BL2B

Performance Test of Z-Stack MCP Mounted in a TOF Mass Spectrometer
C. Huang1, H. Katayanagi1,2, Md. S. I. Prodhan2, H. Yagi1, B. P. Kafle2, K. Mitsuke1,21Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
2
Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan
determined the relative ion counts of C60z+ (z=1-3)
and their concomitant fragment ions. The Ș issaturated when the bias voltage is set to above 2.95
kV.

The microchannel plates detector (MCP) is a kind ofuniversal and traditional tool which is used for
detection of photons and charged particles in a large
varietyofapplications
includingimaging
spectroscopy,
electron
spectroscopy,
mass
spectrometry, astronomy, molecular atomic collision
studies, cluster physics and so on. The detection
efficiency (Ș) of MCP is a very important parameterand is known to be a complicated function of the
kinetic and internal energies and the mass to charge
ratio (m/z) of impinging particles and the bias voltage.
As for the conventional two-stage MCP, the Ș value
decreases rapidly with the increase of m/z at
m/z >150.1 Compared with the two-stage MCP, the Șof z-stack MCP (three-stage MCP) is much easier tobe saturated.
Recently we have replaced the two-stage MCP in
our TOF mass spectrometer by a three-stage MCP
which includes three microchannel plates at theBL2B endstation of UVSOR. Some rare gases, i.e.
helium, neon and krypton are utilized to test theperformance of the new MCP. In the experiment, theoutput signal from the MCP was amplified with apreamplifier (EG&G, Ortec, 9301, gain=10) and an
amplifier (EG&G, Ortec, 9302, gain=20), thendiscriminated by a constant fraction discriminator(EG&G, Ortec, 474). After the discrimination, thesignal was processed by a time-to digital converter(FastComtec, 8867). Ion counts were normalized byphotonflux and sample pressure to evaluate therelative Ș value.
Here we show the results of Kr as an example. In
the experiment, the normalized signal counts ofsingly, doubly and triply charged ions of Kr wereattained using several MCP bias voltage at hȞ=90eV.
Fig 1 shows the dependence of normalized signal
counts of Kr on the discriminator level when the
MCP bias voltage is set to -2.7kV, -2.8kV, -2.9kV
and -3.0kV.
In this figure, the normalized signal counts ofindividual Krz+(z=1-3) ions are almost the same when
the discriminator level increases from 200 mV to 600
mV and the MCP bias voltage changed from -2.7kV
to -3.0kV. At a given discriminator level, the countrate of Krz+(z=1-3) ion signal do not depend on thebias voltage. The same results were found in thehelium and neon results. The ratios between multiply
charged ions and singly charged ions of these raregases were kept almost constant with the change ofthe discriminator level and bias voltage. Therefore,
we concluded that Ș of z-stack MCP is made tooperate in a saturate manner in the broad range of
discriminator level and MCP bias voltage. Finally weFig. 1. The dependence of normalized signal counts
on the discriminator level at different MCP bias
voltages.
[1] K. Mitsuke et al J. Phys. Chem. A 111 (2007)
8336.

61

BL2B

Development of a New Momentum Imaging Spectrometer with the Use of
Synchrotron Radiation Photoionization
C. Huang1, H. Katayanagi1,2, Md. S. I. Prodhan2, H. Yagi1, B. P. Kafle2, K. Mitsuke1,21Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
2
Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan

A novel momentum imaging spectrometer has been
constructed to investigate dissociation mechanism offullerenes molecules combining synchrotron radiation
photoionization in the VUV in BL2B of UVSOR. The
structure of the new apparatus is similar to theconventional velocity map imaging spectrometer
developed by Eppink and Parker [1].
The performance of the spectrometer has beentested by using the He, Ne, Ar, Kr, SF6 and CF4. The
best focusing condition was found to be
V ext  V tub
0 .7 .
V rep  V tub
Here Vext, Vrep and Vtub correspond to voltages whichapplied to TOF extractor plate, TOF repeller plate and
TOF tube respectively.
Figure 1 is the image of the Kr ions which wasmeasured at the best focusing condition at hȞ=35eV.
The inverse abel transform (IAT) [2] was applied toanalyze all the image data. In the data processing,
Welch window was used to suppress backgroundnoise. The velocity distribution of the projected ions
was extracted from the raw data by using IAT
method.
The Maxwell-Boltzman distribution of all of therare gas ions has been calculated at the room
temperature (300K). Comparing the velocity
distribution of rare gas ions with the calculated
Maxwell-Boltzman distribution, we found ourexperimental results agree with the calculated
distribution Fig.2 shows the velocity distribution of
Xez+(z=1-3).
Very recently, the first imaging data of the C60 ionshave been achieved using the imaging setup in BL2B
[3].

Fig. 2. The velocity distribution and theMaxwell-Boltzman distribution at 300K of
Xez+(z=1-3).
[1] A. T. J. B. Eppink, D.H. Paker, Rev. Sci. Instrum.
69 (1997) 3477.
[2] A. J. R. Heck, D. W. Chandler, Annu. Rev. Phys.
Chem., 46 (1995) 335.
[3] H. Katayagagi, et al, in preparation.

