Photofragmentation and Recombination Dynamics of Partially SolvatedAnionic Clusters

byJoshua P. Martin
B.S., Illinois State University, 2004

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This thesis entitled:
Photofragmentation and Recombination Dynamics of Partially Solvated Anionic Clusters
written by Joshua P. Martin
has been approved for the Department of Chemistry and Biochemistry by_______________________________________
W. Carl Lineberger_______________________________________
Robert Parson

Date: ________________

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

iii
Martin, Joshua P. (Ph.D. Physical Chemistry)
Photofragmentation and Recombination Dynamics of Partially Solvated Anionic Clusters
Thesis directed by Professor W. Carl Lineberger

Photofragmentation studies of mass-selected, partially-solvated anionic clusters are
performed to investigate solvent-number specific perturbations to the dissociation dynamics ofanionic chromophores. Two anion solutes are employed in the studies reported here: IBr− and thechemically-related ICN−. While the former has been the subject of earlier photodissociation
studies, ICN− has not been studied previously. Replacing Br with CN allows the possibilities ofisomerization to INC− and rotational excitation of a photofragment. Two quite different solvents,
CO2 and Ar, are utilized in the experiments. The CO2 solvent is bound to the chromophore by~200 meV. Its charge distribution gives rise to substantial electric quadrupole and higher
moments that can modify the electronic structure of the solute ion. The Ar solvent is much more
weakly bound to the solute (~50 meV), has no permanent multipole moments, and no possibilityof rotational, vibrational or electronic excitation in the dissociation process. The contrasting
photodissociation dynamics associated with the different combinations of these solutes andsolvents reported in this Dissertation both provides new understanding and raises questions
concerning photoprocesses in partially solvated ionic complexes.
Following 430 to 650 nm excitation of ICN− to its 2Π1/2 excited state, the predominant
ionic photoproduct is I−, with a minor CN− component. Photodissociation of ICN−(Ar)n, n = 1–5,
results in dominant I− and I−(Ar)n ionic photoproducts, the observation of single-solvent cage

iv
recombination for n = 1, and highly solvated I− and ICN− photoproducts. The experimental
results, electronic structure calculations, and quantum dynamics calculations together indicatethat efficient transfer of a significant amount (> 0.3 eV) of the photoexcitation energy into
rotation of the CN diatom occurs following dissociation of the chromophore.
Photofragmentation studies of ICN−(CO2)n, n = 1–18, following 400, 500, and 600 nm
excitation examine the influence of long-range solute-solvent interactions on the dissociation
dynamics. While the dominant fragment from ICN− excitation is I−, the addition of even one CO2
solvent results in dominant CN−-based photoproducts. Significant cage recombination products
are observed for n = 3, 4, and 7 following 600, 500, and 400 nm excitation. Solvent evaporation
experiments show that the average CO2 solvent binding energy to the ICN− chromophore is~200 meV, consistent with earlier experiments on diatomic solutes.

to my Grandpa and Grandma Kettelkamp and my Grandma Martin

ne sis sapiens apud temet; ipsum time Dominum et recede a malo
Proverbia 3:7

Primitive
Ax. 1
Ax. 2
Th. 1
Def. G
Ax. 3
Ax. 4
Def. Ess.
Th. 2Def. NE
Ax. 5
Th. 3

φ is positiveP(¬φ) ↔ P(φ)P(φ) ˄ □x[φ(x) → ψ(x)] → P(φ)P(φ) → í$xφ(x)G(x) ¨ "φ[P(φ) → φ(x)]P(G)P(φ) → áP(φ)φ Ess. x ↔ φ(x) ˄ "ψ[ψ(x) → □"y[φ(y) → ψ(y)]]G(x) → G Ess. X
NE¨"φ[φ Ess. x→á $xf(x)]P(NE)á$xG(x)– Kurt Gödel

vi

Acknowledgements
I want to thank Him who has given me strength, Christ Jesus my Lord. Without God’s
grace, mercy, and compassion I never would have able to learn so much from so many greatteachers.
To my advisor Prof. Carl Lineberger, I wish to thank you for all your guidance andsupport throughout the years. Even after I stood in front of hundreds of the world’s best physicalchemists and roasted Carl, he took it all in stride. Additionally, I want to thank Carl for all thehelp he gave me in the laboratory, helping me whenever he could. As a member of Carl’s group I
have been able to interact and work with abundantly talented theoreticians, such as Prof. RobertParson and Prof. Anne B. McCoy, and I am grateful.
The past and present members of the Lineberger research group have acted as teachers,
collaborators, and, most importantly, friends. I want to thank Prof. Jack Barbera and Dr.
Vladimir Dribinksi for introducing me to the instruments that would become my tools. Prof.
Joshua Darr provided me the leadership and knowledge to begin performing the experiments on
my own. Dr. Quanli Gu worked hard at continually asking me questions about the instrument
and experiment to keep me continually thinking and learning. Dr. Amanda Case helped me in thelab near the end of my experimental work and read more versions of my dissertation than I canever thank her for. Dr. Elisa Miller and Yu-ju Lu were always just beyond the lasers whenever I
needed them. Both of them provided me with feedback to ideas and helping hands, and were myallies in the fight to keep the Infinity running. Dr. Scott Wren and Dr. Kristen Lemke were theones past the wall that gave me sanctuary from experimental problems for the five minutes Ivii
would need to recuperate. Also, thank you to all the past and present members of the Ion Super
Group who always challenged and listened to me.
I want to thank all the brilliant professors and lecturers whom I have been a student ofsince I first came to CU. Thanks to Prof. Robert Parson, Prof. Casey Hynes, Prof. Niels
Damrauer, and Prof. Rex Skodje for teaching me physical chemistry with such knowledge andenthusiasm. Thanks to Prof. Veronica Bierbaum, Prof. Barney Ellison, and Prof. Dr. Mathias
Weber for all the amazing discussions and questions that came from the Ion Super Group
meetings. As a TA, I learned so much about learning, teaching, and chemistry and I am very
thankful to Prof. Robert Parson, Prof. Joel Eaves, Prof. Margaret Asirvatham, and Dr. Lynn
Gieger for showing me how to teach.
Never was I met with a situation in the lab in which the extraordinary staff of JILA was
not able to help. Krista Beck has kept the Lineberger group running efficiently and always knows
the answer. To the staff of the machine and electronics shops, thank you for all of your skillful
assistance. Each time I went to Tom Foote, James Fung-A-Fat, or Todd Asnicar with my simple
questions, they would treat the situation and me with respect and support, and always save me inthe end. Also, I want to thank all the JILA computing staff. Without the help of Peter Ruprecht
and J.R. Raith I would not have been able to run calculations, let alone do it using Linux.
To all my family and friends outside of JILA, I want to thank you for always distracting
me and keeping me sane. Thanks to my Grandpa Kettelkamp for pushing me to think simply andlogically about problems and not necessarily jump straight to the “textbook” methods. To both of
my Grandmas, thank you for always believing in me and never letting me forget that. To myparents, thank you for providing me with everything essential and not spoiling me with anything

viii
that wasn’t. You have always supported me in all my crazy ideas. To my parents-in-law, I could
not have hoped for a better addition to my family and want to thank you for all your support. To
my sister, thank you for continually pretending that I was the smart one, even though that’s
obviously not true.
To my wife. Without you I would have collapsed a long time ago. Thank you for always
being so understanding and patient when I had to spend long nights in the lab or office. Thankyou for putting up with me after I have been stuck in an office or lab by myself all day. Thankyou for always listening to me talk about work, even though you don’t always know what it isthat I am talking about. Thank you for always reminding me to stop rambling on whenever I get
carried away about something. Thank you for always believing till the very end that I could
finish. Thank you for helping me in whatever way you could, without hesitation. I love you.

ix

Table of Contents
Chapter I: Introduction .................................................................................................................... 1
1.1

Solvation Effects .............................................................................................................. 1

1.2

Photofragmentation Dynamics of Solvated Dihalides ..................................................... 3

1.3

Dissertation Overview ...................................................................................................... 9

Chapter II: Experimental Apparatus and Methods ....................................................................... 11
2.1 Overview ............................................................................................................................. 11
2.2 Cluster Ion Source and Cluster Formation.......................................................................... 12
2.3 Primary Time-of-Flight Mass Spectrometer ....................................................................... 16
2.4 Secondary Mass Spectrometer and Detector ...................................................................... 19
2.5 Laser Systems ..................................................................................................................... 21
2.5.1 Nanosecond Laser System ........................................................................................... 21
2.5.2 Femtosecond Laser System.......................................................................................... 23
2.6 Data Acquisition ................................................................................................................. 26
2.6.1 Experimental Timing Sequence ................................................................................... 26
2.6.2 Data Acquisition Electronics ....................................................................................... 29
2.6.3 Data Acquisition for Static Photofragmentation Experiments..................................... 29
2.6.3.1 Ionic Photoproduct Distributions .......................................................................... 29
2.6.3.2 Action Cross Section for Photodissociation of an Anionic Chromophore ........... 30

x2.6.4 Data Acquisition for Time-Resolved Experiments ...................................................... 32
2.6.4.1 Time-Resolved Absorption Recovery Measurements .......................................... 33
Chapter III: Photodissociation of ICN−(Ar)n ................................................................................ 35
3.1 Introduction ......................................................................................................................... 35
3.2 Experimental ....................................................................................................................... 37
3.3 Theoretical Methods ........................................................................................................... 38
3.4 Experimental Results: Photodissociation of ICN−(Ar)0–5 ................................................... 41
3.4.1 Relative Action Cross Section for Photodissociation of ICN−..................................... 41
3.4.2 Anionic Photoproduct Distributions ............................................................................ 42
3.4.3 Kinematics of Collinear ICN−(Ar) Dissociation .......................................................... 45
3.4.4 Quantum Dynamics ..................................................................................................... 48
3.5 Conclusions ......................................................................................................................... 54
Chapter IV: Photofragmentation Dynamics of ICN−(CO2)n ......................................................... 55
4.1 Introduction ......................................................................................................................... 55
4.2 Experimental ....................................................................................................................... 57
4.3 Photodissociation Action Spectra ....................................................................................... 58
4.4 Photofragmentation of ICN−(CO2)n .................................................................................... 63
4.4.1 Photofragmentation Data at 400, 500, and 600 nm ..................................................... 63
4.4.2 Efficiency of Production of CN−-based Photofragments ............................................. 75
4.4.3 Recombination as a Function of Excitation Energy .................................................... 80

xi
4.4.4 Solvent Evaporation ..................................................................................................... 85
4.5 Comparison of ICN−(CO2)n and ICN−(Ar)n Photofragmentation ....................................... 90
4.6 Recombination following photoexcitation of ICN−(CO2)n, I2−(CO2)n, and IBr−(CO2)n ..... 94
4.7 Summary and Conclusions ................................................................................................. 96
Chapter V: Recombination Dynamics of Photodissociated IBr−(CO2)n ....................................... 98
5.1 Introduction ......................................................................................................................... 98
5.2 Experimental ....................................................................................................................... 99
5.3 Theoretical Methods ......................................................................................................... 100
5.3.1 Electronic Structure ................................................................................................... 100
5.3.2 Molecular Dynamics .................................................................................................. 102
5.3.3 Minimum Energy Structures ...................................................................................... 103
5.3.4 Trajectory Calculations .............................................................................................. 106
5.4 Experimental Results ........................................................................................................ 107
5.4.1 Absorption Recovery Dynamics of IBr−(CO2)n, n = 11–14 ....................................... 107
5.4.2 Simulated Absorption Recovery Dynamics of IBr−(CO2)n, n = 5–16 ....................... 113
5.5 Discussion ......................................................................................................................... 118
5.6 Conclusions ....................................................................................................................... 120
Chapter VI: UV Photofragmentation of ICN−(Ar)n and Future Directions ................................ 122
6.1 Introduction ....................................................................................................................... 122
6.2 Experimental ..................................................................................................................... 125

xii
6.3 Preliminary Results for UV Photoexcitation of ICN−(Ar)n .............................................. 125
6.3.1 Photodissociation Action Spectra .............................................................................. 126
6.3.2 Photofragmentation Data at 300 nm .......................................................................... 128
6.4 Summary of Preliminary Results ...................................................................................... 133
6.5 Future Directions .............................................................................................................. 133
Bibliography ............................................................................................................................... 136

xiii

List of Tables
Table 3.1 Translational energy partitioning as a function of the assumed I–CN separation at thetime of collision of CN with Ar, as determined from a simple kinematic analysis. ..................... 47
Table 3.2 Energy decomposition for a variety of initial states, following excitation to the 2Π1/2
state. The various components of the energy are expectation values of the three terms in the
Hamiltonian................................................................................................................................... 52
Table 4.1 One-photon ionic photoproducts following 500 nm excitation of ICN−(CO2)n. .......... 65
Table 4.2 One-photon ionic photoproducts following 600 nm excitation of ICN−(CO2)n. .......... 69
Table 4.3 One-photon ionic photoproducts following 400 nm excitation of ICN−(CO2)n. .......... 74
Table 5.1 Summary of energetics from ab initio calculations (energies in eV)......................... 102
Table 5.2 Properties of minimal energy clusters of IBr−(CO2)n from 80 K trajectory ensembles..................................................................................................................................................... 105
Table 6.1 One-photon ionic photoproducts following 300 nm excitation of ICN−(Ar)n. .......... 130

xiv

List of Figures
Figure 1.1 Schematic of one-photon photoexcitation of a solvated chromophore to a repulsive
excited state followed by dissociation, charge transfer, or recombination. The solid arrow
represents photoexcitation via a photon with energy hν. ................................................................ 5
Figure 1.2 Schematic illustration of the dynamics of the dissociating cluster following
photoexcitation in a one-color, pump-probe time-resolved absorption recovery experiment. The
pump (hνpump) and the probe (hνprobe) laser pulses are indicted by the solid arrows and Δt
indicates the time delay between the pump and probe. The dashed arrow illustrates a possible
trajectory of a solvated-dissociating chromophore that results in recombination in the groundelectronic state well......................................................................................................................... 8
Figure 2.1 Schematic diagram of the ion source and time-of-flight mass spectrometer. ............ 12
Figure 2.2 Time-of-flight mass spectrum of ICN−(Ar)n clusters, n = 0–3. The unlabeled peaks
present include I(CN)2−, CN−(Ar)n clusters, and I−(Ar)n clusters, for example. ........................... 15
Figure 2.3 Schematic diagram of nanosecond laser system and optical path. ............................. 23
Figure 2.4 Schematic diagram of femtosecond laser system and optical path. ........................... 25
Figure 2.5 Schematic diagram describing the relative timing of the mass spectrometer andnanosecond laser system utilized for static experiments. All delays and pulse widths shown areapproximate................................................................................................................................... 27
Figure 2.6 Schematic diagram describing the relative timing of the mass spectrometer andfemtosecond laser system utilized for time-resolved experiments. All delays and pulse widths
shown are approximate. ................................................................................................................ 28
Figure 2.7 Schematic diagram of the data acquisition system utilized for time-resolved
experiments. .................................................................................................................................. 33
Figure 3.1 Schematic potential energy curves (based on Ref. 66) for the ICN− (2Σ+ in black and2
Π1/2 in purple) as a function of the I–CN center of mass distance. The black dashed line shows
the probability amplitude of the ground-state wave function for ICN−. The black arrow illustrates
photoexcitation and subsequent dissociation (3.1). ...................................................................... 36
Figure 3.2 Two-dimensional contour plots of the ground (2Σ+) and excited (2Π1/2) states of ICN−;
contour lines are shown every 0.05 eV. The X denotes the location of the shallow minimum of
the 2Π1/2 potential (Ref. 66)........................................................................................................... 39
Figure 3.3 Cross sections (σrel(λ)) for the production of I− (blue circles) and CN−(red triangles, x10) photoproducts following 430 to 650 nm photodissociation of ICN−. ........... 42

xv
Figure 3.4 Solvation breakdown for I−- and ICN−-based photoproducts following 2.5 eV
excitation of ICN−(Ar)n. The color of the bar corresponds to the number of Ar atoms in the ionicphotoproduct. Note the change in scale of the y-axis between the two plots. .............................. 44
Figure 3.5 Schematic diagram of the photodissociation of ICN−(Ar) and the subsequent collision
between CN and the solvent Ar atom. The bottom ICN−(Ar) cluster defines the atoms, thecenter-of-mass of ICN−, XCOM, and the distance between the iodine and the center-of-mass of the
CN diatom, RI–CN. The CN diatom is assumed to be a hard sphere with a radius of 1.16 Å, thecalculated bond length of CN. The arrows indicate the direction of the hard-sphere’s trajectory
and the time at which photodissociation and the collision occur is indicated on the left axis.
Additionally, the time range for the pre-collision (light blue shading) and post-collision (lightgray shading) energy values in Table 3.1 are shown on the right................................................. 48
Figure 3.6 Quantum dynamics calculation results for t = 0, 180 and 360 fs following promotionof the ICN− 2Σ+ rovibronic ground-state wave function (Jtot = 0) to the 2Π1/2 surface. The
probability amplitude (black contours) is shown on the 2Π1/2 potential energy surface (color) as afunction of the Jacobi coordinates, R and θ. ................................................................................. 50
Figure 4.1 Linear ICN− potential energy curves (based on Ref. 66) as a function of the distancefrom the iodine atom to the center-of-mass of the CN diatom. The ground-state wavefunction isshown as a black dashed line. ....................................................................................................... 56
Figure 4.2 Relative cross sections (σrel(λ)) for the production of I− and I−(CO2) photoproducts(b) following excitation of ICN−(CO2) to the 2Π1/2 excited state. The relative cross section for the
production of I− photoproducts (a) following excitation of ICN− to the 2Π1/2 excited state isshown as a reference. The error limits are the average percent error for each photoproduct acrossthe entire spectral range and are defined as follows: 10% for I− from photodissociation of ICN−,
and 10% and 20% for I− and I−(CO2) from photodissociation of ICN−(CO2), respectively. ........ 60
Figure 4.3 Relative cross sections (σrel(λ)) for the production of CN− and CN−(CO2)photoproducts (b) following excitation of ICN−(CO2) to the 2Π1/2 excited state. The relative crosssection for the production of CN− photoproducts (a) following excitation of ICN− to the 2Π1/2
excited state is shown as a reference. The error limits are the average percent error for eachphotoproduct across the entire spectral range and are defined as follows: 15% for CN− fromphotodissociation of ICN−, and 10% and 20% for CN− and CN−(CO2) from photodissociation of
ICN−(CO2), respectively. .............................................................................................................. 61
Figure 4.4 Ionic photoproduct distribution of ICN−(CO2)n upon one-photon 500 nm excitation to
the 2Π1/2 excited state. ................................................................................................................... 64
Figure 4.5 Ionic photoproduct distribution of ICN−(CO2)n upon one-photon 600 nm excitation to
the 2Π1/2 excited state. ................................................................................................................... 68
Figure 4.6 Ionic photoproduct distribution of ICN−(CO2)n upon one-photon 400 nm excitation to
the 2Π1/2 excited state. ................................................................................................................... 73

xvi
Figure 4.7 Linear ICN− potential energy curves (based on Ref. 66) for the X̃ 2Σ+, 2Π1/2, and 2Σ+states as a function of the distance from the iodine atom to the center-of-mass of the CN diatom.
The proposed nonadiabatic crossing is represented by two selected diabatic curves shown asdashed lines. The color of the text for the ground and excited state photoproducts correlates toadiabatic dissociation of the chromophore. .................................................................................. 77
Figure 4.8 The average number of CO2 lost from CN−-based photoproducts as a function ofparent cluster size following 500 nm excitation of ICN−(CO2)n (n = 0–10). The number of CO2
lost from Cl−-based products following 644 nm excitation of ICl−(CO2)n are also shown forcomparison (Ref. 33). The solid-black diagonal line corresponds to the evaporation of all solvent
CO2 molecules from the parent cluster and is shown as a guide. ................................................. 80
Figure 4.9 Yield of recombined ICN−-based photoproducts as a function of parent cluster size
following 400, 500, and 600 nm excitation. ................................................................................. 81
Figure 4.10 Schematic of linear ICN− ground state (solid black) and 2Π1/2 excited state (solid
purple) as a function of the distance from the iodine atom to the center-of-mass of the CN
diatom. The proposed well induced by solvation of CN− is shown as a dashed line. The structuresof the ICN−(CO2)2 and I–CN−(CO2)2 complexes are based off of previously calculated (Ref. 39)
minimum energy structures of IBr−(CO2)n. .................................................................................. 83
Figure 4.11 Optimized geometries of (a) the NCCO2− molecule and (b) thesolvent-separated-like complex. The structure of both complexes were calculated at theB3LYP/aug-cc-PVTZ level of theory, with the iodine electron core potential of Thompson(Ref. 102) utilized for (b). ............................................................................................................. 85
Figure 4.12 The average number of CO2 lost from CN−-based photoproducts as a function ofparent cluster size following 500 and 600 nm excitation of ICN−(CO2)n. The number of CO2 lostfrom Cl−-based products following 644 nm excitation of ICl−(CO2)n are also shown forcomparison (Ref. 33). The solid-black diagonal line corresponds to the evaporation of all CO2
solvent molecules from the parent cluster and is shown as a guide.............................................. 86
Figure 4.13 The average number of CO2 lost from ICN−-based photoproducts as a function ofparent cluster size following 500 and 600 nm excitation of ICN−(CO2)n. The solid black
horizontal lines show the average number of CO2 lost for the largest clusters studied. The solid
black diagonal line corresponds to the evaporation of all CO2 solvent molecules from the parent
cluster and is shown as a guide. .................................................................................................... 90
Figure 4.14 Ionic photoproduct distribution of ICN−(CO2)n and ICN−(Ar)n (Chapter III)
following one-photon 500 nm excitation to the 2Π1/2 excited state. ............................................. 91
Figure 4.15 The average number of solvent lost from I−-based and ICN−-based photoproducts asa function of parent cluster size, n, following 500 nm excitation of ICN−(CO2)n and ICN−(Ar)n.
See Chapter III for ICN−(Ar)n photofragmentation results. The solid black line corresponds to theevaporation of all solvent from the parent cluster and is shown as a guide.................................. 93

xvii
Figure 4.16 Yield of recombined photoproducts following excitation of ICN−(CO2)n, andI2−(CO2)n, (Ref. 87) and IBr−(CO2)n (Ref. 51) to the 2Π1/2 excited state. The excitation
wavelengths utilized result in roughly 0.4 eV of kinetic energy release for each solute. The
dashed gray line represents 100% recombined photoproducts. .................................................... 95
Figure 5.1 Potential energy curves for the six lowest spin-orbit states of IBr−. ........................ 101
Figure 5.2 Exemplar calculated minimum energy structures for IBr−(CO2)n, n = 1–16. The
pattern of CO2 filling is first around the bond, next the Br end beginning at n = 3, and then theiodine end beginning at n = 9...................................................................................................... 104
Figure 5.3 Average ΔΦ for 100 configurations of IBr−(CO2)0-16 in the ground state at 60 K. Error
bars represent one standard deviation of the mean. .................................................................... 106
Figure 5.4 Transient illustrating the absorption recovery of IBr−(CO2)11. The circles represent
the experimental data, and the dotted line is the result of a single exponential fit. The black line isused to guide the eye. .................................................................................................................. 109
Figure 5.5 Absorption recovery transient for IBr−(CO2)13, dashed line and squares, andIBr−(CO2)14, solid line and circles. The lines are shown to guide the eye. ................................. 111
Figure 5.6 Schematic diagram illustrating the origin of the overshoot of the asymptote for theabsorption recovery of IBr−(CO2)13,14. The ground state potential is a Morse potential obtainedusing the experimentally determined parameters for the equilibrium bond length and vibrationalfrequency (Refs. 69, 131). The excited state potential is also a model Morse potential with thewell depth chosen to match the experimentally determined value (Ref. 135) and the equilibrium
bond length adjusted to illustrate the origin of the overshoot. The arrow labeled hνovershootrepresents the transition giving rise to the overshoot of the asymptote, and the arrow labeledhνasym represents the transition giving rise to the asymptotic signal. Note the break in the energy
scale along the y-axis. ................................................................................................................. 113
Figure 5.7 Ground-state recombination dynamics for IBr−(CO2)8. The stepped line represents
theoretical results, τMD, and the dots represent experimental data, τexpt. The dotted anddash-dot-dash lines represent single-exponential fits to the experimental and simulated data,
respectively, see Eq. (5.1). .......................................................................................................... 115
Figure 5.8 Comparison of experimental and theoretical ground-state recombination times. The
red solid line and diamonds represent the experimental data, and the blue dashed line andtriangles represent the results of the theoretical calculations. The lines connecting the data points
correspond to the fast component of the absorption recovery in cases where the recovery was
best fit to a bi-exponential function. The data points not connected by a line correspond to theslow component of the absorption recovery. .............................................................................. 116

xviiiFigure 5.9 Visual representation of the solvent coordinate, ΔΦ, using IBr−(CO2)8 clusters. ΔΦ isdefined by the change in energy when the charge is transferred from the bromine atom to theiodine atom as shown on the left and right sides of the figure, respectively. A symmetric solvent
configuration, top, corresponds to a small ΔΦ. An asymmetric solvent configuration, bottom,
corresponds to large values of ΔΦ. ............................................................................................. 117
Figure 6.1 Linear ICN− potential energy curves (based on Ref. 66) as a function of the distance
between the iodine atom and the center-of-mass of the CN diatom. The ground state
wavefunction is shown as a black dashed line. The diabatic curves proposed to take part in the
dissociation dynamics of ICN− are shown for the 2Π1/2 (dashed purple line) and 2Σ+ (dashed blueline) excited states. ...................................................................................................................... 123
Figure 6.2 Normalized relative cross section (σrel(λ)) for the production of I− and CN− following
270 to 330 nm excitation of the ICN− chromophore. The normalized relative cross section for the
production of I− and CN− following 430 to 650 nm excitation of ICN− is shown for reference. All
values are normalized to σrel(λ) for the production of I− products at 300 nm. The error limits are
the average percent error for each photoproduct and are defined as follows: 15% for CN− and
10% for I− following 430–650 nm excitation, and 12% and 5% for CN− and I− following
270–330 nm excitation, respectively. Note the break in the x-axis. ........................................... 127
Figure 6.3 Ionic photoproduct distribution of ICN−(Ar)n following one-photon 300 nm
excitation. The data points are the bold red values in Table 6.1. ................................................ 129

1

Chapter I: Introduction

1.1 Solvation Effects
The vast majority of important chemical reactive processes take place in a condensed
phase of matter. Such multi-particle environments make it extremely difficult to characterize thetruly essential components of the process to obtain a molecular view of the reaction. The identityof these components cannot be uniquely specified in the abstract, but must be determined for
different cases. In general, such components of a condensed phase reaction would include thereactants, possible catalysts, and some key solvent molecules. Indeed, to obtain a molecular level
understanding of many chemical processes, the effects of solvation on the course of a reaction
must be known. The influence that a solvent has on a reaction can be simple, such as creating aphysical barrier to the motion of reactive species or acting as an energy sink stabilizing reaction
products. The solvent’s influence can be more complex, such as perturbing the potential energy
surfaces of the reaction, or creating the opportunity for solvation-induced dynamics not observedfor isolated reactants. The focus of this dissertation is the study of step-wise, solvation-induced
dynamics following photodissociation of an anionic chromophore within a partially-solvated,
size-selected anion-solvent complex.
The effect of solvation on photodissociation dynamics was first observed in the 1930’s by
Franck, Rabinowitch, and Wood.1-3 They observed “caging” of photodissociated I2 in polar
solvents and concluded that collisions between the separating iodine atoms and the solvent(acting as a barrier) led to an increased probability that the dissociated iodine atoms would
recombine to form covalently bound I2. This process is known as geminate recombination.

