measured intensity ratios of the different projectile K x-ray components. Open circles, Ka/Ky, full circles, Ka/Kfi. The theoretical curves are calculations based on [27].

The 'direct vacancy transfer' contribution for this particular sample (shown in figure 3) is an estimate for the upper limit of the dielectronic process proposed in [23]. In this process, the

argon projectile K-vacancy is filled by a silicon K-shell electron and the excess energy is taken

away by a second electron from the silicon L shell. In the calculation of the Si K-vacancy

production, the rate for the dielectronic process was estimated using the Landau-Zener model

as shown previously for L-shell vacancy transfer in the Ar-Si system [39]. To account for the

filling of the hollow argon atom, a complex charge exchange and cascade model calculation, of

the type described previously by Stolterfoht et al [27] was performed. The model was verified

by complementary measurements in which the energy distribution and yield of the emitted

electrons were measured in the same system [29].

From figure 3, we conclude that, even in the case of the Si02 sample where close collisions

between the argon projectiles and the silicon surface atoms were possible, the contribution

from the 'direct vacancy transfer mechanism' is less than 5% of the contribution from x-ray

photoionization. This observation supports the suggestion of Lehnert et al [26]; however, in their measurement the error limits did not rule out the contribution from the 'direct vacancy

transfer mechanism' within the limits of the theoretical estimates.

5.3. Ratios ofprojectile K emission lines

In addition to the silicon K radiation, we have studied the argon K radiation. We obtained

intensity ratios for the Ka, K)3 and Ky x-ray lines in argon from the data fits described earlier

and compared them with the charge exchange and cascade calculation [27]. The positions

and relative strengths of these lines carry information about the number of electrons in the

various shells at the time of the K transition and, in turn, about the dynamics of the electron

capture and cascade. The charge exchange and cascade model calculation accounts for these

ISnST-294
Collisions of slow Ar' ions with Si02 1287

[7] Dufour C, Audouard A, Beuneu F, Dural J, Girard J P, Hairie A, Levalois M, Paumier E and Toulemonde M

1993 J. Phys.: Condens. Matter 5 4573-84

[8] Schenkel T, Hamza A V, Barnes A V and Schneider D H 1999 Prog. Surf. Sci. 61 23-84

[9] Parks D C, Stockli M P, Bell E W, Ratliff L P Schmieder R W, Serpa F G and Gillaspy J D 1998 Nucl. Instrum. Methods B 134 46-52

[10] Amau A et al 1997 Surf. Sci. Rep. 27 1 17-239

[11] Spom M, Libiseller G, Neidhart T, Schmid M, Aumayr F, Winter H P, Varga P, Grether M, Niemann D and

Stolterfoht N 1997 Phys. Rev. Lett. 79 945-8

[12] Burgdorfer J, Lemer P and Meyer F W 1991 Phys. Rev. A 44 5674-85

[13] Hattass M, Schenkel T, Hamza A V, Barnes A V, Newman MW, McDonald J W, Niedermayr TR, Machicoane G A

and Schneider D H 1999 Phys. Rev. Lett. 82 4795-8

[14] Stolterfoht N, Amau A, Grether M, Kohrbruck R, Spieler A, Page R, Saal A, Thomaschewski J and Bleckneuhaus J 1995 Phys. Rev. A 52 445-56

[15] Snowdon K J, Havener C C, Meyer F W, Overbury S H, Zehner D M and Heiland W 1988 Phys. Rev. A 38 2294-304

[16] Limburg J, Schippers S, Hughes L, Hoekstra R and Morgenstem R 1996 Phys. Rev. A 53 4228

[17] Aumayr F, Kurz H, Schneider D, Briere M A, McDonald J W, Cunningham C E and Winter H P 1993 Phys.

Rev. Lett. 71 1943-6

[18] Wmecki S, Cocke C L, Fry D and Stockli M P 1996 Phys. Rev. A 53 4228-37

[19] Briand J P Le Roux V, Bechu N, Dreuil S, Machicoane G, Prior M and Xie Z 1999 Nucl. Instrum. Methods B

154 166-73

[20] Meyer F W and Morozov V A 1999 Phys. Sck T SOB 226-7

[21] Schenkel T, Barnes A V, Hamza A V, Schneider D H, Banks J C and Doyle B L 1998 Phys. Rev. Lett. 80 4325-8

[22] Briand J P Debilly L, Charles P Essabaa S, Briand P Geller R, Desclaux J P Bliman S and Ristori C 1990 Phys.

