PHOTOELECTRON SPECTROSCOPY OF ORGANIC AROMATIC ANIONS

by
Daniel John Nelson
B.S., University of Wisconsin – Madison, 2010

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
Chemical Physics
2017iThis thesis entitled:
Photoelectron Spectroscopy of Organic Aromatic Anions
written by Daniel John Nelson
has been approved for the Committee on Chemical Physics
by___________________________________
W. Carl Lineberger___________________________________
Veronica M. Bierbaum

The final copy of this thesis has been examined by the signatories, and wefind that both the content and the form meet acceptable presentation standardsof scholarly work in the above mentioned discipline.

DATE:___________________ii

Nelson, Daniel John (Ph.D. Chemical Physics)
Photoelectron Spectroscopy of Organic Aromatic Anions
Thesis directed by Professor W. Carl Lineberger
This dissertation reports and interprets the results of experiments in which photoelectron
spectroscopy was performed on a variety of aromatic anions. In addition to these photoelectron
studies, the results and conclusions of an experiment in which HCl is scattered off atomically flatAu (111) surfaces are also presented.
Photoelectron spectroscopy of the isomers of methylphenoxide reveals that thesemolecules display minimal vibrational excitation upon photodetachment, accessing the electronic
ground and first excited state of the corresponding radicals. The photoelectron spectra of pmethylphenoxide reveal a photon energy dependence arising from electron autodetachment. The
slow electron velocity map imaging (SEVI) technique was employed to obtain the electron
affinities (EAs) of these radicals with an uncertainty of 1.4 meV. Combining the measured EAs
with previously measured O–H bond dissociation energies in a thermodynamic cycle allows forthe measurement of the acidities of the methylphenols with an uncertainty that is an order ofmagnitude smaller than any previous measurement.
The full interpretation of the photoelectron spectra of the isomers of methylenephenoxide
presents a far greater challenge with many subtleties. The EAs of o- and p-methylenephenoxyl
were measured and shown to be ~1 eV lower in energy than for the methylphenoxyls, implyingthat the electron withdrawing effect of the CH2 group in the methylenephenoxides attracts
electron density from the oxygen site via resonance, as compared to the methylphenoxides. Theiii

singlet–triplet splittings of the diradicals o- and p-methylenephenoxyl were measured. The
acidities of the methylenephenols were measured by acid bracketing. Combining the EAs of themethylenephenoxyls with these acidities allows for a measurement of the weak O-H bonddissociation energy of the methylenephenols.
The photoelectron spectra of indolide were obtained and interpreted. The structure ofindolide minimally distorts upon electron photodetachment accessing the electronic grounddoublet state of indolyl. The EA of indolyl was measured utilizing the SEVI technique with anuncertainty of 1.7 meV. Ring distortion vibrational modes were found to be excited uponelectron photodetachment. The previously measured acidity of indole is combined with ourmeasurement of the EA of indolyl to determine the N–H bond dissociation energy of indole.

iv

DEDICATION

To the memory of my late grandfather, whose name I bear, Daniel W. Nelson.

v

ACKNOWLEDGEMENTS
The simple truth is that there are far too many people to thank and acknowledge here whohave supported me throughout the sometimes grueling course of my graduate education. It was
only with the considerable compassion, love, and understanding of these people that I was able
to complete this Ph.D.
Of course, my advisor W. Carl Lineberger deserves special recognition. His advice andcounsel on both academics and life over the course of these last few years has been invaluable.
Carl, if you’re reading this, know that I will always treasure what you’ve taught me. In so many
ways, thank you.
Starting at the beginning of my graduate career, I must thank those people who took in anarrogant, idiotic undergrad and transformed him into an experimentalist. Thank you to Joseph
Robert Roscioli, the post–doc who first trained me, starting from how to turn on a laser without
electrocuting myself. In a similar vein, I also thank David Nesbitt who opened my mind andtaught me, for the first time, how to actually understand quantum mechanics. Everyone in the
Nesbitt group at that time also deserves a thank you: Tom Baker, Andrej Grubisic, Grant
Buckingham, Mia Zutz, Nick Dupuis, Julie Fiore, Andy Gisler, Erik Holmstrom, Melanie
Roberts, and Mike Ziemkiewicz. After Rob left the group for new horizons, I would next workwith Monika Grütter Kasumaj, a phenomenal scientist I was privileged with the opportunity tolearn from.
I would subsequently begin my work in the Lineberger group, and specifically, I teamed
up with Wilson K. Gichuhi. Wilson and I would spend the next two years on one of the mostdifficult projects I have ever been challenged with. Thank you so much for your patience andtenacity, Wilson. I found in Wilson one of the kindest and most hard working people I will everviknow. It was also at this time when I started to collaborate with the Bierbaum group, andeveryone there deserves my gratitude, including Charles Nichols, Callie Cole, and especially
Veronica M. Bierbaum, herself! Finally, the last post-doc I worked with was Julia H. Lehman.
Julia is, without question, one of the finest scientists I have ever met. Julia taught me how to be abetter scientist myself, and especially how to be a better writer. Julia would go on to become apersonal friend as well as professional colleague. Thank you, Julia. Your support and friendshipmean the world to me. In this last year of my studies I have worked with Allan Maple deOliveira, a fantastically enthusiastic grad student who has also become a friend of mine. Thankyou Allan, for your help and support- especially as I’ve slowly pulled a disappearing act theselast several months to try and actually complete this ridiculous document. Other members of the
Lineberger group deserve a thank you as well: Elisa M. Miller, Amanda Case, and Yu-Ju Lu.
All the members of the JILA support staff deserve my sincere thanks. You all are far
more talented and skilled than even your considerable reputation suggests. Terry Brown in theelectronics shop is especially deserving of my thanks. I had the pleasure of working extensively
with him to rebuild whole laser systems, and I couldn’t have asked for a better man to help in acrisis.
Finally, I want to thank my friends and family. To my mom and dad, Jenny and Doug
Nelson, as well as my sisters, Katie Nelson Hardt and Sallie Nelson Haugen, thank you for all
the love and support you have given me my whole life. I wouldn’t be here without you. To myold friends, Luke Justman, Nick Doremus, and Dylan Jambrek, thank you for keeping me mostly
sane through high school and undergrad. I love you guys. And to my new friends, Steve
Harshman, Tony Tumminello, Rashawn Griffin, and to the whole Magic club, you’ve all added
so much to my new life out here in Colorado. Thank you.
vii

CONTENTS

Chapter I: Introduction and Background§ 1.1 Principles and Advantages of Anion Phototelectron Spectroscopy………………...1§ 1.2 General Background on Aromatic Organic Molecules and Radicals………......….11§ 1.3 Background on Methylphenoxides and Methylphenoxyls…………………...……15§ 1.4 Background on Distonic Radical Anions and Diradicals:
Methylenephenoxides and Methylenephenoxyls……………………………..…18§ 1.5 Background on Deprotonated Indole and Indoline……………………...………....21§ 1.6 References……………………………………………………………………….…23

Chapter II: Experimental Background and Methods§2.1 Introduction and Background………………………………………………………31§2.2 Overview: Continuous Photoelectron Spectrometer………………………………..36§2.3 Principles of Operation: Flowing–Afterglow Ion Source ……………….....………40§2.4 Principles of Operation: Wien Velocity Filter……………………………………...44§2.5 Principles of Operation: Argon–Ion Laser System…………………………………49§2.6 Principles of Operation: Hemispherical Electron Energy Analyzer………………..68§2.7 Overview: Pulsed Velocity Map Imaging Photoelectron Spectrometer……………73§2.8 Principles of Operation: Dual Pulsed Valve Ion Source………………….......…….76
viii

§2.9 Principles of Operation: Wiley-McLaren Time-of-Flight Mass Spectrometer……..79§ 2.10 Principles of Operation: Velocity Map Imaging Photoelectron Spectrometer...…81§ 2.11 References……………………………………………….………………………..85
Chapter III: Anion Photoelectron Spectroscopy of Deprotonated o-, m-, and p-Methylphenol§3.1 Introduction and Background……………………………………………………....87§3.2 Experimental Specifics……………………………………………………………..89§3.3 Theoretical Methods and Simulations……………………………………………...94§3.4 Expectations and Error Analysis……………………………………………………96§3.5 Electron Affinities and Term Energies……………………………………………..98§3.6 Vibrational Analysis: ortho-Methylphenoxyl……………………………………..108§3.7 Vibrational Analysis: meta-Methylphenoxyl……………………………………...115§3.8 Vibrational Analysis: para-Methylphenoxyl……………………………………...120§3.9 CH3 Deprotonated meta-Methylphenol…………………………………………...123§3.10 Photon Energy Dependence of the para-Methylphenoxide Photoelectron Spectra…………………………………………………………………………………………..127§3.11 Discussion………………………………………………………………………..133§ 3.12 Conclusions……………………………………………………………………...139§3.13 References………………………………………………………………………..140ix

Chapter IV: Photoelectron Spectroscopy and Thermochemistry of o-, m-, and pMethylenephenoxide Anions§4.1 Introduction and Background……………………………………………………..143§4.2 Experimental Specifics……………………………………………………………145§4.3 Theoretical Methods and Simulations…………………………………………….151§ 4.4 Results: Overview………………………………………………………………...152§ 4.5 Results: Photodetachment to the ground S0 Stateof o-, and p-Methylenephenoxyl………………………………………..159§ 4.6 Results: Photodetachment to the T1 State ofo-, and p-Methylenephenoxyl…………………………………………..167§ 4.7 Results: Unassigned Features…………………………………………………….171§ 4.8 Results: Thermochemistry………………………………………………………..176§ 4.9 Discussion…………………………………………….....………………………..179§ 4.10 Conclusions……………………………………………………………………...185§4.11 References…………………………………………………………………….….189
Chapter V: Anion Photoelectron Spectroscopy of Deprotonated Indole and Indoline§5.1 Introduction and Background……………………………………………………..190§5.2 Experimental Specifics……………………………………………………………192§5.3 Theoretical Methods and Simulations…………………………………………….195
x§5.4 Results: Overview…………………………………………………………………199§5.5 Results: Electron Affinity of Indolyl andPhotoelectron Angular Distribution of Indolide..................................................203§5.6 Results: Vibrational Analysis of Indolyl…………………………………………..206§5.7 Discussion………………………………………...……………………………….221§5.8 Deprotonated Indoline…………………………………………………………….224§5.9 Conclusions………………………………………………………………………..229§5.10 References………………………………………………………………………..230
Chapter VI: State-Resolved Velocity Map Imaging of Surface–Scattered Molecular Flux
§6.1 Introduction and Background…………………………………………..…………233§6.2 Experimental Setup and Apparatus………………………………..………………237§6.3 Collected Velocity Maps…………………………………………………………..246§6.4 Velocity Map Analysis……………………………………………………………249§6.5 Rotational State Distributions and Energy Maps……………………….…………258§6.6 Conclusions………………………………………………………………………..264§6.7 References…………………………………………………………………………265
Bibliography…………………………………………………………………………………....268
Appendix………………………………………………………………………………………..284xi

LIST OF TABLES
Table 2.1 Proton Affinities of Gas Phase Bases…………………............………………….…...42
Table 3.1 Calculated Thermochemistry of the Methylphenols…………………………..………90
Table 3.2. Summary of Experimental Results Obtained for the Methylphenoxides...................106
Table 3.3 Diagrams of the Primary Franck-Condon Active
Normal Modes of o-Methylphenoxide Upon Photodetachment......................................113
Table 3.4 Diagrams of the Primary Franck-Condon Active Normal Modes ofm-Methylphenoxide Upon Photodetachment.………………………...………………..118
Table 3.5 Diagrams of the Primary Franck-Condon Active Normal Modes of
p-Methylphenoxide Upon Photodetachment……………….…………………………..122
Table 3.6 Diagrams of the Primary Franck-Condon Active Normal Modes ofm-Methylenephenol………………………………………………………………...…..125
Table 3.7 Summary of Vibrational Results Obtained for the Methylphenoxides…….......…….126
Table 4.1 Summary of Vibrational Results Obtained for the Methylenephenoxides...……..….163
Table 4.2 Diagrams of the Primary Franck-Condon Active Normal Modes for
Each Product Studied. ……………………………………………………………….…164
Table 4.3 Summary of Acid Bracketing Results. ……………………….................…………..177
Table 4.4 Summary of All Results Obtained for the Methylenephenoxides…….......…………178
Table 5.1 Calculated Vibrational Analysis of Indolyl………………………………………….198
Table 5.2 Summary of Experimental Vibrational Assignments………………………………..210
Table 5.3 Diagrams of the Vibrational Normal Modes of Indolyl
in the Ground Electronic State. ……………………………………………………..…212
Table 6.1 Line Positions and State Assignments of(2+1) REMPI Transitions Utilized in HCl…………………………………………...….243