2
Since this initial observation of the cage effect, a multitude of studies have further
investigated this phenomenon in condensed-phase media.4-10 However, the variable local
environment of the solute in the condensed phase prevents elucidation of number- andconfiguration-specific solvation effects on photodissociation dynamics. Thus, only bulk averaged
perturbations of the solute are detected.11-14
Utilization of mass-selected, partially-solvated solute-solvent complexes provides awell-defined solvent environment about the solute and allows the study of molecular level detailsof solvation effects without averaging over bulk conditions. Partially-solvated solute-solvent
complexes are aggregates composed of one chromophore that weakly interacts with, but is notcovalently bound to, solvent atoms or molecules. A primary motivation for the study ofpartially-solvated chromophores is to investigate the properties and dynamics of matter as it
evolves from the gas to condensed phase as the number of solvent particles increases.11, 15-18
These studies work to provide answers to questions such as, how many solvent particles are
required in a cluster to observe bulk-like properties? For example, Leutwyler and coworkers19, 20
utilized two-color resonant two-photon ionization and fluorescence emission spectroscopy ofnaphthol(NH3)n clusters to study proton transfer from naphthol to NH3. They reported that protontransfer was only observed when four or more solvent NH3 molecules interact with naphthol
inducing bulk-like processes.21 Scoles and co-workers22, 23 carried out infrared spectroscopic
studies of Arn·SF6 and reported that solvation with approximately twelve argon atoms induced ashift in the vibrational spectrum that was comparable to values observed in matrices.
McIlroy et al.24 utilized a slit jet, difference frequency laser spectrometer to obtainhigh-resolution infrared spectra of ArnHF (n = 1–4). Their results show a clear red-shift in the

3gas-phase infrared spectra as n increases from n = 1–4, indicating a weakening of the H–F bondby the interaction with “nearest neighbor” solvent Ar atoms.
1.2 Photofragmentation Dynamics of Solvated Dihalides
Mass selection of partially-solvated solute-solvent complexes is readily accomplished byutilization of ionic chromophores and standard mass spectroscopic techniques, such as tandem
time-of-flight25 and Fourier transform ion cyclotron resonance.26 Within this dissertation, allwork presented utilizes partially-solvated dihalide or pseudo-dihalide chromophores to study
step-by-step solvation induced photodissociation dynamics; these anion-solvent complexes arehenceforth referred to as “anionic clusters.” The interactions between the anionic solute andneutral solvent are primarily electrostatic, resulting in binding energies of the solvent to theanionic chromophore that are larger than those observed for the corresponding solvated neutral
solute.27, 28 The properties of anionic clusters, such as ionization potential, electron affinity,
chemical reactivity, and optical spectra are significantly perturbed by stepwise solvation when
the cluster size ranges from one solvent particle to tens of particles.29, 30 Following production
and mass-selection of the desired anionic cluster, the photofragmentation dynamics of the cluster
are examined by employing optical spectroscopic techniques. Due to the low particle densities
generated by ion beam experiments,25, 31 absorption spectroscopy is not feasible and, therefore,
photofragmentation action spectroscopy is employed. In such experiments, a mass-selectedparent anion is promoted, via a photon, from its bound ground state to a repulsive excited state.
This results in direct, adiabatic dissociation of the parent anion and the formation ofphotoproducts (neutral and ionic). The production of ionic photoproducts is then monitored,

4following secondary mass analysis, in order to elucidate the excitation energy and cluster size
dependence of any observed solvation-induced photofragmentation dynamics.
Figure 1.1 illustrates the dissociation dynamics of a solvated chromophore following onephoton photoexcitation to a repulsive excited state. The dashed-blue arrow in Fig. 1.1
corresponds to the direct dissociation of the solvated dihalide. Addition of one solvent particle tothe bare chromophore leads to an increased probability of nonadiabatic transitions (dash-blackline in Fig. 1.1) from the excited state to the ground state at large internuclear distances. A key
difference between neutral and anionic cluster photofragmentation dynamics is the
charge-transfer (dashed-red arrow in Fig. 1.1) process that may occur following this nonadiabatic
transition. This process is solvent-induced and strongly coupled to the motion of the solvent
during dissociation of the chromophore.

5

Figure 1.1 Schematic of one-photon photoexcitation of a solvated chromophore to a repulsive
excited state followed by dissociation, charge transfer, or recombination. The solid arrow
represents photoexcitation via a photon with energy hν.

With one or two solvent molecules, geminate recombination of the dissociating
chromophore can sometimes be observed32-35 (dashed-green arrow in Fig. 1.1). In the type ofgeminate recombination depicted here, the dissociating dihalide must undergo a nonadiabatic
transition to the ground state, possibly with extensive vibrational excitation. Collisions with thesolvent and solvent evaporation may further relax the recombined chromophore. The observationof > 80% recombination for partially-solvated dihalide–solvent complexes32-36 is especiallystriking in light of the low yield of recombined products observed in solution.5, 9 This gas-phase
recombination process is solvent induced, similar to the “caging” of photodissociated I2 insolution reported by Franck, Rabinowitch, and Wood.1-3 However, recombination of a gas-phase,partially-solvated chromophore involves fewer solvent particles than in the condensed phase and6is the result of perturbations of the electronic structure of the chromophore from solvation
leading to the nonadiabatic transitions. Additionally, the dihalide chromophore is stabilized bydissipation of excess energy from photoexcitation to the solvent particles. This leads to solvent
evaporation, a process that does not occur in condensed-phase media.
Previous one-photon photofragmentation studies33-35 of I2−(CO2)n, IBr−(CO2)n, andICl−(CO2)n clusters following excitation to the repulsive Ã′ 2Π1/2 excited state revealed the effectof solvation on the dissociation dynamics of solvated dihalides. Of particular interest is theefficiency of solvent CO2 molecules to induce recombination of the three different
chromophores. Following photoexcitation to the Ã′ 2Π1/2 excited state, the yield of recombined
photoproducts observed increases as n increases; the maximum yield reached following
excitation of ICl−(CO2)n clusters is 80%, whereas the yields reached 100% recombined products
for I2−(CO2)n and IBr−(CO2)n. Additionally, the yield of recombined photoproducts forIBr−(CO2)n and ICl−(CO2)n increases rapidly from zero to > 80% at small values of n. This is instark contrast to the gradual increase observed as n increases from two to sixteen for I2−(CO2)nclusters. The distinct similarities and sharp differences in the yield of recombined products
following excitation of these clusters are a result of not only the difference in size of thechromophore, but also the location at which solvation occurs around a homonuclear versus
heteronuclear dihalide. Within this Thesis, photofragmentation studies of ICN−(Ar)n andICN−(CO2)n clusters provide further understanding of the effects of solvation on
photodissociation dynamics.
Although photofragmentation studies provide information on the effects of solvation,these experiments only investigate the results of dissociation dynamics roughly 10 μs after

7
photoexcitation. In order to investigate the photodissociation dynamics on the timescale of thenuclear motion, time-resolved probe-pump spectroscopic techniques are utilized. Figure 1.2
illustrates the photodissociation dynamics of a solvated chromophore and the one-color,
pump-probe scheme employed in time-resolved absorption recovery measurements. Following
excitation of the cluster to a repulsive excited state, the dissociating chromophore can undergo anonadiabatic transition to the ground state, and vibrationally relax by evaporating some number,m, of solvent atoms or molecules. When sufficient vibrational relaxation has occurred in therecombined chromophore, it will regain significant photoabsorption at the excitation wavelength,
and the probe laser will again be able to excite the recombined chromophore. Two-photon ionicphotoproducts are then detected as a function of the time delay (Δt in Fig. 1.2) between the pumpand the probe pulses. In order to recognize the two-photon signal of recombination andvibrational relaxation, at least one of the two-photon photoproducts must have a mass different
from that of any one-photon product.

8

Figure 1.2 Schematic illustration of the dynamics of the dissociating cluster following
photoexcitation in a one-color, pump-probe time-resolved absorption recovery experiment. The
pump (hνpump) and the probe (hνprobe) laser pulses are indicted by the solid arrows and Δt
indicates the time delay between the pump and probe. The dashed arrow illustrates a possible
trajectory of a solvated-dissociating chromophore that results in recombination in the groundelectronic state well.

An ongoing collaboration between experimental and theoretical studies to examine theeffects of solvation on the timescale of absorption recovery has revealed the drastic changes in
the photodissociation dynamics that accompanies changes to the solute composition.
Papanikolas et al.34, 37 studied the time-resolved absorption recovery of I2−(CO2)n following
excitation to the Ã′ 2Π1/2 excited state. Their results showed a decrease in the recombination timefrom ~30 ps to ~10 ps, as n increased from nine to sixteen. However, Dribinski et al.38 reported
an increase in the recombination time of IBr−(CO2)n from tens of picoseconds to roughly 1 ns as
n increased from five to ten. This dramatic difference in the recombination dynamics gave

9
important clues concerning the influence of the location of the solvent molecules during theprocesses of solute recombination and relaxation. In this case, the important effect related to thesize difference between Br and I, as well as the strong electric multipole moments (quadrupoleand higher) of CO2. Because the ionic radius of bromine is smaller than that of iodine, solvation
begins to occur on the Br end of the IBr− chromophore following the addition of the third CO2
molecule.39 This asymmetric solvation produces a strong electric field at the location of thechromophore, inducing a shallow well on the IBr− Ã′ 2Π1/2 excited state; the well depth increases
as the asymmetry increases. The maximum depth of this well occurs with a half-filled solvent
shell; this maximum depth is sufficient to trap IBr− in the Ã′ 2Π1/2 excited state, and extend therecombination time to the radiative lifetime of the perturbed IBr− Ã′ 2Π1/2 excited state. One thus
might expect increasing recombination times as the number of CO2 solvent molecules isincreased up to the point of maximum solvent asymmetry (roughly a half filled solvent shell).38
Electronic structure calculations and molecular dynamics simulations show that further solvation(n > 10) leads to more symmetric solvation of IBr− and less trapping on the Ã′ excited state.38
Thus, the timescale for absorption recovery might be expected to decrease as the extent ofsolvation is increased beyond a half-filled solvent shell. Performing this test is accordingly oneof the objectives of this Thesis.
1.3 Dissertation Overview
The remainder of this Thesis comprises five chapters. Chapter II details the experimental
apparatus and methods utilized to study the photofragmentation dynamics of partially-solvatedanion-solvent complexes. The photofragmentation dynamics40 of ICN−(Ar)n, n = 0–5, clusters
following 430–650 nm excitation to the 2Π1/2 excited state of the ICN− chromophore and the10subsequent kinematics are discussed in Chapter III. Experimental and theoretical results40
provide evidence for the distribution of a significant amount of energy into the rotation of the CN
diatom following dissociation of ICN−. The results of photofragmentation of ICN−(CO2)n clusters
following 400, 500, and 600 nm excitation are presented in Chapter IV and elucidate the effectsof long-range solute-solvent interactions. Time-resolved absorption recovery studies39 of
dissociating IBr−(CO2)n, n = 11–14, following 790 nm pump-probe excitation to the Ã′ 2Π1/2
excited state are presented in Chapter V. Finally, Chapter VI presents preliminary results of thephotofragmentation of ICN−(Ar)n, n = 0–5, following 270–330 nm excitation and provides ideas
for future experiments that build on the work presented in Chapters III–V.

11

Chapter II: Experimental Apparatus and Methods

2.1 Overview
The experimental apparatus and methods utilized to perform photodissociation studies ofanionic clusters are described in this chapter. Each experiment is performed by interrogating
specific anionic cluster sizes and compositions with laser radiation and detecting thephotoproducts of the ion-laser interaction. The experimental apparatus is discussed as four
sections that perform the following functions: production of anionic clusters, mass separationand selection of cluster size, photoexcitation of the anionic chromophore, and secondary mass
selection and detection of photoproducts. Figure 2.1 is a schematic diagram of the experimental
apparatus. The production of anionic clusters is achieved through electron-impact ionization andattachment of slow-secondary electrons to a chromophore within a pulsed gas expansion. The
anionic clusters are extracted into a Wiley-McLaren time-of-flight mass spectrometer41 where
they are mass separated. Laser radiation intersects the mass-selected clusters, resulting inphotoproducts. Finally, the ionic photoproducts are separated by a secondary reflectron mass
spectrometer and are detected by a microchannel plate detector. The nanosecond andfemtosecond laser systems utilized to investigate the photofragmentation and time-resolved
recombination of solvated anionic chromophores, respectively, are also discussed in further
detail.

12

Figure 2.1 Schematic diagram of the ion source and time-of-flight mass spectrometer.

2.2 Cluster Ion Source and Cluster Formation
The two anionic chromophores studied, cyanogen iodide (ICN−) and iodine
monobromide (IBr−), have neutral counterparts that are stable solids in ambient laboratory
conditions. The IBr crystals are purchased from Aldrich (98% purity, CAS# 7789-33-5);
however, due to a lack of commercial availability of ICN, the ICN sample is synthesized by theexperimental procedure reported by Bak and Hillebert.42 The neutral ICN and IBr, havesufficient vapor pressures at room temperature, ~0.9 Torr (Ref. 43) and ~5 Torr,44, 45
respectively, that neither the container holding the solid sample nor the pulsed valve need to beheated. Additionally, by replacing all stainless steel gas supply lines with Teflon lines, the timerequired to passivate the supply line when performing experiments with IBr− is reduced from a
week35 to ~1 day.46
The production of anionic clusters begins by passing 1–2 atm of neat CO2 (or neat Ar in
the case of ICN−(Ar)n clusters) through a stainless steel reservoir containing the solid sample at13room temperature. The carrier gas, which mixes with the vapor from the solid sample, is pulsed
into the ion source chamber using a Series 9 General Valve. For these experiments, the General
Valve is equipped with an 800 μm orifice conical nozzle, a Kel-F poppet (Parker Hannifin), a
Viton O-ring (Parker Hannifin), and a Teflon-coated armature (Parker Hannifin). The solenoid of
the General Valve is driven by a single-channel Iota One (Parker Hannifin) pulse driver that istriggered by a TTL pulse from a digital delay generator (Stanford Research Systems, model
DG535). For the photofragmentation studies, the valve is operated at 50 Hz, and for thetime-resolved studies the repetition rate of the valve is set to 100 Hz. The source chamber ispumped by a 10” diffusion pump (National Research Corporation) that is backed by a Welchmechanical pump. Without a gas load, the nominal pressure of the source chamber is roughly
1x10−7 Torr. The running pressure in the source chamber reaches roughly 1x10−5 to 1x10−4 Torr,
depending on the optimal conditions for anionic cluster production.
During the expansion, the gas mixture intersects a beam of electrons (~1 keV, ~200 μA)
from a homebuilt electron gun. The electron gun consists of a bent ribbon filament(thoriated iridium, Electron Technology, Inc.) that is floated at −1000 V and generates electrons
via thermionic emission. The tip of the filament is positioned just above the knife-edge of anelectrode held at −200 V with respect to the potential on the filament. The electrons repelled by
the anode are then focused by an einzel lens (−750 to −1000 V) and steered vertically andhorizontally by two pairs of deflectors to optimize ion production. Additionally, a ring magnet isplaced on the faceplate of the pulsed valve, around the nozzle, to direct electrons toward thedensest portion of the gas expansion.

14
Low-energy secondary electrons produced via ionization of the gas mixture can attach tomolecules, forming anions and an overall neutral plasma. Coulomb screening and ambipolar
diffusion reduce the diffusive spread and allow for a high density of ions within the plasma. Theinternal energy of the anionic clusters produced is estimated to be ~40–70 K based on previous
studies.36
Formation of anionic clusters (Eq. 2.1) is dependent on the reaction time and the gas
density in the expansion as it involves a termolecular attachment of the electrons47 and is
described as
X + e−(secondary) + M → X− + M,

(2.1a)

X− + S + M → X−(S) + M,

(2.1b)

X−(S)n + S + M → X−(S)n+1 + M,

(2.1c)

where X− is a general chromophore, S is the neutral solvent, and M is a third body collisionpartner (X or S). Following the formation of an anion (Eq. 2.1a), neutral solvent atoms or
molecules condense around the charge center, X−, and are stabilized by collisions with M,
Eq. 2.1b. This process continues as X−(S)n collides with more S and M to produce cluster anions
of various sizes. For example, Figure 2.2 shows a typical mass spectrum of ICN−(Ar)n clusters,
n = 0–3, formed through this process. Additionally, evaporation of monomer solvent atoms or
molecules and collisions between the X−(S)n with neutral M result in the dissipation of excessenergy. Because this mechanism requires a number of collisions for cluster growth, long drift
distances (long reaction times) and high density expansions are favorable. In the Langevinapproximation,48 the collision rate of an ion with a polarizable neutral is independent of

15
temperature, while the neutral-neutral collision rate is proportional to the square root of
temperature, and, therefore, falls off rapidly with decreasing temperature.25 Thus, at the lowtemperatures of the gas expansion neutral-neutral collisions are negligible, but a large number ofcollisions between ions and neutrals occur. This provides an ideal environment for anioniccluster growth.

Intensity

ICN

ICN  (Ar)

140

160

180

200

ICN (Ar)2

220

240

ICN(Ar)3

260

280

Mass (amu)

Figure 2.2 Time-of-flight mass spectrum of ICN−(Ar)n clusters, n = 0–3. The unlabeled peaks
present include I(CN)2−, CN−(Ar)n clusters, and I−(Ar)n clusters, for example.

16
2.3 Primary Time-of-Flight Mass Spectrometer
A full discussion of the time-of-flight mass spectrometer (TOF MS) has been presented
elsewhere;34 therefore, only relevant details will be discussed here. Following formation in theneutral plasma, the ions drift for roughly 15 cm, after which the anionic clusters are extracted bya transverse, pulsed-electric field into a differentially-pumped Wiley-McLaren TOF MS.41 In
general, a Wiley-McLaren TOF MS achieves mass separation of ions by accelerating themthrough two acceleration regions into a third field-free region. The two accelerating fields areadjusted to obtain a spatial focus at the ion-laser intersection (Wiley-McLaren condition).
Additionally, the double acceleration corrects for the inherent energy spread in the extracted ion
beam due to the initial position of the anions within the extraction region.
The pulsed-extraction of the anions takes place in the primary acceleration stage and therising-edge of the square pulse defines t = 0 in the Wiley-McLaren TOF MS. To produce theextraction pulse, a TTL pulse from a delay generator (Stanford Research Systems, model
DG535) triggers a homebuilt pulse generator, to which high voltage is supplied from an externalsource. The extraction pulse is roughly1 ms in duration and a 10 ns rise time; the long durationand short rise time are required to obtain spatial focusing of the ion beam because all anions
must experience the same uniform electric field while in the extraction region. Typically, boththe negative voltage applied to the extraction plate (−1000 to −1500 V from earth ground) andthe timing of the extraction pulse are varied to achieve the Wiley-McLaren condition.
The extracted anions have roughly 1 keV kinetic energy and enter the second acceleration
region of the TOF MS by passing through a 4 mm aperture. A stack of ten isolated andresistively coupled stainless-steel guard rings form an acceleration stack that accelerates the17anions to a total energy of ~3.5 keV. This is accomplished by applying a DC voltage of 2.5 kV to
the last ring of the stack such that the anions are given an additional 2.5 keV of kinetic energy.
From the end of the acceleration stack it is roughly 1.5 m to the spatial focus (SF1 in Fig. 2.1) of
the TOF MS.
Directly after exiting the acceleration stack, the anions are steered by a pair of verticaldeflectors (floated roughly 0–20 V above the beam potential) before entering an einzel lens
operating in decelerating mode. The einzel lens consists of three stainless steel cylinders; the first
and the last cylinders are held at the beam potential and the middle cylinder is floated at roughly
900 to 1100 V above the beam potential; this creates a focusing lens with the electric field. After
exiting the einzel lens the anions encounter a second pair of deflectors that steer themhorizontally and are floated roughly 0–40 V above the beam potential. A 1.5° horizontal bend in
the mass spectrometer, located after the horizontal deflectors, separates stray neutral atoms or
molecules formed during collision-induced dissociation or metastable decay of the anions in theextraction and acceleration regions from the ion beam.
To avoid floating the detection region of the mass spectrometer at 2.5 kV (the ion beam
potential), a potential switch49 is utilized to re-reference the potential of the anions to earth
ground. The potential switch is composed of a 40 cm long cylindrical wire mesh with a 2.5 cm
diameter aperture at each end. As the desired anions reach the center of the potential switch, apulser (Directed Energy, Inc., model PVX-4140) switches (Δt ≤ 25 ns) the potential on the meshto earth ground, re-referencing the anions to earth ground and providing a mass-to-chargetransmission window corresponding to the flight time in the potential switch cylinder. For mostof the experiments reported here, this time corresponds to a mass range of Δm ~ 130 amu.

