Photodissociation Dynamics and Photoelectron Imaging
Spectroscopy of Anions and Anion Clusters
byTodd J. Sanford
B.S., Purdue University, 1997

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirements for the degree ofDoctor of Philosophy
Department of Chemistry and Biochemistry
2004

ii
This thesis for the Doctor of Philosophy degree byTodd J. Sanford
has been approved for the
Department of Chemistry and Biochemistryby_______________________________________________
W. Carl Lineberger______________________________________________
Robert Parson

Date: ____________________________

iii
Sanford, Todd J. (Ph.D. Physical Chemistry)
Photodissociation dynamics and photoelectron imaging spectroscopy of anions andanion clusters.
Thesis directed by Professor W. Carl Lineberger
The objective of this dissertation is to study the broad topic of solvation by
investigating the effects of a solvent environment on small anion solutes in gas-phaseclusters. The dihalide solute chromophore, IBr⎯, is solvated by a controllable amountof CO2 molecules and photodissociated with tunable laser radiation. The subsequent
solvent-dependent dynamics are monitored by detection of ionic photofragments.
The second solute-solvent system involves Cu⎯ solvated by H2O molecules withsolvent effects on the atomic anion solute studied by photoelectron imaging
spectroscopy to monitor solvent-dependent changes in the photoelectron spectrum of
the solute.
The first part of the dissertation involves a detailed discussion of the two
experimental methods with the previously described cluster ion source and tandem
time-of-flight mass spectrometer briefly discussed focusing on modifications made to
the existing system for the study of IBr⎯(CO2)n cluster ions. The remaining portion of
the section describes in detail the design and operation of a new velocity-map
imaging photoelectron spectrometer.
The second part of the dissertation describes the results of thephotodissociation of mass-selected IBr⎯(CO2)n cluster ions. In one-color experiments,
the clusters are excited to two electronic states of the solute anion and the productdistributions measured as a function of cluster size. These results are compared toiv
previous studies on ICl⎯(CO2)n and I2⎯(CO2)n clusters.

The dynamics after

photoexcitation are shown to be highly dependent on the number of solvent molecules
attached and on the identity of solute anion. The solvent is shown to drive suchprocesses as charge-transfer, spin-orbit relaxation, and vibrational relaxation.

A

femtosecond time-resolved experiment is carried out on IBr⎯(CO2)8 to determine
timescales for these events.
The final part of the dissertation investigates the photoelectron imaging
spectroscopy of Cu⎯(H2O)n (n = 1,2) clusters to determine the effect of solvation on an
atomic anion solute. The technique of velocity-map imaging is used to produce highresolution images from which the photoelectron energy spectra and photoelectron
angular distributions are obtained. Solvation shifts the Cu⎯ features to higher binding
energies along with broadening the spectral features. There are also features present
assigned to the water solvent molecules that are confirmed by deuteration
experiments.

v
Acknowledgements
I would like to first thank my advisor, Prof. Carl Lineberger. He is a superb
scientist with a breadth of knowledge that seems to know no bounds. This was
especially useful to a graduate student who not only did not know the answer to aquestion, but often did not even know where to start looking. Almost as important as
his scientific abilities was his ability to manage his group effectively. Throughout mytime here, he has kept a very supportive, laid back atmosphere to the research groupwhile carrying out high level scientific experiments without sacrificing productivity.
He also has the ability to bring in good people to the group that fit this mold. He has
been a great mentor and has taught me much more than just how to carry out
solvation dynamics experiments. I also thank Prof. Barney Ellison with whom I have
had many interactions through group meetings. Barney’s unbridled enthusiasm forscience in general was an enormous inspiration to me. Whenever I had doubts about
continuing in graduate school, all it took was to hear Barney give a group meeting to
get me excited about not only what he was discussing, but also my own work as well.
I want to acknowledge the great help provided to me by our theoretical collaborators
Prof. Robert Parson and Matt Thompson. Dr. Jim Faeder and Dr. Nikki Delaney
were very helpful early in my graduate career, but Robert and Matt have been
invaluable in helping me understand at a deep level what the cluster results from theexperiments really mean.
I want to thank all of the members of the Lineberger group that I have beenprivileged to know and work with over the years. First on the list, I would like tothank Prof. Andrei Sanov, who is now a rising star in the science world at thevi
University of Arizona. I worked with Andrei, then a postdoc, when I first joined thegroup. He was instrumental in really getting me excited about the experiment andbeing a fantastic teacher of not only the ins and outs of our experiment in particular,
but how to be an experimentalist in general. After he left, he was still there to answer
any questions I had and provided valuable information in designing andtroubleshooting the photoelectron imaging spectrometer. I also thank Dr. Sang-Yun
Han, Dr. Mark Taylor, Dr. Bert Callicoatt, Dr. Becky Schwartz, Prof. Gustavo
Davico, Dr. Jeff Rathbone, Django Andrews, Adam Gianola, Jack Barbera, Dr. Evan
Jochnowitz, Dr. Xu Zhang, Prof. Stephen Blanksby, and Dr. Tanya Ramond for being
fantastic coworkers in JILA and good friends outside of JILA.
One of the major strengths of JILA is the unmatched support staff employed
here. The machine shop and electronics shop, in particular, provided the highest levelof knowledge in solving any electrical or mechanical problems and needs that arise in
the lab. In the electronics shop, I thank Mike Whitmore, James Fung-a-Fat, Terry
Brown, and Paul Beckingham. All of them were always willing to drop what they
were doing and answer any questions I had. They were also very easy to work withon the design of electrical components for the lab. The machine shop also has top
notch people who were willing to help and work well with us in designingcomponents for the lab. In particular, I thank Hans Green for his always enthusiastic
and friendly manner I was greeted with whenever I bothered him with design andmachining problems. Finally, I want to thank Brian Lynch in the supply office andDave Tegart in the electronics shop for being good friends outside of JILA.

vii
Last, but certainly not least, I thank my beautiful wife Kristin for all of
support and love throughout this process. I have only known her for roughly the last
four years of this, but these were definitely the most trying years of my time here.
Although, she didn’t always understand what I was doing or why, I knew I had asympathetic ear and someone who would try to empathize if things didn’t go well or
congratulate me if they did. She was also a sport in putting up with hearing me say,“I think I’m almost done with grad school. Probably next semester it will be wrappedup,” for the past four years.

viii
Table of Contents

I.

Introduction……….…………..…………………………………....................1
1.1

Solvation studies using gas-phase clusters.………………….………...…...1

1.2

Photodissociation dynamics of size-selected clusters……….…...........…....2

1.3

Photoelectron imaging spectroscopy…………………………………..…...8

1.4

Dissertation overview………………………………………………...…...14

Chapter 1 References……………………………………………………….......…18

II.

Experimental Hardware and Methods…………………………..………..….22
2.1

Overview………………………………………………………..……..…..22

2.2

Cluster Ion and Discharge Sources…………………………..………..…..24

2.3

Cluster Ion Formation……………………………………..…………..…..28

2.4

Cluster Ion Source Modifications………………………………..……..…31

2.5

Extraction, Ion Optics, and Time-of-Flight Mass Spectrometer………......36

2.6

Photolysis, Secondary Mass Spectrometer, and Detectors…………..........39

2.7

Photoelectron Imaging Considerations and Potential Switch……..............45

2.8

Electron Imaging Stack Design and Installation Concerns………..……....47

2.9

Electron Imaging Optics and Flight Tube………………………………....50

2.10

Imaging Detector and CCD Camera……………………………..………..51

2.11

Laser Systems and Timing Schemes………………………….……….......55

2.11.1 Nanosecond Laser System…………………….……….…………...…..…55
2.11.2 Femtosecond Laser System…………………………………………...…..60

ix
2.11.3 Nanosecond Timing and Data Collection Schemes………………..……..65
2.11.4 Femtosecond Timing and Data Collection Schemes………..……..…..…75
2.11.5 Photoelectron Imaging Timing and Data Acquisition……………...….…78
Chapter 2 References……………………………...………………..………..…….83

Photodissociation Dynamics of IBr⎯(CO2)n Cluster Anions…………….......85

III.
3.1

Introduction……...…………….………………………………………......85

3.2

Spectroscopic characterization of bare IBr⎯………………...………..…....87

3.3

Photofragmentation of IBr⎯(CO2)n upon excitation to the A' state……..…97

3.4

Photofragmentation of IBr⎯(CO2)n upon excitation to the B state.............110

3.5

Time-resolved dynamics following excitation to the A' state…............…118

3.6

Photofragmentation of ICl⎯(CO2)n upon excitation to A' at 790 nm..........121

3.7

Solute dependent photodissociation upon excitation to A'…...……….…124

3.8

Dissociation following excitation to the 2 B 2Σ+1/2 state……...……...…..130

3.9

Time-resolved recombination dynamics……………...……….........……132

3.10

Solvent evaporation energetics………………...………………..…...…..133

3.11

Conclusions……………...………………………..……………..…..…...141

Chapter 3 References……………….…………………………………….....…..143

IV.

Photoelectron Imaging Spectroscopy ………………... …...………..…....145
4.1

Introduction………………………………………………..….........……145

4.2

Photoelectron Imaging of unsolvated Cu⎯………………….……..….…146

4.3

Photoelectron Imaging of Cu⎯(H2O)n complexes…...…....……...….…..155x
4.4

Summary of increasing solvation on Cu⎯(H2O)n complexes..……......….168

Chapter 4 References………………………….…………………………..…....171

V.

Photodissociation Dynamics of Trihalide Anions……….…………...…...172
5.1

Introduction……………………………………………....…...........……172

5.2

Synthesis and Identification of BrICl⎯ and IBr2⎯…………………......…173

5.3

Calculated Structures and Dissociation Energetics………….........….….176

5.4

Photodissociation of BrICl⎯………………………………….….……....178

5.5

Photodissociation of IBr2⎯………………………………………....…….183

5.6

Vibrational Dynamics of IBr⎯ Photofragments………...……..………....188

5.7

Wave Packet Dynamics near Dissociation Threshold…………...….…..192

Chapter 5 References……….……………………………………………...…...198

VI.

Conclusion………………………………...……...…………………..…...201

Bibliography………………….………………………………….…………..……..206

xi
List of Tables

Table 3.1

Photofragmentation branching ratios for IBr⎯(CO2)n at 790 nm……....100

Table 3.2

Photofragmentation branching ratios for IBr⎯(CO2)n at 760 nm............103

Table 3.3

Photofragmentation branching ratios for IBr⎯(CO2)n at 675 nm……....106

Table 3.4

Photofragmentation branching ratios for IBr⎯(CO2)n at 355 nm……....114

Table 3.5

Photofragmentation branching ratios for IBr⎯(CO2)n at 395 nm……....117

Table 3.6

Photofragmentation branching ratios for ICl⎯(CO2)n at 790 nm……....123

xii
List of Figures

Figure 1.1

Calculated potential energy curves for I2⎯ and ICl⎯ anions...……..…….4Figure 1.2

The photofragmentation products after photodissociationof size-selected IBr⎯(CO2)n clusters………………………............…….7

Figure 1.3

Schematic diagram of the photoelectron spectrum of thephotodetached AB⎯ anion…………………………………….........….10

Figure 1.4

Velocity map imaging electrode configuration and focusing ……...….13

Figure 1.5

Photoelectron imaging spectrometer schematic……………...........….15

Figure 2.1

Cluster ion source and tandem time-of-flight mass spectrometer…….23

Figure 2.2

Time-of-flight mass spectrum of water in the copper
discharge source……………………………………..…….………......27

Figure 2.3

Cross sections and dimensions of the commercial versus
homemade faceplates for a General Valve….………………..……....33

Figure 2.4

Time-of-flight mass spectrum of the midsized IBr⎯(CO2)ncluster ions………………………………………..………………..…35

Figure 2.5

Cluster ion source and tandem time-of-flight mass
spectrometer with potential switch and electron velocity
map imaging components………………………………..….……..…49

Figure 2.6

Electron velocity map imaging schematic…………….....………..….52

Figure 2.7

Side view of the chamber with the imaging stack installed………….53

Figure 2.8

Nanosecond Nd:YAG-pumped OPO laser schematic……………..…56

Figure 2.9

Femtosecond laser system for pump-probe experiments…………….61

Figure 2.10

Ion and laser event timings for the nanosecond experiments……...…67

Figure 2.11

Analog signal processing electronics……...…..……..…………...….71

Figure 2.12

Relative cross section data collection and calculation
schematic……………………………..………………….……………73

xiii
Figure 2.13

Simulated data showing typical analog image data versus
event counted image data……………………………...……..……….80

Figure 3.1

Calculated potential energy curves for the lowest six
electronic states of IBr⎯…………………………...……...……...……88

Figure 3.2

Relative cross sections for the A' absorption band…………………...89

Figure 3.3

Calculated absorption spectrum for unsolvated IBr⎯……..………..….91

Figure 3.4

Neutral Br time-of-flight spectra of photodissociatedIBr⎯ at 790 nm with the laser polarized parallel to theion flight axis and perpendicular to the flight axis……...……..….....94

Figure 3.5

Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the A' 2Π state with 790 nm photons………..…….99

Figure 3.6

Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the A' 2Π state with 760 nm photons………......…102

Figure 3.7

Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the A' 2Π state with 675 nm photons……..........…105

Figure 3.8

Recombination efficiency across the A' absorption bandin IBr⎯(CO2)n cluster ions……………...…………..……..………..…109

Figure 3.9

Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the B state with 355 nm photons………………….113

Figure 3.10 Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the B state with 395 nm photons……….............…116
Figure 3.11 The IBr⎯(CO2)8 recombination probability as afunction of time following excitation to the A' 2Π statewith a 790 nm pump pulse…………………………………..…….....119
Figure 3.12 Photofragmentation products of ICl⎯(CO2)n cluster ions
upon excitation to the A' 2Π state with 790 nm photons…………….122
Figure 3.13 Calculated potential energy curves for I2⎯ and ICl⎯anions…………………………………………..………………..…..125
Figure 3.14

Recombination percentage for the three solutes as afunction of the number of CO2 molecules upon excitation
to the A' 2Π state………………………………………..……..…….126

xiv
Figure 3.15.
Figure 3.16

Representative cluster structures calculated for ICl−, IBr−,and I2− with various numbers of CO2 solvent molecules….……..….128
Recombination percentage for the three solutes as afunction of number of CO2 molecules following excitation
to the B 2 2Σ state………………………………..………………..…131

Figure 3.17

Average number of solvent loss for the three productchannels after excitation with 790 nm to A' as a functionof initial cluster size…………………………………….....……..….135

Figure 3.18

Average solvent loss for the recombined products
after excitation to A' in IBr⎯(CO2)n clusters at the threelabeled wavelengths as a function of initial cluster size…………..…136

Figure 3.19 Average number of solvent loss for the three productchannels after excitation with 355 nm to B as a functionof initial cluster size……………………………………...........……..140
Figure 4.1

Energy level diagram for the ground state of Cu⎯ and the three
low-lying state of Cu neutral………………………………....……...147

Figure 4.2

Photoelectron images of Cu⎯ taken at 964.7 nm (1.285 eV)……......150

Figure 4.3

Energy scale calibration factor as a function of voltage
placed on the extraction plate for Cu⎯ photodetachment
at 964.7 nm………………………………………………...……..….152

Figure 4.4

Photoelectron energy spectrum extracted from the BASEX
reconstructed image of Cu⎯ photodetachment at 532 nm(2.33 eV)…………………………………………………………….153

Figure 4.5

Photoelectron energy spectrum extracted from the BASEX
reconstructed image of Cu⎯ photodetachment at 355 nm(3.49 eV)………………………………………………..………..…..156

Figure 4.6

Photoelectron energy spectrum of Cu⎯(H2O) taken at
568.8 nm (2.18 eV)…………………………………....………….....158

Figure 4.7

Photoelectron energy spectrum of Cu⎯(D2O) taken at
568.8 nm (2.18 eV)……………………………………..……..….....160

Figure 4.8

The potential energy surfaces for the Cu⎯(H2O) anion andneutral complexes………………………………..……………..…..162

xv
Figure 4.9

Photoelectron energy spectrum of Cu⎯(H2O) anion at 355 nm(3.49 eV)……………………………………………..…………......164

Figure 4.10

Photoelectron imaging spectrum of Cu⎯(H2O)2 at 521.2 nm(2.379 eV)………………………………………..…..…………..…166

Figure 4.11

Photoelectron energy spectrum of Cu⎯(H2O)2 at 313 nm(3.961 eV)……………………………...……………....………..….167

Figure 4.12

Composite photoelectron spectrum of the Cu⎯(H2O)n(n = 0,1,2) clusters………………………………………………….169

Figure 5.1

Mass Spectra of precursor anions formed by co-expanding
ICl and CH3Br seeded in Ar carrier gas, and IBr andCH3Br in Ar……………………………………………..……...…..175

Figure 5.2

Energetics of the BrICl⎯, IBr2⎯, and I3⎯ anions…………..……..…..177

Figure 5.3

Photofragment ion mass spectrum obtained in thephotodissociation of BrICl⎯ (the 79BrI35Cl⎯ isotope)
at 350 nm and 392 nm……………………………………..……..…180

Figure 5.4

Delay-dependent pump-probe signals in the photodissociation of BrICl⎯……………………………………...........…..182

Figure 5.5

Photofragment ion mass spectrum of the photodissociation of IBr2⎯ (the 79BrI81Br⎯ isotope) at 402 nm………........184

Figure 5.6

Delay-dependent pump-probe signals in the photodissociation of IBr2⎯……………………………….…………..……186

Figure 5.7

Equilibrium geometries of the BrICl⎯, IBr2⎯, and I3⎯ anions
and their fragments……………………….....……………………...190

Figure 5.8

IBr⎯ absorption signals calculated by propagating wavepackets on the MP2 and QCISD potentials…………………….…...197

Chapter 1
Introduction

1.1

Solvation studies using gas-phase clusters
The process of solvation is of central importance in chemistry and has been

the subject of numerous experimental and theoretical studies. The presence of asolvent influences the outcome of a chemical reaction through the interaction of thelocal environment with the individual species undergoing the reactions. The solvent,on one level, provides an energy bath for the stabilization of energetic products
formed during reactions and provides physical barriers to the motion of the reactive
species. On a more subtle level, the solvent perturbs the potential energy curves thatgovern these reactions. Solvation has been shown to be of fundamental importance insuch diverse areas as biological activity and atmospheric processes.1,2
In condensed phase media, the effects of solvation are easily observed bymonitoring macroscopic changes in solute properties or solvent-dependent reaction
products. On a microscopic level, understanding of the solvation process iscomplicated by the poorly defined local environments around solutes in thecondensed phase. Studies of these effects will necessarily sample very large numbersof solvent molecules in undetermined configurations about the solutes. The goal ofthis dissertation is study solvation at a microscopic, molecular level. To achieve this,the detailed interactions between the solute and solvent along with the solvent-solvent
interactions must be explored. Information on these interactions at the molecularlevel is lost due to the statistical nature of descriptions of condensed phase media.

2
The work presented here attempts to avoid the averaging of molecular details inherent
in condensed phase experiments by using gas-phase clusters to provide a controllable,
well-defined solvent environment that mimics condensed phase behavior.3-7 In ionicclusters,8 the techniques of ion mass spectrometry9 allow for the study of stepwise
microsolvation, since clusters of a desired size can be selected.

Supersonic

expansions10 allow for the production of a wide range of cluster sizes from solutes
with one or two solvent molecules up to clusters for which the solute is completely
enclosed by a solvent cage. The entire ensemble of anionic clusters is introduced intoa Wiley-McLaren time-of-flight mass spectrometer11 where the clusters are then mass
selected, thus allowing for the control of the solvent environment by carrying out
experiments as a function of cluster size.

1.2

Photodissociation dynamics of size-selected clusters
The behavior of a chromophore following photodissociation has been used to

study solvation effects since the work of Franck and Rabinowich12 in 1934. In thatstudy, neutral I2 was photodissociated in a liquid solvent and found on occasion toundergo bond recombination or “caging”, which was attributed to collisions of the
dissociated I atoms with the solvent. This process has also been confirmed bynumerous subsequent studies.13-17

Similar studies on the time-resolved caging

dynamics have been carried out on the I2⎯ anion in liquids by the Barbara group.18,19
As mentioned above, the solvent effects in these studies are averaged over a large
number of solvent configurations, so that the effects of specific solvent configurations
around the solute are unresolved.

3
The Lineberger group carried out solvent-dependent photodissociation studies
in size selected gas-phase clusters, using I2⎯ as the solute chromophore and CO2,
OCS, N2O, and Ar as the solvent.20-26 The I2⎯ anion has a measured bond strength27,28
of 1.01 eV and the typical binding energy of the above solvents to this solute isaround 0.2 eV.22,29 In the presence of four or five solvent molecules, the total solvent
binding energy becomes comparable to the solute bond strength. Accurate potential
energy curves30-32 have been generated which allow for a complete description of theeffects of the solvent molecules on the solute anion. These calculated potential
energy curves30,32 are shown in Fig. 1.1 for unsolvated ICl⎯ and I2⎯.
The first studies on solvated I2⎯ in gas phase clusters involved the excitation of
the solute chromophore to the A' electronic state as shown in Fig. 1.1(a). In this case,the excited state is asymptotically degenerate with the ground state and very little
perturbation to the potentials is needed to induce recombination on the ground state.
The results showed that the extent of recombination on the ground state increases
with increasing cluster size. This recombination channel turns on with the addition ofa sixth CO2 solvent molecule and reaches 100% with the 16th solvent molecule,corresponding to a complete solvent shell around the solute.22 Time-resolved pumpprobe studies determined the timescales for this recombination for I2⎯ in CO2
clusters.22,25,33 Typical timescales were on the order of tens of picoseconds with therecombination time decreasing with increasing cluster size. These recombination
times were comparable to those determined in the similar experiments on I2⎯ inliquids18,19 leading to the conclusion that gas phase clusters, in this case, do indeed
produce solvent-dependent behavior similar to that in condensed phase.

4

(a)

(b)
B 2Σ+1/2

2 +B Σ g,1/2

EnergyI⎯ + I*Cl⎯ + I*1

A' 2Πg,1/2

I⎯ + I

X 2Σ+u,1/2
Internuclear separation

0I⎯ + Cl*A' 2Π1/2

I⎯ + ClCl⎯ + I

X 2Σ+1/2

Internuclear separation

Figure 1.1: Calculated potential energy curves for (a) I2⎯ and(b) ICl⎯ anions. The original calculations are found in references
cited in the text. The states in bold are accompanied by their statelabels.

5
Subsequent studies on the I2⎯ solute centered on UV excitation to the B
electronic state.20,21 This excitation in size-selected clusters leads not only to thesolvent-induced recombination seen in A' excitation, but now includes a spin-orbit
relaxation step that must occur before recombination. The recombination on the
ground state was shown20,21 to be depend even more strongly on the number ofsolvent molecules (and thus the structure of the solvent cage around the solute), due
to the additional perturbations to the potential curves required for spin-orbit
relaxation. Pump-probe studies21 determined that the additional spin-orbit step before
recombination produced no increase in the recombination time as the observed
recombination times for A' and B excitation were very similar. A charge-transfer
mechanism34 has been implicated in the spin-orbit relaxation step, but is not directly
observable in the experiments due to the symmetric nature of the solute.
Photodissociation experiments were also carried out on ICl⎯(CO2)nclusters.26,35 For this asymmetric solute, the two photodissociated products can be
distinguished by ion mass spectrometry. In both excitation to A' and B states (seeFig. 1.1 (b)), the solvent was again shown to drive recombination and spin-orbit
relaxation.

Solvent-induced charge transfer could now be observed directly.

However, ICl⎯(CO2)n clusters showed decreasing recombination with increasingcluster size, in stark contrast to the I2⎯ case for reasons to be discussed in Ch. 3. Thisbehavior coupled with a very low two-photon cross section made ICl⎯(CO2)n clusters
unsuitable for pump-probe experiments to determine recombination timescales.
The photodissociation of IBr⎯(CO2)n clusters is one of the projects presented inthis dissertation. This system is well-suited for photodissociation solvation studies,6because it provides an antisymmetric solute for which time-resolved experiments canbe carried out. The photofragmentation of IBr⎯(CO2)n clusters is shown schematicallyin Fig. 1.2. There are three classes of products that are possible after exciation of theIBr⎯ chromophore. Both the Br⎯-based and I⎯-based products represent dissociationof the solute anion, while the IBr⎯-based products arise from the cage effect observedin the condensed phase and in gas-phase cluster studies of I2⎯.

The anionicphotoproducts are unambiguously detected by a mass-sensitive detection method
described in Ch. 2. The product ratios as a function of cluster size reflect the stepwise effects of increasing solvation on the solute anion. The mass-selective detection
scheme allows for not only differentiation among the three classes of photoproducts,
but also for the determination of the number of solvent molecules evaporated fromthe products. The solvent evaporation data makes it possible to distinguish between
ionic dissociation products that are correlated with different spin-orbit states of theneutral fragments. The Br⎯-based products formed with spin-orbit excited I retain
more solvent molecules than those Br⎯-based products formed with ground state I and
are easily distinguished in the experimental detection.
The one-photon photofragmentation of size-selected clusters clearly
demonstrates the effect of the solvent on the solute photodissociation behavior.
Recombination, charge-transfer, and spin-orbit relaxation were shown to depend on
the number of solvent molecules (and thus the structure of the solvent cages) aroundthe solute. However, these one-photon studies only probe the state of the system atroughly 10 µs after photodissociation when detection of the ionic products occurs.

7hνI⎯-basedBr⎯-based

Uncaged Products

IBr⎯-based
Caged Products

Figure 1.2: The photofragmentation products after photodissociationof size-selected IBr⎯(CO2)n clusters. The amount of solvent molecules
retained on the products identifies the states on which the dissociation
products are formed as well as the amount of vibrational relaxation on
the ground state for the recombined products.

8
Pump-probe studies shed light on dynamics at earlier times after photodissociation.
This method probes the system only when the solute has recombined on the groundstate; the timescales of individual processes such as charge-transfer and spin-orbit
relaxation along with any other excited state dynamics cannot be directly measured.
Such phenomena as potential wells formed on the excited electronic states of thesolute and reorganization of the solvent cage are thought to be critical to the overall
recombination dynamics, but are not observed using the photodissociation methods
described above. Anion photoelectron spectroscopy is ideally suited for these excitedstate dynamics studies. After photodissociation, the excess negative charge resides
on species (IBr⎯, Br⎯, or I⎯ in the case of IBr⎯(CO2)n cluster photodissociation) that areeasily photodetached. The time evolution of the photodetachment signal will yieldsnapshots of the system throughout the entire excited and ground state dynamics andis not limited to a narrow window on the ground state. This will provide a much
more powerful tool in studying the solvation dynamics of these dihalide systems.

1.3

Photoelectron imaging spectroscopy
Anion photoelectron spectroscopy (PES)36 has proven to be an invaluable tool

for the study of the structures and energetics of both anions and neutrals.

An

important parameter obtained from anion photoelectron spectroscopy is the electron
affinity of the neutral molecule, which can be used in a thermochemical cycle for thedetermination of bond strengths.37,38 Vibrational frequencies of neutral molecules canbe extracted from vibrational progressions in the spectra while those of the anions canbe determined from vibrational hot bands, as can term energies of electronic states,9
spin-orbit splittings and bond lengths of both the anion and neutral. A powerfulapplication of anion PES is the study of reactive neutral species formed via thephotodetachment of the precursor anion that cannot otherwise be directly prepared.
Photodetachment of anions is also used to prepare neutral species at the transition
state geometry on the neutral ground state surface, in order to study the subsequent
reactions.39,40
The anion photoelectron spectroscopy experiment is simply described by theprocess:

AB⎯ + hν → AB + e⎯

(1.1)

where AB⎯ is the precursor anion species, hν is the photon energy, AB is the neutral
species formed from photodetachment, and e⎯ is the photodetached electron. The
process is depicted schematically in Fig. 1.3. The photodetached electron kinetic
energies (eKE) are measured to yield the photoelectron spectrum shown in Fig.1.3.
The eKEs are typically converted to electron binding energies (eBE) by subtracting
the eKE from the photon energy yielding spectra independent of the photon energy.
The Franck-Condon Principle, stating that photodetachment of the electron is much
faster than any nuclear motion, constrains the geometries of the anion and neutral tobe equivalent upon photodetachment.

For anions and neutrals with similarequilibrium geometries, the electron affinity can be measured as the transition
between the lowest vibrational energy levels of the ground states of the two species.

10← Electron binding energyhνAB

VDE(AB–)

EA(AB)

AB–
Figure 1.3: Schematic diagram of the photoelectron spectrum of thephotodetached AB⎯ anion at a fixed photon energy, hν. The spectrum
represents transitions to vibrational states of the ground electronic stateof the neutral AB. The transitions representing the EA and VDE discussed
in the text are shown.

11
However, if the equilibrium geometries are significantly different, this transition isvery weak and the vertical detachment energy (VDE) is then reported as the transition
from the lowest vibrational state of the anion to the state in the neutral with same
geometry as the anion. Photoelectron angular distributions are also obtained from thephotoelectron spectra and used to determine the anisotropy parameter, β.41 The valueof β reflects the symmetry of the electronic orbital from which the photoelectron is
detached.

This analysis is rigorous for atomic systems, but is less exact formolecules. The value of β is experimentally obtained by taking photoelectron spectra
at multiple laser polarizations.42
Photoelectron imaging of negative ions43-48 can trace its roots to thepioneering imaging experiments49 of Houston and Chandler on photofragments
formed from the photodissociation of neutral molecules. The images simultaneously
provide the full three dimensional velocity distributions of all of the particles, fromwhich the individual velocities (kinetic energies) and angular distributions can beobtained. The photoelectron energy spectra as described above are extracted fromphotoelectron images that now also directly contain the photoelectron angular
distributions (PAD) necessary for a description of the properties of the orbitals fromwhich the photoelectrons are detached.50 These two observables (kinetic energy andangular distributions) are measured simultaneously adding enormously to theapplication of this technique, which can be extended to the time domain.43,45,51
A key step in the development of photoelectron imaging as an effective
experimental tool was the introduction of velocity-map imaging52 by Parker andEppink.

The ability to obtain high resolution images, and thus well resolved

12
photoelectron energy spectra, had suffered from blurring associated with theextended, non-point source of the anions from which photodetachment occurs. The
quality of the images is determined in large part by how the three-dimensional (3D)
photoelectron distributions are mapped onto the two-dimensional (2D) detector.
Parker and Eppink recognized that the quality of the 2D images, from which the 3D
distributions are reconstructed, depends on the configuration of the electrodes used to
extract the electrons from the photodetachment region to the detector.

A threeelement open electrode configuration was found to minimize distortions to images
caused by grid electrodes as well provide unique focusing characteristics. Parker andEppink determined that when appropriate voltages are applied to the electrodes theparticles with the same initial velocity vector are mapped onto the same point on thedetector regardless of their initial position upon photodetachment, hence the term
“velocity map imaging.” This allows for the acquisition of high quality images
despite the use of an extended ion source. The velocity map imaging electrodeconfiguration and mapping characteristics are depicted in Fig 1.4 along with typicalelectrode voltages used for the imaging experiments described within.
The high quality 2D images detected do not represent the full 3D electron
distribution necessary to obtain the photoelectron energy spectra and angular
distributions. The task is to reconstruct the 3D distribution from the 2D projection on
the detector. For images where the electrons’ photodetachment kinetic energy ismuch less than the kinetic energy acquired in the acceleration stage, cylindricalsymmetry exists and the 2D projection can be transformed into the 3D distribution byuse of the Abel integral.53 This cylindrical symmetry requirement dictates that the

13

-30

0 134.4

Focal Plane

3 mm

Figure 1.4: Velocity map imaging electrode configuration withexperimental voltages applied shown in volts with the focusing
properties of the electrons. The different colors for the trajectories
represent different initial velocities of the photodetached electrons.
The schematic demonstrates that electrons with the same initial
velocity in the focal plane are mapped onto the same point on thedetector despite an extended ion source of 3 mm.

14photodetachment laser polarization be parallel to the plane of the detector. However,this transform can lead to reconstructed images that are very noisy and containsingularities due to the form of the transform integral.

Reisler and coworkers

developed the basis set expansion (BASEX) method53 to produce much higher quality
reconstructions that avoid the pitfalls of Abel transform methods.

This method

expands the 2D image projection in a basis set of well-behaved, Gaussian-likefunctions. The full 3D distribution is then reconstructed as a linear combination ofthese well-behaved functions. The BASEX software takes a 2D slice through the 3D
distribution reconstruction from which the photoelectron spectrum as a function ofcamera pixel (later converted to electron kinetic or binding energy) is produced andthe values of β as a function of pixel are calculated. The imaging process is shown
schematically in Fig. 1.5 as well as the associated sample raw and BASEX
reconstructed images.