xii

LIST OF FIGURES
Fig. 2.1 Early measurement of the O– photoelectron cross section
as a function of photon energy………………………………..………………..…….......32
Fig. 2.2 Diagram of the continuous photoelectron spectrometer.…………………...........……...37
Fig. 2.3 An idealized schematic of the magnetic and electric fields that a negatively charged
particle q, with a velocity v, would encounter upon entering the Wien filter.………………...…46
Fig. 2.4 A schematic of the experiment prior to converting the laser system to be able to perform
UV photoelectron spectroscopy………………………………………………………………….51
Fig. 2.5 Schematic of the full laser system utilized in the current continuous photoelectron
spectrometer…………………………………………………………………………………...…53
Fig. 2.6 A qualitative schematic of how the laser light is frequency modulated as it impinges
upon the build-up cavity…………………………………………………………………………57
Fig. 2.7 An example of the FM-to-AM conversion when the dithered laser is scanned over thecavity fringe of the build-up cavity in frequency by monitoring the transmitted light intensity.
……………………………………………………………………………………………………59
Fig. 2.8 Qualitative schematic of the form of the error signal utilized in the laser locking system
as the laser fringe scans over a build-up cavity fringe. ………….........................................……61
Fig. 2.9 Schematic of the custom bayonet output coupler mount fabricated to accommodate apiezoelectric actuator. ………………………...............................................…………………....63
Fig. 2.10 Annotated circuit diagrams detailing the specific ways in which the laser servo works……………………………………………………………………………………………………67
Fig. 2.11 Schematic of the electron kinetic energy analyzer system with deceleration, input, andoutput lens stacks included………………………………………………………………………69
Fig. 2.12 Schematic of the pulsed VMI photoelectron spectrometer experiment……………….74
Fig. 2.13 A schematic of the ion source utilized in the experiments…………………………….77
Fig. 2.14 Picture of the VMI PE spectrometer, annotated……………………………………….84
Fig. 3.1 The 355 nm photoelectron spectra of the three methylphenoxide anion isomers.
……………………………………………………………………………………………………99

xiii

Fig 3.2 Photoelectron spectra of the products of methylphenol reacted with hydroxide in theflowing afterglow ion source obtained with a photon energy of 3.40814 eV and collected in ahemispherical energy analyzer………………………………………………………………….101
Fig. 3.3 The Slow Photoelectron Velocity-Map Imaging (SEVI) spectra of o-, m-, and pmethylphenoxide, allowing for the determination of the EAs of the corresponding
methylphenoxyls………………………………………...……………………………………...103
Fig. 3.4 The 355 nm anion photoelectron spectra of o-, m-, and p-methylphenoxide, examining
the first excited doublet electronic states (𝐴̃) of the methylphenoxyl radicals.……….………..105
Fig. 3.5. The anion photoelectron spectra of o-, m-, and p-methylphenoxide, examining the
ground doublet electronic states of the methylphenoxyls………………...…………………….110
Fig. 3.6 Photoelectron spectrum of ortho-methylphenoxide taken with a photon energy of 2.822
eV, utilizing the VMI photoelectron spectrometer…….…………….........................................111
Fig. 3.7 The photoelectron spectrum of meta-methylphenoxide taken on the VMI spectrometerwith a photon energy of 2.329 eV………………………………………………………………116
Fig. 3.8 Photoelectron spectrum of methyl deprotonated meta-methylphenol, taken with a photon
energy of 3.408 eV and collected with a hemispherical energy analyzer..……….….................124
Fig. 3.9 Photoelectron spectra of p-methylphenoxide taken at a variety of photon energies,
displaying a photon energy dependence as discussed further in the text.……………….......….128
Fig. 3.10 Photoelectron spectra of para-methylphenoxide obtained with the VMI spectrometerwith photon energies of 2.329 eV and 2.637 eV…………………………………….………….129
Fig. 3.11 The photoelectron spectrum of meta-methylphenoxide taken on the VMI spectrometerwith photon energies of 3.494 eV and 3.832 eV.……………...........................................……..132
Fig. 4.1 Mass spectra collected on the FA-SIFT instrument of selectively deuterated cresols
reacted with O –............................................................................................................................153
Fig. 4.2 Photoelectron spectra taken on the VMI photoelectron spectrometer with a photon
energy of 3.494 eV of the three isomers of methylenephenoxide...............................................157
Fig. 4.3 Photoelectron spectra of the o- and p-methylenephenoxide anions obtained using
Instrument A utilizing a photon energy of 3.408 eV……………..................................……….160
Fig. 4.4 Photoelectron spectra accessing the first triplet electronic excited states of the o- and pmethylenephenoxyl diradicals........................................................................……….......……..168
Fig. 4.5 Photoelectron spectra of m-methylenephenoxide anions obtained on instrument B withtwo different photon energies, 3.494 and 3.832 eV.………………..........................…………..173xiv

Fig. 4.6 Photoelectron Spectra of the three isomers of the methylenephenoxide anion, collectedon the VMI PE spectrometer with a photon energy of 2.330 eV.................................................174
Fig. 4.7 Photoelectron spectra obtained with a photon energy of 2.330 eV on instrument B.
These spectra were obtained by simply alternating the laser timing to either intersect m/z = 106(methylenephenoxide, black curves) or 107 (methylphenoxide, red curves) .............................175
Fig. 5.1 The photoelectron spectrum of indolide obtained with a photon energy of 3.494 eV.............................................................................……………………….................................…201
Fig. 5.2 The Slow Electron Velocity–Map Imaging (SEVI) PE spectrum of the origin transitionof indolide acquired with a photon energy of 2.4500 eV...............................................……….204
Fig. 5.3 Photoelectron spectra of indolide obtained with three separate photon energies, 2.518 eV(upper panel), 2.588 eV (middle panel), and 2.678 eV (lower panel)……...........................…..207
Fig. 5.4 The photoelectron spectrum of the anionic product(s) of F– + indoline with a mass tocharge ratio of 118......................................…………………………………………………….226
Fig. 6.1 Experimental setup of the velocity-map imager (VMI) adapted to probe surface
scattering dynamics......................................................................................................................238
Fig. 6.2 Velocity map images of scattered HCl from a 1% HCl/H2 incident supersonic jet with Einc
= 21(5) kcal/mol...........................................................................................................................247
Fig. 6.3 Integrated velocity distributions parallel to the scattering plane (along vin-plane) for
scattered HCl molecules in J = 2 - 12 originating from a 21 ± 5 kcal/mol incident beam…………………………………………………………………………………………………..250
Fig. 6.4 Tracking the energy content in various degrees of freedom as a function of J-state.
......................................................................................................................................................253
Fig. 6.5 Rotational distribution of molecules scattered from a 1% HCl/H2 beam at 21 ± 5
kcal/mol....………………………………………………………………………………………259
Fig. 6.6 Rotational energy map of a scattered HCl beam (Einc = 21 ± 5 kcal/mol) at every value of
vin-plane and vout-of-plane.……..…..............................................................................................…...261

xv

CHAPTER IINTRODUCTION AND BACKGROUND
Portions of this chapter have been published or will be published in the Journal of Chemical
Physics under the titles of:“Anion Photoelectron Spectroscopy of Deprotonated ortho-, meta-, and para-Methylphenol”
By, Daniel J. Nelson, Wilson K. Gichuhi, Elisa M. Miller, Julia H. Lehman, and W. Carl
Lineberger“Anion Photoelectron Spectroscopy and Thermochemistry of o-, m-, and pMethylenephenoxide”
By, Daniel J. Nelson, Wilson K. Gichuhi, Charles M. Nichols, Julia H. Lehman, Veronica M.
Bierbaum, and W. Carl Lineberger“Anion Photoelectron Spectroscopy of Deprotonated Indole and Indoline”
By, Daniel J. Nelson, Allan M. Oliveira, and W. Carl Lineberger§ 1.1 Principles and Advantages of Anion Photoelectron Spectroscopy
Anion photoelectron spectroscopy is a form of spectroscopy which is quite distinct fromother forms of spectroscopy.1, 2 In principle, this process is understandable in broad strokes: aphoton possessing a known amount of energy interacts with an anion. If the photon possesses
more energy than the binding energy of an electron in the anion, there is a chance that the anion
will eject that electron and simultaneously absorb the photon. This may be summarized by the
deceptively simple formula:hν + A– → A + e– (KE)
Where A– represents an anion, which can take the form of an atom or molecule. Since one
electron is ejected from the anion in question, it is necessarily the case that either the anion or thecorresponding neutral is an open shell radical. As one can see by examining the above formula,
there are several methods one might employ to measure the quantum state distribution of the1

anion or neutral. Perhaps most intuitively, one might measure the attenuation of the light as it
passes through a cloud of anions as a function of the frequency of the light. This is a nearly
impossible measurement, however, as one can practically only make very low densities of ions,such that the attenuation of the light through the ion cloud would be so small as to render thistype of measurement impossible without herculean efforts. The inverse of this scheme has beenutilized however, i.e. measuring the attenuation of the ions as they pass through a radiation field.
This is a plausible experimental design, as it is possible to measure ion currents with highsensitivity with either a Channeltron or Microchannel Plates (MCPs). Historically, this method
has been used to obtain absolute photoelectron cross–sections of anions. As an alternative tothese methods, one could instead collect the electrons produced. As these particles are charged,
it is again possible to measure them on a single event basis with either a Channeltron or MCPs.
In order to obtain quantum state sensitivity, one of two methods must be employed whencollecting electrons. Either the photon energy may be scanned and all of the photodetachedelectrons are collected as a function of photon energy (photodetachment spectroscopy), or a fixed
frequency laser is used and the electrons are gathered and then their kinetic energy is measured(photoelectron spectroscopy). This latter process, photoelectron spectroscopy, is utilized
throughout this work, and a detailed, theoretical explanation follows.
Before detailing the experimental steps taken (Chapter II), we will first focus on thetheory of photoelectron spectroscopy, and how one might analyze and understand the physicalmeaning of a photoelectron spectrum. Assuming that one is able to measure the kinetic energiesof the electrons, and given that we know the photon energy, by simple conservation of energy wemust also know the energy which was left in internal states of the remaining neutral molecule oratom. This is achieved simply by subtracting the electron kinetic energy (eKE) from the photon
energy to arrive at the electron binding energy (eBE):
2

𝑒𝐵𝐸 = ℎ𝜈 − 𝑒𝐾𝐸
The only assumption which has been made so far is that upon photodetachment, all of the
kinetic energy released will be deposited in the electron. How good is this assumption? This is astraightforward classical mechanics problem. One may accurately model the situation thusly:
Upon photodetachment there is a known amount of energy which must be accounted for, namely,
the energy of the absorbed photon. Some portion of this energy must be deposited within theinternal degrees of freedom of the neutral molecule. This, by definition, cannot affect the centerof mass (COM) motion of the particles. This leaves the remaining energy which must be placedin either the kinetic energy of the molecule or the electron. However, there remain two
constraints which govern this: conservation of energy and conservation of momentum.
(Conservation of Energy) 𝐾𝐸𝑇𝑂𝑇 = 𝐾𝐸𝑚𝑜𝑙 + 𝐾𝐸𝑒𝑙𝑒𝑐 =

1
1
2
2
𝑀𝑣𝑚𝑜𝑙
+ 𝑚𝑣𝑒𝑙𝑒𝑐2
2(Conservation of Momentum) 𝑀𝑣⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗
𝑚𝑜𝑙 = 𝑚𝑣
𝑒𝑙𝑒𝑐
It is possible to define the fraction of kinetic energy which will be possessed by the electron.
1
2
𝑚𝑣𝑒𝑙𝑒𝑐𝐾𝐸𝑒𝑙𝑒𝑐2
𝐹==
𝐾𝐸𝑇𝑂𝑇 1 𝑀𝑣 2 + 1 𝑚𝑣 2
𝑚𝑜𝑙
𝑒𝑙𝑒𝑐
2
2
Upon the addition of conservation of momentum and some simplification one arrives at:

𝐹=

𝑚𝑣𝑒2
1
=𝑚
𝑚
𝑚𝑣𝑒2 (𝑀 + 1) 𝑀 + 1

Hence, if one conservatively estimates that the molecular or atomic mass is 50,000 times larger
than the electron mass, then F = 0.99998. This is a conservative estimate considering that
2,000 × 𝑚𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 ~ 𝑚𝑝𝑟𝑜𝑡𝑜𝑛 .3 It may be concluded then that the assumption that all of the3

available kinetic energy is found in the kinetic energy of the electron is appropriate for all cases
which are studied in this work.
Interpreting the measured electron binding energy distribution will first be done from aclassical perspective in order to facilitate intuition, and then will be considered from the quantum
perspective. Assuming that one is considering a molecular anion which is a classical object, one
would expect a smooth, continuous distribution of energy which could be placed into internal
degrees of freedom of the molecule following photodetachment of the corresponding anion, andthus the kinetic energy of the electron would also possess an analogous distribution. This leads
one to question how one might determine into which degrees of freedom this energy is deposited.
There are several physical pictures which may be used to motivate this process. Perhaps the mostintuitive is as follows: Upon photodetachment, all atoms comprising the molecule will take on
new geometrical equilibrium positions, which means that each atom is by definition displaced tovarying degrees from equilibrium. If this were a classical problem, we might be tempted tomodel the force connecting the atoms to each other (the bonds) as harmonic springs each with aseparate spring constant and with the initial condition that each atom is displaced just so from thesprings’ equilibrium positions at time zero. Solving this set of ordinary differential equation(ODE) problems is straightforward, if a bit tedious.4 However, it should be intuitively obviousthat those atoms which have been displaced further from their respective equilibrium position
1

2
will obtain more vibrational energy (~ 2 𝑘(𝑟⃗⃗⃗𝑜 − 𝑟⃗⃗⃗⃗⃗⃗⃗⃗𝑚𝑎𝑥 ) ) than those displaced less.