Rev. Lea. 65 159-62

[23] Takacs E, Onodi-Szucs Z, Ratliff L P Gillaspy J D and Palinkas J 1997 Nucl. Instrum. Methods B 124 43 1-4

[24] Briand J Petal 1996 Phys. Rev. A 54 4136-9

[25] Krause M O 1979 J. Phys. Chem. Ref. Data 8 307

[26] Lehnert U, Stockli M P and Cocke C L 1998 J. Phys. B: At. Mol. Opt. Phys. 31 51 17-22

[27] Stolterfoht N, Bremer J H and Muino R D 1999 Int. J. Mass Spectrom. 192 425-36

[28] Khemliche H, Schlatholter T, Hoekstra R, Morgenstem R and Schippers S 1998 Phys. Rev. Lett. 81 1219-22

[29] Stolterfoht N, Grether M, Niemann D and Hoffmann V 1999 21st Int. Conf. on Physics ofElectronic and Atomic

Collisions (Sendai, 1999) Book of Contributed Papers

[30] Ashley J C and Anderson V E 1981 7. Electron Spectrosc. 24 111

[31] Chantler C T 1995 J. Phys. Chem. Ref Data 24 71-591

[32] Schenkel T, Hamza A V, Bames A V, Schneider D H. Banks J C and Doyle B L 1998 Phys. Rev. Lett. 81 2590-3

[33] Cheng H P and GiUaspy J D 1998 Comput. Mater Sci. 9 285-94

[34] Cheng H P and Gillaspy J D 1997 Phys. Rev B 55 2628-36

[35] Bitensku I S, Murakhmetov M N and Parilis E S 1979 Zh. Tekh. Fiz. 49 1044

[36] Pikin A I, Morgan C A, Bell E W, Ratliff L P Church D A and Gillaspy J D 1996 Rev. Sci. Instrum. 67 2528-33

[37] PeduUa J and Deslattes R D 1993 Production of ion beam sputtered multilayers Multilayer and Grazing Incidence X-Ray/EUV Optics (Proc. SPIE vol 2077j ed R B Hoover and A B C Walker (Bellingham, WA: SPIE)

[38] Campbell J L, Wang J X and Teesdale W J 1989 Nucl. Instrum. Methods B 43 490-6

[39] Stolterfoht N 1993 Phys. Rev. A 47 R763-6

[40] Legros M, Silver E, Schneider D, McDonald J, Bardin S, Schuch R, Madden N and Beeman J 1995 Nucl.

Instrum. Methods A 357 1 10-4

[41] Salem S I, Panossian S L and Krause R A 1974 At. Data Nucl. Data Tables 14 91-109

NIST-295
In situ imaging of highly charged ion irradiated mica

L.P. Ratliff and J.D. Gillaspy

Atomic Physics Division, National Institute ofStandards and Technology, Gaithersburg, MD 20899 USA

Abstract We have studied the modification of mica surfaces due to the impact of Xe"*** ions by imaging the ion-exposed

surfaces with atomic force microscopy in vacuum. By incorporating the microscope into the vacuum chamber where the samples are exposed to the ions, we rule out posterior modification of these features in air. The features, raised bumps

19(2) nm in diameter, are similar to those imaged previously in air, however, their heights appear to be larger than

previously reported.

There is a growing interest in the interactions of

highly charged ions (HCIs) with surfaces, which is driven by a basic scientific interest in the physical processes involved and by an increasing number of

applications that are being developed [1,2]. This

interest stems from the fact that HCIs carry massive

amounts of potential energy (for example, the Xe***^ ions used in the present work have 5 1 keV of potential

energy). Because this potential energy is releeised at the surface [3], while the kinetic energy is released along the ion's path through the solid, the processes

that occur at the surface are dominated by the potential energy. This strong influence of the potential energy

has been demonstrated in studies of particle emission

during HCI impact [4-6] (for example, x-ray emission,

sputtering, secondary ion emission and electron emission), and in studies of surface damage after ion

exposure [7,8]. With these experiments, as well as theoretical investigations [9,10], we have learned a great deal about the evolution of the projectile as it approaches

and penetrates the surface. As described in the

'classical over-the-barrier model' [9], the ion begins to extract electrons from the surface even before impact.

Many of these electrons become bound in high-lying

states of the projectile, forming a 'hollow atom' with

many empty core states. As the ion nears the surface,

it can begin to decay, for instance by Auger ionization. Upon penetration of the siuface, the electrons that are weakly bound are 'peeled off and replaced with

electrons from inside the surface, forming a secondary

hollow atom. While the formation and decay of the hollow atom is well described by models, at least in a general sense,

the response of the surface is still poorly understood.

Several models have been proposed to describe this situation. The first such model asserts that, if electrons

are removed from the surface faster than they can be

replaced from the bulk, there will be a localized charge

imbalance which will cause a 'Coulomb explosion'