1.4

Dissertation overview
The remainder of the dissertation is broken into five chapters. Chapter 2

describes the cluster ion sources and the Wiley-McLaren time-of-flight and reflectronmass spectrometers. Various modifications made to the existing system forproduction of the IBr⎯(CO2)n cluster ions are presented along with descriptions of the
femto- and nanosecond laser systems employed here. This chapter also describes thedesign and implementation of the velocity map imaging spectrometer.

15

MCP and phosphor-30.0 V 0 V 134.4 V
Field free flight tube

CCD
CameraElectronsE

Ion flight path

BASEX reconstructed
image“Raw” 2-D
photoelectron
image

Figure 1.5: Photoelectron imaging system including the electrodes arranged
in velocity-map imaging mode. A typical raw image recorded with the CCD
camera as well as the BASEX reconstruction of the raw image are shown.

16
Chapter 3 discusses the photofragmentation studies carried out on IBr⎯(CO2)nclusters to determine the effect of the CO2 solvent on the dihalide solute anion. The
bare IBr⎯ solute is first spectroscopically characterized to determine absorption crosssections and the bond strength of the solute.

The photoproduct ratios are thenmeasured as a function of cluster size upon excitation to two electronic states of thesolute anion. A time-resolved pump-probe experiment is carried out to determine therecombination time after excitation to the lower electronic state studied.

Thesolvation dynamics of IBr⎯(CO2)n are then discussed in the context of a comparison
with the dynamics of ICl⎯(CO2)n and I2⎯(CO2)n clusters studied previously in thisgroup.
The photoelectron imaging spectroscopy of Cu⎯ and Cu⎯(H2O)n (n = 1,2)
clusters is the subject of Chapter 4. The copper anion is used to provide an atomic
system for characterization and optimization of the new imaging spectrometer. The
photoelectron spectra of the Cu⎯(H2O)n anion complexes are used to determine theeffect of the solvent on the electronic structure of the Cu⎯ solute and to characterize
the neutral complexes formed after the photodetachment of the anion. This work
supplements previous time-resolved experiments and calculations on these systems
that investigate the dynamics of the complex after photodetachment.54-56
Chapter 5 describes experiments involving the photodissociation of mixedtrihalide anions to produce vibrational wavepackets in the dihalide anion
photoproduct. The time-resolved experiments use a second probe photon to monitor
the evolution of the wavepackets on the dihalide ground state potential. Finally,

17
Chapter 6 presents a summary of the dissertation work as well as conclusions andfuture directions that build on the results presented here.

18
Chapter 1 References
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M. A. Johnson and W. C. Lineberger, in Techniques for the Study of Ion
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S. Nandi, A. Sanov, N. Delaney, J. Faeder, R. Parson, and W. C. Lineberger,
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A. Sanov, T. Sanford, S. Nandi, and W. C. Lineberger, J. Chem. Phys. 111(2), 664 (1999).

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J. M. Papanikolas, J. R. Gord, N. E. Levinger, D. Ray, V. Vorsa, and W. C.
Lineberger, J. Phys. Chem. 95 (21), 8028 (1991).

23

J. M. Papanikolas, P. J. Campagnola, V. Vorsa, M. E. Nadal, H. K. Buchenau,
R. Parson, and W. C. Lineberger, in The Chemical Dynamics and Kinetics ofSmall Radicals, edited by K. Liu and A. Wagner (World Scientific PublishingCo., Singapore, 1995), Vol. 6, pp. 616.

24

J. M. Papanikolas, V. Vorsa, M. E. Nadal, P. J. Campagnola, J. R. Gord, andW. C. Lineberger, J. Chem. Phys. 97 (9), 7002 (1992).

25

V. Vorsa, S. Nandi, P. J. Campagnola, M. Larsson, and W. C. Lineberger, J.
Chem. Phys. 106 (4), 1402 (1997).

26

M. E. Nadal, Ph.D. Thesis, University of Colorado, 1996.

27

B. J. Greenblatt, M. T. Zanni, and D. M. Neumark, Chem. Phys. Lett. 258 (5),523 (1996).

28

M. T. Zanni, T. R. Taylor, B. J. Greenblatt, B. Soep, and D. M. Neumark, J.
Chem. Phys. 107 (19), 7613 (1997).

29

J. M. Papanikolas, V. Vorsa, M. E. Nadal, P. J. Campagnola, H. K. Buchenau,
and W. C. Lineberger, J. Chem. Phys. 99 (11), 8733 (1993).

30

P. E. Maslen, J. Faeder, and R. Parson, Chem. Phys. Lett. 263 (1-2), 63(1996).

31

P. E. Maslen, J. M. Papanikolas, J. Faeder, R. Parson, and S. V. ONeil, J.
Chem. Phys. 101 (7), 5731 (1994).

32

J. Faeder, Ph.D. Thesis, University of Colorado, 1998.

20
33

B. J. Greenblatt, M. T. Zanni, and D. M. Neumark, J. Chem. Phys. 112, 601(2000).

34

N. Delaney, J. Faeder, and R. Parson, J. Chem. Phys. 111, 651 (1999).

35

M. E. Nadal, P. D. Kleiber, and W. C. Lineberger, J. Chem. Phys. 105 (2), 504(1996).

36

K. M. Ervin and W. C. Lineberger, in Advances in Gas Phase Ion Chemistry,edited by N. G. Adams and L. M. Babcock (JAI Press, Greenwich, 1992),
Vol. 1, pp. 121.

37

K. M. Ervin, S. Gronert, S. E. Barlow, M. K. Gilles, A. G. Harrison, V. M.
Bierbaum, C. H. DePuy, W. C. Lineberger, and G. B. Ellison, J. Am. Chem.
Soc. 112, 5750 (1990).

38

S. J. Blanksby, T. M. Ramond, G. E. Davico, M. R. Nimlos, S. Kato, V. M.
Bierbaum, W. C. Lineberger, G. B. Ellison, and M. Okumura, J. Am. Chem.
Soc. 123 (39), 9585 (2001).

39

D. M. Neumark, Science 272 (5267), 1446 (1996).

40

D. M. Neumark, PhysChemComm 11, 76 (2002).

41

J. Cooper and R. N. Zare, J. Chem. Phys. 48, 942 (1968).

42

T. M. Ramond, Ph.D. Thesis, University of Colorado, 2001.

43

A. V. Davis, R. Wester, A. E. Bragg, and D. M. Neumark, J. Chem. Phys.
118, 999 (2003).

44

R. Mabbs, E. Surber, and A. Sanov, Analyst 128, 765 (2003).

45

R. Mabbs, K. Pichugin, E. Surber, and A. Sanov, J. Chem. Phys. 121, 265(2004).

46

E. Surber, R. Mabbs, and A. Sanov, J. Phys. Chem. A 107 (40), 8215 (2003).

47

A. Osterwalder, M. J. Nee, J. Zhou, and D. M. Neumark, J. Chem. Phys. 121(13), 6317 (2004).

48

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Phys. 104, 3515 (2004).

49

D. W. Chandler and P. L. Houston, J. Chem. Phys. 87 (2), 1445 (1987).

21
50

K. L. Reid, Ann. Rev. Phys. Chem. 54, 397 (2003).

51

L. Wang, H. Kohguchi, and T. Suzuki, Faraday Discuss. 113, 37 (1999).

52

A. Eppink and D. H. Parker, Rev. Sci. Instr. 68 (9), 3477 (1997).

53

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73, 2634 (2002).

54

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Phys. 121, 5676 (2004).

55

M. S. Taylor, J. Barbera, C. P. Schulz, F. Muntean, A. B. McCoy, and W. C.
Lineberger, J. Chem. Phys. submitted (2004).

56

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Phys. 121, 5688 (2004).

22
Chapter 2
Experimental Hardware and Methods

2.1

Overview
The various experiments carried out in this work involved the use of a single

tandem time-of-flight mass spectrometer. However, numerous modifications weremade throughout the course of these experiments to meet the requirements of thespecific chemical systems used and the various detection methods. The system is firstdescribed in its original configuration with the modifications presented as theyoccurred. The system is used to produce anions and anion clusters of various
precursor species. These ions are injected into a Wiley-McLaren time-of-flight mass
spectrometer1 where they are mass-selected and intersected with laser pulses ofvarying wavelength and pulse duration.

The photodissociated anion and neutralfragments are then detected with two microchannel plate detectors. The significant
modification to the system involves the design and implementation of a velocity-map
imaging2 photoelectron spectrometer into the existing apparatus. The lasers used togenerate the nanosecond and femtosecond pulses along with the various opticalcomponents are also discussed in some detail. An overall schematic of the apparatus
before implementation of the electron imaging system is depicted in Fig. 2.1.
.

23

Pulsed Valve

Off-axis MCP

In-line
MCP

Acceleration Stack
And Ion Optics

Electron
Gun

Mass
Gate

Reflectron

Parentions andneutrals

Laser 200-2000 nm

Components Floated at 2.5 kV
except as mentioned in text.

Figure 2.1: Cluster ion source and tandem time-of-flightmass spectrometer.

2.2

Cluster Ion and Discharge Sources

24
The IBr⎯(CO2)n cluster ions are created by crossing a neutral supersonic
expansion with a high energy beam of electrons.3 The supersonic neutral expansion
is formed by passing a backing gas, which is typically neat CO2 or CO2 diluted withAr, at pressures of 30 to 50 psig over a stainless steel reservoir containing a solid
sample of iodine monobromide (Aldrich).

The methods for the production ofICl⎯(CO2)n and I2⎯(CO2)n cluster ions are described in detail elsewhere.4-7
The most important factor in the formation of the IBr-containing species isproviding sufficient time for the IBr vapor to saturate the lines and valve.

A

passivation time of about a week is required after any thorough cleaning of thestainless steel lines or the pulsed valve.

A qualitative observation is that until

sufficient saturation of the lines and valve is achieved the dominant ions in the mass
spectra are I⎯ along with I⎯-based clusters. There is also an I2⎯(CO2)n progression
along with the ubiquitous (CO2)n⎯ peaks. Initially, there are no IBr⎯-based peaks. As
the IBr saturation of the lines proceeds, the mass spectra show the IBr⎯(CO2)n peaks
growing in at the expense of the corresponding I⎯- and I2⎯-based cluster peaks withthe (CO2)n⎯ ions remaining nearly constant in intensity. Once the saturation has beenachieved, IBr⎯ clusters then persist as the dominant peaks until the next cleaning. The
IBr reservoir is typically heated to about 40°C with the pulsed valve kept at a few
degrees higher to avoid clogging. For the most stable ions, the CO2 was initiallymixed in the reservoir. The outlet is then left open with the inlet closed as the CO2
bypasses the reservoir. The production schemes for the trihalide anions are discussed
in Ch. 5.

25
The supersonic expansion is formed by expanding the gas mixture through a
Series 9 General Valve with an 800 µm aperture on the faceplate. The poppets foundto perform the best were made from KELF and Teflon. The valve is driven at 30, 50,
or 80 Hz by a trigger pulse from a Stanford Delay Generator. The TTL trigger isamplified and varied in width before being sent to the valve solenoid. The voltage ofthe driving pulse can be varied and it is found that the most stable ion production
occurs when a voltage of around +100 V is applied. The driving pulse duration is200–600 µs with the actual gas pulse slightly longer. The production of large clusters
with good intensities is also very dependent on the adjustment of the valve. The
optimal adjustment has the pulse barely discernible to the touch at 30 Hz and a pulse
width of 450 µs.

The body of the valve can be heated or cooled with four

thermoelectric pads along with water cooling lines attached to copper pads that are incontact with the valve housing block. A grounding cable is also attached to the faceplate to prevent charging of the valve due to its electrical isolation from the chamber.
An electron gun produces the high energy beam of electrons via thermionicemission from a bent thoriated iridium filament (Electron Technology, Inc.). Thisfilament is heated by passing approximately 6 amps through it. The electrons are
extracted from the filament region and mildly focused by an anode plate that isnegatively biased from the filament by about 80–200 V. Both the filament and theanode plate are floated at +1000 V. After extraction the electrons encounter an einzel
lens that provides additional focusing of the electrons. The lens consists of two plates
held at ground and a middle plate at around –1000 V. The two ground plates haveapertures of roughly 0.054″ and the middle plate has an aperture of 0.30″. The

26electron beam is steered by a set of horizontal and vertical deflectors that typicallyhave modest voltages of 0–50 V. The electron gun is positioned in a perpendiculararrangement to the expansion nozzle about 5 cm away from the nozzle assembly.
The electron beam axis in the gun is positioned as close as possible to the end of thenozzle in the horizontal direction to ensure that the electron beam intersects a highdensity portion of the expansion close to the orifice.

The vertical and horizontal

deflectors provide a fine adjustment of this condition with the electron beamsometimes skimming the face plate. The electron current is measured by a Faradaycup positioned on the opposite side of the valve with electron currents in the range ofa few hundred µA.
A discharge source is used for the production of Cu⎯ and various clusters
consisting of Cu⎯ solvated by water along with impurities, such as copper hydroxides
and copper oxides.

A representative mass spectrum is shown in Fig. 2.2 with thepeaks of experimental interest assigned and labeled. The discharge replaces theelectron gun as the source for ion production. A General Valve analogous to the one
described above is used, but slightly modified by having four threaded holes drilled
into the face plate. A Macor block is attached to this faceplace with Teflon screws.
The Macor block has a 4 mm orifice in the center. A copper rod and a stainless steel
rod are brought in from the sides of the block through a channel perpendicular to thegas flow channel with the 4 mm orifice. The two rods are separated by typically 1.5
mm in the center of the orifice. The stainless steel rod is kept at ground and thecopper rod is held at −1000 to −3000 V. When the valve is pulsed and the gas
expansion is present, there is a discharge from the copper rod to the grounded

27

65

Intensity

Cu⎯(H2O)

65

Cu⎯(H2O)2

63

Cu⎯
65

Cu⎯

60

70

80

90

100

110

120

130

Mass (amu)
Figure 2.2: Time-of-flight mass spectrum of water in the copper
discharge source. The ions used in the experiments are denoted witharrows along with the two isotopes of copper atomic anion. The otherpeaks represent species such as copper oxides, copper hydroxides, andwater hydroxides.

28
stainless rod. For the copper-water anion experiments, a backing gas mixture of 40
psig is passed over a stainless steel reservoir containing water. The gas mixture
found to be the most effective at making copper-water clusters consists of 40 psig ofAr and 210 psig of Ne (16% Ar). As the gas passes through the discharge region, theAr gas is ionized producing Ar+ and secondary electrons. The positively charged ionsare attracted to the negatively charged copper rod and sputter off neutral, anionic, andcationic copper atoms. The secondary electrons attach to the neutral atoms forming
additional negative copper ions. Clustering around the newly formed negative charge
centers then occurs in a manner similar to that described below for the dihalide
anions. A more detailed account of the dimensions and operation of the dischargesource can be found elsewhere.8

2.3

Cluster Ion Formation
The intersection of a high energy electron beam with the molecules in a free

jet expansion ionizes a portion of the initially neutral molecules to produce cations
and low energy secondary electrons that collisionally cool to around 1 eV.3,9 These
secondary electrons remain in the expansion and then attach to neutral molecules via
collisions.
There are two major mechanisms for formation of the anion clusters observedin the mass spectra.4 The first mechanism is rather straightforward and is responsible
for the formation of the dihalide anion clusters. A given anion, IBr⎯ for example, isformed by attachment of secondary electrons to the neutral dihalide species found in

29
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:
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 growth depends on the presence of neutral CO2
molecules along with third body collision partners. Regions of high density in the
expansion favor large cluster growth.
The second mechanism is responsible for the ubiquitous (CO2)⎯n clusters thatare present in spectra where CO2 is used as a backing gas. In this case, there is not anegatively charged nucleation center as is the case for the dihalide anions, due in partto the CO2⎯ anion being unstable. The (CO2)n⎯ anion clusters are instead formed byattachment of the slow secondary electrons to neutral CO2 clusters already formed in
the expansion. The negative cluster ions are then stabilized by ejection of neutralmonomers and collisions with other molecules in the expansion,(CO2)n + secondary e⎯ → [(CO2)n⎯]* → (CO2)m⎯ + (n-m) CO2.
The formation of these clusters depends less on the position of the high energy
electron beam with respect to the dense part of the expansion, because cluster growthoccurs before interaction with the high energy electrons.
The keys to cluster growth around the dihalide ions of interest are both thedensity of the expansion and the temperature.

The clusters grow based on theinteraction of the newly formed dihalide ion and the neutral CO2 molecules in the30
expansion. Long drift distances are needed for sufficient cluster growth. The drift
distance is about 15 cm, which is the distance from the pulsed valve to the extraction
region in the apparatus. The formation of a neutral plasma keeps the ions coupled to
the neutral CO2 molecules over this long drift distance. Otherwise, the ions would
undergo diffusion from Coulomb repulsion and the densities would be much less than
the 109/cm3 density that is estimated3 in the extraction region. The ion-molecule
reaction rate10 (Langevin reaction rate) determines the collision rate between thedihalide ions and the neutral solvent molecules necessary for cluster growth. This
reaction rate is shown to be independent of temperature and cluster growth isfacilitated by having a very dense expansion over large distances. Temperature does
become important, however, because the clusters expel their excess energy to become
more stable by collisional cooling with neutral molecules in the expansion and also
by evaporation of monomer solvent molecules. Lower expansion temperatures may
not be important for the ion-molecule reaction rates, but they are important forlimiting excess energy that clusters contain thus making the evaporation of solvent
molecules unnecessary and leading to the production of larger clusters. The ideal
expansion is then both as cold and as dense as possible over long distances. The
clusters in this source are estimated to have a temperature of 50−60 K based on
RRKM calculations.11

31
2.4

Cluster Ion Source Modifications
Modifications were made to the existing cluster ion source during the work

carried out on IBr⎯(CO2)n cluster ions.

Initially, large clusters (n>11) of usable

intensities were absent from the mass spectra. Large clusters were present in theICl⎯(CO2)n and I2⎯(CO2)n systems with experiments carried out up to around n = 20 insome cases.

The IBr⎯(CO2)n mass spectra showed an increase in intensity withincreasing n up to a maximum at around n = 7,8,9. The intensities of the clusters thenquickly dropped off with n = 11 usually being the larger cluster of usable intensity.
The first modification is based on the work of the Johnson group.12

Theirelectron gun is operated in a position collinear with the expansion nozzle. This modeof operation guarantees that the high energy electrons interact with the densest part ofthe expansion. The production mechanism for the IBr⎯(CO2)n cluster ions indicatesthat the position of the electron beam with respect to the nozzle orifice is critical tothe production of large clusters. Having the electron beam propagate collinearly
allows steering of the beam down the throat of the nozzle thus guaranteeinginteraction with the absolute densest part of the expansion.
The electron gun is moved from the previous perpendicular position describedabove to a position in-line with the nozzle on the opposite side of the extraction
region. The end of the gun was positioned about 15 cm from the center of theaperture to the time-of-flight portion of the instrument, the same distance as the valve
orifice to the time-of-flight entrance. The electron beam is quite diffuse at the
expansion interaction region after traveling over the 30 cm from the gun to the nozzle
orifice. A small samarium-cobalt ring magnet was attached to the faceplate of the32valve to focus the diffuse electron beam into the nozzle orifice. The electron beam isaligned by adjusting the gun deflectors as the appearance of a visible plasma at thevalve faceplate is monitored.

The deflectors are then fine tuned by monitoring themass spectra on an oscilloscope. The electron beam current is measured by a smaller
Faraday cup that is placed on a vertically retractable arm. The electron beam current
is maximized and then the Faraday cup is removed from the electron beam and ion
path easily.
The second modification to the cluster source involves the redesign of thesupersonic expansion nozzle. Previously, commercially available nozzles employedin the cluster source had valve orifices of 0.8 mm and cone angles of 90º. There isalso a small, narrow channel between the poppet and the orifice in which the poppet
rests and the gas must pass through before expansion. Studies have been publishedby the Even group13,14 where they investigate the properties of rare gas expansions as
a function of the valve geometry. They found that an optimum cone angle of 40°
provides the best spatial confinement of the expansion over long distances. Theirstudies also measure temperatures of the expansions to the tenths of Kelvin range.
With these results, a new nozzle is designed for the cluster ion source that is depicted
in Fig. 2.3. Due to machining constraints, the valve orifice was decreased to only
0.61 ± 0.02 mm from the original 0.8 mm of the commercial valve. The Even nozzles
have much smaller orifices of 0.1 mm. Another parameter found to be important is
the ratio of the total nozzle length to the nozzle orifice. The nozzles that produced
good spatial confinement and low temperatures had ratios of around 30.
machined nozzle for the IBr source has a ratio of only 6.6. Another change in the

The

33

Commercial
GeneralValve
Faceplate

90°

Poppet0.80 mm orificeHomemade
GeneralValve
Faceplate

Poppet40°0.61 ± 0.02 mm
orifice4.03 mmFigure 2.3: Cross sections and dimensions of the commercial
and homemade faceplates for a General Valve.

34
new nozzle is the manner in which the poppet rests on the nozzle. The commercial
valve has poppets with conical shaped ends that rest in a conical shaped area on theback of the valve orifice. A small channel separates this poppet resting area from the
expansion orifice. This small channel leads to significant heating of the gas before
expansion. The newly designed valve no longer has this channel. The orifice isconnected to the poppet region by a knife edge (see Fig. 2.3). The conically shaped
poppet no longer rests easily against the valve orifice, so the poppet now is modified
so that the end of the poppet is a flat surface as opposed to the cone shape. This
ensures that when the valve is closed the poppet rests flush against the orifice andproduces no leaks through the nozzle.
When both of these modifications are implemented in tandem the mass
spectra show an increase in intensity of midrange clusters by about 50%. A typicalmass spectrum of the IBr⎯(CO2)n cluster ions with these new modifications is shownin Fig. 2.4. Clusters of usable intensities are also now present up to n = 14. The
larger clusters (up to n ∼18) seen in the ICl⎯ and I2⎯ systems still are not present after
the modifications. The modifications are not implemented as effectively as possible
and improvements can still be made. First, the electron beam diameter is quite large
at the ring magnet. Although, the electrons are focused the diameter of the electron
beam is much larger than the diameter of the ring magnet. A significant portion of
the electrons are not being focused into the expansion and are simply lost. The
electron gun must be redesigned to provide better focusing of the beam over longer
distances. Second, the new valve did not have the optimal dimensions that aredesired. For maximum spatial confinement and the lowest temperature possible, the35* Isotope ratio 79Br: 81Br ~1:1

Intensity(CO2) ⎯14

n=4

400

5

6

7

500

8

9

600

10

11

12

700

Mass (amu)
Figure 2.4: Time-of-flight mass spectrum of the midsized IBr⎯(CO2)n cluster
ions, where n indicates the number of CO2 molecules attached to IBr⎯. The otherprominent peaks are (CO2)⎯n cluster ions which were used as references for mass
assignment.

36
nozzles have to be fabricated with smaller orifices (∼0.1 mm) along with a larger
nozzle length to orifice diameter ratio. Ratios of 20 to 30 should be more effective at
producing larger clusters than the current ratio of 6.6.

2.5

Extraction, Ion Optics, and Primary Time-of-Flight Mass Spectrometer
After the ions drift for around 15 cm and cluster formation is complete, thenegatively charged ions are extracted into the primary time-of-flight mass
spectrometer by a transverse, pulsed electric field. The pulsed negative voltage istypically −400 to −1000 V and has an adjustable pulse width held at 2 ms. The
homemade extraction plate pulser is triggered externally by a TTL pulse from a
Stanford Research Systems delay generator (Model DG535) and is fed high voltage
from an external power supply source. The final output is a square wave pulse at the
input negative high voltage with a pulse width of 2 ms. The extraction pulse has a
sharp rise time (<10 ns) and a long duration to ensure a uniform extraction field while
the ions are in the extraction region. The timing of the extraction voltage pulse is
optimized by observation of the ions on an oscilloscope.
The extracted ions pass through a 3 mm aperture into the acceleration regionof the time-of-flight mass spectrometer. A majority of the ions in the extraction
region collide with the entrance plate to the time-of-flight held at ground with only asmall cylindrical volume of the anions entering the time-of-flight region.

Thisvolume is quite spatially diffuse, however, due to the diffuse character of the
expansion in the extraction region.

The extracted ions first enter an acceleration

region that consists of 10 stainless steel rings with 2.54 cm diameter apertures each37
spaced 0.43 cm apart with a 1 MΩ resistor between the two. A DC voltage of +2500
V is applied to the final plate giving the extracted ions an extra 2.5 keV of kinetic
energy. A majority of the ion elements and detectors downstream of the acceleration
stage are floated at the beam potential (2.5 kV).
The accelerated ions pass through a set of horizontal and vertical deflectors
immediately after exiting the acceleration stage. The deflectors typically have 0 to
100 V floated above the +2.5 kV beam potential. After the deflectors, the ions enteran einzel lens that consists of three stainless steel cylinders with the first and last
cylinder biased at the beam potential. The middle lens is operated in a deceleration
mode with the voltage applied to this element between +800 V and +2000 V.
Operation in this mode provides a much longer focal length for the anions with thefocus near or at the Wiley-McLaren time-of-flight focus in the laser interaction
region. The ions travel half a meter in a field free region before encountering a final
set of horizontal deflectors. These deflectors usually are at 0 to 80 V above the float
voltage and steer the ions through a 1.5° horizontal bend in the instrument that
separates the energetic ions from metastable decay or collision induced dissociation
products formed in the extraction and acceleration regions.
One of the critical aspects of working with gas phase clusters to study
solvation is the ability to select specific cluster sizes on which to carry out
experiments. This is accomplished by the use of a mass gate that is simply a set ofhorizontal, parallel stainless steel plates just before the laser interaction region. The
top plate is directly connected to the floated back-end of the chamber and isconstantly held at the float voltage of +2.5 kV. The bottom plate is switched between

38
two voltages. A DEI high voltage pulser takes two input voltages and a trigger pulse.
One voltage is the float potential of +2.5 kV. The second potential is slightly below
this voltage at +2.2 kV to +2.45 kV. When the mass gate is off, the bottom plate
simply has the float voltage of +2.5 kV applied. In this mode of operation, all ionspass through the mass gate with no attenuation or deflection. When the mass gate ison, the bottom plate has the second lower potential on it deflecting the ions out of thepath to the detectors. The DEI box pulses the potential back to the beam potential
when it is provided a trigger. This allows a selected ion of a given mass to passthrough unaffected. The voltage then returns to the offset voltage deflecting all otherions after the one selected.

This trigger is provided from the same Stanford

delay/pulse generator mentioned above.
A mass gate driver is fed two TTL trigger pulses from the delay generator.
The first trigger brings the lower plate voltage down from the beam potential to theoffset voltage. The driver then provides the DEI pulser a second trigger pulse with an
adjustable pulse width that brings the plate back up to the beam potential for an
adjustable time. This pulse width was measured to range from 58 ns to 5.6 µs. The
plate is then pulsed back to the offset until the end of the cycle when it returns to thebeam potential. The entire mass gate operation cycle (pulsing down to the offset
voltage, back up to the beam potential for an adjustable time, and back down to theoffset voltage again until the end of the cycle) is roughly constant at 275 µs in
duration after receiving the initial trigger from the Stanford delay generator.

The

delay generator timing is adjusted until the desired ion peak appears on the scope. In
the ideal mode of operation, this is the only ion mass reaching the detectors and thus

39
appearing on the scope. Both the trigger pulse width and the offset voltage need to be
adjusted to minimize the attenuation of the mass-selected peak in conjunction withallowing a minimum amount of neighboring ion masses to get through. It is foundthat an offset voltage of around 200 V provides enough deflection to minimize bothbackground noise at the detector and attenuation of the mass selected ion. Offset
voltages of more than 300 V provides efficient minimization of noise, but begin todistort and attenuate the parent ion. Pulse widths of around 600 to 800 ns provide thesmallest window where no attenuation or distortion was observed. The mass gate issurrounded by an enclosure with entrance and exit apertures. The entrance aperture
diameter is 2.5 mm while the exit is 4.3 mm. These are easily removed if there is achange in the ion trajectory leading to scattering from either aperture plate. The
entrance and exits have diameters of 9.2 mm when the apertures are absent. The
entire assembly is on threaded rods by which the height can be adjusted in case ofchanges in the ion trajectories.

2.6

Photolysis, Secondary Mass Spectrometer, and Detectors
The ions arrive at the spatial focus of the primary time-of-flight mass

spectrometer just after exiting the mass gate. Extensive work was done in the designof the instrument to determine the dimensions and voltages necessary for the spatial
focus of the time-of-flight to be precisely at the laser interaction region.4 The mass
selected ions are intersected perpendicularly by the laser beam about 6 cm after theexit of the mass gate. Upon photolysis, the parent ion and any photofragment ionsproduced enter the secondary time-of-flight mass spectrometer. This spectrometer is40
a single stage, reflectron mass spectrometer15,16 at an initially designed tilt angle of
2.5°. The reflectron consists of 30 equally spaced plates connected by 10 MΩ
resistors. The front of the reflectron has both the entrance and exit apertures and isheld at the beam potential of +2.5 kV. The back plate is held at a variable voltagethat is discussed below. The entrance aperture is collinear with the entrance and exit
apertures of the mass gate. The back plate also contains an exit aperture to allow ionsto travel directly through the reflectron to the in-line detector when desired.
The voltage applied to the back plate of the reflectron reverses the trajectoryof an ion when the energy of the retarding field exceeds the kinetic energy of the ion.
This field separates ions according to their mass as well as focuses them onto the offaxis detector located after the exit of the reflectron. There is an inherent energy
spread in the ion beam of about ±500 V. The distance that an ion penetrates into thereflectron depends on its kinetic energy. Focusing of the ions is achieved due to thefact that ions of the same mass and spatial position, but with different energies
penetrate to different depths in the reflectron. The higher energy ions penetrate
further and thus have a longer flight time through the reflectron to the off-axis
detector. However, in the field free region after the reflectron the more energetic ionsovertake the slower, less energetic ions. The point where this takes place is thespatial focus of the reflectron where the off-axis detector is positioned.
The advantage of the reflectron is that desired ionic fragments of a massselected, photofragmented parent ion are focused onto the off-axis detector by simply
varying the voltage applied to the back plate of the reflectron. The position of thespatial focus depends on the mass of the ionic fragments for a fixed reflectron field.

41
The voltage applied to the back plate must be adjusted to move the spatial focus of afragment ion to that of its parent ion, which is located at the off-axis detector. The
voltage that is applied is proportional to the ratio of the mass of the fragment ion, mf,
to the mass of the parent ion, mp. The reflectron voltage, VR, is given by:

VR =

EP
4D m fL m p e cos(θ )(2.1)

where mf and mp are the masses of the fragment and parent ions, Ep is the energy ofthe parent ion, θ is the tilt angle of the reflectron, L is the free field drift region
distance, and D is the percentage of penetration into the reflectron.

There are twovariables in Eq. 2.1 with all others assumed to be constant for this particularapparatus. The masses of the fragment and parent ions are the variables to be enteredfrom which the voltage applied to the reflectron is given. This is the voltage that
focuses the fragment onto the off-axis detector at the same flight time as the parent
ion. It had been assumed in previous experiments that the optimal penetration depth
for the best focusing and most intense ions on the off-axis detector was 0.86. This
penetration depth corresponds to a reflectron voltage to focus all of the parent ions
onto the off-axis detector of −988 V. However, it was found that the optimized ionsare at reflectron voltages other than the voltage previously used. When the parent
ions are optimized, it is critical that the reflectron voltage is varied to obtain the mostintense, focused parent ions. The reflectron voltage that must be applied varies daily,
sometimes over hundreds of volts meaning that the penetration depth is varying. The

42
prior method of assuming a penetration depth of 0.86 when optimizing the parent ionsand calculating voltages for the fragment ions is inaccurate.
The reflectron voltages for the fragment ions are now found by first
determining the reflectron voltage necessary to produce the most intense, focusedparent ions. From this voltage, the penetration depth was determined by simply
taking the average parent ion kinetic energy divided by the reflectron voltage. The
average parent kinetic energy is found by taking half of the extraction voltage andadding it to the float voltage of +2.5 kV. The reflectron voltage is 2500 V – (voltage
on the reflectron power supply). For instance, if the voltage supplied is −850 V thereflectron voltage is 2500 – (−850) or 3350 V. For an extraction voltage of 500 V,
the penetration depth is 0.82. The voltage necessary to focus a fragment of a given
mass from a parent ion is now calculated by entering the extraction voltage, thepenetration depth, and the masses of the parent and fragment ions. These calculations
provide the correct reflectron settings when changes in the extraction voltage and thepenetration depth occur.
The instrument has two detectors with one in-line behind the reflectron on theion axis and the other off-axis after the exit of the reflectron. Only a brief discussionof the configuration in the floated regime is given here with a full description of thedetectors and their circuit diagrams when floated found elsewhere.4 The in-line
detector consists of two microchannel plates (Hamamatsu Corp. F4612-09 and Burle
part #32733) that are impedance matched and in a “Chevron” configuration. This
arrangement provides a gain of around 107. Each plate has a 20 mm active area. In
the floated configuration of the detector, the front plate was biased at the beam

43
potential of +2.5 kV. The back plate is biased at 1.6 kV above the beam potential
when detecting parent ions and usually 1.8–1.9 kV for detecting less intense
photoproducts with a maximum permitted voltage of 2.0 kV above the float potential
across the plates. There is a mesh held at the beam potential in front of the first plate
to provide a flat, field free potential for the ions and can be used as an acceleration
stage if desired. A vertical deflector is also used to steer the ions into the detector. It
typically has a few hundred volts above the beam potential for maximum ion signal at
the in-line detector. It is also used to deflect the negative ions away from the detector
for the detection of neutrals. In this case, the plate is biased at 1.0–1.5 kV above thebeam potential. The electrons from the microchannel plates (MCPs) are collected byan anode that is biased at +200 V above the voltage of the last plate. This voltage isregulated by a Zener diode that maintains this +200 V between the anode and lastplate as the voltage on the last plate is varied. The ion signal is capacitively coupled
to the data processing electronics due to the detector being biased at high voltages.
The signal from the anode is amplified by a factor of 10 by a pre-amplifier (EG&G
9301) and is then further amplified by a Stanford Research Systems 350 MHz fast
pre-amplifier (SR 240A) that provides a gain of up to 625 before finally being sent to
the data processing electronics.
The off-axis detector is located at the spatial focus of the reflectron about 38
cm from the reflectron exit. This detector consists of three microchannel plates(Galileo, 0.70″ active area diameter). The first two are impedance matched in a
Chevron configuration.

