Negative Ion Photoelectron Imaging Spectroscopy of
Iodine Monobromide and Copper Hydrides

byRyan M. D. Calvi

B.S., University of California Santa Barbara, 2004

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirements for the degree ofMasters of Science
Physical Chemistry
2007

ii

This thesis entitled:
Negative Ion Photoelectron Imaging Spectroscopy of
Iodine Monobromide and Copper Hydrides
Written by Ryan M. D. Calvi
has been approved for the Physical Chemistry program by___________________________
W. Carl Lineberger___________________________
Mathias J. Weber

Date: ______________

The final copy of this thesis has been examined by both the signatories, and we find that boththe content and the form meet acceptable presentation standards of scholarly work in theabove mentioned discipline.

iii
Calvi, Ryan M. D. (M.S. Physical Chemistry)
Negative Ion Photoelectron Imaging Spectroscopy of Iodine Monobromide and Copper
HydridesThesis directed by W. Carl Lineberger

This thesis contains the results of several negative ion photoelectron spectroscopy
experiments performed on two types of anions. The first class of anions studied consisted ofcopper containing complexes, including CuH−, CuD−, CuH2−, CuD2−, and CuCD3O−. The
photoelectron spectra of these molecules allowed us to measure (for the first time) the
electron-binding energies of the ground and excited states of the neutrals as well as theelectron affinities when possible. Franck-Condon simulations were performed on themolecules to determine the bond length change between the anion and neutral. A predicted
but until now unobserved, a 3Σ+ excited state of CuH and CuD is seen in the anion spectra;
the electron affinity is measured as 0.444(6) eV for CuH. The spectra of CuH2− and CuD2−
allow us to study the neutrals for the first time; two geometric isomers are present in the
spectra. The CuH2− photoelectron spectrum is dominated by a strong transition to the linear
excited 2Σg state and accompanied by an extended weak-unresolved vibrational progression
involving the bent ground 2B2 state. Ab initio calculations of the 0.255 eV energy differenceB

between these two states show EA(CuH2) to be 2.60(5) eV. Copper methoxy is only briefly
presented as a detailed analysis will follow in a subsequent publication; however, a measured
electron affinity of 1.077 eV is determined for this molecule from the photoelectron spectrapresented here.
The second type of molecule studied is the heterodiatom IBr. The photoelectron
spectra of the IBr anion are characterized by three distinct electronic states. An extended
vibrational progression is present for the ground state in the neutral, which lacked a distinct

iv
origin; it is caused by a shortening of the bond length between the anion and the neutral.
Several excited states are present in the spectra, providing confirmation to previous
experimental results. The IBr− photoelectron spectrum exhibits transitions to the ground (X1Σ) and lowest-excited triplet A 3Π1 and A 3Π2 states. Previous experimental results of theterm-splitting energy between the ground 1Σ and excited 3Π1 states of 1.525 eV give anelectron affinity of 2.51(1) eV. The vibrational progression observed in the excited statesmatches extremely well to previous results, confirming the correct identification of theelectronic states.

v
Acknowledgments

I would like to thank my parents Wendy and David Calvi who have supported my
educational goals since I started elementary school. They never looked down on my choices
and always allowed me to strive for the best. They have been a real motivating factor in mysuccess and achievements; they have sacrificed some of their ambitions to help me attainmine, and for that I must thank them for all they have done.
I would like to thank Professor Alec Wodtke at the University of California at Santa
Barbara who took me in as a young undergrad and got me excited about the world of
gas-phase physical chemistry. Without the opportunities I obtained working in his lab, I
would not have been able to advance as quickly as I have. I learned a lot, but it was only adrop in the bucket compared to what I ended up learning about experimental design andperforming experiments.
W. Carl Lineberger has been the greatest influence in my educational endeavors I
knew that Carl was the one I wanted to work for when I entered graduate school. He took me
in and let me run wild. It was in his lab that I really learned how to carefully design anexperiment and see it through from conception to completion.

Whenever there was a

problem in lab, he always had an idea for problem solving. Even if we thought we had tried
everything else, there was always something more that could be done. Even with his hectic
travel schedule, he always had time to sit down and figure out what was going on, even if I
had no idea. He has been supportive of my decision to leave graduate school to pursue myother educational goals, while still allowing me to work in his lab. I thank him for that.
I also thank Django Andrews, Jeff Rathbone, and Lenny Sheps. Without these three
individuals, none of this work would have been possible. They are the ones who really took
the time to allow me to learn the experiment and figure out all the intricacies that are usuallyvileft out. Having spent many of hours with me, they have become great friends. They knowwhat they are doing and will succeed in their future endeavors.
None of this work would be possible without support from the JILA instrument andelectronic shops. Tom Foote and Hans Green of the instrument shop have done so much for
our lab. They always provide ideas that are practical and inventive saving us from strategies
that otherwise would be very complex and prone to failure.

James Fung-A-Fat in theelectronics shop has been instrumental in the design of many of the circuits in our lab;
without his help, many things would not have been done as well as they were.

vii
Contents

Introduction to Negative Ion Photoelectron Spectroscopy.......................................................................1
1.1

Overview of Negative Ion Photoelectron Spectroscopy .........................................................1

1.2

Franck-Condon Profiles.........................................................................................................4

1.3

Photoelectron Angular Anisotropy Parameter.......................................................................5

1.4

Photoelectron Imaging ...........................................................................................................6

1.5

Image Reconstruction.............................................................................................................6

1.6

Thesis Overview .....................................................................................................................7

References for Chapter I ......................................................................................................................9
Experimental Setup ................................................................................................................................10
2.1

Velocity Map-Imaging Photoelectron Spectrometer Overview ............................................10

2.2

Velocity-Map Imaging Ion Sources ......................................................................................12

2.3

Ion Beam Formation and Time-of-Flight Mass Selection ....................................................13

2.4

Velocity Map-Imaging Photoelectron Detection ..................................................................17

2.5

Tunable Nanosecond Nd:YAG Laser System........................................................................21

2.6

Image Reconstruction...........................................................................................................22

2.7

Data Analysis .......................................................................................................................24

References for Chapter II...................................................................................................................26
Photoelectron Velocity-Map Imaging Spectroscopy of IBr− ..................................................................28
3.1

Introduction..........................................................................................................................28

3.2

Experiment ...........................................................................................................................32

3.3

Results ..................................................................................................................................36

3.4

Ground Electronic X 1Σ State of IBr ....................................................................................45

3.5

First Excited A 3Π2 State of IBr............................................................................................48

3.6

First Excited A 3Π1 State of IBr............................................................................................51

viii
3.7

Conclusion............................................................................................................................52

References for Chapter III..................................................................................................................54
Photoelectron Velocity Map Imaging Spectra of CuH− and CuH2− .......................................................57
4.1

Introduction..........................................................................................................................57

4.2

Experiment ...........................................................................................................................59

4.3

Results and Discussion.........................................................................................................62

4.3.1

CuH and CuD.......................................................................................................................62

4.3.2

CuH2 and CuD2 ....................................................................................................................70

4.4

Conclusion............................................................................................................................79

References for Chapter IV..................................................................................................................80
Photoelectron Velocity Map-Imaging Spectroscopy of CuCD3O−.........................................................84
5.1

Introduction..........................................................................................................................84

5.2

Brief Results .........................................................................................................................86

References for Chapter V ...................................................................................................................91
Bibliography...........................................................................................................................................92
Appendix A.

Laser Fundamentals...................................................................................................98

Figure A.1

Optical Components of the Infinity XPO Laser System .............................................99

Figure A.2

Energy Levels for a Nd:YAG System and a Four Level System...............................102

Figure A.3

Schematic for 1st Crystal in OPO Cavity ................................................................108

Figure A.4

Diagram of the Maser Oscillator Cavity .................................................................110

Figure A.5

Diagram for Nd:YAG rod assembly, including imaging of beam onto rods............113

References for Appendix A ...............................................................................................................115
Appendix B.

Photoelectron Simulations of CuH ..........................................................................116

Figure B.1

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.600 Å and a neutral bond length of 1.46263 Å. ....................................................117

ix
Figure B.2

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.597 Å and a neutral bond length of 1.46263 Å. ....................................................117

Figure B.3

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.594 Å and a neutral bond length of 1.46263 Å. ....................................................118

Figure B.4

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.591 Å and a neutral bond length of 1.46263 Å. ....................................................118

Figure B.5

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.585 Å and a neutral bond length of 1.46263 Å. ....................................................119

Figure B.6

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.588 Å and a neutral bond length of 1.46263 Å. ....................................................119

Figure B.7

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.582 Å and a neutral bond length of 1.46263 Å. ....................................................120

Figure B.8

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.582 Å and a neutral bond length of 1.46263 Å. ....................................................120

Figure B.9

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.576 Å and a neutral bond length of 1.46263 Å. ....................................................121

Figure B.10

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.573 Å and a neutral bond length of 1.46263 Å. ....................................................121

Figure B.11

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.570 Å and a neutral bond length of 1.46263 Å. ....................................................122

Figure B.12

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.567 Å and a neutral bond length of 1.46263 Å. ....................................................122

Figure B.13

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.564 Å and a neutral bond length of 1.46263 Å. ....................................................123

Figure B.14

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.561 Å and a neutral bond length of 1.46263 Å. ....................................................123

Figure B.15

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.558 Å and a neutral bond length of 1.46263 Å. ....................................................124

Figure B.16

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.555 Å and a neutral bond length of 1.46263 Å. ....................................................124

Figure B.17

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.552 Å and a neutral bond length of 1.46263 Å. ....................................................125

Figure B.18

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.549 Å and a neutral bond length of 1.46263 Å. ....................................................125

Figure B.19

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.546 Å and a neutral bond length of 1.46263 Å. ....................................................126

x
Figure B.20

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.543 Å and a neutral bond length of 1.46263 Å. ....................................................126

Figure B.21

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.540 Å and a neutral bond length of 1.46263 Å. ....................................................127

Figure B.22

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.537 Å and a neutral bond length of 1.46263 Å. ....................................................127

Figure B.23

Simulated spectra of the CuH neutral ground state with an anion bond length of
1.534 Å and a neutral bond length of 1.46263 Å. ....................................................128

Appendix C.

Photoelectron Spectra of Copper Hydrides..............................................................129

Figure C.1

Photoelectron spectra of CuD− recorded at 355 nm................................................129

Figure C.2

Photoelectron spectra of CuD− recorded at 600 nm................................................130

Figure C.3

Photoelectron spectra of CuH2− recorded at 355 nm. .............................................131

xi

List of Figures and Tables
Figure 1.1

Schematic diagram depicting processes and energies present in photoelectron
spectra.................................................................................................................. 3

Figure 2.1

Photoelectron velocity map imaging spectrometer............................................ 11

Figure 2.2

Sputter source utilized in the anion copper studies, a Stainless Steel (SS) rod isheld at ground potential, while the copper (Cu) rod is held at −3 kV................ 11

Figure 2.3

Electronic scheme of the MCP-box, acting as a voltage divider to the MCPs,
and the input from the collection anode. ........................................................... 16

Figure 2.4

Velocity map imaging setup (top of image), with the repeller, ground, andextraction plates, field-free flight tube, and detection system. A typical raw
image is displayed on the bottom of the image, with the reconstructed image
obtained with the BASEX reconstruction.......................................................... 19

Figure 3.1

Schematic diagramming of the potential energy curve of possible
photodetachment states arising from IBr−. Electronic states are labeledaccording to Hund’s case (c) notation. See text for details. ............................. 30

Figure 3.2

Time-of-flight mass spectrum obtained from a pulsed source, peaks are labeledaccordingly. The sample contains IBr and CO2; expansion conditions werechosen to favor the formation of the bare IBr−. ................................................. 33

Figure 3.3

Photoelectron spectra of I− recorded at 300 nm. The two lowest states of I
neutral are observed; both states are utilized in the calibration of thephotoelectron spectra of IBr− and are labeled accordingly. The 2P3/2 peak occurs
at a electron-binding energy of 3.059 eV, with the 2P1/2 peak occurring at anelectron-binding energy of 4.001 eV. ................................................................ 35

xii
Figure 3.4

Photoelectron image of I2− recorded at 266 nm. Three of the six states areobserved in our spectra, allowing for a comparison of binding energies. ......... 37

Figure 3.5

Photoelectron spectra of IBr− recorded at 300 nm. Data are obtained from
integration of the BASEX-reconstructed image in Figure 3.6........................... 38

Figure 3.6(Left) Raw IBr− photoelectron image showing all photoelectron energies from 0
to 1.33 eV. This image was recorded at a wavelength of 300 nm. (Right)
BASEX-reconstructed image. Three electronic states are present, and the image
has an energy span of 2.66 eV. .......................................................................... 40

Figure 3.7(Left) Raw IBr− photoelectron image showing all photoelectron energies from 0
to 0.21 eV. The image was recorded at a wavelength of 300 nm. Electron
optics were adjusted to magnify the excited 3Π1,2 states. (Right) BASEXreconstructed image. The energy scale of the image spans 0.42 eV................. 41

Figure 3.8

Calculated potential energy curves of the ground and seven lowest-excitedpotentials of IBr including the spin-orbit interactions. Energy is relative to theground state minimum. Symbols represent the calculated points..................... 43

Figure 3.9

Potential energy curves of the ground and five lowest-excited potentials of theIBr- molecule. .................................................................................................... 44

Figure 3.10 High-binding energy portion of the 300 nm photoelectron spectrum of IBr−,
showing magnification of the two excited 3Π1,2 states. Data are obtained from
integration of the BASEX-reconstructed image of Figure 3.7. ......................... 47
Table 3.1

Measured values for the IBr anion, neutral, and excited states. ....................... 49

xiii
Figure 3.11

Simulated photoelectron spectra of the excited triplet A states of the IBr.
The red spectrum represents the continuous simulation for the 3Π1 state, withthe blue lines representing the continuous simulation for the 3Π2 state. The
simulation was performed with an anion temperature of 100 K and an anion
bond length of 3.025(.01) Å. ......................................................................... 50

Figure 4.1

Time-of-flight mass spectrum obtained from a pulsed source. Mass peaks arelabeled accordingly. Sputtered copper with methanol-d4. ................................ 60

Figure 4.2

355 nm Cu− photoelectron spectra. Transitions to the three lowest-lying
electronic states of Cu are shown and labeled for reference.............................. 61

Figure 4.3

Photoelectron image of CuH− recorded at 355 nm. The raw image is on the left
side, and the BASEX-reconstructed image is on the right side. ........................ 63

Figure 4.4

Photoelectron spectra of CuH− recorded at 355 nm. They are obtained from theintegration of the BASEX image in Figure 4.3. The Cu 2D3/2 state is displayed
underneath the 3Σ state of CuH. The spectrum has been cut off at 2 eV to focus
the attention on the excited states. ..................................................................... 64

Figure 4.5

Photoelectron image of CuH− recorded at 600 nm. The raw image is on the left
side, and the BASEX-reconstructed image is on the right side. ........................ 66

Figure 4.6

Photoelectron spectrum of CuH− recorded at 600 nm, obtained from integrationof the BASEX image in Figure 4.5.................................................................... 67

Table 4.1

Recommended spectroscopic constants for 63CuH and 63CuD. Values marked
with an asterisk are obtained from spectra reported in this chapter................... 69

Figure 4.7

Photoelectron image of CuD2− recorded at 355 nm. The raw image is on the left
side, and the BASEX reconstructed image is on the right side. ........................ 72

xiv
Figure 4.8

Photoelectron spectrum of 65CuD2− at 355 nm. The intense peak is assigned as
the transition A 2Σg in CuD2, with a symmetric stretch of 1369(40) cm-1. An
extended vibrational progression is present at higher-binding energies and isassigned as the X 2B2 ground state..................................................................... 73

Table 4.2

Recommended spectroscopic constants for 65CuD2 and 65CuH2. Values marked
with an asterisk are obtained from spectra reported in this chapter................... 75

Figure 4.9

Comparison of the photoelectron spectrum of CuD2 with the simulated
spectrum. As a consequence of the large geometry difference between thelinear anion and bent ground state, there is no detectable photoelectron signal at
EA(CuD2), as indicated by a vertical arrow. The simulation does not consider
the major effects of the bent→linear transition and must be considered as being
very qualitative. ................................................................................................. 77

Figure 5.1

Time-of-flight mass spectrum obtained from a pulsed source. Peaks are labeledaccordingly. ....................................................................................................... 85

Figure 5.2

The raw CuCD3O− photoelectron image is displayed on the left hand side, andthe BASEX reconstructed image is presented on the right hand side. The image
was recorded with a wavelength of 355 nm. ..................................................... 87