Fig. 1. The imaging of Kr ions which were measured
at the best focusing condition at hȞ=35eV.

62

BL2B

Evaluation of the Optical Oscillator Strength of C60 in the Valence Region

B. P. Kafle1, H. Katayanagi 1,2, S. I. Prodhan1, H. Yagi 2, C. Huang 2, K. Mitsuke 1,21
Graduate University for Advanced Studies, Okazaki 444-8585, Japan2
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585,
Japan
were given by Piacente et al. in 1995 [8]. By using
their vapor pressure data, we have reevaluated Vabs,Aof Refs. [5] and [6]. As a result the f values wererecomputed to be 178.5 and 230.5 for the hv ranges,
respectively, from 3.5 to 40.8 eV and from 3.5 to 119
eV. These values agree well with those expectedfrom the TKR sum rule and 60×V(C), namely 187.6
for the former range and 233.4 for the latter range.
Partial Cross section V(+) or V(2+) / Mb

4 00

4 .0

p re s e n t d a ta
V (+ )V (2 + )3 .5

V (3 + )

3 00

3 .0
2 .5

2 00

2 .0
1 .5

1 00

1 .0

Partial Cross section V(3+) / Mb

We have reevaluated the absolute partial crosssection Vabs(z+) for the formation of the ions in acharge state z from C60 [1], by applying essentiallythe same method as that employed in our previous
studies for estimating Vabs(z+) (z = 1 - 3) at hȞ = 25 to
120 eV ([2] and refs. therein). Specifically, theabsolute partial cross section V(z+) for the formationof C60z+ can be expressed asR(z)1
V abs ( z  ). (1))nL FW K abs ( z  )Here, R(z+) is the signal count rate of thephotoions in a charge state z produced from C60, ) is
the photon flux of synchrotron radiation, n is thenumber density of C60 in the ionization region, L is
the length of the ionization volume along the light
path, F is the repetition rate of the pulsed electricfield, W is the average residence time of the ions in theionization volume under the field free condition.
Open circles, triangles, and squares in Fig. 1 indicate,respectively, the absolute partial cross sections ı(+),ı(2+), and ı(3+) for the formation of C60+, C602+, andC603+ from C60. Each partial cross section includes thecontribution of not only the parent but also fragment
ions produced by the sequential loss of a C2 unit.
Hence, it is equal to the cross section involving all
the ionic species in a particular charge state.
In Fig. 2, open circles represent the sum of Vabs(+),
Vabs(2+), and Vabs(3+) which is nearly equal to thetotal photoionization cross section Vabs,I. In the hȞ
range from ~ 45 to 120 eV the present curve of Vabs,Iis in fair agreement with the Vabs,I reported by
Reinköster et al. [3] and absolute photoabsorption
cross section curve Vabs,A by Colavita et al. [4]. This
may prove the validity of our data analysis. Our Vabs,Iof 401 Mb at hȞ = 25.5 eV is a little smaller thanVabs,A determined by Jaensch and Kamke [5].
The oscillator strength f was calculated, by
combining the present Vabs,I curve with the Vabs,A data
given in Ref. [5] (hv = 11.425 eV) and Yasumatsu etal. (hv = 3.511.4 eV) [6]. The value integrated overthe hv range from 3.5 to 40.8 eV was 120.4 and thatfrom 3.5 to 119 eV was 156.0. These values are
significantly smaller than the corresponding oscillator
strengths of 187.6 and 233.4 expected from the TKR
sum rule and 60 times the total photoabsorption crosssection of a C atom, 60×V(C) [7]. The insufficiencyof f is entirely predictable because the Vabs,A data
points of Ref. [5] lie much lower than those compiled
by Berkowitz [7]. Many authors have argued that themost dependable equilibrium vapor pressures of C60

0 .5
0

30

40

50

60

70

80

90

10 0

0 .0
120

1 10

P h o to n E n e rg y / e V

Fig. 1. Partial cross sections for single, double andtriple photoionization of C60 determined from eq. (1).
1600
1400

V a b s ,I , P r e s e n t d a ta x 1 .5

1200

V a b s ,A , J a e n s c h a n d K a m k e x 1 .5
V a b s ,A , Y a s u m a ts u e t a l. x 0 .2 51000

6 0 x V (C ) , H e n k e e t a l.
V a b s ,A , C o m p ile d b y B e rk o w itz

800
600
400
200
0

10

20

30

40

50

60

70

80

90

100

110

120

P h o to n E n e rg y / e V

Fig. 2. Total photoionization and photoabsorption
cross sections of C60 in the hv range of 3.5–119 eV.
[1] B. P. Kafle et al., J. Phys. Soc. Jpn. 77 (2008)
014302.
[2] K. Mitsuke et al., J. Phys. Chem. A 111 (2007)
8336.
[3] A. Reinköster et al., J. Phys. B 37 (2004) 2135.
[4] P. Colavita et al., Phys. Chem. Chem. Phys. 3(2001) 4481.
[5] R. Jaensch, W. Kamke, Mol. Mater. 13 (2000)
143.
[6] H. Yasumatsu et al., J. Chem. Phys. 104 (1996)
899.
[7] J. Berkowitz, J. Chem. Phys. 111 (1999) 1446.
[8] V. Piacente et al., J. Phys. Chem. 99 (1995)
14052.