[11]. This explosion can result in the sputtering of a

large nimiber of ions and neutral atoms or, in the case of mica, blistering [12]. A second model says that the ion leaves many excitations near its impact site [13]. These excitations promote atom pairs from bonding

states to antibonding states. If the excitation density is above a threshold, they can lead to a destabilization of

the crystal structure and permanent structural damage

[14,15]. Materials that support self trapped excitons can be described by a third model. In this case, self- trapping can localize the excitations very near the surface until they decay into color centers, which

migrate to the surface and result in sputtering [6]. One approach to studying the surface response to HCI impact is to image the impact sites after ion exposure in order to determine the size and

morphology of the impact features. Such studies can

give us information about the spatial extent of the ion- surface interaction and perhaps about the mechanism

for their formation. In addition, for the various

applications that are under development, it is useful,

and in some cases necessary, to understand and

characterize the damage left by the ion. Furthermore,

if we can image these features, the morphology might

suggest other applications that have not yet been

considered. Because they are very small (approximately 20 nm in diameter and approximately 1 nm high), however, observation of the ion-induced

features has been limited to a few materials on which

atomically smooth surfaces can be prepared easily and

CP576, Application ofAccelerators in Research and Industry - Sixteenth Int'l. Conf, edited by J. L. Duggan and I. L. Morgan

2001 American Institute of Physics 0-7354-0015-6

935

NIST-296
reliably. One such surface is mica. Because of its layered structure, mica is easily cleaved with adhesive

tape to reveal large, single crystalline terraces. Such a

freshly cleaved surface can be imaged with contact mode atomic force microscopy (AFM) in air to reveal

crystal structure. Because of these favorable

characteristics, mica was used as the target for the most

extensive, systematic studies of HCI-induced surface damage [7,8,16].

The studies of surface damage on mica from HCI

impact have shown that each ion impact leaves a bump

on the surface and that the size of these bumps is independent of kinetic energy in the range, 4.4 to 880 keV (Xe*** projectiles) [7,16]. Furthermore, the volume of the bumps increases monotonically with

charge state, and therefore, potential energy [7,8]. In one study, where HCIs of various species and kinetic energies were used, it was found that the volume

increased linearly with projectile charge state from an

apparent threshold at q = 30. Another study used only xenon ions with 100 keV of kinetic energy so that the charge state (and therefore, potential energy) was the only independent variable. In this case, the volume

increased more rapidly than the charge state. In fact, the volume correlated more strongly with the potential energy, which has a non-linear relation to the charge

state. Although the variation of feature size with charge

state seems to be different in the two studies, they are not inconsistent. All of the data points in these two

studies that correspond to the same ion species and

charge state agree to within the stated uncertainty

limits. The apparent difference in slope might arise from the fact that the former work involved ions of

different mass (note that the relationship between

charge state and potential energy varies with mass).

Because the variation in the size of the observed

features is systematic and reproducible, we believe that the method of measuring the features with AFM in air

is sound and useful. That is, the dimensions being

measured are physically meaningful. In order to better understand the nature of these features, however, we

need to look carefully at the imaging technique. There

are some aspects of imaging in air that can change the apparent features from their original form. First, exposing the samples to air allows the features themselves to be modified [17]. For example, the impact sites are likely to contain many broken bonds,

which would make them more reactive than the surrounding single-crystalline material. When such a

surface is exposed to air, hydrocarbons or other contaminants might preferentially attach to the impact

site, significantly changing its morphology from that of

the original feature. Second, in air, there is a layer of water adsorbed on the surface. The c^illary forces

due to this water layer increase the force between the sample and the probe, possibly deforming the surface. The purpose of this study is to better understand the

nature of the features as created by the HCIs. To this end, we have exposed mica surfaces with Xe*^ ions and imaged them in vacuum without allowing them to come into contact with air. We chose Xe"^ ions for our experiments because they were used in both of the previous experiments with which we compare pur

results. Because it has been established that the morphology of these features is independent of kinetic energy [7,16], we used a single kinetic energy, 350 keV, for all of our measurements.

Experimental

The ions are produced in the NIST electron beam ion

trap (EBIT) and extracted into a beam line system that has been described previously [18]. The beam that reaches the target consists of approximately one

million '^^e""* ions per second at 350 keV in a

diameter of 3 mm.

The mica samples were cleaved in air using adhesive

tape and immediately inserted into a load lock

chamber. From the load lock, they were passed

between the various chambers of the apparatus while

remaining under a vacuum of 10'^ Pa. This apparatus

consists of a target chamber, where they are exposed to

the ions and the imaging chamber, which is equipped

with a scanning probe microscope. This microscope was used previously in scanning tunneling mode to image HOPG (highly oriented pyrolitic gr^hite)

samples [19]. Because mica is a good insulator, all of

the imaging was performed using AFM. We used

contact mode AFM with an applied normal force of 1 nN. The samples were imaged, exposed to the HCIs,

then imaged again without exposure to air. In order to study the effect of air exposure on the ion-induced

features, the exposed samples were brought out of the vacuum, reinserted and imaged again.