Figure 5.3

Photoelectron spectra of CuCD3O− recorded at a wavelength of 355 nm.......... 88

Figure 5.4

The raw CuCD3O− photoelectron image is displayed on the left hand side, andthe BASEX reconstructed image is presented on the right hand side. The image
was recorded with a wavelength of 500 nm. ..................................................... 89

Figure 5.5

Photoelectron spectra of CuCD3O− recorded at a wavelength of 500 nm. ........ 90

Introduction to Negative Ion Photoelectron Spectroscopy

1.1

Overview of Negative Ion Photoelectron Spectroscopy

Negative ion photoelectron spectroscopy (PES) has allowed for the study of thestructures and energetics of anion and neutral molecules.1 Important measurements obtained
from negative ion photoelectron spectroscopy are the electron binding energies, more
specifically the electron affinity of the neutral molecule.2 Using the measured electron
affinity allows for the determination of bond strengths in the neutral and anion molecule.
Vibrational frequencies of neutral molecules can be extracted from vibrational progressions
present in the spectra, while the vibrational frequencies of the anion can be determined fromvibrational hot bands, if present. The electronic-term splitting energies of different electronic
states in the neutral molecule such as spin-orbit splittings and bond lengths of both the anion
and neutral can be measured. An advantage of negative ion PES over traditional
spectroscopic methods is in the study of reactive/metastable neutral species formed via thephotodetachment of the precursor anion. These species cannot otherwise be directly
prepared. In photoelectron spectroscopy, the spin of the molecule must change when theelectron is photodetached, ΔS=1/2, which is not the case in optical spectroscopy. Thischange in spin allows access to a greater number of electronic states. The neutral is formedby photodetachment of the anion at the vertical detachment geometry on the neutral surface.
Negative ion photoelectron spectroscopy can be simply described by the process:AB− +hν → AB + e−(eKE),(1.1)

where AB− is the molecular anion, hν is the photon energy utilized in the photodetachment,
AB is the photodetached neutral, and e−(eKE) is the photodetached electron, which receives
all of the recoil kinetic energy. Which knowledge of the energy of the molecular anion and a2well-defined narrow photon source, measurement of the kinetic energy of the photodetachedelectron will give a direct energy separation between the initial electronic states of the anion
and the final electronic states of the neutral. The term reported by the measured electron
kinetic energy is the electronic-binding energy (eBE). The relationship between the electron
kinetic energy and binding energy is given by

eBE = hν − eKE,

(1.2)

where the electron binding energy (eBE) is found by subtracting the electron kinetic energy(eKE) from the photon energy. The electron-binding energy is reported to allow a directcomparison of photoelectron spectra independent of photon energy. Transitions to excitedstates of the neutral molecule are accomplished by changing the photon energy to meet or
exceed the term-splitting energy. This process is diagramed in Figure 1.1. The process canbe conceptualized as; a photon comes into the molecular anion and excites it to a continuum
level, i.e. the photon energy. Subsequently, the molecule relaxes into a given electronic stateand vibrational level. The excess energy between the continuum level and that of the relaxedstate is transferred to the photodetached electron.

3← Electron binding energy (eBE)Electron kinetic energy (eKE) →

hν
A AB

EA (AB)X AB

Figure 1.1

T0

X AB

-

ElectronAffinity

Schematic diagram depicting processes and energies present in photoelectron
spectra.

4

In Figure 1.1, the anion state is represented by AB−. The absorption of the incoming
photon is the upward arrow to the continuum level, labeled hν. The kinetic energy given to
the ejected electron is proportional to the length of the arrows between the continuum level
and the level the neutral molecule resides on. The longer the arrow, the more kinetic energy
is imparted to the ejected electron. The adiabatic electron affinity of the neutral molecule isthe energy measured directly between the two ground vibrational states of the anion andneutral. If the photon energy is sufficient to access higher electronic states of the neutral, thesplitting of the states can be measured (represented as T0 in Figure 1.1). The photoelectron
spectrum on the left side of Figure 1.1 is a schematic diagramming the spacing between thevibrational levels. It is what a “typical” spectrum collected looks like.

1.2

Franck-Condon Profiles
The Franck-Condon principle constrains the geometries of the anion and the neutral

to be equal during photodetachment. For anions and neutrals with similar equilibriumgeometries, the electron affinity can be measured as a transition between the lowest
vibrational energy levels of the ground state of the two species. When the equilibriumgeometries are significantly different, this transition is very weak and usually unobserved in
the spectra. In this case, the vertical detachment energy (VDE) is then reported as thetransition from the ground state of the anion to a vibrationally excited state in the neutral withthe same geometry as the anion. The quantum mechanical version of the Franck-Condon
principle is based on the fact that the intensity of a given transition is proportional to thesquare of the integral overlap between the two states:1,3

Meν =|<Ψ ' | Ψ '' >|2 .

(1.3)

5
The Franck-Condon factor determines how the intensity is distributed among the vibrational
bands. The intensity of the vibrational bands of an electronic transition is determined by the
population of the vibrational levels, the strength of a transition, and the Franck-Condon
factors. Here Ψ’ is the vibrational wave function of the final neutral state, and Ψ’’ is thevibrational wave function of the initial anion state. Governed by symmetry, only transitions
to and from totally symmetric vibrational levels or even quanta of nonsymmetric modes areallowed.
1.3

Photoelectron Angular Anisotropy Parameter
Photoelectron angular distributions are also obtained from photoelectron spectra andused to determine the anisotropy parameter, β. The anisotropy parameter reflects thesymmetry of the electronic orbital from which the electron is photodetached. β is rigorous
for atomic systems, but is less exact for molecular systems. The β parameter is given byI(ν ,θ )=(1)f (ν )(1 + β P2 (cosθ )) ,
4π

(1.4)

where θ is the angle between the ε vector of the photolysis laser radiation and the direction ofphotofragment recoil, f(ν) is the speed distribution of the detected photofragment, β is theanisotropy parameter, and P2(cos θ) =3cos2 θ − 1
is a second-order Legendre polynomial.4
2

β ranges between −1 and +2 depending on the prompt dissociations following, respectively, apurely parallel, ΔΩ =0, or a purely perpendicular ΔΩ=±1, excitation of the parent diatomic
molecule. Electrons photodetached from s orbtials in the case of atomic systems and Σ
orbitals in a molecular system have positive β values. Electrons detached from p or d atomic
orbitals and Π or Δ molecular orbitals have a negative β value.4 We use the β parameter offeatures in the photoelectron spectra to assign transitions to different electronic states of theneutral.

6
1.4

Photoelectron Imaging
Imaging techniques for molecular spectroscopy were first conceived by Houston andChandler for photo fragments formed from the photodissociation of neutral molecules.5 Such
images simultaneously provide full three-dimensional velocity distributions of all
photodetached species imaged. From these, the individual velocities (kinetic energies) andangular distributions can be obtained. The photoelectron images are used to reconstruct thephotoelectron energy spectra. In addition, they contain the photoelectron angular
distributions, which are now observable in the images and necessary for obtaining the βvalues. Both the electron kinetic energies and angular distributions are obtained
simultaneously, allowing for an enormous amount of information to be continuously
collected.
Imaging resolution increased dramatically with the development ofvelocity-map imaging by Parker and Eppink.6 Prior to the development, images were often
blurred because of the extended, nonpoint source of the anions from which photodetachment
occurs. Image quality is dependent on how three-dimensional photoelectron distributions are
mapped onto a two-dimensional detector. A three-element open-electrode configuration was
found to minimize imaging distortions by acting as a lens. Parker and Eppink determinedthat when appropriate voltages are applied to the respective electrodes, particles with thesame initial velocity vector are mapped onto the same point of the detector, regardless of theirinitial position upon photodetachment. This method gives rise to the term velocity-map
imaging. Utilizing a velocity-map imaging configuration allows for high quality images to beacquired, even with the presence of an extended ion source.
1.5

Image Reconstruction
The image acquired is not a full representation of the three-dimensional electron

distribution necessary to obtain the photoelectron spectra and angular distributions. To

7reconstruct the three-dimensional distribution from the two-dimensional projection on thedetector, cylindrical symmetry must exist for the reconstruction of the image by use of theAbel integral. This cylindrical symmetry requirement requires that the photodetachment laser
polarization be parallel to the plane of the detector. The Abel transformation can lead tosignificant artifacts in the reconstruction. However, a new method, was developed by Reislerand co-workers, known as the basis set expansion (BASEX) method.7 The BASEX method
is able to produce higher quality reconstructions while avoiding the mistakes of the Abeltransformation. This method expands the two-dimensional image projection in a basis set ofwell-behaved Gaussian-like functions. The full three-dimensional distribution is thenreconstructed as a linear combination of these well-behaved functions. The BASEX software
takes a two-dimensional slice through the three-dimensional distribution reconstruction fromwhich the photoelectron spectrum as a function of camera pixels is produced, and the valuesof β as a function of pixels are calculated.
1.6

Thesis Overview
This thesis contains the photoelectron spectra of two different types of molecularspecies collected with a pulsed-ion source that utilizes a velocity map-imaging spectrometer
to measure the photodetached-electron kinetic energy. The experimental apparatus and laser
system, along with the theory explaining the experiment are described in Chapter 2.
The first system presented here addresses the solvation dynamics ofIBr(CO2)n, n=0-16 and includes the photoelectron spectra of the bare IBr molecule. An
updated measurement of the electron affinity is reported. The results of this study arepresented in Chapter 3.
The second system presented addresses copper monohydride (CuH−) and copper
dihydride (CuH2−). The electronic structures and vibrational frequencies of the molecules arepresented and compared to high-level theoretical calculations. The electronic affinities of8these molecules are reported here. A low-lying triplet state in the copper monohydride isobserved for the first time is reported in Chapter 4.
The last system presented in this thesis addresses the copper methoxy anion(CuCH3O−). The photoelectron spectrum is briefly mentioned. The binding energies andvibrational frequencies are listed in Chapter 5.

9References for Chapter I

1Advances in Gas Phase Ion Chemistry, edited by N. G. Adams and L. M. Babcock(JAI Press Inc., 1992), Vol. 1.

2

S. J. Blanksby and G. B. Ellison, Accounts of Chemical Research 36 (4), 255 (2003).

3

P. F. Bernath, Spectra of atoms and molecules. (Oxford University Press, New York,
1995).

4

R. N. Zare, Angular Momentum. (Wiley, New York, 1988).

5

Imaging in Molecular Dynamics Technology and Applications, edited by B. Whitaker(Cambridge University Press, 2003).

6

A. Eppink and D. H. Parker, Review of Scientific Instruments 68 (9), 3477 (1997).

7

V. Dribinski, A. Ossadtchi, V. A. Mandelshtam, and H. Reisler, Review of Scientific
Instruments 73 (7), 2634 (2002).

Experimental Setup

2.1

Velocity Map-Imaging Photoelectron Spectrometer Overview
The photoelectron velocity-map imaging apparatus is diagrammed in Figure 2.1,

although it has been described in some detail in previous publications,1-3 an overview will bepresented here. All experiments were performed in a Wiley-McLaren-type time-of-flight
photoelectron-imaging spectrometer4 and a velocity map-imaging photoelectron detection
region.5,6 The apparatus consists of three regions within our chamber. The first region
contains the source of the negative ions, the second contains the ion-optics and a drift tube formass separation, and the third consists of an interaction and detection region, where thevelocity map imaging setup is housed. The detection region is isolated from the first two
sections by a pneumatically operated gate valve. The two anion sources utilized in theseexperiments are described in Section 2.2. The Wiley-McLaren time-of-flight mass
spectrometer is described in Section 2.3 and the velocity map-imaging detector in Section
2.4. A brief overview of the laser system utilized in the experiments is given in Section 2.5,
and the reconstruction of the collected images and data analysis are presented in Sections 2.6
and 2.7, respectively.

11

ΔE/E ≤ 0.01

General Valve

Product
Ions

Electronimaging

Potential Switch

Neutrals

Ion Optics
Electron
Gun

Figure 2.1

210 nm - 2000 nm
3 ns pulse width

Reflectron

Photoelectron velocity map imaging spectrometer

Ar + H2O

SS

Figure 2.2

Cu
D1

Sputter source utilized in the anion copper studies, a Stainless Steel (SS) rod
is held at ground potential, while the copper (Cu) rod is held at −3 kV.

12

2.2

Velocity-Map Imaging Ion Sources
The purpose of the source chamber is to create the negative ions under investigation.

The formation of negative ions consists of mixing a backing gas seeded with trace amounts ofour molecule or precursor of interest. Typical backing pressures range from 5 to 45 psi. The
gas mixture is pulsed into the source chamber using a General Valve (Series 9) with a nozzle
diameter of 800 μm. The General Valve is driven by a single-channel Iota One pulse drive
made by Parker and Hannifin. Typical base pressures in the source chamber prior to pulsing
are 4 x 10-7 torr; typical pressures while pulsing the General Valve are 2 x 10-5 torr.
Formation of the negative ions utilized in the case of the copper complex studies
includes a dc discharge sputtering-ions source, (Figure 2.2) or, in the case of the IBr studies, ahomemade electron gun.
After the dc discharge sputtering-ion source is fitted to the General Valve, theemerging molecular beam initiates a discharge across a gap of 1.0 mm between a copper
cathode at −3 kV and a stainless steel pin, held at ground potential. This discharge producesCu− anions and small clusters. The sputtered-plasma produces significant quantities ofcopper anions, and molecular anions are formed during the expansion of the plasma. The
anions produced in the plasma are cooled and stabilized inside a cone-shaped expansion
channel and then allowed to expand into the source chamber.
The second of our two ion sources is a homemade electron gun, which is positioned
in the chamber to be collinear with the molecular jet pulse from the General Valve. The
electron gun consists of a thoriated-tungsten filament, which is focused utilizing an Einzel
lens and steered using a set of vertical and horizontal deflectors. Electrons are emitted byresistively heating the thoriated tungsten filament by passing 5.5 amps through the filament.
The ejected electrons are repelled by a plate biased at −100 V relative to the filament. The
filament and anode power supplies are floated at 1 kV. The Einzel lens is composed of three13
annuli with the first and third held at ground potential and the middle annulus biased between
900 and 1100 V. The electron beam is steered via a set of vertical and horizontal deflectors.
Different pairs of deflectors have a typical potential difference of 0—30 V. The electron
beam is measured daily with a Faraday cup, yielding an average reading of 320 μA. The
electron beam is focused into the General Valve with a 5 mm thick samarium-cobalt magnet
annulus mounted to the front of the pulsed-valve face plate.
Producing IBr− is done by attachment of slow secondary electrons. The slow
secondary electrons are produced by interaction of an electron beam with a CO2 molecule
acting as moderator. The first mechanism is rather straightforward and is responsible for theformation of dihalide anion clusters. A particular anion, IBr− for example, is formed byattachment of secondary electrons to the neutral dihalide species found in the expansion.
Cluster growth then occurs as the solvent molecules (CO2 in this case) condense around the
newly formed charge centers by the following mechanism:7
IBr + secondary e− + M → IBr− + M
IBr− + CO2 + M → IBr(CO2)− + M
IBr(CO2)− + CO2 + M → IBr(CO2)2− + M
IBr(CO2)n− + CO2 + M → IBr(CO2)n+1− + M
This mechanism shows that cluster formation depends on the presence of neutral CO2
molecules along with third-body collision partners. As the density of the expansion
increases, a shift toward larger cluster formation is favored. The resulting anions formed in
the molecular jet drift approximately 15 cm in the source chamber before being repelledperpendicularly into the acceleration flight tube.
2.3

Ion Beam Formation and Time-of-Flight Mass Selection
The molecular anions produced from the expansion are repelled perpendicularly

into a time-of-flight mass spectrometer by a pulsed-repeller plate. The repeller plate obtains a