The third plate is not impedance matched and is at anorientation of 180° with respect to the second plate. The overall gain is roughly 1010.

44
The detector is floated at the beam potential of +2.5 kV. A potential of around 2.1 kV
above the float potential is applied to the back of the third MCP for the detection ofparent ions. A potential of 2.5–2.6 kV is applied for the less intense one-photon ionicphotoproducts with 2.8–2.9 kV applied for the typically smaller two-photon ionicproducts. The anode is biased +200 V above the potential of the back plate by use of
a Zener diode in the same manner as the in-line detector.

The ion signal iscapacitively coupled to the detector electronics and is amplified by an identical preamplifier as the in-line detector.
As with the in-line detector, the off-axis detector has a mesh held at the beam
potential which is 96% transmissive. After this there is an electron filter that is
simply a grid that is at –200 V to discriminate against any stray, low energy electrons
from hitting the detector. This filter is quite useful when ion production is difficult.
The filter can be turned off to allow the low energy electrons to strike the detector.
These low energy electrons typically depend on the same conditions as the ions whenpresent. The conditions for maximum electron signal on the detector are usually very
close to the conditions for ion production. The filter is turned on and the parent ion
signal can be found. After the electron filter, there is post-acceleration stack. This
stack was designed to provide the ions with an additional 1 to 3 keV kinetic energy
before striking the detector. However, it was determined that this stack is responsible
for a great deal of noise on the detectors, especially noticeable when operating in the
single ion counting (two-photon experiments) regime. Although, the stack does
increase the intensity of the parents the added noise made the use of it undesirable.
The stack is now kept at the float potential making the added noise negligible.

45
2.7

Photoelectron Imaging Considerations and Potential Switch

There are two major design concerns that were considered when
implementing the electron velocity map imaging system as described in the
Introduction. The first is that the imaging system can not be held at a large potential.
The detector region and all the components in electrical contact with the imaging
system can no longer be floated at the existing 2.5 kV beam potential. The second
concern is that the electron imaging apparatus needs to be designed such that it can beinstalled into the pre-existing tandem time-of-flight/reflectron mass spectrometersystem. Full functionality of the Wiley-McLaren time-of-flight and reflectron mass
spectrometers as well as the two detectors needs to be retained with minimal
modifications.
The first task is to reference components that were previously floated at +2.5
kV to ground, which in this case is the detection chamber ground. Accelerated ions
are still desirable, but they need to be referenced to ground after the acceleration
region in a way in which no potential force is felt when encountering the grounded
portions of the apparatus. This is accomplished by the use of a potential switch17
pioneered by the Johnson group. The potential switch is a region that is electricallyisolated from the other components of the instrument. Initially, it is held at the beam
potential of 2.5 kV.

When the accelerated anions of interest enter the switch, thepotential applied to the region is quickly switched down to the chamber ground. The
ions continue to have 2.5 keV of kinetic energy, but now are referenced to groundmaking floating the reflectron and detectors region unnecessary.

46
The switch itself is composed of existing hardware in the time-of-flightchamber. A cylindrical mesh previously connected directly to the acceleration andion optics stack, is now electrically isolated by a Teflon ring and Vespel screws.
Near the beginning of the mesh cylinder (close to the exit of the ion optics stack)there is a stainless steel circular plate with a 1″ aperture in the center. The other endof the mesh is in electrical contact with the moveable stainless steel cylinder housing
the aperture between the second and third differential pumping regions and the 1.5ºbend deflectors. After the deflector region, there is another cylindrical mesh inelectrical contact with the moveable cylinder that forms the last portion of the switch.
The end of this mesh is capped with a circular stainless steel plate with a 1″ aperturein the center. The two end plates with the 1″ apertures define the total length of thepotential switch. The apertures are in place to keep potential fields from other partsof the instrument from penetrating into the switch. It is desirable to have the switch
as long as possible to allow for larger ranges of masses to be present in the switch at agiven time to facilitate peak assignments. The switch in this case is 70 cm allowing
for a range of 65–70 amu of focused ions. This range is actually a bit wider, but ionson the edge of the switch tend to be distorted and not used in experiments.
A problem with the potential switch arranged in this manner is that the 1.5ºbend deflectors are within the switch. These deflectors are electrically isolated fromthe switch and float at voltages different from the switch potential. This effectively
halved the length in which the switch performed properly with the effective length
now from the first stainless steel plate aperture to the aperture separating acceleration
region from the region with the bend deflectors.

This problem is remedied by47completely removing the deflectors from this region. A cylindrical section of thechamber is removed between the detection chamber gate valve and the end of theswitch and fitted with a new set of deflectors. Electrical SHV feed-throughs wereadded on both sides of the cylinder to provide the electrical connection to thedeflectors from the power supplies. A final modification to the system was placing a
stainless steel plate with a 1″ aperture and tabs to hold it in place just after thedetection region gate valve to provide differential pumping between the third
pumping region and the detector region.
The potential switch voltage is pulsed by a DEI PVX-4140 Pulse Generator.
This pulse generator takes two voltage inputs with one from a power supply set to thebeam potential (2.5 kV in this case) and the other input is connected directly to thebackend of the chamber (ground). A TTL trigger pulse from a Stanford Delay
Generator is sent to a homemade DEI controller box. This allows the trigger pulse
sent to the DEI pulser to be adjustable in width from 30 ns to 13 ms. The switch isturned on when the trigger pulse is sent to the pulser. This trigger is set to coincidewith the triggering of the extraction plate. The length of the trigger pulse is varied inwidth to allow a larger mass range to be present in the switch. The pulse width tocleanly include masses of up to around 70 amu is 13 µs. The rise and fall time of theswitch pulse is 15 ns.

2.8

Electron Imaging Stack Design and Installation Concerns

In addition to the implementation of the potential switch, the other major
challenge of the design of the electron imaging system is to incorporate the hardware

48of the imager into the existing apparatus in a way such that none of the originalfunctionality of the time-of-flight and reflectron mass spectrometer instrument iscompromised.

The only suitable location for the electron imaging hardware isvertically in the region between the mass gate and the reflectron. However, it needs
to be carried out in such a way that the parent ion and ionic fragment flight paths
through the mass gate to the reflectron entrance and from the reflectron exit to theoff-axis detector remained unobstructed.
A generalized schematic of this electron imaging system along with thepotential switch installed in the existing system is shown in Fig 2.5. The distancesand sizes are not to scale. Exact dimensions are given below. As mentioned above,the electron imaging system follows the velocity map imaging design2 of Eppink andParker. In their configuration, a repeller plate with a small 1 mm hole is usedfollowed by an extractor and a ground plate. Both of these plates have open aperturesof 20 mm. The imaging system described here has a slightly different electrodeconfiguration based on the Sanov group imaging apparatus.18 The photodetachedelectrons after extraction pass through a field free flight tube before striking a
microchannel plate detector. The electrons from the microchannel plates impinge ona phosphor screen that is then imaged by a CCD camera. The electron imaging stage
must be shielded from stray magnetic fields as well as having accurately timed
voltage pulses applied to various elements of the imaging system.

49

Electron Imaging
Stack

Pulsed Valve
SourceOff-axis MCP
detector

In-lineMCP
detector

Potential
SwitchAcceleration
Stack
And Ion Optics

Electron
Gun

Mass
Gate
Reflectron
Laser 200-2000 nm

Parentions andneutrals

Figure 2.5: Cluster ion source and tandem time-of-flight mass
spectrometer with potential switch and electron velocity map imaging
components.

50
2.9

Electron Imaging Optics and Flight Tube

The electron extraction and imaging optics consist of three plates made ofoxygen-free high-conductivity copper. The plates are 3″ in diameter and 0.03″ inthickness. The bottom plate (repeller) is solid with the ground and extractor plates
having a 1″ aperture in the center of each. The plates are spaced 1″ apart and are heldin place by threaded Molybdenum rods and ceramic washers to electrically insulatethem from the grounded rods.

The plates are connected to power supplies via

insulated wires. The flight tube is made of oxygen-free high-conductivity copper andis 15 cm long with a 2.75″ diameter. The flight tube is in direct electrical contact
with the final electrode plate and the two are held at the same potential. The entire
assembly is mounted onto an 8″ conflat flange containing electrical feed-throughs forthe voltage connections.
The low energy electrons formed from photodetachment are susceptible to the
Earth’s magnetic field along with any other stray magnetic fields. For this reason, theelectron optics and flight tube assembly is enclosed with magnetic shielding. The
shielding is µ-metal (Magnetic Shielding Corp.) and consists of four layers of 0.020″
thickness covering the entire length of the inside of the flight tube. The extraction
plates are covered by a µ-metal cup that is fastened to the flight tube via a Teflon ringto isolate it from the flight tube. The cup contains openings to allow for the parent
ions, laser beam, and fragment ions exiting the reflectron to the off-axis detector topass through unperturbed. The openings were designed to be as small as possible tominimize field penetration into the extraction region [penetration ∝(diameter)2], but
to still allow for unobstructed ion trajectory paths. The laser openings on each side51
are 0.5″. The entrance and exits on the front side are roughly 1 cm and the backconsists of a long slit about 1 cm wide and 3″ long. There is also a 1″ opening on thebottom of the cup for pumping purposes. The assembly is illustrated in Fig. 2.6.
As mentioned before, the entire assembly needed to be installed in the existing
chamber with minimal modifications. The assembly is mounted on an existing 8″flange directly above the laser interaction region of the chamber. The mass gatepreviously was positioned on the middle of a 6″ flange that is just before the laserinteraction region. For the new assembly to be accommodated, the mass gate is
moved 1.5″ away from the center of the flange in the direction of the source region.
The side view of the chamber with the electron imaging system installed is shown in
Fig. 2.7.

2.10

Imaging Detector and CCD Camera

The imaging detector consists of two microchannel plates (MCP) and aphosphor screen mounted onto a 6″ flange with the electrical feed-throughs that itself
mounts onto the 8″ electron flight tube flange. The entire detector assembly wasshipped as one component from Burle Electro-Optics, Inc. The MCPs are impedance
matched in a chevron configuration. The plates have a 40 mm active area and aminimum gain of 4000 for each plate at +1000 V. The front plate is held at the flighttube potential while the back plate is at +1600 V above the front plate.

The backplate is pulsed between +1600 V and +1900 V. The plates have the larger voltage
across them when the photoelectrons are arriving at the detector. Pulsing the plates
down to the lower voltage minimizes the background of stray electrons and ions

52

Magnetic
Shielding

Flight Tube
42 to 1260 V
Middle Plate
GroundPulsed ExtractionPlate–10 to –300 V

Figure 2.6: Electron velocity map imaging schematic. Exact
dimensions and distances are given in text.

53

Electron Imaging Stack

Off-axis Detector Entrance

Mass Gate
Laser Interaction Region

Reflectron

Figure 2.7: Side view of the chamber with the Imaging stack installed.
Components in red are either new or in a modified position compared to theoriginal design in black.

54
arriving at the detector when the photoelectrons are not present. The phosphor
screen is a P47 type and is electrically isolated from the MCPs and the chamber
ground. The screen is at held at around +6 kV and is not pulsed.
The pulsed voltages are provided by homemade controllers similar to that ofthe potential switch mentioned above. The pulsed extraction plate voltage is provided
from a DEI PVM-4210 model dual output pulser module.

The pulser provides

adjustable positive and negative voltages. In the imaging experiments describedwithin, the typical voltages are between −10 and −100 V. The pulser module receives
a trigger of adjustable width from the homemade DEI pulser controllers.

This

controller takes a TTL trigger provided from the Stanford delay generator and sends itto the DEI module with an adjustable width. The extraction plate pulse is set to be 50µs wide and pulses between chamber ground and the desired negative voltage set on
the module itself. The MCPs are likewise pulsed by a DEI PVM-4140 pulser.
However, this module takes not only a trigger pulse, but also two input voltages toswitch between. The input voltages are provided by two external power supplies andthe trigger pulse is produced in an analogous manner as the extraction plate from an
input TTL pulse. The MCP pulse is 200 ns wide and pulses the back plate between+1600 and +1900 V. Both the front MCP plate and the flight tube are held at their
voltages in DC mode by external power supplies. The exact timing of the pulsed
voltages is discussed below.
The CCD camera is a La Vision 3LS model. It has a spectral range of
290−1000 nm that includes the peak wavelength of the P47 phosphor of 410 nm. The
physical dimensions are 1280 x 1024 pixels with an individual pixel size of 6.7 µm x

55
6.7 µm. The exposure time can be varied between 1 and 100,000 ms with a read-out
time of 121 ms. The camera is linked to the PCI-Interface Board by a 10 m fiber
optic cable. Within the camera head is a 12 Bit A/D along with a parallel/serial
converter for serial data transfer to the interface board. The CCD is Peltier cooled toa normal operating temperature of −11 ºC. The camera has a Nikon 12 x 24 mm wide
angle lens.

The entire camera is mounted onto a height adjustable mount which isfastened to the 6″ detector flange. A black metal cylinder is placed between thephosphor screen and the lens along with a black drape over the entire camera
assembly to limit stray light impinging on the CCD that leads to higher backgroundcounts.

2.11

Laser Systems and Timing Schemes

2.11.1 Nanosecond Laser System

For one-photon experiments a nanosecond laser system is used.

This system

consists of a commercial Nd:YAG pumped optical parametric oscillator (OPO). The
laser is a Coherent Infinity with a Coherent XPO and a 2nd harmonic generation stage
and is depicted in Fig. 2.8. The entire XPO and doubling assembly of the Infinity
provides a wavelength range of 210−2000 nm at pulse energies of 100 µJ−600 mJ
with a continuously adjustable repetition rate of 0.1−100 Hz.
The initial pulse is formed by a diode-pumped Nd:YAG prism inside a ring
resonator. The cavity is Q-switched by use of an acousto-optic modulator (AOM)within the oscillator cavity.19

The low energy pulses from this oscillator are thenamplified by use of Xenon flashlamp pumped Nd:YAG rods. There are two trigger

56

OPO SHG

OPO Cavity
XPO
Pump
Dump

SHG Dump
Laser Output
210 – 2000 nm

SHG
THG

SBS CellRod-rod relay

YAG Rods

Spatial Filter

MasterOscillator
Faraday Isolator

Figure 2.8: Nanosecond Nd:YAG-pumped OPO laser schematic.

57
timings involved in this laser that must be coordinated in any combination of external
and internally triggered. The timing between the Q-switch and the firing of the
flashlamp is optimized by observing the power of the fundamental pulse as a functionof this delay between the two. The optimized timing has the Q-switch being triggeredaround 250 µs after the flashlamp is triggered. This value is used when developing
experimental timing sequences described below.

The pulses make two passes

through each of the two Nd:YAG rods that are designed to ensure a pure polarization
state and are then reflected out of the amplifier by a thin film polarizer.
The fundamental pulse (1064 nm) exits the amplifier with vertical polarization
and up to 600 mJ/pulse in energy. The pulse width of the amplified pulse is 3 ns.
The fundamental pulse undergoes type I doubling in an angle tuned BBO crystal toproduce frequency doubled light at 532 nm. The fundamental and 2nd harmonic
pulses are then mixed in another angle tuned BBO crystal to produce the 3rd harmonicof the fundamental at 355 nm. The fundamental, 2nd, and 3rd harmonics are separated
from one another using dichroic mirrors allowing for the use of the three colors forexperiments.
The XPO stage is an optical parametric oscillator (OPO) placed in the Infinity
laser and is pumped by the 3rd harmonic light. The 3rd harmonic pump pulse isintroduced into a cavity with a nonlinear BBO crystal as the gain medium. The
production of the tunable light from the pump pulse is a result of difference frequency
mixing.20 Due to noise in the cavity there is a cavity resonant beam present at low
intensity of a frequency lower than the pump called the signal, ωs. This frequency
interacts with the pump frequency, ωp. The interaction of these two frequencies

58
produces a new frequency called the idler at ωi. This new frequency quickly grows in
intensity and can itself interact with the pump frequency to produce a difference
frequency at the original signal frequency. In this way, both the signal and the idler
are amplified within the cavity at the expense of the pump beam. The overall process
is then ωp = ωs + ωi with the pump photons converted into the signal and idler
photons.

A phase matching condition must be met for the optical parametricoscillator to provide gain for the signal and idler frequencies. The equation kp = ks +ki must hold where k represents the wavevector of each frequency that depends on theindex of refraction of the frequency in the BBO. The crystal is birefringent meaningthat the indices of refraction in the crystal change as the crystal angle is changed. As
the crystal angle is changed, the phase matching condition is met by a new set ofsignal and idler frequencies. The tunability of the OPO thus arises from varying the
crystal angle to allow different sets of signal and idler frequencies to meet the phasematching condition necessary for oscillation.
The cavity is set up to be singly resonant in which only the signal or only theidler can form the output pulse. Selection of either the idler or the signal can easilybe accomplished by the use of different sets of optics in the XPO cavity. There is anarrow wavelength region that is inaccessible around the point where the signal andidler are degenerate in energy (709.4 nm). The degeneracy gap has been measured to
be roughly from 690−730 nm. The signal tuning range is then 420 nm−690 nm andthe idler range is 740−2200 nm. Output pulse energies in the range of 1−20 mJ for
the signal are observed with the pulses becoming broader in width from typically 3 ns
to about 6 ns as the wavelengths approach the degeneracy gap. The idler wavelengths

59
have pulse energies below 1 mJ with somewhat broader pulse widths than the signal
wavelengths. The final element to the Infinity-XPO is a 2nd harmonic generationassembly employing another BBO crystal to produce the doubled frequencies of
either the signal or the idler. There is a gap in the wavelengths between 345−365 nm
reflecting the degeneracy gap in the OPO wavelengths to be doubled.
The velocity map imaging experiments require the laser polarization to be
horizontal with respect to the ion flight axis. The light coming out of the InfinityXPO has polarizations dependent on whether the signal, idler, or the doubled
signal/idler is used. In some cases, the polarization must be rotated 90° before
interaction with the ions. This is simply accomplished by the use of two mirrors in aperiscope arrangement. The input polarization is retained upon exiting the periscope
when the two mirrors have their axes parallel to one another. The polarization isrotated 90°, however, if the mirror axes are 90° apart. The polarization is thenchanged from vertical to horizontal or vice versa by simply changing the periscopearrangement. This allows for polarization changes easily over broad wavelength
ranges without the need for wavelength specific wave plates. Also, in the optical path
is a telescope that consists of a 50 cm focal length lens followed by a −20 cm focal
length lens that is placed within the focus of the first lens. This telescope serves tocollimate the beam as it enters the chamber. This is especially important for the idler
beam that has a very large divergence in the far field. A polarizer is used after thetelescope to ensure a pure polarization state and various lenses are used after this fortighter focusing inside the chamber. Broadband aluminum mirrors (Thorlabs, Inc.)
with a range of 200−1000 nm were used to avoid switching out mirrors when varying

60
the wavelength drastically. One final consideration for the nanosecond laser systemis the fact that numerous components within the electronic imaging stack containcopper that has a work function of 4.7 eV (267 nm). Using wavelengths below thisthreshold produces a great deal of electrons ejected from the copper surfaces leading
to a much higher background in the images. To overcome this, the entrance and exit
window flanges on the chamber were extended on each side by about 7.5″. On eachside in the extended regions, three annuluses were inserted with apertures ofdecreasing diameter. This serves to block scattered UV light from reaching thecopper components thus minimizing background at the detector. Brewster windows
were also installed on the entrance and exits sides cut at 56.44° for 250 nm. This
minimizes scattering from the windows and is effective over a broad wavelength
range. The Brewster’s angle for 200 nm to 800 nm only changes by 2°.

2.11.2 Femtosecond Laser System

The femtosecond laser system generates short pulses for use in the timeresolved pump-probe experiments described below.

A schematic of the overallsystem is shown in Fig. 2.9. The femtosecond pulses are generated by a Coherent
Mira Ti:Sapphire oscillator. The Mira is pumped by a Coherent Verdi diode-pumped
Nd:YVO4 laser producing 5.5 W cw at 532 nm. The Mira produces modelocked
femtosecond pulses at wavelengths tunable between 750−850 nm. The repetition rateof the laser is 76 MHz and is determined by the round-trip time of a pulse from one
end of the Mira cavity to the other.

The laser uses the technique of passivemodelocking via the optical Kerr effect.19 The wavelength is varied by tuning a61

Coherent
Verdi V5532 nm5W CW

Coherent Mira
Ti:Sapphire
Oscillator

750−850 nm
85 fs, 76 MHz
5 nJ/pulse
Quantronix
Titan Amplifier
Stretcher/Compressor

3mJ/pulse, 400 Hz
150 fs, 750−850 nm
Delay Stage
SHG
THG

527 nm
20 mJ/pulse
400Hz
Quantronix
Nd:YLF pump
laser

Pump

Probe

To Ion Chamber
Figure 2.9: Femtosecond laser system for pump-probe experiments.

62
birefringent filter and monitoring the wavelength with an Ocean Optics spectrometer.
The pulse width of the new center wavelength is then minimized by the adjustment of
a Brewster prism to compensate for any group velocity dispersion incurred byelements within the cavity or by self-phase modulation of the pulse. Optimized pulse
widths tend to be 10 or 12 nm on the spectrometer corresponding to around 85 fs.
However, the pulse energies are only 5−10 nJ and need to be amplified for use in thepump-probe experiments. A photodiode is placed behind a mirror outside the Mira toserve as a trigger for the amplification stage.
The output pulses of the Mira enter a Quantronix Titan amplifier that consistsof a stretcher and compressor along with a regenerative amplifier21 and a multipass
amplifier. The input pulses are first stretched out to a few hundred picoseconds andare sent into a regenerative amplifier. The leakage from the mirror just after the Mira
triggers a Medox Pockels Cell driver. This driver serves numerous purposes in thelaser system. First, it divides the 76 MHz repetition rate of the Mira down to 400 Hz.
A trigger at 400 Hz is then provided to the Q-switch of the pump laser for theamplifier. The Medox driver also controls when the stretched pulse is injected into
the regenerative amplifier and when it is switched out by providing a time-dependent
voltage for a Pockels cell within the regenerative cavity. Finally, it provides anexternal trigger for the clusters experiments that is discussed below. When a voltage
is applied to the Pockels cell it rotates the polarization of the laser pulse 90°. This iscombined with a thin-film polarizer to control when the pulse is injected into thecavity and switched out. The timing of this process is determined by observing theintensity build up of the pulse in the regenerative amplifier cavity on an oscilloscope

63
monitoring leakage from an end mirror of the cavity onto a photodiode.

Theamplification medium is a Ti:Sapphire crystal that is pumped by a Q-switched
Quantronix Nd:YLF laser with a 400 Hz repetition rate. The power upon exiting theregenerative amplifier is around 800 mW.
The pulse is then further amplified by a multipass amplifier that consists ofanother Ti:Sapphire crystal pumped by the same Nd:YLF laser. The Nd:YLF light issplit upon entering the Titan between the regenerative and multipass amplifiers. The
pulses make two passes through the second amplifier and exit with a power of 1.8 W.
The pulses are then recompressed using another grating pair to introduce groupvelocity dispersion of the opposite sign introduced in the stretcher stage.

This

compression depends primarily on the distance between the two gratings. The pulse
width is measured by an autocorrelator and is minimized by adjusting the grating
distance while monitoring the autocorrelation trace. The optimized output pulse is150−200 fs in width with a power of 1.2 W corresponding to 3 mJ/pulse at 400 Hz.
Losses in the compressor account for the drop in power from 1.8 W.
The output of the Titan is split into a pump and a probe pulse with a 50:50
beam splitter.

For one-color experiments, the two identical beams are sent on two

separate optical paths. For two-color experiments, the 2nd harmonic of thefundamental is generated by placing a KDP crystal in one of the optical paths. In
both one- and two-color experiments, a computer-controlled delay stage was inserted
into one of the beam paths chosen to be the probe pulse path. The time-delay
between when the pump and the probe pulse arrive at the ion packet is controlled bychanging the relative path length difference between the two optical paths. The delay64
stage provides delays within a tenth of a picosecond with a maximum delay of ±200
ps. The paths are designed to be nearly identical in total length with the picosecond
delays then added by moving the delay stage millimeters (1 mm difference in path
length distance corresponds to a delay of 3 ps between the two pulses). For the longtime delays experiments described below, the delay stage is not used. The long
delays are set by manually changing the length of one optical path with respect to theother.
The two beams are recombined in the one-color experiment by use of apolarizing beam splitter cube. The two beams are identical and thus have the samepolarization. The polarization of one of the beams is rotated 90° by use of a wave
plate. The polarizing beam splitter is then oriented to allow one beam pass straight
through and the other beam of the opposite polarization to be reflected at a 90° angle.
In this manner, the two beams of now horizontal and vertical polarizations arerecombined collinearly. A 1m lens is used for mild focusing of the beams inside thechamber. The pump and the probe beams for the two-color experiments (typicallynear 800 nm and 400 nm) are recombined by simply using a mirror just before thechamber that reflects one color and passes the other color straight through frombehind.

This allows the two beams to be collinearly recombined with any

combination of horizontal or vertical polarizations.
The accurate determination of zero time delay between the pump and the probe
beams is critical for carrying out the time resolved experiments. For one-color
experiments, the zero delay between the two beams is determined by the appearanceof an interference pattern when the two beams are slightly misaligned with respect to65
one another.

The delay stage is moved until the interference pattern appears. The

delay time usually needs to be accurate to one or two tenths of a picosecond for themost prominent interference pattern. The delay stage is then reset to have this newly
determined position read as zero time delay.

For the fundamental and second

harmonic beams, time zero is determined by the appearance of the third harmonic of
the fundamental. This is accomplished by mixing the pump and the probe beams in a
KDP crystal followed by a prism to separate the colors. The delay stage is moved
until the third harmonic appears. Fine adjustments are made until the third harmonic
intensity is maximized which occurs when the pump and probe are overlapped
exactly in time. This represents time zero outside of the chamber. The two colors
have different travel times through the laser window on the chamber. The time zero
inside the chamber is previously measured to be 0.95 ps less than time zero outside
the chamber. The delay stage is then moved by this amount and reset as time zero.

2.11.3 Nanosecond Timing and Data Collection Schemes

The Infinity laser system is used for one-photon experiments to determine
relative and absolute cross sections as well as ionic cluster branching ratios. The
timing scheme is designed to ensure that the mass-selected ion packet arrives at thelaser interaction region at the precise time the laser pulse does. The Infinity laser has
two triggers with one for the flashlamp and the other for the Q-switch. The flashlamp
and the Q-switch can be triggered in any combination of external or internal. The
first triggering scheme involves triggering both the flashlamp and Q-switch externallyfrom the SRS DG 535 Delay Generator with TTL pulses at 50Ω impedance. The

66
overall scheme including ion events is as follows and is shown schematically in Fig.
2.10 with an actual set of timings shown for reference. The delay generator isinternally triggered at 80 Hz. The To output is used to trigger the pulsed valve andrepresents time zero for each ion production cycle.

The A delay triggers theextraction plate and is typically set to be around To + 1 ms where 1 ms represents theflight time of the cluster ions from the valve to the extraction region. The B delay
turns the mass gate on and is equal to A + x where x is the flight time of the ions fromthe extraction region to the mass gate. The timing, x, is adjusted to mass select adesired parent ion as monitored on the scope. The C delay triggers the flashlamp andthe D delay triggers the Q-switch. As mentioned before, the timing between the
flashlamp and the Q-switch was optimized by measuring the output power of theInfinity. This value is represented in the D delay, which in this case is D = C + 251.6µs and remains fixed. This delay may need to be periodically re-optimized, but does
not change throughout the course of experiments. The C delay is the delay used tooverlap the laser pulse with the ion packet and is calculated as follows. It is assumed
that when the Q-switch is triggered, the laser pulse is instantaneously at the ion
packet interaction position. The flight time of the ions from the extraction region to
the mass gate is known from the mass gate timing. There is an added flight time, ∆,that represents the short flight time from the mass gate to the laser interaction region.
The flashlamp and Q-switch have a fixed delay, in this case, of 251.6 µs, which ismuch longer than the flight time of the ions from the extraction region to the laserinteraction region. The flashlamp must be triggered some time before the ions are
extracted to ensure the Q-switch fires when the ions are at the interaction region.

67

A delay reference
Ion arrival at laserinteraction region

917.395 µs

T0: Pulsed Valve69.7µs∆

A: Extraction Plate
A = T0 + 917.395 µs
B: Mass Gate
B = A + 69.7 µs
C: Laser (flashlamp)
C = A – 180.32 µs

180.32 µs

D: Laser (Q-switch)
D = C + 251.6 µs

251.6 µs (fixed)

Figure 2.10: Ion and laser event timings for the nanosecond experiments.
Actual delays from an experiment are included.

68This time is calculated by the equation:

C = Q-switch setting (251.6 µs) – mass gate delay (from B setting) – ∆(2.2).

This C value represents that amount of time before the extraction plate is triggeredthat the flashlamp must be triggered.

The value for ∆ is not known until

photoproducts are actually observed.
• Kinetic Energy Release and Angular Distributions
In

these

experiments,

themass-selectedparentdiatomicanion

photodissociated to produce an atomic anion and an atomic neutral fragment.

isThe

laser timing is adjusted by monitoring the appearance of the photoneutrals on the inline detector. The 8 mm orifice diameter of the entrance to the reflectron truncatesthe spatial distribution of the neutral fragments in the transverse direction thus
limiting the total amount of neutrals striking the detector. The neutral signal at the inline detector is amplified by both the EG&G pre-amplifier (x10) and by either aPhillips 771 amplifier that only provides linearity in gains up to 5x or the SR 240A
amplifier providing gains up to 625. The amplified analog signal is then digitized andaveraged with a Transient Recorder 200AS/Averaging Memory 4101. The samplinginterval is 10 ns rather than the usual 20 ns to provided better temporal resolution for
more accurate determination of peak separations in the neutral spectra. The spectra
are typically average over 3000 or 5000 laser shots depending on signal levels.

69• Absolute Cross Section Measurements
The absolute cross sections are determined from the depletion of a massselected parent ion signal after interaction with the laser. This is not a precise
measurement in that a very small change needs to be measured in a very large signal.
Any fluctuations and high backgrounds degrade the quality of the measurements, so
numerous data sets are taken and averaged. The measurement made is the intensityof the parent ion with the laser on compared with the intensity with the laser off todetermine the depletion of the parent ion as a result of photoabsorption. The percent
depletion is measured as the ratio of depleted parent ion intensity over the parent ion
intensity without laser absorption.

Each intensity measurement consists of two

measurements to take into account backgrounds.

The depleted parent signal isdetermined by measuring the intensity with the both the reflectron and the laser on.
The background for this is the intensity of the parent ion with the reflectron off, butwith the laser on. This gives the background due to the presence of the laser itself
being there. The non-depleted parent ion intensity has the reflectron on with the laser
off. From this is subtracted the background with both the laser and the reflectron off.
This ratio with the background subtractions is then inserted into Beer’s Law. The
process is summarized as:

Parent ion depleted = I(on, on) – I(off, on) = Io

Parent ion = I(on,off) – I(off, off) = Ioff

70

σ=

1 ⎡ I on ⎤ln ⎢⎥Φ ⎣⎢ I off ⎦⎥(2.3)

where the (on/off, on/off) corresponds to the state of the reflectron and laser(reflectron, laser). Equation 2.3 is Beer’s law with σ being the cross section in cm2
and Φ is absolute photon flux that is described in a later chapter. The reflectron andlaser state are controlled by the 12 bit DAC. The DAC controls a mechanical shutter
placed in the laser path with a maximum repetition rate of 40 Hz. The reflectron
receives its voltage from a programmable power supply that itself is also controlled
by the 12 bit DAC. Each data cycle consists of making each of the measurements ofthe four states of the reflectron/laser at 40 Hz to accommodate the maximum
repetition rate of the shutter.
A schematic of the analog signal processing is shown in Fig. 2.11. The signal
from the off-axis detector is amplified and sent to the Dual Gated Integrator that istriggered from the second SRS Delay Generator.