Of course, molecules are inherently quantum mechanical systems and not classical ones.
As such, we must treat the internal states of the molecule as quantized, not continuous, andrework the same problem with this understanding.5-7 Critically, the same intuitive result from theexamination of the classical case will hold. In the anionic state, the atoms within the molecule4experience a potential and from this a particular Hamiltonian may be written. Assuming that the
Hamiltonian is separable and the Born-Oppenheimer approximation holds, then the electronic,
̂𝑡𝑜𝑡 ≈ 𝐻
̂𝑛𝑢𝑐𝑙𝑒𝑎𝑟 + 𝐻̂𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑖𝑐 +nuclear, and spin coordinates may be treated separately (𝐻̂𝑠𝑝𝑖𝑛 ).Thus, we may consider only the portion of the Hamiltonian which concerns itself with the𝐻nuclear motion of the molecule or anion. Disregarding eigenfunctions/eigenvalues representing
either translation or rotation, then solutions may be found for the eigenvalues physicallycorresponding to the vibrations of the normal modes of the molecule. If at this point an electron
is removed from the system, the newly formed neutral molecule exerts a new potential on the
nuclei and electrons remaining in the system. This potential defines a new Hamiltonian andconsequently new eigenfunctions and eigenvalues as well. Hence, the previous wavefunction of
the system is no longer an eigenfunction of the newly formed Hamiltonian. Physically,
therefore, the system must have the current state be described by the neutral eigenfunctions.
Since the neutral eigenfunctions of the Hamiltonian represent a complete orthonormal set, this isa straightforward process. The anionic eigenfunction must be expanded in the complete
orthonormal basis set of the neutral eigenfunctions.
|𝛹𝑓𝑖𝑛𝑎𝑙 > =∑< 𝛹𝑛𝑒𝑢𝑡𝑟𝑎𝑙 |𝛹𝑎𝑛𝑖𝑜𝑛 > |𝛹𝑛𝑒𝑢𝑡𝑟𝑎𝑙 >𝑛𝑒𝑢𝑡𝑟𝑎𝑙 𝑠𝑡𝑎𝑡𝑒𝑠

While a full derivation here is not appropriate, this explanation is sufficient to motivate the factthat the probability that a certain vibrational state will be populated is therefore proportional to|< 𝛹𝑛𝑒𝑢𝑡𝑟𝑎𝑙 |𝛹𝑎𝑛𝑖𝑜𝑛 > |2. This value is commonly referred to as a Franck-Condon Factor(FCF).6, 7 Recall that since each of these newly populated quantized states have specificeigenvalues (disregarding degeneracy), or energies, associated with each, then the final quantized5

population state distribution of the molecules will be encoded necessarily on the eKE distributionof the measured electrons.
The FCF is an essential tool for predicting and interpreting photoelectron spectra, and has
several properties that are important to underscore. First, consider the case where the geometry
change between an anion and neutral is very small, with no bond or internal angle changing by
more than ~0.01%. From a classical picture, it is easy to anticipate that there will be virtually no
vibrational excitation. This is borne out in the FCFs as well. Assuming the majority of the anion
population is in the vibrational ground state, the overlap integral between the anion and theground vibrational neutral eigenfunction will approach unity, while the overlap with other states
will approach zero. An easy way to intuit this result is as follows: In the extreme case that thepotential energy surfaces are the same between the anion and the neutral, then the anion andneutral species will share the same complete orthonormal set of eigenfunctions. Since weassume the anion population is in the vibrational ground state, then by virtue of the orthogonalityof the set, the overlap with anything other than the ground state of the neutral will be identicallyzero. If one imagines making only slight changes to the potential energy surface from thisextreme case, it is easy to imagine that, while the overlap between the ground state of the anion
and non-ground states of the neutral are no longer identically zero, they remain small comparedto the overlap between the two ground states.
The second, and most common case of those molecules studied in this thesis, is that mostatoms in the molecule do not change their equilibrium geometry relative to each other, or only do
so to an insignificant degree, while only the geometry of a minority of the atoms change to somedegree. From the classical picture, it is intuitive to see that the normal modes which incorporate
motion along the changed bond lengths/angles will be vibrationally excited, while other normal

6

modes will remain dormant. This is again consistent with what the FCFs predict in such a
scenario. For the internal coordinates with very little change, only the ground vibrational statesof both the anion and neutral will have significant overlap with all other states resulting in verysmall overlap, comparatively. For modes with significant change, the neutral eigenfunction withthe greatest overlap with the anion ground vibrational state may correspond to eigenvalues
several or even many quanta above the ground state. This will lead to a clear progression of one
or several vibrational modes in the photoelectron spectra.
Finally, there is the case of a large geometry change for many of the atoms in themolecule upon photodetachment. This might correspond to a “floppy” molecule whose potential
energy surfaces are extremely flat, or a molecule for which several bonds severely weaken upon
photodetachment. As one would expect, this will result in a very significant amount of
excitation in virtually every normal mode. Indeed, under these circumstances, the normal
mode/harmonic approximation is almost certainly incorrect. That is to say Taylor expanding thepotential energy surface of the anion and neutral out to only second order about the equilibrium
position of each atom comprising the molecule and then solving the Schrödinger equation withthis simplified version of the potential energy surface is woefully inadequate to the task ofmodeling the behavior of the molecule. Even in the classical picture, non-rigid motion is all but
virtually impossible to model, often leading to chaotic motion. In the FC picture, this would beequivalent to the opposite of the first case; namely, the overlap between the ground anionicvibrational state and the ground neutral vibrational state will be small compared to the overlap
between the ground vibrational anionic state and other non-ground vibrational states of theneutral. This will, in general, result in far more transitions having significant FCFs than in theother two cases, and practically will result in a spectrum wherein no single transition may be
individually resolved.
7

Now that the basic principles of how to interpret a photoelectron spectrum areunderstood, one might question what benefit this form of spectroscopy offers? The answer tothis is several fold. First, by the very nature of PE spectroscopy, one does not necessarily need atunable laser. (We do utilize several such lasers in this work, but this is due to a design necessity,rather than a fundamental requirement of PE spectroscopy. This is discussed in greater detail in
Chapter II.) This is in contrast to most other spectroscopic techniques which require one to scan
the frequency of a laser over the spectroscopically active region of a molecule and observe either
light absorption or emission as a function of the laser frequency. In principle, for a given photon
energy utilized in a PE spectroscopy experiment, the investigator will receive data on all Franck–
Condon active transitions with binding energies less than that photon energy. This can be a majoradvantage, as obtaining tunable light sources over large (1-2 eV) ranges of photon energies canbe quite challenging. The second advantage of this form of spectroscopy lies in the molecules
which may be investigated. As was previously explained, the information gathered in a PE
spectroscopy experiment yields data primarily on the neutral molecule which is formed upon
photodetachment. The only requirement on the lifetime of a compound, therefore, is on the
lifetime of the anion. Thus, we may investigate short-lived neutrals, e.g. neutrals which aretransition states, so long as the anionic form of that neutral is long lived.8 This allows one toperform spectroscopic measurements of transition states which would be otherwise nearly
impossible to measure directly. The third advantage stems from the fact that this form ofspectroscopy necessarily removes a source of spin in the molecule, and as such, opticallyinaccessible neutral states may be investigated. For example, if an anion begins in the groundelectronic state with doublet multiplicity, after photodetachment it would be possible to accessboth the singlet and triplet electronic states of the resulting neutral molecule.

8

Photoelectron spectroscopy has one final advantage. However, this advantage is notdirectly due to the spectroscopic data which are collected. Instead, photoelectron spectra
inherently allow for another measurement- the photoelectron angular distribution. This
distribution and its interpretation was studied first in atomic anions by Cooper and Zare9 andlater expanded to Molecular Orbital Theory (MOT) by Sanov.10 This distribution, whenmeasured for molecular systems, often reveals the nature of the molecular orbital from which thephotoelectron was detached. The interpretation of this phenomenon may be understood as
follows: By its fundamental nature, an electron may possess angular momentum. Certainly, anelectron bound within an atomic system will possess different quantities of angular momenta,
depending upon the atomic orbital which describes its motion. However, the total angularmomentum of an electron bound in a molecule is not a good quantum number. If the molecule
happens to be a diatomic molecule, then the projection of the angular momentum on theinternuclear axis is a good quantum number. Unfortunately, none of the molecules which arestudied in this collection of works will be diatomic, and so this case will not be of use toconsider. Hence, we must approximate. MOT suggests that it is possible to describe a moleculeby the symmetry-adapted linear combination of the atomic orbitals of the atoms which comprisethe molecule in question.11 This leads to the formation of molecular orbitals, which may beapproximated to confer angular momentum analogous to the atomic total angular momentum
associated with the electrons which occupy them, despite this not being rigorously accurate.
When an electron is photodetached, it may therefore possess some amount of angularmomentum, limited by the angular momentum characteristic of the molecular orbital from which
it was detached. Practically, this translates into a difference in the probability of an electron
ejecting oriented either along the plane of the electric field of the laser which photodetached it orperpendicular to the electric field. This difference is characterized as the anisotropy and defined9

by the anisotropy parameter, β. In general, ejecting electrons from different electronic states(molecular orbitals) will result in a different value of β. This allows for one to distinguishfeatures in a photoelectron spectrum which arise from different electronic states. This is anespecially attractive feature as frequently a spectroscopist must rely on fitting quantum chemical
calculations to experimental spectra in order to identify electronic and vibrational transitions, but
this allows for an independent verification of different electronic states being accessed in a given
spectrum without the reliance on theoretical prediction.

10§ 1.2 General Background on Aromatic Organic Molecules and Radicals
In this work, the predominant class of molecules which have been studied are cyclic or
polycyclic aromatic anions and radicals. Aromatic molecules are a class of molecules which arecharacterized by possessing fully occupied cyclic π bonded molecular orbitals, such as in the
proto-typical case of benzene. This molecular orbital configuration offers a variety of qualitative
differences in reactivity and chemical behavior when compared to non-aromatic species, such as
an alkane. Taking benzene as an exemplar, the π bonds which connect each sp2 hybridized
carbon center to its nearest neighbor make the overall ring structure extremely rigid and flat, thus
attaining the high symmetry point group D6h.12 Cyclohexane, the immediate analog of benzene,
but a non-aromatic alkane, is far less rigid and possesses almost none of the symmetry ofbenzene. Cyclohexane is a bent molecule, with either a classic “chair” or “boat” structuralconfiguration. Upon the addition of functional groups onto an aromatic species, the aromatic π
structure allows for chemical resonance between the aromatic ring and the functional groups.
When two separate functional groups are attached to a benzene ring, this resonance effect allows
for the two moieties to influence the others behavior. For example, adding an electron donatinggroup to benzene, such as NH2, and an electron withdrawing group, such as COOH, to the paraor 3 position on the benzene ring, will cause electron density to be pulled onto the COOH groupfrom the NH2 group, thus preparing the COOH group for electrophilic attack. This represents justone way in which an aromatic molecule might behave differently from a non-aromatic species,
and the reader is directed to Ref. 12 or another textbook covering organic chemistry for an
exhaustive review of this topic.
Radicals are critical to understanding nearly, if not all reaction mechanisms, and play a
key role in all fields of chemistry as a result.13-15 Molecular radicals are easiest to understand11

from a molecular orbital standpoint. They are simply molecules that possess a partially filledmolecular orbital, commonly a valence orbital. Due to this orbital being only partially filled,
radical species are extremely reactive with short lifetimes, often possessing a reaction rate on theorder of the collision rate, if not faster if they undergo unimolecular reactions.16-18 Thus, it isfrequently an experimental challenge to observe them directly, despite radicals frequently
playing the role of reaction intermediates. As mentioned previously in § 1.1, when one conducts
photoelectron spectroscopy, either the anion or the neutral that results from photodetachment of
the anion must be a radical. In the case of the anions studied in this work, the neutrals which
result from photodetachment are all radicals, which allows for our experiments to directly probe
what would otherwise be a difficult species to spectroscopically observe. The umbrella of“radical” covers many other specific cases. Studied in this work, in addition to the standard
radical case, are diradicals, i.e. molecules with two molecular orbitals only partially filled, anddistonic radical anions, i.e. an anion which is also a radical, but the radical site is localized on adifferent part of the molecule from the site containing the negative formal charge. As one might
imagine, these molecules have reactivities which defy simple physical models of their behavior.
These will be explained in detail on a case by case basis in the following chapters.
The molecules studied in this dissertation were chosen deliberately for a variety ofscholarly reasons. First, cyclic and polycyclic organic aromatic molecules are vitally importantto numerous fields of physical chemistry,19-22 analytical chemistry,23-25 astrochemistry,26, 27
biochemistry,28, 29 combustion chemistry,23, 24 organic chemistry,30, 31 and biophysics.32, 33 While
the full list of applications wherein organic aromatics are key reaction intermediates is too large
to go into detail, here are several examples: In astrochemistry, polycyclic aromatic hydrocarbons(PAHs) are believed to be major contributors in Diffuse Interstellar Absorption Bands (DIBs).26,
27

In combustion chemistry, they have been shown to be critical intermediates with respect to the12modeling of the removal of oxygen from biomass in order to generate biofuels.23, 24 In
biochemistry, cyclic and polycyclic organic aromatic molecules comprise the backbone of many
biologically relevant molecules and pharmaceuticals.32-35 Second, cyclic and polycyclic organic
aromatic structures are geometrically rigid due to the π-bonded aromatic electronic structure
which typically requires these molecules to have small geometry changes upon
photodetachment.36-38 Practically, this translates into photoelectron spectra which are minimallycongested, as explained in the previous section. This is especially interesting as this allows forthe investigation of large molecules (~15 atoms/molecule), which have typically been avoided in
the past since larger molecules possess heavily congested PE spectra. Finally, the aromatic
structure allows for the investigation of how substituents affect the electronic structure of thesecompounds. This would be expected to be different from the behavior of a more standardhydrocarbon, such as an alkane, in that the resonance structures are fundamentally different
between aromatic and non-aromatic organic molecules.12 Due to the π-bonded electronic
structure of the molecular orbitals in aromatics there are many resonance structures which may
be constructed, and as such, functional groups which are attached to the aromatic structure willinteract in far different, and often more long-rang ways than simple alkanes.
This general background now laid out, we will go into more detail about each of themolecules which have been studied in this work and why they are of interest to the scientificcommunity. What follows is an introduction to the molecules studied in this dissertation and acts
as an outline for the upcoming chapters. First, the methylphenoxides and methylphenoxyls are
introduced. This will be followed by the methylenephenoxides and methylenephenoxyls, as well
as a more general discussion of distonic radical anions and diradicals. The final molecules
considered are deprotonated indole and indoline.