14negative voltage pulse generated from a home-built switch and triggered by a TTL pulse,which, in turn, is generated by a Stanford Research Systems Delay Generator with a dc input
voltage varied between 400–700 volts. The input voltage is varied to help obtain the best
possible signal on the inline detection multichannel plates (MCP) further down the chamber.
The ions are extracted into an acceleration stack consisting of 10 metal annuli
separated by 0.43 cm. Each annulus is 7.62 cm in diameter with a 2.54 cm aperture for theions to pass through. A voltage of 2.3 kV is connected to the last annulus on the acceleration
stack and resistively connected to ground with 1 MΩ resisters across the 10 annuli, which
creates a field that accelerates the ions to a kinetic energy of 2.8 keV. The acceleration stackmarks the beginning of the Wiley-McLaren time-of-flight mass spectrometer.4 Upon exiting
the acceleration stack, the ions encounter a set of horizontal and vertical deflector plates, eachwith a potential difference between 0–60 V above the float voltage of 2.3 kV. The deflectors
are followed by an Einzel lens, which is composed of three elements. Each element is 3.81
cm long with a 2.54 cm aperture for the ions to pass through. The first and third elements areheld at the float voltage of 2.3 kV, with the middle element operating in a deceleration mode
at a static 800–1000 V above ground.
Upon exiting the Einzel lens, the ions enter a 70 cm long potential-switch drift tube,
electrically isolated from the ion options and chamber. When the repeller plate is triggered,
the potential of the drift tube is switched from ground to the float voltage using a Directed
Energy Inc. PVX-4140 pulse switch. When the anion of interest is in the drift tube, thepotential is rapidly switched to ground. The potential switch allows for a defined mass rangeof ~200 amu to make it through the drift tube without being deflected upon exiting. The
entrance and end of the potential switch are fitted with an annulus with a 2.54 cm hole tolimit the field penetration into the potential switch and deflect the ions. Upon exiting the drift
tube, the ions are steered through a 1.5 degree bend introduced in the flight tube to separateout any neutral particles formed from collisions in the drift tube. After the 1.5 degree bend, a15
deflector is introduced to steer the ions down the remaining portion of the flight tube; theneutrals collide with the side of the chamber.
After the 1.5 degree deflector, the ions encounter a mass gate composed of three setsof parallel horizontal plates. The mass gate is arranged in a configuration where the first andthird set of plates are held at ground, while one of the middle pair of plates is held at a highpotential (600 V) and pulsed down to ground when the anion of interest is between the plates.
It is pulsed back to the high potential once the anion of interest has passed. After exiting themass gate the ion enters a differentially pump chamber containing the photoelectron velocity
map-imaging setup, a reflectron (not used), and a horizontal deflector before it impinges on
the front face of a microchannel plate detector. The inline MCP detector consists of two
impedance-matched plates housed in a chevron configuration (Hamamatsu Corp. F4612-09
and Burle 32733). Electrons emitted from the microchannel plates are collected by an anodeplate biased at 200 V above the back face of the MCPs. A diagram of the voltage separator
for the MCP’s and anode plate is diagrammed in Figure 2.3.
The anode signal is amplified by a Stanford Research Systems (SR 240A) 350 MHz
fast preamplifier, increasing the gain of the signal by a factor of 10. This signal is monitored
on a 200 MHz oscilloscope (Tektronix 2445B). This signal is monitored as a time-of-flightmass spectrum by recording the anion arrival time in reference to the pulse of the repeller
plate.

16

to preamp
input

Figure 2.3

Electronic scheme of the MCP-box, acting as a voltage divider to the MCPs,
and the input from the collection anode.

17

2.4

Velocity Map-Imaging Photoelectron Detection
The photoelectron-imaging spectrometer, displayed in Figure 2.4, is based on thevelocity map-imaging technique developed by Eppink and Parker.5 Our spectrometer is a
modified version of the spectrometer employed by Sanov and co-workers.6 The imaging
setup is composed of a three-element copper lens, a copper flight tube, a MCP stack housed
in a chevron configuration, a P47 phosphor screen (Burle) with a μ-metal cup covering thethree copper elements, and 4 rolled sheets of μ-metal, forming a sleeve inside of the copper
flight tube. The three copper electrodes are arranged vertically in a stack separated from one
another by 2.54 cm.
The first lens consists of a negative repelling voltage, the second element is at ground
potential, and the third element is a positive extraction voltage. All lenses are 7.62 cm indiameter, and 0.762 mm thick, with 2.54 cm apertures on the ground and extraction plateallowing the electrons to fly through; the repelling plate has no apertures. The electrodes arecontained in a μ-metal (Magnetic Shielding Corp.) enclosure with a 2.54 cm aperture for theentrance and exit laser beam and entrance for the ion beam. A slit 1.25 cm wide and 3.6 cm
long on the exit path allows ions to be directed towards the reflectron (not used in theseexperiments). An aperture 1.25 cm in diameter on the bottom of the enclosure allows for
efficient pumping of the imaging region. Typical voltages on the repeller plate range
between −15 and −200 V at the time of photodetachment. The middle plate is held at ground
potential, and the third plate is held at a positive voltage between 45 and 600 V, helping to
extract the electrons into the flight tube. The extraction plate is in physical contact with a
15-cm-long—7-cm-diameter—oxygen-free high-conductivity—copper-field free-flight tube.
The flight tube is mounted on an 8” conflate flange.
The inner surface of the flight tube is lined with four layers of a 0.5 mm thick μmetal sheet rolled to line the flight tube. At the exit of the flight tube is a 40 mm active

18
diameter-imaging dual-impedance-matched chevron-configured microchannel plate detector
and a P47 (Y2SiO5:Ce3+) type phosphor screen coupled by a fiber optic bundle to a chamberwindow (Burle Electro-Optics, Inc.). This window is imaged by a 1280 x 1074 pixel CCD
camera (LaVision 3Ls, 6.7 μm x 6.7 μm pixel size, Peltier cooled). This camera is interfacedwith a desktop PC, which is utilized in data collection.

19

MCP and phosphor
-15 V

0V

63 V
Field free flight tube

CCD
CameraElectronsE

Ion flight path

BASEX Reconstruction

2-D photoelectron
image

Figure 2.4

2-D Slice
Through 3-D
Reconstruction

Velocity map imaging setup (top of image), with the repeller, ground, andextraction plates, field-free flight tube, and detection system. A typical raw
image is displayed on the bottom of the image, with the reconstructed image
obtained with the BASEX reconstruction.

20

Photodetachment of the electron from the molecule occurs approximately half way
between the repeller and ground plate in the electron-imaging assembly. The repelling plateis pulsed to a negative voltage by an externally controlled switched-power supply (Directed
Energy Inc. PVM-4210) ~ 10 μs prior to the arrival of the laser pulse. The voltage applied to
the repeller must be optimized for each laser wavelength used for photodetachment becauseof minute changes in the position of the laser pulse. These changes in position affect thefocusing of the electrodes on the photodetached electrons. The voltages are optimized bymonitoring the full width at half maximum of a well-known electronic transition in thephotoelectron spectra of an atomic anion. In the studies conducted here, the 2S ← 1S
transition in copper was monitored in the cases utilizing copper. In the case of IBr−, theP3/2 ← 1S transition in I− was monitored.

2

The microchannel plates in the imaging setup are arranged to help eliminate
background noise. The front of the imaging detector is held at the flight tube voltage, while
the back of the detector is pulsed from 1700 to 2300 V above the frond face by an externallyswitched high-voltage pulse generator (Directed Energy Inc. PVM-4140). The voltages are
externally provided. The switching between 1700 and 2300 V produces a ringing on the front
face of the microchannel plate. The ringing causes a transient in the electron lens that leads
to the image size changing with the delay introduced in the switching time. To alleviate thisimage size discrepancy, the back of the microchannel place is switched high 1 μs before thearrival of the photodetachment laser pulse. By this time, the ringing has subsided, allowing
the background signals to continue to be reduced. The phosphor screen is held 3000 V above
the high voltage of the microchannel plate. The electrons emitted from the MCPs impinge
upon the phosphor screen, fluoresceing at 400 nm. Images of the phosphor screen are
acquired by a CCD camera, which is interfaced to a PC and operates in a digital event-21
counting mode. The CCD is exposed between 50 – 500 ms. At this point an event recordedwith a intensity profile greater than our background threshold is interpreted as aphotoelectron. A typical data set contains 5000–100,000 images, which are continuously
summed together to create the final image. The exposure time for each image is set to record
the greatest number of events without stacking events on top of each other. Otherwise, theevent counting software would count the two events as a single event, thereby reducing thedata collected.
We have a resolution detection limit of approximately 2% of the electron kinetic
energy. We determine this limit by measuring the full width at half maximum of theelectron’s kinetic energy divided by a single electron’s kinetic energy (ΔeKE/eKE). For
electron kinetic energies below 200 meV, electron-binding energies can be determined withan accuracy of ±0.005 eV or better. To date, the best resolution obtained has been thedetection of a Cu 2S peak where the ΔeKE/eKE was measured at 1%. The energy resolution
is limited because the ion propagation is perpendicular to electron extraction. Ion
propagation introduces a Doppler shift in the ion velocity. This shift in velocity introduces anirregular anisotropy profile in the image, stretching out the image and making it more
elliptical as oppose to circular. This problem has been observed when using relatively low
mass atoms and low repeller voltages for electron extraction. It can be corrected by utilizing
higher mass atoms and molecules and higher repeller voltages for photodetachment.
2.5

Tunable Nanosecond Nd:YAG Laser System
The laser system utilized in our experiments has a fundamental laser radiation of1064 nm with a variable repetition rate of 0.1–100 Hz and a characteristic pulse duration of 3
ns. The pulse is generated from a Coherent Infinity laser system, described previously.8 The
1064 nm pulse is created in a diode-pumped Nd:YAG prism in a ring resonator. The initial1064 nm pulse is amplified in a dual-pass—flash-lamp-pumped Nd:YAG rods. The

22
amplified 1064 nm laser light has a maximum pulse energy of 600 mJ/pulse. The
fundamental laser pulses undergo frequency doubling in an angle-tuned beta barium borate(BBO) crystal to produce 532 nm pulses with 50% efficiency. These 532 nm pulses are
mixed with the residual 1064 nm pulses in another angle tuned BBO crystal to produce thethird harmonic of the 1064 nm pulse, yielding 355 nm pulses with 50% efficiency. The
resulting laser pulses consist of the remaining 1064 and 532 nm pulses plus the newly formed
355 nm laser pulses, which are separated by use of the appropriate mirrors. The 1064 nm and532 nm pulses are steered into a beam dump, while the 355 nm pulses are either utilized forphotodetachment or used to pump an optical parametric oscillator (OPO). The 355 nm pulse
produces signal and idler pulses in a BBO crystal by optical parametric generation.
Combined signal and idler beams can be scanned between 410 and 2000 nm, with thewavelength tuning accomplished through the changing of the angle of the BBO crystal that
produced the beams. Selection of the signal or idler beam is done optically. The typicalsetup uses a signal beam, which is passed through a second angle-tuned BBO crystal
undergoing frequency doubling. At 710 nm, the OPO produces both signal and idler beams;
this is the “degeneracy” of the OPO. As the BBO crystal angle is tuned away from thedegeneracy, the line width decreases, until it increases dramatically when degeneracy isreached. A more detailed look at the laser system is presented in Appendix A.
2.6

Image Reconstruction
The image recorded with the CCD camera is interfaced to a desktop PC running the

DaVis imaging software by LaVision. Because the center of the image is offset in the frameof the camera, the center of the recorded image must be found. This is accomplished bymasking a circle over the image until the recorded image is completely masked by the circle.
This is all done by eye and has proved to be the most accurate and reliable method of finding
the center.8 After the initial center is found, the pixels surrounding the center are also utilized23
as a center until the best fit is found. The image is then imported into the BASEX program,developed by Dribinski and Reisler to allow a reconstruction of the full 3D distribution.9 A
slice through the center of the 3D distribution from the center of the image outward is
outputted as a radial integration of the 2D slice. The output represents a pixel distributionthat is indicative of electron velocity. In addition to producing a velocity plot of thephotoelectron energy spectra, the image contains the angular distribution described by the
angular anisotropy (β) parameter.10
Calibration of the speed distribution is done in a two step manner. The relationshipbetween the pixels and the electron kinetic energy (eKE) is given by the relationshipeKE ∝ c * P2 ,(2.1)

where c is the proportionality constant used for the calibration, and P is the distance in pixels
from the center of the image outward. Since the electrons are traveling at a given velocity,
the further out from the center of the image they are, the faster their velocity. The relationshipbetween pixels and velocity is

P ∝ v.

(2.2)

The proportionality constant between the velocity and eKE are lumped together and found byplotting the electron kinetic energy vs. P2 for a known atomic transition measured in thephotoelectron spectrometer just prior to calibration. A linear function is fitted to the plot,
with the slope of the plot being equal to the proportionality constant. Once the constant isfound, the plot can be converted from a pixel scale to an electron kinetic energy scale. The
scale is then converted to electron-binding energy (eBE) by subtraction of the electron kinetic
energy from the photon energy, i.e.,eBE = hν - eKE ,(2.3)

Plotting the spectra in terms of eBE allows for a direct comparison between several spectra
independent of the photodetachment wavelength.

24
2.7

Data Analysis
The analysis of the photoelectron spectra is complemented with theoretical studies,

which aid in the spectral assignments. More often than not, features that were not expected
are present in the photoelectron spectra; the theoretical calculations aid in the determinationof these features. The primary method utilized in the study of these systems calculates theground state geometry and vibrational frequency of the anion, neutral, and any excited state
present using the Gaussian suite of programs.11 Most information is obtained through energy
calculations, that aid in estimating electron-binding energies.
To aid in the determination of the spectral assignments, the Gaussian output files areconverted for input into the FCFGaus03 program by running a FCF Gaussian job. The FCF
Gaussian job is used to extract the geometries and vibrational frequencies from the Gaussian
output file, where the new FCF output files are imported into the FCFGaus03 program.
FCFGaus03 uses the normal coordinates and frequencies to calculate the Dushinsky rotation
matrix J and the displacement vector K. The Dushinsky rotation matrix and displacement
vector are transformed into the Sharp and Rosenstock arrays A, B, C, D, and E. These new
outputs are matrices that are used to calculate a spectral simulation in the PESCAL
program.12
The PESCAL program is utilized to calculate a simulated spectrum from the initialgeometry of the anion and neutral species. The parameters of the anion and neutral
geometries, and frequencies can by manually inputted or they can be imported from theDushinsky rotation matrix previously calculated using the FCFGaus03 program. Several of
the parameters such as anion temperature, the intensity of the origin, and eKE of the origin
are adjustable in the PESCAL program. These adjustments are used to create a representative
spectrum of the collected photoelectron spectrum. The software also allows for the electron
kinetic energy vs. intensity data to be imported for direct comparison to the simulated spectra.

25
For further optimization of the simulated spectra, a statistical weighting on a Chi-square limit
can be run against the simulated spectra and imported data to produce a cohesive match
between the data and simulation. Once a match between the two spectra is accomplished, thesimulation is exported in two formats, a stick spectrum where the intensity and positions of
the transitions occur, and a convolution of the data. A comparison of the simulated spectra to
the photoelectron spectra allows for assignments to be made, that are independent of theindividual states present in the spectra; thus each electronic state can be independently
simulated and compared.

26

References for Chapter II

1

M. E. Nadal, P. D. Kleiber, and W. C. Lineberger, The Journal of Chemical Physics
105 (2), 504 (1996).

2

G. J. Rathbone, T. Sanford, D. Andrews, and W. C. Lineberger, Chemical Physics
Letters 401 (4-6), 570 (2005).

3

A. Sanov, T. Sanford, S. Nandi, and W. C. Lineberger, The Journal of Chemical
Physics 111 (2), 664 (1999).

4

W. C. Wiley and I. H. McLaren, Review of Scientific Instruments 26 (12), 1150
(1955).

5

A. T. J. B. Eppink and D. H. Parker, Review of Scientific Instruments 68 (9), 3477
(1997).

6

E. Surber, R. Mabbs, and A. Sanov, Journal of Physical Chemistry A 107 (40), 8215
(2003).

7

T. Sanford, S. Y. Han, M. A. Thompson, R. Parson, and W. C. Lineberger, Journal ofChemical Physics 122 (5), 054307 (2005).

8

D. H. Andrews, Ph.D. Thesis, University of Colorado, 2006.

9

V. Dribinski, A. Ossadtchi, V. A. Mandelshtam, and H. Reisler, Review of Scientific
Instruments 73 (7), 2634 (2002).

10

R. N. Zare, Angular Momentum. (Wiley, New York, 1988).

27

11

M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, J. A. Montgomery, Jr. , T. Vreven, K. N. Kudin, J. C. Burant, J. M.
Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegama, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J.
Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M.
Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J.
A. Pople, Gaussian 03, Revision B.05 (Gaussian, Inc., Pittsburgh, PA, 2003).