18
A pulsed mass gate is positioned 4 cm before the anions reach the Wiley-McLaren spatial
focus (SF1 in Fig. 2.1). The purpose of the mass gate is to transmit only anionic clusters of a
selected mass-to-charge ratio and reduce background noise by vertically deflecting unwanted
ions from the beam path. The mass gate consists of three pairs of horizontal, parallel stainlesssteel plates enclosed in a stainless steel housing with a 4 mm diameter entrance aperture and5 mm diameter exit aperture. The lower plate in each pair is at earth ground, while the upper
plates are pulsed between −200 V and earth ground. When −200 V is supplied to the upper
plates, all anions entering the mass gate housing are deflected. When the voltage is pulsed off,
the top plates are at earth ground and the desired anions are allowed to pass through the massgate undeflected. To switch the upper plates of the mass gate between the two potentials, a pulser(Directed Energy, Inc., model GRX, referred to as the DEI box) is supplied with two input
voltages (−200 V and earth ground) and a trigger pulse. The trigger pulse is generated by ahomebuilt, NIM-rack mounted, mass-gate driver that is supplied two TTL trigger pulses from a
digital delay generator (Stanford Research Systems, model DG535). The first TTL pulse triggers
the DEI box to switch the potential on the mass gate to −200 V, deflecting all the anions. The
second pulse from the driver triggers the DEI box to switch the potential on the mass gate toearth ground. This second pulse, supplied by the mass gate driver, has an adjustable temporal
width typically set between ~200 and 500 ns. Following the second pulse, the DEI box reapplies−200 V to the mass gate deflecting the anions. Successful implementation of the pulsed massgate permits unambiguous selection of a single cluster anion size.
After traveling roughly 4 cm from the mass gate, the anions are intersected by pulsed
laser radiation at the Wiley-McLaren spatial focus (SF1 in Fig. 2.1). Optimization of thetemporal overlap of the anions and laser radiation is accomplished by adjustment of the timing

19
delay between the extraction pulse and laser pulse, while monitoring the intensity of an ionicphotoproduct (see Sec. 2.6.1). At the spatial focus, the ion packet has been measured to have aroughly 4 mm waist and is predicted to be roughly 4 mm in length along the beam axis.
Additionally, the ion packets are estimated to consist of 103 to 104 ions.
2.4 Secondary Mass Spectrometer and Detector
Following photodissociation of a parent ion at the ion-laser interaction, the photoproducts(ionic and neutral) enter a single-stage reflectron mass analyzer50 roughly 10 cm upstream fromthe exit of the mass gate with the same velocity the parent ion had at the time ofphotodissociation. The reflectron consists of a stack of ten concentric cylindrical electrodes,
which are separated by Mylar washers and separated by resistors so as to maintain a uniform
retarding electric field. The reflectron is mounted such that the deflection angle between theretarding field and the ion beam propagation direction can be varied from ~5° to ~10°; this angle
is typically set at 8°. As the photoproducts traverse the reflectron, the ions experience a retarding
electric field and only penetrate to a specific depth in the reflectron, determined by each ion’s
kinetic energy. As an ion penetrates the retarding field, the ion decelerates until its velocity along
the beam axis goes to zero and its trajectory is reversed. Consequently, ions with the same massand spatial position, but different energies, will penetrate to different depths and exit thereflectron at different times and positions. After exiting the reflectron, the ions enter a roughly
20 cm field-free drift region in which the more energetic ions (the ions that spent more time in
the reflectron) will overtake the less energetic ions of the same mass. The point in space at which
this occurs is the second spatial focus (SF2 in Fig. 2.1) and is the optimal location for the off-axis
detector. Therefore, the reflectron provides secondary mass-selection of the ionic photoproducts

20
and increased mass resolution. The energetic neutral photoproducts, however, pass unperturbed
through the reflectron and after exiting are monitored on an inline channeltron detector(Burle Electro-Optics, Model 32685).
As described above, the spatial focus following the reflectron, SF2, is the position in
space at which the more energetic ions overtake the less energetic ones. The distance from theexit of the reflectron to SF2 will vary according to the mass of the ion and will be different for anionic photoproduct and the parent ion. The position of SF2 for an ionic photoproduct is found by
taking the derivative of the total travel time of the ionic photoproduct within the reflectron withrespect to the parent ions average kinetic energy, Ek, and setting it equal to zero to obtain
,

(2.2)

where d is the distance from the exit of the reflectron to the second spatial focus (SF2), mf is themass of the photofragment, q is the elementary charge, Er is the magnitude of the reflectron
electric field, and mp is the mass of the parent ion. As a result, to detect photofragments theelectric potential on the reflectron is adjusted so that SF2 of the ionic photoproduct is located at
the off-axis detector face. Additionally, by adjusting the potential on the reflectron, the ionicphotoproducts will penetrate to the same depth and have the same travel time through thereflectron to the detector as the parent ion. This provides an unambiguous identification of theionic photoproduct mass.
The off-axis detector, positioned at SF2, utilizes dual microchannel plates (Burle ElectroOptics, model 31558, referred to as MCP), that are in a chevron configuration and 25 mm indiameter. Just prior to the MCPs, the ions encounter three mesh grids. The first grid is at earth

21
ground and the second is biased at −200 V to eliminate low energy electrons, thereby reducing
detector noise. The final grid, also at earth ground, is resistively coupled to three concentric
stainless steel rings by three 10 M resistors, allowing acceleration of the product ions by anadditional 3 keV. The acceleration stack is utilized to increase the detection efficiency of the
MCPs by providing the ions with an additional 3 keV of energy. The final ring in theacceleration stack is resistively coupled to the face of the first MCP, which is constantly
maintained at the beam potential (3 kV). The second MCP is coupled to the collector anode by a
Zener diode that provides a 200 V drop in the voltage between the MCP and the anode. Whendetecting parent anions, the collector anode biased at 4.7 kV and the face of the second MCP
biased at 4.5 kV. When detecting one-photon photoproducts during photofragmentation
experiments, the MCPs are operated with a higher voltage applied to the collector anode toincrease the gain of the MCPs; the second MCP is biased at 4.8 kV and the collector is at 5.0 kV.
For time-resolved experiments, a still higher gain in required to observe the two-photon
products; therefore, the face of the second MCP is biased at 5.1 kV and the anode is at 5.3 kV.
2.5 Laser Systems
2.5.1 Nanosecond Laser System
A nanosecond laser is used to study the photofragmentation of ICN− and solvated ICN−
(Chapters III, IV, and VI). The laser system employed has been described in detail previously51
and, therefore, only an overview will be presented here.
The nanosecond laser employed in these experiments is a Coherent Infinity laser, with a
Coherent XPO and 2nd harmonic generation stage. A Q-switched master oscillator pumps two

22
Nd:YAG (neodymium-doped yttrium aluminum garnet, Nd:Y3Al5O12) rods resulting in 1064 nm
fundamental light. The 1064 nm radiation can be frequency doubled and tripled to produce
532 nm and 355 nm light, respectively, by using two type I BBO (β-barium borate) crystals(SHG and THG in Fig. 2.3). The 355 nm light is then directed into a build-up cavity that pumps athird type I BBO crystal in an optical parametric oscillator (henceforth referred to as XPO). The
BBO crystal in the XPO produces both signal (420–680 nm) and idler (730–900 nm) radiationfrom the 355 nm light. The output pulse energies of the signal radiation range from 1–20 mJ,
with an energy resolution of < 8 cm−1. Additionally, the light generated in the XPO can befrequency doubled by another type I BBO crystal (SHG(2) in Fig. 2.3) to generate 210–340 nm
light from the signal and 365–450 nm light from the idler radiation.
The photofragmentation experiments require a laser beam with relatively uniform spatialintensity and known size. A 2:1 telescope collimates the laser beam and a periscope is utilized toraise the elevation of the laser beam to the entrance of the TOF MS. Depending on thepolarization of the light required in the experiment, the setup of the periscope can be altered torotate the polarization of the light. An additional optical path is constructed using a removablemirror that picks off the laser beam and allows for measurement of the laser beam diameter(dashed path in Fig. 2.4). The length of this path is such that a knife-edge, used for determining
the beam diameter, is at the same distance as the ion-laser interaction from the last turning mirror
to ensure measurement of the diameter at the interaction. The diameter of the laser beam isdetermined by fitting the curve produced by measuring the transmitted pulse energy as the
knife-edge is moved in the x or y plane of the beam with an analytic equation reported by
Khosrofian and Garetz.52 Typically, the laser beam is optimized by adjusting the focusing

23
conditions of the telescope in order to have a diameter of roughly 8 mm at the ion-laserinteraction.

Figure 2.3 Schematic diagram of nanosecond laser system and optical path.

2.5.2 Femtosecond Laser System
A femtosecond laser system, described in detail previously,36, 53 is utilized in the study ofthe time-resolved absorption recovery of photodissociated IBr−(CO2)n clusters (Chapter V). Onlya brief overview of details relevant to these experiments is presented here.
A Ti:Sapphire oscillator (Coherent Mira Basic) is pumped by a Nd:VO4(neodymium-doped vanadium oxide) laser (Coherent Verdi V5, λ = 532 nm) to generate
750–850 nm light at a repetition rate of ~76 MHz. The temporal pulse width of the MIRA output
beam is ~85 fs and the pulse energy is ~7–8 nJ. The output of the Mira is then amplified in a
Ti:Sapphire regenerative multipass amplifier (Quantronix, Titan). The Titan amplifier is pumped
by a Nd:YLF (neodymium-doped yttrium lithium fluoride) laser (Quantronix, model 527 DQ,

24
λ = 527 nm) to produce 795 nm light (~3 mJ/pulse, bandwidth Δλ ≈ 10 nm ) at a repetition rateof 400 Hz. The temporal pulse width of the Titan output is measured by optical autocorrelation(INRAD, model 5-14BX) to be roughly 120 fs.
The time-resolved experiments, discussed in Chapter V, are performed with a one-color
pump-probe scheme and optical path that utilizes two delay stages, see Fig. 2.4. The 795 nm
pump pulse photoexcites IBr−(CO2)n clusters to the Ã′ 2Π1/2 excited state, on which thechromophore begins to dissociate. Following dissociation of IBr−, the anionic cluster istransparent to 795 nm light. Only after a solvent-induced nonadiabatic transition to the groundstate, vibrational relaxation into the ground state well, and evaporation of solvent CO2 molecules,
the recombined chromophore regains absorption cross section. The probe pulse (at some timedelay relative to the pump pulse) then excites the recombined chromophore to the Ã′ 2Π1/2
excited state. This leads to either dissociation of the chromophore on the Ã′ state into I−-based
photoproducts or a second nonadiabatic transition to the ground state and recombination in the
ground state well resulting in IBr−-based products. By detecting the total number of two-photon
ionic photoproducts, the absorption recovery is measured as a function of pump-probe delaytime.
To perform this measurement, the output of the Titan amplifier must be split, utilizing a
50/50 beam splitter, into the pump and probe beams. The pump beam traverses a
computer-controlled delay stage (Parker, 008-9129) with a range of 800 ps and time resolution of~100 fs. The probe beam is directed through an optical rail delay stage with a range of 3.5 ns anda time resolution of ~20 ps. The poor time resolution of the optical rail compared to thecomputer-controlled delay stage is a result of manual positioning of the stage on the optical rail

25
to adjust the range of pump-probe delay time. The two delay stages are used in conjunction toallow for both small and large time steps; the computer-controlled delay stage is utilized, withthe manual delay at a fixed position, to obtain data for short pump-probe delay and small timeintervals. For large pump-probe delay, the position of the delay stage on the optical rail was
adjusted, with the computer-controlled delay scanning ±400 ps at each position. After traversing
the delay stages, the two halves of the beam are then recombined collinearly in a 50/50 beamcombiner, directed through a periscope, and focused using a single lens before entering the TOF
MS. The typical laser beam is roughly 1 mm in diameter at the intersection of the anions with thelaser, with a pulse energy of ~500 μJ.

Figure 2.4 Schematic diagram of femtosecond laser system and optical path.

26
2.6 Data Acquisition
2.6.1 Experimental Timing Sequence
For the experiments presented in this thesis, two to three digital delay generators(Stanford Research Systems, model DG535) are utilized to control the timing sequence of theexperiments. When taking data, the master clock is provided to the digital delay generators by
the laser system in use. When measuring the ion photoproduct distribution or action cross section
for photodissociation of ICN−, ICN−(Ar), or ICN−(CO2), the Q-switch of the master oscillator in
the Infinity nanosecond laser provides the external trigger for the digital delay generators. The
trigger in the time-resolved experiments is provided by the pulse train from the Ti:Sapphireoscillator. A portion of the 76 MHz pulse train is continually monitored on a photodiode and theresulting signal is divided to 400 Hz. The 400 Hz signal triggers the Pockels Cell driver(Medox Electro-Optics, model DR85-A), which is utilized to trigger the Titan amplifier and
Nd:YLF laser. The 400 Hz output signal from the Pockels Cell driver is further divided by ahomebuilt Modulo-N divider to 100 Hz and utilized as the external trigger for the digital delay
generators.
Figures 2.5 and 2.6 show a schematic representation of the relative timing sequence forthe various experimental components of the static and time-resolved experiments, respectively.
For both types of experiments, following the trigger from the lasers, the opening of the pulsed
valve is the first in a sequence of events to produce, mass separate, and photoexcite ions. The
extraction pulse to extract anions into the TOF MS is the next event to occur. Following
extraction, the potential switch is pulsed when the desired ion is in the middle to re-reference theions to earth ground. The mass gate is pulsed next, allowing for the desired anionic cluster size to27pass through, while deflecting the other ions from the beam axis. Finally, when performing
time-resolved experiments, four time-gates, used for the integration of the ionic photoproducts,
are triggered. The relative timing for each event in this sequence is varied to optimize the spatial
focus of the parent anionic clusters at SF1 (Wiley-McLaren condition) and the temporal overlapof the ion packet and the laser pulses.

Figure 2.5 Schematic diagram describing the relative timing of the mass spectrometer andnanosecond laser system utilized for static experiments. All delays and pulse widths shown areapproximate.

28

Figure 2.6 Schematic diagram describing the relative timing of the mass spectrometer andfemtosecond laser system utilized for time-resolved experiments. All delays and pulse widths
shown are approximate.

29
2.6.2 Data Acquisition Electronics
In order to measure the photoproducts of one or two-photon experiments, the output
signal from the collector anode of the off-axis detector is monitored and collected. This output
transient signal is capacitively coupled to the data acquisition electronics to isolate the output
signal from the high voltage applied to the collector anode. The output signal is amplified by a
10x fast preamplifier (Ortec, model 9301) that is directly connected to the output capacitors. The
signal is further amplified by an adjustable gain (1x to 10x) quad bipolar amplifier(Phillips Scientific, model 771) set to scale the signal by a factor of ten. The subsequent dataacquisition methods, presented below, are different for the two types of experiments described inthis thesis.
2.6.3 Data Acquisition for Static Photofragmentation Experiments
For the static photofragmentation experiments, the amplified photoproduct signal ismonitored on an oscilloscope (Tektronix, model DPO 2024) to allow for optimization of the ion
intensities. Two measurements (described below) are performed in the static experiments: themeasurement of the ionic photoproduct distributions following photoexcitation of a parent
anionic cluster and the relative action cross section for photodissociation of ICN−, ICN−(Ar), andICN−(CO2).
2.6.3.1 Ionic Photoproduct Distributions
The significance of photofragmentation dynamics occurring on a single or multiple
excited states following excitation of a chromophore is elucidated by the ionic photoproductdistributions as a function of wavelength and cluster size. To distinguish between ionic30
photoproducts, the electric potential on reflectron is adjusted to focus the desired ionicphotoproduct, i, on the off-axis MCP detector.
The ionic photoproduct distributions are obtained by measuring the production ofmass-selected ionic photofragments following excitation and fragmentation of parent anionicclusters. Optimization of the temporal overlap of the ion packets with the laser pulses isperformed by adjusting the laser timing with respect to the extraction timing (Fig. 2.5); during adata set, the laser timing is not adjusted. Additionally, a manually-activated shutter(Uniblitz, model VS14S2TO) is utilized to block the laser beam to allow for backgroundsubtraction. A minimum of ten data sets are collected to reduce statistical fluctuations andsystematic effects. The spectrum of the ionic photoproduct i is averaged over all data sets,
integrated, and divided by the sum of all observed ionic photofragment average peak areas. This
yields the final ratio for ionic photoproduct i from the photofragmentation of one parent cluster
size. Laser power dependence studies on the production of ionic photoproducts following
photofragmentation of a parent cluster are performed to ensure the measured ionic photoproductdistribution results entirely from one-photon photodissociation. Following the analysis of ten
data sets, the typical statistical errors in the ionic photoproduct distribution are about 5%.
2.6.3.2 Action Cross Section for Photodissociation of an Anionic Chromophore
The measurement of an absolute cross section necessitates determining the same
quantities as the relative cross section, with the addition of the spatial overlap of the laser pulse
and ion packet, and the detection efficiency of the off-axis MCP detector. However, because of
the difficulty in measuring the ion-laser spatial overlap and detection efficiencies neither of thesefactors is measured. Therefore, the cross section data presented in this thesis are relative cross31
sections for the production of ionic photoproduct i. Action spectroscopic techniques are utilized
to measure the relative photodissociation cross section by monitoring the production of a
selected ionic photoproduct i, the parent ion signal with no laser interaction, and the laser
fluence.
For each ionic photoproduct i, the average total peak area, ̅ , on the off-axis detector as
a function of wavelength is determined by
̅

Φ ̅

,

,

(2.3)

where Φ is the fluence of photons per laser pulse (in photons/pulse*cm2), ̅ is the average total
precursor anion peak area on the off-axis with the laser blocked, and χ is the product of theunmeasured spatial overlap and detection efficiency. The fluence of photons per laser pulse isgiven by
Φ

,

(2.4)

where Elaser is the measured pulse energy of the laser, h is Planck’s constant, ν is the frequency ofthe laser light, and Alaser is the cross-sectional area of the laser beam at the ion-laser interaction.
The peak area of i,,, for a single laser pulse j and data set k (k ≥ 10) is roughly equivalent to̅ when summed over 128 laser pulses at 50 Hz. Therefore, the relative cross section for the
production of i as a function of excitation wavelength summed over all laser pulses and data sets
is determined by

,

∑

∑

,

∑

,

,

(2.5)

32
where χ is assumed to be unity. A minimum of ten data sets is measured to reduce statisticalfluctuations and systematic effects. Typical statistical fluctuations in the relative cross section
measurements are roughly 5-15%.
2.6.4 Data Acquisition for Time-Resolved Experiments
Figure 2.7 shows a schematic of the data acquisition system used for the time-resolved
experiments. Following amplification of the signal from the off-axis MCP detector, the resulting
signal is split by a splitter (Minicircuits, model ZSC-2-2); one half of the signal is monitored on
an oscilloscope (Tektronix, model DPO 2024) and the other half is input into a 300 MHz
discriminator (Phillips Scientific, model 6904). The discriminator threshold is set to ~0.150 V,
which separates of the ion signal from the detector noise. From the discriminator, the signal
proceeds to a quad scalar (DSP, model QS-450), which counts the pulses associated with ionicproducts that impinge upon the MCPs within one of four adjustable time-gates. The position of
the time-gates is controlled by a digital delay generator (Stanford Research Systems, model
DG535) and set to open during the arrival times of the desired photoproducts. A computer, run in
MS-DOS (Microsoft Disk Operating System), then obtains the signal from the quad scalarthrough a GPIB (General Purpose Interface Bus) crate controller (DSP, model 6001) that isinterfaced with a CAMAC crate (Kinetic Systems, model 1510-P2C), in which the quad scalar is
held.

33

Figure 2.7 Schematic diagram of the data acquisition system utilized for time-resolved
experiments.

2.6.4.1 Time-Resolved Absorption Recovery Measurements
The time-resolved absorption recovery data for photodissociated IBr−(CO2)n clusters arecollected by scanning a selected set of pump-probe time delays; the procedure utilized is detailed
elsewhere54 and only a brief description is reported here. One scan sweeps the time-delay range,
collecting data for 1000 laser shots per time delay, until all selected time delays are sampled.
Typically, the pump-probe delays are stepped by roughly 20–100 ps for small pump-probe
delays and roughly 500 ps for large delays. Scans are repeated, in both the forward and backward
scanning direction, until data for roughly 5x104 laser shots per time delay is collected. To allow
for background subtraction to obtain the two-photon time-dependent signal, baseline traces fromthe pump and probe laser beams are obtained by a series of measurements withcomputer-controlled shutters in the open and closed positions. This also enables analysis of the34contribution of the pump and the probe beams to the ionic photoproduct signal. The number ofphotoproducts detected per laser pulse is dependent on the size of the parent cluster anion, but
typical two-photon signals range from 1–4 photoproducts per laser shot.

35

Chapter III: Photodissociation of ICN−(Ar)n

3.1 Introduction
This chapter describes experimental studies40 of the dissociation/recombination dynamics
of ICN−(Ar)n, n = 0–5, following photoexcitation to the dissociative 2Π1/2 state. The ICN
molecule is a prototype for the study of dynamics at conical intersections.55-63 Trajectory
calculations run on high-level potential energy surfaces64, 65 established that bending motion,
which occurs along with the I–CN bond elongation, maps into torque on the CN diatom as ICN
passes through a conical intersection, giving the notably high degree of rotational excitation
observed in concurrence with I (2P3/2). In the absence of a conical intersection, as is the case for
ICN− photodissociation, the shape of the excited-state potential energy surface still plays a largerole in the rotational state distribution of the photofragments. This chapter focuses on the role of
the CN fragment rotation in the dynamics of photoexcited ICN− with and without Ar solvation.
Figure 3.1 sketches potential energy curves adapted from Ref. 66 for the electronic ground stateof the ICN− and INC− isomers (2Σ+, black) and the first optically accessible excited state of theseisomers (2Π1/2, purple). Figure 3.1 also depicts photoexcitation of ground-state ICN−, which
undergoes dissociation (3.1) through the following channels:
ICN−(Ar)n + hν → ICN−(Ar)m < n + (n − m)Ar

(3.1a)→ I−(Ar)k ≤ n + CN + (n − k)Ar

(3.1b)→ CN− + I + nAr(3.1c).

36
Beyond the photodissociation products observed following excitation of previous dihalides33-35 to
the 2Π1/2 excited state, the addition of a single Ar atom (which is only bound by ~50 meV(Ref. 67)) results in the formation of recombined ICN−. Experimental and theoretical evidence
suggests a large amount of internal energy must be deposited into the neutral or anion product.
For the photon energies used in these experiments, most of the excess energy is in the I–C bond.
Energy transfer between the low-frequency I–C stretch and the high-frequency C–N stretch will
be inefficient. Furthermore, quantum dynamics calculations indicate quick and efficient energy
transfer into CN rotation following photoexcitation. Thus, it is likely that CN rotation plays avital role in storing considerable amounts of internal energy.

22

 1/2 /  3/2

3.0



~ 2

I + CN (A )

Energy (eV)

2.5
2.0

2

1.5

2

1.0

2



+

I* + CN  (X 1 )~I  + CN (X 2 )
~

 1/2
 3/2

hν

I + CN  (X 1 )
~

~ 2 +

X 

0.5
0.0
3

4

5

6

7

8

R(I-CN COM) (Å)
Figure 3.1 Schematic potential energy curves (based on Ref. 66) for the ICN− (2Σ+ in black and2
Π1/2 in purple) as a function of the I–CN center of mass distance. The black dashed line shows
the probability amplitude of the ground-state wave function for ICN−. The black arrow illustrates
photoexcitation and subsequent dissociation (3.1).

37
3.2 Experimental
This section presents an overview of the experimental methods utilized in this study; only
relevant details, beyond those presented in Chapter II, are provided. Solid ICN is synthesizedusing the procedure reported by Bak and Hillebert.42 Neat Ar (20–30 psig) passes over solid ICN
and then expands from a 0.8-mm orifice of a pulsed General Valve nozzle into the ion-sourcechamber. The neutral molecular beam encounters a focused and magnetically-guided 1-keVelectron beam, forming a variety of anions. A pulsed extraction field, the first component of the
Wiley-McLaren41 time-of-flight mass spectrometer (TOF MS), extracts the anions into the
TOF MS, where they separated by their mass-to-charge ratio. While in the TOF MS, a fast
potential switch49 re-references the anions to an earth ground and an einzel lens focuses theanions into the interaction region.
A mass gate located just prior to the anion spatial focus selects the desired anion,
allowing only this single species to continue into the interaction region. The output of a
Nd:YAG-pumped optical parametric oscillator (OPO) is collimated into the interaction region.
Both the surviving parent anions and the photodissociation products (neutral and anioniccomponents) generated from the ion-light interaction enter a single-stage secondary reflectronmass spectrometer. The neutral photoproducts continue through the reflectron unperturbed. The
reflectron separates the anionic photoproducts by mass and directs them toward a dual set of
microchannel plates (MCPs) located 8° off-axis of the parent-ion beam. Changing the reflectron
potential allows different product anion masses to impinge upon the detector; a recording
oscilloscope collects the signal from the detector for various reflectron potentials, thereby

38
permitting a determination of the ion photoproduct distribution and photodissociation crosssection, as detailed in Chapter II.
It is also critically important to note that the observed ionic fragments are fragments thathave remained in their detected form from the time of photodissociation through the time thatthey are detected. While this time could be calculated for each photoproduct, it is adequate tonote that it is ~10 μs for all of the products discussed here. Thus, e.g., when recombined
photoproducts are observed, the observation only implies stability on a 10 μs time scale, notabsolute stability.
3.3 Theoretical Methods
Initial one-dimensional ICN− electronic structure calculations of the ground and valence
excited states were carried out by the author,40 in collaboration with Professor Anne McCoy andhave been detailed elsewhere.66 Further investigations showed that in order to describe
adequately the photodissociation process, it would be essential to increase the dimensionality toinclude large amplitude bending motions. This was done and the ICN− electronic structure wasobtained as a function of two Jacobi coordinates, R and θ.66 In this context, R is the distance
between the iodine atom and the center-of-mass of the CN diatom and θ is the angle between thevector R and the CN bond. As excitation of CN vibrations are not expected in thephotodissociation process reported in this thesis, it was acceptable to constrain the CN bondlength (r) to its v = 0 value, 1.16 Å. We define θ = 0° to be the linear ICN− configuration and the
INC− configuration then corresponds to θ = 180°. Thus the θ = 0° limit of this calculation
corresponds to the one-dimensional calculations mentioned above.

39
We display the resulting X̃ 2Σ+ and 2Π1/2 surfaces in Figure 3.2, plotted as functions of thetwo Jacobi coordinates described above. As can be seen, the 2Π1/2 state has a shallow minimum ata slightly bent geometry. Analysis of the potential energy surface for this state shows that it isbound by 0.27 eV, relative to the dissociated products, and that this minimum occurs at
R = 4.74 Å and θ = 24°.

Figure 3.2 Two-dimensional contour plots of the ground (2Σ+) and excited (2Π1/2) states of ICN−;
contour lines are shown every 0.05 eV. The X denotes the location of the shallow minimum of
the 2Π1/2 potential (Ref. 66 )To explore the energy partitioning following 500 nm excitation of ICN− to thedissociative 2Π1/2 state, we propagate eigenstates of the ground-state surface on this excited-state
potential. Based on the potential energy surfaces shown in Figure 3.2, the ground-statevibrational wave is obtained using the approaches described in Ref 66. For these calculations, theHamiltonian is
,

,

40
where

…

,

and

…

̂

2

,

and V(R, θ) represents the potential energy for the 2Π1/2 state. For the purpose of the analysis, theaverage potential energy at the end of the calculation is defined to be zero. The masses represent
the reduced mass, μ, of I–CN and CN. In these calculations, we set J = M = 0 for calculations ofstates with even quanta in the bend and J = M = 1 for calculations involving states with odd
quanta in the bend. Here J represents the total angular momentum of the ICN− molecule, while M
gives its projection onto R.
The calculations use 1000 evenly spaced grid points in R, ranging from 2 to 50 a0(1.06–26.5 Å), and 75 grid points in θ, which are based on the zero’s in the Legendre polynomialwith l − m = 75 (with m = 0 for even quanta in the bend and m = 1 for odd quanta in the bend).
Again, the CN bond length is held constant at 1.16 Å. At t = 0, the wave function for the state ofinterest is promoted to the 2Π1/2 excited-state potential, and the dynamics are followed as afunction of time. We propagate the dynamics using a Lanczos scheme with 20 iterations for each
25 au time step. The approach is similar to that employed in an earlier study of Cu(H2O).68 We
test the parameters to ensure convergence of the calculations for the 1000 time steps over which
the results are reported.