13

14

§ 1.3 Background on Methylphenoxides and Methylphenoxylso-methylphenol

m-methylphenol

p-methylphenol

Methylphenols, commonly referred to as cresols, are an important class of molecules inmany fields of chemistry.19, 23-25, 39-67 They are commonly used as model systems forhydrodeoxygenation in catalytic chemistry for refining biofuels, 23-25, 40, 41 while the internalrotations of the functional groups are of great interest to many spectroscopists.42-46, 68, 69 Theirthermochemistry and kinetics have also been studied, e.g. King et al.58 measured the bonddissociation energies of the O-H bond in each isomer of methylphenol to high accuracy (± 0.14
kcal/mol), while McMahon and Kebarle66 implemented proton-transfer equilibria studies tomeasure their gas phase acidities. The study of the thermochemistry of methylphenols is often
motivated by the desire to understand the effect of different benzene substitutions on reactivity,
particularly in comparison with their simpler counterparts, phenol and toluene. Through thesecomparisons, insight can be gained into the relative contribution of the OH group (which isstrongly electron donating in aromatic systems) compared to the CH3 group (weakly electron
donating), and how these two functional groups work in concert to affect reactivity andelectronic structure. Methylphenols have been previously spectroscopically studied utilizing a15

large variety of spectroscopic techniques including microwave absorption,42, 68 Laser Induced
Fluorescence (LIF),43, 44, 46, 53, 69 Resonance Enhanced Multiphoton Ionization (REMPI),19, 52, 56
stimulated Raman-UV optical double resonance,55 and Hole Burning spectroscopy. In addition topurely spectroscopic probes, these compounds have been ionized and detected using Time of
Flight (TOF),18,24 and Fourier-Transform Ion Cyclotron Resonance (FT–ICR) mass
spectrometry.65 However, very little experimental information on the methylphenoxyl radicals or
their thermochemistry was previously available prior to the studies contained in this dissertation.
Despite the numerous studies and the reliance on methylphenols as a model species in catalytic
and combustion chemistry, prior to this work, the electron affinity of p-methylphenoxyl had only
been measured to an accuracy of ~100 meV,66 while there are no measurements for the otherisomers, and the gas phase acidities of all three isomers have been measured with a fairly large
uncertainty (at least 2 kcal/mol).
This dissertation will present a study utilizing photoelectron spectroscopy to determine
accurate electron affinities of the methylphenoxyl radicals, and, in concert with quantum
chemical calculations, provide physical insight into the structures of both the methylphenoxideanions and methylphenoxyl radicals. Combining the highly accurate (error of ± 0.14 kcal/mol)
O-H bond dissociation energy for each methylphenol isomer from King et al.,58 as modified by
Karsili et al.,67 with the EAs measured in this study allows for the construction of athermodynamic cycle that provides accurate gas phase acidities.70 The results will also becompared with the previous work on the phenoxide36, 71 and benzyl anions, 36 which were alsostudied using anion photoelectron spectroscopy. One might ask if the methylphenoxyls will
display a combination of attributes previously shown by those two anions, or will the chemistry
and spectroscopy have qualitatively different behavior? For example, one major difference

16

between the ortho, meta, and para isomers of methylphenol is based on possible resonancestructures, which will likely influence the derived chemical properties.

17§ 1.4 Background on Distonic Radical Anions and Diradicals: Methylenephenoxides andMethylenephenoxylso-methylenephenoxidem-methylenephenoxidep-methylenephenoxide

The next set of molecules studied with anion photoelectron spectroscopy in thisdissertation are the three isomers of methylenephenoxide. These anions are in fact distonic
radical anions which form diradicals upon photodetachment. The study of diradicals is of interest
to the many diverse fields of chemistry, biology, and physics.34, 72-79 Due to their frequently
unstable and short lived nature, as well as their complicated electronic structure,80-89 they arechallenging to investigate both experimentally and theoretically. Despite this difficulty, they
have proven important as reaction intermediates as seen in experiments, and predicted to beimportant for many more. For instance, the Bergman cyclization reaction is used to produce thediradical p-benzyne from enediyne, and this diradical proceeds to strand cleavage in DNA,leading to cell apoptosis.34 Thus, a diradical is responsible for one of the most potent subsets ofanticancer drugs to date.
The field of distonic radical ions, which is to say an ion that has a radical site localized
separate from the charged site on the ion, is of equal interest to the scientific community.90-103
Radical ions may react as either a radical or an ion, opening the door to any number of reaction18

schemes. Their initial study is largely due to the pioneering work of Kenttämaa and coworkers
and their efforts towards understanding distonic radical cations in the gas phase.102 Distonic
radical anions have also been extensively studied.92, 94, 97 As the study of these radical ionscontinues, they have been shown to be relevant in reaction mechanisms, often in zwitterion
chemistry, with a famous example being the McLafferty rearrangement of molecular ions incarbonyl compounds.
In some of the initial studies, distonic radical anions were created in the condensed phase,and studied with Electron Spin Resonance (ESR) spectroscopy. 91 This, however, was limited tothose distonic radical anions and/or diradicals which were long lived in the condensed phase, on
the order of minutes or hours. Later, both diradicals and distonic radical anions became the focus
of gas phase studies, with many groups in the mass spectrometry field taking an interest. Time offlight mass spectrometry, Fourier Transform Ion Cyclotron Resonance mass spectroscopy, andCollision Induced Dissociation (CID) have all been utilized to this end.91, 92, 102
The study presented in this dissertation is an investigation via photoelectron spectroscopyof all three isomers of methylenephenoxide. While a wealth of information may exist on the
methylphenols (see § 1.3), the methylenephenoxides are not well understood as of yet, andpresent an opportunity to study both distonic radical anions and diradicals by way ofphotodetachment.
In addition to anion PE spectroscopy, the thermodynamic properties of themethylenephenoxide isomers are studied using a Flowing Afterglow – Selected Ion Flow Tube
(FA-SIFT) mass spectrometer in conjunction with the acid bracketing procedure,28 thus
providing information on the acidities of the methylenephenol radicals. Together with the19Electron Affinities (EAs) provided by PE spectroscopy, acidities from the FA-SIFT machine,
and a thermodynamic cycle, bond dissociation energies are also measured.

20

§ 1.5 Background on Deprotonated Indole and Indoline
IndoleIndoline

The final molecules which were investigated with PE spectroscopy are deprotonatedindole and indoline. The bicyclic molecules indole and indoline are of interest to the fields ofbiophysics and biochemistry.32, 33 This is primarily due to the fact that indole acts as achromophore in tryptophan and therefore dominates much of the spectroscopy performed on
numerous biological systems. Indoline is less studied, and has primarily been utilized as ananalog of indole with a less complex electronic structure.104 The importance of indole as achromophore of tryptophan has symbiotically inspired numerous spectroscopic studies of indole
and indole clustered with various partners.20, 21, 32, 35, 105-119 While this scrutiny from thespectroscopic community is partially due to the related biological interest, indole also provides
unique spectroscopic challenges due to the complex nature of its vibronic structure.120, 121 Indole
is a low symmetry molecule belonging to the Cs point group, and the two lowest level electronic
states ( 1𝐿𝑎 and 1𝐿𝑏 ) have been shown to be strongly coupled, due in part to belonging to thesame representation. This has made indoline an attractive analog of indole as the 1𝐿𝑎 and 1𝐿𝑏states of indoline are further separated in energy than in the case of indole.
Despite this interest in both indole and indoline, there is relatively little known of their
thermodynamic properties. Similarly, neither deprotonated indole or indoline have been studied
21

extensively, spectroscopically or otherwise. The gas phase acidity of indole, referring todeprotonation from the N–H site, was studied in 1988 by means of proton transfer equilibria,122,123∆𝑎𝑐𝑖𝑑 Ho600 K (N–H) = 352 ± 2 kcal/mol, and has not been updated since that time. The gas

phase acidity of indoline has not been studied. The electron affinity (EA) of indolyl may beinferred by means of a thermodynamic cycle124 from the work of Taft et al.122 with relatively
large uncertainty, 2.52 ± 0.20 eV, and was updated recently, though with only a modest
improvement in the error bar, by McKay et al.,125 2.31 ± 0.15 eV. The EAs of any of the otherradical isomers associated with deprotonated indole have not been studied. Finally, an upperbound was placed on the bond dissociation energy of the N–H bond in indole by Nix et al.126 by
way of photofragment translational spectroscopy: D0(N–H) ≤ 91.2 kcal/mol. No analogous
studies of indoline have been conducted to date that directly measure these thermodynamic
properties. We believe that these previous studies present an opportunity to obtain the EAs ofthese molecules with significantly improved error bars and explore both the electronic andvibrational structure by means of anion photoelectron (PE) spectroscopy.

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30

CHAPTER II
EXPERIMENTAL BACKGROUND AND METHODS§ 2.1 Introduction and BackgroundPhotoelectron (PE) spectroscopy has been performed by researchers for a number ofyears, though the technology and methodologies have evolved considerably. Perhaps one of theearliest examples of the use of PE spectroscopy, in the form of a photodetachment study, wasperformed by Branscomb and Smith1 in 1955. At the time, the electron affinity (EA) of theoxygen atom was a contested issue with most of the experiments that attempted to measure thisquantity relying on calculating the EA from thermodynamic measurements of electron
attachment to hot tungsten filaments.2 History is a harsh judge; these measurements would beshown to be quite inaccurate, far beyond the error bars reported in those early works. Branscomb
and Smith resolved to measure the EA more directly by exposing a mass selected beam of O–
anions to a bright lamp (a 1 kW movie theater projector) with a water-cooled doped-glass plate,capable of filtering out known wavelengths of light, interposed between the lamp and the anionic
oxygen beam. If the photons which passed through the filter were energetically capable ofdetaching the electrons, then the electrons would be ejected, collected, and measured by lock-in
detection. By changing the wavelength filtering plates, they were able to measure a crude
version of the photoelectron cross-section as a function of photon energy. See Fig. 2.1.

31

Fig. 2.1 Early measurement of the O– photoelectron cross section as a function of photon energy.
This figure is reprinted from Ref. 1.

32

As one can see from Fig. 2.1, the experimental data was quite sparse and had large errors
associated with the measurements. Even more extraordinary is the theoretical fit to the data; so
extraordinary, in fact, that it likely had been viewed with healthy skepticism as the authors used
this fit to extrapolate out to the threshold of photodetachment, which would be equal to the EA.
By this method, the authors claimed to measure the EA of the oxygen atom as 1.45 ± 0.15 eV.
Disregarding the validity of such an extrapolation, this measurement would be later confirmed ascorrect, within its reported error, by far more sensitive and rigorous investigations.3
After this initial foray into utilizing a light source to photodetatch electrons from anions,several attempts to refine the methodology of lamps and filters were made;4 however this quickly
proved to be fatally limited by both the low power non-coherent light which could not be
efficiently focused and the broad continua of wavelengths which filters must inherently transmit.
At this same time, a new technology was waxing strongly which would revolutionize the way
photodetachment spectra were collected: organic dye lasers. These lasers allowed for the firstmeasurements of photodetachment spectra of atomic anions with enough resolution to resolve
the fine structure of those atoms.5 Later, this would allow for the measurement of vibrational
states in molecular anions as well.
While photodetachment studies offer the same information as photoelectron spectra, inprinciple, they differ in several important ways. Photodetachment studies require a tunable lightsource whose frequency is scanned across and then above the detachment threshold of aparticular anion, and all ejected electrons are collected as a function of the photon energy. The
kinetic energies of these electrons (eKE) are not analyzed directly in this process. PE spectra areinstead obtained with a fixed frequency of laser light, after which, the ejected electrons arecollected and then their kinetic energy is measured by some means. This has the advantage of not33

requiring a tunable laser which must be accurately and smoothly scanned across some frequency
range, and, in the case of organic dye lasers, Amplified Spontaneous Emission (ASE) must also
be effectively eliminated lest the laser turn back into a lamp. Considering the advantages
available to PE spectroscopy the only obstacle was how to accurately measure the kinetic energyof photodetached electrons. This was first overcome by means of the device known as ahemispherical electron energy analyzer, then called an “electron monochromator,”6 and thus thefirst photoelectron spectra were obtained in 1972 and subsequently in 1974.7, 8
In principle, the primary limitation on the resolution in PE spectroscopy is set by themethod used to analyze the electron kinetic energy. As such, considerable effort would be
expended in the following years to improve and refine the hemispherical electron energy
analyzer, allowing it to attain a resolution of ~3 meV under the correct circumstances andconditions. While improving the hemispherical electron energy analyzer was pursued, so werealternative methods. The magnetic bottle electron energy analyzer9 and the Velocity Map
Imaging (VMI) photoelectron spectrometer10, 11 were later developed.
These various methods all have unique advantages and weaknesses. The hemisphericalelectron energy analyzer has the advantage of possessing a constant spectral resolution,
regardless of electron kinetic energy. As such, this method enjoys the full benefit of PE
spectroscopy in that one need only employ the highest photon energy laser possible, with
virtually no need to ever change the laser color. However, this piece of equipment is greatly
limited by the collection efficiency of the photoelectrons. In the hemispherical electron analyzer
utilized in the studies presented in this dissertation, for example, of the photoelectrons being
ejected in the total 4π steradians of solid angle, only those electrons within ~1/2000 steradians
are collected for analysis. This places a strong restriction on the apparatus that the source of34