12

K. M. Ervin, J. Ho, and W. C. Lineberger, Journal of Chemical Physics 91 (10), 5974
(1989).

Photoelectron Velocity-Map Imaging Spectroscopy of IBr−
3.1

Introduction

Much of the chemistry that governs chemical reactions takes place in solution. A
solvent has many important effects on reactivity, including acting as a medium for energy
exchange. In effect, the solvent alters the potential energy surface of the reacting moleculesby raising or lowering the barriers to the reaction. Studying the effects of solvent molecules
on chemical reactions at the molecular level in solution provides many challenges, including
the lack of a well-defined solvent environment. However, molecular ionic clusters can offera picture of a solvent molecule in a well-defined microenvironment that provides
characteristics of the condensed phase moiety. Because we can vary the cluster size, thestudy of these gas phase clusters enhances our ability to investigate properties that bridge thegas-phase to condensed phase in a controlled environment.
Much work using photoelectron spectroscopy has focused on the effects of CO2
solvation in X −2 (CO2)n clusters, where n is the cluster size and X is one of the halogens (F,Cl, Br, or I).1-3 This work led, in turn, to investigations of mixed dihalide anions (i.e., ICl−,IBr−).4-8 IBr− and other halogen molecules are useful in spectroscopic studies, because they
absorb visible and ultraviolet light quite easily. Because these molecules exhibit a large
number of low-lying excited states due to spin-orbit splitting, interesting studies haveemerged, especially on IBr.9-15 Our ability to study these systems with negative ion
photoelectron spectroscopy allows us to measure potential energy curves and investigate
molecular photodissociation dynamics. Previous experiments in the Lineberger group16-19
investigated the photodissociation dynamics of the negative ion cluster IBr−(CO2)n (n=0–14).
In these experiments, a long-lived excited state was found for the IBr−(CO2)8 cluster. Its

29
recombination time was two orders of magnitude longer than previously predicted byrecombination studies on I −2 (CO2)8. This very slow recombination was hypothesized to becaused by the trapping of the cluster anion in a shallow well formed on the otherwise
repulsive A’ state potential-energy surface as solvent molecules were added. An investigation
was subsequently proposed to characterize the solvent-induced well through a pump-probe
photoelectron-imaging experiment.2,4,20,21 Before this experiment could be done, however, asingle photon photoelectron-imaging experiment had to be conducted on the system to map
the binding energies of the various electronic states of the solvated IBr−(CO2)n molecule.
Because the binding energy of a specific electronic state at a given cluster size will notchange, identification of the binding energies for these states prior to the pump-probe studies,
will allow for identification of the final electronic states in future studies. A schematic ispresented in Figure 3.1 outlining the electronic states to be accessed in these studies, theground X 1Σ state, and the two excited A 3Π1 and A 3Π2 states.

30

A 3Π1 IBr (2431)
A 3Π2 IBr (2431)
T0

T0
1.525 eV1.410 eV T0
1

X Σ IBr (2440)
EA (AB)
X 2Σ IBr (2441)

Figure 3.1

Electron Affinity

Schematic diagramming of the potential energy curve of possible
photodetachment states arising from IBr−. Electronic states are labeledaccording to Hund’s case (c) notation. See text for details.

31

Identification of the electronic states in the photoelectron spectra of IBr is aided byusing the previously measured photoelectron spectra of I −2 by Neumark et al.22 Changing themolecule from a homo-diatomic molecule (I2), to a hetero-diatomic molecule (IBr), should
yield a photoelectron spectra that contains the same type of electronic state features.
Adapting the features of the I −2 spectra to IBr− should result in a shift in the electronic states
toward higher-binding energies, even with a reduction in symmetry. This shift in binding
energy is caused by the change in electron affinity between I and Br at 3.059 and 3.363 eV,
respectively.23,24
The single photon studies of solvated IBr− clusters will be geared toward accurately
measuring the binding energy of the first excited A 3Π1 and 3Π2 states of IBr using aphotodetachment wavelength of 300 nm. A 300 nm photon has an energy of 4.132 eV, whichis enough energy to access the excited A states in I2. Even with a shift toward higher-binding
energies in IBr, the photon energy used will allow the ground X 1Σ and excited A 3Π1 and 3Π2states of IBr to be accessible. These relevant electronic states are presented in a schematic in
Figure 3.1. The electronic states are labeled according to the electron configuration fromwhich they arise, in conjunction with Hund’s case (c) notation. The …(σg)m(πu)p(πg*)q(σu*)nnotation for the electron configuration is abbreviated as mpqn.25
In this chapter, we report the use of negative-ion photoelectron spectroscopy todetermine a much improved value for the electron affinity of IBr and the bond dissociationenergy of the anion. The rest of this chapter has six sections: (3.2) a brief experimental
section, (3.3) an overview of the results, (3.4) a detailed analysis of the IBr ground state, (3.5)
and (3.6) analyses of the excited A 3Π2 and A 3Π1 states, respectively, and (3.7) conclusion.

32

3.2

Experiment
IBr anions are formed by secondary electron attachment, as described in Chapter 2.

Photoelectron spectroscopy of IBr− is carried out with a velocity map-imaging photoelectron
spectrometer that has also been described previously.26 IBr− clusters are formed in a pulsed
supersonic expansion containing IBr and CO2. The IBr anions are produced in this expansion
by low-energy electron attachment to IBr by the following mechanism:18
IBr + secondary e− + M → IBr− + M.
A sample holder was constructed to house IBr (Aldrich, 224847, Iodine monobromide) withthe greatest amount of surface area of IBr exposed. CO2 flows over the IBr at a pressure of
0–30 psi, while the IBr sample is gently heated to 30 ºC. This configuration relies on thevapor pressure of IBr to seed the carrier gas. The line connected from the IBr sample holder
to the General Valve is made of a polytetrafluoroethylene (PTFE), “Teflon,” tube. The
amount of metal coming into contact with the IBr sample has been minimized, because IBrreacts with the metal, causing corrosion to occur and producing undesirable by-products. The
CO2 gas mixture is passed through a General Valve (series 9) with an 800 μm orifice
expanding into our source chamber where it is crossed with an electron gun, producing thenegative ions. The anions are then repelled perpendicularly into a Wiley-McLaren27 typetime-of-flight mass spectrometer. Typical anion signals of 800 mV are detected from themicrochannel plates (MCP) combined with a preamplifier. A typical time-of-flight mass
spectrum is shown in Figure 3.2.

33

127

I

Intensity (arb)

IBr anion

IBr(CO2)2
81
79
Br

Br

IBr2

81Br(CO2)(CO2)
679Br(CO2)

IBr(CO2)

I(CO2)(CO2)4

75

100

125

150

175

(CO2)5

200

225

250

275

300

Mass (m/z)

Figure 3.2

Time-of-flight mass spectrum obtained from a pulsed source, peaks arelabeled accordingly. The sample contains IBr and CO2; expansion conditions
were chosen to favor the formation of the bare IBr−.

34

Photoelectron spectra are recorded at an ultraviolet photodetachment wavelength of
300 nm using the doubled light of the optical parametric oscillator (OPO) pumped by the
Infinity laser system. Two photoelectron images of IBr− were recorded at 300 nm, with each
image focusing on a different part of the spectra. In each image, the optics are optimized tofocus and maximize the size of the photoelectron image. This optimization is done to collectthe greatest amount of photodetached electrons from IBr−. It is done using a reference ion, I−.
The well-known electron affinity of the I atom observed in the photoelectron spectrum of theI− 2P3/2 ← 1S transition is used to focus the photoelectron-imaging optics and calibrate theenergy scale.24 A 300 nm photoelectron spectrum of I− is presented in Figure 3.3. For
electron kinetic energies below 200 meV, electron-binding energies can be determined withan accuracy of ±0.005 eV or better.26 The ground (X 1Σ) and excited (A 3Π1,2) states of IBr
are collected in the first image (Figure 3.8). The photoelectron-imaging optics were then
optimized to magnify the excited (A 3Π1,2) states in the second photoelectron image (Figure3.9). An image is magnified by decreasing the repeller and extraction voltages. This reduces
the potential felt on the photodetached electrons, allowing them to expand more by the timethey reach the detector. All images are recorded in event-counting mode with 250 ms chargecoupled device (CCD) exposures at very moderate laser pulse energies of less than 100 μJ at
300 nm.
The pulse energy is chosen to reduce the background signal as much as possible
while still allowing the ground and excited triplet states to be accessible with the desired
photon energy.

35

Photoelectron counts

2

4.1

4.0

P1/2

3.9

3.8

3.7

3.6

3.5

3.4

3.3

2

P3/2

3.2

3.1

3.0

Electron binding energy (eV)Figure 3.3

Photoelectron spectra of I− recorded at 300 nm. The two lowest states of I
neutral are observed; both states are utilized in the calibration of thephotoelectron spectra of IBr− and are labeled accordingly. The 2P3/2 peakoccurs at a electron-binding energy of 3.059 eV, with the 2P1/2 peak occurring
at an electron-binding energy of 4.001 eV.24,28

36

3.3

Results
A photoelectron spectrum of IBr− was recorded with a photodetachment wavelengthof 300 nm and compared to that of I −2 . Identification of the electronic states in IBr wereaided by using the previously measured photoelectron spectra of I −2 of Neumark et al.22 As
can be seen in the photoelectron spectra for I −2 (Figure 3.4), seven electronic states areobserved. With the electronic states of IBr occuring at higher-binding energies than I2 thetrend in the electronic states can easily be followed. The I −2 photoelectron spectrum lacks avibrational progression in the excited states. In contrast, a vibrational progression is observed
for both the ground and excited states in the photoelectron spectra for IBr.
The photoelectron spectra of IBr− is shown in Figure 3.5. Three electronic states areobserved. A 300 nm photon has an energy of 4.132 eV, which is slightly higher than the A
states observed in the I2− photoelectron spectra, giving an indication of the electronic statesthat will be observed in the IBr spectra. The IBr spectra were generated by integrating thephotoelectron image (Figure 3.6) from the center out, over all angles.

37

100

A

Photoelectron Counts

80

A'
B
60

40

X
B''

20

B'

0
4.4

4.2

4.0

3.8

3.6

3.4

3.2

3.0

Electron binding energy (eV)Figure 3.4

Photoelectron image of I2− recorded at 266 nm. Three of the six states areobserved in our spectra, allowing for a comparison of binding energies.22

Photoelectron counts

38

A 3Π1

X 1Σ

3

A Π2

EA(IBr)

4.0

3.8

3.6

3.4

3.2

3.0

2.8

2.6

2.4

Electron binding energy (eV)Figure 3.5

Photoelectron spectra of IBr− recorded at 300 nm. Data are obtained from
integration of the BASEX-reconstructed image in Figure 3.6.

39

The raw photoelectron images and BASEX29-reconstructed images of IBr− arerecorded at 300 nm and presented in Figures 3.6 and 3.7. The images in Figure 3.6 containthe raw and reconstructed image of IBr− and show the ground X 1Σ and first-excited triplet A
3

Π1 and 3Π2 states. The raw and reconstructed images display an energy span of 2.66 eVfrom one side of the image to the other. The rings found in the image correspond to the threeobserved electronic states of IBr. A vibrational progression is observed in the image for eachof the three electronic states. Very little detachment occurs at lower electron velocities(center of image), indicating that we are not on the threshold of an electronic state. A distinct
ring occurring at higher electron velocities (outer edge of image) corresponds to the groundstate of the neutral.
The second set of raw and reconstructed images (Figure 3.7) magnifies the firstexcited A 3Π1 and 3Π2 states of IBr. The images display an energy span of 0.42 eV from one
side of the image to the other A vibrational progression is clearly resolved for the twoelectronic states, with each state having a different vibrational spacing, indicating different
electronic states and subsequently different equilibrium bond lengths for each state. These
vibrational progressions are used to measure the bond length of the anion. The measurementof the binding energy of these excited A states will be used extensively when a pump-probe
experiment is carried out in these systems, as it will be the only accessible state from theanion electronic configuration.

40

BASEX-Reconstructed Image

2.66 eVRaw Image

Figure 3.6(Left) Raw IBr− photoelectron image showing all photoelectron energies
from 0 to 1.33 eV. This image was recorded at a wavelength of 300 nm.
(Right) BASEX-reconstructed image. Three electronic states are present,
and the image has an energy span of 2.66 eV.

41

BASEX-Reconstructed Image

0.42 eVRaw Image

Figure 3.7(Left) Raw IBr− photoelectron image showing all photoelectron energies
from 0 to 0.21 eV. The image was recorded at a wavelength of 300 nm.
Electron optics were adjusted to magnify the excited 3Π1,2 states. (Right)
BASEX-reconstructed image. The energy scale of the image spans 0.42 eV.

42

The anion starts out in a 2441 configuration. When photodetachment occurs, anelectron from the σu* orbital is removed, resulting in IBr in the ground X state, a 2440
configuration. If photodetachment of an electron occurs from the πg* orbital, this results inthe neutral 2431 configuration, producing the first excited A 3Π1 and 3Π2 states observed in
the spectra. Accurate energy levels of the anion and neutral have been previously
calculated.18,30
The potential energy curves for IBr were calculated by Jungwirth (Figure 3.8),30 andthe anion potential energy curves were calculated by Thompson and Parson (Figure 3.9).18
These previous calculations not only gave a starting point for determining what electronic
states would be accessible, but also were used in the photoelectron simulations using the
PESCAL program developed by Ervin.31 The PESCAL program calculates the FranckCondon overlap between the anion and neutral structures of the molecule of interest, allowing
a photoelectron spectrum to be calculated. The calculated photoelectron spectrum is used inassigning the spectral features and spectroscopic constants. In the calculated IBr
photoelectron spectra, the neutral configurations are held constant to previously measured
spectroscopic constants14,32,33 while the anion parameters are adjusted. Holding the neutral
structure constant allows us to determine an accurate change in the bond length from theanion to neutral.

43

Figure 3.8

Calculated potential energy curves of the ground and seven lowest-excitedpotentials of IBr including the spin-orbit interactions. Energy is relative tothe ground state minimum. Symbols represent the calculated points.30

44

2.0

Energy (eV)

1.5
⎯

1.0

B 2 2Σ+1/2

Br + I*

0.5

a' 2 2Π1/2
a 2 2Π3/2

I + Br*

⎯

3

0.0

⎯

A' 2Π1/2

I + BrA 2Π3/2

Br + I

3/

⎯

-0.5

X 2Σ+1/2
-1.0
2

3

4

5

6

7

8

R (I-Br), Å

Figure 3.9

Potential energy curves of the ground and five lowest-excited potentials of
the IBr- molecule.18

45

3.4

Ground Electronic X 1Σ State of IBr
The photoelectron spectrum presented in Figure 3.5 was obtained by integrating thereconstructed (right hand column) image in Figure 3.6. Present in this image are both theground and first-excited triplet states of IBr. The individual electronic states are labeledaccordingly in the photoelectron spectra. The ground electronic state of the neutral isobserved in the raw and reconstructed image in Figure 3.6 as the outermost ring of the image.
A vibrational progression is observed in the reconstructed image as a series of less intenserings radiating towards the center of the image originating from the outermost ring.
Measurement of the binding energy of the ground state only yields a vertical detachment
energy of ~3.3 eV. The restriction for measuring a vertical detachment energy is due to ageometry change between the anion and neutral state that is too great to allow for overlap of
the electronic states, an inherent problem in photoelectron spectroscopy.
The technique of photoelectron spectroscopy limits our ability to measure theadiabatic electron affinity in certain systems where the Franck-Condon overlap between theanion and neutral state geometries is nonexistent. This poor Franck-Condon overlap is due tothe excessive bond-length change between the anion and neutral. The equilibrium bondlength of the neutral has been previously measured at 2.468 Å.34
The change in bond length results from the photodetachment of an electron from theantibonding σu* orbital. Photodetachment of an electron from an antibonding orbital resultsin greater bonding character, yielding a shorter bond. Because of the shifted potential energy
curve of the neutral to a shorter equilibrium bond length, the Franck-Condon overlap only
becomes significant at higher vibrational levels. This overlap is observed at higher-binding
energies relative to the origin in the photoelectron spectra. For instance, a vibrational
progression is observed in the spectrum with a spacing of 201(80) cm-1. Previous

46
measurements placed the vibration spacing at 267.01 cm-1.14 This discrepancy is caused byaccessing such a high vibrational level in the ground state and by our imaging method where
the ground state is on the edge of the detector. Obtaining meaningful information on the
ground state isn’t favorable, even if a proper wavelength (~340 nm) is chosen to access theground state and focused properly onto the imaging detector. This difficulty stems from theanharmonicity of the ground state and the high vibrational level that is accessed between theanion and neutral states. Even so, to extract additional information from the ground state, wemust use the excited states.
Using the excited triplet A states observed in the spectra, useful thermochemical
information from the ground state can be extracted. We use these two features to determine
the adiabatic electron affinity, (as opposed to using the generalized vertical detachment
energy) and calculate the bond dissociation energy of the anion. Both values are reported in
the following sections.