63

BL2B

Velocity Map Imaging of C60 Molecular Beams

H. Katayanagi1.2, C. Huang1, S. I. Prodhan2, H. Yagi1, B. P. Kafle2, K. Nakajima3,
K. Mitsuke1,2
1
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585,
Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan
3
Faculty of Letters, Hosei University, Chiyoda-ku, Tokyo 102-8160, Japan
The photoion images of fullerene (C60) molecularbeams were observed with the velocity map imaging
technique [1]. The temperature of the beam wasderived from the images and was consistent with thatof the fullerene oven.
The design of our imaging setup was described
elsewhere [2]; here we explain the experimental
procedure briefly. The C60 sample was loaded in acylindrical quartz cell and heated up by a heater
around 700-800 K in vacuum. The C60 vapor passed
through two apertures and reached the ionization
region, where the C60 molecular beam intersected themonochromatized synchrotron radiation. Ions
produced at the ionization region were extracted by avelocity map imaging electrode assembly andprojected on to a position sensitive detector (PSD) of
25 mm in diameter and 375 mm away from theionization region. No mass selection was made. We
thus obtained two-dimensional (2D) projections of
three-dimensional (3D) scattering distributions of theions on the PSD. Focusing conditions of thiselectrode assembly were determined in advance usingrare gases at room temperature as samples.
An image of the ions produced by photoionizationof C60 at the photon energy of 70 eV is shown in Fig.
1a. The synchrotron radiation passes parallel to theabscissa and the C60 beam to the ordinate of theimage. The image is parallel to the plane spanned by
the synchrotron radiation and the C60 beam. Spatial
distributions of C60+ directly reflect those of parent
C60 in the beam. An acute vertical intense part isclearly observed at the center of the image. This partcan be assigned to a projected shape of the C60 beam,
or a 2D projection of 3D velocity and angular
distribution of the beam, because the intensity of thispart changed with changing the sublimation rate of
C60 which can be controlled with the adjustment of
the oven temperature. Around the “base” of the acutecomponent, a subtle round component also exists. We
considered that this component originated fromionized residual gases and, if any, fullerene vapor
which stagnates around the ionization region. The
acute component shows cylindrical symmetry around
its major axis since the fullerene beam source
mechanically has cylindrical symmetry.
We tried to derive the velocity distribution, or thetemperature of the C60 beam from the present results.
We measured a background image experimentally
without fullerene evaporation and subtracted it fromthe image with fullerene evaporation. We regarded

the major axis of the acute component as acylindrically symmetric axis and applied the inverse
Abel transform with which we can calculate a 2D
section of the 3D distribution [3]. On the section,
distance from an origin of the image is directly
proportional to velocity. The origin and a coefficient
to convert the distance into velocity were obtained
from the ion images of Kr at room temperature. The
velocity distribution of C60 beam is shown in Fig. 1b.
Solid curve in Fig. 1b shows shifted
Maxwell-Boltzmann (MB) distribution of which
temperature was set to the same as that of thefullerene oven. The experimental distribution agreed
well with the MB distribution with a small shift of 15
m/s.
As a result of the present experiment, weconcluded that our imaging setup worked well evenwith fullerenes. We will observe photofragment ion
images from fullerenes in order to investigate thedynamics of dissociative photoionization of fullerenes.
In addition, an establishment of the present beamdiagnostic method of evaporating fullerenes willallow us to design a new type of beam monitor for
MBE or CVD experiments.

(a)㪠㫅㫋㪅㩷㩿㪸㫉㪹㪅㪀

㪉

㪈

(b)

㪇

㪇

㪈㪇㪇

㪉㪇㪇

㪊㪇㪇

㪭㪼㫃㫆㪺㫀㫋㫐㩷㪆㩷㫄㫊

㪋㪇㪇

㪌㪇㪇

㪄㪈

Fig. 1. (a) Ion image of C60 beam. (b) Velocity
distribution of C60 beam. Black curve shows velocity
distribution of C60 beam derived from (a), red curveshows MB distribution of 785 K with a shift of 15
m/s.
[1] A. T. J. B. Eppink, D. H. Parker, Rev Sci. Instrum.
68 (1997) 3477.
[2] B. P. Kafle et al. AIP Conf. Proc. 879 (2007)
1809.
[3] S. M. Candel et al. Comput. Phys. Commun. 23(1981) 343.