photoelectrons must be large enough to overcome this limitation. Practically, this requires anintense light source. (This will be discussed further in § 2.4) The magnetic bottle, on thecontrary, is nearly the inverse of the hemispherical electron energy analyzer in terms of strengths
and weaknesses. The magnetic bottle, or Time-Of-Flight (TOF) electron spectrometer, enjoys
nearly 100% collection efficiency of photoelectrons. However, the spectral resolution of thismethod decreases with increasing electron kinetic energy and generally possesses lower spectral
resolution than the hemispherical energy analyzer. For a magnetic bottle to attain resolution on
par with the hemispherical electron energy analyzer requires a TOF Ultra-High-Vacuum (UHV)
tube often several stories tall. The VMI photoelectron spectrometer offers an attractive
compromise between the two previous instruments. While the VMI PE spectrometer also has aspectral resolution which is a function of the kinetic energy of the photoelectrons, it is also
capable of meeting or exceeding the resolution of the hemispherical electron energy analyzer at
low kinetic energies.12 Additionally, the VMI PE spectrometer is a physically small anduncomplicated device, especially when compared to the magnetic bottle regarding size or thehemispherical electron energy analyzer regarding complexity.
The photoelectron studies presented in this dissertation have been collected on two
separate machines. The first instrument combines a Flowing–Afterglow (FA) ion source with a
Wien Velocity Filter (WVF), which mass selects the anions of interest, a hemispherical electronenergy analyzer, and finally a light build-up cavity locked to an argon ion laser system. The
second experiment utilizes a novel dual pulsed valve plasma entrainment ion source, a Wiley–
McClaren TOF mass spectrometer, VMI PE spectrometer, and a nanosecond tunable pulsed laser
system. The following sections will describe and discuss all of these components in detail, first
addressing the former instrument followed by the latter.

35

§ 2.2 Overview: Continuous Photoelectron Spectrometer
The first instrument to be discussed is best summarized in Fig. 2.2.

36

Fig. 2.2 Diagram of the continuous photoelectron spectrometer. Continuous ion generation takes
place on the left hand side of the instrument in a flowing afterglow microwave plasma source.
The ions are extracted through a small orifice, accelerated to a kinetic energy of 735 eV, mass
selected in a Wien velocity filter, decelerated to a kinetic energy of 35 eV, intersected by a cw Ar
ion laser, and the photoelectrons collected and their kinetic energies analyzed via a hemisphericalphotoelectron energy analyzer.

37

The anions to be studied are generated in a flowing afterglow ion source (§ 2.3) and thenextracted into a differentially pumped region and accelerated to a kinetic energy of 735 eV. The
ion beam, consisting of many different anions, is introduced to a Wien velocity filter (§ 2.4)
where anions of a single mass-to-charge ratio (m/z) are selected. The mass selected anions arethen decelerated to a kinetic energy of 35 eV and intersect a build-up cavity of light generated
with an Ar ion laser. (§ 2.5) Upon encountering the radiation within the build-up cavity, theanions undergo photodetachment, and the photoelectrons are subsequently collected and fed intoa hemispherical energy analyzer. (§ 2.6) All of these components are discussed in detail in theirrespective sections, which follow this section.
Taken as a whole, the strengths and weakness of this experimental design may besummarized as follows. Utilizing a flowing afterglow ion source gives this experiment access tomulti-step rational ion synthesis, by sequentially introducing reagents to the gas flow. The FAsource is also capable of generating large continuous ion currents, up to 40 nA prior to mass
selection, however this comes at the cost of a very heavy gas load of up to 25 sccm of helium.
This is both monetarily expensive, and requires a roots blower based pumping system toadequately remove the helium. The Wein velocity filter benefits from being spacially compact,
does not require exceptionally powerful magnetic or electric fields, and offers low loss of ion
current through the device, compared to a mass selecting quadrupole, for instance. The limitationof the Wien velocity filter is found in the fact that it generally possesses a lower mass resolution
than other mass selecting devices. The hemispherical electron energy analyzer is a complex
device with a long history of scientific use, as discussed in the previous section, but the primaryadvantage could arguably be said to be the constant resolution of the device across the entire
photoelectron spectrum of a given anion. The primary disadvantage is the low collection38

efficiency of photoelectrons, necessitating the use of a laser build-up cavity to boost thephotoelectron yield.
A discussion of the specific components utilized in this experiment and their respective
principles of operation follows.

39§ 2.3 Principles of Operation: Flowing–Afterglow Ion Source
A Flowing–Afterglow plasma has been shown to be an effective method for thegeneration of a continuous flow of atomic and molecular anions possessing many advantages.13,
14

The first key advantage is temperature control. The FA He plasma used in these experiments

is at high pressure, ~ 0.5 Torr, with only trace amounts of reagent. If the formation of an anion
of interest is exothermic, the excited ion can be collisionally cooled by He, which is thermallyequilibrated with the walls of the chamber, for several milliseconds. This implies that any anion
of interest generated in the apparatus will have a well-defined thermal temperature in all degrees
of freedom. The second advantage is the ability to perform sequential rational ion synthesis byinjecting reagents separated spatially along the flow. In theory, multi-step syntheses are possible
via this route, and may be easily controlled by varying the spacing of the reagent inlet ports inrelation to one another in the flow.
In the experiments examined in this dissertation, the specific flowing afterglow apparatus
utilized has been described in detail previously, and as such only a generalized discussion will begiven here.15 This FA ion source utilizes a flow of helium which is converted into a plasma by aresonant microwave cavity which engulfs the flow tube, which itself is made of glass. The
plasma of He+ and e– is then utilized to perform any number of chemical reactions with a given
reagent. While it is sometimes possible to encounter a reagent which is capable of directlyinteracting with the plasma to form the desired product anion, this is rare and not the case for anyof the studies displayed in this work. Instead, the first step in the anion synthesis is thegeneration of a strong Lewis base in the gas phase which is then reacted with a neutral moleculethat will subsequently be deprotonated. Some common reactions to generate these bases are as
follows:
40

N2 O + e– → N2 + O–
O2 + e– → O + O–
CH4 + O– → CH3 + HO–
NF3 + e– → NF2 + F –NH3 + e– → NH2– + H
Note that in the case of generating hydroxide, this must be done in a two-step process; first
generating atomic oxygen anions via either N2O or O2 and then subsequently introducing
methane into the flow after the first step, adjusting the methane flow to fully titrate the oxygenanions.
The only question which remains is how to choose the proper base for a given molecularsystem which is to be investigated. As was discussed in the prior chapter, aromatichydrocarbons were the focus of the studies of this dissertation, and as such, many of thesemolecules have multiple sites which could, in principle, be deprotonated. This implies that the
choice of base used may be critical to the success of an experiment, as choosing too strong of abase might result in multiple deprotonated isomers, which cannot be distinguished via mass
selection. To this end, the acidity of all of the sites on the molecules studied were theoreticallycalculated (see the individual chapters regarding any specific molecule for more detail) andcompared to the known proton affinities of the bases produced in the reactions shown above.

41

Table 2.1 This table details the proton affinities of the gas phase bases which were mostcommonly utilized to generate the anions of interest in the studies of this dissertation.16
Proton Affinities of Common Bases in the Gas PhaseF–

372 kcal mol–1

O–

382.6 kcal mol–1

OH–

390.3 kcal mol–1

NH2–

403.4 kcal mol–1

42

Following the ion generation step in the FA plasma source, the negatively charged
products are extracted through a nosecone (r ~ 0.5 mm) and into a differentially pumped region.
Subsequently, the newly formed beam of anions are focused, steered and accelerated to 735 eV
towards the Wien velocity filter. Of course, the synthesis of the anions which are intended to bestudied are not unique and a number of byproducts are also produced in the FA plasma source.
As such, it is necessary to physically separate the anions of interest from all other anions before
performing PE spectroscopy. This is accomplished by way of the Wien velocity filter, which will
be discussed next.

43

§ 2.4 Principles of Operation: Wien Velocity Filter
Wien Velocity Filters (WVFs) have been utilized for some time with the express purposeof separating out particles of a single velocity from a continuous beam of charged particles.17
There are several features of the WVF which make it particularly attractive in the application of
PE spectroscopy. First, as was previously discussed, the collection efficiency of a hemisphericalelectron energy analyzer, which is the detection instrument employed on the continuous
experiments, is quite low, and as such we cannot afford to lose any possible signal. The WVF
offers high throughput, meeting this design need. Additionally, the WVF ideally does not deflect
or in any way alter the course of the ions of the desired mass, which further reduces any designneeds to steer and focus the mass selected ion beam after passing through the filter.
The other two obvious options for mass selecting a continuous beam of anions were not
employed for several reasons. The first choice is the quadrupole mass filter.18 This is a device
which alternates between two large (Vmax ~ 15 kV) saddle point potentials orthogonally oriented
to the direction of motion of the incoming ion beam at radio frequencies. The two potentials are
simply the same potential rotated π/2 radians from each other about the axis of the incoming ion
motion. This causes the ions to travel in a helical trajectory through the quadrupole, where thetrajectories’ radius is a function of the mass of the ions, assuming all of the ions which entered
the filter had the same kinetic energy. By also applying a DC field, one is able to select a
specific mass to have a stable trajectory though the filter.19 However, this method has two key
flaws for this application. First, the exit trajectory of the mass selected ions is helical andtherefore quite difficult to re-collimate and focus without loss of overall ion signal following
mass selection. Second, the throughput of quadrupole mass filters is generally lower than for
WVFs. The other option for mass selecting the ion beam would be a magnetic bend.20 This type44

of setup would enjoy the same high throughput of the WVF, but large magnetic fields (far larger
than those employed in the WVF) are required to operate this equipment. As we are performing
PE spectroscopy, and therefore trying to measure the kinetic energy of electrons, having an
external magnetic field acting upon the photoelectrons to any appreciable degree would be
unacceptable. For example, the hemispherical electron energy analyzer must be shielded fromthe magnetic field of the earth with μ-metal, as that is already disruptive to the electron
trajectories. Additionally, this method is in general a larger and more costly setup than a WVF.
The WVF setup is intuitive and relies on basic physical ideas to operate. The setup is asfollows. Charged particles, each moving at an arbitrary speed, though all have the same directionof motion, enter a region of space intersected by two orthogonally oriented fields; one electricand one magnetic. See Fig. 2.3.

45

Fig. 2.3 An idealized schematic of the magnetic and electric fields that a negatively charged
particle q, with a velocity v, would encounter upon entering the Wien filter. Also displayed are
the resulting force vectors acting upon the negatively charged particle resulting from thesefields.

46

As Fig. 2.3 demonstrates pictorially for the idealized case of a negatively charged particle
q entering a Wien filter with a velocity v, the Lorentz force on the particle will be given as⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗
𝐹
𝐿𝑜𝑟𝑒𝑛𝑡𝑧 = 𝑞𝐸 + 𝑞𝑣 × 𝐵
The ion beams used in the experiments are comprised of a collection of particles (molecularanions) all possessing -1e of total charge, accelerated to the same kinetic energy, 735 eV, andpossessing different masses. Since all of these particles possess the same kinetic energy, buthave different mass, particles of different masses will attain different speeds.
12𝐾𝐸
𝐾𝐸𝑖 = 𝑚𝑖 𝑣𝑖2 → |𝑣⃗⃗⃗𝑖 | = √2
𝑚𝑖Rewriting the Lorentz force with this substitution
2𝐾𝐸⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗⃗
𝐹𝐿𝑜𝑟𝑒𝑛𝑡𝑧 = 𝑞𝐸⃗ + 𝑞√𝑣̂ × 𝐵𝑚𝑖 𝑖
Since we design the apparatus such that the electric and magnetic forces are antiparallel to eachother, by setting a constant electric field and therefore constant electric force, and then choosing
a magnetic field such that the Lorentz force on particles of a particular mass is zero, we haveselected one set of particles of mass mi to pass through the Wien filter without being deflected atall. Necessarily, this requires all other particles of mass not equal to mi to be deflected. An
aperture is placed ~30 cm after the exit of the Wien filter, and all particles which have been
deflected by an amount greater than or equal to the radius of this aperture are thus discarded.
This aperture defines the mass resolution of the instrument, which in our case is m/Δm ~ 70.

47

Of course, the real instrument deviates from the idealized model on display in Fig. 2.3.
The largest obstacle in the design of a Wien filter lies in the construction of truly parallel electricfield lines. Truly parallel field lines may only be achieved with infinitely large conducting plates
laid parallel to each other and charged to different potentials. Infinitely large plates are notpossible in the experiment, and finite plates, for which even approximating them as infinite
plates is inappropriate, are used instead. Due to the effects of fringe fields in non-infinite plates,
one must use other methods to create these parallel fields within the limited space of the Wien
filter. This was achieved in this case by placing glass plates that were coated in vapor-deposited
silicon along the edges of and perpendicular to the metal plates providing the electric field. This
provided a gradient of potential along the resistive silicon which worked to counteract the fringe
fields exerted by the edges of the metal plates. This setup was the focus of a previous graduate
student’s work and is explained in detail elsewhere.21 This is not as significant a problem for thecreation of parallel magnetic field lines, as magnetic fields penetrate common metals such as 304
steel without significantly attenuating or distorting due to low magnetic susceptibility, and assuch, one may place a large electromagnet exterior to the vacuum chamber without being limited
by chamber dimensions. This allows for greatly increased surface area of the magnet faces, andso the parallel field approximation is valid.
Following mass selection in the Wien filter, the ion beam is steered, focused, anddecelerated into the interaction region of the hemispherical electron energy analyzer.