The input analog signal ismonitored along with the integrated signal on an oscilloscope. The integrator trigger
timing is adjusted by observing the integrated signal on the scope. The gate from theintegrator is then used to trigger the quad gate generator. From the quad gate
generator, two gates are sent to the LeCroy ADC. The integrated signal is sent to thetwo gated channels of the ADC. The gates are set to be less than 100 ns to provide
the largest dynamical range for the ADC. The gate timing is adjusted for each suchthat Gate 1 resides on the higher plateau of the integrated signal and the Gate 2 on thelower side of the signal (see Fig 2.11). The signal within each gate is digitized and71

EG&G 9301
Preamplifier

Dual Gated Integrator

Quad Gate Generator

Gate

Input

TriggerGate 1

Outputs

Gate 2

LeCroy ADC
Channel 2

Q-switch Input

Channel 4
CAMAC Interface

SRS (DG 535)
A delay

Oscilloscope
to monitor

386 PCIon Signal
Integrated
OutputGate 1Gate 2

Figure 2.11: Analog signal processing electronics.

72
sent to the computer where the area of the ion peak is simply the digitized signal from
Gate 1 minus the signal from Gate 2. The computer program then measures these
digitized peak areas at the various reflectron/laser configurations and calculates thepercent depletion from which the absolute cross section is calculated. The laser
fluence is measured at a photodiode during each cycle to monitor fluctuations in laser
intensity.
• Relative Cross Section Measurements
The relative cross section measurements are carried out by measuring theintensity of the photofragment ion divided by the parent ion intensity multiplied by
the laser intensity. The fragment ions are measured at the off-axis detector with boththe laser and the reflectron on. The reflectron on setting corresponds to placing a
voltage on the reflectron to focuses the photofragment ion onto the off-axis detector.
The off setting corresponds to placing the back plate of the reflectron at the float
potential allowing the parent ion to travel through the reflectron to the in-line
detector. The laser off position simply has a shutter closed blocking the laser. The
various signals to be measured along with the relative cross section calculation aresummarized in Fig. 2.12. The laser signal in the calculation is taken to be themeasured laser power multiplied by the wavelength. The data acquisition program
records the laser signal at a photodiode and includes it in the data output to determine
the magnitude of fluctuations in the laser power. This signal, however, is not directly
used in calculating the relative cross sections.
controlled as described above.

The reflectron and shutter are

73

Reflectron 20 Hz

Laser Shutter 40 Hz

Ion Beam 80 Hz

Reflectron/Laser(ON/ON) (ON/OFF) (OFF/ON) (OFF/OFF)One cycle of experiments

σrel =

Fragment
Parent x Laser

At off-axis detector: Fragment = (on,on) – (off,on)
At in-line detector: Parent = (off, off) – (on, off)
Figure 2.12: Relative cross section data collection and calculation
schematic.

74
The timing of the events within each cycle is depicted in Fig. 2.12. This
scheme is used to maximize the repetition rate of both the ions (80 Hz) and the shutter(40 Hz).

Each cycle consists of taking measurements at each of the fourreflectron/laser configurations. Due to the 80 Hz repetition rate, each reflectron/laser
configuration setting consists of two laser shots per cycle. Numerous runs are taken
at each wavelength with typically 800 or 1000 cycles per run. The largest crosssection across the band of wavelengths is set to equal 1 and the cross sections at theremaining wavelengths are then scaled accordingly.

The absolute cross section

measurement taken using the depletion method is used to determine the absolute crosssections across the band if desired.
• Cluster Photofragmentation Product Distributions
The photofragmentation distributions are carried out by intersecting the laserof a particular wavelength with a mass selected cluster ion. The full set of reflectron
voltages are calculated for every possible photoproduct ion from a given parent
cluster ion. The products present were determined by setting the reflectron to thevoltage to focus a suspected fragment ion onto the off-axis detector. The appearanceof a laser dependent signal on the oscilloscope at the exact flight time of the parent
ion confirms the presence of the fragment ion upon dissociation of the parent cluster
ion.
The fragment ion signal from the off-axis detector, that is amplified if desired,
is sent to a transient recorder and averaged for 3000−5000 laser shots. A reference
peak is chosen (typically the most intense fragment peak) from the photoproduct ionsfor a given parent cluster ion. The magnitude of this peak is determined at the onset

75
of a data run. Each data run consists of an averaged mass spectrum for each production present. Before each photofragment is averaged, the reference peak is checked
and the ions and laser overlap are optimized if needed to achieve the originalreference signal level. This should ensure that all the product ions are collected under
the same ion and laser overlap conditions. Multiple runs are undertaken for eachparent cluster that are then averaged together to yield the final product ratios.
Previously characterized results put the statistical error at about 3% for a given
fragment yield.4

2.11.4 Femtosecond Timing and Data Collection Schemes

The time-resolved experiments are carried out with the femtosecond laser
system described above. The 400 Hz trigger from the Medox box is sent to ahomemade modulo-N-divider that divides this trigger pulse down to 80 Hz. This 80
Hz pulse is used as the external trigger for the SRS delay generator that had beenpreviously triggered internally for the nanosecond experiments. The timing for allother ion events is referenced to the 80 Hz external trigger; T0 of the SRS delay
generator. When the delay generator receives the 80 Hz laser trigger, the laser pulsethat served as the trigger is instantaneously (on the timescale of the ion events) at thelaser interaction region and thus unusable for absorption by the ions.
The events are timed so that the mass-selected ion packet formed by the triggerof the first laser pulse arrives at the laser interaction region as the subsequent laser
pulse arrives. The system triggers off of every fifth laser shot (80 Hz), but laser
pulses are arriving at the chamber at 400 Hz or every 2.5 ms. The three delays that

76
are used from the SRS delay generator are the B delay for the pulsed valve, the C
delay for the extraction plate, and the D delay for the mass gate. The C delay serves
as the laser timing, i.e. this delay is adjusted to overlap the ions and the laser. The
entire sequence occurs in the 2.5 ms period between laser pulses. The mass gatetiming, D, is C + x where x is the flight time between the extraction region and themass gate. This value is determined by adjusting x until the ion of interest is mass
selected. The valve timing, B, is C – y where y is the flight time of the ions from thevalve to the extraction region that is optimized by monitoring the parent ion
intensities. The extraction plate, C, is T0 + z where z is the adjustable timing toensure the ion packet and laser are overlapped temporally. The value for z cancoarsely be calculated by subtracting the value of the D delay, the flight time from theextraction region to the mass gate, from 2.5 ms. However, there is a short ion flighttime between the mass gate and the laser interaction region that must be accounted forby simply adjusting the C timing and observing the appearance of laser-dependent
photofragment ion signal on the oscilloscope.
The off-axis detector is operated in a single ion counting mode for the typicallysmall two-photon signal. The detector is biased at 2.8−2.9 kV with a large amount ofelectronic noise present at these large voltages.

The two-photon signal is pre-amplified and sent to a 300 MHz Phillips 6904 Discriminator. The threshold voltageof the discriminator is set to pass the two-photon signal and reject the smaller
electronic background noise. The output of the discriminator is sent to the Quad
Scaler that counts the ion signal for up to four different two-photon products. The
scaler has four separate channels that are each given a timing gate from a home built

77pulse delay driver. The delay driver is triggered by the A output of the second SRS
delay generator. Each individual timing gate has an adjustable delay and width. The
timing gates have a TTL monitor that is sent to the oscilloscope.
discriminator outputs is likewise sent to the same oscilloscope.

One of the

The gates arecoarsely adjusted by the A delay of the second SRS box by observing their position
with respect to the two-photon signal on the scope. Fine adjustments of each gate
delay and width are made with respect to the two-photon product signals on thescope. The Quad scaler is connected to the data acquisition program through the
CAMAC and GPIB interfaces. The parent ions that do not interact with the laser aremonitored on the in-line detector for normalization of the two-photon signal. The inline detector signal is sent to the Dual Gated Integrator and monitored on an
oscilloscope in a manner analogous to the cross section measurements mentioned
above.
The data acquisition program measures the counts in each of the four set gates
at a series of five delay pump-probe times. The desired delay times are inputtedalong with a reference delay time. The program moves the delay stage through the 12
bit DAC along with shutters in the pump and the probe optical paths for backgroundcollection. The reference signal is chosen to be at a point where there is significant
two-photon signal if possible. Within each data cycle, three backgrounds are taken.
Two backgrounds have either the pump or the probe blocked and the thirdbackground has both pump and probe blocked. The ion counts at the reference aretaken after the backgrounds and then counts at each of the five delays are taken.
Finally, the counts at the reference are again taken. If the reference counts before and78
after the delay points vary by more than 10%, the data set is discarded. Also, if the
background at the three pump/probe configurations (off/on + on/off – off/off) is morethan 30 or 40 % of the reference signal then the data set is also discarded. Typically,
3000 or 5000 laser shots are taken at each delay or background point over multiple
cycles. At each point, the integrated parent ion intensity from the in-line detector isrecorded and used for normalization of the data. A very detailed description of the
femtosecond pump-probe experiment is given elsewhere.22

2.11.5 Photoelectron Imaging Timing and Data Acquisition

The photoelectron imaging experiment uses the Infinity nanosecond laser
system, but is triggered in a manner similar to the femtosecond experiments. The
SRS delay generator is triggered by the Q-switch synch output from the Infinity
power supply after being divided down to 50 Hz by the modulo-N-divider from the
100 Hz repetition rate of the Infinity laser. The delays are the same as for the
femtosecond experiment with the C delay being the extraction and “laser” delay, B is
the pulsed valve delay and D is the mass gate delay. The timing of C is calculated in
the same manner, but now the D delay is subtracted from 10000 µs, reflecting the
new repetition rate of the laser. The new delay introduced in the imaging experimentis the electron extraction plate timing that is controlled by delay A. The A delay is
set to be roughly C + 15 µs. The extraction plate is turned well before the ions arrivein the laser interaction region and is set for width of 50 µs. Initially, the extraction
plate was turned on a few nanoseconds before the laser intersected the mass-selected
ion packet and stayed on for only 200 ns. The extraction plate timing was also tuned

79
to optimize the images. However, it was determined that the ring diameters in theimages (and thus the energies) change as this timing is varied at these very small
extraction plate pulse widths. Although, not visually noticeable when observing theimages, small changes in the extraction timing lead to significant changes in thephotoelectron energy spectra. The longer extraction plate pulse width turned on wellbefore the ions reach the laser interaction region eliminates this ring size dependence
on the extraction timing.
The images are acquired in two modes with both being controlled by the
DaVis data acquisition software that accompanied the CCD camera and are depicted
in Fig 2.13. The first mode is the so-called analog mode with images being acquired
for a variable exposure time (0.1 to 100000 µs). These analog images contain not
only the photoelectron counts, but also any background counts present. Backgroundimages are taken with the laser blocked to determine the parent ion background andwith the ions blocked (gate valve closed) to determine the background from the laser.
The laser background is typically low at wavelengths longer than 300 nm with the ion
background being much larger. The backgrounds are subtracted from thephotoelectron images using routines programmed in LabView. Multiple images arealso summed together after background subtractions using these same routines.
The second acquisition method is carried out by using the event counting
mode of the data acquisition software. In the analog method, an event appeared as aspot that was roughly 6 x 6 square pixels in area along with a background of various
magnitudes throughout the remaining pixels. Event counting looks for the maximum
number of counts within a 6 x 6 square pixel area and puts a value of one in the center

80

Analog Data

Event Counted Data

Figure 2.13: Simulated data showing typical analog image data versus
event counted image data.

81
pixel if the counts are above a set threshold. The threshold is determined by astatistics function within the DaVis software. This takes images with events andwithout events (the phosphor screen voltage set to zero) and recommends a thresholdsetting and an offset value. The offset value is the constant background counts acrossthe pixel array that is subtracted from the event peaks. The threshold value is then set
to only allow photoelectron events to be counted that are presumably greater than any
background counts. In this way, pixel resolution is achieved on a zero background.
This method leads to higher quality images with improved resolution as compared tothe analog method.
A concern with event counting is that signal loss can occur if the CCD camera
exposure time is not set to be short enough. If this exposure time is long, it isprobable that multiple, real photoelectron events occur within the same 6 x 6 pixelarea. However, as the event counting procedure is defined, only the event withhighest intensity is counted with the other events being discarded. It is found thatusing exposure times of 100 or 250 µs minimizes signal loss. A full image is thenacquired by taking 3000 to 100000 short time exposures and summing them.
Multiple full images can themselves be summed using the LabView routines
mentioned above.
In either the analog or event counting mode, the images at this point represent
the raw 2-D representations of the 3-D photoelectron spheres. The 3-D spheres arereconstructed using the BASEX method developed in the Reisler group23 from whicha slice is taken through the center. The BASEX software takes the event counted raw
image .dat file as the input.

Accurate determination of the pixel coordinates of the82
center of the image is critical. The second version of BASEX contains an algorithm
that calculates the center of inputted image. This is checked by overlaying a circle on
the raw image and determining the coordinates of the center visually. The BASEX
calculated center is generally accurate, although having the center coordinates wrong
by a few pixels is enough to degrade the reconstructions severely.
The BASEX software provides the raw image, the reconstructed image, andthe speed distribution as the outputs. The speed distribution is the angle integrated
intensity as a function of a selected pixel step size (usually 1 pixel) in the radial
direction from the center of the image.

The speed scale is then converted to electron

kinetic energy (eKE) by using a calibration spectrum of known peak energies. It is
typically desirable to use an atomic species for calibration due to much narrower
photoelectron peaks widths in atoms.

A new calibration is needed when theextraction voltage is changed, as the extraction voltage determines the pixel energy
scale. The electron kinetic energy is then simply converted to electron binding
energy (eBE) by use of the equation eBE = Ehν – eKE, where Ehν is the photon energy.
Additionally, BASEX calculates the anisotropy parameter, β, at each radial pixel or
eKE. The exact definition of β is discussed below in a following chapter.

83
Chapter 2 References
1

W. C. Wiley and I. H. McLaren, Rev. Sci. Instr. 26 (12), 1150 (1955).

2

A. Eppink and D. H. Parker, Rev. Sci. Instr. 68 (9), 3477 (1997).

3

M. A. Johnson and W. C. Lineberger, in Techniques for the Study of Ion
Molecule Reactions, edited by J. M. Farrar and J. W. Saunders (Wiley, New
York, 1988), pp. 591.

4M. E. Nadal, Ph.D. Thesis, University of Colorado, 1996.

5

M. E. Nadal, P. D. Kleiber, and W. C. Lineberger, J. Chem. Phys. 105 (2), 504(1996).

6

J. M. Papanikolas, J. R. Gord, N. E. Levinger, D. Ray, V. Vorsa, and W. C.
Lineberger, J. Phys. Chem. 95 (21), 8028 (1991).

7

V. Vorsa, S. Nandi, P. J. Campagnola, M. Larsson, and W. C. Lineberger, J.
Chem. Phys. 106 (4), 1402 (1997).

8

M. A. Taylor, Ph.D. Thesis, University of Colorado, 2004.

9

A. W. Castleman, Jr. and R. G. Keesee, Chem. Rev. 86, 589 (1986).

10

J. I. Steinfeld, J. S. Francisco, and W. L. Hase, Chemical Kinetics andDynamics. (Prentice Hall, Englewood Cliffs, New Jersey, 1989).

11

N. E. Levinger, Ph.D. Thesis, University of Colorado, 1990.

12

W. H. Robertson, J. A. Kelley, and M. A. Johnson, Rev. Sci. Instr. 71 (12),4431 (2000).

13

U. Even, J. Jortner, D. Noy, N. Lavie, and C. Cossart-Magos, J. Chem. Phys.
112, 8068 (2000).

14

M. Hillenkamp, S. Keinan, and U. Even, J. Chem. Phys. 118, 8699 (2003).

15

M. L. Alexander, N. E. Levinger, M. A. Johnson, D. Ray, and W. C.
Lineberger, J. Chem. Phys. 88 (10), 6200 (1988).

16

B. A. Mamyrin, V. I. Karataev, D. V. Shmikk, and V. A. Zagulin, Sov. Phys.JEPT 37 (1), 45 (1973).

84
17

L. A. Posey, M. J. DeLuca, and M. A. Johnson, Chem. Phys. Lett. 131 (3),170 (1986).

18

E. Surber, R. Mabbs, and A. Sanov, J. Phys. Chem. A 107 (40), 8215 (2003).

19

A. E. Siegman, Lasers. (University Science Books, Mill Valley, CA, 1986).

20

M. H. Dunn and M. Ebrahimzadeh, Science 286 (5444), 1513 (1999).

21

J. D. Kafka, M. L. Watts, and J.-W. J. Pieterse, IEEE Journal of Quantum
Electronics 28 (10), 2151 (1992).

22

J. M. Papanikolas, Ph.D. Thesis, University of Colorado, 1994.

23

V. Dribinski, A. Ossadtchi, V. Mandelshtam, and H. Reisler, Rev. Sci. Instr.
73, 2634 (2002).

85

Chapter 3
Photodissociation Dynamics of IBr⎯(CO2)n Cluster Anions

3.1

Introduction

Photodissociation of mass-selected IBr⎯(CO2)n, n = 0–14, cluster ions iscarried out to study the effects of the presence of the CO2 solvent on the IBr⎯ soluteanion. One-color experiments focus on the determination of anion photoproductbranching ratios as both a function of excitation wavelength and initial cluster size.
Photoproduct branching ratios illustrate the degree of perturbation to the internal
electronic structure of the IBr⎯ solute upon solvation by a known number of solutes
molecules in well-defined configurations. The steric effects of the solvent cage as aphysical barrier to dissociation that may be present in these systems are also
highlighted by the photoproduct distributions.
The IBr⎯(CO2)n clusters are excited to the A' 2Π1/2 and B 2 2Σ+1/2 electronic
states of the IBr⎯ chromophore. The photoproducts for excitations to these two states
are determined for the bare IBr⎯ solute. The effects of the solvent are then studied by
the types of photoproducts formed as a function of initial cluster size. The solvent isshown to drive nonadiabatic transitions in the chromophore solute involving chargetransfer between the dissociating atoms of the solute, spin-orbit relaxation within theneutral photoproducts, and vibrational relaxation followed by solute recombination on
the ground state. The photoproduct ratios provide the data needed to carry out a timeresolved pump-probe experiment to determine the timescale for recombination ofIBr⎯(CO2)8 on the ground 2Σ+1/2 state, following excitation to the A' state.

86
Prior to the solvation studies of IBr⎯(CO2)n clusters, the IBr⎯ chromophore is
first spectroscopically characterized to determine the properties of the solute before
solvation as well as to determine the excitation wavelengths to be used in the clusterphotodissociation experiments. Relative cross section measurements are carried out
across the A' 2Π1/2 ← X 2Σ+1/2 absorption band to determine the wavelength profile aswell as to identify the photoproduct(s) present after excitation to the A' state. A
depletion measurement is employed near the band maximum to record the absolute
cross section measurement that is then used to scale the relative cross section
measurements at the other wavelengths. The absolute cross sections are used todetermine the feasibility of two-photon, time-resolved experiments. Translationalenergy release measurements of the photodissociated IBr⎯ provide a bond strength
measurement for the solute. The experimental results are compared to theoretical
calculations of the potential energy curves and absorption spectrum.1,2
The photodissociation dynamics of IBr⎯(CO2)n clusters is discussed in thecontext of comparisons with results of similar experiments previously carried out inICl⎯(CO2)n and I2⎯(CO2)n cluster ions. These previous results along with the resultsfor IBr⎯(CO2)n clusters present an overall picture of the role of the various dihalide
solutes in determining the solvation dynamics. The IBr⎯(CO2)n clusters, also, present
a mixed halide system that is well-suited for time-resolved experiments.

87
3.2

Spectroscopic characterization of bare IBr⎯

The calculated potential energy curves for the six low-lying electronic statesof IBr⎯ anion are shown in Fig. 3.1.

The ground state is bonding in nature and has a

deep, attractive well. The other five electronic states are either dissociative or havevery shallow wells. The IBr⎯ anion has 11 valence electrons, one short of a fullvalence shell. The anion behaves as a one-electron molecule with the symmetry of
the electronic state determined by location of the electron hole. The state labels in
Fig. 3.1 correspond to Hund’s case (c) where strong spin-orbit mixing is present andis experimentally confirmed below. The strong spin-orbit coupling leads to four
distinct dissociation limits for the six valence electronic states. The dissociative
states with neutral iodine as a photoproduct are asymptotically separated by the 0.94
eV spin-orbit splitting in iodine.3

The curves corresponding to formation of theneutral bromine photoproduct are separated by 0.457 eV, reflecting the spin-orbit
splitting in bromine.4 The IBr⎯ anion has a formal bond order of 1/2 and has acalculated 5 equilibrium bond length of 3.053 Å. The measured vibrational frequency
of IBr⎯ is 136 cm-1.6
The photoabsorption spectrum of the A' 2Π1/2 ← X 2Σ+1/2 absorption band inIBr⎯ is measured to determine the shape, location, and magnitude of this band.
Characterization of this band is crucial in that excitation on the A' electronic state is afocus of experiments described below. The spectrum characterization is carried out
by the measurement of relative cross sections at wavelengths across the band as
discussed in Ch. 2. The measured spectrum for A' is shown in Fig. 3.2. The band is a

88

2.0

1.5

+

B 2 2Σ

Energy, eV

1.0Br⎯ + I*

1/2

I⎯ + Br*a' 2 2 Π 1/2

0.5

a 2 2 Π 3/2

I⎯ + BrA' 2 Π 1/2
0.0

3/
3

Br⎯ + IA 2 Π 3/2
-0.5
+

X 2Σ

1/2

-1.0
2

3

4

5

6

7

8

R (I-Br), Å
Figure 3.1: Calculated potential energy curves for the lowest six
electronic states of IBr⎯. Arrows indicate the studied transitions to the A'
and B states.

89

Relative Cross Section (arb. units)

2.5
X
2.02

A'(2Π1/2)

1.5

1.01
0.5
00
600
600

650
650

700
750
800
700
750
800
Wavelength (nm)

850
850

900
900

Figure 3.2: Relative cross sections for the A' absorption band. The
solid line is a Gaussian fit to the data points. The error bars represent
one standard deviation of the statistical uncertainty.

90
broad, featureless Gaussian-shaped curve centered around 740 nm with a full-width
half-maximum of ~50 nm. The absence of points between 690–740 nm is due to thedegeneracy gap in the Infinity-XPO laser system described in Ch. 2. A Gaussian
curve is fit to the experimental data points and is superimposed on the data in Fig.
3.2. The error bars for each point represent the statistical fluctuations in the relative
cross section measurements and are taken to be one standard deviation of the mean.
The photoproduct I⎯ is the sole product formed across the band measured confirmingthat only one electronic state is excited, with dissociation taking place exclusively on
the A' electronic state.
The calculated absorption spectrum2 is shown in Fig. 3.3. The peak of the A'band is calculated to be 765 nm, which is slightly red-shifted from the experimentallymeasured band maximum of 740 nm. There is also calculated to be negligible
overlap between the A' band and the absorption bands of adjacent electronic states.
Transitions to the B electronic state are also investigated in experiments described
below. The spectroscopic characterization of the B band was not carried out. The
wavelengths that are used for excitation to this state are inferred from the calculated
spectrum.

The experimentally measured A' band was blue-shifted from thecalculated, so it is assumed that peak transitions to the B state are slightly blue-shiftedfrom the calculated band maximum of 405 nm.
A depletion measurement that is described in Ch. 2 is carried out at 760 nm
near the measured A' band maximum. The depletion measurement yields the absolute
cross section via Beer’s Law. The photon flux, Φ, is given by the measured laser
pulse energy divided by hν times the area of the laser spot where ν is the frequency of

91

2 +

Σ 1/2 (B)22

Π1/2 (A')

2

Π1/2 (a')Π3/2 (a)

2

200
200

4400
00

6600
00

8800
00

Π3/2 (A)

11000
000

11200
200

11400
400

Wavelength (nm)Figure 3.3: Calculated absorption spectrum for unsolvated IBr⎯.

92
the laser light. The photon flux has the units of photons/pulse∗cm2. For instance, 760
nm light with a pulse energy of 2.1mJ/pulse and a spot diameter of 10.5 mm has aflux of 9.28 x 1015 photons/pulse∗cm2. The measured depletion was found to be 6%
yielding an absolute cross section of 6.9 x 10-18 cm2 at 760 nm. Using the measured
relative cross sections, the absolute cross section is calculated to be 3 x 10-18 cm2 at
790 nm. The cross section at this wavelength is calculated to determine the feasibilityof two-photon experiments on IBr⎯. The femtosecond laser system performs mosteffectively at wavelengths around 790 nm. In the pump-probe absorption recovery
experiments, the cross section for the two-photon absorption process is approximatedas the cross section for one-photon absorption squared. The measured cross section
for IBr⎯ at 790 nm from above is 3 x 10-18 cm2. Similar measurements carried out on
I2⎯ and ICl⎯ found the cross sections at 790 nm to be 2.5 x 10-17 and <<5 x 10-19 cm2,
respectively.7,8 The two-photon cross section for IBr⎯ is 100 times less than for I2⎯
and ICl⎯ is even weaker at about 1000 times less than for I2⎯.

Two-photon

experiments have been successfully carried out for I2⎯ solvated systems.9-12 Twophoton experiments are also shown in a following section to be successful at thismuch lower two-photon cross section for IBr⎯. However, based on these cross section
arguments, ICl⎯ two-photon experiments are probably not feasible at wavelengths
around 790 nm.
The near-IR absorption spectrum was measured for unsolvated IBr⎯, but notmeasured for the IBr⎯(CO2)n clusters. The absorption spectrum of ICl⎯(CO2)4 has
been measured previously13 to determine the effect of solvation on the absorption
characteristics of the ICl⎯ solute. This cluster size was chosen due to the prediction

93
that the largest solvent electric field (largest asymmetric solvation) on the ICl⎯ soluteelectronic states is produced at this size. The solvated spectrum shows virtually noshift when compared to the bare ICl⎯ spectrum. The electronic structure of the ICl⎯solute is not significantly perturbed by the presence of the solvent at internuclear
separations close to the equilibrium bond length, which is the region that is being
probed by the one-photon cross section measurements.

This agrees well withdynamics simulations14 that show perturbations to the electronic states occurring atlarge internuclear separations (~6–8 Å). It is assumed that the perturbations to theIBr⎯ solute electronic states only occur at large internuclear separations, as well.
Translational energy spectroscopy15 is used to measure the kinetic energy
release on the A' state during the photodissociation of IBr⎯ with multiple excitation
wavelengths.
backward

The measurement of the time splitting between the forward andscatteredneutralfragmentsonthein-linedetectorfrom thephotodissociated IBr⎯ is used to determine the kinetic energy release on the A' state.
The kinetic energy release is then related to the bond strength, D0, for the ground stateof IBr⎯. The angular distributions of the recoiling neutral Br photofragments are also
used to determine the direction of the A' 2Π1/2 ← X 2Σ+1/2 electronic transition upon
excitation with linearly polarized light.
The time-of-flight spectra for the photodissociated Br atoms are shown in Fig.
3.4 with (a) corresponding to dissociation with light polarized parallel to the ion flightaxis and (b) is light polarized perpendicular to the flight axis. Figure 3.4(a) shows aspectrum consisting of a doublet of peaks representing neutral fragments ejected withvelocities toward and away from the in-line detector. The case of the light

94

(a)
∆t

Br backward

Intensity

Br forward

Flight Time

Intensity(b)Flight Time
Figure 3.4: Neutral Br time-of-flight spectra of photodissociatedIBr⎯ at 790 nm with the laser polarized (a) parallel to the ion flightaxis and (b) perpendicular to the flight axis.

95
polarization perpendicular to the flight axis leads to a single peak in the neutral
spectrum. These results show that the fragments are ejected along the polarization
axis and the transition dipole moment is parallel to the internuclear axis of IBr⎯. This
characterizes the A' 2Π1/2 ← X 2Σ+1/2 electronic transition as a parallel transition.
In the absence of spin-orbit coupling, Hund’s case (a) can be applied in which
the transition dipole moment lies along the internuclear axis for a purely paralleltransition.

This is the case for the ∆Λ = 0, Σ←Σ or Π←Π.

For a purely

perpendicular transition, the dipole moment lies perpendicular to the internuclear
axis. This occurs for ∆Λ = ±1, Σ ←Π. The A' 2Π1/2 ← X 2Σ+1/2 transition should be
perpendicular under Hund’s case (a), however it is experimentally found to beparallel. Strong spin- orbit coupling leads to a Hund’s case (c) description where thetransition moment does not have to lie along a symmetry axis. This also leads to thetwo spin-orbit states for both I and Br and as seen by the presence of four distinct
product channels in the potential energy curves shown in Fig. 3.1.
The photodissociation of IBr⎯ with parallel polarization laser light leads todistinct forward and backward scattered peaks as labeled in Fig. 3.4(a). The flighttime splitting between the two is measured and used to determine the kinetic energy
release on the A' electronic state. The kinetic energy of the parent IBr⎯ anion is foundby taking half of the extraction voltage and adding this to the beam potential of 2500
eV. The velocity of the center of mass,VCOM, is then:

VCOM =

2E
M

(3.1)

96
where E is the kinetic energy of the IBr⎯ in joules and M is the mass of IBr⎯ inkilograms. The recoil velocity of the Br fragment, VBr, is found by the expression:

VBr =(VCOM ) 2 ∆t
2x

(3.2)

where VCOM is the center of mass velocity found from Eq. 3.1, ∆t is the measured
time difference between the forward and backward scattered peaks, and x is the
distance from the laser interaction region to the in-line detector (0.448 m for thisapparatus). The kinetic energy release (KER) is:⎡ m ⎤1
KER = VBr2 ∗ mBr ∗ ⎢1 + Br ⎥
2
mI ⎦
⎣

(3.3)

where VBr is the recoil velocity of the Br fragment from Eq. 3.2, mBr and mI are themasses of Br and I, respectively. Finally, the bond strength of IBr⎯, D0, is found by:

D0 = hν − ⎡⎣ KER + ( EA( Br ) − EA( I ) ) ⎤⎦(3.4)

where hν is the A' excitation photon energy, KER is found from Eq. 3.3, and EA(Br)and EA(I) are the electron affinities of Br and I, 3.3636 and 3.0591 eV, respectively.
The D0 calculations are performed at four different wavelengths within the A' band
and averaged to give a value of the bond strength of IBr⎯ of 1.10 ± 0.04 eV, where the97
error represents one standard deviation of the mean. This makes the bond strength ofIBr⎯ comparable to that of ICl⎯ at 1.07 eV and I2⎯ at 1.01 eV.8,16

3.3

Photofragmentation of IBr⎯(CO2)n upon excitation to the A' 2Π1/2 state

The photodissociation of mass-selected IBr⎯(CO2)n cluster ions is investigatedat a series of wavelengths providing excitation to the spectroscopically characterized
A' 2Π1/2 electronic state of IBr⎯. The photodissociation is carried out at 790 nm, 760
nm, and 675 nm. These wavelengths span the absorption band of A' and provide
kinetic energy releases on A' of 0.3, 0.35, and 0.55 eV, respectively.

The 760 nm

wavelength is chosen due to its proximity to the peak of the absorption band, 790 nm
due to the possibility of use in time-resolved experiments, and 675 nm to provide alarger kinetic energy release on the A' state. From the absorption studies of IBr⎯,these wavelengths are shown to exclusively excite A' with I⎯ and Br as the solephotoproducts.
Solvation of IBr⎯ by CO2 yields three distinguishable product channels:
IBr⎯(CO2)n→I⎯(CO2)n-m + mCO2 + Br

(A)→IBr⎯(CO2)n-k + kCO2

(B)→

Br⎯(CO2)n-p + pCO2 + I(C)Channel (A) corresponds to direct dissociation on the A' state, giving the expected I⎯based dissociation products. Channel (B) corresponds to cage recombination,
reforming the dihalide anion on the ground electronic state after dissociation. The
number of solvents retained on the recombined products (B) reflects the extent of

98vibrational relaxation on the ground state and is discussed in detail below. Channel(C) corresponds to a dissociative event following charge transfer from the initiallyformed I⎯ to the neutral Br. Both Channels (B) and (C) represent processes that
require solvent participation. For the larger clusters (n ≥ ~ 4, an additional channel
producing spin-orbit excited iodine, I*, is energetically accessible through theadditional process
IBr⎯(CO2)n→Br⎯(CO2)n-(p+4) + (p+4)CO2 + I*.

(D)

The photofragmentation data show no evidence for product formation in this channel.
Molecular dynamics simulations17 have shown product formation in this channel for
ICl⎯(CO2)n cluster photodissociation.