47

3

Photoelectron counts

A Π1

138 cm

3

A Π2

-1

150 cm

ν0

-1

ν0

4.10

4.05

4.00

3.95

3.90

Electron binding energy (eV)Figure 3.10

High-binding energy portion of the 300 nm photoelectron spectrum of IBr−,
showing magnification of the two excited 3Π1,2 states. Data are obtained from
integration of the BASEX-reconstructed image of Figure 3.7.

48

3.5

First Excited A 3Π2 State of IBr
The IBr molecule has been studied by many authors since the 1920s, but there wereno reports of the measurement of the first excited 3Π2 state of IBr until the 1990s. The A 3Π2state is elusive in conventional optical spectroscopy because of the ΔJ=0±1 selection rule.
Yet it is easily accessible using negative ion photoelectron spectroscopy. The first reported
spectra of the 3Π2 state came from Tellinghuisen in 1992.15,33,35 He used state-selective
spectroscopy to measure the D’ → A’ transition and determined the spectroscopic constants
that will be compared to our reported values.
The most direct measurements obtained in our photoelectron spectra of the A 3Π2state is of the binding energy and the vibrational and electronic-state splittings. The binding
energy of the A 3Π2 state origin (Figure 3.10) appears at 3.912(5) eV between the A 3Π2 ← XΣ electronic states. A splitting energy of 1.401 eV between the A 3Π2 ← X 1Σ transition was

2previously reported by Tellinghuisen.33 This splitting allows the splitting energy between the
A 3Π2 and 3Π1 states of 124 meV to be calculated.14,33
Six distinguishable vibrations with a peak spacing of 150(5) cm-1 are observed in the
spectra. This compares well with Tellinghuisen’s previous measurement of
147.631(9) cm-1.33 Using the vibrational spacing in the spectra, a bond length change fromthe anion to the neutral-excited state can be determined. Using a Franck-Condon analysis,
the neutral parameters were held constant to known literature values of re(A 3Π2)= 2.842 Å
and ωe=147.631.33 The anion parameters were adjusted until the vibrational progression in
the spectra had a coherent fit. This fit was then repeated using the A 3Π1 state parameters to
confirm these results. The spectra were found to have a best fit with an equilibrium bondlength for the anion of 3.025(.01) Å and ωe of 115(20) cm-1. Simulated spectra containing the49
Franck-Condon overlap of the excited A 3Π1 and 3Π2 states of IBr are shown in Figure 3.11.
The data from the photoelectron spectra is presented as a black trace, the simulation of the A3

Π1 state is presented as a red trace, and the A 3Π2 state is presented as a blue trace. The

simulation was run with an anion temperature of 100 K. Hot bands are observed in the
spectra, giving rise to the feature below the origin of the peak. A collection of reported
values appears in Table 3.1.

Table 3.1
State
A 3Π1
A 3Π2
X 1Σ
X− 2Σ

Measured values for the IBr anion, neutral, and excited states.
T0-0 (eV)Results Literature
1.525141.4103300-2.51(1) -2.5537,38

ω1-0 (cm-1)
Results Literature
138(5) 135.214
150(5) 147.63133
201(80) 268.6814
115(20) 13618

re (Å)
Results Literature
2.858336
2.841733
2.46834
3.025(.01) 2.937

50

3Photoelectron counts

A Π1

ν0

A 3Π2
ν0

0.00

0.05

0.10

0.15

0.20

0.25

Electron kinetic energy (eV)Figure 3.11

Simulated photoelectron spectra of the excited triplet A states of the IBr.
The red spectrum represents the continuous simulation for the 3Π1 state, withthe blue lines representing the continuous simulation for the 3Π2 state. The
simulation was performed with an anion temperature of 100 K and an anion
bond length of 3.025(.01) Å.

51

3.6

First Excited A 3Π1 State of IBr
Of the three prominent features in our spectra, the first-excited A 3Π1 state contains

the most accurate measurements in our spectra. The A 3Π1 state has been previously studied
by several groups for many years now.14,15,32 This state appears in our spectra with anelectron binding energy of 4.041(5) eV that corresponds to an electron kinetic energy of
72 meV. Using our measured electron kinetic energy and the measurement by Suzuki14 of theseparation between the A 3Π1 ← X 1Σ states, of 1.525 eV, we are able to provide a moreaccurate electron affinity than from the 3Π2 state. This measurement allows for an updated
value of the electron affinity of IBr of 2.51(1) eV. Previous measurements using chemiionization of heteronuclear halogen collisions previously placed the electron affinity at 2.55
eV.37,38 Our measurement is internally consistent between the two excited triplet states. A
vibrational progression with a spacing of 138(5) cm-1 is present in our photoelectron spectra,
which compares well to Suzuki’s previous measurement of 135.2 cm-1.14 A reported
equilibrium bond length of 2.8583 Å36 for the 3Π1 state allows us to confirm the anion bondlength using the previous measurement from the 3Π2 state. The spectroscopic information
obtained from these excited triplet states is used in determining thermochemical properties of
the anion.
Measurement of the electron affinity allows for thermochemical properties to be
calculated and compared to previous measurements. The bond dissociation energy (D0) for
IBr− can be calculated by using the equation
D0(IBr−) = EA(IBr) + D0(IBr) – EA(Br).

(3.1)

Our measured electron affinity for IBr− is 2.51(1) eV. The best determined measurement of
the bond dissociation energy (D0) of IBr is 1.8184(2) eV,8 and the electron affinity of Br is3.363583(4) eV.23 These measured values place the bond dissociation energy of the anion at52
0.97(1) eV. This measurement differs by 136 meV from a previous measurement of 1.10(4)eV by the Lineberger group obtained with a crude photofragment translation energy releasemethod.18 It compares well to a previous calculation of 0.963 eV.18 We assign thediscrepancy between bond dissociation energies to the technique previously used to measure
the anion bond dissociation energy.
Upon dissociation of the neutral I–Br bond, the products are the same when startingfrom the ground (X) electronic or excited (A) triplet states. All yield iodine and bromine intheir ground electronic state (2P3/2).6,30 In the case of the anion when the I–Br bonddissociates, the Br atom will contain the extra electron (1S). This occurs because bromine hasa higher electron affinity than iodine.
3.7

Conclusion
Using a negative-ion photoelectron spectrometer, we recorded a set of photoelectron

spectra of IBr− and measured vibrational frequencies and splitting energies of three electronic
states. Using the information collected from the photoelectron spectra, we measured an
updated electron affinity of 2.51(1) eV by using the measured-splitting energy between theanion and excited triplet states and the known splitting energy between the excited triplet
states and the neutral ground state. This new electron affinity measurement allowed us tomeasure a bond dissociation energy of the anion of 0.97(1) eV. This value compares well tothe calculated value of 0.963 eV and updates the previous measurement of 1.10(4) eV.
The two photoelectron spectra shown in Figures 3.5 and 3.10 were recorded at
separate times but correspond well to each other. Thus the binding energies of the electronic
states are confirmed with two separate measurements. The values of the excited A 3Π1 and3Π2 states are reported from the magnified photoelectron spectra and confirmed in the full

spectra. The full photoelectron spectrum is utilized in the measurement of the vertical
detachment energy of the ground X state. The absence of an origin in the ground state

53
confirms the large bond length, resulting in poor Franck-Condon overlap and the large
progression observed in the transition to the neutral. The vibrational spacing in the excited3Π1,2 states agrees extremely well to those of previous measurements, confirming ourelectronic states.
The spectrum of IBr− (Figure 3.5) is similar to that of I −2 (Figure 3.4),22 which wesuspected would be the case. We also predict that when IBr is solvated with CO2, thespectrum will be similar to that of solvated I2.22 Solvation of IBr should result in an increaseof binding energies and a spreading of the peak distribution as the electron-binding energy is
distributed over the molecule and solvent. This set of spectra for the bare IBr− will lead tofurther discoveries once a set of spectra are collected for the solvation of IBr−(CO2)n.

54

References for Chapter III

1

D. W. Arnold, S. E. Bradforth, E. H. Kim, and D. M. Neumark, Journal of Chemical
Physics 102 (9), 3510 (1995).

2

B. J. Greenblatt, M. T. Zanni, and D. M. Neumark, Journal of Chemical Physics 112(2), 601 (2000).

3

A. Sanov, T. Sanford, S. Nandi, and W. C. Lineberger, Journal of Chemical Physics
111 (2), 664 (1999).

4

R. Mabbs, K. Pichugin, and A. Sanov, Journal of Chemical Physics 122 (17), 174305(2005).

5

R. Mabbs, E. Surber, and A. Sanov, Journal of Chemical Physics 122 (5), 054308(2005).

6

W. S. McGivern, R. J. Li, P. Zou, T. Nguyen, and S. W. North, Chemical Physics
249 (2-3), 237 (1999).

7

A. Stolow, A. E. Bragg, and D. M. Neumark, Chemical Reviews 104 (4), 1719
(2004).

8

E. Wrede, S. Laubach, S. Schulenburg, A. J. Orr-Ewing, and M. N. R. Ashfold,
Chemical Physics Letters 326 (1-2), 22 (2000).

9Y. S. Kim, Y. J. Jung, and K. H. Jung, Journal of Chemical Physics 107 (10), 3805
(1997).

10

K. P. Lawley, D. Austin, J. Tellinghuisen, and R. J. Donovan, Molecular Physics 62(5), 1195 (1987).

11

M. Shapiro, M. J. J. Vrakking, and A. Stolow, Journal of Chemical Physics 110 (5),2465 (1999).

55
12

P. Vogt, M. Schmitt, and W. Kiefer, Chemical Physics Letters 243 (1-2), 64 (1995).

13

M. J. J. Vrakking, D. M. Villeneuve, and A. Stolow, Journal of Chemical Physics 105(13), 5647 (1996).

14

T. Yukiya, N. Nishimiya, and M. Suzuki, Journal of Molecular Spectroscopy 214 (2),132 (2002).

15

X. N. Zheng, M. C. Heaven, and J. Tellinghuisen, Journal of Molecular Spectroscopy
164 (1), 135 (1994).

16

V. Dribinski, J. Barbera, J. P. Martin, A. Svendsen, M. A. Thompson, R. Parson, andW. C. Lineberger, Journal of Chemical Physics 125 (13), 133405 (2006).

17

T. Sanford, D. Andrews, J. Rathbone, M. Taylor, F. Muntean, M. Thompson, A. B.
McCoy, R. Parson, and W. C. Lineberger, Faraday Discussions 127, 383 (2004).

18

T. Sanford, S. Y. Han, M. A. Thompson, R. Parson, and W. C. Lineberger, Journal ofChemical Physics 122 (5), 054307 (2005).

19

T. J. Sanford, Ph.D. Thesis, University of Colorado, 2004.

20

A. V. Davis, R. Wester, A. E. Bragg, and D. M. Neumark, Journal of Chemical
Physics 118 (3), 999 (2003).

21

E. Surber, R. Mabbs, and A. Sanov, Journal of Physical Chemistry A 107 (40), 8215
(2003).

22

H. Gomez, T. R. Taylor, and D. M. Neumark, Journal of Chemical Physics 116 (14),6111 (2002).

23

C. Blondel, P. Cacciani, C. Delsart, and R. Trainham, Physical Review A 40 (7),3698 (1989).

24

D. Hanstorp and M. Gustafsson, Journal of Physics B-Atomic Molecular and Optical
Physics 25 (8), 1773 (1992).

25

R. S. Mulliken, Journal of Chemical Physics 55 (1), 288 (1971).

56
26

G. J. Rathbone, T. Sanford, D. Andrews, and W. C. Lineberger, Chemical Physics
Letters 401 (4-6), 570 (2005).

27

W. C. Wiley and I. H. McLaren, Review of Scientific Instruments 26 (12), 1150
(1955).

28

L. Minnhagen, Arkiv for Fysik 21 (5), 415 (1962).

29

V. Dribinski, A. Ossadtchi, V. A. Mandelshtam, and H. Reisler, Review of Scientific
Instruments 73 (7), 2634 (2002).

30

J. Pittner and P. Jungwirth, Chemical Physics Letters 321 (3-4), 281 (2000).

31

K. M. Ervin, T. M. Ramond, G. E. Davico, R. L. Schwartz, S. M. Casey, and W. C.
Lineberger, Journal of Physical Chemistry A 105 (48), 10822 (2001).

32

D. R. T. Appadoo, P. F. Bernath, and R. J. Leroy, Canadian Journal of Physics 72(11-12), 1265 (1994).

33

D. T. Radzykewycz, C. D. Littlejohn, M. B. Carter, J. O. Clevenger, J. H. Purvis, and
J. Tellinghuisen, Journal of Molecular Spectroscopy 166 (2), 287 (1994).

34

W. Holzer, W. F. Murphy, and H. J. Bernstein, Journal of Chemical Physics 52 (1),399 (1970).

35

X. N. Zheng, M. C. Heaven, and J. Tellinghuisen, Chemical Physics Letters 195 (23), 273 (1992).

36

J. O. Clevenger, Q. P. Ray, J. Tellinghuisen, X. Zheng, and M. C. Heaven, Canadian
Journal of Physics 72 (11-12), 1294 (1994).

37

D. J. Auerbach, M. M. Hubers, A. P. M. Baede, and J. Los, Chemical Physics 2 (1),107 (1973).

38

A. P. M. Baede, Physica 59 (3), 541 (1972).

Photoelectron Velocity Map Imaging Spectra of CuH− and CuH2−
4.1

Introduction
While conducting studies on the copper-methoxy and copper methanol anions,1 webecame interested in studying copper hydride anions. We attempted to identify all possible
species in the mass spectrum around the copper anion. The hydrides stood out as having
possible contamination from the bare copper anion. After recording the photoelectron spectraof the copper hydride anions, we performed a literature search that revealed a photoelectron
spectra of these simple molecules has not been previously reported. CuH has been studied
previously by many different groups dating back to the 1920s. The most extensive studies
were performed by Ringström in 1966. A series of publications by the Andrews groupreported studies of copper hydrides utilizing H2 matrix infrared spectra. These matrix
infrared spectra were assigned using DFT calculations of the corresponding neutral
molecules.
Reactions of the form M + H2 → MH + H are important prototypes for the processof hydrogen splitting on transition metals. These reactions can proceed through anintermediate MH2. One class of intermediates, the copper hydrides, has been the subject ofnumerous theoretical and experimental studies. These hydrides are important forunderstanding the absorption of hydrogen onto metal surfaces in heterogeneous catalysis.2
As a result, there have been a number of spectroscopic3-10 and computational2,9,11-19 studies ofcopper monohydride, while the copper dihydride has been primarily limited to theoreticalstudies.
Copper monohydride emission and absorption spectra were first observed more thaneighty years ago.4 The next major advance was Ringström’s 1966 study,7 in which hecharacterized five CuH electronic states. Since then, several groups have characterized anumber of CuH singlet and triplet states.6,8-10,20-23 The most recent work was performed by

58
Andrews and co-workers on laser-ablated Cu atoms with hydrogen in low temperaturematrices.3,9 The experimental studies were accompanied by increasingly sophisticatedcalculations of the energies and properties of the low-lying states of CuH.9,12-15,18,24,25 Notably
missing in the experimental work of copper hydride, however, has been detection of thelowest triplet state, a 3Σ+. This state is a prominent feature of the CuH− photoelectron
spectrum reported here.
Copper dihydride has proven much more difficult to investigate experimentally andtheoretically than the monohydride. The difficulty is the complex electronic structure of
CuH2, which includes bent and linear excited states, conical intersections, and multiple
important electronic configurations. Only recently have elaborate calculations begun toprovide consistent electronic structures of CuH2.13 Most theoretical work has addressedbarriers to CuH2 formation, CuH2 energies relative to Cu (2S) + H2 and CuH + H, adiabatic
correlations, and conical intersections.2,3,9,13,16,17 Theory has shown that the ground state sits
approximately 0.45 eV above the Cu (2S) + H2 asymptote, with a barrier of about 1.4 eV fromthe reactants.7,8,10,13 However, calculations have shown the CuH2 molecule to be unstablewith respect to decomposition to CuH + H.
On the experimental front, Andrews and co-workers recently detected infraredabsorptions in a hydrogen matrix that they could attribute to CuH2 and CuH2−.3,9 However,there have been no reports for the detection of isolated CuH2. Previous matrix attempts toobtain copper dihydride only provided evidence for CuH.6 A major finding was that theformation of copper dihydride must be accomplished by reacting excited Cu (2P) atoms andH2 molecules to form the metastable intermediate CuH2.6,12,13,26
In this chapter, I report the production and photoelectron spectra of CuH− and CuH2−
anions. The CuH2− photoelectron spectra give electron affinities of the copper dihydride and59
the direct observations of a previously unobserved linear HCuH intermediate in thehydrogen-splitting reaction and the lowest triplet state of CuH.
4.2

Experiment
The photoelectron spectroscopy of copper hydrides is carried out using a velocity

map-imaging photoelectron spectrometer, described previously in Chapter 2.27 The sputter
ion source gas consists of argon saturated with water vapor at 45 psi. The gas mixture isexpanded into the chamber using a pulsed General Valve (series 9). The emerging molecularbeam initiates a discharge between a copper cathode (−3 kV) and a stainless steel pin held at
ground potential, producing Cu− anions and small clusters. The anions react with the gas
mixture producing a variety of anions, including CuH− and CuH2−.28 The ion of interest ismass selected by a time-of-flight mass spectrometer. A typical mass spectrum is displayed in
Figure 4.1. The ion selected is intersected with a 2 mJ (355 or 600 nm) laser pulse in avelocity map-imaging photoelectron spectrometer.27,29 The three dimensional photoelectron
image is reconstructed using the BASEX inversion algorithm.30 The photoelectron energy
scale is calibrated using well known Cu 2S, 2D3/2,5/2 ← Cu− 1S transitions31 and is displayed in
Figure 4.2. For electron kinetic energies below 200 meV, electron-binding energies can be
determined with an accuracy of ±0.005 eV or better.27
The interpretation of the experimental results was aided by DFT calculations by theGaussian suite of programs32 using the B3LYP method with the 6-311++G(3dp,3df) and SDD
basis sets for hydrogen and copper, respectively. These calculations supplement theextensive earlier theoretical studies.5,6,9,13-15,17 The optimized geometries and vibrationalfrequencies of the anion and neutral obtained in the earlier calculations are utilized in FranckCondon simulations of the photoelectron spectra using the PESCAL program.33 Only theabsolute value of Δr can be determined from the harmonic Franck-Condon simulations, andthe sign of Δr must be obtained from calculated structures of the anion and neutral.