48

§ 2.5 Principles of Operation: Argon–Ion Laser System
Once the ion beam has been decelerated to a kinetic energy of 35 eV, down from 735 eV,
and is focused into the interaction region of the hemispherical electron energy analyzer, the beam
is intersected by a linearly polarized UV cw laser beam generated by an argon–ion laser
orthogonally oriented to the ion beam. This causes the desired photodetachment of electrons,
which are then collected and their kinetic energies analyzed.
As previously discussed, the low collection efficiency of the analyzer necessitates a highlevel of photoelectron production in the interaction region in order to obtain practically usefullevels of photoelectron signal. Photoelectron production of a well aligned ion and laser beam in
the interaction region is proportional to the product of the laser power and the ion beam current.
Considering this, one seeks to maximize both of these quantities. This has already beenaccomplished with regard to the ion beam, by utilizing a high amperage ion source in the flowing
afterglow and a high throughput, minimum deflection mass selector in the Wein velocity filter.
This now leaves the task of maximizing the laser power in the interaction region of the analyzer.
A high power cw laser beam, at least on the ~50 W, and ideally on the ~100 W scale, has been
determined as necessary in order to obtain practically useful levels of photoelectron signal, fortypical ion beam currents. For context, when the instrument is configured to select only O–,
typical ion currents in the interaction region range from 10–20 nA.
Generating a cw UV laser beam with 50-100 W of output power is not practical by
conventional methods, such as purchasing a commercial laser with such a rated output power. In
the past incarnations of this experiment, this obstacle was overcome by using an argon–ion laser
for green (488 nm) light. While commercial lasers of this type can achieve ~50 W at thiswavelength, the previous design sought to drastically increase this power further. By removing
49

the output coupler of the laser and placing the remaining components in front of the windows
into the vacuum chamber and finally placing a spherical high reflector after the hemisphericalelectron energy analyzer interaction region, one was able to essentially create a laser withextremely low output power, but extremely high circulating in-cavity power, ~ 1 kW.22 See Fig.
2.4. In essence, this made the vacuum chamber housing the interaction region of thehemispherical electron energy analyzer a lossy component of a laser cavity. This was possible to
do with an Ar–ion plasma tube as a gain medium for 488 nm laser light, as the gain is extremely
high at this wavelength, and so lasing was achievable even with the added loss that the vacuum
chamber represented. To give a sense of the amount of gain available at this wavelength, typical
output couplers of green Ar–ion lasers are only ~ 66% reflective.

50

Fig. 2.4 A schematic of the experiment prior to converting the laser system to be able to perform
UV photoelectron spectroscopy. Reprinted from Ref. 22.

51

This technique is not an option, however, if one wishes to perform UV photoelectron
spectroscopy, as the gain of an Ar–ion laser plasma tube on either of the two strongest UV lasingtransitions (λ = 364 or 351 nm) is far lower than for the green wavelengths. For reference, theoutput coupler on a typical Ar–ion laser lasing in the UV is ~ 97% reflective.
Given this fact, the system was designed to instead transform the windows of the vacuum
chamber into a near-confocal cavity and then to lock the frequency of the UV laser light to afringe of the chamber cavity. See Fig. 2.5. By injecting the frequency servo-locked laser lightinto the chamber with proper mode matching, one may achieve a large buildup of power within
the chamber cavity: 𝑃𝑐𝑎𝑣𝑖𝑡𝑦 ~𝑃𝑙𝑎𝑠𝑒𝑟 𝔉, where 𝔉 is the finesse of the chamber cavity. The fulldetails of how this was accomplished are explained below.

52

Fig. 2.5 Schematic of the full laser system utilized in the current continuous photoelectron
spectrometer.

53

The first component in this laser system is a commercial Ar-ion laser. This laser has 3
Brewster windows in order to stabilize the horizontal laser polarization; two windows fused to
the 1.5 meter long beryllium-ceramic plasma tube and one after the output coupler. There is aninternal iris aperture that can be constricted to maximize the TEM(0,0) component of the beam.
In order to precisely select the laser frequency, two components are required; a prism forselecting the lasing transition and an etalon to select the specific cavity fringe on which to lase.
The etalon is necessary because we will be attempting to lock this cavity to another cavity, andpower not contained within the overlap of the two fringes of the laser and chamber cavities infrequency space will be lost. Once the lasing transition and cavity fringe have been selected,
typically the 364 nm lasing transition, the light is fed into the chamber build-up cavity.
A general description of the behavior of laser light coupling into an optical cavity
follows: The cavity formed about the interaction region of the electron energy analyzer,
designated the build-up cavity, intrinsically has a free spectral range (𝐹𝑆𝑅 = 2𝐿𝑐𝑐𝑎𝑣𝑖𝑡𝑦) and finesse

associated with forming a standing light wave within it. In order to accumulate photons within
the cavity such that the photon flux within it increases to a steady state value larger than that of
the input laser, one must inject photons whose frequency is resonant with the maximum of one of
the cavity fringes associated with the FSR of the build-up cavity. If the frequency of the laser
light overlaps with a cavity fringe of the build-up chamber in frequency space to within the Full
Width at Half Maximum (FWHM) of the build-up cavity fringe, ~ 400 kHz, and the laser
FWHM, ~10 kHz, then the two cavities may be said to be resonant and photons will rapidly
accumulate and reach a steady state population within the build-up cavity. Under thesecircumstances, considering that the finesse of the build-up cavity is ~ 450, the steady-state power
within the build-up cavity will be approximately 𝑃𝑐𝑎𝑣𝑖𝑡𝑦 ~𝑃𝑙𝑎𝑠𝑒𝑟 450. If the lasing and build-up
54

cavity fringes do not overlap, the cavities will not be resonant, and comparatively few photons
will accumulate within the buildup cavity.
In an idealized world then, one might consider that one could manufacture the laser andbuild-up cavity lengths to be such that the lasing cavity fringe and a cavity fringe of the build-up
cavity are perfectly resonant, and thus the problem of locking the frequencies of the cavities tobe resonant would be solved. This, however, is not possible as it does not account for the realworld problems of noise and frequency drift. While one might be able to manufacture both
optical cavities with enough precision to have them be resonant, length changes within either thelaser or the build-up cavity on the order of one micron are enough to lose the resonance. Since
there will always be acoustic noises in a lab, which might easily vibrate a cavity mirror with adisplacement of one micron or more, plasma density fluctuations causing slight frequency shiftsof the amplified photons through the gain medium of the Ar-ion laser, or any other of a host ofsmall disturbances which might break the resonance of the two cavities, another method must be
developed. We seek a method to actively adjust the FSR of both the laser and build-up cavities inreal time with piezoelectric actuators to counteract these spurious motions utilizing home-built
electronics in conjunction with an Acousto-Optic Modulator (AOM) to more finely tune thefrequency of the laser light at high speed.
The method used to perform this active intervention has been described previously and is
known as Pound-Drever-Hall (PDH) frequency stabilization.23 The first step to utilizing thistechnique is to create an electronic error signal. This must take the form of some voltage which
the electronics will check against a desired value and then send voltage(s) to outputs whichconnect to devices that alter aspects of the experiment which thereby change the error signal.
This cycle continues indefinitely and ideally maintains the error signal at some value. In this55

case, an error signal must be generated that is only a function of the overlap of the laser andbuild-up cavity fringes and is not a function of the output power of the laser. Additionally, theerror signal must be built in such a way that the value of the error voltage indicates specifically
how the electronics must act to correct the error signal back to the desired value. Accomplishing
all of these mandatory criteria is challenging, but was accomplished in the following way.
First, we manipulate an AOM with a Voltage Controlled Oscillator (VCO) in order todither the laser frequency as it impinges upon the buildup cavity. We modulate the laser
frequency at 1MHz, which is to say we move the laser in frequency space ~ 50 kHz in one
microsecond. This Frequency Modulated (FM) light is then fed into the build-up cavity. This
scenario is depicted in Fig. 2.6.

56

gain of thebuildupcavityFrequency of buildup light

Modulation
Laser lightintensity

Span ~50 kHz
every microsecond

Frequency of the laser light

Fig. 2.6 A qualitative schematic of how the laser light is frequency modulated as it impinges
upon the build-up cavity. Recall that Δflaser ~ 10 kHz while Δfcavity ~ 400 kHz.

57

Acoustic noise in the lab typically shifts the laser frequency by far more than 400 kHz,the FWHM of a fringe of the build-up cavity. Recall that whenever the fringe of the laser isoverlapped with a fringe of the build-up cavity, the reflected light of the front cavity mirror
decreases and the light transmitted from the rear cavity mirror is increased. Thus, as noise causes
a laser cavity fringe to cross over a build-up cavity fringe in frequency space, due to the 1 MHz
dither on the laser, reflected light off the front cavity mirror or the transmitted light off the backcavity mirror will fluctuate in amplitude (Amplitude Modulation, AM) in time with the dither
frequency and may be read by a photodiode. In other words, the cavity acts as an FM-to-AM
converter. See Fig. 2.7

58

Fig. 2.7 An example of the FM-to-AM conversion when the dithered laser is scanned over thecavity fringe of the build-up cavity in frequency by monitoring the transmitted light intensity.

59Thus, we feed the amplitude from either the reflected or transmitted light, read by aphotodiode, from the build-up cavity and the original wavefunction that drives the AOM into adouble wave mixer and this will generate the product of the two functions. Since the light fromthe build-up cavity will be phase shifted from the dither function, this may be modeled as:
1sin(𝜔𝑡) sin(𝜔𝑡 + 𝜑) = 2 {cos(𝜑) − cos(2𝜔𝑡) cos(𝜑) + sin(2𝜔𝑡) sin(𝜑)} Notice that there isa part of the left hand side of the equation that depends only on φ and another part that depends
on 2ω. Critically, φ is a function of time as the value of φ will increase as the laser frequency ismoving towards the peak center of the build-up cavity fringe, reach a maximum of 2ω at the
maximum of the fringe, and then flip negative as the laser frequency scans over the opposite
flank of the build-up cavity fringe. Thus, by monitoring the output of the double wave mixer andinserting a low pass filter that eliminates the 2ω or higher signal, we generate the desired error
signal. See Fig. 2.8.

60

Lock Point
Output fromdouble0wave mixer

time
Fig. 2.8 Qualitative schematic of the form of the error signal utilized in the laser locking system
as the laser fringe scans over a build-up cavity fringe. The red arrow points to the lock point of
the servo loop filter system.

61

This is now a usable signal that can be servo-locked to, and is only sensitive to theresonance of the two cavities. By configuring the electronics to lock at the baseline voltage, thenafter crossing a build-up cavity fringe the servo will attempt to lock on the slope of the above
curve, and the sign of the error signal informs the system whether to correct “up” or “down.”
Thus, as the voltage changes and indicates that the two cavities are falling out of resonance witheach other, the system will send a signal to one of three different devices which alter thefrequency of the laser light or the FSR of the build-up cavity in order to bring the error voltage
back to baseline.
This system relies primarily on two methods to adjust the frequency of the laser. The
first is the ring piezo mounted inside the custom output coupler mount of the laser, onto which isaffixed the output coupler itself (by way of Torr Seal). This necessitated a custom design andfabrication of the output coupler mount of the laser. See Fig. 2.9. The second is the AOM
combined with the VCO.

62

Fig. 2.9 Schematic of the custom bayonet output coupler mount fabricated to accommodate apiezoelectric actuator, onto which the output coupler optic is fused with Torr Seal adhesive.

63

The strategy used to achieve locking is as follows. The slew rate at which the two cavities
come into and out of resonance allows for a window of opportunity of ~ 50 μs to counteract the
motion of the two cavities in frequency space. This necessitates a device that can operate at ~20
kHz in order to maintain the resonance. In general, this restriction rules out a piezoelectric
actuator. The AOM, however, has significant gain in this region of frequency space. The AOM,
then, is responsible for the initial lock, however it is limited in range in frequency space; it can
only shift the input laser light by a max of 40 MHz before it runs out of range and must reset.
This will extend the time over which the cavities remain resonant from 50 μs up to 500 μs, but it
cannot lock them permanently. To accomplish this, the piezoelectric actuator on the output
coupler of the laser acts in concert with the AOM, and has far more range in altering thefrequency of the laser. While the piezoelectric actuator has a very low gain response at 20 kHz,it has excellent response at 2000 Hz, corresponding to the time over which the cavities now stayresonant due to the intervention of the AOM. Once the lock is established, the AOM is
responsible for countering high frequency noise that could disrupt the lock, while the piezo
handles the lower frequency noise. Correctly balancing the Bode plots of the circuits which
control these two devices and their associated phases such that we do not drive the circuits tooscillation is a critical factor, but will not be extensively discussed here. See Fig. 2.10 for details
on these circuits.
Finally, one final set of piezoelectric actuators are affixed to the build-up cavity mirrors.
Rather than have these respond to the error signal directly, these piezoelectric actuators servo off
the voltage of the laser piezoelectric actuator and attempt to set that voltage to the baseline bias
level. Effectively, this extends the overall range in frequency space of the laser that thepiezoelectric actuator inside it is able to scan over. This final set of piezoelectric actuators are64only able to act at very low frequencies, DC–10 Hz, and so are also primarily effective atcounteracting the lowest frequency noise sources, such as temperature changes.