64

BL2B

Total Electron Yield Spectroscopy and Electron-Signal-Triggered TOF
Mass Spectrometry Developed for the Studies on Photodissociation
Dynamics of Fullerens

H. Katayanagi1.2, C. Huang1, S. I. Prodhan2, H. Yagi1, B. P. Kafle2, K. Nakajima3,
K. Mitsuke1,2
1
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585,
Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan
3
Faculty of Letters, Hosei University, Chiyoda-ku, Tokyo 102-8160, Japan
In order to detect all the photoelectrons fromfullerenes, we modified a threshold electron analyzer[1] which could be attached easily at the opposite sideof ion time-of-flight (TOF) and imaging
spectrometers [2]. The performance of the modified
electrodes as a total electron detector was inspectedby measuring total electron yield (TEY) spectra andelectron-signal-triggered ion TOF mass spectra of Kr.
We originally designed and produced the thresholdelectron analyzer to perform threshold electron
photoion coincidence (TEPICO) experiments in ordertoclarify
thedynamics
ofdissociative
photoionization of fullerenes (C60, C70, etc.) [1]. The
TEPICO experiments will allow us to determine theinternal energies of parent ions uniquely. Meanwhile,
we utilize the total electron detector for the following
purposes: (1) supplying start triggers in the imaging
experiments to select masses of ions imaged, (2)measuring the absolute photoionization cross sectionsof fullerenes from their TEY spectra. This method ismore advantageous from the total ion yield (TIY)
measurements, because the detection efficiency of a
microchannel plate detector depends on the
mass-to-charge ratios (m/z) of fullerene ions.
We have revised the former electron analyzer so as
to make it applicable to the total electron detector byenlarging the diameters of the holes on lenses andapplying higher voltage to the lenses and even to astray electron shield covering the electrodes.
We measured TEY spectra of Kr in the photon
energy range of 90-96 eV. The Kr sample wasintroduced to the chamber through a needle valve. A
typical pressure in the chamber was 2×10-6 Torr.
Figure 1 shows a typical TEY spectrum of Kr. This
spectrum agrees very well with the TIY spectra
measured at BL2B previously [3], except a small riseof the baseline because of stray electrons.
We also tried to measure TOF spectra of Krz+ ionsusing the electron signal as a start trigger. Figure 2
shows a thus obtained TOF spectrum of Krz+ at thephoton energy of 55 eV. Peaks of singly and doubly
charged Kr ions are clearly observed. The peaks havetails toward the longer flight time, which indicates afiner voltage adjustment is required. The peak area
ratio of (Kr2+)/(Kr+) is much larger than that we
expected according to Ref. 4. This can be explained
as a composite effect of the following reasons: doubly

charged ions emit two electrons and thus double theprobability of triggering, and the present electron
detector has higher efficiency for the slower electrons
produced by double ionization. The earlier peaks atflight times of 5.0 and 6.2 Ps are probably due toresidual water and nitrogen in the chamber. Although
N2 has almost the same m/z ratio as that of Kr3+, thepeak at 6.2 Ps can be assigned to N2+ since thethreshold for the triple ionization of Kr is 75 eV.
In summary, we concluded that this electron
detector can be used to observe electrons fromfullerenes of which number density is at most oneorder of magnitude smaller than those of rare gases,
although further tuning of the detector will be needed.

㪠㫅㫋㪅㩷㩿㪸㫉㪹㪅㪀

㪊

㪉

㪈

㪇
㪐㪇

㪐㪈

㪐㪉

㪐㪊

㪐㪋

㪧㪿㫆㫋㫆㫅㩷㪼㫅㪼㫉㪾㫐㩷㪆㩷㪼㪭㪐㪌㪐㪍

Fig. 1. Total electron yield spectra of Kr in the photon
energy range of 90-95.6 eV. Peaks can be assigned tothe 3d-15/2 and 3d-13/2 series.
㩷

㪎
2+

Kr

㪌

+

Kr

㩿N2+㪀

㪋

㩷

㪠㫅㫋㪅㩷㩿㪸㫉㪹㪅㪀㪍㪊㪉

㩿H2O+㪀

㪈
㪇

㪇

㪌

㪈㪇

㪫㪦㪝㩷㪆㩷P㫊㪈㪌㪉㪇

Fig. 2. Electron-signal-triggered TOF spectra of Krz+at the photon energy of 55 eV.
[1] H. Katayanagi et al., UVSOR Activity Report 33(2006) 46.
[2] B. P. Kafle et al., AIP Conf. Proc. 879 (2007)
1809.
[3] M. Ono et al., Nucl. Instrum. Methods Phys. Res.
A 467-468 (2001) 577.
[4] N. Saito, H. Suzuki, Int. J. Mass Spectrom. Ion
Proc. 115 (1992) 157.

65

BL2B

Momentum Imaging of the Photofragments from C60 and Data Analysis of
the Image
S. I. Prodhan1, H. Katayanagi1,2, B. P. Kafle1, C. Huang2, H. Yagi2, K. Mitsuke1,2
1
The Graduate University for Advanced Studies Okazaki 444-8585, Japan
2
The Institute for Molecular Science, Okazaki 444-8585, Japanthe initial C60 beam temperature by analyzing theimage data.
We are carrying out experiments with C60 sample
for checking the performance of the newly installed
imaging spectrometer. The C60 and backgroundimages recently measured are shown in Fig. 2. The
experimental conditions are given in the caption.