Results and Discussion

Before exposure to the ions, the mica surfaces were

featureless except for crystal structure observed in scans of small areas. In the exposed regions of the

surfaces, we consistently found bumps that are 19(2) nm in diameter (the uncertainties given in this paper

are single standard deviation and are based on repeated

sampling) (see Figure 1) and each bump appears to correspond to a single ion impact; this is in agreement

with previous measurements [7,8,16]. The fact that these features are convex, even before removal from

the vacuum, means that this morphology is a direct

result of the ion impact rather than, for

936

lSfIST-297
FIGURE 1. AJFM image showing the topography of three Xe ^ impact sites on mica. The image is 50 nm x 80 nm

and the features are approximately 20 nm in diameter and 1 nm high.

instance, contaminant adsorption at the impact sites [17]. Therefore, our observation is consistent with the

interpretation of these features as blisters [7,16]. The

heights measured in the current work, however, are somewhat larger than those previously reported. In the images taken before exposure to air, the features were

1.0(3) nm high, approximately double the heights

measured previously with AFM in air [7,8,16]. The

feature heights that were measured after the samples were exposed to air and returned to the vacuum for imaging, varied somewhat but remained in the range between the height of those not exposed to air (approximately 1 nm) and the height of those imaged

in air (approximately 0.5 nm [7,8,16]). Quantitative

measurement of feature height is difficult, however,

because the topography is convoluted with other

characteristics of the probe and of the surface, such as hardness and ftiction. In order to correctly interpret the images, it is important to understand the lateral forces. The term

lateral force refers to the torque exerted on the

cantilever as it is scanned, for example, over a feature of elevated friction. When scanning in the forward

direction (the direction perpendicular to the symmetry

axis of Jie cantilever, referred to below as the 0°

direction) this torque manifests itself as a lateral displacement of the beam of light in the 'bouncing

beam' method of detection £20J. When imaging an

area that is flat, but has a modified coefficient of

friction, if the lateral force signal decreases when

scanning in the forward direction it will increase when

scanning in the reverse direction (this is because the

frictional force is opposite to the direction of travel). If

this surface is scanned in the direction along the axis of

symmetry of the cantilever (90"), there will be no

features in the lateral force image because the torque on the cantilever will tend to move it up and down,

modifying the normal force, and thus mimicking

topography. Such an increase in friction has been

observed in tracks formed in mica by fast ions 121].

While the lateral and normal forces are nominally

independent, there can be a coupling between the two

that interferes with the measurement of feature height.

Parks et al. 116] found that, in imaging mica exposed

with Xe""*, the lateral force played an important role in the measurement of the heights of the ion induced

features. The heights measured when scanning in one

direction were larger than those measured when

scanning in the opposite direction; the quoted height

values are averages of the two measurements. In order

to minimize this effect, the normal force was kept to its minimum stable value (recall that the magnitude of the

frictional force is proportional to the normal force). Interestingly, Ruehlicke et al. IT] did not observe this

effect. In the present work, we find that many of our lateral force images have an asymmetric character (see Figure

2) while others have the character of flat areas of

modified ftiction (see Figure 3) that was seen by Parks

et al. [16]. With respect to the asymmetric images,

when the scanning direction is changed, their orientation does not rotate, as if the features are truly

asymmetric. We interpret this phenomenon as

resulting from the torque on the cantilever as it traverses the changing slope of the ion-induced bumps

[20]. While many of our lateral force images show the type of frictional character seen in fig. 3, in addition to asymmetric character seen in fig. 2, the images taken

before exposure to air are dominated by the later. With

b.

FIGURE 2. Lateral force images taken before the sample was exposed to air. The images are 100 nm across and were

scanned at a) 0° and b) 90°. The asymmetric nature of these

features is interpreted as resulting from a torque on the

cantilever as it traverses the changing slope of the ion- induced bumps.

937

NIST-298
FIGURE 3. Lateral force images taken after the sample was

exposed to air. The images are 100 nm across and were

scanned at a) 0°, b) 90° and c) 180°. The features appear to have a modified coefficient of ftiction as compared to the background.

the possible exception of images taken very soon after exposure to air (less than one hour in the vacuum), we

do not see the difference in forward and reverse heights

observed by Parks et al. [16].

It was previously reported that repeated scanning

tends to 'erase' the features; that is, the bumps flatten [16] or even peel open to reveal a crater [7]. While

there were examples in the present work where slight

modification in the features weis observed after repeated scanning, generally the feature heights

remained constant. One f>ossible explanation for this discrepancy is that the adsorption of water and other contaminants in the air plays a role in the erasure. Alternatively, perhaps the imaging in the present work

is done with a normal force that is sufficiently weak

that the features are not significantly affected by the probe. It is not surprising that the effective normal

force in vacuimi is less than that in air because the

capillary force due to adsorbed water is reduced. A

weaker normal force could also explain the apparent

increase in feature height over those measured in air; the tendency of the probe to compress the features

diiring scanning is reduced, allowing the true topography to be measured.

Conclusion

We have confirmed that the features created by HCI

impact on mica are protrusions as previously observed

in air. This supports the assertion that these features are in fact blisters, and that they are caused by a delamination of the layers of the mica due to the deposition of the ion's large potential energy

[7,8,12,16]. The heights of the features appear larger

than those measured previously in air. This difference could reflect an actual modification of the surface when in air or an artifact of the imaging. In either case, because the vacuum environment allows more control over atmospheric conditions that are known to influence imaging with AFM, images taken in vacuum

are more likely to reflect the true topography of the features created by the ions. Because of the similarity

of features imaged in vacuum to those imaged in air, this study supports the general conclusions of the previous systematic studies [7,8,16].