65

66

Fig. 2.10 Annotated circuit diagrams detailing the specific ways in which the laser servo works.
67§ 2.6 Principles of Operation: Hemispherical Electron Energy Analyzer
Following photodetachment, a small portion of the electrons is collected and their kinetic
energy is analyzed. This is done by way of a hemispherical electron energy analyzer, thespecifics of which have been discussed at length previously.24 A brief overview of theoperational methodology is given here.
The electrons which are gathered through the collection pin hole in the molybdenuminteraction region are first guided through a series of lens which are computer controlled. See
Fig. 2.11. The purpose of these lenses is several fold. First, the analyzer will only transmit
electrons which possess a kinetic energy within a small range of a central transmission energy,
T0. Typically, this transmission energy is set to T0 ~ 1.5 eV. Therefore, in order to scan throughthe full range of possible kinetic energies, the input lens stack is used to accelerate or decelerate
the collected electrons so as to add or subtract precise amounts of kinetic energy before theelectrons enter the annulus of the hemispheres. Thus, stepping the voltages on the lens stack andscanning through the added or subtracted kinetic energies, one is able to effectively measure acomplete spectrum of electron kinetic energies. This critically provides the basis for the fact that
this instrument will have a constant spectral resolution, regardless of the kinetic energy of theelectrons which it measures. Second, the input lens stack is used to focus the electrons at theentrance to the annulus of the hemispheres, regardless of the added or subtracted kinetic energy.
This is necessary, as the electrons’ deflection through the annulus will only disperse in such a
way as to be proportional to their respective kinetic energies if the electrons are generated from apoint-source at the entrance to the annulus, equidistant from either hemisphere.

68Fig. 2.11 Schematic of the electron kinetic energy analyzer system with deceleration, input, andoutput lens stacks included.

69

The theory of the motion of the electrons through the hemispherical analyzer is now
discussed. As can be seen in Fig. 2.11, the analyzer is comprised of two hemispheres, one
concave and one convex, the convex hemisphere partially inserted into the cavity of the other.
These two pieces are then held at different voltages. This results in an electrostatic potential, inkspherical coordinates, Φ = r + 𝐶, where k is a constant associated with the voltage difference
between the hemispheres and C is a constant of integration. This potential might be immediately
recognizable as isomorphic with the famous central force field of celestial mechanics as laid out
by Kepler. The equations of motion of a body in such a potential have long since been solved.25
1𝑚𝑒𝑘= 2 {1 − 𝜀 sin(𝜃 + 𝜙)}
𝑟(𝜃)
𝑙
𝑙2
(1 −
)
𝑚𝑒𝑘𝑟𝑖𝑛𝜙 = sin−1 []𝜀𝜀 = √1 +2𝑙 2 (𝐸 + 𝑒𝐶)
𝑚𝑒 2 𝑘 2

Where e is the electric charge of an electron, m is the electron mass, k is the constant determinedby the voltage difference of the hemispheres, 𝑙 = 𝑣𝑚𝑟 cos(𝜙), v is the velocity of the electron, φis the divergence angle between the electron’s velocity vector and the component of the velocity
tangent to its radial position vector, ε is the eccentricity, rin is the radius at which the electrons
entered the electrostatic field, and E is the total energy of the electron in the central field.

70

Considering the annulus of the hemispheres will not permit particularly eccentric orbits topass through, then the cases where circular orbits are formed are of interest. These orbits will𝑒𝑘

𝑙2

occur when the minimum effective potential energy 𝑉𝑒𝑓𝑓 = − 𝑟 − 𝑒𝐶 + 2𝑚𝑟 2 is equal to thetotal energy of the electrons in the central force field. The kinetic energy of an electron in such𝑒𝑘

an orbit is 𝑇 = 2𝑟 . Electrons possessing these orbits and this kinetic energy, where 𝑟 = 𝑟0 =(𝑟1 +𝑟2 )2with r1 and r2 being the hemispheres’ respective radii, therefore define the central

transmission energy, T0, of the analyzer. Electrons possessing kinetic energy equal to T0 will
therefore also be the electrons most efficiently transmitted through the analyzer.
Solving more specifically for the radius of electrons at the output (θ = π) of thehemispherical analyzer in terms of the starting input radius (θ = 0) gives−1
2𝑇0 𝑟0𝑟𝑜𝑢𝑡 = 𝑟𝑖𝑛 [− 1]𝑇𝑟𝑖𝑛 cos2 (𝜙)While this equation is rigorously correct, it offers little obvious physical insight. In order toextract practical information from it, one may perform a multivariate Taylor expansion evaluatedabout r0, T0, and 𝜙 = 0 keeping only the first non-zero term past a constant. Following
simplification and some rearrangement, this leads to
Δ𝑟𝑜𝑢𝑡 2Δ𝑇 3Δ𝑟𝑖𝑛≈+− 2𝜙 2𝑟0
𝑇0
𝑟0
Where the terms with Δ refer to the difference between either the reference radius, r0, or thereference energy, T0, and the variable in question. Since the leading term in φ is of order φ2 andthis represents the divergence of the electrons at the output of the analyzer, the hemisphericalanalyzer is said to be perfectly focusing through first order. The critical insight, however, is that71

the dispersal of the output electrons will be linear in kinetic energy. Namely, by ensuring that
Δrin is zero, which is to say that the electrons all enter the electrostatic field at the same radiusequal to r0, which the input lens stack ensures, and that the divergence of the electrons is small
and therefore the φ2 term may be treated as negligible, then the dispersion of the output electrons
is simply
D = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑓𝑟𝑜𝑚 𝑟0 𝑎𝑡 𝑒𝑥𝑖𝑡 ≈

2Δ𝑇𝑟0
𝑇0

While the Taylor expansion is useful to gain insight in the workings of the instrument, it
is an approximation and fails to capture several other implications and only holds true under
specific operating conditions. For example, this does not capture the linear magnification of the
instrument, and assumes the electrons are generated at a point source in the entrance of theanalyzer and each electron does not influence the motion of another. Nevertheless, this picture issufficient to motivate the principles under which the hemispherical analyzer is operated.
Following the kinetic energy analysis and spatial dispersion of the electrons, an output
lens stack interacts with the electrons. The purpose of this stack of lenses is to first magnify theimage of the electrons, and then to accelerate this image onto a position sensitive detector
without distortion. The position sensitive detector in this instrument takes the form of a stack offive Microchannel Plates (MCPs) and position detecting electronics.
The calibration of this instrument in order affix the energy scale is achieved by measuring
the O(3P2) + e– ← O– (2P3/2) transition in the atomic O– photoelectron spectrum using the known
EA of the O atom, 1.46112 eV.3 Additionally, a small energy scale compression factor (0.7%) isapplied based on the photoelectron spectrum of O2–, which is determined by comparing
measured peak positions to the well-known photoelectron spectrum of O2–.26
72

§ 2.7 Overview: Pulsed Velocity Map Imaging Photoelectron Spectrometer
The other experiment utilized in this dissertation is now discussed. This experiment has
been discussed at length previously.27 The primary difference between the previous experimentand this one is that this experiment is pulsed. See Fig. 2.12. The ions are generated in a dual
supersonic pulsed valve ion source. (§ 2.8) Following generation of the ions, they are extracted
into a Wiley–McLaren time of flight mass spectrometer, mass separating the ions by theirrespective mass to charge ratios. (§ 2.9) The flight path of the ions places them into theinteraction region of a velocity map imaging device. (§ 2.10) An appropriately timed nanosecond
pulsed laser then interacts with only the ions of a chosen m/z, thus photodetaching electrons. The
VMI repeller plate is then pulsed to operating voltage, causing the photoelectrons to be velocity
mapped onto the two dimensional plane of the MCPs. Each of these components is discussed indetail in the following sections.

73Fig. 2.12 Schematic of the pulsed VMI photoelectron spectrometer experiment.

74

As with the continuous instrument, this experimental setup has a series of advantages anddisadvantages. Namely, as will be discussed, the dual pulsed valve ion source is capable of bothrational ion synthesis, and is also capable of rapidly cooling and stabilizing even weakly boundreaction products. This allows for the generation of many novel ions and clusters. The Wiley–
McClaren time of flight system presents a distinct improvement, by approximately an order ofmagnitude, in mass resolution over the Wien velocity filter found on the continuous apparatus.
This reduces or eliminates the possibility of mass contamination in any collected photoelectron
spectra. The VMI photoelectron spectrometer itself possesses 100% collection efficiency ofphotodetached electrons, allowing for a standard commercial pulsed Nd:YAG laser system,
combined with a visible-light optical parametric oscillator (OPO) or dye laser, to be entirely
sufficient. The disadvantage of the VMI photoelectron spectrometer comes in the form of thevariable spectral resolution, which is a function of photoelectron kinetic energy. This will be
discussed in detail in § 2.10, but in broad terms, this necessitates that several photoelectron
spectra must be collected for a given anion utilizing multiple photon energies. Alternatively, thisproperty may be used advantageously, as for very low kinetic energy electrons (< 100 meV) thisspectrometer achieves a resolution of ~ 3 meV. Compared to the constant resolution of 10 meVon the continuous experiment, this allows one to obtain higher resolution photoelectron spectra.
This technique is known as slow electron velocity–map imaging (SEVI) photoelectron
spectroscopy.12
The individual components comprising this experiment will now be detailed and theirprinciples of operation explained.

75§ 2.8 Principles of Operation: Dual Pulsed Valve Ion SourceThis experiment begins with ion generation which takes the form of two Parker-Hannifin
Series 9 General Valves placed such that they face perpendicular to each other. One is designatedthe main valve, and is operated as a normal supersonic expansion. For the studies presented inthis thesis, this valve will always use argon as a carrier gas seeded with trace amounts of anaromatic organic compound to be investigated. The other valve is designated the side valve andhas electric discharge plates (ΔV ~ –1000-2000 V) placed immediately in front of the exit
aperture of the valve. The tension of the side valve is adjusted so that it accounts for ~ 10% of
the base line pressure rise in the vacuum chamber upon pulsing. This valve is used to generate aplasma whose composition depends on the gas backing the valve. Typically, this is a mixture ofoxygen, argon, and hydrogen. This plasma is then entrained in the primary supersonic expansion
envelope, and subsequent collisions with the aromatic molecules seeded in the primary
expansion effect reactions forming the anions of interest for a given experiment. See Fig. 2.13
for a schematic of this ion source reprinted from Ref. 27.

76

Fig. 2.13 A schematic of the ion source utilized in the experiments. Reprinted from Ref. 27.

77

The general theory behind the use of this experimental setup has been discussed andcharacterized at length previously.28 For the works considered in this dissertation, this ion source
was utilized in such a way as to use similar chemical reaction schemes as were found in § 2.3. Asin the FA ion source, the strategy employed for all experiments presented here was to create astrong Lewis base in the side valve which is then entrained in the main supersonic expansion andwill subsequently deprotonate the aromatic molecules seeded therein. The Lewis bases used in
the experiments were: O– via discharge of O2 in Ar, OH– via discharge of O2, H2, and Ar, and F–
via discharge of NF3 in Ar. Following collisions and reactions with whichever base was chosen
for a given experiment, the products are subsequently collisionally cooled by the argon carrier
gas and extracted (utilizing an electrically pulsed metal plate) into a Wiley-McLaren Time-ofFlight (TOF) mass spectrometer to mass separate the product anions prior to performing PE
spectroscopy on the anions of interest.

78§ 2.9 Principles of Operation: Wiley-McLaren Time-of-Flight Mass Spectrometer
The Time-of-Flight mass spectrometer has been utilized and studied extensively,
including the specific variation known as the Wiley-McLaren TOF.29 In general, the principle
under which a TOF mass spectrometer operates is a simple one. A charged particle which isaccelerated by and toward an electrode attains a kinetic energy equal to its charge times thevoltage of the electrode once it reaches the position of that electrode in space. Notice that this1

kinetic energy is not dependent on the mass of the particles, and recalling that 𝐾𝐸 = 2 𝑚𝑣 2 , thenparticles accelerated by the same electrode and which possess the same charge but different
masses will therefore possess different velocities. Thus, if one is able to generate an ensemble ofions, such as using the ion source described in §2.6, which are then simultaneously accelerated tothe same kinetic energy and allowed to then travel in space unperturbed for some period, the ionswill necessarily separate along the axis of motion according to their respective masses. In thecase of the experiments presented here, these mass separated ions will pass into the laserinteraction region of a VMI PE spectrometer, allowing for an appropriately timed nanosecond
laser pulse to photodetatch electrons from the ions of the appropriate mass.
Practically, several other factors must be accounted and compensated for. First we
require that the ions not only be mass separated, but spatially focused into the smallest possible
volume when they arrive at the VMI PE spectrometer’s laser interaction region. In principle, theWiley-McLaren TOF setup specifically allows for the ions being mass separated to also come toa single spatial focal point, solving this problem, so long as the experiment is contrived to locate
this focus at the interaction volume of the VMI PE spectrometer. Realistically, more intervention
is required for adequate focusing and thus several other ion optics have been added along thedrift tube. Specifically, an Einzel lens30 and several deflector plates are used to both focus and79steer the ions. Second, a method is required to allow for the collection of TOF mass spectra. Thisis accomplished by placing a double stack of MCPs immediately after the VMI PE spectrometerintersecting the ion beam, the output of which is read on an oscilloscope. This allows for realtime optimization of the TOF parameters for the best possible spatial and temporal focusing of
the ions during the course of an experiment.