60

55

60

65

70

CuD2−

75

65

(CD3O)CD3OD−

CuD2−Intensity

63

CuD−

65

CuD−
−
Cu

63

65

63

Cu

55

−

60

65

70

75

Mass (m/z)
Figure 4.1

Time-of-flight mass spectrum obtained from a pulsed source. Mass peaks
are labeled accordingly. Sputtered copper with methanol-d4.

Photoelectron counts

61

2

2

D3/2

S

2

D5/2

3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6

Electron binding energy (eV)Figure 4.2

355 nm Cu− photoelectron spectra. Transitions to the three lowest-lying
electronic states of Cu are shown and labeled for reference.

62

4.3

Results and Discussion

4.3.1

CuH and CuD
The CuH 1Σ+ ground state arises from the combination of the Cu 3d104s and the H 1s

orbitals giving a σ2 bonding HOMO. Adding an additional electron forms the anion in the
2 +Σ state, a σ2σ* configuration.14 The lowest energy excited a 3Σ state of CuH arises frommixing of the Cu 3d104s and H* 1s orbitals. It is formed by photodetachment from the σorbital in the anion, giving rise to a σσ* configuration. Both states are accessible by single
electron photodetachment from σ2σ* X 2Σ+ CuH−. The lowest-excited CuH singlet state, A
1

Σ, does not arise from this configuration, but rather is formed from the Cu* 3d94s2 and H 1s

orbitals, giving rise to a d9σ2σ* configuration,14 which is also accessible by photodetachmentof a Cu d-electron from CuH−. The energy difference between the X 1Σ+ and A 1Σ states iswell known,7 but the a 3Σ state has not previously been observed. Figure 4.4 depicts thephotoelectron spectra of CuH− recorded at 355 nm; the CuH− photoelectron spectra arecompletely consistent with the qualitative expectations described above. Comparison of theobserved peak separations with the known CuH A 1Σ← X 1Σ splitting enables theidentification of the transitions shown in Figures 4.4 and 4.6.

63

Raw Image

Figure 4.3

BASEX Reconstructed Image

Photoelectron image of CuH− recorded at 355 nm. The raw image is on theleft side, and the BASEX-reconstructed image is on the right side.

Photoelectron counts

64

a 3Σ+

A 1Σ+
Cu
2
D3/2
3.4

3.2

3.0

Cu
2
D5/2
2.8

2.6

2.4

2.2

2.0

Electron binding energy (eV)Figure 4.4

Photoelectron spectra of CuH− recorded at 355 nm. They are obtained fromthe integration of the BASEX image in Figure 4.3. The Cu 2D3/2 state isdisplayed underneath the 3Σ state of CuH. The spectrum has been cut off at 2
eV to focus the attention on the excited states.

65

As the lowest electron kinetic energies are most accurately determined, I derived theelectron affinity of CuH from the energy of the transition to produce the A 1Σ CuH state. The
result is EA(CuH) = 0.444(6) eV. Two peaks are seen, corresponding to ν=0 and 1 of X 1ΣCuH, with a separation of 1835(50) cm-1, in reasonable agreement with the very accurate
1866 cm-1 obtained by Brault and Bernath.10 This short vibrational progression allows the useof the Franck-Condon simulation program to obtain |Δr|(X CuH – X CuH−) = 0.104(3) Å. As
the added electron in the anion is antibonding, I obtain re(X CuH−) = 1.567(3) Å. A
measurement of the anion’s equilibrium bond length is placed at 1.567(3) Å. Through the useof the Franck-Condon simulations, the neutral bond length was fixed, while the anion bondlength was adjusted until a representative spectrum was reproduced.

66

Raw Image

Figure 4.5

BASEX Reconstructed Image

Photoelectron image of CuH− recorded at 600 nm. The raw image is on theleft side, and the BASEX-reconstructed image is on the right side.

Photoelectron counts

67

1 +

X Σ
EA 0.444 eVCu
2S
1.235 eVFWHM
57 meV
227 meV

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

Electron binding energy (eV)Figure 4.6

Photoelectron spectrum of CuH− recorded at 600 nm, obtained from
integration of the BASEX image in Figure 4.5.

0.0

68

Measurement of the electron affinity allows for the measurement of the dissociationenergy (D0) of the anion. Utilizing the value D0(CuH) = 2.84(4) eV, measured by Rao34 in athermochemical cycle with EA(CuH), gives D0(X CuH− → Cu− + H) = 2.05(6) eV; theweaker anion bond is expected because of the antibonding character of the added electron.
Though not shown here the photoelectron spectrum of CuD− was recorded to confirm ourspectral assignments. It is included in Appendix C. Table 4.1 summarizes our view of the“best” thermochemical values for those states of CuH and CuH− reported here. I use thepresent results where appropriate.

69

Table 4.1

Recommended spectroscopic constants for 63CuH and 63CuD. Values marked
with an asterisk are obtained from spectra reported in this chapter.

State
63
CuH A 1Σ+63
CuH a 3Σ+63
CuH X 1Σ+63
CuH− X 2Σ+T0-0 (eV)2.890b
2.418(6)*0-0.444(6)*ω1-0 (cm-1)
1610.4c
1762a,f
1866.4d
1564f

re (Å)
1.5663g
1.567(5)e,*1.46263c
1.567(3)*63

2.892b
2.425(6)*0-0.439(6)*1171.9b1174a,f
1346.2c
1115f

1.566(3)b
1.567(5)e,*1.4625(5)c
1.567(3)*CuD A 1Σ+CuD a 3Σ+63
CuD X 1Σ+63
CuD− X 2Σ+63

a – Calculated at the CASSCF level by Marian 15.
b – Measured by Ringström 7.
c – Reported in Herzberg 35.
d – Measured by Brault using Fourier transform emission spectroscopy 10.
e – based upon non-observation of a Δν = +1 transition in the photoelectron spectrum.
f – DFT calculations obtained from the B3LYP method and the6-311++G(3dp,3df)/SDD basis sets for hydrogen and copper respecively
g- Measured by Bernath in an infrared emission spectra 8

70

Contamination from the copper anion appears in our photoelectron spectra. It occurs
because of the mass separation of 1 amu between Cu− and CuH−, combined with the large
photodetachment cross section of copper. The 600 nm spectra displays the Cu 2S state, andthe Cu 2D5/2 state is present in the 355 nm spectra. The copper 2D3/2 state is encompassed by
the a 3Σ state of CuH and is shown in the spectra (Figure 4.4) as a trace located underneaththe a peak. The excitation energy of the previously unobserved a 3Σ state, 2.425(6) eV, isdetermined by measurement of the A 1Σ+ - a 3Σ splitting and compared it to the ~2.29 eV
obtained in several calculations.14-16 A vibration progression is not observed in our spectra,
indicating that the detached electron was nonbonding and re(a 3Σ+)=1.567(5) Å, the same as
for the anion. This result compares well with the calculated bond length of 1.54 Å.15
The peak with the highest-binding energy corresponds to the known separation
between the ground state and the first-excited state of the singlet state, A 1Σ+.7,15 The photon
energy was insufficient to access the ν=1 transition, precluding measurement of either thebond length or vibration frequency. Previous experimental studies give re(A 1Σ+ CuH) =1.572 Å, with ωe = 1610cm-1.7,35
4.3.2

CuH2 and CuD2
The ground state configuration of copper dihydride has C2v symmetry with a(5a1)2(3b2)1 orbital configuration, yielding a bent 2B2 state. When an extra electron is added,B

the lowest energy configuration of the anion rearranges into a linear (3σg)2(2σu)2configuration, giving rise to the X 1Σ+ state. The sputter ion source yields intense beams of2 +CuH2−. Electron photodetachment produces both the bent 2B2 CuH2 and a low-lying A Σg
B

excited state with a geometry very similar to that of the anion.13 Thus, both of these statesshould be observed in the CuH2− photoelectron spectrum: the ground state should have

71
extensive, but poorly resolved, vibrational progressions, and the excited state should have a
sharp near-diagonal transition dominating the spectra. These qualitative expectations areexactly what is observed in the 355 nm photoelectron spectrum of CuD2−, as seen in Figure
4.8. The velocity map image is presented in Figure 4.7. Photoelectron spectra of CuH2− andCuD2− were recorded to confirm the spectral assignments, but we report only on the latter, as
this provided a sharper photoelectron spectrum. The photoelectron spectrum of CuH2− isincluded in Appendix C.

72

Raw Image

Figure 4.7

BASEX Reconstructed Image

Photoelectron image of CuD2− recorded at 355 nm. The raw image is on theleft side, and the BASEX reconstructed image is on the right side.

Photoelectron counts

73

A 2Σg+

170 meV

X 2B2

3.4

3.3

3.2

FWHM
18 meV

3.1

3.0

2.9

2.8

2.7

2.6

Electron binding energy (eV)Figure 4.8

Photoelectron spectrum of 65CuD2− at 355 nm. The intense peak is assigned
as the transition A 2Σg in CuD2, with a symmetric stretch of 1369(40) cm-1.
An extended vibrational progression is present at higher-binding energies andis assigned as the X 2B2 ground state.
B74

The photoelectron spectrum is dominated by a strong transition to the linear-excited+A Σg state of CuD2. Photodetachment occurs from the 3σg orbital in the anion, as this has
2been shown to be lower in energy than detachment from the 2σu orbital. This result contrasts
with other IB transition metal hydrides where the 2σu orbital is lower.11-13 A shortprogression in the symmetric stretch mode is observed with a frequency of 1370(40) cm-1; theassignment to a vibrational progression was confirmed by hydrogen substitution. A binding
energy of 2.841(5) eV is measured for the A state in the photoelectron spectra. Photoelectron
ejection is strongly peaked parallel to the laser polarization vector, with an anisotropy β of
1.8(1), consistent with photodetachment removing an electron from the 3σg orbital in theanion.11-13 With the observed ν0-1 vibration and the use of the PESCAL program, theequilibrium bond length of the anion is calculated using a harmonic Franck-Condon intensity
analysis. The A state bond length is fixed to the values calculated by Chambaud at 1.52 Å.13
From these calculated values, a change in the equilibrium bond length is calculated at|Δr(Cu-H)| to be 0.04 Å. Our DFT calculations show the anion to have the longer Cu-H bond2 +length, providing the sign associated with |Δr(Cu-H)| above. As A Σg CuH2 has not been
2 +previously observed, we employ the much higher level A Σg CuH2 bond length calculated

by Chambaud (1.52 Å)13 to obtain the anion bond length in the anion as 1.56(2) Å. A
simulation of the spectra calculated in PESCAL using the calculated geometries andfrequencies is shown in Figure 4.9. Table 4.2 summarizes my view of the “best”
thermochemical values for those states of CuH2 and CuH2− reported here, using the present
results where appropriate.

75

Recommended spectroscopic constants for 65CuD2 and 65CuH2. Values
marked with an asterisk are obtained from spectra reported in this chapter

Table 4.2

ν (cm-1)

Electronbindingenergy(eV)
2.841(5)*

ν1

ν2

ν3

ν4

H-Cu-H re(Å)

592.8f

592.8f

1211f

1369(40)d*

1.52a

CuD2 X 2B2

2.60(5)b

1196f

1291f

1 +
CuD2− X Σg

0*

470f

450(60)d(458.5c)
470f

1065f

1107.3c1.56(3)*

2 +CuH2 A Σg

2.853(5)*

817g

817g

1808g

1884(40)d*

1.52a

65

CuH2 X 2B2

2.60(5)a

1935e

1799e

1 +
CuH2− X Σg

0*

655f

636.5c(685e)
655f

State
2 +
CuD2 A Σg

65

65

B

65

65

B

65

1065f

1.468a1.468a1517.8c

1.56(3)*a – From Chambaud X-A separation, MRCISD.13
b – Chambaud [12], X-A separation as a above, corrected for H-D zero point differences,using the calculated harmonic CuH2 vibrational energies.
c - IR absorption in a hydrogen matrix.9
d – Measured in photoelectron spectra
e – Chambaud anharmonic frequencies calculated at the MRCISD level.13
f – DFT calculations obtained from the B3LYP method and the6-311++G(3dp,3df)/SDD basis sets for hydrogen and copper respecively
g – MP2-MP4 calculations performed by Fitzpatrick.17

76
The extended, unresolved vibrational progression present in the spectrum arises fromtransitions to high vibrational levels in the 2B2 ground state. The extent of the progression
Barises from the large geometry difference between the linear anion and the bent neutral
ground state. This progression is observed as a series of concentric rings radiating outward
from the center of the photoelectron image toward the outer ring, which is assigned to the A
state, as shown in Figure 4.7. Calculations by Chambaud13 place the 2B2 ground stateB

2 +~0.2 eV below the A Σg state, at least 0.3 eV below the binding energy where we firstobserve the extended progression. We cannot quantitatively simulate this progression,
because the potential energy surfaces are to poorly known to obtain vibrational energy levelsin the bent, linear, and transition regions. Moreover, the experimental spectra exhibit no clear
vibrational peaks. Even a vertical detachment energy cannot be obtained from the data. Even
though fully resolved vibration peaks are not observed, one can estimate a frequency of450(60) cm-1 for the bending mode. This frequency compares well to the 458 cm-1 measured
by Andrews and co-workers in a D2 matrix, which provided the few experimentally measured
values for copper dihydrides.9 Nevertheless, the extended vibrational progression isconsistent with the large predicted geometric change. If we use the calculated geometries forboth states, employ harmonic vibrational frequencies, and assume the electronic transition
matrix elements are the same for both ground and excited states, then we obtain thesimulation shown in Figure 4.9. The only point to this qualitative CuH2 X 2B2 simulation isBthat both the absence of signal at the 2B2 origin (2.6 eV) and the intensity of the extendedB

progression in the energy range observed are consistent with expectations. With thissimulation, the ground state of the copper dihydride should possess a symmetric H−Cu−H
bond length of 1.468 Å with a bond angle of 121.6°.13

77

Photoelectron counts+

65

CuD2 Data

2

+

A Σg

+
++

CuD2 Linear Simulation++

CuD2 Bent Simulation

+

+

+
+
+
+ +
+

2

+

X B2

+
+
+
+
++
+
++
+++++++++++++
+++
+
++
+++
+++ +++++++++++
+ ++
+
+++ +
+++
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ ++++++ + +++ +++++
++
+++++
+++
+ ++
+++++++++ +++++++++ +++ +++++++++ + ++
+ +
++++
++
+ ++
++
++++
++++++++++ ++++ +++++++++++++++++++++++++++++++ ++ +++++++++++++ ++ ++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++
+++

3.4

3.3

3.2

3.1

3.0

2.9

+

+

EA(CuD2)+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

2.8

2.7

2.6

Electron binding energy (eV)Figure 4.9

Comparison of the photoelectron spectrum of CuD2 with the simulated
spectrum. As a consequence of the large geometry difference between thelinear anion and bent ground state, there is no detectable photoelectron signal
at EA(CuD2), as indicated by a vertical arrow. The simulation does notconsider the major effects of the bent→linear transition and must be
considered as being very qualitative.