We have developed a time-of-flight (TOF) based
mass-selected momentum imaging spectrometer toobserve the momentum distributions of the scatteredfragments
producedfromdissociative
photoionization of gaseous C60 at the BL2B in the
UVSOR facility. We have adopted the Eppink-Parker
type three-element velocity focusing lens system [1](Repeller, Extractor, and Tube electrodes) to achieve
the high kinetic energy (KE) resolution on thephotofragment images. It is known that two possible
mechanisms are proposed for the fragmentation
processes [2,3], namely the sequential ejection of C2
units as C60+ĺ C58+ + C2, C58+ĺ C56+ + C2, and thefission of C60+ to directly produce a final fragment as
C60+ĺ C56+ + C4.
Several groups tried to distinguish the above two
mechanisms by measuring the total average KE
release in the decomposition of C60 into its fragments[4,5], though such efforts have met with failure
because the two mechanisms give almost the same
total average KE release. It is likely that expanding
C56+ ion clouds arising from different reaction
mechanisms give different shapes of image or
different momentum distributions of C56+. We
examined this statement by means of computer
simulation and designed the momentum imaging
apparatus [3] which is now installed in our
experimental end station of BL2B.
In the momentum imaging technique the 3D
momentum distributions of fullerene photofragment
ions are projected on the 2D position sensitive
detector (PSD). We performed the Monte Carlosimulation to generate the momentum images under
the following conditions: TOF, 50Ps; number of ionshitting at the surface of PSD, 8×105; temperature of
the C60 molecular beam, 0K and 273K. The KE
release in the C2 or C4 ejection takes a constant valueof 0.4 eV, to a first approximation. Angular
distribution is assumed to be isotropic and angles ofejection are selected randomly.
The simulated images of the fragment ion C56+
from C60 through the C2 loss and C4 loss mechanisms
are depicted in Fig. 1. Obviously the two mechanisms
are well distinguished at 0 K, but with increasing
temperature they are hardly distinguishable. The
experimental image of C56+ is a convolution of the
momentum distribution of the parent C60 in the beamand that of C56+ produced by the reaction. We canobserve the image of the parent C60+ (momentum
distribution of C60 in the beam), as well as that of
C56+. Thus, we are trying to deconvolute the effect of

Fig. 1. Simulated 2D projections of the 3D scattering
(a)

(b)

C2 loss

C4 loss(c)(d)

C2 loss

C4 loss+

distributions of C56 fragments (image size:
40×40mm). (a) and (c), sequential ejection of C2
units; (b) and (d), fission of C60+. The beam
temperature is 0K for (a) and (b) and 273K for (c)and (d).
(a)(b)

Fig. 2. (a) The experimental C60 image at beam
temperature, 505.2oC (b) Background image at room
temperature, 27oC. Photon energy,90eV; VMCP, 2.2KV; Vtube, -350V; Vrep, 500V; Vext, 250V; Vdeft ,25V; data acquisition time, 60s.
[1] A.T. J. B. Eppink, D. H. Parker, Rev. Sci. Instrum.
68 (1997) 3477.
[2] K. Mitsuke et al., AIP Conf. Proc. 811 (2006) 161.
[3] B. P. Kafle et al., AIP Conf. Proc. 879 (2007)
1809.
[4] H. Gaber et al., Z. Phys. D 24 (1992) 302.
[5] D. Muigg et al., J. Chem.Phys. 108 (1998) 963.

66

BL3U

X-Ray Absorption Spectra of Small Kr Clusters: Mass-Selected Ion Yields
with a TOF Spectrometer
M. Nagasaka, T. Horigome, T. Hatsui, N. Kosugi
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, JapanThe cluster is an intermediate phase between
isolated and condensed ones and has both properties
of surface and bulk dependent on its size. Inner-shell
spectroscopies such as X-ray absorption andphotoelectron (XAS, XPS) are promising methods toinvestigate local, or site-selected, electronic structures[1, 2]. XAS and XPS of large clusters (average size N> 1000) show dominant bulk and surface features ofclusters [1]. On the other hand, XPS of small clusters(N < 100) efficiently reveals several kinds of surface
sites [2]. The site dependence in XAS of small
clusters may be possible in neither total electron nor
ion yield, because the amount of clusters is relatively
low compared to monomers and dimers involved
inherently in the adiabatic gas expansion. A
component of small clusters should be extracted fromthe cluster/monomer/dimer mixture. In the present
work, we have developed a compact time-of-flight(TOF) mass spectrometer to measure Kr2+ yields
emitted from small clusters and then to obtain
mass-selected ion yield spectra, which correspond toselective or enhanced XAS for small Kr clusters.
Kr clusters were produced by the adiabatic gas
expansion. The gas was expanded through a 50 ȝm
nozzle into vacuum with the stagnation pressure of
0.5 MPa, where the average size of Kr clusters was
about 30. The Kr clusters were ionized or excitednear the Kr 3d5/2 and 3d3/2 edges by using
monochromatic soft x-rays from the high-resolution
undulator beamline BL3U. In the present work, to gethigher photon fluxes, photon energy band path was
set to ~40 meV. To get XAS of small Kr clusters, theKr2+ ions emitted from clusters after photoabsorption
were collected as a function of photon energy byusing a newly developed Wiley-McLaren type TOF
mass spectrometer [3]. Figure 1 shows a schematic
TOF mass spectrometer, where Kr2+ was triggered bypulsed voltage apparatus with the repetition ratio of
10 kHz, secondly accelerated, traveled in the drift
tube, and collected by the MCP detector. The Kr2+
ions were extracted from the total signals by using
delay-pulse
generator
andconstant-fraction
discriminator.
Figure 2 shows XAS of Kr clusters obtained in thepresent method. The spectra show contributions fromboth Kr atoms and Kr clusters. The peaks for Kr
atoms also include contributions from Kr dimers,
because the electronic state of dimers is close to thatof atoms [2]. In the region of the lowest excitation
3d5/2-15p, the surface peak in the small clusters isclearly observed with 0.17 eV blueshift from theFig. 1. Schematics of TOF mass spectrometer.