Acknowledgements

We thank Doug Alderson for his technical assistance.

References

1. Hamza. A.V. etal., J. Vac. Sci. Technol. A, 17. 303-305 (1999).

2. Marrs, R.E. eial.. Rev. Sci. Instrum., 69, 204-209 (1998).

3. Hattass, M. etal., Phys. Rev. Lett.. 82, 4795-4798 (1999). 4.Schenkel, T. etal.. Prog. Surf. Sci.. 61, 23-84 (1999).

5. Amau, A. etaL Surf. Sci. Rep., 27, 117-239 (1997).

6. Aumayr, F. etal.. Int. J. Mass Spectrom., 192, 415^24 (1999).

7. Ruehlicke, C. et al., Nucl. Instrum. Methods B, 99, 528-531

(1995).

S.Parks. D.C. etal, Nucl. Instrum. Methods B, 134, 46-52 (1998). 9.Burgdorfer. J. etal.. Phys. Rev. A. 44. 5674-5685 (1991).

lO.Stolterfoht. N. et al., Int J. Mass Spectrom., 192, 425-436 (1999).

1 1 .Parilis, E.S., in Proceedings of the International Conference on Phenomena in Ionized Gasses, edited by 1969, 94.

12.Parilis, E., Nucl. Instrum. Methods B, 116, 478-481 (1996). 13.Schenkel, T. etal., Phys. Rev. Lett., 81, 2590-2593 (1998). 14.Stampfli, P. and Bennemann, K.H., Appl. Phys. A, 60, 191-196

(1995).

15.Hernnann. R.F.W. etal., Appl. Phys. A, 66, 35-42(1998).

16. Parks, D.C. etal., i. Vac. Sci. Technol. B, 13, 941-948 (1995). n.Neumann, R., Nucl. Instrum. Methods B, 151. 42-55 (1999).

18.Radiff, LP. etal.. Rev. Sci. Instrum.. 68. 1998-2002 (1997).

19.Minniti. R. etal.. Physica Scripta. . In Press (2000).

20.Colton, R.J. et at.. Procedures in Scanning Probe Microscopies. John Wiley and Sons, Chichester. 1998.

21. Hagen. T. etal.. J. Vac. Sci. Technol. B, 12, 1555-1558 (1994).

938

NIST-299
Physica Scripta. T92, 22-26, 2001

In-situ Observation of Surface Modification Induced by Highly

Charged Ion Bombardment

R. Minniti, L. P. Ratliff, and J. D. Gillaspy

National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Received July 31 2000: accepted September 29. 2000

Abstract

Nanoscale dots created on the surface of a highly oriented pyrolitic graphite sample by individual highly charged xenon projectile ions were observed

using an in-situ scanning tunneling microscope. This is the first time that such features have been created and imaged without exposure to air. The dots vary in size with the charge state of the ion, up to a diameter of 6.6 nin

for Xc***. The results are discussed in terms of the energy density deposited

in the near surface region.

The interaction of ions with surfaces has been extensively studied and forms the basis of many powerful tools used

in modem industrial processes [1], particularly in the area of microelectronics [2]. Until recently, however, this field of study virtually always involved ions of low charge state. In this regime, the mechanism imderlying the ion-surface

interaction involves the kinetic energy of the ion. The advent of compact sources of highly charged ions (HCIs) with reasonably high brightness has allowed investigators to study a new regime in which the internal potential energy dominates the interaction with the surface. The kinetic energy of ions extracted from these sources is generally

in the range of several hundred keV but can be accelerated or decelerated to suit the experiment. Using an electron beam ion trap (EBIT), it is possible to produce ions with charge states beyond q = 70+ [3] with a device that is small enough to fit in a closet; a larger version of EBIT has been shown to produce even the highest possible charge state ion among the naturally occurring elements i.e. U'^"*" [4]. When a single such ion neutralizes itself on a surface, it releases approximately 700 keV of potential energy (the sum of the binding energies of all electrons removed in for- ming the HCI). In contrast, a conventional Ar'""" ion releases approximately 10 eV of potential energy. For the purposes

of this paper, we refer to the "highly charged regime" as that for which the ion's potential energy is at least a few

percent of the U^^"*" potential energy. Most of the possible

states of ionization on the periodic table fall into this regime, but relatively few of them have been investigated, leaving a

vast territory unexplored. Because a single HCI deposits a tremendous amount of energy into a small voliune (nanometers) in a very short time (femtoseconds) [5] it has been hypothesized that investigation of the high charge

regime will reveal mechanisms of ion-surface interaction

that rely on the ion's potential energy rather than its kinetic energy. This is of fundamental interest and might have practical application [6,7]. As an HCI approaches a surface, it extracts many

electrons. Some of these electrons become bound in high

Physica Scripta T92

lying states of the projectile, forming a so-called hollow

atom. The formation and decay of the hollow atom have

been the subject of a considerable amount of theoretical and experimental work [8-11]. In addition, one would

expect a single HCI to produce a very substantial disruption on a surface that would be observable using modem

scanning probe microscopy techniques. To date, however,

microscopic evidence of such disruptions has been pre- sented for only a few selected surfaces: polymers (CR-39