41
3.4 Experimental Results: Photodissociation of ICN−(Ar)0–5
The cross sections for the production of I− and CN− following photoexcitation of ICN−,
along with the results of 500 nm photodissociation of ICN−(Ar)n (n = 1–5), are given below.40
Argon was chosen as the solvent because of the weak electrostatic interactions it shares with thesolute and with other Ar atoms. The short-range, ion-induced dipole interaction between Ar andthe ICN− chromophore results in an Ar solvation energy67 of ~0.05 eV and is not expected tolead to significant perturbations of the ICN− electronic structure. Additionally, Ar–Ar
interactions are only slightly attractive and result in a loose cage around the chromophore, which
is not anticipated to lead to high yields of caged photoproducts for the small (0–5) numbers of Ar
solvent molecules studied. Comparing the results of ICN−(Ar)n photodissociation with previous
dihalide results elucidates the role of the additional degrees of freedom available for ICN− and its
photoproducts.
3.4.1 Relative Action Cross Section for Photodissociation of ICN−
The experimental relative cross sections (σrel(λ)) for the production of I− and CN−
products following 430 to 650 nm excitation of the ICN− chromophore are shown in Fig. 3.3.
This spectral region corresponds to excitation to the 2Π1/2 state which results in two dissociation
channels: ICN− ⟶ I− + CN (97%) and ICN− ⟶ I + CN− (3%). The peak of the I− spectrum isnear 490 nm and the full-width-at-half-maximum (FWHM) is ~0.31 eV. Similar experiments
were performed following photoexcitation of ICN−(Ar)n. As expected, because of the weaksolvent-solute interaction, solvation of ICN− with one Ar atom does not lead to a significant
change in the cross section for the production of ionic photoproducts.

42

Figure 3.3 Cross sections (σrel(λ)) for the production of I− (blue circles) and CN−(red triangles, x10) photoproducts following 430 to 650 nm photodissociation of ICN−.

3.4.2 Anionic Photoproduct Distributions
Studies of the photodissociation product distributions of mass-selected ICN−(Ar)1-5
following 500 nm excitation to the 2Π1/2 electronic-excited state of ICN− show that I−-based
products dominate for all cluster sizes. For example, I−(Ar) and I−(Ar)2 are by far the mostabundant products of ICN−(Ar)2 photodissociation. Figure 3.4 shows the degree of Ar solvationof the I−- and ICN−-based products (I−(Ar)m and ICN−(Ar)m), plotted as a function of the numberof Ar atoms in the parent ICN−(Ar)n. Both an unexpectedly large solvent retention andrecombination with a single Ar solvent are observed. From these two plots in Figure 3.4, it isapparent that the product distributions are in stark contrast to previous studies on dihalides,33-36, 69

43
where the solvent loss from caged products follows the evaporative ensemble model, in which
excess energy rapidly results in the loss of neutral solvent.70 This result unequivocallydemonstrates that the large amount of energy released following photodissociation is notcompletely converted into evaporation of the weakly bound Ar, but instead must remain ininternal degrees of freedom of the I− and CN or, if recombination occurs, ICN−.

44
Ionic Photoproduct Yield

1.0

0.8

0.6

0.4

0.2

0.0
0.10

Ionic Photoproduct Yield

I -Based Photoproducts

ICN  -Based Photoproducts

0 Ar atoms
1 Ar atom
2 Ar atoms
3 Ar atoms
4 Ar atoms

0.08

0.06

0.04

0.02

0.00
0

1

2

3

4

Number of Ar Atoms in Parent (n)5Figure 3.4 Solvation breakdown for I−- and ICN−-based photoproducts following 2.5 eV
excitation of ICN−(Ar)n. The color of the bar corresponds to the number of Ar atoms in the ionicphotoproduct. Note the change in scale of the y-axis between the two plots.

The photodissociation studies also find a roughly 6% recombination yield for ICN−(Ar).
Cage recombination for a single Ar solvent is particularly surprising, especially with such a
weakly bound solvent. Moreover, the observation that the cage fraction depends very weakly onn for 1  n  5 strongly indicates that a single, labile Ar is critical for cage recombination in this45
size range. The Ar–ICN− potential is relatively isotropic, allowing the Ar to sample large regionsof configuration space about the chromophore. This leads to an occasional collision between the
Ar and the departing CN fragment, the kinematics of which are described below. Recombination
to stable ICN− products cannot occur on a dissociative state. Yet, the ICN− photoproduct survives
for at least 10 μs after photoexcitation, indicating that substantial energy is stored in themolecular ion for a long period of time. The weak binding energy of the Ar solvent (~0.05 eV) isinsufficient to bridge the large energy separation between the 2Π1/2 state and the bound groundstate (ΔE = 0.80 eV). While, ab initio calculations do find a shallow (~0.27 eV) potential well on
the 2Π1/2 surface of ICN−,66 the only way that recombined anions can be trapped on the excited2

Π1/2 surface is for a large fraction of the available energy to be tied up in rotation of the CN

diatom, forming a complex between the I− and the trapped, quasi-free rotor CN diatom(Be(CN) = 1.89974 cm−1).71, 72 This [I−–CN] complex formation allows sufficient time forradiative decay back to the ground state, resulting in a bound (but with significant internal
excitation) ground state ICN− photodissociation product.
3.4.3 Kinematics of Collinear ICN−(Ar) Dissociation
A simple hard-sphere, center-of-mass frame model that only considers translational
degrees of freedom is used to describe the kinematics of dissociation for ICN−(Ar). The
photodissociation of ICN− and the elastic collision of the CN fragment with the Ar atom areconsidered as two separate actions. Additionally, the collision is only evaluated in a collinear
geometry. Accordingly, the numbers given in this section represent the maximum translationalenergy transfer to the Ar atom.

46
Photodissociation of the ICN− initiated with a 500 nm photon produces I− andground-state CN, leaving 0.87 eV of available energy to be partitioned into the products.
(This excess energy is obtained by taking the difference between the photon energy and theexperimentally determined D0(I–CN−) = 0.83 eV (Ref. 67) and EA(CN) − EA(I) = 0.80 eV).73, 74
This simple kinematic model finds that 0.15 eV and 0.72 eV of energy distributes into translationof I− and CN, respectively. The subsequent CN–Ar collision further divides the 0.72 eV of CN
translation energy into 0.44 eV and 0.28 eV of energy for translation of the CN and Ar,
respectively. This model results in roughly one third of the translational energy available to thethree species (I−, CN, and Ar) leaving with the Ar atom. By obtaining the potential energy of the
I−–CN system from the sum of D0(I–CN−) and [EA(CN) − EA(I)], we determine the amount oftranslational energy available to the dissociation products and, thus, the energy transfer between
the CN fragment and Ar atom, at an infinite I−–CN separation.
Although the ICN−–Ar potential is rather flat, the Ar atom is located roughly 3.5 Å
beyond the N atom of the ICN− chromophore. As a result, it is likely that the CN–Ar collision
will occur at a I−–CN separation of roughly 4.5 Å. To consider the quantity of energy that leaves
with the Ar atom for finite I−–CN separations, we use a 1D potential patterned after the
calculated 2Π1/2 potential to approximate the translation energy available to the dissociating ICN−
for varying I−–CN separations. The results of these calculations are given in Table 3.1; RI–CN isthe distance between the iodine atom and the center of mass of the CN diatom, Eavail is theamount of energy that becomes available for translation of the fragments, and Et is thetranslational energy that is partitioned into the given fragment. The translational energies arebroken into pre- and post-collision energies, representing the energy deposited into the I− and CN

47
fragments from the dissociation and then the transfer of energy from the CN fragment to the Ar
atom upon collision.

Table 3.1 Translational energy partitioning as a function of the assumed I–CN separation at thetime of collision of CN with Ar, as determined from a simple kinematic analysis.
RI–CN (Å)
3.27
4.50
9.00

Pre-Collision Energies (eV)
Et(CN)
Et(Ar)
Eavail (eV) Et(I−)
0.00
0.00
0.00
0.00
1.02
0.17
0.85
0.00
0.87
0.15
0.72
0.00

Post-Collision Energies (eV)
Et(I−)
Et(CN)
Et(Ar)
0.00
0.00
0.00
0.17
0.51
0.33
0.15
0.44
0.28

At an I−–CN separation of 4.5 Å (approximately the separation for which the CN
fragment will collide with the Ar atom), the available energy is 1.02 eV. Figure 3.5 is aschematic of the photodissociation of ICN−(Ar) and the subsequent collision between the CN
diatom and the collinear, solvent Ar atom. From this simple kinematic model, we find that the
departing Ar removes 0.33 eV of energy via translation and another 0.05 eV is expended to breakthe ICN−–Ar bond. There is an additional loss of 0.03 eV as the CN fragment changes its
trajectory following the collision with Ar and the center of mass of the ICN− begins to translate(see Fig. 3.5). This leaves the I−–CN system with 0.61 eV of energy unaccounted for on an
excited-state surface (2Π1/2) that contains a 0.27 eV potential well. For this simplified model, theamount of energy that is left in the I−–CN system implies that dissociation will be rapid. This
rapid dissociation leaves no opportunity for a radiative transition back to the ICN− ground state,
which is essential to undergo recombination. As such, it is apparent that the energy partitioned to
the rotation of the CN fragment is crucial to the observation of recombined ICN− from ICN−(Ar).

48

Figure 3.5 Schematic diagram of the photodissociation of ICN−(Ar) and the subsequent collision
between CN and the solvent Ar atom. The bottom ICN−(Ar) cluster defines the atoms, thecenter-of-mass of ICN−, XCOM, and the distance between the iodine and the center-of-mass of the
CN diatom, RI–CN. The CN diatom is assumed to be a hard sphere with a radius of 1.16 Å, thecalculated bond length of CN. The arrows indicate the direction of the hard-sphere’s trajectory
and the time at which photodissociation and the collision occur is indicated on the left axis.
Additionally, the time range for the pre-collision (light blue shading) and post-collision (lightgray shading) energy values in Table 3.1 are shown on the right.

3.4.4 Quantum Dynamics
Additional evidence for the importance of the free internal rotation of the CN diatom in[I−–CN] comes from quantum dynamics calculations of the photodissociation of bare ICN− on
the 2Π1/2 potential energy surface.40 Promoting the rovibronic ground-state wave function(Jtot = 0) for the anion onto the 2Π1/2 surface results in an increase in the I–CN distance49(R coordinate) due to the dissociative nature of the excited state. The calculations also show thatthe wave function spreads quickly along the CN rotational degree of freedom (θ coordinate). The
anisotropy of the 2Π1/2 potential efficiently couples R and θ, allowing the excited ICN− to explore
both internal coordinates as it dissociates. As illustrated in Figure 3.6, at t = 0 the wave function
only has amplitude for values of θ less than 50°; however, by 180 fs the wave function has
amplitude over all I–CN bend angles, effectively making the CN diatom a free rotor(Be(CN) = 1.89974 cm−1).71, 72 After 360 fs, the I–CN separation has increased to ~12 Å, andessentially all CN orientations are equally probable. The calculations also show that ~20% of theavailable energy is partitioned into the CN diatom rotation; furthermore, this fraction increases
with initial bend excitation. Additional calculations, artificially altering the time scale of the CN
rotation with respect to the ICN− dissociation, illustrate that the longer CN samples theanisotropic portions of the potential energy surface, the more energy will go into rotation.

50

Figure 3.6 Quantum dynamics calculation results for t = 0, 180 and 360 fs following promotionof the ICN− 2Σ+ rovibronic ground-state wave function (Jtot = 0) to the 2Π1/2 surface. The
probability amplitude (black contours) is shown on the 2Π1/2 potential energy surface (color) as afunction of the Jacobi coordinates, R and θ.

Photodissociation of ICN− on the 2Π1/2 surface produces I− and ground-state CN, leaving
0.87 eV of kinetic energy to be partitioned into the products. In the analysis of the dispersal ofthis kinetic energy, we focus on the expectation values of the three operators that make up the
Hamiltonian. Thus at t = 600 fs (〈R〉 = 18.1 Å), of the 0.87 eV of kinetic energy, 19% or 0.17 eV
is in CN rotation; the remaining 0.71 eV is in translation. The time when 〈V〉 is a minimum,t = 45 fs, gives 〈R〉 = 4.1 Å , 〈Ttrans〉 = 0.77 eV and 〈Trot〉 = 0.25 eV. This corresponds to the
propagation time for which the 〈V〉 is minimized.

51
To elucidate the mechanism for the substantial energy deposited into CN rotationalmotion, we repeat the wave packet propagation but change the mass of the iodine atom to that ofa hydrogen atom. This change in mass leads to a much more rapid dissociation of the ICN−
relative to the timescale for CN rotation. As a result, the CN does not have the opportunity tosample the anisotropic portions of the potential energy surface and a smaller fraction of the
kinetic energy ultimately appears in CN rotation (1% when 〈R〉 = 18 Å). Likewise in anotherpropagation, the CN rotation is made fast relative to the timescale of the ICN− dissociation bychanging the reduced mass of the CN fragment to that obtained for a carbon and nitrogen eachhaving the mass of hydrogen; thereby increasing the rotational constant for the CN diatom byroughly a factor of 20. This modification has the effect of allowing the CN to more fully sample
the anisotropy of the potential energy surface as the ICN− dissociates, and as a result, there is alarger fraction of the kinetic energy in CN rotation (70% of the kinetic energy). For thiscalculation, we do not alter the mass of the fragmenting partners for the determination of thetranslational energy. Furthermore, for both of these calculations, the initial wave packet and thepotential energy surfaces are the same as for the true ICN− simulations. Based on these results,
we conclude that the large degree of rotational excitation in the CN fragment following
photoexcitation to the 2Π1/2 excited state reflects the anisotropy of the repulsive wall and therelatively slow dissociation of ICN−.
An important characteristic of ground-state ICN− is a very low frequency, 70 cm−1 bend
vibration, arising in part from the very weak I–CN bond.66 At the temperature of the experiment(see Chapter II Sec. 2.2), there will be significant population in the ICN− bend states. Not
surprisingly, additional excitation in this mode will lead to a larger fraction of the excess energy
being partitioned into CN rotation. Thus, 〈Trot〉 exceeds 0.3 eV when the initial anion vibrational

52
wave packet is prepared with one quantum of excitation in the bend. In Table 3.2, we list the
expectation values of the three terms that comprise the Hamiltonian for a variety of energeticallyaccessible initial states of ICN−. We use νb to denote the number of quanta in the ICN bend, andνs for the number of quanta in the I–CN stretch. The results are analyzed at the initial excitation(top), at the point where 〈V〉 is minimized (middle), and after 600 fs (bottom). Note that theseenergies are averages based on the wave packet, and tails of the distribution will have more orless energy in rotation or translation.

Table 3.2 Energy decomposition for a variety of initial states, following excitation to the 2Π1/2
state. The various components of the energy are expectation values of the three terms in the
Hamiltonian.
Ttrans (eV)Trot (eV)Initial Excitation
3.25
0.87

≤ 0.01≤ 0.01

45
40
41
40
40
46

Region Near Potential Minimum
4.10
−0.14
3.91
−0.13
3.91
−0.11
3.85
−0.12
3.86
−0.11
4.11
−0.14

0.77
0.65
0.64
0.59
0.59
0.79

0.25
0.36
0.34
0.38
0.37
0.23

600
600
600
600
600
600

Asymptotic Region
18.08
0.00
16.50
0.00
16.11
0.00
15.60
0.00
15.36
0.00
18.02
0.00

0.71
0.57
0.54
0.50
0.48
0.74

0.17
0.30
0.33
0.36
0.37
0.15

State (νb, νs)Time (fs)(0, 0)

0(0, 0)(1, 0)(2, 0)(3, 0)(4, 0)(0, 1)(0, 0)(1, 0)(2, 0)(3, 0)(4, 0)(0, 1)

R (Å)V (eV)53
From the quantum dynamics calculations, the degree of rotational excitation imparted to
the CN fragment in the dissociation process is revealed. It seems quite possible to deposit~0.3 eV into CN rotation (see Table 3.2). This would lead to an upper limit for the CN rotational
levels, jCN, of ~35. Further, it is expected that some of the CN–Ar collision energy imparted to
the CN fragment will also end up in rotation. In addition, the CN–Ar collision is likely to occur
near the potential minimum on the excited-state surface. For the [I−–CN] complex formed,
R–T transfer should be slow, which effectively sequesters the rotational energy for a period oftime longer than the radiative lifetime of the excited state. Thus, there appears to be a mechanism
allowing the [I−–CN] complex to stay together for a sufficient time to radiatively decay to theground state, as is required to stabilize the anion and yield the observed one-atom caging in thephotodissociation of ICN−(Ar). Additional dynamics studies to further elucidate the mechanism
for energy transfer are anticipated.
The calculations of bare ICN− dynamics point to a mechanism for the one-atom cage
effect that we observe in ICN−(Ar). In a small portion of the excited population, collision of the
CN fragment with Ar reduces the excess energy remaining with the I− and CN components.
Additionally, the relatively slow dissociation of ICN− allows ample time to sample theanisotropic portions of the excited-state potential energy surface, thus populating a reasonable
degree of rotational excitation in the CN fragment. Taken together, these two processes tie up aconsiderable amount of energy, allowing the [I−–CN] complex a substantial excited-state lifetime
and, thus, a non-negligible radiative transition probability. This mechanism gains support fromphotodissociation studies of neutral ICN in a cryogenic matrix, detecting emission following
cage-induced trapping on a weakly bound excited-state surface.57

54
3.5 Conclusions
Despite the lack of a conical intersection, the dynamics of photodissociated, linear ICN− resultin a highly rotationally excited CN fragment. The ability of the ICN− chromophore to sequester
substantial amounts of energy in CN rotation enables both the single-solvent caging and the
production of highly solvated photodissociation products. In all cases, CN rotation (hindered orfree) is a crucial part of the intramolecular energy flow pathways and a major sink for internalenergy.

55

Chapter IV: Photofragmentation Dynamics of ICN−(CO2)n4.1 Introduction
To further understand the role solvent plays in dissociation dynamics of an anioniccluster, the photofragmentation of mass-selected ICN−(CO2)n was studied following
400, 500, and 600 nm excitation to the dissociative 2Π1/2 excited state of the chromophore(see Fig. 4.1). Previous studies33-35, 39, 69, 75-81 of anionic clusters report an increased probabilityof nonadiabatic dynamics contributing to the photofragmentation process due to changes in theelectronic structure of the chromophore. As seen in Chapter III, solvation of ICN− with one to
five Ar atoms leads to the observation of one-atom caging (n = 1) and highly-solvated I−-based
and ICN−-based photoproducts, but CN−-based products were absent. These observations are theresult of weakly-solvating labile Ar atoms and the significant amount (~20%) of the excitationenergy that is partitioned into CN rotation. Because Ar is readily polarized (α = 1.664 Å3),82 but
has neither a permanent dipole nor quadrupole moment, the dominant solute-solvent electrostaticinteraction is a short-range 

ion-induced dipole interaction. The Ar solvent binding energy

to the ICN− chromophore was measured67 to be ~50 meV, in agreement with experimental
studies in which Ar is used to reduce the internal energy content of a cluster by evaporative
cooling.83-85 With such a small solvent binding energy, solvation of the ICN− chromophore with
Ar results in only minor perturbations to potential surfaces at the ground state equilibriumgeometry of ICN−.

56
2

2

 1/2 /  3/2

I  + CN (A 2)+

I* + CN  (X 1 )3.0~

Energy (eV)

2.5
2.0

2

1.5

2

1.0

2

~

I  + CN (X 2 )
~

 1/2
 3/2

I + CN  (X 1 )~~X 2 +

0.5
0.0
3

4

5

6

7

8

R(I-CN COM) (Å)

Figure 4.1 Linear ICN− potential energy curves (based on Ref. 66) as a function of the distancefrom the iodine atom to the center-of-mass of the CN diatom. The ground-state wavefunction isshown as a black dashed line.

By performing similar photofragmentation experiments to those presented in Chapter III,the effect of long-range electrostatic interactions on the photodissociation of the ICN−
chromophore is examined using a CO2 molecular solvent. The similar mass of an Ar atom
(44 amu) and a CO2 molecule (40 amu) leads to similar solute-solvent kinematics. Thus,
differences in the dissociation dynamics of ICN−(Ar)n and ICN−(CO2)n are attributed to theelectrostatic interactions between the ICN− chromophore and the solvent. The CO2 molecule hasa large quadrupole moment ((−4.31±0.74)x10−40 C m2),86 with an approximate charge
distribution of +1 on the carbon atom and −½ on each oxygen. This results in not only along-range 

ion-quadrupole interaction between the ICN− chromophore and the CO2

solvent, but also a strong CO2–CO2 interaction that results in the formation a rigid cage around57
the chromophore. Previous studies of CO2 solvated dihalides34, 35, 39, 87, 88 report a CO2 solvent
binding energy of ~200–300 meV, approximately a factor of 4–5 times larger than that of asolvent Ar atom. As a result, the dynamics involved in the photofragmentation of ICN−(CO2)nare expected to differ significantly from those observed for ICN−(Ar)n. Following
photoexcitation of ICN−(CO2)n to the 2Π1/2 excited state, CN−-based and ICN−-based
photoproducts are observed as a result of increased probability of nonadiabatic transitions due tosolvation, similar to IBr−(CO2)n and ICl−(CO2)n.33, 35 However, in contrast with IBr−(CO2)n, andICl−(CO2)n, following photofragmentation of ICN−(CO2)n the yield of X−-based (X = Br, Cl, orCN) photoproducts dominates the ionic photoproduct distribution for most values of n studied
and recombined products are a minority. Comparison of the results of photofragmentation
studies on different solutes within a cluster of CO2 molecules provides evidence for the role ofinternal energy of the solute in the dissociation dynamics.
4.2 Experimental
The photofragmentation studies of ICN−(CO2)n were performed utilizing a dualtime-of-flight mass spectrometer (TOF MS) in conjunction with a nanosecond laser system. A
detailed description of the TOF MS and the laser system was presented in Chapter II; only detailsof the experiment relevant to ICN−(CO2)n photofragmentation studies will be presented. A gas
mixture, formed by passing CO2 at a pressure of 1–2 atm over solid ICN (synthesized by theexperimental procedure reported by Bak and Hillebert42) is pulsed through an 800 m orifice of aGeneral Valve nozzle (Series 9) operated at 50 Hz. Following electron impact from a 1 keVelectron beam, anions are formed via slow secondary-electron attachment. Anions are formed via
collisions with the neutral CO2 solvent and are then extracted perpendicularly into a58
Wiley-McLaren TOF MS41 by a pulsed electric field. After entering the TOF MS, the ions areaccelerated, steered, and focused before a fast potential switch49 re-references the anions to earth
ground. Directly before the Wiley-McLaren spatial focus, a pulsed mass gate deflects all ions,
except the desired anionic clusters. The mass-selected anions are then photoexcited via ananosecond laser pulse at the Wiley-McLaren spatial focus.
Pulsed 430 to 650 nm light is generated via a nanosecond laser system (Coherent Infinity
XPO), with output energy of up to 5 mJ/pulse. The photoproducts that result from interaction of
the parent cluster with laser radiation enter a single-stage secondary reflectron mass spectrometerand are detected by a dual microchannel plate (MCP) detector 8 off the parent ion beam axis.
The electric potential applied to the reflectron is scaled in proportion to the mass ratio of theionic photofragment and parent ion. This adjustment refocuses the trajectories of the ionicphotoproducts onto the MCP detector. A mass spectrum of the ionic photofragments detected on
the off-axis MCPs is amplified, digitized, and averaged after which the area under of each ionicphotoproduct peak is determined. The mass spectrum is again obtained under the same
conditions, but with the laser radiation blocked to provide a background trace for subtraction.
The ionic photoproduct distributions and the relative cross section for production of ionicphotoproducts reveal the effects of solvation on the photodissociation dynamics of theICN−(CO2)n.
4.3 Photodissociation Action Spectra
The effects of solvation with CO2 on the electronic structure of ICN− were investigatedby measuring the photodissociation action spectra of ICN−(CO2). Analysis of the action spectra
provides the relative cross section for the production of each ionic photoproduct following

59
excitation of ICN−(CO2)n to the 2Π1/2 excited state. As detailed in Chapter II, the cross section
reported is a relative cross section due to unmeasured scaling factors such as detection efficiencyof the photoproduct and parent ions and spatial overlap of the ion-laser interaction.
Figures 4.2 and 4.3 show the relative cross section, σrel(λ), for the production of eachionic photoproduct following 430 to 650 nm excitation of ICN− and ICN−(CO2). The error limits
are the average percent error for each photoproduct across the entire spectral range and thevalues are given in the captions for Fig. 4.2 and 4.3. Comparison of the cross sections reveals aslight shift in the location of the peak and an increased width for the I− and CN− products from
ICN−(CO2)n photodissociation compared to the I− products from ICN− photodissociation. For
example, the Gaussian-like shape of the relative cross section for production of I− photoproducts
from ICN− photodissociation has a full-width-at-half-the-maximum (FWHM) of roughly 60 nm,
whereas the FWHM for the relative cross section for I− from ICN−(CO2) photodissociation isroughly 80 nm.

60
3.0


(a)



ICN : I Products

 rel () (arb. units)

2.5

2.0

1.5

1.0

0.5

0.0
1.0



(b)



 rel ( ) (arb. units)

ICN (CO2 ): I Products
ICN  (CO2 ): I (CO2 ) Products

0.8

0.6

0.4

0.2

0.0
430

450

470

490

510

530

550

570

590

610

630

650

Wavelength (nm)Figure 4.2 Relative cross sections (σrel(λ)) for the production of I− and I−(CO2) photoproducts(b) following excitation of ICN−(CO2) to the 2Π1/2 excited state. The relative cross section for the
production of I− photoproducts (a) following excitation of ICN− to the 2Π1/2 excited state isshown as a reference. The error limits are the average percent error for each photoproduct acrossthe entire spectral range and are defined as follows: 10% for I− from photodissociation of ICN−,
and 10% and 20% for I− and I−(CO2) from photodissociation of ICN−(CO2), respectively.

61
0.14
0.12

 rel () (arb. units)(a)

ICN  : CN Products

0.10
0.08
0.06
0.04
0.02
0.00

ICN  (CO2 ): CN  Products (b)0.5 rel () (arb. units)

ICN  (CO2 ): CN  (CO2 ) Products
0.4

0.3

0.2

0.1

0.0
430

450

470

490

510

530

550

570

590

610

630

650

Wavelength (nm)Figure 4.3 Relative cross sections (σrel(λ)) for the production of CN− and CN−(CO2)photoproducts (b) following excitation of ICN−(CO2) to the 2Π1/2 excited state. The relative crosssection for the production of CN− photoproducts (a) following excitation of ICN− to the 2Π1/2
excited state is shown as a reference. The error limits are the average percent error for eachphotoproduct across the entire spectral range and are defined as follows: 15% for CN− fromphotodissociation of ICN−, and 10% and 20% for CN− and CN−(CO2) from photodissociation of
ICN−(CO2), respectively.