Fig. 2. Kr2+ photo-ion yield spectra of Kr~30 clusters,
where a and c mean the Kr atoms/dimers and Kr
clusters, respectively.
atomic/dimer peak. No bulk peak is observed. This
blueshift is larger than the blueshift (0.13 eV) in large
clusters (N ~ 4700) [1]. The cluster peaks in the
3d3/2-15p region are also blue-shifted by 0.19 eV. On
the other hand, the cluster peaks in the 3d5/2-16p and3d3/2-16p regions are red-shifted by 0.29 eV and 0.32
eV, respectively, whereas 0.45 eV redshifts arereported in the 3d-16p peaks of large clusters [1].
The 3d-15p cluster peak (blue-shifted) widths are
similar to the atomic/dimer ones, but the 3d-16p
cluster peaks are broad with shoulder features c1, c2,
and c3 at the lower-energy part (red-shifted). In order
to elucidate origin of these blue and red shifts, we are
analyzing the present results by combining XPS,
resonant XPS, and theoretical simulations.
[1] For example, M. Tchaplyguine et al., J. Chem.
Phys. 127 (2007) 124314.
[2] T. Hatsui et al., J. Chem. Phys. 123 (2005)
154304.
[3] W. C. Wiley, I. H. McLaren, Rev. Sci. Instrum. 26(1955) 1150.

67

BL3U

Resonant Auger Electron Spectra of Small Kr Clusters
near the Kr 3d Thresholds
M. Nagasaka, T. Hatsui, N. Kosugi
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
Although a 40.9 eV feature in Fig. 1(b) has not yet
been successfully assigned, the peaks of 37.8 eV and
40.0 eV observed in Fig. 1(a) may be assigned to(4p)4np 1D and 1S with n >> 0 around the double
ionization threshold. They are comparable to 37.0 and
39.3 eV double 4p ionization energies in the large
cluster surface [4]. These discrepancies correspond to
the difference in the stabilization energy of doubly 4p
ionized states 0.6 eV as above discussed.

We have measured x-ray absorption spectra (XAS)of small krypton clusters (average size N ~ 30) by
Kr2+ ion yields and successfully observed
site-dependent surface 3d-1np peaks [1]. In the present
work, we applied resonant Auger electron
spectroscopy (RAES) to the same small Kr clusters
on the photoexcitation energies of 3d-15p and 3d-16p
with distinct atom-surface energy shifts.
Kr clusters were produced by the adiabatic gas
expansion through a 50 Mm nozzle into vacuum withthe stagnation pressure of 0.5 MPa, where the average
size is estimated to be ~30, indicating only two or
three bulk atoms and mostly surface atoms [1,2]. Kr
clusters were excited by monochromatic soft x-rays
from the undulator beamline BL3U. The photon
energy band path was set to be 40 meV. RAES
spectra were measured by using an electron energy
analyzer, SES-200 combined with A-1 lens anddetection system, with the pass energy of 100 eV [2].
Figures 1(a) and 1(b) show RAES spectra of small
Kr clusters (with almost no bulk sites) by choosing
the photoexcitation energies of cluster surface
3d5/2-15p and 3d5/2-16p transitions. In the clusters,
difference between the 3d5/2-15p and 3d5/2-16p RAES
spectra is relatively small. This may be mainly
because the atomic 5p and 6p orbitals are strongly
distorted and mixed in clusters. We could not remove
atomic contributions in the cluster spectra.
The atomic RAES peaks are assigned according to
the work of Jauhiainen et al [3]. We observed
dominant np Rydberg series converging to 3 P, 1D and1
S (4p)4 states, where the branching ratio is 16:53:31.
This situation is also applicable to the cluster spectra;
as compared with the atomic peaks, the 4p45p cluster
peaks are red-shifted by 0.6 eV and the 4p46p cluster
peaks are red-shifted by 2.1 eV. The stabilizationenergy in singly 3d ionized states is 0.5 eV in thesame small clusters [1] and is 0.8 eV in the large
clusters N = 4700 [4]. Thus, the stabilization energyof doubly 4p ionized states in the small cluster is 2.6
eV (2.1+0.5) and is 0.6 eV smaller than thecorresponding atom-surface energy difference 3.2 eV
in the large Kr clusters [4].
Similarly to the Kr 3d excitation spectra [1], Fig. 1
shows that the higher np Rydberg states (n > 6) arealmost quenched in the (4p)4 manifold in the cluster.
Then, there may be n>>0 band-like features(“conduction band” [4]) around the single/double
ionization thresholds. Normal Auger electrons fromthe single core hole state to doubly ionized statescannot be observed below the ionization threshold;however, resonant Auger spectra near the thresholdsometimes show normal Auger-like features [4,5].