[12] and PMMA [13]), mica [14,15] and alkanethiols [16]. Of these, only mica and alkanethiols show features that are evident without chemically etching the surface after bombardment with ions. Only in mica has a systematic

dependence of the feature size on potential energy been

demonstrated. This dearth of evidence, coupled with the

fact that other surfaces such as siUcon have been studied

in air with null results [17], leaves open the question of whether the formation of surface features is a general

characteristic of HCI-surface interactions, or whether it is a phenomena which appears in only a few special types

of surfaces. Answering the question posed above has been hampered

by the lack of appropriate tools. In the case of sihcon,

for example, oxide overgrowth can be expected to obscure any nanoscale surface features that might be induced by

HCIs, unless the entire experiment is performed in vacuum.

Furthermore, even for normally inert surfaces, the HCI

impact site is likely to contain broken bonds to which

atmospheric gasses might attach [18,19]. This nucleation could create bumps at impact sites that were originally

craters. In order to overcome these limitations, we have

recently developed a unique combination of tools in our

laboratory at NIST: an EBIT [20] equipped with a highly

efficient ion beamline [21] that terminates in a UHV

chamber equipped with an in-situ scanning tunneling

microscope (STM). This paper presents our first results using this system.

The surface we have chosen to study first with this system

is highly oriented pyrolitic graphite (HOPG), both for its similarity to mica (layered structure) and for its ease of sur- face preparation. Many groups have studied single ion impact sites on HOPG with STM in air and have observed

nanoscale features ([19,22-27], and the references therein), but none of these explored the highly charged regime.

Furthermore, there is an apparent lack of reproducibility

in the size of these features, perhaps due to the different atmospheric conditions under which the various experiments were carried out. In one study, the structures were studied as a function of charge state at a constant kinetic energy of 2 keV; the diameters (full width at half max) were observed

© Physica Scripta 2001

NIST-300
In-situ Observation of Surface Modification Induced by Highly Charged Ion Bombardment 23

to increase from 0.23 nm to 0.67 nm as the potential energy was increased from 14 eV to 138 eV [26]. Our study extends

this potential energy study by more than two orders of

magnitude and reaches well into the highly charged regime,

where mechanisms that depend on the projectile's potential energy have been hypothesized to dominate.

In the present work, HOPG samples were cleaved with

adhesive tape and inserted into a load lock chamber. From

the load lock they were transferred, under vacuum, to a

target chamber, where they were exposed to the ion beam,

and to a surface analysis chamber equipped with an STM.

The pressure in the system was approximately 10"^ Pa

(10"' torr). The samples were imaged with the STM prior

to exposure to the ion beam using Pt-Ir tips, with tunneling

currents between 0.5 nA and 0.9 nA and gap voltages

around 0. 1 Volt. The unexposed surface displayed large flat terraces and atomic structure. The samples were then transferred to the target chamber

and exposed to beams of xenon ions from the NIST EBIT

with 276 keV of kinetic energy and with charge states of

q = 23+ and ^ = 44+ [20,21]. For a typical ion beam

current, 10 pA (as measured with both a Faraday cup

and a channel electron muliiplier), the exposure time was approximately 60 minutes.

After exposure, the samples were again imaged with the STM. Figure 1(a) shows a typical image of an HOPG sample

after exposure to a Xe'*^"'' ion beam. A number of HCIinduced nanoscale protrusions, which we refer to hereafter

as dots, can be observed. The density measured on several exposed regions is, on average, about 2500 dots/nm^. This

is in good agreement with the estimated ion fluence and

is consistent with the assertion that each of the observed dots corresponds to a single ion impact. To confirm that we were

observing topographical, as opposed to electrical, features, we imaged them at both positive and negative bias voltages

and found that, in both cases, the features appeared convex

[25]. In contrast, Mochiji et al. [26] found that their ion induced features were due, at least in part, to a modification

of the partial density of states at the impact sites; they illus- trate this with I-V curves from both the impact site and

the background. Furthermore, they did not see any features

with contact mode AFM, and they concluded that the fea- tures were due to,a sputtering of surface atoms which left a defect in the crystal. It is not surprising that our features

are somewhat different from those seen by Mochiji et al. [26] because, in their case, the ion's kinetic energy and potential energy were both an order of magnitude less, and the surfaces were imaged in air. Figure 1(b) is an image of a dot produced by a Xe**"*" ion. The region around the dot is unperturbed, displaying the expected crystal structure. The image was obtained using