78

While we cannot resolve transitions in the extended vibrational progression in the
ground state of copper dihydride, we can obtain EA(CuH2) as the difference between the+measured electron-binding energy of HCuH A Σg (2.853(5) eV) and the calculated
2+

separation between the A Σg state and CuH2 X 2B2 ground state (0.255 eV).13 This energy2B

difference is obtained from Chambaud’s MRCISD calculation of the splitting between thesestates (0.192 eV), but increased by 63 meV to account for zero point energies in both statesby using Chambaud’s CuH2 X and A state harmonic frequencies.13 If we estimate theuncertainty in the calculation to be 20%, or 0.05 eV, we obtain EA(CuH2) = 2.60(5) eV. The
mixing of measured and calculated energies is largely justified by the fact that the measured
energy is an order of magnitude larger than the calculation correction used to obtain theelectron affinity. Similarly, we find EA(CuD2) = 2.60(5) eV.
This determination of EA(CuH2) allows an estimate of the anion dissociation energy
using a similar mix of calculation and measured energies. This strategy is justified by the factthat the theoretically determined quantities make a much smaller contribution to thedissociation energy than do the experimental ones. Chambaud reports De(CuH2 X 2B2 – CuHB+ H) to be 1.32 eV.13 Again, zero point corrections give D0 to be 1.05 eV. Here we crudely
estimate that the error in the calculated dissociation energy might be 0.2 eV. This dissociationenergy, coupled with the electron affinities of CuH and CuH2, form a thermochemical cycle
to give D0(HCuH− → CuH− + H) = 3.2(2) eV. Even with the large uncertainty, it is clear that
this bond is much stronger in the anion than in the ground state of the neutral molecule.
Since we know from the first study that D0(CuH− → Cu− + H)=2.05(6) eV, this result also
allows us to conclude that removing the first hydrogen atom from HCuH− is significantly
easier than removing the second one.

79
4.4

Conclusion
The use of a dc-sputtering discharge source provides a way to produce significant

quantities of copper hydride anions. The negative ion velocity map-imaging photoelectron
spectra provide quantitative electron-binding and dissociation energies, yielding new insight
into the bonding and structure of these small molecules. The photoelectron spectra of CuH−
exhibit three electronic transitions from the anion to the neutral state. The most prominent
feature of our spectra is the excited a 3Σ+ state, which contrasts with previous experimental
work, lacking this feature.
An extended weak vibrational progression consistent with the bending mode of theground state of CuH2 is observed in the photoelectron spectra. However this progression
provides an unreliable measurement of the vibrational frequency. The absence of a definedorigin corresponding to the ground state is consistent with previous geometry calculations
and spectral simulations. The distinct intense feature of our photoelectron spectra is assigned+

to the first excited A Σg state of copper dihydride, because of predictions resulting from2

previous geometry calculations. We have shown previous theory on copper hydrides
correlates well with our experimental results. More work on the copper dihydrides still need
to be done to provide a better splitting energy between the X and A states and a measurement
the vibrational frequencies of the ground state. The ion production technique employed here
will be useful for studies of other transition metal hydrides and may provide new insights into
the use of transition metals for hydrogen-splitting processes.

80

References for Chapter IV

1

D. H. Andrews, Ph.D. Thesis, University of Colorado, 2006.

2

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3

L. Andrews and X. F. Wang, Journal of the American Chemical Society 125 (38),11751 (2003).

4

R. Frerichs, Z. Phys. 20, 170 (1923).

5

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6

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7

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8

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9

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10

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11

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12

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13

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14

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81
15

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16

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17

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18

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19

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20

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21

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22

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23

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24

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25

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26

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27

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82
28

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29

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31

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32

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Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M.
Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J.
A. Pople, Gaussian 03 (Gaussian Inc., Pittsburgh PA, 2003).

33

K. M. Ervin, T. M. Ramond, G. E. Davico, R. L. Schwartz, S. M. Casey, and W. C.
Lineberger, Journal of Physical Chemistry A 105 (48), 10822 (2001).

34

V. M. Rao, M. L. P. Rao, and P. T. Rao, Journal of Quantitative Spectroscopy &Radiative Transfer 25 (6), 547 (1981).

83
35

K. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, Constants ofDiatomic Molecules. (Van Nostrand Reinhold, New York, 1979).

Photoelectron Velocity Map-Imaging Spectroscopy of CuCD3O−
5.1

Introduction
The studies on copper methoxide developed out of curiosity during the investigationof the photoelectron spectra of copper methanol.1 Our interest began with the idea that otherspecies may be present in the mass spectra, shown in Figure 5.1. In an attempt to identify all
relevant species in our mass spectra, a photoelectron spectrum was collected for the species
on either side of the copper methanol anion, the molecule of interest. Upon examination of
the spectra, we realized that copper methoxy was the relevant species, since it is the
deprotanated form of the copper methanol complex. It is hypothesized to have a similar
structure to that of copper hydroxide CuOH−,2,3 where the methyl group has been replaced byhydrogen.
A photoelectron spectrum of copper hydroxide was obtained by Misaizu,4 but the
spectra contained contamination from copper water, as they are only separated by one amu.
This is the closest related system that has been previously reported. The copper hydroxideanion has a structure consisting of a bond between the copper and oxygen atom, with thehydrogen pointed towards the electronegative copper in a bent structure. The copper
methoxide has a similar structure with a bond between the copper and oxygen with the copper
atom sitting between two of the deuterium atoms, the third points away from the copper.
Because a complete analysis has not been performed on copper methoxy, it will only bereported on very briefly here. A publication at a later date will report on copper methoxy indetail.

85

55

60

65

70

75

80

85

90

95

100

105

110

105

110

CuD2−

65

−

63

Intensity

CuD2
−

65

CuD

−

63

CuCD3OD−

CuD
Cu−

65

65

Cu−

CuCD3O−

63

55

63

60

65

70

75

80

85

90

95

100

Mass (m/z)
Figure 5.1

Time-of-flight mass spectrum obtained from a pulsed source. Peaks arelabeled accordingly.

86

5.2

Brief Results
Two spectra were recorded, the first at a wavelength of 355 nm, as shown in

Figure 5.3. The second was at 500 nm, as shown in Figure 5.5. Four electronic states wereobserved in the 355 nm spectra. A ground X 1A’ state, the first-excited triplet A 3A’ state, andtwo unidentified excited states. Only the ground X 1A’ state was observed in the 500 nm
spectra. The ground state originated at 1.077(5) eV, which corresponds to the electron
affinity. A vibrational progression was observed with a vibrational spacing of 528(50) cm-1.
This frequency has not yet been attributed to any part of the molecule.
The term-splitting energy between the ground state and first-excited state is 1.701(5)eV. We observed it with an electron binding energy of 2.778(5) eV. An observed vibrational
progression of 520(50) cm-1 is present in the first-excited state of copper methoxy. As thisvibrational progression is close to that of the ground state, the progression may correspond to
the same type of vibration present in both the ground and excited states. A second-excitedstate is present with an electron-binding energy of 3.133(5) eV, corresponding to a splittingenergy of 2.056(5) eV. Only one mode is observed for this second-excited state, and it isn’t
as prominent as the other electronic states. A third electronic state is observed in the spectrawith the origin at 3.271(5) eV. This is separated from the ground state origin by 2.194(5) eV.
A vibration is observed with a spacing of 335(50) cm-1, that differs from the previous
vibrations, indicating a new mode has been observed.
Aside from the electron-binding energies observed in the spectra and the vibrational
spacing of the ground and first-excited states, not much is known about the copper methoxide
molecule. A greater understanding of this molecule will be presented at a later date. Copper
methoxy will be compared to the copper hydroxide molecule and investigated with ab initio
calculations. The goal is to find the optimized geometries and energetics of the molecule.

87

Raw Image

Figure 5.2

BASEX Reconstructed Image

The raw CuCD3O− photoelectron image is displayed on the left hand side,and the BASEX reconstructed image is presented on the right hand side. The
image was recorded with a wavelength of 355 nm.

88

X 1A’

Photoelectron counts

A 3A’

3
2

3.5

3.0

2.5

2.0

1.5

1.0

0.5

Electron binding energy (eV)Figure 5.3

Photoelectron spectra of CuCD3O− recorded at a wavelength of 355 nm.

0.0

89

Raw Image

Figure 5.4

BASEX Reconstructed Image

The raw CuCD3O− photoelectron image is displayed on the left hand side,and the BASEX reconstructed image is presented on the right hand side. The
image was recorded with a wavelength of 500 nm.

90

Photoelectron counts

X 1A’

2.0

1.5

1.0

0.5

Electron binding energy (eV)Figure 5.5

Photoelectron spectra of CuCD3O− recorded at a wavelength of 500 nm.

0.0

91

References for Chapter V

1

D. H. Andrews, Ph.D. Thesis, University of Colorado, 2006.

2

S. Y. Wang, A. Paul, N. J. DeYonker, Y. Yamaguchi, and H. F. Schaefer, Journal ofChemical Physics 123 (1) (2005).

3

C. G. Zhan and S. Iwata, Chemical Physics Letters 232 (1-2), 72 (1995).

4

F. Misaizu, K. Tsukamoto, M. Sanekata, and K. Fuke, Surface Review and Letters 3(1), 405 (1996).

92

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98

Appendix A.

Laser Fundamentals

The laser system utilized in the photoelectron imaging spectrometer is a Coherent
Infinity XPO laser system, utilizing a pulsed Nd:YAG laser. The laser system has its
fundamental laser pulse created in a diode-pumped solid state (DPSS) laser, producing 1064
nm using an Nd:YAG prism in a ring resonator, capable of producing a repetition rate
between 0.1 – 100 Hz. The pulse exiting the DPSS takes a dual pass through a flashlamppumped Nd:YAG rods amplifying the 1064 nm laser pulse. Amplification of thefundamental is controlled by the user between 1 – 600 mJ. The amplified pulse undergoes
frequency doubling in an angle tuned BBO crystal producing 532 nm pulses. The residual
1064 nm pulses is combined with the 532 nm pulses in another angle tuned BBO crystal
producing 355 nm pulses, the third harmonic of the laser. At this point the 355 nm laser
pulse is the most intense, and is either utilized in a optical parametric oscillator (OPO)
followed by frequency doubling allowing for the formation of 210 – 600 nm pulses or thepulses out of the OPO are utilized as is after being separated from the residual 1064 and 532
nm laser pulses.
The 355 nm pulses that are used for pumping the OPO undergo a difference
frequency mixing in a double pass BBO cavity, producing a signal and idler beam. The idler
beam is tunable from 710 – 2000 nm, while the signal beam is tunable between 410 – 710
nm. The signal beam is then doubled in an angle tuned BBO crystal and compensator. The
OPO system has a limitation as the signal and idler beams approach each other, reaching thedegeneracy of the OPO. As the OPO approached the degeneracy the line width of the laser
pulses increases exponentially.

99

OPO
Cavityopo cavi
ty

dump

Dump

THG
Pump
t hg pump
Dump
dump
THG
t hg

SHG
shg
sbs

r od- r odr el ay
Rod-Rod
Relay

Amplifieramp headspat i alf i lFiltert erSpatialmast er osci l l at or

Master Oscillator
Faraday
Isolatori sol at or

Figure A.1

Optical Components of the Infinity XPO Laser System

100

The heart of the Infinity system, consists of two Neodymium-doped Yttrium
Aluminum Garnet (Nd:YAG) rods. They are used as the gain medium for the laser system.
The pumping of the Nd:YAG rods with a flash lamp promotes electron in the gain medium
into an excited state. The configuration of the electrons is limited according to the laws ofquantum mechanics which also limit how the energy of the excited electrons can be released.
The excited system can be stimulated to return to the ground state. When this occurs, aphoton with identical properties to the incoming one is emitted. The process is called
stimulated emission and it is the underlying mechanism of all laser action. It provides a wayof generating large numbers of identical photons in a controlled way. Stimulated emission
occurs with a probability of B21 that is related to the spontaneous emission A21 in equation
A.1:

A21 8π n 2 ng hν 3
.
=
B21
c3

(A.1)

If there are N2 numbers of excited systems, the de-excitation rate due to stimulated emission
can be written as B21 N2 P, where P is the pumping rate, which is related to the energy density
in the external field.
If the system is in the ground state when it is exposed to an external field oscillating
at its resonance frequency, it can absorb an energy quantum and end up in an excited statethrough a process called stimulated absorption. The two processes, stimulated absorption andemission are symmetrical with respect to each other. This symmetry means that an incoming
photon does not distinguish between the upper and lower energy states; hence they occur withequal probability, i.e., B21 = B12.
The processes just described are called radiative since they involve the creation or
annihilation of a photon. In addition to these processes, there are also various nonradiative
de-excitation and excitation mechanisms such as collisions with other systems and electron

101
impact. Under normal conditions all of these processes occur simultaneously.
Since stimulated emission is the process by which laser light is generated, we would
like to arrange things so that this process dominates over the others. The only way to do thisis to have a greater number of systems in the higher-lying state so that B21 N2> B12 N1. Inother words, N2> N1. The situation when there are more systems in the higher-lying state iscalled population inversion. It is a necessary condition for lasing action to take place. It is
not possible to achieve population inversion in a two-level system1,2. A higher-level system
is required to achieve population inversion, with a minimum of three and preferably fourlevels.
The Nd:YAG system has a four level scheme (Figure A.2). It has a narrow
fluorescence line width that displays a low-lasing threshold and high gain. By pumping the
Nd:YAG rods with 1.53 eV of energy to an excited state, the electron quickly relaxes to the4

F3/2 level. This is a long lived state, τ ≈ 230 μs. The transition taking place is theI11/2 ← 4F3/2. It has an energy separation of 1.16 eV, corresponding to a wavelength of 10644

nm. From the 4I11/2 state the electron quickly decays down to the 4I9/2 ground state.

102

Figure A.2

Energy Levels for a Nd:YAG System and a Four Level System

103

Placing the Nd:YAG rods within a resonator allows for the feedback necessary for a
stimulated emission process. It also confines the laser field within the gain medium. In thecase of our laser system, the resonator consists of several optics and cells used for theconfinement of the beam and control of the beam properties.
The flash lamp for pumping the Nd:YAG rods creates a thermal gradient across therod diameter, because radiation from the flash lamp is absorbed into the rods. The
temperature gradient, in turn, gives rise to a stress gradient because the hotter regions in thecenter of the rod will try to expand more than the cooler surface regions. The resulting stressgradient gives rise to a refractive index gradient across the rod surface. Since the radial andtangential stress components are different, the refractive indices along these two directions
will be different as well. As a consequence the electromagnetic vector components of theincoming radiation along the radial and tangential directions will experience different
refractive indices, resulting in the two waves travel at different speeds through the rods.
The pulse exhibits horizontal polarization as it enters the first amplifying rod.
Because of the difference in refractive index along the radial and tangential directions, theexiting pulse has acquired an elliptical polarization in all points across its spatial mode except
for those that coincide with the two diameters that are parallel and perpendicular to thepolarization state of the incoming pulse. If the pulse exiting the first rod, exhibiting ellipticalpolarization, is allowed to directly propagate through the second rod, the degree of ellipticity
will increase, i.e., double.1
After the pulse has been reflected in the Stimulated Brillouin Scattering Cell (SBS
Cell) and passed through the two rods a second time, the vertically polarized component will
be reflected out of the laser amplifier by the thin-film polarizer. If the polarization state ispoorly defined as the beam strikes the thin-film polarizer, a large fraction of it will end up in
the Faraday isolator. This energy is not available in the output beam, and it can also disturb

104
the stability of the oscillator. It is important that the beam is in a well-defined polarization
state as it goes through the quarter-wave plate positioned between the second rod and the SBS
cell.
To compensate for the phase difference induced by the birefringence in the first rod,a 90° polarization rotator is positioned between the two rods. Light entering the first rod(from the left), which has vertical polarization, will experience a phase retardation between
the radial and tangential components. Before the pulse enters the second rod, the polarizationis rotated 90°. The component that was radial in the first rod is now tangential, and thetangential component in the first rod is now the radial. The phase retardation that was
induced in the first rod will be compensated by the second rod; the net result will be light thatis again linearly polarized, but now rotated 90°. The same correction will take place as thepulse goes through its second pass through the rods; the net result will be vertically linearlypolarized light.
When light waves are scattered by density waves in a medium, a backwards-travelingand frequency-shifted component can be created in a process called Brillouin scattering. The
density waves can be excited thermally or by the laser field itself. In the latter case, thisphenomenon is called Stimulated Brillouin Scattering, or SBS. As is typical for processes
that are self amplifying, a threshold must be exceeded for it to occur. As a long as thefluency is below this threshold value, the process does not take place at all. When the fluency
exceeds the threshold value, the signal grows exponentially until the onset of saturation.
When the leading edge of the laser pulse returns through the rods, it will decrease the gain.
The effect of this can be seen in the trailing end of the laser pulse where there is a sudden
drop in intensity.3,4
Typically a Chlorofluorocarbon (CFC) is used in a SBS Cell. In the case of theInfinity laser, CFC113 is used (CFC113 is CCl2FCClF2). The laser beam is focused out of the
Nd:YAG rod into the SBS Cell, while in the cell the field generated in the backwards

105
direction by SBS is also the phase conjugate of the incident field. The beam profile isrestored to its original shape as opposed to a distorted beam shape. No polarization
information is retained in the phase conjugate process. The polarization state of the beam is
not reversed or corrected; these effects occur in the oscillator and beam path.
Primarily 1064 nm radiation is emitted from the Nd:YAG rods. However, mostapplications require wavelengths in the visible to ultraviolet range. We achieve thiswavelength shift with a fundamentally nonlinear processes known as harmonic generation.
With the advent of lasers, it became possible to create electric fields large enough to saturate
the polarization of a dielectric medium. This technique did not permit the induction of apolarization on the dielectric medium that related to the electric-field strength in a linear way.
As a result, the polarization relationship can be thought of as a Taylor-series expansion,
giving rise to nonlinear terms.