Fig. 1. RAES of Kr clusters (black line) following the(a) 3d5/2-15p and (b) 3d5/2-16p excitations in clusters[1], together with the atomic spectra (red line).
[1] M. Nagasaka et al., in this volume.
[2] T. Hatsui et al., J. Chem. Phys. 123 (2005)
154304.
[3] J. Jauhiainen et al., J. Phys. B 29 (1996) 3385.
[4] M. Tchaplyguine et al., J. Chem. Phys. 127(2007) 124314.
[5] A. Kivimäki et al., Phys. Rev. A 71 (2005)
033204.

68

BL4B

Formation of Metastable Carbonyl Sulfide (OCS) Dications
Following Core-Hole Creation

M. Ito1,2, K. Soejima2, T. Kaneyasu1, Y. Hikosaka1, E. Shigemasa1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Science and Technology, Niigata University, Niigata 950-21 Japan
1

While doubly charged molecular ions (or dications)
are inherently unstable owing to the Coulomb
repulsion between the nuclei, metastable molecular
dications having lifetimes of at least Ps order are
known to exist from mass spectroscopic studies. In an
inner-shell study of Carbonyl Sulfide (OCS), it has
been reported that Auger decays of the S 2p core-holestates favorably populate metastable OCS2+, incontrast to the case of the C 1s core-hole states [1]. In
this study, we have used an Auger-electron-ion
coincidence method to investigate formation
mechanism of the metastable dications following thecore-hole creations in OCS molecules.
The experiment was performed at BL4B, using theelectron-ion coincidence spectrometer [2]. The
electrons were analyzed in energy by a double
toroidal electron analyzer, while ions were extracted
from the interaction region into an ion momentum
spectrometer by a pulsed electric field according to
the detection of the electron. As an example, thetime-of-flight (TOF) spectrum obtained in this way isshown in Fig. 1. To obtain sufficient Auger-electron
counts, the coincidence data sets were recorded atthree different photon energies as indicated in Fig. 1.
The pass energy of the analyzer was set to Epass=100
eV for the S 2p ionization, while for the C and O 1s
ionization they were set to Epass=240 eV. The
resolving power of the energy analyzer was estimatedto be better than Epass /'E=100.
Figure 2 shows the coincidence and conventional
Auger spectra plotted in a binding energy scale. In
order to obtain the coincidence spectra, theelectron-ion coincidences are filtered by the time
window corresponding to the flight time of themetastable OCS2+ (~3.5 Ps). In this analysis, false
coincidence signals due to uncorrelated particles were
carefully subtracted from the raw data. The red curves
depict conventional spectra while the blue curves
represent the coincidence Auger spectra related to themetastable dications. The intensities are normalized
to the conventional spectra, and thus the spectral
intensities directly reflect the decay branching ratios.
The coincidence observation has revealed
metastable character of the Auger final doubly
charged states lying in the range 30 - 35 eV which are
attributed to the valence two-hole 3S-2 states [3]. The
metastable yields show dramatic increase in the S 2p
core-hole creation. This intense production isqualitatively explained by considering the spatial
distribution of the 3S orbital involved in the decay
processes. Since the 3Svalence orbital is mostly

localized around the S atom in the OCS molecule [4],the 3S electron preferentially fills the S 2p core-holedue to a large overlap between the core and valenceorbitals. Thus the Auger transition involving the 3S
electrons is favored in the decay of the S 2p core-holestates rather than in those of the C and O core-holestates.

Counts

3000

2+

OCS

2000
+

CO
1000

+

+

C
0
2000

S
+

O

2500

3000

3500

+

CS

+

OCS

4000

4500

Time-of-flight / ns

Fig. 1. TOF spectrum of ions measured incoincidence with sulfur LVV Auger electrons.
S LVV

1.0

hQ=174eV

Intensity / arb. units

0.5

0.0
1.0

C KVV
hQ=306eV

0.5

x10

0.0
1.0

O KVV
hQ=550eV

0.5

x10
0.0
25

30

35

40

45

Biniding eneryg / eV

Fig. 2. Conventional (red curves) and coincidence(blue curves) Auger spectra of OCS. The coincidence
spectra are extracted from the Auger-electron-ion
coincidence data sets by filtering the Auger electron
signals related to the formation of metastable OCS2+.
[1] R. Feng et al., Chem. Phys. 255 (2000) 353.
[2] T. Kaneyasu et al., J. Electron Spectrosc. Relat.
Phenom. 156-158 (2007) 279.
[3] T.X. Carroll et al., J. Electron Spectrosc. Relat.
Phenom. 51 (1990) 471.
[4] K.T. Leung, C.E. Brion, Chem. Phys. 96 (1985)
241.