Experimental results in previous work13

likewise show this channel not to be present after excitation to A' in ICl⎯(CO2)nclusters. However, results on ICl⎯(CO2)n clusters at 790 nm exciation presented inSec. 3.6 may show this channel to be accessible.
The photofragmentation products from excitation of IBr⎯(CO2)n clusters with
790, 760, and 675 nm photons as a function of cluster size are shown in Fig. 3.5, Fig.
3.6, and Fig. 3.7, respectively.

The exact photoproducts formed with theirpercentages are shown in Tables 3.1, 3.2, and 3.3.

The total percentages in bold areplotted in the figures. In each case, the photodissociation of the bare IBr⎯ solute leads
solely to I⎯ as the ionic photoproduct. For 790 and 760 nm excitation, the addition of
the first solvent molecule allows a nonadiabatic transition to the ground electronic
state, yielding the charge transfer Br⎯ product ion. The addition of the second CO2
solvent molecule increases the yield of the charge transfer Br⎯-based products and99

Ionic Product Percentage

100

80

I- basedBr- based

60

IBr- based

40

20

0
0

2

4

6

8

10

12

14

Number of CO2 molecules (n)Figure 3.5: Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the A' 2Π state with 790 nm photons and an energy
release of 0.3 eV on the state.

100
Table 3.1: Photofragmentation branching ratios for IBr⎯(CO2)n at 790 nm.
Parent Ion
IBr⎯
IBr⎯(CO2)1
IBr⎯(CO2)2

IBr⎯(CO2)3

I⎯-basedBr⎯-basedI⎯: 100%

NONE

NONE

I⎯: 36.3%
I⎯(CO2)1: 60.5%[96.8%]
I⎯: 4.5%
I⎯(CO2)1: 65.1%
I⎯(CO2)2: 3.9%
[73.5%]I⎯(CO2)1: 48.9%
I⎯(CO2)2: 4.1%

Br⎯: 3.2%

NONE

IBr⎯(CO2)4[53.0%]I⎯(CO2)1: 2.4%

IBr⎯(CO2)5[2.4%]
NONE[3.2%]
Br⎯: 5.1%
Br⎯(CO2)1: 12.8%[17.9%]Br⎯(CO2)1: 12.2%
Br⎯(CO2)2: 2.5%
Br⎯(CO2)3: 4.8%
[19.5%]Br⎯(CO2)1: 6.6%
Br⎯(CO2)2: 16.2%[22.8%]Br⎯(CO2)2: 3.1%
Br⎯(CO2)3: 5.6%

NONE

[8.7%]Br⎯(CO2)3: 3.6%

IBr⎯(CO2)7NONE[3.6%]Br⎯(CO2)4: 3.0%

IBr⎯(CO2)8NONE[3.0%]
NONE

IBr⎯(CO2)9

NONE

NONE

IBr⎯(CO2)6

IBr⎯-based

IBr⎯: 8.6%
[8.6%]
IBr⎯: 23.2%
IBr⎯(CO2)1: 4.3%
[27.5%]
IBr⎯: 72.8%
IBr⎯(CO2)1: 2.0%
[74.8%]
IBr⎯: 49.4%
IBr⎯(CO2)1: 35.7%
IBr⎯(CO2)2: 6.2%
[91.3%]
IBr⎯: 5.8%
IBr⎯(CO2)1: 58.0%
IBr⎯(CO2)2: 31.0%
IBr⎯(CO2)3: 1.6%
[96.4%]
IBr⎯(CO2)1: 16.0%
IBr⎯(CO2)2: 50.8%
IBr⎯(CO2)3: 22.4%
IBr⎯(CO2)4: 7.8%
[97.0%]
IBr⎯(CO2)1: 2.1%
IBr⎯(CO2)2: 25.9%
IBr⎯(CO2)3: 48.6%
IBr⎯(CO2)4: 20.2%
IBr⎯(CO2)5: 3.2%
[100%]
IBr⎯(CO2)2: 5.6%
IBr⎯(CO2)3: 37.4%
IBr⎯(CO2)4: 38.6%
IBr⎯(CO2)5: 16.6%
IBr⎯(CO2)6: 1.8%
[100%]

101
IBr⎯(CO2)10

NONE

NONE

IBr⎯(CO2)11

NONE

NONE

IBr⎯(CO2)12

NONE

NONE

IBr⎯(CO2)13

NONE

NONE

IBr⎯(CO2)14

NONE

NONE

IBr⎯(CO2)3: 11.8%
IBr⎯(CO2)4: 38.2%
IBr⎯(CO2)5: 37.9%
IBr⎯(CO2)6: 12.1%[100%]
IBr⎯(CO2)4: 7.4%
IBr⎯(CO2)5: 50.1%
IBr⎯(CO2)6: 42.5%[100%]
IBr⎯(CO2)5: 22.2%
IBr⎯(CO2)6: 48.7%
IBr⎯(CO2)7: 29.1%[100%]
IBr⎯(CO2)7: 87.4%
IBr⎯(CO2)8: 12.6%[100%]
IBr⎯(CO2)7: 53.7%
IBr⎯(CO2)8: 46.3%[100%]

102

Ionic Product Percentage

100

80

60

I- basedBr- based
IBr- based

40

20

0
0

2

4

6

8

10

12

14

Number of CO2 molecules (n)Figure 3.6: Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the A' 2Π state with 760 nm photons and an energy
release of 0.35 eV on the state.

103
Table 3.2: Photofragmentation branching ratios for IBr⎯(CO2)n at 760 nm.
Parent Ion
IBr⎯
IBr⎯(CO2)1
IBr⎯(CO2)2

IBr⎯(CO2)3

IBr⎯(CO2)4
IBr⎯(CO2)5

IBr⎯(CO2)6

IBr⎯(CO2)7I⎯-basedBr⎯-basedI⎯: 100%

NONE

NONE

I⎯: 31.1%
I⎯(CO2)1: 64.7%[95.8%]
I⎯: 5.0%
I⎯(CO2)1: 72.0%
I⎯(CO2)2: 7.0%
[84.0%]I⎯(CO2)1: 41.8%
I⎯(CO2)2: 10.8%

Br⎯: 4.2%

NONE[52.6%]I⎯(CO2)1: 1.0%
I⎯(CO2)2: 2.2%
[3.2%]
NONE

NONE

NONE

[4.2%]
Br⎯: 6.2%
Br⎯(CO2)1: 7.3%
[13.5%]Br⎯(CO2)1: 21.6%
Br⎯(CO2)2: 2.4%
Br⎯(CO2)3: 0.3%
[24.3%]Br⎯(CO2)1: 6.3%
Br⎯(CO2)2: 23.2%[29.5%]Br⎯(CO2)2: 5.9%
Br⎯(CO2)3: 6.5%
[12.4%]Br⎯(CO2)3: 12.0%[12.0%]Br⎯(CO2)4: 1.7%
Br⎯(CO2)5: 1.0%
[2.7%]

IBr⎯(CO2)8

IBr⎯-based

NONE

NONE

IBr⎯: 2.5%
[2.5%]
IBr⎯: 22.1%
IBr⎯(CO2)1: 1.0%
[23.1%]
IBr⎯: 65.6%
IBr⎯(CO2)1: 1.7%
[67.3%]
IBr⎯: 57.2%
IBr⎯(CO2)1: 27.5%
IBr⎯(CO2)2: 2.9%
[87.6%]
IBr⎯: 19.1%
IBr⎯(CO2)1: 57.0%
IBr⎯(CO2)2: 11.0%
IBr⎯(CO2)3: 0.3%
[88.0%]
IBr⎯: 0.9%
IBr⎯(CO2)1: 29.5%
IBr⎯(CO2)2: 51.8%
IBr⎯(CO2)3: 12.9%
IBr⎯(CO2)4: 2.2%
[97.3%]
IBr⎯(CO2)1: 3.3%
IBr⎯(CO2)2: 34.4%
IBr⎯(CO2)3: 45.2%
IBr⎯(CO2)4: 15.8%
IBr⎯(CO2)5: 1.3%
[100%]

104
IBr⎯(CO2)9

NONE

NONE

IBr⎯(CO2)10

NONE

NONE

IBr⎯(CO2)11

NONE

NONE

IBr⎯(CO2)12

NONE

NONE

IBr⎯(CO2)13

NONE

NONE

IBr⎯(CO2)14

NONE

NONE

IBr⎯(CO2)2: 8.9%
IBr⎯(CO2)3: 42.4%
IBr⎯(CO2)4: 36.3%
IBr⎯(CO2)5: 11.2%
IBr⎯(CO2)6: 1.2%
[100%]
IBr⎯(CO2)3: 13.9%
IBr⎯(CO2)4: 45.5%
IBr⎯(CO2)5: 30.7%
IBr⎯(CO2)6: 9.9%
[100%]
IBr⎯(CO2)4: 21.1%
IBr⎯(CO2)5: 43.9%
IBr⎯(CO2)6: 27.5%
IBr⎯(CO2)7: 8.6%
[100%]
IBr⎯(CO2)5: 34.0%
IBr⎯(CO2)6: 48.4%
IBr⎯(CO2)7: 17.6%[100%]
IBr⎯(CO2)7: 86.9%
IBr⎯(CO2)8: 13.1%[100%]
IBr⎯(CO2)7: 59.3%
IBr⎯(CO2)8: 40.7%[100%]

105

Ionic Product Percentage

100

80

60

I- basedBr--based

40

IBr--based

20

0
0

2

4

6

8

10

12

14

Number of CO2 molecules (n)Figure 3.7: Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the A' 2Π state with 675 nm photons and an energy
release of 0.55 eV on the state.

106
Table 3.3: Photofragmentation branching ratios for IBr⎯(CO2)n at 675 nm.
Parent Ion
IBr⎯
IBr⎯(CO2)1
IBr⎯(CO2)2

IBr⎯(CO2)3
IBr⎯(CO2)4I⎯-basedBr⎯-based

IBr⎯-basedI⎯: 100%

NONE

NONE

I⎯: 18.0%
I⎯(CO2)1: 82.0%[100.0%]I⎯: 4.0%
I⎯(CO2)1: 83.0%
I⎯(CO2)2: 8.0%
[95.0%]I⎯(CO2)1: 57.2%
I⎯(CO2)2: 25.0%[82.2%]I⎯(CO2)1: 11.5%
I⎯(CO2)2: 36.8%
I⎯(CO2)3: 2.8%

NONE

NONE

Br⎯: 3.0%
Br⎯(CO2)1: 2.0%

NONE

IBr⎯(CO2)5[51.1%]I⎯(CO2)3: 14.1%

IBr⎯(CO2)6[14.1%]I⎯(CO2)3: 2.0%

IBr⎯(CO2)7[2.0%]
NONE

IBr⎯(CO2)8

IBr⎯(CO2)9

NONE

NONE

[5.0%]
Br⎯: 4.0%
Br⎯(CO2)2: 10.2%[14.2%]Br⎯(CO2)1: 16.1%
Br⎯(CO2)2: 4.1%
Br⎯(CO2)3: 1.7%
Br⎯(CO2)4: 2.3%
[24.2%]Br⎯(CO2)2: 27.0%
Br⎯(CO2)3: 2.8%
Br⎯(CO2)4: 8.5%
Br⎯(CO2)5: 3.0%
[41.3%]Br⎯(CO2)3: 6.9%
Br⎯(CO2)4: 2.2%
Br⎯(CO2)5: 31.4%
Br⎯(CO2)6: 13.1%[53.6%]Br⎯(CO2)4: 2.2%
Br⎯(CO2)5: 1.9%
Br⎯(CO2)6: 39.0%
Br⎯(CO2)7: 5.0%
[48.0%]Br⎯(CO2)7: 34.5%[34.5%]Br⎯(CO2)7: 5.3%
Br⎯(CO2)8: 10.0%[15.3%]

IBr⎯: 3.6%
[3.6%]
IBr⎯: 20.0%
IBr⎯(CO2)1: 1.6%
IBr⎯(CO2)2: 3.1%
[24.7%]
IBr⎯: 39.7%
IBr⎯(CO2)1: 4.9%
[44.6%]
IBr⎯: 23.9%
IBr⎯(CO2)1: 15.6%
IBr⎯(CO2)2: 1.8%
IBr⎯(CO2)3: 3.1%
[44.4%]
IBr⎯: 7.2%
IBr⎯(CO2)1: 31.9%
IBr⎯(CO2)2: 8.5%
IBr⎯(CO2)3: 2.2%
IBr⎯(CO2)4: 2.2%
[52.0%]
IBr⎯(CO2)1: 8.9%
IBr⎯(CO2)2: 36.3%
IBr⎯(CO2)3: 20.3%[65.5%]
IBr⎯(CO2)1: 7.7%
IBr⎯(CO2)2: 36.8%
IBr⎯(CO2)3: 32.5%
IBr⎯(CO2)4: 7.8%
[84.7%]

107
IBr⎯(CO2)10NONEBr⎯(CO2)8: 2.7%

IBr⎯(CO2)11NONE[2.7%]
NONE

IBr⎯(CO2)12

NONE

NONE

IBr⎯(CO2)13

NONE

NONE

IBr⎯(CO2)14

NONE

NONE

IBr⎯(CO2)2: 12.2%
IBr⎯(CO2)3: 46.4%
IBr⎯(CO2)4: 30.9%
IBr⎯(CO2)5: 5.6%
IBr⎯(CO2)6: 2.2%
[97.3%]
IBr⎯(CO2)3: 21.6%
IBr⎯(CO2)4: 50.4%
IBr⎯(CO2)5: 25.4%
IBr⎯(CO2)6: 2.6%
[100%]
IBr⎯(CO2)4: 35.1%
IBr⎯(CO2)5: 46.1%
IBr⎯(CO2)6: 17.9%
IBr⎯(CO2)7: 0.9%
[100%]
IBr⎯(CO2)5: 43.3%
IBr⎯(CO2)6: 47.0%
IBr⎯(CO2)7: 9.7%
[100%]
IBr⎯(CO2)5: 3.0%
IBr⎯(CO2)6: 48.3%
IBr⎯(CO2)7: 41.7%
IBr⎯(CO2)8: 7.0%
[100%]

108
produces the first IBr⎯-based recombined products on the ground state.

As thenumber of CO2 solvents is increased, the recombined products quickly dominate, andthe “direct,” I⎯-based dissociation products disappear. The charge transfer Br⎯-based
products persist in small quantities up to n = 8, after which the only products present
correspond to recombination on the ground electronic state. Although Br⎯-based
products are observed only for 1−7 solvent molecules, solvent driven charge transfer
is clearly demonstrated to occur before recombination.
Photofragmentation is also carried out at a bluer wavelength in the A' band to
determine the effect of a larger kinetic energy release on the photoproductdistributions.

The wavelength 675 nm provides a kinetic energy release of

approximately 0.2 eV greater than for 760 and 790 nm excitation. The photoproductdistributions at 675 nm are shown in Fig. 3.7. As in the case of excitation with theother two wavelengths, unsolvated IBr⎯ produces only I⎯ as a photoproduct. With theaddition of the second solvent, the charge transfer Br⎯-based products begin to appear.
The first recombined IBr⎯-based products are formed with the third solvent molecule.
The I⎯-based products persist until n = 7. The Br⎯-based products reach percentagesof near 60% of the total products before disappearing by n = 11 when recombinationon the ground state becomes 100%.
The recombination efficiency as a function of cluster size for the three
excitation wavelengths is shown in Fig. 3.8. The approximate locations of eachwavelength within the A' band are also shown. The curves for both 760 and 790 nm,
which have nearly equal kinetic energy release on A', are nearly identical. Both show
recombination at n = 2 and reach 100% recombination at n = 8. The 675 nm curve

109

100

Percent Recombined

80

60

40
760 nm

790 nm20675 nm

760 nm

790 nm

675 nm

0
0

2

4

6

8

10

12

14

16

Number of CO2 molecules (n)Figure 3.8: Recombination efficiency across the A' absorption bandin IBr⎯(CO2)n cluster ions. The excitation wavelengths are 790, 760,
and 675 nm corresponding to energy releases on A' of 0.30, 0.35, and0.55 eV, respectively. The locations of the wavelengths on the A' band
are also shown in the inset.

110shows the first recombined products at n = 3 and does not reach 100% recombination
until n =11. The additional 0.2 eV in kinetic energy release is roughly the binding
energy of one CO2 to IBr⎯. This binding energy value is determined below. This
leads to the prediction that the 675 nm recombination curve should be shifted one
CO2 solvent higher. This is indeed the case for the onset of recombination on the
ground state where this occurs at n = 2 for 790 and 760 nm and at n = 3 for 675 nm.
However, the clusters in the n = 5−11 range show very different behavior for 675 nm.
The solvent shift is almost four solvents compared to the other wavelengths and theBr⎯-based products reach a much higher percentage of the total products. The simple
energetic arguments thought to be applicable cannot account for this larger than
expected solvent shift. Detailed simulations, similar to those previously reported18,19
for I2⎯, will need to be carried out to provide a complete explanation for the large
solvent shift in the recombination dynamics.

3.4

Photofragmentation of IBr⎯(CO2)n upon excitation to the B 2 2Σ+1/2 state

Photodissociation studies are carried out on mass-selected IBr⎯(CO2)n cluster
ions upon excitation to the B 2 2Σ+1/2 state in a manner analogous to that of A' 2Π1/2
excitation. The B state in IBr⎯ is not spectroscopically characterized to the extent that
A' was above. The calculated spectrum shown in Fig. 3.3 is used, along with theexperimental absorption band for A', to determine the excitation wavelengths forphotodissociation on B.

Two convenient wavelengths for both the femto- andnanosecond laser systems are used for excitation to the B state. The wavelength 355
nm is the third harmonic of the Nd:YAG Infinity and has large pulse energies with

111
superior mode quality as compared to the XPO wavelengths. There is also predicted
to be substantial absorption cross section at this wavelength based on the calculated
spectrum with the observed experimental blue shift taken into account. The 395 nm
light is the second harmonic of the 790 nm fundamental wavelength characterized
above and would be used in a two-color pump-probe scheme to study the timeresolved dynamics of the IBr⎯(CO2)n clusters.

The appropriate blue-shift of thecalculated spectrum also puts it very near the band maximum for B state excitation.
Excitation of bare IBr⎯ at both wavelengths produces only Br⎯ and I* as thephotoproducts with kinetic energy releases on the B state of 1.75 and 1.4 eV for 355
and 395 nm, respectively.
The addition of CO2 solvent molecules provides a total of five possible
product channels. Channels (A), (B), and (C) that are observed for excitation to theA' state are present as well as the now energetically accessible channels:
IBr⎯(CO2)n→Br⎯(CO2)n-m + mCO2 + I*

(D)→I⎯(CO2)n-k + kCO2 + Br*(E)

Channel (D) corresponds to direct dissociation on B, giving the expected Br⎯-based
photoproducts. Channel (E) represents a dissociation preceded by a charge transferfrom the initially formed Br⎯ to the spin-orbit excited I* leaving Br in a spin-orbit
excited state. These two channels are separated by around 0.5 eV corresponding to
the binding energy of two CO2 solvents. Products formed on these two channels are
expected to differ in two solvent molecules retained.

Further solvation is then

expected to lead to product formation on Channels (A), (B), and (C).

112
The photofragmentation products from excitation of IBr⎯(CO2)n clusters with
355 and 395 nm photons as a function of cluster size are shown in Fig. 3.9 and Fig.
3.10, respectively. The exact photoproducts formed with their percentages are shownin Tables 3.4 and 3.5.

The total percentages in bold are plotted in the figures. The

cluster n = 5 is the largest size studied for 395 nm with n = 11 the largest cluster for
355 nm.

In both cases, the unsolvated IBr⎯ produces exclusively Br⎯ as thephotoproduct. The addition of the first solvent molecules leads to the production of
the charge-transfer product I⎯, likely accompanied by Br*. For 355 nm dissociation,
the I⎯-based products have reached 80% of the total products by n = 4, then level off
to an apparent asymptotic value of around 65% at the largest cluster size studied.
Likewise, the Br⎯-based products drop in percentage quickly as the number ofsolvents is increased, but level out at roughly 25−30% of the total products by n = 8
up to the largest cluster. The recombined products on the ground state appear at n =7, but never reach 10% of the total products. The product distributions at 395 nm aretaken over a much smaller range of cluster sizes (up to n = 5), but show the same
basic trends as the 355 nm data. Although, the increase in I⎯-based products is not as
steep as for 355 nm, the total percentage of I⎯-based products is greater for 395 nm.
The recombination product channel also turns on one solvent faster than for 355 nm
photodissociation. The solvent loss for each product is used to determine the identityof the product channel on which it is formed and is discussed below.

113

Ionic Product Percentage

100

80

60

I- basedBr- based

40

IBr- based

20

0
0

2

4

6

8

10

Number of CO2 molecules (n)Figure 3.9: Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the B state with 355 nm photons and an energy releaseof 1.75 eV on this state.

114
Table 3.4: Photofragmentation branching ratios for IBr⎯(CO2)n at 355 nm.
Parent Ion

I⎯-based

IBr⎯

NONE

IBr⎯(CO2)1

I⎯: 34.1%[34.1%]
I⎯: 48.6%
I⎯(CO2)1: 27.8%[76.4%]
I⎯: 26.9%
I⎯(CO2)2: 44.7%[71.6%]
I⎯: 20.0%
I⎯(CO2)1: 60.1%
I⎯(CO2)2: 2.9%
[83.0%]
I⎯: 12.1%
I⎯(CO2)1: 59.1%
I⎯(CO2)2: 10.6%[81.8%]
I⎯: 6.8%
I⎯(CO2)1: 54.9%
I⎯(CO2)2: 15.4%
I⎯(CO2)3: 0.6%
[77.7%]
I⎯: 2.4%
I⎯(CO2)1: 29.0
I⎯(CO2)2: 39.1%
I⎯(CO2)3: 3.6%

IBr⎯(CO2)2
IBr⎯(CO2)3
IBr⎯(CO2)4

IBr⎯(CO2)5

IBr⎯(CO2)6

IBr⎯(CO2)7

IBr⎯(CO2)8

IBr⎯(CO2)9[74.1%]
I⎯: 0.5%
I⎯(CO2)1: 7.7%
I⎯(CO2)2: 30.7%
I⎯(CO2)3: 28.8%
I⎯(CO2)4: 0.9%
[68.6%]I⎯(CO2)1: 1.3%
I⎯(CO2)2: 11.8%
I⎯(CO2)3: 39.0%
I⎯(CO2)4: 15.0%
I⎯(CO2)5: 0.4%
[67.5%]

Br⎯-based

IBr⎯-basedBr⎯: 100%

NONE

Br⎯: 65.9%[65.9%]
Br⎯: 20.5%
Br⎯(CO2)1: 3.1%
[23.6%]
Br⎯: 7.3%
Br⎯(CO2)1: 21.1%[28.4%]
Br⎯: 1.2%
Br⎯(CO2)1: 12.8%
Br⎯(CO2)2: 3.0%
[17.0%]Br⎯(CO2)1: 6.3%
Br⎯(CO2)2: 9.7%
Br⎯(CO2)3: 2.2%
[18.2%]Br⎯(CO2)1: 1.3%
Br⎯(CO2)2: 10.1%
Br⎯(CO2)3: 8.8%
Br⎯(CO2)4: 1.5%
[21.7%]Br⎯(CO2)1: 0.8%
Br⎯(CO2)2: 6.1%
Br⎯(CO2)3: 12.3%
Br⎯(CO2)4: 4.5%
Br⎯(CO2)5: 0.3%
[24.0%]Br⎯(CO2)1: 0.6%
Br⎯(CO2)2: 1.4%
Br⎯(CO2)3: 11.2%
Br⎯(CO2)4: 11.1%
Br⎯(CO2)5: 2.7%
[27.0%]Br⎯(CO2)1: 0.6%
Br⎯(CO2)2: 1.4%
Br⎯(CO2)3: 5.8%
Br⎯(CO2)4: 12.1%
Br⎯(CO2)5: 4.9%
Br⎯(CO2)6: 0.6%
[25.4%]

NONE
NONE
NONE
NONE

NONE

IBr⎯: 0.6%

[0.6%]
IBr⎯: 1.9%

[1.9%]
IBr⎯: 4.4%

[4.4%]
IBr⎯: 6.7%
IBr⎯(CO2)1: 0.4%

[7.1%]

115
IBr⎯(CO2)10IBr⎯(CO2)11

I⎯(CO2)1: 0.4%
I⎯(CO2)2: 3.3%
I⎯(CO2)3: 19.7%
I⎯(CO2)4: 35.5%
I⎯(CO2)5: 7.0%
[65.7%]I⎯(CO2)2: 0.2%
I⎯(CO2)3: 8.6%
I⎯(CO2)4: 26.6%
I⎯(CO2)5: 28.5%
I⎯(CO2)6: 2.2%
[66.1%]Br⎯(CO2)3: 3.3%
Br⎯(CO2)4: 10.8%
Br⎯(CO2)5: 13.2%
Br⎯(CO2)6: 2.6%

IBr⎯: 3.4%
IBr⎯(CO2)1: 1.0%

[29.9%]Br⎯(CO2)2: 0.3%
Br⎯(CO2)3: 4.7%
Br⎯(CO2)4: 7.2%
Br⎯(CO2)5: 11.6%
Br⎯(CO2)6: 3.6%
[27.4%][4.4%]
IBr⎯: 3.0%
IBr⎯(CO2)1: 2.9%
IBr⎯(CO2)2: 0.6%
[6.5%]

116

Ionic Product Percentage

100

80

60

I- basedBr- based

IBr- based

40

20

0
0

1

2

3

4

5

Number of CO2 molecules (n)Figure 3.10: Photofragmentation products of IBr⎯(CO2)n cluster ions
upon excitation to the B state with 395 nm photons and an energy releaseof 1.4 eV on this state.

117
Table 3.5: Photofragmentation branching ratios for IBr⎯(CO2)n at 395 nm.
Parent Ion

I⎯-based

IBr⎯

NONE

IBr⎯(CO2)1

I⎯: 32.5%[32.5%]
I⎯: 27.4%
I⎯(CO2)1: 32.2%[59.5%]
I⎯: 5.7%
I⎯(CO2)1: 68.7%[71.6%]
I⎯: 15.8%
I⎯(CO2)1: 64.1%
I⎯(CO2)2: 9.3%
[91.2%]
I⎯: 5.2%
I⎯(CO2)1: 61.5%
I⎯(CO2)2: 23.1%[89.8%]

IBr⎯(CO2)2
IBr⎯(CO2)3
IBr⎯(CO2)4

IBr⎯(CO2)5Br⎯-based

IBr⎯-basedBr⎯: 100%

NONE

Br⎯: 67.5%[67.5%]
Br⎯: 19.1%
Br⎯(CO2)1: 21.4%[40.5%]
Br⎯: 3.6%
Br⎯(CO2)1: 22.0%[25.6%]Br⎯(CO2)1: 7.8%
Br⎯(CO2)2: 3.0%

NONE

[8.1%]Br⎯(CO2)1: 2.4%
Br⎯(CO2)2: 5.7%
Br⎯(CO2)3: 1.5%
[9.6%]

NONE
NONE
NONE

IBr⎯: 0.6%
[0.6%]

118
3.5

Time resolved dynamics of IBr⎯(CO2)8 following excitation to the A' state

As mentioned in Ch.1, the time-resolved absorption recovery experiments
provide an excellent probe for the timescales of photodissociation and subsequent
recombination on the ground electronic state. After photodissociation of the IBr⎯
chromophore within the cluster, the cluster anion is transparent to absorption of either
near-IR or UV light until the chromophore has recombined on the ground state with
sufficient vibrational relaxation. This provides an overall timescale for dissociation
on an excited electronic state and recombination on the ground state. By tuning theexcitation wavelength, other processes such as charge-transfer and spin-orbit
relaxation can be included in the overall absorption recovery time.
A one-color pump-probe experiment is carried out here on the IBr⎯(CO2)8cluster. A 790 nm pump pulse excites the chromophore within the cluster to theA' 2Π1/2 dissociative state. A second 790 nm probe pulse is then absorbed by thechromophore after it has recombined and vibrationally relaxed on the ground state.
The two-photon signal as a function of the delay between the pump and the probe
pulses then gives the timescale for photodissociation, charge transfer from I⎯ to Br,
and recombination and vibrational relaxation on the ground state.

The cluster

IBr⎯(CO2)8 is chosen due to the 100% recombination after excitation to A'. This was
also the most intense cluster in the mass spectra for which 100% recombination waspresent. Based on similar experiments10 in I2⎯(CO2)n clusters, the absorption recovery
time after excitation to the A' state is predicted to be 10−30 ps.
The time-resolved data for IBr⎯(CO2)8 are shown in Fig. 3.11. The pump andthe probe wavelengths have polarizations perpendicular to one another due to beam

119

Nomralized two-photon
counts

0.8

0.6

0.4

0.2

0.0

0

200

5000

8000

Pump-probe delay (ps)Figure 3.11: The IBr⎯(CO2)8 absorption recovery as afunction of time following excitation to the A' 2Π state with a
790 nm pump pulse. A break is shown to indicate the substantialchange in scale. The dashed line is simply to guide the eye.

120
recombination restrictions. The one-photon photofragmentation results presented inSec. 3.3 are used to determine that I⎯(CO2) is the two-photon product ion of thelargest intensity for this cluster and excitation scheme. This product ion is monitored
as a function of pump-probe delay to yield the absorption recovery curve. The
appropriate backgrounds are subtracted to show that this two-photon product depends
upon the presence of both the pump and the probe pulses and represents true twophoton signal.
The striking feature of the absorption recovery curve is that there is no
detectable absorption recovery signal over the first 200 ps, the maximum delay time
available using the precision delay line. The one-photon photodissociation branchingratios from above require that there must be complete absorption recovery withinaround 10 µs, which is the flight time from the laser interaction region to detection on
the off-axis detector. A crude delay-line is set up to allow for the use of longer than
200 ps pump-probe delay times. Two data points were taken at 5 ns and 8 ns withthis longer delay-line. At these long delay times, the overlap between the pump andprobe pulses is poorly controlled resulting in a large uncertainty in the two data
points. The curve shows the two-photon signal to be increasing from 5 ns to 8 ns,
but within the experimental uncertainty it is impossible to determine if this is the caseor a complete absorption recovery plateau has been reached. Nevertheless, somedegree of recombination on the ground state is occurring by 5 ns with none before atleast 200 ps. This is in stark contrast to the predictions made based on the ~20 psrecombination time observed for the smallest I2⎯(CO2)n cluster (n = 12) that shows
100% recombination.10

121
3.6

Photofragmentation of ICl⎯(CO2)n upon excitation to A' at 790 nm

The photodissociation dynamics of ICl⎯(CO2)n cluster ions has beenthoroughly investigated previously8,13 with numerous excitation wavelengths.
However, photofragmentation studies at wavelengths near 790 nm were not carriedout.

In the event that time-resolved experiments are carried out on ICl⎯(CO2)nclusters, 790 nm will be an important excitation wavelength due to the enhanced
performance of the femtosecond laser system near this wavelength. The
photofragmentation as a function of cluster size is carried out in an analogous mannerto that described for IBr⎯(CO2)n clusters described in Sec. 3.3. Only the results areprovided here as detailed explanations on the product channels and dynamics can befound elsewhere.8,13
The photofragmentation products as a function of cluster size after excitation
with 790 nm photons are shown in Fig. 3.12. The complete product distributions arefound in Table 3.6. The kinetic energy release on A' is roughly 0.25 eV.
show

qualitatively

similarbehavior tothosepreviously

The results

observed8,13

forphotodissociation at 644 nm and 740 nm. The photodissociation of unsolvated ICl⎯produces the expected I⎯ + Cl photoproducts. The addition of the first CO2 leads torecombination on the ground state. Recombination reaches 100% at n = 2 and is
100% for n = 3. The fourth solvent molecule begins to yield Cl⎯-based products thatthen increase at the expense of the recombined products with increasing solvation.
The Cl⎯-based products reach 100% by n = 9 with no other photoproducts present atlarger clusters. Molecular dynamics simulations17 show a Cl⎯-based product channel

122

Ionic Product Percentage

100

80

60

I--basedCl--based

ICl--based

40

20

0
0

2

4

6

8

10

Number of CO2 molecules (n)Figure 3.12: Photofragmentation products of ICl⎯(CO2)n cluster ions
upon excitation to the A' 2Π state with 790 nm photons and an energy
release of 0.25 eV on the state.