62
The small red shift and increase in the width of the relative cross section for the
production of I− and CN− products from photodissociation of ICN−(CO2) are likely the result of achange in the charge distribution near the ground state equilibrium geometry of ICN− following
excitation from the ground state to the 2Π1/2 excited state. Because the asymptotes of the X̃ 2Σ+and 2Σ+ states both correspond to CN− photoproducts and the photon energy following
430 to 650 nm excitation is insufficient to result in a 2Π1/2/2Π3/2 ← X̃ transition (2Π1/2/2Π3/2 mixed
state designation used as defined by McCoy66), observation of the I− and I−(CO2) photoproducts
must be the result of dissociation on the 2Π1/2 excited state (purple in Fig. 4.1) that corresponds toI− + CN photoproducts. Multi-reference configuration-interaction calculations performed by
McCoy66 show that, near the equilibrium geometry on the ground state of the ICN−
chromophore, roughly 44% of the excess charge is localized on CN. For this same geometry on
the 2Π1/2 excited state, ~80% of the excess charge is localized on the CN, even though theasymptotic photoproducts for this state are I− + CN. (This change in charge distribution from theground-state equilibrium geometry to the asymptote of the 2Π1/2 excited state has a drastic effecton the photofragmentation dynamics, as discussed in Sec. 4.4.) Because solvation of a smaller
anion is energetically favored,33, 77, 78 solvation of CN− (ionic radius = 1.77 Å)89, 90 is favored
over solvation of I− (ionic radius = 2.20 Å).91 Since the charge is more localized on the CN
diatom on the 2Π1/2 excited state, the 2Π1/2 state should be more stabilized than the ground state,
and a red shift in the cross section is expected. Alternatively, if the charge localized on theiodine, a blue-shift in the cross section would occur because of solvation of the larger iodine isenergetically unfavorable. Therefore, it is proposed that solvation of the ICN− chromophore with
CO2 results in a solvated 2Π1/2 excited state that is stabilized at large internuclear distances. This

63
result illustrates the relative effect of the long-range electrostatic interactions between ICN− andCO2 on the ground state and 2Π1/2 excited state of the ICN− chromophore.
4.4 Photofragmentation of ICN−(CO2)nAs with previously studied dihalide clusters,33-35 it was expected that thephotofragmentation of ICN−(CO2)n would result in three dissociation products following
excitation to the 2Π1/2 excited state: I−-based products from direct adiabatic dissociation,CN−-based products from nonadiabatic transitions, and recombined ICN−-based photoproducts.
The distribution of these ionic photoproducts is predicted to vary with excitation energy (due todifferent amounts of kinetic energy release) and the size of the parent cluster anion. Similar tophotofragmentation studies36 of I2−(Ar)n and I2−(CO2)n, the caging efficiencies of the Ar and CO2
solvents were expected to be drastically different.
4.4.1 Photofragmentation Data at 400, 500, and 600 nm
The photoproduct distributions of each ionic dissociation channel (I−-based, CN−-based,
or ICN−-based) following 500 and 600 nm excitation of ICN−(CO2)n as a function of the parent
cluster size are shown in Fig. 4.4 an 4.5, respectively. The total yield for each ionic photoproductchannel, in bold red in Tables 4.1 and 4.2, is plotted in the figures. Additionally, the yield of all
photoproducts observed following excitation of ICN−(CO2)n at these two wavelengths istabulated in Table 4.1 and 4.2. All excitation energies utilized are larger than the measured bonddissociation energy67 of ground electronic-state ICN−, Do(I–CN−) = 0.84(+0.04/−0.02) eV, andlaser power dependence studies are performed to ensure photofragmentation was the result of a64
one-photon process. Additionally, typical statistical errors in the ionic photoproduct distributions
are roughly 5%.

Ionic Photoproduct Yield

1.0

0.8

I  -basedCN -based
ICN -based

0.6

0.4

0.2

0.0
0

2

4

6

8

10

12

14

16

18

Number of CO2 in Parent Cluster, nFigure 4.4 Ionic photoproduct distribution of ICN−(CO2)n upon one-photon 500 nm excitation to
the 2Π1/2 excited state. The error bars correspond to a 5% standard deviation and are plotted foreach ionic product distribution.

65
Table 4.1 One-photon ionic photoproducts following 500 nm excitation of ICN−(CO2)n.
Parent Cluster
ICN−

CN−-Based

ICN (CO2)

I− = 97%3%

97%

CN = 31%CN−(CO2) = 1%

I = 61%I−(CO2) = 7%

32%

68%

CN = 29%
CN− (CO2) = 42%

I = 12%I−(CO2) = 15%

ICN− = 2%

71%

27%

2%

CN = 19%CN−(CO2) = 58%

I (CO2) = 7%

ICN− = 16%

77%

7%

16%

CN = 3%CN−(CO2) = 39%
CN−(CO2)2 = 14%

I (CO2)2 = 4%

ICN− = 40%.

56%

4%

40%

0%

13%

84%

0%

16%

CN (CO2) = 1%
CN−(CO2)2 = 10%
CN−(CO2)3 = 78%
CN−(CO2)4 = 4%CN−(CO2)5 = 1%

I = 1%
I−(CO2) = 1%

ICN− = 2%
ICN−(CO2) = 2%

94%

2%

4%

−

ICN−(CO2)2−

ICN−(CO2)3−

ICN−(CO2)4−

ICN−(CO2)5

−

−

−

−

CN (CO2) = 3%CN−(CO2)2 = 56%
CN−(CO2)3 = 24%
CN−(CO2)4 = 1%−

ICN−(CO2)7−

CN (CO2) = 17%
CN−(CO2)2 = 69%
CN−(CO2)3 = 1%

87%
ICN−(CO2)6

ICN−-BasedCN− = 3%−−

I−-Based

−

0%

0%

ICN− = 12%
ICN−(CO2) = 1%

ICN− = 14%
ICN−(CO2) = 2%

66

ICN−(CO2)8

CN−(CO2)3 = 28%
CN−(CO2)4 = 35%
CN−(CO2)5 = 20%
CN−(CO2)6 = 3%I−(CO2)2 = 1%

ICN− = 7%
ICN−(CO2) = 4%
ICN−(CO2)2 = 2%

86%

1%

13%

CN (CO2)3 = 1%
CN−(CO2)4 = 34%
CN−(CO2)5 = 6%CN−(CO2)6 = 50%
CN−(CO2)7 = 2%

I (CO2)2 = 1%

ICN− = 2%
ICN−(CO2) = 2%
ICN−(CO2)2 = 2%

93%

1%

6%

CN (CO2)4 = 1%
CN−(CO2)5 = 4%CN−(CO2)6 = 43%
CN−(CO2)7 = 46%

I (CO2)4 = 1%

ICN− = 1%
ICN−(CO2) = 1%
ICN−(CO2)2 = 2%
ICN−(CO2)3 = 1%

94%

1%

5%

CN (CO2)6 = 1%
CN−(CO2)7 = 86%
CN−(CO2)8 = 9%

I (CO2)5 = 1%

ICN−(CO2) = 1%
ICN−(CO2)2 = 1%

96%

2%

2%

98%

0%

2%

CN (CO2)6 = 1%
CN−(CO2)7 = 1%
CN−(CO2)8 = 64%
CN−(CO2)9 = 13%

I (CO2)6 = 3%

ICN−(CO2)2 = 1%
ICN−(CO2)3 = 5%
ICN−(CO2)4 = 10%
ICN−(CO2)5 = 3%

79%

3%

18%

0%

23%

−

ICN−(CO2)9−ICN−(CO2)10−

ICN−(CO2)11−

ICN−(CO2)12−

ICN−(CO2)14−−

CN (CO2)7 = 11%
CN−(CO2)8 = 87%−

ICN−(CO2)13−−

CN (CO2)8 = 3%CN−(CO2)9 = 71%
CN−(CO2)10 = 3%

77%

ICN−(CO2)2 = 1%
ICN−(CO2)3 = 1%

ICN−(CO2)3 = 2%
ICN−(CO2)4 = 11%
ICN−(CO2)5 = 7%
ICN−(CO2)6 = 3%

67

ICN−(CO2)15

CN−(CO2)9 = 17%
CN−(CO2)10 = 33%
CN−(CO2)11 = 8%

I−(CO2)5 = 1%
I−(CO2)6 = 2%
I−(CO2)7 = 2%

ICN−(CO2)4 = 8%
ICN (CO2)5 = 17%
ICN−(CO2)6 = 11%
ICN−(CO2)7 = 1%

58%

5%

37%

CN (CO2)10 = 6%CN−(CO2)11 = 35%
CN−(CO2)12 = 15%

I (CO2)8 = 2%

ICN−(CO2)4 = 1%
ICN−(CO2)5 = 10%
ICN−(CO2)6 = 16%
ICN−(CO2)7 = 13%
ICN−(CO2)8 = 2%

56%

2%

42%

CN (CO2)12 = 42%
CN−(CO2)13 = 14%

I (CO2)7 = 2%
I−(CO2)8 = 2%
I−(CO2)9 = 2%

ICN−(CO2)5 = 2%
ICN−(CO2)6 = 14%
ICN−(CO2)7 = 17%
ICN−(CO2)8 = 5%

56%

6%

38%

CN−(CO2)12 = 2%
CN−(CO2)13 = 37%
CN−(CO2)14 = 12%

I−(CO2)7 = 1%
I−(CO2)8 = 1%
I−(CO2)9 = 1%

ICN−(CO2)6 = 5%
ICN−(CO2)7 = 19%
ICN−(CO2)8 = 15%
ICN−(CO2)9 = 5%
ICN−(CO2)10 = 2%

51%

3%

46%

−

ICN−(CO2)16−

ICN−(CO2)17

ICN−(CO2)18

−

−

68

Ionic Photoproduct Yield

1.0

0.8

I  -basedCN -based
ICN -based

0.6

0.4

0.2

0.0
0

2

4

6

8

10

12

14

16

Number of CO2 in Parent Cluster, nFigure 4.5 Ionic photoproduct distribution of ICN−(CO2)n upon one-photon 600 nm excitation to
the 2Π1/2 excited state. The error bars correspond to a 5% standard deviation and are plotted foreach ionic product distribution.

69
Table 4.2 One-photon ionic photoproducts following 600 nm excitation of ICN−(CO2)n.
Parent Cluster
ICN−

CN−-Based

ICN (CO2)

I− = 85%15%

85%

CN = 43%
CN−(CO2) = 23%

I = 34%66%

34%

CN = 35%
CN−(CO2) = 49%

I (CO2) = 4%

ICN− = 12%

84%

4%

12%

−

ICN−(CO2)2−

ICN−(CO2)3

ICN−(CO2)5

−

0%

0%

0%

ICN− = 43%43%

CN− = 1%CN−(CO2) = 12%
CN−(CO2)2 = 64%

I− = 3%

77%

3%

20%

CN−(CO2) = 5%CN−(CO2)2 = 42%
CN−(CO2)3 = 33%
CN−(CO2)4 = 4%

84%

ICN−(CO2)6−

CN = 10%CN−(CO2) = 45%
CN−(CO2)2 = 2%

57%

ICN−(CO2)4

ICN−-BasedCN− = 15%−−

I−-Based

ICN− = 19%
ICN−(CO2) = 1%

ICN− = 13%
ICN−(CO2) = 3%CN−(CO2) = 2%
CN−(CO2)2 = 3%CN−(CO2)3 = 59%
CN−(CO2)4 = 10%
CN−(CO2)5 = 9%I−(CO2)2 = 1%

0%

16%

83%

1%

16%

ICN− = 10%
ICN−(CO2) = 4%
ICN−(CO2)2 = 2%

70

ICN−(CO2)7

CN−(CO2) = 1%
CN−(CO2)2 = 2%
CN−(CO2)3 = 1%
CN−(CO2)4 = 49%
CN−(CO2)5 = 11%
CN−(CO2)6 = 19%

I−(CO2)3 = 1%

ICN− = 6%
ICN−(CO2) = 6%
ICN−(CO2)2 = 3%
ICN−(CO2)3 = 1%

83%

1%

16%

CN (CO2)3 = 1%
CN−(CO2)4 = 1%
CN−(CO2)5 = 5%CN−(CO2)6 = 55%
CN−(CO2)7 = 27%

I (CO2)4 = 1%
I−(CO2)5 = 1%

ICN− = 1%
ICN−(CO2) = 4%
ICN−(CO2)2 = 2%
ICN−(CO2)3 = 2%

89%

2%

9%

CN (CO2)3 = 2%
CN−(CO2)4 = 1%
CN−(CO2)5 = 1%
CN−(CO2)6 = 2%
CN−(CO2)7 = 80%

I (CO2)5 = 2%

ICN−(CO2) = 3%
ICN−(CO2)2 = 4%
ICN−(CO2)3 = 3%
ICN−(CO2)4 = 2%

86%

2%

12%

−

ICN−(CO2)8−ICN−(CO2)9ICN−(CO2)10

CN−(CO2)3 = 2%
CN−(CO2)4 = 2%
CN−(CO2)5 = 2%
CN−(CO2)6 = 2%
CN−(CO2)7 = 12%
CN−(CO2)8 = 68%
CN−(CO2)9 = 2%

90%−

ICN−(CO2)11−−

ICN−(CO2) = 1%
ICN−(CO2)2 = 3%
ICN−(CO2)3 = 4%
ICN−(CO2)4 = 2%

10%

0%

CN (CO2)5 = 2%
CN−(CO2)6 = 3%CN−(CO2)7 = 3%CN−(CO2)8 = 66%
CN−(CO2)9 = 7%

I−(CO2)6 = 1%

ICN−(CO2) = 1%
ICN−(CO2)2 = 2%
ICN−(CO2)3 = 6%
ICN−(CO2)4 = 4%
ICN−(CO2)5 = 1%
ICN−(CO2)6 = 3%

81%

2%

17%

71

ICN−(CO2)12

CN−(CO2)6 = 4%CN−(CO2)7 = 6%CN−(CO2)8 = 5%CN−(CO2)9 = 58%

I−(CO2)7 = 3%

ICN−(CO2)3 = 5%
ICN−(CO2)4 = 10%
ICN−(CO2)5 = 5%
ICN−(CO2)6 = 2%
ICN−(CO2)7 = 2%

73%

3%

24%

CN (CO2)8 = 2%
CN−(CO2)9 = 27%
CN−(CO2)10 = 23%

I (CO2)7 = 2%
I−(CO2)8 = 1%

ICN−(CO2)3 = 2%
ICN−(CO2)4 = 15%
ICN−(CO2)5 = 19%
ICN−(CO2)6 = 9%

52%

3%

45%

51%

0%

49%

CN (CO2)9 = 7%
CN−(CO2)10 = 6%CN−(CO2)11 = 6%CN−(CO2)12 = 31%

I (CO2)7 = 1%
I−(CO2)8 = 1%

ICN−(CO2)5 = 1%
ICN−(CO2)6 = 20%
ICN−(CO2)7 = 18%
ICN−(CO2)8 = 5%

54%

2%

44%

−

ICN−(CO2)13−

ICN−(CO2)14

CN (CO2)8 = 5%CN−(CO2)9 = 7%
CN−(CO2)10 = 11%
CN−(CO2)11 = 28%−

ICN−(CO2)15−−

ICN−(CO2)3 = 1%
ICN−(CO2)4 = 2%
ICN−(CO2)5 = 19%
ICN−(CO2)6 = 16%
ICN−(CO2)7 = 8%
ICN−(CO2)8 = 3%

Upon absorption of a 500 or 600 nm photon, the ICN− chromophore is excited to the 2Π1/2
state, for which the adiabatic asymptote corresponds to the formation of I− and CN (X̃ 2Σ)photoproducts. Thus, direct dissociation following 500 or 600 nm excitation of ICN− results in adominant I− photoproduct; 97% of the observed ionic photoproducts following 500 nm excitationof ICN− are I−. Following the addition of one solvent CO2 molecule, excitation to the 2Π1/2
excited state via 500 or 600 nm light results in the observation of an increased yield ofCN−-based products, with a corresponding decrease in the yield of I−-based products. Addition of
the second CO2 solvent leads to the dominance of CN−- based products and the observation of

72
recombined ICN− for both excitation wavelengths. As the number of CO2 solvents is increased,
the CN−-based products continue to dominate the ionic photoproduct distribution, and the yieldof I−-based photoproducts decreases rapidly, reaching zero by n = 5 for both wavelengths.
Remarkably, the yield of recombined products reaches a local maximum of roughly 40% at n = 4
and n = 3 for 500 and 600 nm excitation, respectively. Following the maximum yield in therecombined products, the yield begins to decrease to roughly 10–20% for 5 ≤ n ≤ 11. This trend
in the yield of recombined products is similar to results observed following photofragmentationof ICl−(CO2)n, n = 1–10.33, 77 For ICN−(CO2)12–15, the yield of recombined photoproducts begins
to increase again, but never dominates the observed ionic photoproducts for either wavelength.
This is in stark contrast to the results of photofragmentation of IBr−(CO2)n ≥ 8,35 for which only
recombined products were observed following photoexcitation to the Ã′ 2Π1/2 excited state ofIBr−.
Additionally, photofragmentation studies were also performed following 400 nm
excitation of ICN−(CO2)n (n = 0–8) to the determine the effect of an increased kinetic energy
release on the yield of recombined photoproducts. Figure 4.6 and Table 4.3 report the ionicphotoproduct distributions following 400 nm excitation of ICN−(CO2)n; the values in bold red inTable 4.3 are plotted in Figure 4.6 as a function of parent cluster size. Excitation with 400 nm
provides roughly 0.6 eV more asymptotic kinetic energy release on the 2Π1/2 excited state than
500 nm, for a total of roughly 1.45 eV. Interestingly, excitation of ICN− using 400 nm light
results in roughly 65% I− and roughly 35% CN− photoproducts. The decrease in the yield of I−
photoproducts from photodissociation of the ICN− chromophore compared to 500 nm excitationsuggests that more than one excited state is taking part in the dissociation dynamics following
400 nm excitation. In marked contrast to the results for 500 and 600 nm, the addition of one CO2

73solvent molecule results in an increase in the yield of I−-based photoproducts. Furthermore,
recombined ICN− photoproducts are not observed until n = 3. Following the addition of the fifth
CO2 solvent, the trends observed following 400 nm excitation are qualitatively similar to thoseobserved following 500 and 600 nm excitation, i.e. CN−-based products dominate the ionicphotoproduct distribution. Explanation and comparison of the photofragmentation results foreach excitation wavelength are presented in Sections 4.42, 4.4.3, and 4.4.4.

Ionic Photoproduct Yield

1.0

I  -basedCN -based
ICN  -based

0.8

0.6

0.4

0.2

0.0
0

1

2

3

4

5

6

7

8

9

Number of CO2 in Parent Cluster, nFigure 4.6 Ionic photoproduct distribution of ICN−(CO2)n upon one-photon 400 nm excitation to
the 2Π1/2 excited state. The error bars correspond to a 5% standard deviation and are plotted foreach ionic product distribution.

74
Table 4.3 One-photon ionic photoproducts following 400 nm excitation of ICN−(CO2)n.
Parent Cluster−

ICN

CN−-Based

I−-Based

ICN−-BasedCN− = 37%

I− = 63%37%

63%

CN = 28%

I = 72%28%

72%

CN− = 16%CN−(CO2) = 3%

I− = 28%I−(CO2) = 53%

19%

81%

CN = 7%CN−(CO2) = 14%

I = 10%I−(CO2) = 58%

ICN− = 11%

21%

68%

11%

CN = 4%CN−(CO2) = 29%

I = 3%
I−(CO2) = 26%
I−(CO2)2 = 19%

ICN− = 10%
ICN−(CO2) = 9%

33%

48%

19%

CN (CO2) = 39%
CN−(CO2)2 = 4%

I (CO2) = 13%
I−(CO2)2 = 23%
I−(CO2)3 = 3%

ICN− = 11%
ICN−(CO2) = 7%

39%

18%

−

−

0%

−

ICN (CO2)

ICN−(CO2)2−

ICN−(CO2)3−

ICN−(CO2)4−

ICN−(CO2)543%

ICN−(CO2)6

CN−(CO2) = 46%
CN−(CO2)2 = 13%
CN−(CO2)3 = 3%

62%

ICN−(CO2)7

−

−

−

I−(CO2) = 2%
I−(CO2)2 = 7%0%0%ICN− = 17%
ICN−(CO2) = 6%
ICN−(CO2)2 = 6%CN−(CO2) = 21%CN−(CO2)2 = 34%
CN−(CO2)3 = 2%

I−(CO2)2 = 1%
I−(CO2)3 = 1%

9%

16%

57%

2%

41%

ICN− = 19%
ICN−(CO2) = 7%
ICN−(CO2)2 = 7%
ICN−(CO2)3 = 8%

75

ICN−(CO2)8

CN−(CO2) = 6%CN−(CO2)2 = 48%
CN−(CO2)3 = 18%
CN−(CO2)4 = 1%

I−(CO2)2 = 1%
I−(CO2)3 = 1%

ICN− = 11%
ICN−(CO2) = 4%
ICN−(CO2)2 = 4%
ICN−(CO2)3 = 6%

73%

2%

25%

4.4.2 Efficiency of Production of CN−-based Photofragments
Photofragmentation experiments reveal that the dissociation dynamics of solvated ICN−
are significantly affected by excitation wavelength and the solvent species utilized. Contrary to
the results of the photofragmentation of ICN−(Ar)n (Chapter III), the CN−-based photoproducts
dominate the ionic photoproduct distribution following 400, 500, and 600 nm excitation of
ICN−(CO2)n. In previous photofragmentation studies33, 35 of IBr−(CO2)n and ICl−(CO2)n,excitation to the Ã′ 2Π1/2 excited state, which correlates to dissociation into I− photoproducts,
resulted in a significant fraction of Br−-based and Cl−-based photoproducts observed for n = 1–6.
Observation of the charge-transfer Br−-based photoproducts following excitation of IBr−(CO2)1-6
suggested that the energy gap between the 2Π1/2 excited state and the ground state wassufficiently reduced as solvation increased to lead to nonadiabatic transitions and dissociation on
the ground state. Conversely, molecular dynamics simulations by Faeder and Parson77 showedthat following excitation of ICl−(CO2)n to the 2Π1/2 excited state the observed Cl−-based products
are accompanied by I*(2P1/2) products as a result of dissociation on the 2Σ+ excited state. The
differing dynamics for IBr− and ICl− are a result of the contrasting differential solvation of theionic fragments of the chromophore. Similar processes are proposed following photodissociationof ICN−(CO2)n which contribute to the dissociation dynamics as a function of parent cluster size.

76
To provide a rationale for the observation of an increased yield of CN−-based products, it
is informative to examine the ICN− potential energy curves. Calculations of the ICN− potential
energy curves performed by McCoy66 show that roughly 80% of the excess charge is localizedon the CN diatom on the 2Π1/2 excited state near the ground state equilibrium geometry. For thesame geometry, roughly 80% of the excess charge in ICN− is localized on the iodine when on the
2 +Σ excited state. However, as the calculations also show, the asymptotic dissociation products ofthese excited states correspond to I− and CN on the 2Π1/2 state and I*(2P1/2) and CN− on the 2Σ+
excited state. Therefore, as shown in Fig. 4.7, it is proposed that a nonadiabatic crossing exists
between the 2Π1/2 and 2Σ+ states at roughly 5 Å.
Photoexcitation of ICN− to the 2Π1/2 excited state leads to the observation of a minor
contribution from CN− products. For example, following 500 nm excitation, roughly 3% of theionic photoproducts observed are CN− and excitation of ICN− with 400 nm light results inroughly 35% CN− products. Because the energy gap between the 2Π1/2 excited state and theground state is too large (0.8 eV) for nonadiabatic transitions to be favorable, the observation ofCN− products suggests that following excitation of ICN− the dissociating chromophoretransitions to the 2Σ+ excited state from the 2Π1/2 state and dissociates into I*(2P1/2) and CN−
photoproducts. Following excitation of either IBr− or ICl−, 100% of the ionic photoproducts
observed were I− as a result of direct dissociation on the 2Π1/2 excited state.33, 35, 39 However,
because of the nonadiabatic crossing between the 2Π1/2 and 2Σ+ excited state of ICN−, a minor
contribution of charge-transfer CN− photoproducts are observed, even without solvation of theICN− chromophore.

77
3.0

Energy (eV)

2.5
2



2.0
1.5

+

~

I* + CN  (X 1 )~ 2

I + CN (X  )

2

 1/2

1.0

I + CN  (X 1 )
~

0.5

~

X 2 +

0.0
3

4

5

6

7

8

R(I-CN COM) (Å)

Figure 4.7 Linear ICN− potential energy curves (based on Ref. 66) for the X̃ 2Σ+, 2Π1/2, and 2Σ+states as a function of the distance from the iodine atom to the center-of-mass of the CN diatom.
The proposed nonadiabatic crossing is represented by two selected diabatic curves shown asdashed lines. The color of the text for the ground and excited state photoproducts correlates toadiabatic dissociation of the chromophore.