69

BL4B

Stability and Dissociation of OCS2+ Formed via Auger Decaysof S 2p Core-Hole States in OCS

T. Kaneyasu1, M. Ito1,2, Y. Hikosaka1, E. Shigemasa1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Science and Technology, Niigata University, Niigata 950-21 Japan1

is found that the 3S-2 states are mostly metastable,
whose lifetimes should be much longer than thetime-of-flight of OCS2+ (~3.5 Ps) in the present
spectrometer. The majority of the Auger final statesaround 36 eV corresponding to the 9V-13S-1, 8V-13S-1
and 2S-13S-1 states dissociates into CO++S+. The onsetof this band structure exactly accords with theappearance potential (~34 eV) for the CO++S+
formation in valence double photoionization [4]. In
the binding energy region above 40 eV, metastable
OCS2+ can hardly be seen. The 9V-12S-1 states in 40 43 eV predominantly dissociate into CO++S+. The
high-lying states break apart into both the CO++S+and O+C++S+ fragments. The three-body dissociationarises only from the high-lying states.

Auger decay is the main relaxation process
following core ionization in molecules composed oflight atoms. In the decay process, a valence electron
fills the core-hole while another valence electron isejected. Doubly charged molecules with two holes inthe valence orbital, thus formed, often dissociate dueto the Coulomb repulsion between the nuclear
charges. The coincidence detection between theenergy-selected Auger electrons and fragments ions is
one of the most useful methods to gain a clear insightin the dissociation mechanism of the Auger final
doubly charged states. We have developed anAuger-electron-ion coincidence spectrometer inwhich a toroidal electron energy analyzer and an ion
momentum spectrometer are installed. In this report,
we present a coincidence study on stability anddissociation of the Auger final states following the S
2p ionization of OCS.
The experiment was carried out on the beamline
BL4B at the UVSOR facility. A detailed descriptionof the spectrometer and data acquisition scheme were
given in a previous paper [1]. An electron-ion
coincidence data set was obtained for OCS at a
photon energy of 174 eV corresponding to 2.15 eV
above the S 2p1/2 threshold. Pass energy of theelectron analyzer was set to Epass=100 eV and theresolving power was evaluated to be better than
Epass/'E = 100. From the coincidence data set, wehave extracted sulfur LVV Auger spectra for
individual dissociation channels.
Figure 1(a) shows a conventional sulfur LVV
Auger spectrum. Due to the spin-orbit splitting of theS 2p core-hole states (1.2 eV), the Auger spectrum iscomposed of two spectral components associatedwith individual core-hole states. The binding energy
scale is calculated by subtracting the kinetic energy ofAuger electrons from the ionization energy of the S
2p1/2-1 core-hole state. Thus the scale is correct for theAuger lines related to the S 2p1/2-1 state, but is shiftedby 1.2 eV for those from the S 2p3/2-1 state.
The spectrum shows band structures which are
attributed to valence two-hole doubly charged states[2,3]. The assignment of the structures are denoted on
the top of the spectrum. Auger electron spectra
filtered by coincidences with OCS2+, O++CS+ andO+C++S+ are shown in Fig. 1(b)-1(d). The verticalbars in Figs. 1(c) and 1(d) indicate thermochemical
thresholds for the ground states of the products. The
intensities of the coincidence Auger spectra arenormalized to that of the conventional spectrum, andthus the spectral intensity directly reflects the decaybranching rations of the Auger final two-hole states. It

-2

-1

3S

1.0

-1

-1

-1

-1

-1

8V 3S ,9V 3S ,2S 3S

0.5

-1

-1

9V 2S

(a) total

-2

-1

-1

9V ,8V 9V

0.02+(b) OCS

Intensity / arb. units

1.0

0.5

0.0
+

+

(c) CO +S

1.0

0.5

+

2 +

+ 4

CO (X 6 )+S ( S)

x5

0.0
+

0.5

0.0
26

+

(d) O+C +S

1.0

3

+ 2

x5

+ 4

O( P)+C ( P)+S ( S)

28

30

32

34

36

38

40

42

44

46

48

Binding energy / eV

Fig. 1. (a) Conventional sulfur LVV Auger spectrum.
Coincidence Auger spectra related to the formation of(b) metastable OCS2+, (c) CO++S+ and (d) O+C++S+.
[1] T. Kaneyasu et al., J. Electron Spectrosc. Relat.
Phenom. 156-158 (2007) 279.
[2] T. X. Carroll et al., J. Electron Spectrosc. Relat.
Phenom. 51 (1990) 471.
[3] D. Minneli et al., J. Chem. Phys. 107 (1990)
6070.
[4] T. Masuoka, H. Doi, Phys. Rev. A 47 (1993) 278.

70