123
Table 3.6: Photofragmentation branching ratios for ICl⎯(CO2)n at 790 nm.
Parent Ion

I⎯-basedCl⎯-based

ICl⎯-based

ICl⎯

NONE

NONE

NONE

ICl⎯(CO2)2

I⎯: 100%[100%]I⎯(CO2)1: 56%
[56%]
NONE

ICl⎯(CO2)3

NONE

NONE

ICl⎯(CO2)4

NONE

ICl⎯(CO2)5NONE

ICl⎯: 44%
[44%]
ICl⎯: 100%
[100%]
ICl⎯: 89.5%
ICl⎯(CO2)1: 10.5%[100%]
ICl⎯: 48.5%
ICl⎯(CO2)1: 35.0%
ICl⎯(CO2)2: 9.5%
[93.0%]
ICl⎯: 10.1%
ICl⎯(CO2)1: 43.3%
ICl⎯(CO2)2: 31.1%
ICl⎯(CO2)3: 4.2%
[88.7%]
ICl⎯(CO2)1: 19.0%
ICl⎯(CO2)2: 24.0%
ICl⎯(CO2)3: 13.0%[56.0%]
ICl⎯(CO2)1: 2.1%
ICl⎯(CO2)2: 11.0%
ICl⎯(CO2)3: 8.8%
ICl⎯(CO2)4: 2.8%
[24.7%]
ICl⎯(CO2)3: 7.4%
ICl⎯(CO2)4: 4.6%
[12.0%]NONE

ICl⎯(CO2)1

ICl⎯(CO2)6

NONE

ICl⎯(CO2)7

NONE

ICl⎯(CO2)8

NONEICl⎯(CO2)9NONE

ICl⎯(CO2)10

NONE

NONE

Cl⎯: 7%[7.0%]Cl⎯(CO2)3: 2.9%
Cl⎯(CO2)4: trace
Cl⎯(CO2)5: 8.4%
[11.3%]Cl⎯(CO2)4: 1.0%
Cl⎯(CO2)5: 1.0%
Cl⎯(CO2)6: 42.0%[44.0%]Cl⎯(CO2)7: 75.3%[75.3%]Cl⎯(CO2)7: 29.4%
Cl⎯(CO2)8: 58.6%[88.0%]Cl⎯(CO2)8: 100%
[100%]Cl⎯(CO2)9: 26.8%
Cl⎯(CO2)10: 73.2%[100%]

NONE

124
corresponding to formation of spin-orbit excited I*. Products formed on this channel
would be expected to not lose any solvents. It appears that this channel may turn on
at n = 5 where the solvent loss decreases dramatically to one or zero solvents from then = 4 solvent loss. This decreased solvent loss is more apparent than in the solvent
loss data after 644 nm and 740 nm excitation.8,13 The decrease in kinetic energy
release on A' at 790 nm may favor products on this spin-orbit excited I* channel.

3.7

Solute dependent photodissociation dynamics upon excitation to A' 2Π1/2

The results presented in Sec. 3.3 demonstrate the dramatic effects that thepresence of a small number of CO2 solvent molecules has on the photodissociation
dynamics of IBr⎯(CO2)n clusters after excitation on the A' electronic state. The
dynamics of these photodissociation processes can be discussed in terms of acomparison with previous results carried out on excitation to the A' electronic statefor ICl⎯(CO2)n and I2⎯(CO2)n cluster ions.8-10,13

The calculated potential energy

curves17,20 for ICl⎯ and I2⎯ are shown in Fig. 3.13. The photodissociation dynamics ofthe three solutes are compared at wavelengths that correspond to kinetic energy
releases of 0.3 eV on the A' state for each. The recombination efficiency as afunction of cluster size for the three solute ions is shown in Fig. 3.14. The presenceof a single CO2 molecule is sufficient to allow for a nonadiabatic transition to theground electronic state via a charge-transfer for both ICl⎯ and IBr⎯. Recombinationon the ground state is also present for ICl⎯.with the addition of the first solvent. The
second CO2 molecule increases the fraction of ions reaching the ground state andrecombining for ICl⎯ as well producing the first ground state recombined IBr⎯

125

(a)

(b)
B 2Σ+1/2

B 2Σ+g,1/2

EnergyI⎯ + I*Cl⎯ + I*1

A' 2Πg,1/2

I⎯ + I2 +X Σ u,1/2
Internuclear separation

0I⎯ + Cl*A' 2Π1/2

I⎯ + ClCl⎯ + I

X 2Σ+1/2
Internuclear separation

Figure 3.13: Calculated potential energy curves for (a) I2⎯ and(b) ICl⎯ anions. The original calculations are found in references citedin the text. The states in bold are accompanied by their state labels.

126

Percent Recombined

100

80

60

40

I2-(CO2)nICl-(CO2)nIBr-(CO2)n

20

0
0

2

4

6

8

10

12

14

16

18

Number of CO2 molecules (n)Figure 3.14: Recombination percentage for the three solutes as afunction of the number of CO2 molecules upon excitation to theA' 2Π state. The excitation wavelengths are chosen to provide
0.3 eV energy release for each solute.

127
products. Increasing solvation up to n = 4 leads to increasing recombination on the
ground state for both solutes.
The behavior for IBr⎯ and ICl⎯ diverge greatly for clusters larger than n = 4.
The recombination fraction begins to drop dramatically eventually reaching zero by n
= 10 for ICl⎯(CO2)n clusters with Cl⎯-based products being exclusively formed forclusters with n>9.

The IBr⎯(CO2)n clusters show increasing recombination withincreasing cluster size reaching 100% recombination by n = 8. This recombination
behavior is similar to that of I2⎯(CO2)n clusters where recombination reaches 100%with increasing cluster size. However, recombined products do not appear until n = 6
and 100% recombination is not reached until n = 16 for I2⎯(CO2)n.
This photodissociation behavior of the three solutes can partially be explainedin terms of the properties of the solvent cages depicted in Fig. 3.15 for the three
dihalide solutes. After excitation on the A' state for the three solutes, 0.3 eV must be
removed from the dissociating dihalide to prevent prompt, direct dissociation on thisexcited electronic state.

The IBr⎯(CO2)n and I2⎯(CO2)n have the first few CO2

molecules (n ≤ 4) bind near the waist of the dihalide solute and only begin to fully
enclose one end of the solute (the smaller Br end in the case of IBr⎯) with increasing
solvation (Fig. 3.15). For the ICl⎯(CO2)n clusters, the first few solvents bind to themuch smaller Cl end of the solute8,17 with subsequent solvation continuing around the
Cl end (Fig. 3.14). The large recombination efficiency for small ICl⎯(CO2)n clusters
is a result of the CO2 molecules location on the Cl end of the solute. These solventsare directly in the pathway for dissociation and the favorable mass match between Cl

128ICl⎯(CO2)4

IBr⎯(CO2)4

I2⎯(CO2)4ICl⎯(CO2)8

IBr⎯(CO2)8

I2⎯(CO2)8ICl⎯(CO2)14

IBr⎯(CO2)11

I2⎯(CO2)16

Figure 3.15: Representative cluster structures calculated for ICl−, IBr−,and I2− with various numbers of CO2 solvent molecules. The smaller
halogen is always on the right side. The reference for the original
calculations is cited in the text.

129(35 amu) and CO2 (44 amu) provides effective energy transfer between the
dissociating Cl and the solvent. The 0.3 eV of kinetic energy present is easily
removed suppressing direct dissociation and allowing for nonadiabatic transitions tolower electronic states.
Recombination in IBr⎯(CO2)n is not observed until n = 2 due to the fact that
there are no solvent molecules directly in the path of the dissociating dihalide at small
cluster sizes. However, there are likely cluster structures slightly higher in energy
present that have some solvation directly on the smaller Br end. This coupled withthe slightly favorable mass match between Br (79 amu) and CO2 (44 amu) leads torecombination at smaller cluster sizes. The I2⎯(CO2)n clusters do not have solvents
bound on one end of the solute nor is there a favorable mass match between I (127amu) and CO2 (44 amu). Recombination does not occur until n = 6, the size at which
a solvent cage begins to form around one end of the I2⎯ solute. Recombination also
does not reach 100% until the 16th CO2 molecule caps the remaining I end closing thefinal pathway to direct dissociation. The recombination efficiency for the threesolutes at small cluster sizes is highly dependent on both the position of the solvent
molecules and the mass match between the solvents and the dissociated atoms.
The most striking difference between the recombination behavior of the threesolutes is the termination of recombination on the ground state for ICl⎯(CO2)n at large
cluster sizes.

Both I2⎯(CO2)n and IBr⎯(CO2)n clusters demonstrate the intuitive

behavior of increasing recombination with increasing solvation. This agrees wellwith the picture of a larger solvent cage providing a stronger physical barrier to direct
dissociation of the dihalide anion. Upon excitation to the A' state in ICl⎯(CO2)n, there

130
are two deep minima on the potential surface.17 The global minimum corresponds tosolvated ICl⎯.

However, the system likely accesses a deep local minimum

corresponding to solvated Cl⎯. The minimum well deepens with increasing solvation
making it less likely for the system to surmount and produce recombined products on
the timescale of the experiment (~ 10 µs). This leads to the observation of only Cl⎯based products at large cluster sizes. The IBr⎯ electronic structure calculations1
indicate that the A' is not purely repulsive with a shallow well present at 3.5 Å. The
presence of a solvent would deepen this well and trap the dissociating anion at smallinternuclear distances not allowing it to reach internuclear separations wheretransitions to lower electronic states occur. The well present accounts for the much
longer recombination time observed for IBr⎯(CO2)8 as compared to I2⎯(CO2)n clusters.
However, the well is not as deep as for ICl⎯(CO2)n clusters due to the differences insolvation energies for Cl¯ and Br¯. The atomic radii of Cl¯ and Br¯are 1.81Å and1.96Å , respectively. These correspond to solvation energies for each of –347 kJ/moland –321 kJ/mol obtained from the Born equation.21 This shallower well for IBr¯
allows for recombination times much shorter than 10 µs. Sanov and co-workers havealso observed the minimum in IBr⎯ using time-resolved photoelectron imaging studies
of IBr⎯ excited to the A' 2Π1/2 electronic state.22

3.8

Dissociation following excitation to the 2 B 2Σ+1/2 state

The recombination efficiency as a function of solute following excitation to
the B electronic state is shown in Fig. 3.16. The recombination percentage for IBr⎯ ismuch less than that of ICl⎯ and I2⎯ at similar cluster sizes. Recombined products

131

Percent Recombined100

I2-(CO2)n80

ICl-(CO2)nIBr-(CO2)n

60

40

20

0
0

5

10

15

20

Number of CO2 molecules (n)Figure 3.16: Recombination percentage for the three solutes as afunction of number of CO2 molecules following excitation to theB 2 2Σ state. The excitation wavelengths and corresponding energy
releases on the B state are 1.2 eV for I2⎯(CO2)n, 1.5 eV for ICl⎯(CO2)n,and 1.75 eV for IBr⎯(CO2)n.

132
initially appear at the same cluster sizes for IBr⎯ and I2⎯ with the recombined channel
turning on three CO2 solvents earlier for ICl⎯.

The ICl⎯(CO2)n clusters show

increasing recombination on the ground state with increasing solvation in sharp
contrast to the recombination behavior after excitation to the A' state. Molecular
dynamics simulations17 of the recombination dynamics after excitation to the B stateshow that the dissociated ICl⎯ does not access the local minimum present in A'
dissociation allowing for efficient recombination with increasing cluster size.

The

IBr⎯(CO2)n clusters show very little recombination at the largest cluster sizes studied
with less than 10% of the total products at n = 11, the largest cluster studied.
Excitation on A' conversely reaches 100% recombination by n = 8. The I2⎯(CO2)nrecombination shows the overall trend of increasing with increasing cluster size
similar to ICl⎯(CO2)n behavior.

The recombination rate for I2⎯(CO2)n does not

smoothly increase as for ICl⎯(CO2)n, however. The recombination is more sensitive
to the number of solvents and thus the structure of the CO2 solvent cage. The
symmetric I2⎯ solute likely depends more on the solvent structure to facilitate spinorbit relaxation of I before recombination than the asymmetric solutes leading to amuch stronger dependence on the number of solvent molecules present.

3.9

Time-resolved recombination dynamics

The time-resolved recombination dynamics for IBr⎯(CO2)8 after excitation to
the A' 2Π1/2 state show a recombination time of more than 200 ps. Measurable
recombination is observed on the 5 to 8 ns timescale. This cluster size produces
recombination on the ground state for every cluster ion excited. When compared to133similar experiments10,12 carried out on I2⎯(CO2)n clusters, the IBr⎯ recombination time
is some two orders of magnitude slower than for the corresponding I2⎯(CO2)n clusters
that exhibit complete recombination. The discrepancies in the recombination times
for these two systems must be due to the presence of the previously mentioned
minimum well present in the IBr⎯ A' 2Π1/2 potential that is absent for I2⎯.
The well thought to be present in ICl⎯ is deeper than that found on IBr⎯ due tothe larger solvation energy for Cl⎯ compared to Br⎯. This deeper well does not allow
for recombination on the 10 µs timescale necessary for detection on the off-axis
detector. This is manifested in the observation of only Cl⎯-based products at large
cluster sizes in ICl⎯(CO2)n. The IBr⎯ well is sufficient to provide early trapping of theanion, but is shallow enough to allow the anion to escape and produce recombinationon the ground state on the nanosecond timescale. This picture of a shallow wellaccounts for the very slow, but complete recombination for IBr⎯(CO2)n≥8 after
excitation to the A' 2Π1/2 state. This will need to be comfirmed through extensive
molecular dynamics simulations similar to those performed for A' excitation inICl⎯(CO2)n and I2⎯(CO2)n clusters.17,18,23,24

3.10

Solvent evaporation energetics

Analysis of the number of solvent molecules attached to the ionicphotoproducts provides important information on the approximate ion-solvent
binding energies as well as identification of the electronic states from which
dissociated photoproducts arise. The first step is the determination of the averageenergy removal upon evaporation of a single CO2 molecule from the IBr⎯ solute134anion. Analysis of the number of solvent molecules remaining on recombined IBr⎯based products after excitation to A' with three excitation energies provides the totalenergy (solvent binding energy + kinetic energy) removed by evaporation of a CO2
solvent molecule.
The average solvent loss for the recombined products, as well as the otherproduct channels, can be determined from the photofragmentation branching ratios
shown in Tables 3.1, 3.2, and 3.3 for excitation of IBr⎯(CO2)n clusters to the A' state.
The average number of solvents lost through evaporation for each of the three typesof products after excitation to A' at 790 nm is shown in Fig. 3.17. The data in the
figure is obtained from the branching ratios found in Table 3.1. Similar graphs can beconstructed from the branching ratios for 760 nm and 675 nm excitations, but are notshown here. To determine the average energy loss for each CO2, the number ofsolvents lost for the recombined IBr⎯-based products must be determined for large
initial cluster sizes.

For an accurate determination of the energy removal uponsolvent evaporation, the IBr⎯ must undergo sufficient vibrational relaxation on the
ground state potential to reach the vibrational levels occupied prior tophotoexcitation. Larger numbers of solvents initially present in the clusters should
ensure that complete vibrational relaxation is taking place on the ground state. The
solvent loss for the recombined products at the three A' excitation wavelengths areshown as a function of initial cluster size in Fig. 3.18. The average number of solvent
lost is determined in the n = 10−14 cluster range for each wavelength. This number isdivided by the photon energy to yield the average energy removed by each solvent
evaporated. The values for each wavelength are then averaged to yield an overall

135

7

Average CO2 solvent loss

6
5
4
3
2

I--basedBr--based1

IBr--based

0
0

2

4

6

8

10

12

14

16

Number of CO2 molecules (n)Figure 3.17: Average number of solvent loss for the three productchannels after excitation with 790 nm to A' as a function of initialcluster size (n).

136

Number of solvents evaporated

8
7
6
5
4
3

790 nm
760 nm
675 nm

2
1

0

2

4

6

8

10

12

14

16

Parent cluster size (n)Figure 3.18: Average solvent loss for the recombined products
after excitation to A' in IBr⎯(CO2)n clusters at the three labeled
wavelengths as a function of initial cluster size.

137
average energy removal of 263 ± 12 meV per CO2 evaporated from IBr⎯. Thisquantity is consistent with the average CO2 energy removal observed9 of 250 meV for
I2⎯(CO2)n clusters. The solvent binding energy is determined by the use of a statisticalmodel25 to estimate the kinetic energy associated with solvent evaporation. This
kinetic energy amounts to around 40 meV for IBr⎯(CO2)n clusters giving an averagesolvent binding energy of 223 meV for 7−15 solvent molecules attached to IBr⎯. This
number will increase slightly at smaller cluster sizes when there is preferential
binding around the smaller Br end of the solute. However, this model adds nouncertainty to the assignment of the electronic states on which dissociation takes
place as only the total energy removal (kinetic energy + solvent binding energy) is
required.
Excitation to the A' 2Π1/2 state

The photofragmentation branching ratios for the excitation to the A' electronic
state in IBr⎯(CO2)n clusters were presented in Sec. 3.3. The excitation yields thephotodissociation products I⎯ + Br for the unsolvated IBr⎯ solute. Solvation produced
not only the expected I⎯-based products, but Br⎯-based products on the ground state
following a charge-transfer event as well as recombined IBr⎯-based products on the
ground state. The excitation with 760 and 790 nm photons leads to a kinetic energy
release of about 0.3 eV or roughly the removal energy of one CO2 solvent molecule
based on the discussion above. The solvent loss for each product is used to determine
the identity of the electronic state on which the products are formed. With only 0.3
eV kinetic energy release during dissociation on A', the I⎯-based products are
expected to only lose a single CO2 molecule. However, the data in Tables 3.1 and 3.2

138
show more than one solvent loss for clusters sizes up to n = 4 where the I⎯-based
product channel turns off. This larger solvent loss is a result of direct dissociation ofI⎯ from the IBr⎯ clusters occurring faster than the solvent is able to rearrange tosolvate the newly formed atomic anion. The I⎯-based products are then formed withfewer than expected solvents. A statistical equilibrium model is not applicable tothese small clusters.
For the larger clusters studied (n ≥ 13), the average solvent loss has reached aplateau at around six CO2 solvents lost. This is the expected solvent loss for complete
vibrational relaxation on the ground electronic state for the IBr⎯-based products. For
instance, after excitation on A' with 790 nm, complete vibrational relaxation on the
ground state requires dissipating the entire 790 nm photon energy of 1.57 eV. This isequal to the energy of the removal of six CO2 solvent molecules using the value of
263 meV from above. Complete vibrational relaxation then requires the removal of
six CO2 molecules. It should be noted that at n = 8, the cluster size for which thetime-resolved experiment was carried out, there has been only a loss of five CO2
leaving the recombined products vibrationally excited.

However, the additional

vibrational exciation equal to the removal of one CO2 does not significantly shift theabsorption spectrum to affect the time-resolved experiments.
Excitation to the B 2 2Σ+1/2 state

The solvent evaportation analysis becomes critical for product state
identification when products can be formed in multiple spin-orbit states of the neutral.
For instance, excitation with 355 nm light to the B 2 2Σ+1/2 state produces Br⎯-basedthat could be associated with either an I atom product (X 2Σ state dissociation) or an

139
I* product (B 2 2Σ state dissociation). The solvent loss as a function of initial cluster
size for 355 nm excitation on the B state is shown in Fig. 3.19 based on the data from
Table 3.4. Excitation on B with 355 nm leads to a kinetic energy release of 1.75 eV.
With the average energy removal of around 260 meV per CO2 solvent, dissociation on
B leading to Br⎯-based products is expected to show a loss of 6−7 CO2 solvents
corresponding to the amount of kinetic energy release. Dissociation on the X state
producing Br⎯-based would lead to a loss of 10−11 solvents reflecting the additionalloss equivalent to the spin-orbit splitting energy of I atom. Analysis of the solvent
loss for the Br⎯-based products at 355 nm shows, for instance, that the Br⎯ solvent
distribution for the largest cluster studied, IBr⎯(CO2)11, peaks at Br⎯(CO2)5 with a lossof six solvents.

This confirms, based on the argument above, that the Br⎯

photodissociation products are formed exclusively via direct dissociation on theinitially excited B state. The same conclusion is true for the Br⎯-based products at allother cluster sizes studied.
The I⎯-based products from excitation to the B 2 2Σ+1/2 state have very similar
behavior, with the average solvent loss being about one CO2 solvent less than for theBr⎯-based products. This result shows that the I⎯-based products are formed on the a'2 2Π1/2 state, leading to I⎯ + Br*. The alternative product channel corresponding todissociation on the A' 2Π1/2 state producing I⎯ + Br is much closer in energy to the I⎯ +Br* channel than are the two Br⎯ channels with an asymptotic energy spacing of only
0.47 eV, or less than two CO2 solvents. It could be argued that this close energy
spacing makes the product channel assignment ambiguous. However, the very small
recombination fraction does not support this alternative assignment. The A'

140

Average CO2 solvent loss

12
10
I--basedBr--based

8

IBr--based

6
4
2
0

0

2

4

6

8

10

12

Number of CO2 molecules (n)Figure 3.19: Average number of solvent loss for the three productchannels after excitation with 355 nm to B as a function of initialcluster size (n).

141photofragmentation branching ratios demonstrated that recombination on the groundstate is very efficient for even small cluster sizes after excitation on A'. In the case of
B state excitation, if the I⎯-based products were being formed on A' then a larger
fraction of recombined products should accompany this than observed.
The dynamics following excitation to the B state for all cluster sizes withmore than two CO2 solvents is determined by the initial curve crossing between the B
2 2Σ+1/2 and a' 2 2Π1/2 states. This is very similar to the Marcus-like picture of the
dynamics used to describe the results for excitation of I2⎯(CO2)n clusters to the B
state.19,24 There is an initial step in the dynamics involving either a charge transferfrom Br⎯ to I* or solvent rearrangement around the newly formed Br⎯ charge center.
After the branching ratio is determined for solvent reorganization and direct
dissociation on B versus a charge transfer followed by dissociation on a' at aminimum cluster size, subsequent solvation does not affect this branching ratio.

3.11

Conclusions

The photodissociation dynamics of IBr⎯(CO2)n clusters were studied todetermine the effects of solvation on a mixed dihalide anion.

The bare solute, IBr⎯,

was first spectroscopically characterized and compared to theoretical calculations.
These results were used to determine wavelengths for excitation to the A' 2Π1/2 and B
2 2Σ+1/2 electronic states of IBr⎯ corresponding to I⎯ + Br and Br⎯ + I* photoproducts,
respectively. The IBr⎯ anion was also shown to possess a modest bond strength of
1.10 eV, making the solvent binding energy of a small number of CO2 molecules
comparable to the bond energy. The addition of CO2 solvent molecules demonstrated

142
the central role of the solvent configuration in determining the photodissociation
dynamics. The solvent was shown to drive such processes as charge-transfer, spinorbit relaxation, and recombination and vibrational relaxation on the ground state.
Solvent evaporation was used to determine the binding energy of a CO2 solvent to theIBr⎯ solute as well as to identify product channels where the production of an anionicproduct may take place on multiple electronic corresponding to the different spinorbit states of the accompanying neutral product. The photofragmentation branchingratios were used to carry out a time-resolved experiment on IBr⎯(CO2)8 to determine
the timescale for recombination on the ground electronic state after A' excitation. The
recombination time was found to be unexpected long at 5 ns when compared tosimilar experiments in I2⎯(CO2)n. The presence of a shallow well on A' seems to trap
the dissociating anion solute leading to the longer than expected recombination time.
The photodissociation behavior was compared to results from ICl⎯(CO2)n and
I2⎯(CO2)n clusters demonstrating the importance of the solute properties indetermining the dynamics of solvation.

143
Chapter 3 References
1

T. Sanford, S.-Y. Han, M. A. Thompson, R. Parson, and W. C. Lineberger, tobe published (2004).

2

M. A. Thompson and R. Parson, unpublished results (2004).

3

C. E. Moore, Atomic Energy Levels. (National Bureau of Standards,
Washington, D.C., 1949).

4

M. W. Chase, Jr., C. A. Davies, J. R. Downey, Jr., D. J. Frurip, R. A.
McDonald, and A. N. Syverud, JANAF Thermochemical Tables(J.Phys.Chem. Ref. Data 14), 421 (1985).

5

A. Sanov, T. Sanford, L. J. Butler, J. Vala, R. Kosloff, and W. C. Lineberger,
J. Phys. Chem. A 103, 10244 (1999).

6

E. Gershgoren and S. Ruhman, private communication.

7

J. M. Papanikolas, Ph.D. Thesis, University of Colorado, 1994.

8

M. E. Nadal, Ph.D. Thesis, University of Colorado, 1996.

9J. M. Papanikolas, J. R. Gord, N. E. Levinger, D. Ray, V. Vorsa, and W. C.
Lineberger, J. Phys. Chem. 95 (21), 8028 (1991).

10

J. M. Papanikolas, V. Vorsa, M. E. Nadal, P. J. Campagnola, H. K. Buchenau,
and W. C. Lineberger, J. Chem. Phys. 99 (11), 8733 (1993).

11

J. M. Papanikolas, V. Vorsa, M. E. Nadal, P. J. Campagnola, J. R. Gord, andW. C. Lineberger, J. Chem. Phys. 97 (9), 7002 (1992).

12

V. Vorsa, S. Nandi, P. J. Campagnola, M. Larsson, and W. C. Lineberger, J.
Chem. Phys. 106 (4), 1402 (1997).

13

M. E. Nadal, P. D. Kleiber, and W. C. Lineberger, J. Chem. Phys. 105 (2), 504(1996).

14

P. E. Maslen, J. M. Papanikolas, J. Faeder, R. Parson, and S. V. ONeil, J.
Chem. Phys. 101 (7), 5731 (1994).

15

G. E. Busch, R. T. Mahoney, R. I. Morse, and K. R. Wilson, J. Chem. Phys.
51 (1), 449 (1969).

144
16

M. T. Zanni, T. R. Taylor, B. J. Greenblatt, B. Soep, and D. M. Neumark, J.
Chem. Phys. 107 (19), 7613 (1997).

17

J. Faeder, Ph.D. Thesis, University of Colorado, 1998.

18

J. Faeder, N. Delaney, P. E. Maslen, and R. Parson, Chem. Phys. 239 (1-3),
525 (1998).

19

N. Delaney, J. Faeder, and R. Parson, J. Chem. Phys. 111, 651 (1999).

20

P. E. Maslen, J. Faeder, and R. Parson, Chem. Phys. Lett. 263 (1-2), 63(1996).

21

Y. Marcus, Ion Solvation. (Wiley, New Yord, 1985).

22

R. Mabbs, K. Pichugin, E. Surber, and A. Sanov, J. Chem. Phys. 121, 265(2004).

23

N. Delaney, Ph.D. Thesis, University of Colorado, 1999.

24

R. Parson, J. Faeder, and N. Delaney, J. Phys. Chem. A 104, 9653 (2000).

25

P. C. Engelking, J. Chem. Phys. 87 (2), 936 (1987).

145

Chapter 4
Photoelectron Imaging Spectroscopy of Cu⎯ and Cu⎯(H2O)1,2 Anions

4.1

Introduction

The photofragmentation branching ratio experiments described in Ch. 3
demonstrate the detailed nature of the solvation effects observed in IBr⎯(CO2)nclusters as a function of cluster size. However, these studies probe the state of thesystem upon detection some 10 µs after the dynamics are initiated. The time-resolved
absorption recovery experiments shed a more light on the early time
photodissociation dynamics, but the probing of the system is limited to the narrowwindow when the chromophore is recombined on the ground state as discussed in Ch.
1. The photoelectron imaging spectrometer was designed and installed to provide amethod to measure the state of the system at any given time during the dynamics.
The photodetachment of charged species in the clusters is not limited to a specificregion of the potential energy surfaces and provides snapshots of the dynamics at any
time after initiation by photoabsorption of the chromophore. The pulsed nature of theexperiment allows the use of pulsed, tunable lasers to provide higher photon energies
than those available with cw lasers for the study of higher energy electronic states.
The first photoelectron imaging studies are carried out on Cu⎯ and Cu⎯(H2O)nclusters for three reasons. The new imaging spectrometer initially needed to have its
performance optimized. This includes the determination of electrode voltages for thebest focusing of the images on the detector. An atomic system was desirable due tothe narrow lineshapes for atoms.

Also, the Cu⎯ photoelectron spectrum had

146previously been measured1 in this group and could be used as a benchmark for the
spectra obtained from the new spectrometer. The second reason is that the Cu⎯ solutesolvated by H2O molecules provides a simple solvent system in which to test theapplicability of the photoelectron imaging spectrometer to solvation studies ingeneral. The Neumark group has previously shown the effect of increasing solvation
on an atomic anion represented by the shift of spectral features to higher binding
energies as the number of solvent molecules is increased as well as changes in thevibrational progressions of the solvent molecules as a result of perturbations by the
charged solute.2,3

Similar solvation affects should be present in the Cu⎯(H2O)ncomplexes. The final reason for the imaging study of Cu⎯(H2O)n complexes is tobuild on work carried out on Cu⎯(H2O)1,2 which includes a low resolution
photoelectron spectroscopy study,4,5 rearrangement dynamics of the complexes
following photodetachment,6 and calculations on the complexes.7,8

4.2

Photoelectron Imaging of unsolvated Cu⎯

The copper atomic anion has an electron configuration of 3d104s2 leading to a
1

S0 ground state. The energy level diagram of the anion ground state, as well as thethree low-lying electronic states of the neutral, is shown in Fig 4.1. The electron
affinity (EA) of Cu corresponding to the Cu 2S1/2 (3d104s1) ← Cu⎯ 1S0 (3d104s2)
transition has been measured9 as 1.23579(4) eV in agreement with the earlier
measurement1 of 1.235(5) eV. Higher energy photons with the tunable laser can be
employed to detach d shell electrons leading two transitions at higher binding
energies corresponding to Cu 2D5/2 (3d94s2) ← Cu⎯ 1S0 (3d104s2) and Cu 2D3/2 (3d94s2)

147

3d9 4s2 2D5/2,3/2

0.25 eV

1.39 eVhνCu

3d10 4s1 2S1/2

EA=1.236 eV

10

21

3d 4s S

Cu-

Figure 4.1: Energy level diagram for the ground state of Cu⎯ and the three
low-lying states of Cu neutral.

148← Cu⎯ 1S0 (3d104s2). The term energy of the 2D5/2 neutral state is 1.3889 eV with thehigher spin-orbit state 2D3/2 0.2533 eV greater in energy.10
The Cu⎯ bare anion is easily produced in the discharge source and the slightly
more intense

63

Cu⎯ isotope is mass-selected and used for the bare anion

photodetachment. The first step is to determine the electrode voltage ratio to provide
the best focusing of the image.

This ratio is the absolute value of the flight tube

voltage divided by the extraction plate voltage. In this case, the Cu⎯ anion is detachedusing 964.7 nm (1.285 eV) photons producing photodetached electrons with a kinetic
energy of around 50 meV. The electron kinetic energy (eKE) scales as velocity
squared, so the best resolution occurs for slower electrons. The extraction voltage is
set to –30 V to ensure that the images are completely on screen and the flight tube
voltage is then varied and images acquired. The photoelectron spectra are extracted
from the images and the 2S1/2 peak is fit to a Gaussian line-shape and the full-width
half-maximum of the fit determined. The flight tube voltage is varied until this
FWHM is minimized, which is typically less than 5 meV in width for Cu⎯ at these
low electron kinetic energies. The optimum ratio is found to be 4.48 meaning that if
the extraction plate is at –10 V the flight tube must be set to +44.8 V to ensure thebest focusing of the image.
The energy resolution, ∆E/E, of the instrument is measured as the FWHM of apeak divided by the electron kinetic energy of the peak. These values are around0.025 with the best ∆E/E value of 0.017 recorded for photodetachment at 532 nm,
which is a fundamental wavelength from the Infinity with a narrower line-width thanthose for the OPO wavelengths. The image can now be magnified by changing by

149
the voltage on the extraction plate and thus the flight tube to keep the determinedvoltage ratio constant. Two raw images at extraction plate voltages of –5 V and –50
V are shown in Fig. 4.2 to demonstrate the effects of varying the extraction voltage
and thus the magnification of the image. Both the raw and reconstructed images areshown in Fig. 4.2 as well to highlight the improvement in quality between the two.
The center line noise in Fig. 4.2 b,c is an artifact from the reconstruction. It should be
noted, however, that the energy resolution, ∆E/E, remains approximately independentof the magnification. The magnification determines the calibration scale as electrons
with the same kinetic energy are located at different pixels on the CCD camera
depending on the extraction voltage used. The electron kinetic energy and thus theresolution are determined largely by the photon energy used. The spectra must be
calibrated for each extraction voltage used in the experiments.
The photoelectron spectra extracted from the reconstructed images arepresented in counts versus pixel. The pixel axis must be calibrated and converted toan electron kinetic energy scale which can then be converted to electron binding
energy if desired. The pixel, which represents the velocity of the photoelectron, mustfirst be squared due to the velocity squared dependence of the kinetic energy. The
pixel squared value with the most counts is determined, which is assigned as theelectron affinity in the case of Cu⎯ photodetachment. This pixel squared value is thenassigned the eKE value of the electron affinity determined from previous
photoelectron assignments.

The calibration factor for this particular extraction

voltage is given by the assigned eKE divided by the pixel location squared. This

150

(a)

(b)

(c)

Figure 4.2: Photoelectron images of Cu⎯ taken at 964.7 nm (1.285 eV).
Image (a) is the raw image taken an extraction plate voltage of –5 V andimage (b) is the BASEX reconstruction. Image (c) is the raw image taken
at an extraction plate voltage of –50 V demonstrating the magnification
properties of images with the same eKE.