a positive gap voltage of 0.095 V on the sample, a tunneling current of 0.6 nA and a scan speed of 1 line/s. Figure 2 shows cross sections of dots such as the one in Fig. 1(b); the dot produced by the higher charge state ion is consider- ably wider than that produced by the lower charge state ion. This is consistently observed for dots on different regions of the exposed surfaces as seen in the histograms

of the basal wndths of dots, plotted in Fig. 3. The mean

basal widths and standard deviations of the dots produced

by the Xe^^^ ions are 3.0 (10) nm, while for those produced

by Xe'"*"*^ is 6.6 (1 0) nm, indicating that the size of these dots

is correlated with the charge state of the ion. This variation in size can be understood in terms of the deposition of a high local potential energy density into the surface. The potential energy of a Xe''^'*" ion is 51 .3 keV,

nearly 8 times greater than the potential energy of a Xe^^"*" ion (6.58 keV); this energy is transferred to the sohd, as the ion neutralizes, within several nanometers of the surface

[5]. In contrast, assuming that the stopping power is inde- pendent of the ion's initial charge state, the 276 keV of

kinetic energy is deposited over a region extending about

100 nm into the solid and only 16 keV is deposited in the first 5 nm of its path (4 keV in electronic stopping

and 12 keV in nuclear stopping) [28]. This indicates that, in the near surface region, the Xe''^^ ion deposits signifi- cantly more potential energy than kinetic energy, whereas

the Xe^''"'" ion deposits mostly kinetic energy. As a result, the transfer of potential energy from the Xe'*^^ ion plays

a dominant role during the ion-surface interaction, and

is the expected cause of the observed change in the size of the dots.

]^ST-301
24 R. Minniti, L. P. Ratliff and J. D. Gillaspy

0 2 4 6 8 10 12 width (nm)

Fig. 2. Cross sectional cuts of STM images ofdots produced by bombardmeDt

with a Xe^"*" ion (lower curve) and a Xe*'*'*" ion (upper curve).

0123456789 basal width (nm)

Fig. 3. Histograms ofthe basal widths of ion induced dots obtained from STM

images.

Three models attempt to explain the response of surfaces to HCI impact. The first such model [29,30] asserts that the

surface becomes locally charged because of the massive

removal of electrons by the projectile. This charged volume

can 'Coulomb explode', ejecting ions from the surface and

initiating a shock wave which causes the sputtering of

neutral surface atoms. This process was simulated in a moleciilar dynamics computation .by Cheng and Gillaspy [31]. A second model is similar to the structural instability model

used to describe surface sputtering by intense laser pulses

[32]. In this model, the HCI impact produces a localized region of multiple electronic excitation. These excitations can cause the covalent bonds in the lattice to convert to antibonding potentials, leading to the expulsion of atoms

from the surface. This mechanism was suggested as the cause of the high sputter yields observed from gallium

arsenide under HCI impact [33]. A third model asserts that, in certain materials, HCIs can produce self-trapped excitons that decay into color centers leading to enhanced

sputtering of surface particles [34]. This model was used

to explain the observations on LiF, but appears to be rel- evant only to a limited class of materials such as alkali halides and Si02. In the present work, it seems likely that

the ion impact would leave both holes and excitations, so the Coulomb explosion model and the structural instability model should both be considered further. All of the models predict the removal of material from

the surface, and have been presumed to lead to the formation of craters. The data presented in this work show

the formation of protrusions. Similar protrusions, observed

Physica Scripta T92

previously in mica after HCI impact, have been interpreted

by Parilis as a result of potential energy deposition [35]. The model is an extension of the Coulomb explosion model

in which the structure of the surface plays an important

role. Mica has a layered structure and the layers are held

together by ionic bonds. As the ion approaches, electrons are removed from between the surface layers leaving a- net positive charge. The ionic bonds are thus destroyed,

leading to delamination of the layers. In extending the Coulomb explosion model to describe

the response of HOPG to HCI impact, one needs to account

for some differences between mica and HOPG. While both

mica and HOPG have highly anisotropic layered structures, the nature of the bonds between the layers is different: in mica, the layers are held together by ionic bonds while

in HOPG the layers are held together by much weaker van der Waals bonds. This might allow the delamination

to occur even more readily in HOPG than in mica, and

allow the Coulomb explosion to result in a protrusion as seen in the present work. A similar argument could be made

with respect to the structural instability model; that is, a

local expansion of the target due to stretching of bonds

from electronic excitation could result in a delamination

of the layers of HOPG. It remains to be determined if one of these mechanisms (or a combination of the two)

is responsible for the observed dots. We therefore consider

these models in more detail below. With respect to the Coulomb explosion model, it is well

established that the impact of an HCI can remove many

electrons from a surface. In measurements of the total elec- tron yields, it has been shown that a single HCI with an

initial charge, q, can extract a number of electrons into

the vacuum that is several times larger than q [36]. However, the Coulomb explosion can be quenched if the charged region is replenished with electrons before the tar- get nuclei have been displaced. HOPG is a semimetal