Following solvation of the ICN− chromophore with CO2, the yield of CN−-based
photoproducts increases to ~30% for 500 nm. The increased yield of CN−-based products forn ~ 1–3 is likely the result of stabilization of the charge-transfer asymptotes with respect to the I−
asymptotes via the differential solvation of I− and CN−, similar to ICl−(CO2)n.33, 77 Simulations by
Faeder and Parson77 showed that adiabatic dissociation occurred on the 2Σ+ excited state
following excitation of ICl−(CO2)n to the 2Π1/2 state. They concluded that the energy difference inthe solvation of I− versus Cl− (~0.14 eV per CO2) lead to the stabilization of the 2Σ+ asymptote atlarge internuclear distances with respect to the 2Π1/2 asymptote resulting in I*(2P1/2) and Cl−
products. Because the size difference between I− and CN− (discussed in Sec. 4.3) is similar to the78size difference between I− and Cl−, solvation of ICN− with CO2 will likely result in similardifferential solvation. Thus, the I*(2P1/2) and solvated CN− asymptote of the 2Σ+ excited state is
expected to be stabilized, which likely leads to the increased CN− yield observed. In addition, thepresence of the nonadiabatic crossing between the 2Π1/2 and 2Σ+ excited states may further
increase the yield of CN−-based products. Both of these processes can occur at values ofn (n ~ 1–3) for which nonadiabatic transitions to the ground state are still energeticallyunfavorable due to the large energy gap (0.8 eV) between the 2Π1/2 excited state and the groundstate asymptotes. Therefore, the observation of the significant yield of CN−-based photoproducts
following excitation of ICN−(CO2)n (n ~ 1–3) is likely a result of excited-state transitions
induced by differential solvation that lead to I*(2P1/2) and CN− products following dissociationon the 2Σ+ excited state, similar to ICl−(CO2)n.77
For all n studied (n = 1–12), Faeder and Parson showed77 all Cl−-based products are theresult of adiabatic dissociation on the 2Σ+ excited state for ICl−(CO2)n. Evidence for this adiabatic
process is revealed by analysis of the average number of solvent CO2 lost for the Cl−-based
products as a function of the parent cluster size, as seen in Fig. 4.8. Because only one or twosolvent CO2 are lost on average for n = 1–10, dissociation must occur on the 2Σ+ excited state andnot on the ground state because a nonadiabatic transition would require dissipation of roughly
0.7 eV of additional energy. However, as seen in Fig. 4.8, the CN−-based products lose roughly
2–4 solvent CO2 molecules following 500 nm excitation of ICN−(CO2)3-10. This suggests a
different dissociation mechanism is utilized and that the CN−-based products are observed forn ≥ 3 as a result of nonadiabatic transitions from the 2Π1/2 excited state to the ground state andthe coincident solvent evaporation for ICN−(CO2)n ≥ 3. Thompson et al.39 showed that
nonadiabatic transitions from the 2Π1/2 excited state the X̃ 2Σ+ state was the sole dynamical

79
process that contributed to the observation of Br−-based products following excitation ofIBr−(CO2)n. With only one solvent CO2 molecule, Br−-based products are observed and the yield
increases with increased solvation. Similar dynamics are likely to occur following
photofragmentation of ICN−(CO2)n; however, because of the large energy gap between the 2Π1/2
and X̃ 2Σ+ asymptotes (~0.8 eV for ICN− compared to ~0.3 eV for IBr−),39 nonadiabatic
transitions to the ground state are not likely to occur until n ~ 3–4. Thus, evidence for two
dynamical processes, dissociation on the 2Σ+ excited state and nonadiabatic transitions to theground state, resulting in the observation of CN−-based products is observed following excitationof ICN−(CO2)n to the 2Π1/2 excited state, with contribution of each dissociation mechanism
varying as a function of parent cluster size.

80

Average Number of CO2 Lost

5

4

3

2

1

CN  -based: 500 nm
Cl -based: 644 nm
0
0

1

2

3

4

5

6

7

8

9

10

11

Number of CO2 in Parent Cluster, nFigure 4.8 The average number of CO2 lost from CN−-based photoproducts as a function ofparent cluster size following 500 nm excitation of ICN−(CO2)n (n = 0–10). The number of CO2
lost from Cl−-based products following 644 nm excitation of ICl−(CO2)n are also shown forcomparison (Ref. 33). The solid-black diagonal line corresponds to the evaporation of all solvent
CO2 molecules from the parent cluster and is shown as a guide. The error bars on the CN−-based
photoproduct distribution correspond to a 5% standard deviation.

4.4.3 Recombination as a Function of Excitation Energy
Photofragmentation of ICN−(CO2)n following 400, 500, and 600 nm excitation results in a
rapid increase in the yield of recombined products to a peak, followed by a subsequent decrease
as n increases. (For brevity, nmax is defined as the value of n at which the maximum in the yieldof recombined products occurs). Additionally, the maximum in the yield shifts to larger values of
nmax as excitation energy is increased. This result is contrary to what might be expected assolvation of the chromophore increases, leading to an increasingly caged chromophore.

81
Figure 4.9 shows the yield of recombined photoproducts following 400, 500, and 600 nm
excitation of ICN−(CO2)n.

Yield of Recombined Products

0.6

400 nm
500 nm
600 nm

0.5

0.4

0.3

0.2

0.1

0.0
0

1

2

3

4

5

6

7

8

9

Number of CO2 in Parent Cluster, nFigure 4.9 Yield of recombined ICN−-based photoproducts as a function of parent cluster size
following 400, 500, and 600 nm excitation. The error bars correspond to a 5% standard
deviation.

For recombination of the dissociating chromophore to occur following excitation to the2Π1/2 excited state, the chromophore must undergo a transition from the excited state to theground state and recombine in the ground state well. Therefore, a maximum in the yield ofrecombined photoproducts is not expected until solvation of the ICN− chromophore with CO2
sufficiently reduces the 0.8 eV energy gap between the 2Π1/2 and X̃ 2Σ+ states to enable atransition. As proposed in Sec. 4.4.2, nonadiabatic dissociation on the 2Σ+ excited state asymptote
is the dominant source of CN−-based products until n ≈ 4 when nonadiabatic transitions to the82
ground state are proposed to be probable. This expectation is in good agreement with the peak of
the yield of recombined products occurring at nmax = 7, 4, and 3, Fig. 4.9, following
400, 500, and 600 nm excitation, but does not explain the source of the shifting peak.
Examination of the proposed energetics of solvation provides a qualitative explanation of
the shifting maximum in the yield of recombined photoproducts as a function of excitationenergy. Since solvation of the smaller CN− is energetically favored over solvation of I−, it is
proposed that a well is formed on the ground state, at large internuclear distances, corresponding
to solvated CN−, see Fig. 4.10. As the number of CO2 solvents around the CN− increases, thedepth of the well will correspondingly increase, trapping the dissociating chromophore following
a nonadiabatic transition. As the excitation energy increases, the photofragment gains sufficientkinetic energy to overcome the barrier to recombination; escaping the solvated CN− well.
However, the solvated CN− well increasingly traps the dissociating chromophore as the numberof CO2 solvents increases, resulting in the eventual decrease in the yield of recombined
photoproducts. A similar process was proposed by Nadal et al.33 in the photofragmentation study
of ICl−(CO2)n. They proposed that a significant amount of the dissociating chromophores
following excitation of ICl− to the 2Π1/2 excited state access the well on the ground state
introduced by solvation of Cl−. Additionally, their qualitative simulations33 showed that theconcerted motion of all solvent molecules that is required to reach the ground state global
minimum, corresponding to recombined products, from the solvated Cl− well, is unlikely to occur
on the time scale of the experiment (~10 μs). Although the dynamics are similar, the additional
internal degrees of freedom in the ICN− chromophore may act as an energy sink, impacting theprobability of escape of dissociating clusters from the solvated CN− well. Furthermore, an83additional process may contribute to the development and depth of a well on the ground statefrom solvation of CN−; namely, the photo-initiated intracluster reaction of CN− and CO2.

2

Π1/2

Energy (eV)Nonadiabatic Transition

Three
Particle~

X 2 +

Solvated CN
Well

Recombined ICN−
R(I-CN COM) (Å)
Figure 4.10 Schematic of linear ICN− ground state (solid black) and 2Π1/2 excited state (solid
purple) as a function of the distance from the iodine atom to the center-of-mass of the CN
diatom. The proposed well induced by solvation of CN− is shown as a dashed line. The structuresof the ICN−(CO2)2 and I–CN−(CO2)2 complexes are based off of previously calculated (Ref. 39)
minimum energy structures of IBr−(CO2)n.

Following photoexcitation of ICN−(CO2)n, formation of the NCCO2− molecule is
proposed to occur on the ground state of the ICN− chromophore and alters thephotofragmentation dynamics of the dissociating cluster. Several experimental30, 92-98 andtheoretical93, 95, 96, 98, 99 studies examined the solute-solvent interactions in X−(CO2)(X = F, Cl, Br, I). Notably, Arnold et al.93 utilized photoelectron spectroscopy to investigate thestructure and energetics of the X−(CO2) and reported the F–CO2 bond in FCO2− is covalent in84
nature. While Cl−(CO2), Br−(CO2), and I−(CO2) are all reported to be weakly-boundvan der Waals complexes, Do(X−–CO2) ≈ 0.2–0.3 eV, the FCO2− molecule is reported93 to have abond dissociation energy of 1.37 eV. Additionally, the excess charge is covalently shared by the
F and CO2 to such an extent that the O–C–O bond angle was reported93 to be roughly 135°.
Utilizing density functional theory (DFT) calculations, the bond dissociation energy of theground electronic state of NCCO2− is calculated to be 0.8 eV. The O–C–O bond angle in
NCCO2− is shown to be roughly 131°, see Fig. 4.11 (a), suggesting that the CN and CO2 are notacting as a solute or solvent, respectively, but rather as a molecular anion. Because solvation of
CN− is favored, encounters between CN− and the CO2 solvent are likely to occur, increasing thechance of NCCO2− formation. This interaction would result in an increased yield of CN−-based
products observed.
If, during photodissociation of the ICN− chromophore, a CO2 solvent moved in between
the iodine and the CN diatom, it is possible that a solvent-separated complex100, 101 could form.
The internuclear distances between the dissociating chromophore is expected to be large where
nonadiabatic transitions are favored.39 Therefore, the probability of a CO2 solvent, positioned
around the waist of ICN− (see Sec. 4.4.4), moving between the iodine atom and CN is likely
nonzero, resulting in a solvent-separated complex. Since the NCCO2− molecule is energeticallyfavored, it is possible that a solvent-separated-like complex forms in which the oxygen atoms of
NCCO2− point toward the iodine; Figure 4.11 (b) shows the calculated structure of the proposed
solvent-separated-like complex. Due to the inability of the time-of-flight mass spectrometer todistinguish a solvated, recombined ICN− photoproduct versus the solvent-separated-like complex
in the present experiments, a solvent-separated-like complex may likely contribute to the yield ofrecombined photoproducts reported. Further experimental and theoretical studies are necessary

85
to determine the role of the NCCO2− molecule and the solvent-separated-like complex in the
dissociation dynamics of ICN−(CO2)n.

Figure 4.11 Optimized geometries of (a) the NCCO2− molecule and (b) thesolvent-separated-like complex. The structure of both complexes were calculated at theB3LYP/aug-cc-PVTZ level of theory, with the iodine electron core potential of Thompson(Ref. 102) utilized for (b).

4.4.4 Solvent Evaporation
To investigate the photofragmentation of ICN−(CO2)n further, it is informative todetermine the solvation mechanism by examining the average number of solvent molecules lostfor various cluster sizes. Figure 4.12 shows the average number of CO2 solvent lost as a functionof parent cluster size for the CN−-based photoproducts following 500 and 600 nm excitation of
ICN−(CO2)n. Additionally, the average number of CO2 solvents lost for the Cl−-based products
following 644-nm excitation88 of ICl−(CO2)n, is shown for reference. As seen in Fig. 4.12, allthree curves show two distinct plateaus in the number of CO2 solvent lost at intermediate andlarge values of n, and a positive slope in the transition region in between the plateaus. Although

86
no calculations have been performed to date to determine the minimum energy structures of theICN−(CO2)n, the behavior of the curves in Fig. 4.12 suggests solvation of ICN− with CO2 follows
a process qualitatively similar to that previously reported for ICl−(CO2)n.33, 39, 77

Figure 4.12 The average number of CO2 lost from CN−-based photoproducts as a function ofparent cluster size following 500 and 600 nm excitation of ICN−(CO2)n. The number of CO2 lostfrom Cl−-based products following 644 nm excitation of ICl−(CO2)n are also shown forcomparison (Ref. 33). The solid-black diagonal line corresponds to the evaporation of all CO2
solvent molecules from the parent cluster and is shown as a guide.

Solvation of a heteronuclear dihalide (IX−, X = Cl, Br) with CO2 has been shown33, 39, 77
to begin around the waist of the chromophore. This location is energetically favored as a result of
the quadrupole moment of CO2 interacting with the delocalized excess charge on the

87chromophore in the ground state equilibrium geometry. Subsequent solvation with CO2 packs
around the smaller end of the chromophore. Once the binding sites on the smaller end are filled,
further solvation occurs around the iodine. Solvation on the iodine end of the chromophore isenergetically unfavored and evaporation of CO2 from the iodine end is expected to occur before
evaporation from the smaller halide end.
Solvation of the CN end of the chromophore is expected to be energetically favoredbecause of the difference in the ionic radii of CN− and I− (see Sec. 4.3). Thus, the first plateau in
Fig. 4.12, 6 ≤ n ≤ 9, is proposed to correlate with evaporation of CO2 from the CN end of thechromophore, and, the second plateau, 13 ≤ n ≤ 18, is proposed to be the result of evaporationfrom the larger iodine end of the chromophore. The transition region, 10 ≤ n ≤ 12, corresponds tothe solvent evaporation from both the iodine and CN ends of the chromophore, as solvation isshifting to the iodine end because all binding sites on the CN end are full. The slope of the curvein the transition region is expected to be positive because solvation of the iodine end of thechromophore will be less energetically favorable, resulting in a higher number of CO2 lost
following photoexcitation. As reported by Nadal,88 the analysis of the curve for the average lossof CO2 following photofragmentation of ICl−(CO2)n is performed in a similar fashion; the firstand second plateaus correspond to solvation on the Cl and iodine end, respectively.
By considering the loss of CO2 as a function of cluster size for the recombined
photoproducts of ICN−(CO2)n photofragmentation, an approximate CO2 solvent binding energy
can be determined. This approximation must be determined from parent cluster sizes large
enough to dissipate the excitation energy into solvent evaporation without resulting in an excitedchromophore; therefore, data from the 400 nm study will not be included. Additionally, in the88case of ICN−(CO2)n, it was assumed that no energy was partitioned to CN rotation in the large
clusters and that all energy is distributed to the process of solvent evaporation; therefore, theapproximation of the solvent binding energy is only an upper bound.
Following relaxation of the chromophore in the ground state well, the energy fromphotoexcitation is available for solvent evaporation.36 The amount of solvent evaporated by theremaining energy depends on the solvent binding energy, the excitation energy, and the amountof energy partitioned to the kinetic energy of the evaporated monomer. The upper limit of theapproximate solvent binding energy is determined by dividing the excitation energy by the
number of solvent molecules lost for large cluster sizes.
A similar procedure has been utilized previously to estimate the monomer70, 103-106 solvent
binding energy in various clusters. Previously, the upper limit solvent binding energies forI2−(CO2)n, IBr−(CO2)n, and ICl−(CO2)n were determined to be ~250 meV,34, 87 ~260 meV,35 and~300 meV,88 respectively. However, while studying Arn+ and (CO2)n+, Engelking107 developed astatistical quasi-equilibrium theory/RRK model that determined roughly 20% of the energy in theupper limit approximation is partitioned to the kinetic energy of the departing solvent.
Figure 4.13 shows the average number of CO2 molecules lost as a function of parent cluster sizefor the ICN−-based photoproducts following 500 and 600 nm excitation of ICN−(CO2)n. The
solid black diagonal line in Fig. 4.13 corresponds to the loss of all solvent from the parent
cluster. It is apparent from the Fig. 4.13 that solvent loss from the larger clusters (6 ≤ n ≤ 15–18)
is strongly dependent on the excitation energy. Additionally, the solvent binding energy is
expected to vary depending on the location of the solvent molecule with respect to thechromophore. To determine the change of the solvent binding energy with a change in location

89
on the chromophore an experimental binding enthalpy for I−–CO2 and CN−–CO2 is required.
Presently, the binding enthalpy for CN− to a CO2 molecule is unknown and, therefore, only theaverage CO2 binding energy is calculated. Following 500 nm excitation, the average number of
CO2 molecules lost (represented by the solid black horizontal lines at large values of n) is 10;
following 600 nm excitation an average of 8 CO2 were lost. Therefore, the upper limit of the CO2
solvent binding energy to ICN− is determined to be ~246 meV and ~248 meV for
500 and 600 nm, respectively. By including the kinetic energy release of a CO2 monomer, theapproximate average CO2 binding energy for the two wavelengths is roughly198 meV, with aloss to the kinetic energy of the monomer of roughly 49 meV. These quantities are consistent
with the average CO2 binding energies determined for I2−(CO2)n, ICl−(CO2)n, andIBr−(CO2)n.34, 35, 87, 88 More experimental and theoretical studies are required to determine theaverage binding energy of a CO2 molecule to the ICN− chromophore when the CN rotation is
included.

90

Figure 4.13 The average number of CO2 lost from ICN−-based photoproducts as a function ofparent cluster size following 500 and 600 nm excitation of ICN−(CO2)n. The solid black
horizontal lines show the average number of CO2 lost for the largest clusters studied. The solid
black diagonal line corresponds to the evaporation of all CO2 solvent molecules from the parent
cluster and is shown as a guide.

4.5 Comparison of ICN−(CO2)n and ICN−(Ar)n Photofragmentation
Comparison of the ionic photoproduct distributions as a function of cluster size following
500 nm excitation of ICN−(CO2)n and ICN−(Ar)n to the 2Π1/2 excited state provides a microscopic
picture of ICN−–solvent interactions. Figure 4.14 shows the ionic photoproducts distributions of
ICN−(CO2)n and ICN−(Ar)n (n = 1–5) following 500 nm excitation. Following 500 nm excitationof ICN−(CO2)n, the CN−-based photoproducts quickly dominate the ionic photoproductdistribution and the yield of recombined products varies as a function of n. Additionally, the91
I−-based channel for ICN−(CO2)n quickly disappears, illustrating the dominance of nonadiabatic
transitions. Conversely, following 500 nm excitation of ICN−(Ar)n, the I−-based photoproducts
dominant the ionic photoproduct distribution, with only a roughly 6% contribution fromrecombined products for 1 ≤ n ≤ 5.

Ionic Photoproduct Yield

1.0



ICN  (Ar)n: I  -based

ICN (CO2 )n: I -based

0.8





ICN (CO2 )n: CN -based

0.6





ICN (CO2 )n: ICN -based

0.4

0.2

ICN  (Ar)n: CN -based

ICN  (Ar)n: ICN  -based
0.0
0

1

2

3

4

5

Number of Solvent in Parent Cluster, nFigure 4.14 Ionic photoproduct distribution of ICN−(CO2)n and ICN−(Ar)n (Chapter III)
following one-photon 500 nm excitation to the 2Π1/2 excited state. The error bars correspond to a
5% standard deviation and are plotted for each ionic product distribution.

The striking difference in the photofragmentation dynamics can be qualitatively
understood by comparing the dominant electrostatic interactions between the two solvents andthe ICN− chromophore. The CO2 solvent interacts strongly with the ICN− chromophore becauseof its large quadrupole moment. This ion-quadrupole interaction has previously33-36, 39, 76, 77 been

92
attributed to the initial CO2 solvation around the bond axis of the chromophore. Additionally, theCO2–CO2 interactions are strong due to the quadrupole moment and result in the formation of arigid cage around a chromophore, which is expected to promote recombination of a dissociating
chromophore for large values of n. Conversely, Ar has neither a permanent dipole nor
quadrupole moment and Ar–Ar interactions are weak, forming a loose network of solvent atoms.
Figure 4.15 shows the average number of solvent lost from the ICN−-based
photoproducts as a function of the parent cluster size following 500 nm excitation of ICN−(CO2)nand ICN−(Ar)n. The solid black line in Fig. 4.15 represents the loss of all solvent in the parent
following excitation. It is apparent from Fig. 4.15 that more solvent is evaporated following
500 nm excitation of ICN−(CO2)n; this observation is seemingly in opposition to the fact that theenergy required to evaporate CO2 is approximately 4 times that of Ar. As proposed in Sec. 4.4.3,
recombination of the chromophore following excitation of ICN−(CO2)n (n ≥ 4) is likely the resultof nonadiabatic transitions from the 2Π1/2 excited state to the ground state introduced byperturbation of the electronic structure of the ICN− chromophore from the solute-solvent
interaction, leading to recombination in the ground state well. Similar photodissociation
dynamics were observed33, 35 following excitation of ICl−(CO2)n and IBr−(CO2)n to the 2Π1/2
excited state. However, as presented in Chapter III, recombination following excitation of
ICN−(Ar)n is likely to occur following a radiative transition from a well in the 2Π1/2 excited state
to the ground. Therefore, the linear behavior of the number of CO2 solvent molecules lost as afunction of n (see Fig. 4.15) for ICN−(CO2)n adheres to a more statistical evaporation, likeICl−(CO2)n and IBr−(CO2)n, in which recombination occurs via nonadiabatic transitions withcoincident evaporation. In contrast, the number of Ar solvent atoms lost following 500 nm
excitation of ICN−(Ar)n does not follow the statistical model, revealing the alternative

93
recombination dynamics accessed by these clusters. Furthermore, the stronger ion-quadrupole
interaction between the ICN− chromophore and the CO2 solvent and the more rigid CO2 cage
may hinder the rotation of the CN diatom following photodissociation. This may result in moreenergy partitioned to evaporation of solvent following 500 nm excitation of ICN−(CO2)ncompared to ICN−(Ar)n. The current experimental results do not provide a definitive answer
concerning the CN rotation without the aid of further experimental and theoretical studies.

Average Number of Solvent Lost

6

ICN (CO2 )n
ICN (Ar)n

5

4

3

2

1

0
0

1

2

3

4

5

6

Number of Solvent in Parent Cluster, nFigure 4.15 The average number of solvent lost from I−-based and ICN−-based photoproducts asa function of parent cluster size, n, following 500 nm excitation of ICN−(CO2)n and ICN−(Ar)n.
See Chapter III for ICN−(Ar)n photofragmentation results. The solid black line corresponds to theevaporation of all solvent from the parent cluster and is shown as a guide. The error bars
correspond to a 5% standard deviation.

94
4.6 Recombination following photoexcitation of ICN−(CO2)n, I2−(CO2)n, and IBr−(CO2)nComparison of ICN− photofragmentation with dihalide photofragmentation provides aninformative perspective on the recombination of photodissociated chromophores. The bonddissociation energy of the chromophores, the solvent binding energies, the ionic radii, and theasymptotic dissociation products are all qualitatively similar for ICN−, I2−, and IBr−; however,
the yield of recombined photoproducts following excitation to the 2Π1/2 excited state varies
dramatically between solutes. Figure 4.16 shows the yield of recombined photoproducts
following 600 nm excitation of ICN−(CO2)n, 720 nm excitation of I2−(CO2)n,87 and 760 nm
excitation of IBr−(CO2)n.51 At these wavelengths, roughly 0.4 eV of kinetic energy is released toeach cluster at the 2Π1/2 excited state asymptote. (The yield of recombined products forICl−(CO2)n is not shown in Fig. 4.16 because a study utilizing an excitation energy that results in
0.4 eV of energy release has not been performed to date). As can be seen in Fig. 4.16, theaddition of two solvent molecules is sufficient to allow for nonadiabatic transitions from the2

Π1/2 state to the ground state, resulting in recombined products for both ICN− and IBr−. Additionof the third CO2 results in a peak in the ICN−-based product yield and an increase in the yield of
IBr−-based products. However, subsequent solvation results in a decrease in the yield of
ICN−-based products, while the yield of IBr−-based products continues to increase. When n = 5,
roughly 88% of the observed ionic photofragments are IBr−-based products, but ICN−-based
products only account for roughly 20% of the ionic photoproducts; no recombined photoproducts
are observed for I2−(CO2), n = 0–5. For 4 ≤ n ≤ 11, the yield of recombined ICN−-based products
varies between ~10–20%, but does not dependent on parent cluster size. When n ≥ 13, the yieldof ICN−-based products increases, but never overcomes roughly 50% of the ionic photoproducts
observed. Recombined I2− photoproducts are first observed when n = 6 and the yield of

95
recombined I2− products increases as n increases. The yield of recombined products for bothIBr−(CO2)n and I2−(CO2)n reaches 100% at n = 8 for IBr−(CO2)n and n = 17 for I2−(CO2)n.

Yield of Recombined Products

1.0

ICN
I2 

0.8

IBr

0.6

0.4

0.2

0.0
0

2

4

6

8

10

12

14

Number of CO2 in Parent Cluster, nFigure 4.16 Yield of recombined photoproducts following excitation of ICN−(CO2)n, andI2−(CO2)n, (Ref. 87) and IBr−(CO2)n (Ref. 51) to the 2Π1/2 excited state. The excitation
wavelengths utilized result in roughly 0.4 eV of kinetic energy release for each solute. The
dashed gray line represents 100% recombined photoproducts. The error bars on the ICN−(CO2)nphotoproducts correspond to a 5% standard deviation.