151

factor is then used to scale the rest of the pixel squared axis to eKE. This calibration
factor is determined for various extraction voltages (but retaining the same voltage
ratio for each) at 964.7 nm. The plot of calibration factor versus extraction voltage isshown in Fig. 4.3. The plot is nearly linear and a linear fit to the data is shown. This
plot can then be used to determine the calibration factor for any extraction plate
voltage used making recalibration of the spectrum unnecessary whenever the voltage
is changed. The final data adjustment deals with the counts to be plotted as a functionof eKE. A Jacobian correction must be made to account for the fact that the BASEX
reconstruction program artificially produces more counts at large pixel radii where
the integrated area is much larger than for the smaller pixel radii. This correction is
simply the counts at a pixel divided by the pixel. This Jacobian correction is thennormalized to the value at the most intense peak (typically the EA or VDE) andplotted versus the calibrated eKE to yield the final photoelectron energy spectrum.
The raw image and the reconstruction along with the photoelectron energy
spectrum are shown in Fig. 4.4 for the 532 nm (2.33 eV) photodetachment of Cu⎯.
Both the electron binding (eBE) and the electron kinetic energies (eKE) are shown.
The electron binding energy is simply the calibrated eKE subtracted from the photon
energy, 2.33 eV in this case. The spectrum shows one intense peak at the EA of Cu of
1.236 eV and a labeled FWHM of 19 meV at an eKE of 1.095 eV and is assigned as
the 2S1/2 state of the neutral. This leads to a ∆E/E of 0.017. The value for β is 2.00(1)where the uncertainty is one standard deviation of the mean across the FWHM of the
peak.

152

1.6e-5

Calibration factor

1.4e-51.2e-51.0e-58.0e-66.0e-6
4.0e-62.0e-6
0.0
0

20

40

60

80

100

Electron Extraction Plate Voltage (volts)
Figure 4.3: Energy scale calibration factor as a function of voltage
placed on the extraction plate for Cu⎯ photodetachment at 964.7 nm.
The extraction plate to flight tube voltage ratio is kept constant.

153

Photoelectron Kinetic Energy (eV)
0.0

0.2

0.4

0.6

0.8

-

Cu

1.0

1.2

1.4

1.236 eV VDE
β = 2.00

Photoelectron Countshν = 2.33 eV∆E/E = 0.017

19 meV

2.2

2.0

1.8

1.6

1.4

1.2

1.0

Electron Binding Energy (eV)Figure 4.4: Photoelectron energy spectrum extracted from the BASEX
reconstructed image of Cu⎯ photodetachment at 532 nm (2.33 eV). The
peak is assigned as the 2S1/2 state of neutral Cu.

154
The value of β represents the anisotropy of the photoelectron distribution forthis 2S peak and describes the nature of the orbital from which the photodetachment
occurs. The angular distribution of photoelectrons within the dipole approximation11
is given by the expression,dσ σ tot=(1 + β P2 (cos Θ)) ,d Ω 4π(4.1)

where P2(cosΘ) is the second Legendre polynomial given by

1P2 (cos Θ) = (3cos 2 Θ − 1) .
2

(4.2)

The value of β is the asymmetry parameter and can range in value between –1 and 2.
The symmetry character of the orbital from which photoelectrons originate isreflected in the angular distributions and is most easily demonstrated for atomic
systems. The transition of an electron between electronic states must obey the ∆l =±1 orbital angular momentum selection rule. Photodetachment from an s orbital (l =0) must impart one unit of angular momentum thus producing a p partial wave. A p
partial wave has a cos2Θ distribution corresponding to a β value of +2. Detachmentfrom a p orbital can lead to either s or d waves from the ∆l = ±1 selection rule. Near
threshold detachment leads to the isotropic s wave dominating and β = 0. Further
from threshold, the d wave contribution increases leading to a mixing of the two andvalues closer to β = –1. The assignments become less clear for molecules due to the155
mixing of atomic orbitals, but the complexes studied here can be approximated withthe charge residing on the atomic copper with a weakly interacting solvent. The
value of β for photodetachment of Cu⎯ at 532 nm was measured to be 2.00
corresponding to detachment from an s orbital producing a p partial wave.
The laser wavelength can be tuned to the blue to provide higher photon
energies that now make the two 2D states of the neutral Cu accessible.

The

photoelectron spectrum of Cu⎯ taken at 355 nm (3.49 eV) is shown in Fig. 4.5. The
2

S peak is still present, though slightly broader in width at 54 meV than the 532 nm

spectrum due to the higher eKE of the photoelectrons associated with this peak at thiswavelength. It still appears at the previously measured EA of 1.236 eV. The twoaccessible 2D states appear at 1.39 eV higher eBE with the splitting between the twostates reflecting the spin-orbit splitting of 250 meV. Both peaks have FWHMs ofaround 35 meV, narrower than the 2S state reflecting the lower eKEs of the 2D
electrons. The 2S state still retains β = 2 and the 2D states have values of 0.07 and–0.11. These states correspond to detachment of d orbital electrons leading to p and fpartial waves.

This mixture of partial waves leads to near isotropic angular

distributions and β values close to 0.

4.3

Photoelectron Imaging of Cu⎯(H2O)n complexes

The imaging photoelectron spectrometer was optimized and the spectra
determined for the unsolvated Cu⎯ anion. These spectra will be used as a baseline todetermine the effects of solvation of the Cu⎯ solute by H2O solvent molecules. The
Cu⎯ data also provide an energy calibration scale for all subsequent spectra of the156

Photoelectron Kinetic Energy (eV)
0.0

0.5

1.0

-

2.0

2

S
1.236 eV VDE
β = 2.00

Cuhν = 3.49 eV

Photoelectron Counts

1.5

2

2
D3/2
D5/2
β = – 0.11 β = 0.07

54 meV250meV

35 meV

3.0

2.5

2.0

1.5

1.0

Electron Binding Energy (eV)Figure 4.5: Photoelectron energy spectrum extracted from the BASEX
reconstructed image of Cu⎯ photodetachment at 355 nm (3.49 eV). The
main peak is assigned as the 2S1/2 state of neutral Cu and now the two
2
D states are accessible. The term energy of the first 2D is reported in
the text. The 250 meV splitting between the two 2D represents the
spin-orbit splitting.

157
complexes.

As mentioned above, the neutral states of the complexes are best

characterized by the atomic state of the metal plus water molecules.

The state

notation used for the complexes is, for example, [Cu (2S), H2O] to represent the
ground electronic state of the neutral Cu(H2O) complex. In both Cu⎯(H2O) andCu⎯(H2O)2, the 65Cu isotope complex is mass-selected to avoid contamination from
CuO⎯ and Cu⎯(OH) peaks that are present in the mass spectra.
Cu⎯(H2O)

The photoelectron image and spectrum of Cu⎯(H2O) are shown in Fig. 4.6
taken at 568.8 nm (2.184 eV). The electron kinetic energy scale is also shown tohighlight the degree of energy resolution of the spectral features. The raw image isoverlaid with the direction of the laser polarization with respect to the plane of theimage shown as well. In this case, a lower energy photon is used for detachment toincrease the resolution of the lower binding energy features. The dominant peak inthe spectrum is assigned as the [Cu (2S), H2O] state of the neutral complex. The
vertical detachment energy (VDE) is measured to be 1.637(4) eV where the 4 meVuncertainty is largely due to the error in the peak fitting of a Gaussian line-shape tothe peak in the spectrum. The FWHM is found to be 40 meV with β = 2.00(1). A
smaller feature at slightly higher electron binding energy appears at 194 meV(1568(8) cm-1) higher from the VDE with β = 1.94(1) and is roughly 10% of theintensity of the main peak. This feature is just barely discernible in the raw image ata smaller radius from the main ring feature. The FWHM is roughly found to be 38

158

Photoelectron Kinetic Energy (eV)
0.0

0.2

0.4

hν = 2.18 eV

0.6

0.8

1.0

1.637 eV VDE
β=2.00E[Cu (2S), (H2O)]
FWHM 40 meV

H2O bending mode
194 meV

2.18

1.98

1.78

EA(CuOH2)
1.48 eV

1.58

1.38

1.18

Electron Binding Energy (eV)Figure 4.6: Photoelectron energy spectrum of Cu⎯(H2O) taken at 568.8 nm(2.18 eV). This photon energy accesses only the ground 2S state of theneutral complex. The bending mode of the H2O solvent is labeled. The raw
image is shown in the inset along with laser polarization direction.

159
meV, although the peak fit is somewhat uncertain due to the weak intensity of thefeature.
The low intensity feature is assigned as the H2O bending mode. The energy
separation between this peak and the VDE of the [Cu (2S), H2O] state is close to themeasured ν2 bending mode frequency of isolated H2O, 1595 cm-1.12 The studies of
Cu atoms and H2O molecules condensed in a cryogenic Ar matrix13 provide a valueof 1573 cm-1 that matches the photoelectron imaging value within the experimental
uncertainty. The assignment of this feature to the H2O bending mode is further
confirmed by the measurement of the photoelectron spectrum of the deuterated
complex, Cu⎯(D2O). The photoelectron spectrum of Cu⎯(D2O) is shown in Fig. 4.7
and is taken at 568.8 nm (2.18 eV). This spectrum exhibits a low intensity feature at
150 meV (1210(80) cm-1) above the intense peak assigned as the [Cu (2S), D2O]
transition. The energy difference between the two peaks is close to the D2O ν2bending frequency12 of 1178 cm-1 further validating the assignment of the small peakin the Cu⎯(H2O) to the excitation of a local water bending mode in the complex. The
VDE of the Cu⎯(D2O) is 1.638(4) eV, the same as that in Cu⎯(H2O), within the error
bars.
The appearance of the photoelectron spectrum for Cu⎯(H2O) with a single,
narrow intense peak with what may be a small vibrational progression is expected forphotodetachment of a nonbonding electron and production of a neutral molecule
whose geometry is very close to that of the anion molecule. In this case, the mostintense peak in the spectrum should provide the adiabatic electron affinity of the160Electron Kinetic Energy (eV)

0.0

0.2

Photoelectron Countshν = 2.18 eV

0.4

0.6

1.638 eV VDE
β=1.78[Cu (2S), (D2O)]

54 meVD2O bending mode
150 meV

2.0

1.8

1.6

Electron Binding Energy (eV)Figure 4.7: Photoelectron energy spectrum of Cu⎯(D2O) taken at 568.8 nm(2.18 eV). This photon energy accesses only the ground 2S state of theneutral complex. The bending mode of the D2O solvent is labeled.

161
complex. However, there are no features found at the adiabatic electron affinity
whose location is labeled in Fig. 4.6.
The Cu–O bond length in the Cu⎯(H2O) complex is quite long at a calculated
value7 of 3.276 Å. Vertical photodetachment of the anion complex produces a neutral
Cu(H2O) complex with a large Cu–O separation leading to very weak interactions
between the Cu 2S and the H2O molecule. Calculations7 have been carried out on theneutral complex and show that the potential energy difference between the neutral
complex at the anion geometry and the separated Cu and H2O is around 26 meV.
This result is used in a simple thermodynamic cycle to determine the dissociationenergy of the anion complex,

D0[Cu⎯–(H2O)] = VDE[Cu⎯(H2O)] – EA[Cu] + small correction(4.3)

Where EA[Cu] is the electron affinity9 of copper (1.236 eV), VDE[Cu⎯(H2O)] is themeasured vertical detachment energy found to be 1.637(4) eV, and the correction
term is +20 meV, which is approximately 80% of the 26 meV calculated well depth.
The value of D0[Cu⎯–(H2O)] is then found to be 0.42(2) eV which is in very good
agreement with the calculated value of 0.44 eV.7 The quantities in this calculation are
shown in Fig. 4.8.
The large geometry difference between the anion and the neutral complexes isalso demonstrated in that the most intense peak in the Cu⎯(H2O) spectrum is thevertical detachment energy and not the adiabatic electron affinity. The electron
affinity of Cu(H2O) is determined from a calculation7 of the Cu(H2O) ground state

162

Cu-H2O

1.4

26 meV

V(RCu-O) [eV]

1.2

1.0
0.0

EA(Cu)

Cu-OH2-

VDE (Cu H2O)

-0.2-

Cu H2O
-0.4
2

3

4

5

RCu-O (Å)

6

7

Figure 4.8: The potential energy surfaces for the Cu⎯(H2O) anion andneutral complexes. The quantities mentioned in the text for the calculationof the anion complex dissociation energy are labeled. The structures forthe anion ground state and the neutral complex after vertical detachment
are shown.

163
surface from which the dissociation energy of the neutral complex, 0.16 eV, is found.
The adiabatic electron affinity is given by this dissociation energy of the neutral
complex subtracted from the VDE[Cu⎯(H2O)] represented by the intense peak in the
Cu⎯(H2O) spectrum. This gives an EA[Cu(H2O)] of 1.48(3) eV. The error estimate islargely due to uncertainties in the neutral dissociation energy calculation.

This

spectrum has no detectable intensity where the electron affinity peak is expected tooccur.
The photoelectron spectrum of Cu⎯(H2O) with 355 nm (3.49 eV) is shown in
Fig. 4.9. Photodetachment at this wavelength now accesses the two excited electronic
states [Cu 2D3/2,5/2, H2O] of the neutral complex.

The two peaks representing

transitions to these two 2D states are quite broad at roughly 150 meV FWHM
compared to a FWHM of 40 meV for the 2S state taken at low eKE. The greater
width of these 2D states is a result of the greater size of Cu 2D compared to Cu 2S.
These peaks appear at electron binding energies of 3.050(2) and 3.313(2) eV,
respectively, and show the expected spin-orbit splitting energy of 250 meV. The βvalues for these two peaks are 0.16(1) and 0.19(1), values that are quite different fromβ = 2 that is expected for detachment from an s orbital.
Cu⎯(H2O)2

The photoelectron imaging spectroscopy of Cu⎯(H2O)2 is carried out to study
the effect of additional solvation on the Cu⎯ solute. This complex is very similar to
Cu⎯(H2O) in that the size difference between Cu⎯ 3d104s2 1S and Cu 3d104s1 2S allows
for electron photodetachment to access portions of the neutral potential energy
surface that have relatively weak interactions between the metal solute and solvent

164

Photoelectron Kinetic Energy (eV)
0.0

0.5

1.0

Photoelectron countshν = 3.49 eV

1.5

2.0

[Cu (2S), (H2O)]2

D3/2 3.31 eV eBE
β = 0.19
FWHM = 150 meV2

D5/2 3.05 eV eBE
β = 0.16

3.0

2.5

2.0

1.5

1.0

Electron Binding Energy (eV)Figure 4.9: Photoelectron energy spectrum of Cu⎯(H2O) at 355 nm (3.49 eV).
The 2D states are now accessible and are labeled accordingly. The values forthe 2S ground state do not appear as they are found in Fig. 4.6. The electron
binding energies (eBE) at which the 2D states appear are labeled along withthe β values. The FWHM of each peak is roughly 140 meV.

165
water molecules.

The orientation of the two water molecules is important indetermining the observed peak widths in the spectra.

The 40 meV peak widthobserved for Cu⎯(H2O) would be expected to roughly double if the two H2O solvent
molecules were on opposite sides of the Cu⎯. However, if the two H2O solvent
molecules are on the same side in a “quasi hydrogen-bonded dimer” configuration,
photodetachment would lead to significant changes in the water-water interactions
resulting in unresolved vibrational structure in the ground state peak with a width
more than twice that of Cu⎯(H2O).
The 521.2 nm (2.379 eV) photoelectron spectrum of Cu⎯(H2O)2 is shown in
Fig. 4.10 and only accesses the [Cu (2S), 2 H2O] state. The VDE is assigned as
2.003(8) eV with β = 1.86(2) consistent with detachment from an s orbital of Cu⎯.
The width of the peak is measured to be 97 meV, which is more than twice the widthof the Cu(H2O) VDE peak. This suggests the two H2O solvents are interacting
significantly on one side of the solute and agrees with structure calculations of thecomplex.14 There is also a partially resolved feature at 1540(80) cm-1 above the VDE,
which is again assigned as the ν2 bending mode of H2O. A photoelectron spectrum isalso taken at 313 nm (3.961 eV) to access the higher energy [Cu (2D3/2, 5/2), 2 H2O]
states of the complex and is shown in Fig. 4.11. The [Cu (2D5/2), 2 H2O] state has a
VDE of 3.50(7) eV with β = 0.4(1). The higher spin-orbit 2D3/2 state has a VDE of
3.72(7) eV and β = 0.3(1). These transitions are even broader at roughly 200 meVthan the ground state transition of Cu(H2O)2.

166

Photoelectron Kinetic Energy (eV)

0.0

0.2

Photoelectron Countshν = 2.379 eV

0.4

0.6

2.003 eV VDE
β=1.86[Cu (2S), 2(H2O)]

FWHM 93 meV

H2O bending mode
∼195 meV

2.2

2.0

1.8

Electron Binding Energy (eV)Figure 4.10: Photoelectron imaging spectrum of Cu⎯(H2O)2 at 521.2 nm(2.379 eV). This wavelength only accesses the ground 2S state of the complex.
A very small feature at roughly 195 meV higher in binding energy is assigned
as the H2O bending mode.

167

Photoelectron Kinetic Energy (eV)
0.0

0.5

1.0

1.5

hν = 3.961 eV

2.0[Cu (2S), 2(H2O)]

Photoelectron Counts

2

D3/2 3.72 eV eBE
β = 0.3
FWHM = 200 meV2

D5/2 3.50 eV eBE
β = 0.4

3.5

3.0

2.5

2.0

1.5

Electron Binding Energy (eV)Figure 4.11: Photoelectron energy spectrum of Cu⎯(H2O)2 at 313 nm(3.961 eV). The 2D states are now accessible and are labeled accordingly.
The values for the 2S ground state do not appear as they are found in
Fig. 4.10. The electron binding energies (eBE) at which the 2D states appear
are labeled along with the β values. The FWHM of each peak is roughly
200 meV.

168
4.4

Summary of increasing solvation on Cu⎯(H2O)n complexes

The performance of the new photoelectron imaging spectrometer was firstcharacterized and optimized using the Cu⎯ atomic anion. The results on the bare Cu⎯“solute” provide the peak widths and energy resolution, vertical detachment energy(in this case the electron affinity) and term energies of the 2S1/2 ground state and the2

D3/2, 5/2 excited states of the neutral Cu from which the effects of solvation by H2O

molecules can be determined. The subsequent spectra on the Cu⎯(H2O)1,2 complexes
demonstrate that the solvation is best characterized as the metal anion solute being
perturbed by the H2O adducts in these weakly bound complexes. A composite of thespectra for Cu⎯(H2O)0,1,2 is shown in Fig. 4.12. The bare Cu⎯ was taken previously on
a separate photoelectron spectrometer.1 The broadening patterns can be understood interms of the size relationship R(Cu⎯ 3d104s2) ≅ R(Cu 3d94s2) > R(Cu 3d104s).
Photodetachment of an electron from the diffuse, doubly occupied 4s2 orbital leaves
the neutral Cu with a 4s electron in an orbital with a much smaller spatial extent
which interacts less with the H2O solvents at the anion geometry due to an increased
bond length between the Cu and the H2O molecules.

This leads to less peakbroadening than that observed upon photodetachment of a 3d electron. In this case,the diffuse, doubly occupied 4s2 orbital is still present which interacts more strongly
with the H2O solvents leading to broader peaks. The excess charge in the Cu⎯(H2O)1,2
is predominantly localized on the Cu⎯ anion solute, however, there is sufficient charge
delocalization onto the solvent and reduction of the H-O-H bond angle to excite theH2O bending mode upon photodetachment. This bending mode appears as the smallfeatures to higher electron binding energies of the 2S ground state peaks. The ground

169

2

S

Photoelectron Counts

Cu⎯
2

D

Cu⎯(H2O)
40 meV

Cu⎯(H2O)2
93 meV

3.5

3.0

2.5

2.0

1.5

1.0

Electron Binding Energy (eV)Figure 4.12: Composite photoelectron spectrum of the Cu⎯(H2O)n (n = 0,1,2)
clusters. The individual spectra for Cu⎯(H2O)1,2 can be found in previous
figures. The Cu⎯ photoelectron spectrum is taken from Ho et al. referenced
in the text. The progressive solvation retains the atomic level structure withbroadening of peaks and shifts to higher electron binding energies.

170
state of the Cu⎯(H2O) spectrum was shown to possess an intense, narrow peak withperhaps a short vibrational progression, which will be further investigated in future
experiments. These traits are normally indicative of a neutral geometry that is very
similar to the anion precursor geometry. However, the adiabatic electron affinity isabsent from the spectrum and the intense peak is instead assigned as the vertical
detachment energy. The absence of the adiabatic electron affinity is a result of thelarge size difference between Cu⎯ 3d104s2 and Cu 3d104s1 and the large geometry
change between the anion and neutral complexes upon photodetachment.

Thevertical detachment energy is related to the dissociation energy of the anion complex
which is calculated through a simple thermodynamic cycle.

171
Chapter 4 References
1

J. Ho, K. M. Ervin, and W. C. Lineberger, J. Chem. Phys. 93, 6987 (1990).

2

D. W. Arnold, S. E. Bradforth, E. H. Kim, and D. M. Neumark, J. Chem.
Phys. 97, 9468 (1992).

3

D. W. Arnold, S. E. Bradforth, E. H. Kim, and D. M. Neumark, J. Chem.
Phys. 102, 3510 (1995).

4

F. Misaizu, K. Tsukamato, M. Sanekata, and K. Fuke, Laser Chemistry 15,
195 (1995).

5

F. Misaizu, K. Tsukamoto, M. Sanekata, and K. Fuke, Surf. Rev. Lett. 3, 405(1996).

6

F. Muntean, M. S. Taylor, A. B. McCoy, and W. C. Lineberger, J. Chem.
Phys. 121, 5676 (2004).

7

M. S. Taylor, F. Muntean, W. C. Lineberger, and A. B. McCoy, J. Chem.
Phys. 121, 5688 (2004).

8

A. Antusek, M. Urban, and A. J. Sadlej, J. Chem. Phys. 119, 7247 (2003).

9

R. C. Bilodeau, M. Scheer, and H. K. Haugen, J. Phys. B 31, 3885 (1998).

10

J. Sugar and A. Musgrove, Journal of Physical Chemistry Reference Data 19,
527 (1990).

11

J. Cooper and R. N. Zare, J. Chem. Phys. 48, 942 (1968).

12

T. Shimanouchi, Tables of Molecular Vibrational Frequencies Consolidated
Volume I. (National Bureau of Standards, Washington, DC, 1972).

13

J. W. Kauffman, R. H. Hauge, and J. L. Margrave, J. Phys. Chem. 89, 3541
(1985).

14

M. S. Taylor, J. Barbera, C. P. Schulz, F. Muntean, A. B. McCoy, and W. C.
Lineberger, J. Chem. Phys. in press (2004).

172

Chapter 5
Photodissociation Dynamics of Trihalide Anions

5.1

Introduction

Trihalide anions offer precursors for the study of dihalide anion wavepacket
dynamics.

The selective photodissociation of these trihalide anions produces avibrationally excited wavepacket in the dihalide anion products. The triiodide anion,
I3⎯, has been the subject of numerous of these wavepacket dynamics studies in both
solution1-10 and in the gas phase.11,12 However, the triiodide anion is symmetric withboth bonds being identical making control of the dissociation process and selective
bond breaking impossible. It is also difficult to determine on which atom the charge
resides after dissociation and the exact product formation mechanism.
These problems are reduced in the case of a non-symmetric trihalide anion.
The reaction pathways and charge location are unambiguously identifiable and the
dynamics are richer due to the appearance of additional reaction pathways. The
subjects of these mixed trihalide studies are BrICl⎯ and IBr2⎯, which were easily
synthesized in the pulsed valve source. Characterization of these species is firstcarried out by theoretical calculations to determine ground state structures andenergies

along

with

vibrationalfrequencies.

One-photon

photodissociation

experiments are carried out to determine the products formed and then comparisons
are made with the expected photodissociation products predicted from thecalculations. The vibrational wavepacket dynamics of the diatomic anion formed
from the photodissociation of the triatomic anion is studied by time-resolved pump-173
probe experiments. The triatomic anions are photodissociated with a UV pump
photon to produce a vibrationally excited diatomic anion. The vibrational wavepacket
dynamics of the diatomic anion product is monitored by the absorption of an IR probe
photon. The time-dependent signal observed when the delay between the pump andthe probe pulses is varied results from the motion of the wavepacket.

Theseexperimental results are compared to wavepacket dynamics simulations performednear dissociation threshold of the diatomic anion to determine the relationshipbetween the proximity of this threshold to the level where the wavepacket is initiallyformed on the subsequent dynamics of the wavepacket.

5.2

Synthesis and Identification of BrICl⎯ and IBr2⎯

The apparatus on which these experiments are carried out has been describedin detail in Ch. 2. The ions are formed and analyzed in the same manner, but themethod for preparation was not discussed previously. Separate mixtures are used topreferentially form each of the two trihalide anion species studied. For the formationof BrICl⎯, the gas sample is prepared by passing a mixture of 5% CH3Br seeded in Ar
over a sample of solid ICl at room temperature. The resulting mixture is expanded
through the supersonic nozzle at a backing pressure of 3 atm. The neutral expansion
is crossed perpendicularly by the 1 keV beam of electrons. To preferentially form theIBr2⎯, the ICl sample is replaced by IBr which is heated to about 40ºC to increase thevapor pressure. The same 5% mixture of CH3Br in Ar is used for the backing gas.
Though not the subject of experiments presented here, I2Br⎯ was also preferentially174
formed in an analogous manner as for BrICl⎯ and IBr2⎯ by using I2 as the solid sample
with the same backing gas mixture.
The mass spectra of these two mixtures are shown in Fig. 5.1. The two
trihalide anions of interest are the dominant peaks in each spectrum. The exact
mechanism for the formation of these ions is not known, but it likely involves theattachment of slow, secondary electrons to complex neutral clusters as solvent
evaporation and collisions remove the excess energy.13 An alternative mechanism forthe formation is a solvent-assisted association of ICl or IBr with Br⎯ generated bydissociative electron attachment to CH3Br or IBr itself (i.e. via a diabatic electron
capture mechanism).13-17
The peaks in the spectra were easily assigned due to the various isotope ratios of theconstituent Br and Cl atoms in the ions of interest. The triplet structures of the BrICl⎯
and IBr2⎯ peaks in Fig. 5.1(a) (masses 241, 243, 245, and 285, 287, 289, respectively)reflect the existence of the isotopes of 79,81Br and 35,37Cl (while 127I is the only major
natural isotope of iodine). The natural abundance ratios of 79Br : 81Br ≈ 1:1 and35

Cl : 37Cl ≈ 3:1 dictate the following intensity patterns for the BrICl⎯ and IBr2⎯ peaks:

79

BrI35Cl⎯ : (79BrI37Cl⎯ + 81BrI35Cl⎯) : 81BrI37Cl⎯ ≈ 3:4:1 and 79BrI79Br⎯ : 79BrI81Br⎯ :81

BrI81Br⎯ ≈ 1:2:1. The isotope patterns for these and other ions are helpful inassigning the peaks in the mass spectra. For example, the group of four peaks (m =379, 381, 383, and 385) in Fig. 5.1(b) labeled as IBr2⎯·CH3Br reveals deviation fromthe statistically expected isotope ratio of 1:3:3:1. The excessive intensity of the m =381 peak is most likely due to a contribution from I3⎯.

175

BrICl⎯(a) ICl/CH3Br

I⎯

ICl⎯ Br⎯·CH3Br
ICl2⎯
IBr⎯
×20

254, 256,
258, 260

×10
IBr2–

(b) IBr/CH3Br

Br⎯·(CH3Br)2×20

Br⎯·CH3Br
×100

120

160

333, 335,
337, 339,
341
×20

IBr2⎯

IBr⎯

200

I2⎯

240

IBr⎯·CH3Br

IBr2⎯·CH3Br

×10 I Br⎯ ×10
2

280

320

360

Mass (a.m.u.)
Figure 5.1: Mass spectra of precursor anions formed by co-expanding (a)ICl and CH3Br seeded in Ar carrier gas, and (b) IBr and CH3Br in Ar.

400

176
5.3

Calculated Structures and Dissociation Energetics

The zero-order energetics and structures of the BrICl⎯ and IBr2⎯ anions areinvestigated using density functional theory (DFT). The calculations are carried out
with Gaussian 9818 at the level of Barone and Adamo’s Becke-style one-parameterexchange functional mPW1PW.19 For open-shell and closed-shell systems, the spinunrestricted and spin-restricted methods are used, respectively. For iodine, therelativistic effective core potential of Hay and Wadt,20 combined with a double-zeta
valence basis set (LanL2DZ) is used. This basis is augmented with uncontracted
diffuse s and p functions (exponents 0.0569 and 0.0330, respectively) and d and fpolarization functions (exponents 0.292 and 0.441, respectively).21 For bromine andchlorine, the 6-311+G(d) all-electron basis sets are used. For all species larger thanan atom, the energies are calculated after full geometry optimizations.
The calculated energetics for the various possible structures of BrICl⎯ andIBr2⎯, along with the energies corresponding to the various dissociation pathways, isshown in Fig. 5.2. As a comparison, the same energetics is also shown for I3⎯ at thesame level of theory. Spin-orbit coupling was not accounted for in these calculations
and the zero-point vibrational energy corrections were not included. As seen in Fig.
5.2, the expected linear isomer BrICl⎯ is the most stable form of BrICl⎯.
For all of the trihalide anions, the ground state anion geometry is linear withmost of the charge localized on the two end atoms. The BrICl⎯ isomer is stabilized
relative to the two other forms that have the I atom on an end position due to thegreater electronegativity of Br and Cl compared to I. The larger I atom in the middle
position also serves to screen the two partially charged Cl and Br atoms further,

177

4.5

BrICl –

IBr 2 –

I3–

Br+I – +Cl (4.25)

4Br – +I+Cl (4.00)
Cl– +I+Br (3.89)

Br+I – +Br (4.00)
Br – +I+Br (3.76)

3.5

I+I – +I (3.34)

Energy (eV)32.5

IBr – +Cl (2.72)
ICl– +Br (2.58)
BrCl– +I (2.46)

2

I – +BrCl (2.09)

1.5
1

–

Cl +IBr (1.80)
Br – +ICl (1.74)

IBr – +Br (2.47)
Br 2 – +I (2.32)
I – +Br 2 (1.91)

I 2 – +I (2.02)

Br – +IBr (1.67)
I – +I 2 (1.40)

IClBr – (0.88)

0.5

IBrCl – (0.57)

0BrICl– (0.00)

IBrBr – (0.49)
BrIBr – (0.00)

I 3 – (0.00)

Figure 5.2: Energetics of the BrICl⎯, IBr2⎯, and I3⎯ anions, calculatedusing the mPW1PW density functional method, as described in the

text. Numbers in parentheses are energies in electron volts relative
to the most stable isomer of the corresponding trihalide anion

178
stablilizing the lowest energy isomer. The next lowest energy isomer, IBrCl⎯, is
calculated to be 0.57 eV higher and the least stable form, with the mostelectronegative Cl in the middle position, is 0.88 eV above the most stable BrICl⎯
isomer. Similarly, the most stable form of IBr2⎯ is the form containing the lesselectronegative I in the middle position with the two Br atoms occupying the end
positions. The isomer BrBrI⎯ is 0.49 eV higher in energy.
The energetics analysis does not include spin-orbit effects that play animportant role in the spectroscopy and dynamics. The spin-orbit splittings for each
atom lead to a multiplicity in all of the dissociation channels involving open-shellfragments in Fig. 5.2. Also, the DFT calculations overestimate the bond dissociationenergies of the open-shell species due in part to the omission of spin-orbit coupling.
For instance, Fig. 5.2 shows the dissociation energy of I2⎯ as being 1.32 eV. Thisvalue was measured experimentally to be 1.01 eV.22,23 The accuracy is better inclosed-shell species.

The bond strength of I3⎯ is calculated to be 1.40 eV for

dissociation to I2 + I⎯ compared to the experimental value of 1.31 ± 0.06 eV.24 The
calculations, though not completely accurate in all cases, do provide a helpful guidefor interpreting the experimental data.

5.4

Photodissociation of BrICl⎯

The photodissociation experiments are carried out on the mass selected
79

BrI35Cl⎯ (m = 241) anion, corresponding to the first isotope peak in the BrICl⎯

triplet in Fig. 5.1(a). The photodissociation of this mass selected anion was carriedout at two UV wavelengths. The photofragmentation mass spectra are shown in Fig.