and its relatively high carrier density might make it susceptible to such a quenching. The time scale for the electron motion, as extracted from the plasma frequency, is less than one femtosecond. The highly anisotropic crystal structure, however, leads to a conductivity that is orders of magnitude

larger in the plane of the surface than normal to the surface

[37]. This anisotropy could affect the availability of elec- trons to neutrahze the impact site. Furthermore, it is not clear that the properties measured for the bulk material

are relevant on the time scales and length scales involved

here. For example, the presence of two holes in a single

atom of a solid can increase the hopping time of the holes by orders of magnitude [38]. This argimient assumes that the two holes represent a small perturbation to the lattice and can not be directly applied to the situation here where

tens or even hundreds of holes are created at the impact

site. It has been observed, however, that when sihcon is excited with high intensity picosecond laser pulses, carrier diffusion decreases with increasing fluence [39]. These

examples illustrate the point that the Coulomb explosion

model can not be ruled out simply on the basis of bulk

properties such as conductivity. In addition to the charge that the HCI impact creates on

the surface, there is likely to be a large number of electronic excitations that could lead to a structural instabiUty. It was shown recently that a single HCI impact on a siUcon surface © Physica Scripta 2001

]N[IST-302
In-situ Observation of Surface Modification Induced by Highly Charged Ion Bombardment 25

can deposit as much as 40% of its potential energy in elec- tronic excitations [40]. A critical issue that remains to be

addressed is whether or not these excitations are highly

localized. Stampfli predicted that structural instabilities occur in graphite when at least 7% of the valence electrons are excited, or at an excitation density of 32

excitations /nm''. This excitation density requires the full 51 keV of potential energy from the Xe"""*" ion to be

deposited in excitations (assuming that the average exci- tation energy is 5 eV) vidthin a hemisphere of radius 5.4 nm. Atomic displacement due to this instability can begin

in as Httle as 50 femtoseconds, much faster than the time

that it takes for the electrons and atoms to thermalize,

which is about half of a picosecond [41]. Note that 50 fs

is comparable to the response time indicated by the Coulomb explosion simulations [31]. Therefore, if the HCI's potential energy is converted to electronic excitations with sufficient efficiency, the surface could suffer a struc- tural instability that would leave the surface locally

deformed.

The conditions in the present work are right for both

Coulomb explosion and structural instability and both of

these processes can take place in the same ion impact event. Our experiment does not distinguish between these mechanisms because, at the level of precision that the models have achieved, both would result in the same type of surface damage. In order to determine the relative importance of the two mechanisms, one would need to do more extensive modeling and/or perform experiments

specifically designed to address this issue. Another component of the ion/ surface interaction that can afiFect the size of the dots is a charge-state dependence

in the stopping power. This can be understood in terms

of a fast ion entering a solid; the kinetic energy is mainly

deposited by ionizing atoms in the solid. The efficiency of this primary ionization is determined in a competition

between the equilibrium charge state of the ion inside the solid, which increases with speed, and the time that the ion interacts with the target electron, which decreases with increasing speed. In the case of slow HCI impact, both

the charge state and the impulse are large just as the ion

reaches the surface, maximizing the primary ionization

rate. Schenkel et al. studied this eifect by passing ions of various charge states through thin carbon foils and

measuring the (kinetic) energy loss; they found that the energy loss increased with increasing charge state for xenon and gold ions [42]. The energy loss of Xe^^ is several keV

less than that for Xe'**^; for Xe^^"^, the energy loss is likely to be still less. While this measured energy discrepancy

is small compared to the total energy, it is deposited in the first several nanometers so it contributes significantly to the local energy density, and it is likely to contribute

to the creation of surface features. According to the ion explosion spike model [43], which is used to explain tracks in solids from fast ions, the Coulomb

repulsion between ionized target ions causes nuclear dis- placement and there is a threshold for track formation with

respect to the primary ionization from a fast ion. This is similar to the Coulomb explosion model described above

and, if the surface does suffer a Coulomb explosion, the charging might be due to both neutralization of the

projectile and increased electronic stopping power. The

© Physica Scripta 2001

presently observed variation in feature size, therefore, might come, at least in part, from such a synergistic effect between the projectile's kinetic energy and charge state. In summary, we bombarded HOPG samples alternately

with slow xenon ions of the same kinetic energy but in two widely differing charge states. The observed

ion-induced dots vary in size with the charge state of

the ions. The damage formation is discussed in terms of

Coulomb explosion and structural instability. We expect

that both of these processes can be involved in a single

ion impact event and we suggest that their relative import- ance should be the subject of further investigation. We also expect that a charge state dependence in the stopping power

can contribute to the high energy density deposited at the surface and, thus, the damage observed. Our measurements

provide the first in-situ images of direct surface modification due to a highly charged ion beam.

Acknowledgements

We thank Doug AJderson for his valuable technical assistance and Dan Parks and Larry Hudson for many fruitful discussions. R. Minniti gratefully acknowledges a fellowship from the National Research Council.

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NIST-304

© Physica Scripta 2001
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