The marked contrast in the yield of recombined photoproducts can be qualitatively
understood by comparing the structures of the three clusters as a function n. Papanikolas et al.108
performed Monte Carlo simulations to report the minimum energy structure of I2−(CO2)n. Theirsimulations showed that for n ≤ 6, the CO2 solvent binds to I2− around the waist of thechromophore and equally solvates both iodine atoms. This structure does not allow

96
recombination for n  6, as the dissociation of the chromophore is unimpeded by the solvent.
However, for n ≥ 7, the solvation of I2− begins to occur at one end of the chromophore and theyield of recombined photoproducts begins to increase as n increases. Similar to I2−(CO2)n, thefirst few (n ≤ 4) CO2 molecules solvate IBr− around the bond axis.35, 39 Additional solvation
results in the Br end of the chromophore becoming enclosed by a CO2 cage. Solvation of the Br
end of the chromophore increases the yield of recombination significantly, and the only
photoproducts observed by n = 8 are IBr−-based. The solvation of ICN−(CO2)n is expected tooccur in a similar fashion to ICl−(CO2)n, see Sec. 4.4.4; initial solvation occurs around the waistof ICN− and continues around the smaller end of the chromophore. However, as Fig. 4.16 shows,following the addition of the fourth CO2, photoexcitation of ICN−(CO2)4 with 600 nm light
results in a decrease in the yield of recombined products. This decrease is the result of thefavorable solvation of CN− and the likely formation of the NCCO2− molecule which becomestrapped in the ground state well at large internuclear distances introduced by the CO2 solvating
CN− (detailed in Sec. 4.4.3). The yield of recombined products reduces to ~10–20% for
4 ≤ n ≤ 11, but for n = 13–15, it again begins to increase. It is proposed that this increase is theresult of solvation beginning to significantly enclose the iodine end of ICN− giving rise to anincreased yield of recombined products. This explanation is the same proposed in Sec. 4.4.4 toaccount for the average number of CO2 lost as a function of cluster size in the large cluster limit.
4.7 Summary and Conclusions
Results of photofragmentation studies of ICN−(CO2)n following 400, 500, and 600 nm
excitation show a disparity in ionic photoproduct distributions compared to previously studied
dihalides due to intrinsic properties of the solute and the location of solvation about the

97chromophore. Furthermore, comparison of the results of photofragmentation of ICN−(CO2)n andICN−(Ar)n reveal the importance of solute-solvent interactions in photofragmentation dynamics.
Results of photodissociation action spectra reveal the stabilization of the 2Π1/2 excited state uponsolvation of the ICN− chromophore with CO2. Following 500 or 600 nm excitation of
ICN−(CO2)n to the 2Π1/2 excited state, CN−-based photoproducts dominate following the additionof roughly two CO2 solvent molecules. This result is likely due to nonadiabatic transitions fromthe 2Π1/2 state to the 2Σ+ excited state for n = 1–3, and nonadiabatic transitions from the 2Π1/2
states to the X̃ 2Σ+ state are most probable when n ≥ 4. The yield of recombined ICN−-based
photoproducts observed reaches a maximum at small values of n; nmax increases with an increasein excitation energy. The sudden decrease in the yield of ICN−-based products following thislocal maximum is the result of the favorable solvation of CN− and the formation of the NCCO2−
molecule or a solvent-separated-like complex. Analysis of the average number of CO2 molecules
lost as a function of cluster size for the CN−-based products provides evidence of the location of
the solvent about ICN− as n increases. Additionally, examination of the average number of CO2
molecules lost for the ICN−-based products as a function of cluster size provides anapproximation of the CO2 solvent binding energy (~198 meV), but does not include thepossibility of energy partitioned into CN rotation. Comparison of the photofragmentation of
ICN−(CO2)n and ICN−(Ar)n illustrates the central role of the solvent in the dissociation dynamics,
while comparison to I2−(CO2)n, IBr−(CO2)n, and ICl−(CO2)n elucidates the role of the solute.

98

Chapter V: Recombination Dynamics of Photodissociated IBr−(CO2)n5.1 Introduction
The study of ultra-fast cluster dynamics has long been a field of interest allowing for theinvestigation of the behavior of molecules influenced by their environments.109, 110 Simplificationof the local environment by using gas phase cluster ions provides one the chance to study these
influences and gain a fundamental understanding of the effects solvation has onmolecules.109, 111-113 These effects can be seen in simple processes such as solvent molecules
being used to dissipate energy or more complex interactions involving perturbation of theelectronic character of the solvated chromophore. Phenomena resulting from solvent perturbed
potential energy surfaces, such as solvent-induced recombination and “cage effects,” can bestudied using molecular dynamics with the application of simple, first-order kinetic models.
Over the past 15 years, the Lineberger and Parson groups have carried out a collaborative
theoretical and experimental investigation into the dynamics of photodissociation of diatomic
molecular ions embedded in mass-selected molecular clusters.33, 34, 37, 76, 114-120 A common theme
that runs through much of this work has been the systematic measurement of the cluster size
dependence of product branching ratios, such as the relative probabilities of dissociation vs.
solvent-induced recombination, and of the time constants for the associated processes. Earlier
work in this series reported38 a surprising disparity between the overall propensity forsolvent-induced recombination following near-IR excitation, and the rate of this recombination
process. For example, as the number of CO2 solvents around IBr− was increased from 5 to 8, therecombination probability increased35 from 90% to 100%, but the timescale for recombination

99
increased from 12 ps to 900 ps.38 This result is in dramatic variance with earlier investigationsof the homonuclear dihalide cluster I2−(CO2)n, where somewhat more kinetic energy wasavailable to the photofragments and for which recombination times were of order 10 ps anddecreased with increasing cluster size.37, 120 The results of nonadiabatic molecular dynamic
simulations, which reproduced these experimental trends, were also included in the earlier
papers.76, 114, 118, 121 On the basis of the IBr−(CO2)n simulations, it was predicted that the trend inrecombination time would reverse as the cluster size was increased further. The experiments
presented in this chapter involve the recombination of photodissociated, size-selected
IBr−(CO2)n, n = 11–14, anionic clusters;39 they were designed to test the above prediction, and toprovide additional physical understanding of this very unexpected result. Results of earlier
experiments and theory are presented only to the extent required to provide a context for these
new results and the resulting fuller interpretation.
5.2 Experimental
A pump-probe technique is utilized to study solvation-induced recombination dynamics
of photodissociated IBr−(CO2)11-14. The experimental apparatus, laser system, and dataacquisition methods corresponding to this experiment were previously discussed in Chapter II.
Briefly, a one-color (795-nm) pump-probe scheme is employed to measure the absorption
recovery times for the IBr−(CO2)n cluster anions.38 Photoexcitation of the dihalide at thiswavelength excites IBr− to the dissociative Ã′ 2Π1/2 state, leading to I − and Br products with
~0.29 eV kinetic energy release. As CO2 solvent molecules are added, the ionic photoproducts
change to Br−- and IBr−-based products with varying degrees of solvation, indicating dissociationon the ground state surface or recombination of the chromophore, respectively.35 With eight or100
more solvent molecules attached to the chromophore, the only observed photoproducts are
IBr−-based, indicating 100% recombination.35 Once recombined and sufficiently vibrationallyrelaxed on the ground state, the chromophore regains near-IR absorption and can again be
excited to the Ã′ 2Π1/2 state via the 795 nm probe pulse. Following the absorption of a second
photon, the ionic photoproducts have a mass-to-charge ratio different from any of the one-photon
products due to solvent evaporation. Two-photon products are then monitored as a function of
pump-probe delay time, taking advantage of the fact that the electric potential of the secondaryreflectron mass spectrometer can be adjusted to detect only products of pump and probe laser
absorption.38 Appropriate background corrections are applied to assure that the reported
two-photon signal results only from the absorption of one photon from each of the pump andprobe laser pulses.36, 37, 122
5.3 Theoretical Methods
The theoretical work presented in this section was performed by Matthew A. Thompson
and a detailed description can be found elsewhere;102 therefore, only a brief description of thoseresults that are required to understand the experimental data is presented here.
5.3.1 Electronic Structure
The potential energy curves, Fig. 5.1, and associated electronic properties of the isolated
IBr− solute were obtained using MOLPRO 2002,123 with the energy-consistent effective corepotential (ECP) of Stoll et al.124 The electronic structure calculations use theinternally-contracted multireference configuration interaction (icMRCI) methods developed by
Knowles and Werner.125-127 The reference orbitals and configurations are obtained from

101
state-averaged complete active-space self-consistent field calculations128, 129 that consist of
15 electrons in the lowest two Σ, Πx, and Πy states that arise from the 2P state of the neutral and1

S state of the ion. A spin-orbit calculation of all six states is then performed using spin-orbit

ECPs.130 Other electronic properties of IBr− are tabulated and compared with experiment inTable 5.1.

2.0
~2

1.5

Energy (eV)+B  1/2
~ 2
a'
1/2

1.0

0.5

~
a 2

–
I* + Br–I + Br*–I + Br3/2~ 2–
I + BrA'  1/2

0.0~

A 2  3/2
-0.5~

X 2  +1/2

-1.0
3

4

5

6

7

R(IBr) (Å)

Figure 5.1 Potential energy curves for the six lowest spin-orbit states of IBr−.

8

102
Table 5.1 Summary of energetics from ab initio calculations (energies in eV)

Spin-Orbit SplittingBr
I
ΔEA(I–Br)Calc.
0.42370.89320.3156

D0(IBr−)0.948dEA(IBr)Re (Å)

2.494
3.05

Expt.
0.4569a0.9427a0.3045b1.10(4)c
0.966e2.51(2)e3.06(3)eΔ(Calc. – Expt.)−0.0331−0.0495
0.0111−0.152−0.018−0.016−0.01

aRef. 74
Ref. 91dCalculated using ωe = 118 cm-1, as obtained by analysis of the ground state potential energy
curve with LeRoy’s LEVEL program, Ref. 131
eRef. 35
f
Ref. 69c

5.3.2 Molecular Dynamics
The nonadiabatic molecular dynamics (MD) simulations for IBr−(CO2)n are based on theeffective Hamiltonian method developed by Maslen et al.132 and previously
applied76, 115, 116, 118, 121 to I2−(CO2)n. Briefly, the electrostatic and induction interactions between
the IBr−solute and the CO2 solvent are calculated from an operator including electrostatic andinduction terms for the solute and solvent. For the solute, a distributed multipole analysis (DMA)(Ref. 133) of the isolated solute electronic structure is calculated. At each time step of amolecular dynamics simulation, the effective Hamiltonian for the solvated ion is diagonalized toyield the energies, forces, and nonadiabatic transition probabilities needed for the next step.

103
5.3.3 Minimum Energy Structures
Long time simulations on the ground electronic state surface are used to obtain energies
and structures for the solvated chromophore. Minimum energy structures for IBr−(CO2)n(n = 1–16) are constructed by sampling 201 configurations from a 1 ns trajectory on the IBr−ground state having an energy corresponding to a temperature of 80 K. These configurations arethen quenched to local minima.134 The lowest energy structure obtained for each cluster size isshown in Fig. 5.2, and the corresponding energetic properties are reported in Table 5.2. While
this procedure is not expected to find the lowest energy configuration for any but the smallest
clusters, the structures reported in Fig. 5.2 and Table 5.2 are characteristic of the minimum
energy configurations of the cluster anions. The most striking visual feature of these minimum
energy structures is the changing asymmetry of the solvent configurations, with maximum
asymmetry near a half-filled first solvation shell. This asymmetry plays a major role in theunusual recombination dynamics.

104

Figure 5.2 Exemplar calculated minimum energy structures for IBr−(CO2)n, n = 1–16. The
pattern of CO2 filling is first around the bond, next the Br end beginning at n = 3, and then theiodine end beginning at n = 9.

105
Table 5.2 Properties of minimal energy clusters of IBr−(CO2)n from 80 K trajectory ensembles
n
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

PE (meV)a
−205
−406
−627
−852
−1074
−1301
−1525
−1775
−2009
−2248
−2483
−2680
−2917
−3157
−3380
−3588

Δn (meV)b
205
201
222
225
222
227
224
251
233
240
234
198
237
240
223
208

−PE/n (meV)
205
203
209
213
215
217
218
222
223
225
226
223
224
225
225
224

# local min.
1
23
22
47
65
98
104
63
64
112
135
111
126
143
131
140

a

Not including IBr− bond energy (0.956 eV).
b
See Eq. (1).

To quantify this concept, we define the asymmetry of a solvent configuration, ΔΦ, as theenergy required to move a charge of −e from the iodine end to the Br end of a cluster. Negativevalues mean solvation favors bromine, while positive values favor iodine. Figure 5.3 shows thesolvent asymmetry for ground state IBr−(CO2)n averaged over an ensemble of clusters at 60 K.
This asymmetry is maximum near a half-filled solvent shell, consistent with the geometricalstructures.

106

Figure 5.3 Average ΔΦ for 100 configurations of IBr−(CO2)0-16 in the ground state at 60 K. Error
bars represent one standard deviation of the mean.

5.3.4 Trajectory Calculations
Ensembles for MD simulations are constructed as follows. First, starting from thecalculated minimum energy structures, the cluster is warmed for 40 ps at a temperature of 60 K,
followed by a 100 ps run on the ground state to test that the cluster energetics are stable. After
that, the ensembles are constructed using a 2 fs time-step run on the electronic ground state thatsamples every 5 ps until 100 configurations are constructed. A step-size of 1 fs is used for all
trajectories. A trajectory is classified as “dissociated” if the solute bond length ever exceeded
40 a0 on any electronic surface, and as “recombined” if it ever became less than 3.3 Å on the
ground state. As discussed in the next section, the timescales for recombination in theintermediate-sized clusters (n = 8–10) are extremely long,38 and in these cases it is not feasible tofollow every trajectory in an ensemble all the way to its final destination. In these cases, we
propagate the ensemble until enough trajectories have recombined that we can calculate a107
recombination time – this requires as much as 2 ns in extreme cases – and then extrapolate thebranching ratio for recombination on the basis of the fraction of trajectories that have dissociatedor recombined at this time.
5.4 Experimental Results
5.4.1 Absorption Recovery Dynamics of IBr−(CO2)n, n = 11–14
The absorption recovery times of the smaller IBr−(CO2)n clusters, n = 5−10, following
795 nm excitation to the Ã′ 2Π1/2 state were previously reported by our group.38 It was found thatthe absorption recovery time increased as the number of CO2 molecules in the cluster increased
from tens of picoseconds for IBr−(CO2)5,6 to almost 1 ns for IBr−(CO2)8,10. While this cluster size
dependence is the opposite of that observed37, 120, 122 for I2−(CO2)n, I2−(OCS)n, and I2−(Ar)n, it has
been corroborated by theoretical predictions. Furthermore, these predictions indicated that foreven larger clusters, n = 12 and 13, the absorption recovery lifetime should return to tens ofpicoseconds as with IBr−(CO2)5,6. At that time, experimental difficulties prevented the formationof the larger ion clusters. The cluster ion production difficulties have now been resolved andexperimental results for these and other larger clusters are presented here.
Figure 5.4 shows the time-dependent absorption recovery of IBr−(CO2)11. The transient isobtained by using a weighted average of the signal resulting from four days of data acquisition of
the IBr− and IBr−(CO2) two-photon products. Plotted individually, these two products exhibitedvery similar absorption recovery transients. However, the appearance of the transients changedslightly on a day-to-day basis. The y-axis in Fig. 5.4 represents the two-photon, pump-probe
signal normalized to unity at the signal level observed at a 500 ps time delay. The transient108
recombination signal shows a steep, initial rise from slightly greater than 0 ps to 30 ps and then amore gradual rise until 650 ps. While all the data sets show the steep, initial rise from 0 ps to30 ps, the more gradual rise out to 650 ps is only observed in two of the four data sets. Given thatthe existence of this very slow rise is almost totally dependent on the 50 ps data, its existence
must be considered as tentative at best. Because both the pump and probe pulses are the same
wavelength, the signal is symmetric about t = 0. When the pump and probe beams overlap
temporally, we see a large “coherence peak” whose shape is essentially the autocorrelation
function of the 795 nm laser pulse. When fitting the absorption recovery data, however, we avoid
this artifact with the physically compelling assumption that there can be no recombination attime zero,
1

(5.1)

where τ represents the absorption recovery lifetime. Fitting to this model yields a value of 7(2) psfor τ. This fit is shown as the dotted line in Fig. 5.4. The experimental data also hint at thepossibility of a long recovery time for a small (possibly 10 %) component of the IBr−(CO2)11
clusters; however, given the day-to-day fluctuations associated with the data sets, a definitive
conclusion cannot be made. Regardless of the exact functional form of the fit that is utilized,
though, it is clear that the principal absorption recovery occurs much more rapidly than wasobserved for the IBr−(CO2)8 and IBr−(CO2)10 clusters, 900(100) ps. This result is in accord withtheoretical predictions that the absorption recovery time should decrease for larger IBr−(CO2)nclusters.38

109

Normalized Intensity (arb. units)

1.8

Experimental Data, n = 11
Single Exponential Fit

1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0

50

100

150

200

Pump-Probe Time Delay (ps)Figure 5.4 Transient illustrating the absorption recovery of IBr−(CO2)11. The circles represent
the experimental data, and the dotted line is the result of a single exponential fit. The black line isused to guide the eye.

The IBr−(CO2)12 absorption recovery dynamics are qualitatively similar to those observedfor IBr−(CO2)11. In this case, however, only the IBr−(CO2) two-photon product was detected.
Furthermore, no evidence was observed for a second time constant. A single exponential fit ofthe data yielded a lifetime of 10.3(8) ps, in further support of the theoretical predictions for thelarger IBr−(CO2)n clusters.38
The transient depicting the absorption recovery of IBr−(CO2)13 is illustrated with filledsquares in Fig. 5.5. This transient was obtained by summing data from the IBr−(CO2),
IBr−(CO2)2, and IBr−(CO2)3 two-photon products and then normalizing the (constant) signal levelat 100 ps to unity. Clearly, this transient is much more complicated than that observed forIBr−(CO2)11. There is a sharp initial rise from 0.4 to 1 ps, then a plateau at 3–5 ps, and finally a110decrease and leveling off of the signal by 10–20 ps. Data were recorded for time delays as longas 500 ps, and no significant changes in the signal intensity were observed after 20 ps. It isinteresting to note that the signal at 3–5 ps actually exceeds that at 100 ps. A qualitative
explanation for this behavior is given below. It is clear, however, that unlike the data forIBr−(CO2)11 and IBr−(CO2)12, this transient cannot be accurately fitted with a simple exponential
rise. We are left, then, to make only a qualitative estimate of the absorption recovery time, as
discussed below.
As shown by the filled circles in Fig. 5.5, a transient was also recorded for IBr−(CO2)14.
This transient was acquired by summing the IBr−(CO2)2, IBr−(CO2)3, and IBr−(CO2)4 two-photon
products. The intensity was again normalized to unity at the signal level at 100 ps. Note that thistransient is qualitatively similar to that observed for IBr−(CO2)13. A sharp rise is observed from
0.4 to 1.0 ps, along with a peak at 4 ps, and a gradual decay and leveling off of the signal by
10 ps. As with IBr−(CO2)13, data were taken out to a 500 ps time delay, and no significant
changes in the signal level were observed at long time delays. Thus, the experiments do notconfirm the predictions by theory that the absorption recovery of IBr−(CO2)14 should consist ofboth a fast and slow component (see Section 5.4.2). The peak of the transient at 4 ps is much
more pronounced in the absorption recovery of IBr−(CO2)14 than in the recovery of IBr−(CO2)13,
as is evidenced by the magnitude of the y-scale; the signal level at 5 ps for IBr−(CO2)13 isapproximately 1.05, while that observed at 4 ps for IBr−(CO2)14 is approximately 1.4.
Interestingly, depending on the particular product detected for IBr−(CO2)14, the magnitude of thepeak also varied. Higher mass products gave rise to a larger peak in the transient. Theseobservations are consistent with our picture of the absorption recovery for IBr−(CO2)13 andIBr−(CO2)14, as discussed below. Finally, as with IBr−(CO2)13, it is impossible to get an accurate

111
absorption recovery lifetime by fitting the data to an exponential rise. Thus, we are again left toqualitatively assign an absorption recovery lifetime, as discussed in the following paragraph.

Normalized Intensity (arb. units)

1.6
1.4
1.2
1.0
0.8
0.6
0.4

Experimental Data, n = 13
Experimental Data, n = 14

0.2
0.0
0

5

10

15

20

Pump-Probe Time Delay (ps)Figure 5.5 Absorption recovery transient for IBr−(CO2)13, dashed line and squares, andIBr−(CO2)14, solid line and circles. The lines are shown to guide the eye.

The nonexponential behavior observed in the transients for IBr−(CO2)13 and IBr−(CO2)14
shown in Fig. 5.5 is attributed to population traversing vibrationally excited states of the groundelectronic state after recombination. The absorption cross section of the Ã′ ← X̃ transition forIBr− peaks at 740 nm.51 This implies that excitation from a 795 nm laser pulse, the wavelength
used in these experiments, will have a greater cross section from vibrationally excited levels in
the ground electronic state than from the lowest vibrational levels. Figure 5.6 illustrates this witha schematic of the excitation of the chromophore from the lowest vibrational level, hνasym, and112from an excited vibrational level, hνovershoot. The sharp rise observed in the transients forn = 13 and 14 from 0.4 ps to 1.0 ps, then, is attributed to population traversing high vibrational
states in the ground electronic state. The transient then peaks above the normalization point at
4 ps, at which time recombined population is traversing intermediate vibrationally excited levels.
This is illustrated by the hνovershoot arrow in the figure. Finally, the transient reaches theasymptotic level at 10 ps when the population has reached low vibrational levels, as shown by
the hνasym arrow. Based upon this description of the transients observed for IBr−(CO2)13 andIBr−(CO2)14, we assign their absorption recovery lifetimes as 5(3) and 4(2) ps, respectively. This
picture also supports our observation that the higher mass products for IBr−(CO2)14 give rise to alarger peak in the transient because higher mass products are less likely to make it back down to
the lowest vibrational levels in the ground state. Thus, they will be more prevalent in theintermediate vibrational levels which correspond to the signal responsible for the peak in thetransient. Of note is that the theoretical methods used in simulating the absorption recovery
dynamics of these clusters, as outlined in the following section, do not account for the change in
the IBr− cross section as a function of vibrational level and therefore will not reproduce the peakin the transient.

113
1.60

Energy (eV)1.40h asym

1.20

h overshoot0.08

X 2  +1/2~

A' 2  1/2

~

0.04
0.00
3

4

5

6

R(I–Br) (Å)

Figure 5.6 Schematic diagram illustrating the origin of the overshoot of the asymptote for theabsorption recovery of IBr−(CO2)13,14. The ground state potential is a Morse potential obtainedusing the experimentally determined parameters for the equilibrium bond length and vibrationalfrequency (Refs. 69, 131). The excited state potential is also a model Morse potential with thewell depth chosen to match the experimentally determined value (Ref. 135) and the equilibrium
bond length adjusted to illustrate the origin of the overshoot. The arrow labeled hνovershootrepresents the transition giving rise to the overshoot of the asymptote, and the arrow labeledhνasym represents the transition giving rise to the asymptotic signal. Note the break in the energy
scale along the y-axis.

5.4.2 Simulated Absorption Recovery Dynamics of IBr−(CO2)n, n = 5–16
Long-time nonadiabatic MD simulations of IBr−(CO2)5-16 were carried out to obtain
simulated time constants for ground-state recombination. All simulations used a time step of 1 fs,
while the length of simulations ranged from 20 ps for IBr−(CO2)5 to 2 ns for IBr−(CO2)8,10;
ensemble sizes ranged from 100 trajectories for IBr−(CO2)8,10 to over 1700 for IBr−(CO2)5. A
trajectory was considered to have “recombined” when the IBr− bond length reached 3.3 Å on the
ground state.

114
In most cases, it was possible to fit the time dependence of the recombination with asimple exponential (Eq. 5.1). An example of this fit is provided in Fig. 5.7 which plots both thetheoretical and experimental results along with their respective single exponential fits. The
exceptions were IBr−(CO2)14, which required a bi-exponential fit, and IBr−(CO2)5,6, for which atime delay of several ps elapsed before recombination began. In the latter case, the timedependence was fit to the expression:
1

(5.2)

where the time delay t0 is treated as an additional fit parameter. The simulated ground-state
recombination times are summarized along with the experimental results in Fig. 5.8. Bothexperiment and simulation show an increase in ground-state recombination time of 2–3 orders ofmagnitude from IBr−(CO2)5 to IBr−(CO2)8−10.

115

Figure 5.7 Ground-state recombination dynamics for IBr−(CO2)8. The stepped line represents
theoretical results, τMD, and the dots represent experimental data, τexpt. The dotted anddash-dot-dash lines represent single-exponential fits to the experimental and simulated data,
respectively, see Eq. (5.1).

Absorption Recovery Time (ps)

116

1000

Experiment
Theory
100

10

1
4

6

8

10

12

14

Number of CO 2 Molecules on IBr–

16

Figure 5.8 Comparison of experimental and theoretical ground-state recombination times. The
red solid line and diamonds represent the experimental data, and the blue dashed line andtriangles represent the results of the theoretical calculations. The lines connecting the data points
correspond to the fast component of the absorption recovery in cases where the recovery was
best fit to a bi-exponential function. The data points not connected by a line correspond to theslow component of the absorption recovery.

The extraordinarily long relaxation times observed in both simulation and experiment forn = 8–10 strongly suggest that in the solvated molecular ion, the potential energy surface
corresponding to the Ã′ state contains a deep well. In order to elucidate the topography of thismultidimensional surface, a technique used previously121, 136 was adopted, in which trajectories
are analyzed in terms of two coordinates, solute bond length and a collective solvent coordinate.
In IBr−, a negative ΔΦ broadly represents a solvent configuration that is concentrated around thebromine atom, while a positive ΔΦ corresponds to a configuration concentrated around iodine. A
larger absolute value of ΔΦ represents more solvent asymmetry, Fig. 5.9 (bottom), while a ΔΦnear zero represents a more symmetric solvent configuration, Fig. 5.9 (top). Note that, in contrast

117
to a symmetric solute system like I2−(CO2)n, ΔΦ = 0 does not correspond exactly to a symmetricsolvent configuration, as the two solute atoms are not the same. Also, many different
solute-solvent configurations correspond to the same value of ΔΦ.