179
5.3 for photodissociation at 350 and 392 nm, respectively. For all wavelengths
studied (350−410 nm), the IBr⎯ and ICl⎯ photodissociation products account for >95%of the one-photon photofragments and the BrCl⎯ (m = 114) photoproduct is notproduced. These products indicate that only the BrICl⎯ isomer is formed in the ion
source. This is in agreement with calculations that found the BrICl⎯ isomer to belowest in energy.
At these dissociation wavelengths, IBr⎯ is the dominant photoproduct with theyield decreasing slightly with increasing wavelength from 0.8 at λ = 405 nm to <0.7
at λ = 350 nm. The other prominent product is ICl⎯, which increases in yield withincreasing photon energy from about 0.2 at λ = 405 nm to 0.3 at λ = 350 nm. The
atomic anion photoproducts, Cl⎯, Br⎯, and I⎯, are present at the shorter wavelength(350 nm).

However, these atomic products account for about 5% of the total

photoproduct yield and completely disappear at the longer wavelength.
The time-resolved experiments are now carried out based on the one-photon
results above.

The diatomic photoproducts IBr⎯ and ICl⎯ are probed using thefundamental wavelengths (780−820 nm) to perform a second photodissociation stepthat is delayed in time with respect to the photodissociation of the trihalide anion.
The photodissociation cross sections for IBr⎯ and ICl⎯ in this wavelength range havebeen characterized in a preceding chapter for IBr⎯ and previously for ICl⎯.25,26 For
both dihalide products, absorption at these wavelengths produces I⎯ as the dominant
product.
The pump-probe experiment consists of photodissociation of mass-selected
BrICl⎯ using second harmonic wavelengths (around 400 nm) of the fundamental

180

IBr−

(a) 350 nm

Br−

Fragment Ion Signal

Cl−

×5

I − ICl−IBr−(b) 392 nm

×10

20

50

80

ICl−

110 140 170 200 230

Fragment Ion Mass (a.m.u.)
Figure 5.3: Photofragment ion mass spectrum
obtained in the photodissociation of BrICl⎯ (the79
BrI35Cl⎯ isotope) at (a) 350 nm and (b) 392 nm.

181
output of the femtosecond laser system. These wavelengths are used due to theimproved performance of the system near 800 nm. The initial photodissociation
produces the IBr⎯ and ICl⎯ photoproducts shown in Fig. 5.3(b). These diatomic
fragments are then photodissociated with the fundamental IR (~800 nm) femtosecond
pulse to produce I⎯ photoproducts that are detected as a function of delay between thepump (UV) and probe (IR) pulses. The time-resolved data for this process are shownin Fig. 5.4 representing two different pump-probe schemes with the wavelengths
shown.

Both the IBr⎯ and ICl⎯ photoproducts are produced in highly excited

vibrational states. These excited states have absorptions that are red shifted from the
maximum absorption of vibrationally relaxed IBr⎯ and ICl⎯. The contributions fromeach of the dihalide anions to the two-photon I⎯ signal are not resolved, but it isassumed that most of the signal originates from IBr⎯ due to the relative product ratiosfrom BrICl⎯ dissociation and the relative absorption cross sections between ICl⎯ andIBr⎯ mentioned in a preceding chapter.
The first noticeable feature of the signals is the sharp initial peak at very shortpump-probe delays and its rise time is determined by the time resolution of theexperiment. This feature can partially be due to excited state absorption of BrICl⎯
before any dissociation of the trihalide has taken place. This excited state correlates
to IBr⎯ + Cl and/or ICl⎯ + Br channels if direct dissociation is occurring. The peak
could then be attributed to probe absorption of IBr⎯ (ICl⎯) being perturbed at earlytimes by the neutral Cl (Br) fragments that have not yet left the vicinity of the excited
photoproduct anion. The second feature is the oscillatory structure following theinitial sharp peak. The time-dependent signal is superimposed with sine waves with

182

Two-Photon I− Yield(a) BrICl− 400/800 nm(b) BrICl− 410/820 nm

0

1

2

3

4

5

Pump-Probe Delay (ps)Figure 5.4: Delay-dependent pump-probe signals in the photodissociation of BrICl⎯ as described in the text with 400 nm
pump/800 nm probe in (a) and 410 nm pump/820 nm probe in(b). The superimposed sine waves highlight the oscillations andhave periods of 600 fs in (a) and 530 fs in (b).

183
periods of 600 fs in Fig. 5.4(a) and 530 fs in Fig. 5.4(b). These oscillations are
attributed to the newly formed IBr⎯ vibrational wavepacket and are discussed below.

5.5

Photodissociation of IBr2⎯

The photodissociation experiments on IBr2⎯ were carried out on the 79BrI81Br⎯(m = 287) isotope that corresponds to the most intense peak in the triplet in Fig.
5.1(b). The one-photon photodissociation products for photodissociation at 402 nm
are shown in Fig. 5.5. The IBr⎯ fragments account for >95% of the total products,
similar to BrICl⎯. The photoproduct Br2⎯ is not seen confirming the suspected identityof the lowest energy isomer predicted by the calculations. A small amount ofdissociation leads to the atomic anion products Br⎯ and I⎯. These atomic products do
not disappear at lower photon energies as in the case of BrICl⎯ photodissociation.
The product mass spectrum shown for 402 nm photodissociation is representative of
the product mass spectra over the entire wavelength range studied.
The time-resolved experiments for IBr2⎯ are carried out in an analogous
manner to BrICl⎯. The pump photon produces vibrationally excited IBr⎯ that is thenprobed using the fundamental output wavelengths (~800 nm) by monitoring thepump-probe delay-dependent I⎯ two-photon product.

The I⎯ product necessarily

originates from IBr⎯ unlike BrICl⎯ pump dissociation that yields both IBr⎯ and ICl⎯photoproducts. However, the trihalide used in the experiments consists of both 79Br
and81

Br isotopes. The observed two-photon I⎯ signal then arises fromphotodissociation of both I79Br⎯ and I81Br⎯ isotopes. The vibrational frequencies of
the two isotopes differ by only 0.77% making the short term dynamics (<10ps)

184

Br

−

I

−

IBr

−

Fragment Ion Signal

30 ps

×5
0.1 ps− 5 ps

40

80

120

160

200

240

Fragment Ion Mass (a.m.u.)
Figure 5.5: Photofragment ion mass spectrum of the photodissociation of IBr2⎯ (the 79BrI81Br⎯ isotope) at 402 nm. The
superimposed lines indicate the mass spectra with 402 nm
pump/804 nm probe at the indicated delay times between the
two.

185
unaffected by the superposition of the two frequencies.

The mismatch of thefrequencies becomes important on the tens of picoseconds timescale.27
The two-photon I⎯ signal is superimposed on the one-photon I⎯ signal from
IBr2⎯ dissociation by the pump beam only. The superimposed lines in Fig. 5.5 show
portions of the photofragment mass spectra obtained from the pump (402 nm) andprobe (804 nm) measurements at the indicated delays between the two. The bottom
spectrum, recorded with the probe pulse arriving 5 ps before the pump, is similar tothe pump-only mass spectrum, because the probe pulse is not absorbed by IBr2⎯. The
middle trace, obtained at a 0.1 ps delay, shows an increase in the I⎯ intensity resulting
from the two-photon formation of this fragment. The Br⎯ product intensity remains
unaffected, because I⎯ is exclusively produced in IBr⎯ dissociation in the near-IR. At
longer delays, the intensity of the I⎯ peak reduces again, but stays above the onephoton background level.
The delay dependent results are shown in Fig. 5.6 for three different
wavelength combinations of pump and probe pulses. These time-resolved signals
include background subtraction of the one-photon I⎯ signal that is present at all timedelays. Qualitatively, the overall pattern of the data is similar to that of BrICl⎯.
However, the intensity of the initial peak compared to the long time signal is muchgreater in IBr2⎯. This ratio drops significantly with increasing pump and probe
wavelengths. This could be due to either a decrease in excited state absorption in
IBr2⎯ or to an increase in probe absorption by vibrationally excited IBr⎯.
Periodic oscillations in the I⎯ signal are present, as they are in the BrICl⎯ data,
and are attributed to IBr⎯ vibrational wavepacket motion. The oscillations are

186(a) IBr 2 − 391/782 nm−

Two-Photon I− Yield(b) IBr 2 402/804 nm−(c) IBr 2 407/814 nm

0

1

2

3

4

Pump-Probe Delay (ps)Figure 5.6: Delay-dependent pump-probe signals in the photodissociation of IBr2⎯as described in the text with 391 nm pump/
782 nm probe in (a), 402 nm pump/804 nm probe in (b), and407 nm pump/814 probe in (c). The superimposed sine waves
highlight the oscillations and have periods of 800 fs in (a), 650 fs
in (b), and 620 fs in (c).

187

highlighted with superimposed sine waves on the experimental curves. The sine
waves have varying periods to best fit the experimental data. The trend in these three
data sets is that decreasing pump and probe photon energies correspond to decreasing
oscillation periods. As can be seen from Fig. 5.6, 391 nm pump/782 nm probe has an
oscillation period of roughly 800 fs, 402/804 nm corresponds to 650 fs, and 407/814
nm with 620 fs. This trend is the result of exciting and probing progressively lower
vibrational levels in the IBr⎯ photoproduct.
The IBr2⎯ system presents an attractive subject for the study of wavepacket
dynamics. It possess only one distinct intermediate fragment from which to probe thedynamics, unlike BrICl⎯ where the observed time dependent I⎯ signal can originate
from both ICl⎯ and IBr⎯ making the results less straightforward to interpret. As
mentioned above, the presence of two isotopomers in the IBr⎯ one photon product canaffect the wavepacket dynamics at longer pump-probe delays. The I79Br2⎯ species isused when looking for long time recurrences in the wavepacket dynamics to ensurethat only one vibrational frequency was present in the dihalide photoproduct.
However, no recurrences in the vibrational wavepackets are observed around 18 and29 ps where they are predicted to occur based on wavepacket simulations discussed
below. In addition to insufficient time resolution that may smear out the weak revival
signal seen at short delays, there may be a smearing of the revivals themselves due tothe proximity of the wavepackets to the dissociation limit of the dihalide anion at
longer pump-probe delays, which is described in detail below.

188
The photodissociation of I2Br⎯ was briefly studied at 392 nm. This produced
the dominant diatomic anion products of I2⎯ and IBr⎯ with trace amounts of atomic
anion products. The I2⎯ photoproduct was five or six times as large as the IBr⎯
product. The appearance of I2⎯ as a photodissociation product implies a structure withan I atom residing in the center position of the trihalide. The structure energetics was
not calculated, however this experimentally determined structure follows from theresults for BrICl⎯ and IBr2⎯ in which the more electronegative atom resides on an end
position. The I2Br⎯ trihalide anion provides an interesting precursor for studying the
I2⎯ wavepacket dynamics which have been carried out starting from I3⎯ precursor.11,12
Photodissociation of I2Br⎯ places the I2⎯ wavepacket closer to the dissociationthreshold of I2⎯ than photodissociation of I3⎯. The effects of the proximity of the
dissociation threshold to the wavepacket dynamics in I2⎯ can now be studied bysimply tuning the precursor trihalide anion on which experiments are carried out.

5.6

Vibrational Dynamics of IBr⎯ Photofragments

The photodissociation of a trihalide anion to a diatomic anion and a neutral
atom takes an excess anti-bonding electron delocalized across two bonds in the parent
trihalide anion and localizes it onto one bond in the product diatomic anion. Both of
the bonds in IBr2⎯ and I3⎯ have a bond order of ~3/4, while the bond order in the two
diatomic fragment anions IBr⎯ and I2⎯ is reduced to only 1/2. Therefore, the I-Br andI-I bonds are initially compressed in the trihalides relative to the relaxed diatomic
anion products. The dihalide fragments are formed in vibrationally excited states at ashort bond length. This is also true for both photofragment dihalides arising from the189dissociation of BrICl⎯ and I2Br⎯.
The equilibrium geometries of the trihalide anions are shown along with thecorresponding photofragment geometries in Fig. 5.7. These geometries are calculatedwith the DFT method described above. The values in parentheses indicate the bondelongation in angstroms upon photodissociation of each of the trihalides.

Thefragments IBr⎯ and ICl⎯ from photodissociation of BrICl⎯ and IBr2⎯ show a greaterbond elongation than in the I2⎯ fragment from I3⎯ photodissociation. It is expectedthat the IBr⎯ and ICl⎯ products are formed with higher vibrational excitation than the
I2⎯ fragment formed from I3⎯.
The observed oscillations in the I⎯ signal are attributed to the motion of this
newly created wavepacket on the IBr⎯ potential surface. The fragment formed is acoherent superposition of vibrational levels due to the duration of the pump pulse(~180 fs) being less than that of the fragment vibrational period. The diatomic
fragment is able to absorb the probe photon when the initially compact wave packet
passes through an absorption window most likely localized on the outer wall of theIBr⎯ potential. As the wavepacket makes several round-trips across the well, theanharmonicity of the potential causes the wavepacket to dephase and the absorption
signal then disappears. In the case of BrICl⎯, there is also a contribution from ICl⎯ asa wavepacket is also formed on the corresponding ICl⎯ potential and provides a timedependent signal contribution to the overall observed I⎯ signal. However, thesecontributions from ICl⎯ are neglected due to the smaller amount of it formed from
BrICl⎯ dissociation (~4:1 for IBr⎯:ICl⎯) and the much smaller absorption cross section
for ICl⎯ in the near-IR. Both of these factors lead to a negligible contribution to the I⎯

190

–
–
I

Br

+

I

Br
(0.346)

Cl

–

2.572

2.723

Cl

3.069

Br

+

I

Cl

2.920
(0.348)

–
I

Br

2.727

–
Br

Br

+

I

Br

3.069

2.727

(0.342)

–
I

I

I

2.934

2.934

–
I

+

I

I

3.258
(0.324)

Figure 5.7: Equilibrium geometries of the BrICl⎯, IBr2⎯, and I3⎯ anions
and their fragments, calculated using the mPW1PW method, as describedin the text. Bond lengths are in angstroms. Numbers in parentheses
indicate the bond elongation upon dissociation of the respective trihalide
anion.

191
signal from ICl⎯. The vibrational periods observed range from about 530 to 600 fs for
BrICl⎯ and 620 to 800 fs for IBr2⎯. These values correspond to frequencies of 63 to
56 cm-1 and 54 to 42 cm-1, respectively.
The ground state vibrational frequency of IBr⎯ has been measured28 to be ωo =136 cm−1. The classical vibrational frequency scales as:

ω(E) = ωo(E/Do)1/2(5.1)

in the limit of the Morse oscillator model, where Do is the dissociation energy and E
is the energy below the I⎯ + Br threshold. The vibrational energy, Ev, is given by theexpression Ev = Do – E. Using Eq. 5.1, the BrICl⎯ frequencies of 63 to 56 cm-1
correspond to average vibrational excitations from 0.79 to 0.83 of the IBr⎯dissociation energy, Do, which is about 1 eV. For IBr2⎯, the observed frequencies of
54 to 42 cm-1 lead to excitations of 0.84 to 0.91 of Do. The I2⎯ wavepacket wasobserved11,12 to have a period of 550 fs (60 cm-1) after photodissociation of I3⎯ at 390
nm. The ground state vibrational frequency23 of I2⎯ is 110 cm−1 which corresponds toan average vibrational excitation of 70% of the dissociation energy after dissociationof I3⎯.
The overall picture at this point is that BrICl⎯ and IBr2⎯ show a larger
geometry change upon dissociation to the dihalide species IBr⎯ and ICl⎯ than is seenin the corresponding I3⎯ to I2⎯ photodissociation process.

This larger geometry

change leads to the mixed dihalide anion products having more vibrational excitation
than the I2⎯ product. This geometry change results in the mixed dihalide anion

192
wavepackets being produced closer to the dissociation threshold, diminishing theprospects of detecting wavepacket revivals at long time delays.

5.7

Wave Packet Dynamics near Dissociation Threshold

One of the goals of studying the vibrational wavepacket dynamics of these
dihalide anion photoproducts is to observe the wave packet revivals at long pumpprobe delays. The wavepacket motion was only observed at short times (<10 ps) after
formation, but then rapidly dephased due to the potential anharmonicity and signal
was lost. At a later time, the wavepackets were expected to rephase and the signal torevive. However, these signal revivals were not detected. Experimental factors suchas insufficient time resolution along with rotational dephasing and multiple spin-orbit
surfaces that can be accessed during dissociation could in principle explain the lack of
the revival observation. However, revivals were observed in the I3⎯ experiments
under similar experimental conditions.
The time-dependent wavefunction of a wavepacket is given by:ψ ( x, t ) = ∑ cvuv ( x) exp[−iE (v)t / =]

(5.2)

v

where v is the vibrational quantum number, and cv and uv(x) are the level amplitudes
and spatial wavefunctions. The wavepacket initially created is real and localized.
For short times after its creation, the wavepacket behaves classically.29-31 During thistime the position of the wavepacket can be finitely measured and produces the initial
oscillations in the pump-probe signal (Figs. 5.4 and 5.6). However, the energy levels

193
E(v) of a quantum system are not uniformly spaced32,33 leading to a delocalization ordephasing of the wave packet and its subsequent revivals. The energy levels of thesystem can be expanded in a Taylor series as a function of v to account for theanharmonicity of the potential well:E ( v ) = E ( v ) + ( v − v ) ∂E +∂v v

( v − v) 2 ∂ 2 E( v − v) 3 ∂ 3 E+ ...
+2 ! ∂v 2 v
3! ∂v 3 v

(5.3)

where v is the average level quantum number of the wavepacket. Equation 5.3 canalso be written in the form32,33:

E ( v) = E ( v) + 2π=( v − v)
( v − v) 2
( v − v) 3
± 2π=
± 2π=
+ ...
Tcl
Trev
Td

(5.4)

where the ± denotes the sign of the derivatives from Eq. 5.3. The value of Tcl is theclassical vibrational period, Trev is the wavepacket “revival” period, and Td is theperiod for second-order revivals.32-34 The higher order time scales in Eq. 5.4 can be
defined in a similar manner, but are not discussed here.
For a Morse oscillator, the series in Eqs. 5.3 and 5.4 are terminated after thequadratic term which describes first-order anharmonicity. The resulting wavepacket
dynamics displays two timescales: the linear term represents the classical period Tcl
and the quadratic term gives the revival period Trev.32-34 When higher order terms areincluded, additional timescales appear in the dynamics. Much as the classicaloscillations cease after a certain time, the periodic revivals should terminate as well

194
after a given propagation time as the higher order terms in Eq. 5.4 come into play.
This smearing of the revival structure is described by the cubic term in Eq. 5.4. The
value of Td then represents the period of second-order revivals.
The frequency scaling with energy for a Morse oscillator is given by Eq. 5.1.
In the case of the Morse potential, the wavepacket dynamics are described entirely interms of oscillations and periodic revivals due to the value of Td being infinite.34
However, for a general potential, the higher order terms “explode” near dissociationthreshold and smear out these expected revivals. For this revival picture to be valid,
the cubic and higher order terms in Eq. 5.4 must be neglected. The condition forwhich this is valid34 is:

1/Trev >>| v − v | /Td

(5.5)

or
E >> F 2=α |v − v|I
H 3n K

2n
n−2

(5.6)

where α is a constant, n is the parameter describing the long-range scaling of thepotential, and v and v are described in Eq. 5.3. As the system nears the dissociationthreshold, E approaches 0 and the condition of Eq. 5.6 breaks down. This leads to abreakdown of the revival picture of the wavepacket dynamics as one nears thedissociation threshold.

The condition must be valid for all the terms that havesignificant amplitude in Eq. 5.2, so the case of |v − v| ≈ N where N is the number ofvibrational levels in the wave packet must be considered. For potentials with n>2, thenumber of bound levels is finite and N scales as N ∝ ∆E/ω ∝ |dV/dR|/ω.

195
The potential gradient is taken at the initial position where the wavepacket isformed. In these experiments, the initial position of the wavepacket is on the innerwall of the IBr⎯ potential. Both the energy range (∆E) and the level density, which is
proportional to 1/ω, are increasing with Ev. This means that N is rapidly increasingnear the dissociation threshold. The simplest approximation involves the reflection
principle which entails projecting the ground state wavefunction of IBr2⎯ onto thequantum levels of the IBr⎯. A ballpark estimate of the spread ∆E is roughly 0.5 eV.
The long range potential for the near-threshold IBr⎯ is best approximated by
the ion-neutral interaction which scales as R-4. For this potential, the number ofbound levels between Ev = 0.8D0 and the dissociation level is N ≈ 102. The righthand side of Eq. 5.10 is found to be ∼0.08 eV when the other parameters are
estimated. Overall, this analysis shows that the revival picture of the wavepacket
dynamics is only valid when Ev << 0.92 eV, which is likely not the case in the BrICl⎯
and IBr2⎯ systems. The wavepackets in the experiments are formed too close to thedissociation threshold for the revival picture of the dynamics to be valid, thus therevivals are not observed in the time-dependent I⎯ signals.
The

IBr⎯

vibrational

excitation(Ev~ 0.9Do)

observed

in

IBr2⎯

photodissociation corresponds to the near-threshold regime where higher ordertimescales are important. On the other hand, in the Neumark experiment on gas
phase I3⎯,11,12,32,33 the vibrational excitation of I2⎯ (~0.7Do) corresponds to theintermediate regime where the smearing of the early wavepacket revivals only begins
to be important.
Time-dependent wavepacket simulations were carried out on two sets of196
calculated IBr⎯ potentials34 to determine the accuracy of the above analysis. The
results of the simulation are shown in Fig. 5.8 with parts (a) and (b) representing thetwo different levels of theory for the potential calculations. The difference in revival
times between the two levels of theory highlights the dependence of the revival times
on the details of the potentials. The signals show the expected wavepacket revival
structures32,33 along with partial revivals of various orders for moderate vibrational
excitation (Ev ≤ 0.6Do).

As the vibrational excitation is increased, the revival

structures become less intense (Ev ≈ 0.8Do) and are completely lost by Ev = 0.93Do.
This clearly agrees with the hypothesis that as the vibrational excitation in the
wavepacket approaches the dissociation limit, the higher order timescales become
more important and the wavepacket revival structures smear out. In the case of IBr⎯,the vibrational excitation is around 0.9Do. The simulations suggest that this close
proximity to the dissociation limit should smear the revivals to the point of being
unobservable. The work11,12 of the Neumark group on I3⎯ produced wavepackets in
I2⎯ with ~0.7Do vibrational excitation from which they observe a wavepacket revival
structure. According to the simulations, this intermediate regime should produce
observable revivals where the smearing is just becoming important.

As mentioned

above, the I2Br⎯ system could directly test the above hypothesis by forming the I2⎯
wavepacket closer to the dissociation limit and determining if the previously observed
revival structure from the I3⎯ work is still present.

197(a) MP2

Ev = 0.6D0

Ev = 0.8D0

Ev = 0.93D0

0

5

10

15

20

25

30

35

40

45

15

20

25

30

35

40

45

(b) QCISD

Ev = 0.55D0

Ev = 0.82D0

Ev =93D00

5

10Time (ps)Figure 5.8: IBr⎯ absorption signals calculated by propagating wavepackets on the (a) MP2 and (b) QCISD potentials calculated in thereference in the text. The degrees of vibrational excitation are labeled
above each set of absorption signals.

198
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2

U. Banin, R. Kosloff, and S. Ruhman, Chem. Phys. 183, 289 (1994).

3

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4

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5

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6

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7

E. Gershgoren, E. Gordon, and S. Ruhman, J. Chem. Phys. 106, 4806 (1997).

8

T. Kühne, R. Kuster, and P. Vöhringer, Chem. Phys. 233, 161 (1998).

9

T. Kühne and P. Vöhringer, J. Chem. Phys. 105, 10788 (1996).

10

T. Kühne and P. Vöhringer, J. Phys. Chem. A 102, 4177 (1998).

11

M. T. Zanni, B. J. Greenblatt, A. V. Davis, and D. M. Neumark, Proc. SPIE
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12

M. T. Zanni, B. J. Greenblatt, A. V. Davis, and D. M. Neumark, J. Chem.
Phys. 111, 2991 (1999).

13

M. A. Johnson and W. C. Lineberger, in Techniques for the Study of Ion
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York, 1988), pp. 591.

14

D. Klar, M. W. Ruf, and H. Hotop, Australian Journal of Physics 45, 263(1992).

15

D. Klar, M. W. Ruf, and H. Hotop, Chem. Phys. Lett. 189, 448 (1992).

16

J. P. Ziesel, I. Nenner, and G. J. Schulz, J. Chem. Phys. 63, 1943 (1975).

17

J. P. Ziesel, G. J. Schulz, and J. Milhaud, J. Chem. Phys. 62, 1936 (1975).

199
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M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
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O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.
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Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B.
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Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M.
W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. HeadGordon, E. S. Replogle, and J. A. Pople, Gaussian 98, Rev. A.1 (Gaussian,Inc., Pittsburgh, PA, 1998).

19

C. Adamo and V. Barone, J. Chem. Phys. 108, 664 (1998).

20

W. R. Wadt and P. J. Hay, J. Chem. Phys. 82, 284 (1985).

21

M. N. Glukhovtsev, A. Pross, and L. Radom, J. Am. Chem. Soc. 117, 2024
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22

B. J. Greenblatt, M. T. Zanni, and D. M. Neumark, Chem. Phys. Lett. 258,
523 (1996).

23

M. T. Zanni, T. R. Taylor, B. J. Greenblatt, B. Soep, and D. M. Neumark, J.
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24

K. Do, T. P. Klein, C. A. Pommerening, and L. S. Sunderlin, J. Am. Soc.
Mass Spectrom. 8, 688 (1997).

25

M. E. Nadal, P. D. Kleiber, and W. C. Lineberger, J. Chem. Phys. 105, 504(1996).

26

M. E. Nadal, Ph.D. Thesis, University of Colorado, 1996.

27

I. S. Averbukh, M. J. J. Vrakking, D. M. Villeneuve, and A. Stolow, Phys.
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28

E. Gershgoren and S. Ruhman, private communication.

29

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30

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33

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201

Chapter 6
Conclusion

The overall goal of this dissertation was to study the process of solvation at
the molecular level by the use of gas-phase ionic clusters. These ionic clusters
provide weakly bound solutes surrounded by a controllable number of solvent
molecules. This solvent control allowed for the step-wise study of the effect ofincreasing solvation on the solute. It also provides well-defined solvent environments
not found in condensed phase. The statistical nature to the observations in condensed
phase environments made solvent studies in condensed phase media incomplete at themolecular level of understanding. Initially, the photodissociation dynamics of sizeselected IBr⎯(CO2)n cluster ions are studied as a method to understand solvation at the
microscopic level. This provides numerous insights into the detailed nature of thesize-dependent effects of the solvent on the anion solute. However, the complete
picture of the time-resolved dynamics is inaccessible using this method.
Photoelectron imaging spectroscopy is then carried out on Cu⎯(H2O)n complexes toboth characterize the new imaging spectrometer and determine the effect of a few
H2O solvents on the electronic structure of the atomic anion solute.
The second chapter of the dissertation described in detail the existing cluster
ion source and tandem time-of-flight mass spectrometer. The modifications on theapparatus for the production and study of IBr⎯(CO2)n clusters were highlighted. Of
central importance in this chapter was the design and description of the newphotoelectron imaging spectrometer. The spectrometer was designed such that it

202
could be implemented into the existing apparatus with no loss of the originalfunctionality. The image collection scheme along with the processing of the image
data was described in some detail.
The photodissociation study of IBr⎯(CO2)n clusters built on previous work in
the Lineberger group on ICl⎯(CO2)n and I2⎯(CO2)n cluster ions and was the focus ofCh. 3. The IBr⎯ solute was attractive to study, because it provides a mixed halide
solute with absorption cross sections and size-dependent recombination behavior thatwas thought to be ideally suited for time-resolved solvation dynamics studies. The
spectroscopic properties of the bare IBr⎯ solute were characterized. The A' absorption
band was measured and found to provide substantial cross section in the near-IR
around 750 nm; ideal for the femtosecond laser system. The bond dissociation energy
was found to be comparable to that of the two previously studied dihalide solutes ataround 1.10 eV. The size selected IBr⎯(CO2)n clusters were excited to the A' and B
electronic states of the solute anion. Photodissociation on A' yielded the expected I⎯
photoproduct in the unsolvated case. As the solvation was increased, the formation of
the charge transfer Br⎯-based products occurred with recombination on the groundstate beginning soon after with the addition of the second CO2 solvent molecule.
Recombination on the ground state reached 100% at n = 8 representing the smaller Br
completely enclosed by the solvent molecules. The IBr⎯(CO2)n clusters exhibited
behavior similar to ICl⎯(CO2)n clusters in that recombination first occurred at verysmall cluster sizes, unlike the recombination behavior observed for I2⎯(CO2)n.
However, IBr⎯(CO2)n clusters did demonstrate increasing recombination withincreasing cluster size unlike that found in ICl⎯(CO2)n in which the charge-transfer

203Cl⎯-based products are formed exclusively at large cluster sizes with recombination
reaching zero.
The unexpected recombination behavior in ICl⎯(CO2)n clusters is due to thepresence of an excited state well corresponding to solvated Cl⎯ that increases withincreasing cluster size. A similar well is thought to be present on the likewise
asymmetric IBr⎯ potentials.

A pump-probe experiment was carried out on

IBr⎯(CO2)8, which exhibits 100% recombination, to determine the timescale forrecombination on the ground state after excitation on A'. No recombination on the
ground state was observed before 200 ps, with significant recombination occurring at
5 and 8 ns. This result is in stark contrast to results in I2⎯(CO2)n clusters after similar
excitations that show recombination occurring on the tens of picoseconds timescales.
This large difference is attributed to the presence of an excited state well similar tothat found in solvated ICl⎯.

The well is not as deep, however, allowing forrecombination to be observed on the nanosecond timescale. At large cluster sizes, thewell traps the ICl⎯ for at least 10 µs (the time for detection on the off-axis detector)
due to the absence of observed recombined products. The experiment is limited,
however, in that the time-resolved dynamics can only be probed in the narrowwindow when the solute chromophore is recombined on the ground state.
Photoelectron imaging spectroscopy is thought to provide a much more
powerful tool for probing the excited state dynamics at all times after photoexcitation.
The new imaging spectrometer was characterized by the study of Cu⎯(H2O)ncomplexes. The bare Cu⎯ solute was used to optimize the spectrometer as well as
provide a baseline to determine the effects of the addition of a few H2O solvent

204
molecules. The spectrometer was shown to provide very good energy resolution in
the spectra. A pulsed laser was used for the photodetachment and allowed for a wide
range of wavelength tunability. This tunability allowed the study of selected featuresin the spectra at low photoelectron kinetic energies, thus providing higher resolution.
The overall trend of increased solvation was to shift the unsolvated, atomic features tohigher electron binding energies. Substantial peak broadening was also observed andis understood in terms of the sizes of the atomic anion versus the atomic neutral
solute depending on the orbital from which the electron is detached. Some charge
delocalization onto the H2O solvent was observed by the presence of small features
just above the VDE of the complexes that were assigned to the H2O bending mode.
The Cu(H2O) complex ground state was shown to be surprisingly narrow and wasused to determine a dissociation energy of the complex.
Future work will center on the extension of the photoelectron imaging
spectrometer to more complex and larger solvent systems as well as to address
outstanding issues from previous work. The excited state dynamics of the IBr⎯(CO2)nmust be probed in more depth. The recombination time at this point is still quite
uncertain. It was found to be greater than 200 ps, which was itself a valuable
discovery, but the exact recombination time is unknown over a few orders ofmagnitude. Time-resolved imaging spectroscopy can potentially follow the dynamics
at all times from initial photodissociation through recombination on the ground state,including monitoring the motion of the solvent and the charge-transfer between the
dissociating atoms. Additional solvation was shown to produce shifts in the electron
binding energies in Cu⎯(H2O)n complexes. These shifts will also be present in the

205dihalide systems as the solvent rearranges to solvate a newly formed charge center,
for instance. The nature and location of the excited state well in IBr⎯(CO2)n can also
now be probed. Time-resolved dynamics can be carried out for the first time on
ICl⎯(CO2)n clusters, which was inaccessible due to the constraints of the absorption
recovery experiments. Another exciting prospect is the study of solute anions which
produce molecular dissociation photoproducts. Clusters involving solutes such as
ICN⎯ or BrCN⎯ are of great interest in that photoproducts based on CN⎯ can beproduced from which the vibrational state distributions may be able to be probed
using the imaging spectrometer.

Finally, the trihalide anions could provide aninteresting system on which to study solvation. Some clustering of the trihalides has
been observed in these systems, though not reported here. The wavepacket studies on
the dihalide anions formed from photodissociation of the precursor trihalides
demonstrated the critical importance of the location where the dihalide is formed on
the ground state potential in determining the subsequent wavepacket dynamics. Dueto the nature of the photodissociation of the trihalides studied, the dihalide products
were formed quite high on the well smearing out the expected revivals in the
wavepackets. The presence of a solvent could lead to vibrational excitation transferfrom the dihalide allowing for some degree of vibrational relaxation.

The

reappearance of the revival structure could then be probed as the degree of solvation
is increased leading to formation of the dihalide at lower vibrational energy levels.

206

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