P = εΧE,
P = ε (Χ1E + Χ 2 E 2 + Χ3 E 3 + ...),(A.2)

where E is the electric field vector, ε is the permittivity of free space, and Χ is the electricalsusceptibility that is characteristic for the polarized material. When the polarization
relationship is expanded, it gives rise to higher-order nonlinear susceptibilities. Thesehigher-order susceptibility terms are typically small compared to the first-order term unlessthe electric field is very large; then the nonlinear terms have an effect on the field.2
The oscillations of the electric field can be described by the function

E = E0 sin(ωt ).

(A.3)

By substituting this value into the polarization term in A.2 we obtain

1
P = εΧ1E sin(ωt ) + εΧ 2 E 2 (1 − cos(2ωt )) + ... .
2

(A.4)

106
The second-order term shows that a wave having twice the frequency of the fundamental
wavelength can be formed in a nonlinear medium. This is Second Harmonic Generation, or
SHG.
To generate power in the second harmonic, the phase of the two waves must match.
This is where phase matching must come into play. It is achieved through the birefringenceof the materials being used. There is an angle Θ m for which the refractive index of thefundamental beam is equal to the refractive index of the frequency-doubled beam. Bychanging the angle at which the beam propagates into the crystal, we allow phase matching tooccur in the crystal via frequency doubling. The refractive indices for both the incoming andfrequency-doubled beams, are temperature dependent. Thus phase matching of the beams
can be tuned by adjusting the temperature of the crystal. If the temperature of the crystal
changes once phase matching has been achieved, the efficiency of the doubling will decrease
because of improper overlap of the phases. The heating of the crystal to a constanttemperature followed by phase matching of the waves, reduces the chances of the crystal
heating and reduce the efficiency of the doubling due to a change in the refractive indexes.
Higher-order harmonic generation is accomplished by frequency mixing in a second
crystal. Third Harmonic Generation is accomplished by frequency mixing of 1064 nm
radiation and frequency-doubled radiation in a second crystal that is phase matched with thetwo incoming beams. Since energy is conserved, the resulting wavelength is given by1
1
1+1064 532,

qual to 355 nm radiation. Fourth Harmonic Generation is accomplished via doubling of the
532 nm radiation, yielding 266 nm radiation. The crystal utilized for producing the Second,Third and Fourth Harmonic generation is a BBO Crystal.2,5,6

107
At the heart of the system is a tunable Optical Parametric Oscillator (OPO). The
OPO consists of a nonlinear crystal. In this case it is a beta barium borate (BBO) crystal.
The Third Harmonic (355 nm) pump laser beam is passed through the first of two BBO
crystals, and two wavelengths come out (Figure A.3). The two are the signal (shorter
wavelength) and idler beams (longer wavelengths). The relationship between the
wavelengths is strictly given by the equation:1

λ pump=1λsignal+1λidler or in frequency ν pump = ν signal + ν idler .

(A.5)

As long as this condition is met and the phase matching condition is satisfied, the parametricdevice will provide tunable gain.1
By passing the signal output beam through a second BBO crystal we produce asecond Harmonic effect on the signal beam. All the conditions for frequency doubling of thesignal output must be met as was true when the 1064 nm light was doubled. Phase matchingof the signal beam and of the output beam must occur. This is done through tuning the angleof the crystal, to maximize the phase matching.

108355 nmPump

Figure A.3

BBO Crystal

Signal Output

420 – 709.4 nm

Idler Output

709.4 – 2300 nm

Schematic for 1st Crystal in OPO Cavity

109

The simplest type of optical parametric oscillator consists of a crystal placed
between two mirrors that provide optical feedback at either the signal or idler wavelengths.
An oscillation is created within the cavity to provide the feedback, just like in a laser.
For gain to be present in a parametric device, there has to be proper synchronizationof the optical phases of the three interacting waves. The optical phases differs from the gain
medium of a laser where the gain is only dependent on the intensity of the pump beam, withno dependence on its phase or direction. The phase matching condition in OPOs is typicallyobtained by using a birefringent crystal. By changing the orientation of the crystal, wechange the index of refraction of the crystal. Since the wavelength is dependent upon theindex of refraction, improper crystal orientation eliminates wavelengths that don’t fulfill thecondition; thus this method is a means of tuning to different wavelengths.
The Infinity laser contains a Porro prism as an end mirror and a flat output coupler
comprising the OPO cavity. The Porro prism allows the two beams to pass through different
parts of the nonlinear crystal; the end result is that the forward and backward signal beams
see different regions of gain. The active gain angle space is restricted and produces an output
of high brightness and coherence.
To pulse the laser, the Infinity laser system consists of a diode-pumped master
oscillator consisting of two concave mirrors, a Nd:YAG prism, and an acousto-opticalmodulator arranged to form a ring resonator (Figure A.4). This ring cavity allows for two
counter-propagating waves to be present. Only one of these waves is useful; to use the otherbeam, a loss mechanism is introduced. This mechanism can also be used to Q-switch thecavity. An acousto-optical modulator (AOM) is used in the pulsing of the laser.

110

Figure A.4

Diagram of the Maser Oscillator Cavity

111

The acousto-optical modulator serves as the Q-switching mechanism of the laser byregulating the round-trip cavity losses. The way it does this is by diffraction of the opticalwave against the acoustical wave induced in the material of the AOM. When the modulatoris switched and acoustic wave becomes resonant within the material, periodic variations are
set up in the refractive indices of the materials. When the optical wave strikes this refractive
index grating, two beams are created. One is shifted up and the other down by the frequencyof the acoustic wave, because of photon-phonon interactions within the medium.1,7-10
By switching the acoustic field, energy is diffracted out of the main beam, resulting
in the loss mechanism. As the transducer is switched off, the acoustic wave decays, allowing
the beam to pass through the crystal without any loss, thereby creating a laser pulse until thetransducer is switched back on. Since there are two waves within the oscillator, one wave isable to pass through undisturbed, creating a continuous wave laser with only minimal power.
This second wave acts as a seed for allowing the next laser pulse to build up in power.
A Faraday isolator is utilized in the scheme of the laser to prevent back-scattering
light from the amplifier from going into the oscillator and damaging it. The Faraday isolator
rotates the polarization of light 45º in a uniform manner. The amount of polarization rotation
is given by θ = VfH z dz , where θ is the polarization rotation angle, V is the Verdet constantof the optical element being used, f is the frequency of radiation, Hz is the longitudinalcomponent of the magnetic intensity vector, and dz if the optical path length. The Faraday
isolator consists of two polarizers sandwiched in between a high-strength rare earth
permanent magnet. The incoherent light traveling in the forward direction becomes polarized(vertically in our case) by the input polarizer. The Faraday rotator will rotate the polarization
45° to the right, while the output polarizer will allow all the light to escape and continue.
Light traveling in the backward direction becomes polarized (45° to the right in this case) by

112
the output polarizer. The Faraday rotator will rotate the polarization 45° more to the right sothat it is horizontally polarized. The input polarizer, which is vertically aligned, will block
this light. The reason to use a Faraday isolator as opposed to a 1/4 wave plate is the rotationof the radiation is insensitive to the direction of propagation. This feature allows for better
isolation of the light polarization, allowing for greater beam stability.1
By placing an amplifier after the oscillator high power output is obtained. The
optical setup in the Infinity is a multipass configuration. This allows the amplifier to be usedboth as a small signal amplifier (first pass) and as a low-gain and high-energy-extraction
efficiency amplifier (second pass). Amplifying the beam noise in the form of amplificationof spontaneous emission (ASE) is a problem. ASE will decrease the energy available to thelasing beam, because of saturation of the amplification medium, producing an incoherent
beam. The ASE is minimized through spatial filters. Because ASE diverges quickly, it caneasily be eliminated from the laser emission.
As stated before, the Infinity system utilizes two Nd:YAG rods. It is a challenge tocontrol the pulse traveling through the amplifier, since both rods must experience the same
relative environment. Assuming that the two rods are close to identical and symmetricallyloaded, the beam in the first rod must be imaged into the second rod. The imaging,performed by two lenses and a concave mirror, is done so that the transverse coordinates
remain unaltered while the coaxial coordinates are flipped around the center of the rods
(Figure A.5). This arrangement not only performs part of the polarization control, but also
compensates for small residual asymmetries in the pumping of the rods. The relay cell isinserted in the beam path to avoid laser-induced air breakdown at the focal points formed by
the imaging system.

113

Figure A.5

Diagram for Nd:YAG rod assembly, including imaging of beam onto rods

114

Several types of optical components readily found in other laser systems can also befound in the Infinity XPO laser system. However, their arrangement is unique to this laser
system. By utilizing the space in the laser head, an efficient laser cavity was developed.
Through the use of an AOM and a Q-switch, pulses on the order of 3 ns and a repetition rate
ranging from 10–100 Hz can easily be created. High gain is obtained because of the dual
Nd:YAG rods. This ability to obtain high gain in a short pulse was coupled to a tunable OPO
cavity. The ability for continuous tunablility from 210–2300 nm through the OPO systemallows this laser system to be very efficient in producing almost any wavelength needed
without requiring a dye laser.

115

References for Appendix A

1

Coherent, Coherent Laser Systems. (Coherent, Santa Clara, CA, 2000).

2

J. Verdeyen, Laser Electronics, 3 ed. (Prentice Hall, Upper Saddle River, 1995).

3

S. H. V. Bruckner, S. Ludwig, Journal of Modern Optics 40 (2), 187 (1993).

4

J. M. Yu. Anikeev, Optical and Quantum Electronics 31, 545 (1999).

5

W. Hong, L. E. Halliburton, D. Perlov, K. T. Stevens, R. K. Route, and R. S.
Feigelson, Optical Materials 26 (4), 437 (2004).

6

N. V. Kondratyuk, A. A. Shagov, K. L. Demidchik, A. M. Yurkin, and A. E. Kokh,
Quantum Electronics 30 (3), 253 (2000).

7Z. G. Figen and O. Aytur, Optics Express 13 (13), 4896 (2005).

8

M. Ostermeyer, K. Mittler, and R. Menzel, Physical Review A 59 (5), 3975 (1999).

9

H. Su, S. H. Tang, Y. Q. Qin, W. J. Zhang, and A. L. Liu, Journal Of The OpticalSociety Of America B-Optical Physics 21 (12), 2102 (2004).

10

L. Zhang, D. H. Li, Q. L. Zhang, C. Y. Li, Z. Y. Wei, B. H. Feng, P. M. Fu, and Z. G.
Zhang, Optics Communications 250 (1-3), 174 (2005).

116

Appendix B.

Photoelectron Simulations of CuH

The following section contains several photoelectron simulations performed in the
PESCAL program. All simulations are of the neutral CuH molecule in its ground electronic
state. All neutral parameters were held constant, with an equilibrium bond length of
1.46263 Å and a vibrational frequency of 1835 cm-1. The only parameter held constant on theanion was that of the calculated frequency of 1564 cm-1. With the anion bond length varied
between 1.534 and 1.600 Å in 0.003 Å steps, a best fit was obtained by comparing the second
vibration and the level of the background noise.
All graphs have the 2S state of copper neutral present, and the ground electronic
1

Σ state of copper hydride plotted in black. All graphs are plotted with the electron kinetic

energy (eKE) on the x-axis and the photoelectron counts on the y-axis. Each graph has thecorresponding simulated anion bond length on the graph, with the fit plotted in red and acorresponding stick spectrum indicating the position of the peak plotted in blue.
The graphs are arranged in decreasing order starting with 1.600 Å and ending with
1.534 Å. Each simulation was performed as an independent Morse oscillator, with each peakhaving a Gaussian profile with a 10 meV distribution at full-width at half-maximum. Fromthese graphs the, anion bond length is estimated to be 1.567(3) Å.

117CountsAnion 1.600

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.1

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.600 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.597

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.2

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.597 Å and a neutral bond length of 1.46263 Å.

118CountsAnion 1.594

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.3

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.594 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.591

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.4

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.591 Å and a neutral bond length of 1.46263 Å.

119CountsAnion 1.585

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.5

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.585 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.588

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.6

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.588 Å and a neutral bond length of 1.46263 Å.

120CountsAnion 1.582

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.7

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.582 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.582

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.8

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.582 Å and a neutral bond length of 1.46263 Å.

121CountsAnion 1.576

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.9

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.576 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.573

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.10

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.573 Å and a neutral bond length of 1.46263 Å.

122CountsAnion 1.57

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.11

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.570 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.567

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.12

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.567 Å and a neutral bond length of 1.46263 Å.

123CountsAnion 1.564

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.13

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.564 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.561

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.14

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.561 Å and a neutral bond length of 1.46263 Å.

124CountsAnion 1.558

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.15

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.558 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.555

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.16

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.555 Å and a neutral bond length of 1.46263 Å.

125CountsAnion 1.552

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.17

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.552 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.549

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.18

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.549 Å and a neutral bond length of 1.46263 Å.

126CountsAnion 1.546

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.19

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.546 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.543

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.20

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.543 Å and a neutral bond length of 1.46263 Å.

127CountsAnion 1.540

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.21

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.540 Å and a neutral bond length of 1.46263 Å.

CountsAnion 1.537

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.22

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.537 Å and a neutral bond length of 1.46263 Å.

128CountsAnion 1.534

0.0

0.5

1.0

1.5

2.0

2.5

eKE (eV)Figure B.23

Simulated spectra of the CuH neutral ground state with an anion bondlength of 1.534 Å and a neutral bond length of 1.46263 Å.

129

Appendix C.

Photoelectron Spectra of Copper Hydrides

Appendix C contains the photoelectron spectra of CuD− and CuH2− as they were not

Photoelectron counts

included in Chapter 4.

3 +

a Σ
2.864 eVβ = 1.21

1 +X Σ
0.443 eV1 +A Σ
3.331 eVβ = 1.00

3.0

2.5

2.0

1.5

1.0

0.5

Electron binding energy (eV)Figure C.1

Photoelectron spectra of CuD− recorded at 355 nm.

0.0

130

Photoelectron counts

Cu
2S
1.235 eV1 +X Σ
0.443 eVβ = 0.42
170 meV

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

Electron binding energy (eV)Figure C.2

Photoelectron spectra of CuD− recorded at 600 nm.

0.2

0.0

Photoelectron counts

131

2

+

A Σg
2

X B2

3.4

3.3

3.2

3.1

3.0

2.9

2.8

2.7

2.6

Electron binding energy (eV)Figure C.3

Photoelectron spectra of CuH2− recorded at 355 nm.

2.5

2.4