Figure 5.9 Visual representation of the solvent coordinate, ΔΦ, using IBr−(CO2)8 clusters. ΔΦ isdefined by the change in energy when the charge is transferred from the bromine atom to theiodine atom as shown on the left and right sides of the figure, respectively. A symmetric solvent
configuration, top, corresponds to a small ΔΦ. An asymmetric solvent configuration, bottom,
corresponds to large values of ΔΦ.
While these plots allow analysis of the concerted motion of the solute and solvent overthe length of the trajectory, the energy landscape has to be inferred from the limitations of thismovement on the 2D surface. Wells and valleys of the energy landscape are indicated by batchesof trajectories that are trapped either in small, compact areas (wells) or long, slim areas (valleys)
on the 2D surface. Since a more extensive discussion of the physical significance of this118
solvent-coordinate view of the recombination dynamics has appeared in the major publication39of this work, it will not be repeated here.
5.5 Discussion
The principal results of the experiments and simulations are presented in
Figs. 5.4, 5.5, 5,7, 5.8, and 5.9, and may be summarized as follows:(i)

In these clusters, recombination is a highly efficient process, in the sense that given
enough time, the probability of recombination is high. It occurs to a measureable
degree in clusters having as few as two solvent molecules and reaches 100% forclusters having eight or more solvent molecules, significantly less than a closed
solvent shell. This is very similar to the behavior seen for I2−(CO2)n clusters.37, 120(ii)

While recombination is efficient, it is not necessarily fast, and the long time
probability of recombination is not simply related to the rate. The timescale forrecombination increases with increasing cluster size, up to n = 10, and then decreases
as n increases further. This is drastically different from the behavior seen in I2−
clusters.37 In particular, the recombination times for n = 8–10 are orders of magnitude
longer than those seen in the homonuclear system.

(iii)

The solvent coordinate Φ plays a decisive role in recombination. The timescale forrecombination is strongly correlated with the early-time distribution of Φ, andtrajectories that lead to recombination all follow similar pathways when plotted in(r, Φ) space.

119
The increase in recombination times for n = 5–10 appears to be roughly exponential withn, which strongly suggests that the recombination mechanism involves an activated process.
Figure 5.9, in turn, elucidates the molecular origin of this process: recombination requires acollective motion of the solvent cage from the vicinity of the Br atom to that of the iodine atom.
Collecting together the observations detailed in the preceding section, a coherent description of
the excitation, dissociation and recombination process can be presented.
In clusters having 5 to 10 solvent molecules, absorption of a photon leads to an excitedstate in which the charge is predominantly localized on the Br atom (in contrast to the barechromophore). The solvent molecules stabilize this charge distribution, more so since the
I–Br bond is extended in this excited state. Increasing the number of solvent molecules increases
the degree of stabilization, thereby lowering the free energy of the excited state “well.” After asufficiently long time, however, thermal fluctuations carry the solvent cage close to a symmetricgeometry. The charge can then transfer to the iodine atom, and the solvent then reorganizes so as
to stabilize this charge distribution. This solvent-driven adiabatic electron transfer event is therate determining step in the recombination mechanism. The well in the excited state surface
corresponding to solvated I− is energetically close to a barrier in a lower lying excited state,
however, so nonadiabatic transitions to this state and from it to the ground state are rapid. Onceon the lower states, the chromophore can either dissociate (in small clusters) or recombine(in large clusters).
In clusters having more than ten solvent molecules, a further increase in n does notfurther stabilize the Br-localized configuration, since the additional solvent molecules add to theiodine side of the anionic chromophore. Indeed, one can expect that by stabilizing the I−120
localized configuration, the additional solvents may actually pull down the barrier to solvent
reorganization. The free-energy barrier to solvent reorganization has both an energetic and an
entropic component. In the largest clusters, both components are small, since a small amount ofenergy is required to move any given solvent molecule, and only a few solvent molecules need tobe moved, in order for the solvent cage to reach a configuration at which nonadiabatic electronic
quenching can occur.
Finally, to address the conundrum that opened this chapter: how does it come about thatthe ionic photoproduct distribution and the rate constant for recombination do not track one
another? The answer is that the two processes are determined by dynamics on different potential
surfaces. Except in the very smallest clusters (n < 4) there is essentially no dissociation on theinitially excited surface. Instead, the simulations imply that the distribution between dissociation
and recombination takes place on the Ã and X̃ surfaces, following solvent reorganization andelectronic quenching. As n increases from 5 to 10, the timescales for both dissociation andrecombination increase because the solvated Br− configuration becomes more stable. Oncesolvent reorganization and electronic quenching have occurred, however, the larger clusters canmore effectively cage the fragments on the lower surface.
5.6 Conclusions
Experimental studies and nonadiabatic molecular dynamics simulations of thephotodissociation and recombination of size-selected IBr−(CO2)n anionic clusters following
excitation in the near-IR have been carried out. The simulations rely upon an effective
Hamiltonian model that accounts, in a self-consistent fashion, for the interaction between thepolarizable charge distribution on the anionic chromophore and the electrostatic forces due to the121solvent molecules. The surprising results of recent experimental studies38 of thissystem – recombination times that increase with cluster size, resulting in timescales of the orderof nanoseconds in clusters having 8–10 solvent molecules – are reproduced by the simulations,which furthermore predict that the timescale decreases again as the cluster size is increased
further. New experimental results, presented in this chapter, confirm this latter prediction. A
detailed analysis of the simulation trajectories leads to a comprehensive physical explanation for
the experimental trends: recombination is found to be an activated process, with therate-determining step being collective motion of the solvent cage from a configuration thatstabilizes a charge localized on Br− to one that favors I−.

122

Chapter VI: UV Photofragmentation of ICN−(Ar)n and Future Directions

6.1 Introduction
In order to elucidate further the electronic structure and photodissociation dynamics ofICN− and solvated ICN−, preliminary photofragmentation studies of mass-selected ICN−(Ar)nwere performed following ultraviolet (UV) excitation. Argon atoms were utilized as the solvent
to prepare the ICN− chromophore with minimal initial internal energy before photoexcitation.
Additionally, only minor perturbations to the electronic structure of the ICN− chromophore result
from solvation with Ar atoms. Unlike previously studied dihalides,33-35, 39 the 2Σ+ excited state of
the ICN− chromophore is lower in energy than the 2Π1/2/2Π3/2 excited states, where the mixed
state designation 2Π1/2/2Π3/2 is defined by McCoy.66 Figure 6.1 shows the calculated potential
energy curves of linear ICN− as a function of the distance between the iodine atom and thecenter-of-mass of the CN diatom. The energy separation between the 2Π1/2 and the 2Π1/2/2Π3/2
excited states, corresponding to the excitation energy of the CN Ã 2Π ← X̃ 2Σ transition(1.146 eV),71 is larger than the spin-orbit splitting energy of iodine (0.94 eV).74 This leads to the2 +Σ excited state being stabilized in energy compared to the 2Π1/2/2Π3/2 state. Therefore, thedissociation channel calculated66 to be highest in energy corresponds to dissociation intoI− + CN (Ã 2Π) and not I* (2P1/2) + CN−, as with the dihalides.33-35, 39 One of the results of thedifferent ordering of the excited states is the nonadiabatic dynamics between the 2Π1/2 and 2Σ+excited states with the appearance of an avoided crossing (dashed lines in Fig. 6.1). This avoided
crossing is the result of a change in the charge distribution66 on the chromophore on the 2Π1/2 and2 +Σ excited state as a function of the distance between iodine and the center-of-mass of CN(see Chapter IV).

123
2

2

 1/2 /  3/2

I  + CN (A 2)+

I* + CN  (X 1 )3.0~

Energy (eV)

2.5
2.0

2

1.5

2

1.0

2

0.5

~ 2 +

~



~ 2

I + CN (X  )

 1/2
 3/2



~ 1

I + CN (X  )

X 

0.0
3

4

5

6

7

8

R(I-CN COM) (Å)

Figure 6.1 Linear ICN− potential energy curves (based on Ref. 66) as a function of the distance
between the iodine atom and the center-of-mass of the CN diatom. The ground state
wavefunction is shown as a black dashed line. The diabatic curves proposed to take part in the
dissociation dynamics of ICN− are shown for the 2Π1/2 (dashed purple line) and 2Σ+ (dashed blueline) excited states.

One consequence of this crossing is that the ionic photoproduct distribution arising from
300 nm excitation of ICN−(Ar)n should be different from the 500 nm distribution reported in
Chapter III. In that case, 500 nm excitation40 of ICN−(Ar)n, 1 ≤ n ≤ 5, resulted in the major
photoproducts being I−-based photoproducts (> 90 %). Recombined ICN− constituted 5–10 % of
the total, and CN−-based products are absent. Additionally, the I−-based and ICN−-based
photoproducts are highly solvated. This last observation, coupled with calculations, shows thatthe excitation energy is not solely partitioned into solvent evaporation; instead, roughly 20% of
the 1 eV excess energy is partitioned into CN rotation. Furthermore, recombined photoproducts
are observed for n = 1–5. Due to the small Ar solvent binding energy, ~50 meV,67 and the124flatness of the energy landscape around ICN−, an Ar solvent atom can explore configurations
different from its minimum energy configuration above the I–C bond. Specifically, an Ar solvent
atom can sometimes be found near the CN end of the ICN− chromophore. A collision between
the departing CN diatom and the solvent Ar atom following photodissociation can convert asubstantial amount of CN translational energy into CN rotational energy. This results insufficient energy removed from CN translation to trap I…CN− on the excited state surface,possibly for a sufficiently long time for a radiative transition from the 2Π1/2 excited state to theX̃ 2Σ+ state.40 This radiative process is invoked because there is no other way to eliminate thismuch energy when a single solvent molecule is present.
Following photoexcitation of ICN−(Ar)n with UV light, the dissociation dynamics could
possibly involve the avoided crossing between the 2Σ+ and 2Π1/2 excited states. Excitation to anddissociation on the 2Σ+ excited state without the contribution of nonadiabatic dynamics would
result in observation of only CN−-based photoproducts. If the avoided crossing contributes to thedissociation dynamics, I−-based and ICN−-based photoproducts are expected to be observed aswell. It is likely that observation of I−-based photoproducts is the result a nonadiabatic transitionand dissociation on the 2Π1/2, while the ICN−-based products are observed following a radiativetransition from the 2Π1/2 excited state to the X̃ 2Σ+ state. While solvated I−-based photoproducts
are observed following UV excitation of ICN−(Ar)n, the degree of solvation is not as extensive asthat following 500 nm excitation. Furthermore, photoproducts as a result of one-atom caging arealso not observed following UV excitation. However, a significant contribution ofCN− photoproducts is observed and illustrates multi-state dissociation dynamics of ICN−(Ar)nfollowing UV excitation.

125
6.2 Experimental
The photofragmentation studies of ICN−(Ar)n are performed utilizing a tandem
time-of-flight mass spectrometer (TOF MS) in conjunction with a nanosecond laser system. The
sample of ICN is synthesized in the same manner described in Sec. 3.2 of Chapter III.
Additionally, the TOF MS and experimental procedures are detailed in Chapter II; therefore,
only information relevant to the generation of UV radiation will be presented here. The data
presented here is the result of only five sets of data and is, therefore, only preliminary data;
further photofragmentation studies following UV excitation of ICN−(Ar)n are ongoing.
For the photofragmentation studies presented in this chapter, UV radiation is utilized. As
detailed in Chapter II, the nanosecond laser (Coherent, model Infinity) is equipped with acommercial optical parametric oscillator (henceforth referred to as XPO), providing signal
radiation between 420 and 680 nm. For the experiments presented in this chapter, a second
harmonic generation stage (Coherent) positioned following the XPO, frequency doubles thesignal radiation to generate UV light (260–340 nm). The light is collimated in a 2:1 telescope
before entering the TOF MS. The energy content of a single pulse is ~260 μJ throughout thisspectral range. The cross-sectional area of the laser beam is measured utilizing the knife-edge
technique detailed in Chapter II.
6.3 Preliminary Results for UV Photoexcitation of ICN−(Ar)n
Preliminary studies of the photodissociation dynamics of ICN−(Ar)n following
270 to 330 nm excitation have been performed in order to elucidate the role of multiple excitedelectronic state dynamics in the UV dissociation dynamics. Preliminary calculations performed

126
by McCoy137 show the transition moments between the X̃ 2Σ+ state and the 2Π1/2, 2Σ+ and2

Π1/2/2Π3/2 excited states allow the possibility of both states playing a significant role in the

dissociation dynamics. If a transition from the X̃ 2Σ+ state to the 2Σ+ occurs, the nonadiabatic
crossing between the 2Σ+ and 2Π1/2 excited states discussed in Chapter IV likely plays asignificant role in the dissociation dynamics.
6.3.1 Photodissociation Action Spectra
Preliminary measurements of the relative cross section for the production of ionicphotoproducts following 270 to 330 nm excitation of ICN− show a peak in the cross section near
300 nm. These results are determined in a similar manner to those obtained from visible
photolysis. As described in Chapter II, the cross sections reported are relative cross sections
because detection efficiencies of the photoproducts and parent ions as well as the spatial overlapof the ion-laser interaction are not measured.
Figure 6.2 shows the relative cross sections, σrel(λ), for the production of I− and CN−
photoproducts following 270 to 330 nm excitation of ICN− with no solvent. Additionally, therelative cross sections for the production of I− and CN− following 430 to 650 nm excitation of theICN− chromophore are shown as a reference. All values in Fig. 6.2 are normalized to the relative
cross section for the production of I− at 300 nm. The error limits are the average percent error foreach photoproduct after five data sets; their values are defined in the caption for Fig. 6.2. The
full-width-at-half-maximum (FWHM) of σrel(λ) for the production of I− products from
270–330 nm is ~50 nm, compared to ~60 nm from 430–650 nm. The increased relative crosssection for the production of I− products following UV excitation indicates a stronger transition
is being probed than that observed following 430–650 nm excitation. A sudden increase in the127cross section of CN− at 280 nm is shown in Fig. 6.2; however, experiments presently being
performed show a contribution from the CN− photoproduct following 270 nm excitation andsuggest that this sudden onset is not as drastic as indicated in Fig. 6.2.138

Normalized  rel ()1.0I Products
CN Products

0.8

0.6

0.4

0.2

0.0
260 280 300 320 340 420 440 460 480 500 520 540 560 580 600 620 640 660

Wavelength (nm)Figure 6.2 Normalized relative cross section (σrel(λ)) for the production of I− and CN− following
270 to 330 nm excitation of the ICN− chromophore. The normalized relative cross section for the
production of I− and CN− following 430 to 650 nm excitation of ICN− is shown for reference. All
values are normalized to σrel(λ) for the production of I− products at 300 nm. The error limits are
the average percent error for each photoproduct and are defined as follows: 15% for CN− and
10% for I− following 430–650 nm excitation, and 12% and 5% for CN− and I− following
270–330 nm excitation, respectively. Note the break in the x-axis.

The dominance of I− photoproducts following 270–330 nm excitation of the ICN−
chromophore suggests that photoexcitation is either accessing the 2Π1/2/2Π3/2 or the 2Σ+ excitedstates. If excitation to the 2Σ+ excited state occurs, I− photoproducts are produced following a
nonadiabatic transition to the 2Π1/2 state asymptote. Preliminary calculations by McCoy137 show

128
that the 2Σ+ and the 2Π1/2/2Π3/2 states lie close in energy at the ground state equilibrium geometryof ICN− following 300 nm excitation. Additionally, similar to IBr−,102 the 2Σ+ ← X̃ 2Σ+ transition
has the largest transition moment, followed by the 2Π1/2 ← X̃ 2Σ+ transition, with the smallest of
the three moments being the 2Π1/2/2Π3/2 ← X̃ 2Σ+ transition. Thus, the I− products are proposed toresult from excitation of the ICN− chromophore to the 2Σ+ excited state and a subsequent
nonadiabatic transition. Due to the possible contribution of nonadiabatic dynamics, the CN−
photoproducts observed following 270 to 330 nm excitation of the ICN− chromophore are likely
produced following a nonadiabatic transition to the 2Π1/2 state and dissociation on the adiabatic
2 +Σ excited state asymptote. This results in a cross section for the production of CN−

photoproducts that is roughly a factor 40 times larger than the cross section following
430 to 650 nm excitation of the ICN− chromophore, for which only a minor yield of CN−
products are observed (e.g. ~3% following 500 nm excitation).
6.3.2 Photofragmentation Data at 300 nm
The ionic photoproduct distribution for the photofragmentation of ICN−(Ar)n following
300 nm excitation is shown in Fig. 6.3. Additionally, Table 6.1 tabulates the yield of each ionicphotoproduct observed following 300 nm excitation of ICN−(Ar)n; the bold red values inTable 6.1 give the percentage of each dissociation channel (i.e. I−-based, CN−-based, or
ICN−-based) and are plotted in Fig. 6.3. Laser power dependence studies were performed toconfirm that all ionic photoproducts were the result of one-photon photodissociation of
ICN−(Ar)n.

129

Ionic Photoproduct Yield

1.0

0.8

I -basedCN -based
ICN -based

0.6

0.4

0.2

0.0
0

1

2

3

4

5

Number of Ar in Parent Cluster, nFigure 6.3 Ionic photoproduct distribution of ICN−(Ar)n following one-photon 300 nm
excitation. The data points are the bold red values in Table 6.1. The error bars correspond to a
5% standard deviation and are plotted for each ionic product distribution.

130
Table 6.1 One-photon ionic photoproducts following 300 nm excitation of ICN−(Ar)n.
Parent Cluster−

ICN

CN−-Based

I−-Based

ICN−-BasedCN− = 19%

I− = 81%19%

81%

CN = 18%

I = 41%I−(Ar) = 41%

18%

82%

CN = 13%

I = 32%I− (Ar) = 54%

ICN− = 1%

13%

86%

1%

CN = 10%I = 20%I− (Ar) = 39%
I− (Ar)2 = 28%

ICN− = 3%

10%

87%

3%

8%

89%

3%

5%

94%

1%

−

ICN−(Ar)−

ICN−(Ar)2
−

ICN−(Ar)3
CN− = 8%ICN−(Ar)4
CN− = 5%ICN−(Ar)5−−−

I− = 15%I−(Ar) = 37%
I−(Ar)2 = 37%
I− = 9%I−(Ar) = 22%
I−(Ar)2 = 42%
I−(Ar)3 = 17%
I−(Ar)4 = 4%0%0%ICN− = 3%

ICN− = 1%

Following 300 nm excitation of ICN−, roughly 81% of the ionic photoproducts observed
are I−. For I− photoproducts to be observed, the chromophore must dissociate on either the2

Π1/2/2Π3/2 excited state or on the 2Π1/2 state following a nonadiabatic transition from the 2Σ+

excited state. As stated in Sec. 6.3.1, preliminary calculations137 show that the transition from theX̃ 2Σ+ state to the 2Σ+ excited state has the largest transition moment, and it is therefore likely that

131
ICN− is predominantly excited to the 2Σ+ state, after which the dissociating chromophore
undergoes a nonadiabatic transition to the 2Π1/2 state, resulting in I− products. Within the limits ofa one-dimensional Landau-Zener approximation139-141 of the transition probability between two
states in an avoided crossing, the high amount of kinetic energy supplied to ICN− following
300 nm excitation (roughly 2.36 eV at the 2Σ+ excited state asymptote) favors a nonadiabatic
transition from the 2Σ+ state to the 2Π1/2 state and, thus, dissociation into I− + CN (X̃ 2Σ). Though
the hopping probability is greater due to the large kinetic energy, the observation of CN−
products following UV excitation implies that a small portion (~19%) of the ionic photoproducts
are possibly a result of adiabatic dissociation on the 2Σ+ excited state into I* (2P1/2) and CN−. This
result is in agreement with the proposal that more than one excited state contributes to thedissociation dynamics of the ICN− chromophore following UV excitation. Additionally, a minor
contribution from dissociation on the 2Π1/2/2Π3/2 excited state may occur, but the present
experimental setup cannot confirm this proposal and theoretical work is in the preliminary
stages.
Upon addition of Ar solvent atoms, the ionic photoproduct distribution is altered in favorof a larger yield of I− photoproducts, a smaller yield of CN− products, and a minor contribution
from recombined products. With the addition of one Ar solvent atom, the only observed ionicphotoproducts following UV excitation of ICN−(Ar) are I−-based and CN−; recombined ICN− isnot detected. Following the addition of a second Ar solvent, interestingly, recombined ICN− isobserved. Qualitatively similar to the results of 500 nm excitation of ICN−(Ar)n (see Chapter III),the yield of recombined products remains at roughly 3% for all values of n studied, but now withthe excitation energy increased from 2.5 to 4 eV. Production of recombined products following
300 nm excitation might occur via a mechanism similar to that proposed for 500 nm excitation,

132
with the exceptions that a nonadiabatic transition from the 2Σ+ state to the 2Π1/2 state must occur
first and there is much more excess energy that must be taken away by either radiation or the two
departing Ar atoms. It might be that, following 300 nm excitation to the 2Σ+ excited state, thedissociating chromophore undergoes a nonadiabatic transition to the 2Π1/2 state and becomestrapped in a relatively shallow (~270 meV) well. The trapping time would have to be sufficiently
long to allow a radiative transition to the ground state. In contrast to 500 nm excitation, therecombined photoproducts observed following 300 nm excitation are predominantly ICN−, witha small contribution from ICN−(Ar). However, even a single Ar atom remaining attached to therecombined ICN− is astonishing.
As the number, n, of Ar solvent atoms is increased, the yield of I−-based products steadily
increases from roughly 82% of the ionic photoproducts for n = 1 to roughly 94% for n = 5.
Following 300 nm excitation of ICN−(Ar)n, all CN− photoproducts observed are unsolvated,
whereas the I−-based products are highly solvated. For example, following 300 nm excitation of
ICN−(Ar)2, I− (~32%) and I−(Ar) (~54%) are observed but only unsolvated CN− is observed(13%). This is qualitatively similar to 500 nm excitation of ICN−(Ar)n, in which highly solvatedI−-based products were observed, but the dominant photoproduct was often the highest solvated(largest number of Ar solvent) products. These observations are tantalizing, but do not point tothe underlying mechanism; thus, more work is needed.
More recent photofragmentation studies138 of ICN−(Ar)n following UV excitation inwhich more sets of data have been measured observe additional weak photoproducts not shownin these preliminary results. Although the ionic photoproduct distributions and the cross section
for the production of CN− following 270 and 280 nm excitation are slightly different, thequalitative results are the same for these preliminary results and the current experiments.

133
6.4 Summary of Preliminary Results
Photofragmentation of ICN−(Ar)n (n = 0–5) following UV excitation results in dynamics
that are possibly affected by an avoided crossing between the 2Σ+ and 2Π1/2 excited states. The
relative cross section for the production of I− and CN− following 270 to 330 nm excitation of theICN− chromophore reveal the dominance of I− photoproducts, but also show the presence of CN−
products. The I−-based and unsolvated CN− photoproducts following UV excitation of ICN−(Ar)nsuggest multiple excited state are accessed, but are the result of an undetermined mechanism.
Additionally, only a small contribution (~3%) from recombined products is observed for n = 2–5.
6.5 Future Directions
Future studies of the photodissociation dynamics of solvated ICN−will focus on theenergy flow within the chromophore and the effect of different solvents on the dynamics.
Photofragmentation studies following 400, 500, and 600 nm excitation of ICN−(Ar)n (n > 5)
clusters would complete the comparison between the short-range ion-induced dipole interaction
between ICN− and Ar versus the long-range ion-quadrupole interaction of ICN− with CO2.
Additionally, analysis of the average number of Ar solvent atoms lost following photoexcitationof ICN−(Ar)n in the large cluster limit would elucidate the location of solvation as n is increased
and provide another experimental measurement of the solvent binding energy of Ar to the ICN−
chromophore. Photofragmentation studies of the ICN− chromophore solvated by molecules withstrong permanent dipole moments would further develop the understanding of dynamics induced
by long-range interactions. Solvents such as water, ammonia, and carbonyl sulfide, could be
utilized to provide a range of strength of the solvent’s dipole moment. Increased solute-solventand solvent-solvent interactions, as a result of solvent dipole moments, may result in an134
increased yield of recombined ICN− photoproducts. Furthermore, theoretical studies simulating
the photodissociation dynamics of the ICN− chromophore on the 2Σ+ excited state, involving thenonadiabatic avoided crossing, and including solvation of ICN− would help elucidate the detailsof the photodissociation dynamics.
Time-resolved studies of solvated ICN− would also provide insight into the dissociation
dynamics of this triatomic pseudo-dihalide. Performing time-resolved photoelectron
spectroscopy studies could potentially provide experimental evidence for the involvement of the
avoided crossing and monitor the appearance time of various photoproducts following
photodissociation of ICN−(CO2)n. However, because of the small Ar solvent binding energy toICN− (~50 meV),67 identification of the spectral features corresponding to each photoproductfollowing photodissociation and photodetachment of ICN−(Ar)n is unfeasible. Time-resolved
absorption recovery experiments on ICN−(CO2)n would elucidate the presence or absence of the
NCCO2− molecule or solvent-separated-like complexes following photoexcitation of larger
clusters and potentially measure the timescale of recombination of the dissociating chromophore.
Unfortunately, because the yield of recombined one-photon photoproducts never exceeds
roughly 50% (for all wavelengths studied), the signal intensity of two-photon photoproducts is
exceedingly small, making time-resolved experiments difficult. Furthermore, because the effects
of differential solvation in ICN−(CO2)n are proposed to be similar to those observed forICl−(CO2)n,77 the time scale for absorption recovery of ICN−(CO2)n is expected to surpass that ofIBr−(CO2)n, possibly overreaching the limit of the present experimental apparatus.
Photodissociation studies of other triatomic solutes within molecular clusters would
provide further understanding of the importance of the internal degrees-of-freedom in the135dissociation dynamics. Experimental studies of the photofragmentation of BrCN−(Ar)n and
ClCN−(Ar)n would elucidate the role of the CN rotation following photoexcitation. As the mass
match between the halogen and the CN diatom becomes closer, the translational and rotationalenergy partitioned into the CN fragment following photodissociation decreases. Therefore,
solvent evaporation may play an increased role in the dissociation dynamics as the mass of thesolute is decreased from ICN− to BrCN− to ClCN−.

136

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