80§ 2.10 Principles of Operation: Velocity Map Imaging Photoelectron Spectrometer
The VMI PE Spectrometer has been discussed extensively in the past27 and a brief
description will be given here. A VMI apparatus works on the principle that if one places anensemble of charged particles each with a distinct velocity vector into an appropriately
engineered electric field, they may be accelerated and focused onto a two dimensional plane suchthat the position of each particle at the detector is proportional to the 2D projection of its 3D
velocity vector.11 Unlike the hemispherical electron energy analyzer, the mathematics
underpinning the design of the electrodes necessary to generate the appropriate electric fields are
not conducive to analytical solutions which can be easily physically interpreted. Indeed, thedesign of first VMI apparatus was largely possible due to advances in computers and simulation
software capable of calculating on-the-fly ion trajectories in the presence of electric fields.
The VMI setup used in the experiments presented here is comprised of three electrodeelements: the repeller plate, the ground plate, and the extractor plate. The ground and extraction
plates have a hole bored through their center to allow for the electrons to pass through. See Fig.
2.14. Prior to collecting a PE spectrum, both the repeller and ground plate are held at groundvoltage, to which the ions in the Wiley-McLaren TOF have been referenced to. When the ions ofinterest enter the interaction region of the VMI apparatus, two actions are triggered. First, an
appropriately timed nanosecond laser pulse will intercept the ions, photodetaching electrons.
Second, the repeller plate is switched to the voltage necessary to form the appropriate electricfields to achieve VMI. By altering the voltage of the extraction and repeller plates one is able tochange the effective range of electron kinetic energies which are observed by altering themagnification of the electron image projected onto the detector. The photodetached electrons
will then be accelerated and imaged onto the detector where their positions are directly81proportional to their respective velocities. The position sensitive detector utilized here is a stackof MCPs coupled to a phosphor screen. Following an electron impacting a specificmicrochannel, the electron current through the impacted channel will amplify by a factor of ~107
and will impinge on the phosphor screen causing it to phosphoresce at the position of thatmicrochannel, and this is then recorded by a CCD camera.
This photoelectron image is analyzed first by reconstructing the two dimensional image
into a three dimensional velocity distribution using an inverse Abel transform as part of the
BASEX program package. This three dimensional distribution is then converted to a one
dimensional electron speed distribution, and finally an electron Kinetic Energy (eKE)distribution by a Jacobian transformation. This spectrum is converted to an electron Binding
Energy (eBE) distribution by subtracting eKE from the laser photon energy, yielding the reported
photoelectron spectra. This spectrum must then be calibrated; typically, this is done utilizing the
known PE spectrum of an atomic anion, however the specifics of the calibrant used in a given
experiment will be discussed in the chapter appropriate to that experiment.
In contrast with the hemispherical analyzer, the VMI spectrometer has a spectral
resolution that is a function of eKE, namely the resolution decreases with increasing eKE. This issimply due to the conversion to kinetic energy, which goes as the square of the velocity. This
resolution is determined by the eKE and the Full Width at Half Maximum (FWHM) of a peak ina photoelectron spectrum due to a single transition. Typically, a fine structure transition of an
atomic anion is used to determine the resolution as a function of electron kinetic energy, as wellas calibrate the energy scale. Typically, the resolution is ~2–3% (resolution ~ FWHM/eKE).
However, if the photon energy is within ~50 meV of the binding energy of a transition, thisresults in low kinetic energy electrons, and the instrument attains a resolution of ~3 meV. This82

technique is commonly referred to as Slow Electron Velocity Map Imaging (SEVI),12 and will be
used throughout the works presented in this dissertation. This will be accomplished by changing
the photon energies employed in a given experiment, with details of the specifics found in thefollowing chapters.

83

Fig. 2.14 Picture of the VMI PE spectrometer, annotated.

84§ 2.11 References
1) L. M. Branscomb and S. J. Smith, Phys. Rev. 98, 1127-1128 (1955).
2) D. T. Vier and J. E. Mayer, J. Chem. Phys. 12, 28-34 (1944).
3) D. M. Neumark, K. R. Lykke, T. Andersen and W. C. Lineberger, Phys. Rev. A 32, 18901892 (1985).
4) L. M. Branscomb, D. S. Burch, S. J. Smith and S. Geltman, Phys. Rev. 111, 504-512 (1958).
5) W. C. Lineberger and B. W. Woodward, Phys. Rev. Lett. 25, 424-427 (1970).
6) C. E. Kuyatt and J. A. Simpson, Rev. Sci. Instrum. 38, 103-111 (1967).
7) A. Kasdan and W. C. Lineberger, Phys. Rev. A 10, 1658-1664 (1974).
8) R. J. Celotta, R. A. Bennett, J. L. Hall, M. W. Siegel and J. Levine, Phys. Rev. A 6, 631-642(1972).
9) H. Handschuh, G. Ganteför and W. Eberhardt, Rev. Sci. Instrum. 66, 3838-3843 (1995).
10) X. Wu, Z.-b. Qin, H. Xie, X.-h. Wu, R. Cong and Z.-c. Tang, Chin. J. Chem. Phys. 23, 373380 (2010).
11) A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477-3484 (1997).
12) D. M. Neumark, J. Phys. Chem. A 112, 13287-13301 (2008).
13) D. B. Dunkin, F. C. Fehsenfeld, A. L. Schmeltekopf and E. E. Ferguson, J. Chem. Phys. 49,
1365-1371 (1968).
14) C. H. DePuy and V. M. Bierbaum, Acc. Chem. Res. 14, 146-153 (1981).
15) D. G. Leopold, K. K. Murray, A. E. S. Miller and W. C. Lineberger, J. Chem. Phys. 83,
4849-4865 (1985).

8516) D. R. Burgess and J. E. Bartmess, Thermochemical Data in NIST Chemistry WebBook, NIST
Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard,
http://webbook.nist.gov/chemistry/17) L. Wåhlin, Nucl. Instr. Meth. 27, 55-60 (1964).
18) P. H. Dawson, International Journal of Mass Spectrometry and Ion Physics 17, 423-445(1975).
19) J. M. Van Doren, S. E. Barlow, C. H. Depuy and V. M. Bierbaum, Int. J. Mass Spectrom. Ion
Processes 81, 85-100 (1987).
20) S. Penner, Rev. Sci. Instrum. 32, 150-160 (1961).
21) S. W. Wren, (2011). Photoelectron Spectroscopy of Organic Anions (Doctoral Thesis).
22) M. W. Siegel, R. J. Celotta, J. L. Hall, J. Levine and R. A. Bennett, Phys. Rev. A 6, 607-631(1972).
23) E. D. Black, Am. J. Phys. 69, 79-87 (2001).
24) C. S. Feigerle, (1983). Laser Photoelectron Spectroscopy of Transition Metal Negative Ions
(Doctoral Thesis).
25) L. D. Landau and E. M. Lifshitz, Mechanics (Elsevier Ltd., 2010), 3rd edn., Vol. 1, Courseof Theoretical Physics.
26) K. M. Ervin, I. Anusiewicz, P. Skurski, J. Simons and W. C. Lineberger, J. Phys. Chem. A
107, 8521-8529 (2003).
27) L. Sheps, E. M. Miller and W. C. Lineberger, J. Chem. Phys. 131, 064304 (2009).
28) Y.-J. Lu, J. H. Lehman and W. C. Lineberger, J. Chem. Phys. 142, 044201 (2015).
29) W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum. 26, 1150-1157 (1955).
30) G. H. N. Riddle, J. Vac. Sci. Tech. 15, 857-860 (1978).

86

CHAPTER III
ANION PHOTOELECTRON SPECTROSCOPY OF DEPROTONATED ORTHO-, META-,AND PARA-METHYLPHENOL
Sections of this chapter have been published in the peer-reviewed Journal of Chemical Physics
under the same title, by, Daniel J. Nelson, Wilson K. Gichuhi, Elisa M. Miller, Julia H. Lehman,
and W. Carl Lineberger§ 3.1 Introduction and BackgroundAs was discussed in § 1.3 the methylphenoxyls and methylphenoxides are of interest to avariety of fields within chemistry. These reasons have motivated the photoelectron (PE)spectroscopy study which was conducted and is presented here. The strategy to study the
methylphenoxides and their corresponding neutrals, the methylphenoxyls, was as follows. Bothexperimental setups described in Chapter II were employed. The continuous experiment–

generated the anions of interest in the Flowing Afterglow ion source (§ 2.2) utilizing OH or–

NH2 as a Lewis base to deprotonate either ortho, meta, or para-methylphenol. The products ofthis reaction were then mass selected and the photoelectron spectra of these molecules obtained.
While this experimental setup is advantageous due to the constant spectral resolution (~10 meV)of the hemispherical electron energy analyzer, it suffers from an inability to change wavelengths
in order to observe any wavelength dependence of the spectra, or obtain high resolution (~3meV) spectra. Thus the pulsed instrument is also employed. This experiment utilized the same
synthesis pathway in the dual pulsed valve plasma entrainment source, i.e. the reaction of the–neutral methylphenol isomer of interest with the Lewis base OH . Taking advantage of both theincreased mass resolution, compared to the continuous instrument, and the increased spectral

87resolution, due to a tunable laser light source, as discussed in § 2.8, additional high resolution
and SEVI spectra were collected and additional physical insights were able to be drawn.

88§ 3.2 Experimental Specifics
The experimental setups used to conduct these studies have already been discussed ingeneral in Chapter II of this dissertation. However, specific details of how these instruments
were employed for this experiment will now be explained.
In both instruments, methylphenol samples were obtained from Sigma Aldrich (≥ 98%
purity) and used without further purification. Their chemical structure and chemical purity wereverified with 1H NMR.
In the continuous experiment, the methylphenoxide anions are prepared in a flowing
afterglow ion source by proton abstraction from the parent methylphenol molecules using thehydroxide anion, OH¯, or the amide anion, NH2–, in approximately 0.5 Torr of helium buffer
gas. The flow tube is cryogenically cooled to ~200 K. This collisional environment allows thereactants and products to evolve toward chemical equilibrium, so the detected anion products
need not be the nascent deprotonated products. The dominant anion product observed
corresponds to deprotonation of the most acidic site, with trace amounts of other products. BothOH¯ and NH2¯ are energetically capable1, 2 of deprotonating either the hydroxyl or the methyl
sites for all three isomers, although the hydroxyl site is the most acidic site by over 30 kcal/mol,
according to calculations at the B3LYP/aug-cc-pVQZ level. See Table 3.1. The NH2¯ anion is
also energetically capable of deprotonating directly from the benzene ring, but this anion productchannel is not observed.

89

Table 3.1 The enthalpies for the reactions removing a proton from either the OH or CH3 groups
on the methylphenols were computed and are presented here. The electron affinity for thecorresponding radical of the deprotonated methylphenol is also reported. These were all
calculated at the B3LYP/aug-cc-pVQZ level. In all cases the OH site is the most acidic site on
the molecule by ~30 kcal/mol, with m-methylphenol showing the least acidic OH as compared to
the CH3. Note that the acidities of H2O and NH3 are 390 and 403 kcal/mol.3
Methylphenol
Isomero-methylphenol

m-methylphenol

Deprotonation
Site

Calculated𝑜∆𝑎𝑐𝑖𝑑 𝐻298𝐾(kcal/mol)

Calculated EA
(eV)

OH

347.6

2.135

CH3

372.3

1.279

OH

348.9

2.167

CH3

378.8

1.013

OH

349.6

2.065

CH3

385.2

0.710

p-methylphenol

90Following ion formation, the ions are extracted into a differentially pumped chamber(~10-6 Torr). In this chamber, the ions are focused, steered, and accelerated to 735 eV, mass
selected with a Wien velocity filter (§ 2.3), and then decelerated to 35 eV before entering thelaser-anion interaction region. In the interaction region, the mass-selected anions are intersectedby single frequency radiation (363.806 nm, 3.40814 eV) obtained from an Ar-ion laser (§ 2.4). A
small solid angle of the photodetached electrons is collected in a direction perpendicular to thelaser and ion beams. The kinetic energies of these electrons are then measured by ahemispherical electron energy analyzer (§ 2.5) with a constant electron energy resolution of 10
meV. The photoelectron spectra reported here from this machine were obtained with the laser
polarization oriented at the magic angle of 54.7o with respect to the photoelectron collection
plane. This orientation yields photoelectron intensities that are proportional to the total
photodetachment cross-section.4 The laser polarization was scanned to measure the anisotropy inthe photoelectron angular distribution, specifically for the peak corresponding to the origin
transition in the methylphenoxide anion photoelectron spectra. For the methyl deprotonatedproduct for the meta isomer, the reported anisotropy parameter was averaged across the
spectrum.
The pulsed experimental apparatus is now discussed. The anions of interest are produced
in a dual pulsed valve plasma entrainment source (§ 2.6), again from the deprotonation of
methylphenol by reaction with OH¯.5 The primary supersonic expansion is operated with abacking pressure of 10 psig, ~1% methylphenol in Ar, while the side valve is operated with abacking pressure of 35 psig, 1% O2, 30% H2 and the balance Ar. The OH– generated in theplasma source undergoes reactions with the methylphenol isomer of interest contained in themain expansion, generating products which are collisionally cooled with Ar. While not necessary91to be the case, the most thermodynamically favorable products (methylphenoxide anions, Table
3.1) were found to be by far the dominant deprotonated methylphenol produced.
Following the initial expansion, the anions are directed into a Wiley-McLaren TOF mass
spectrometer by a pulsed extraction plate, where the ions are separated by their mass-to-charge
ratio (m/z) and spatially focused in the center of a VMI interaction region (§ 2.8).6 Anappropriately timed nanosecond laser pulse (discussed below) intersects the anion packet,
photodetaching electrons. All of the photodetached electrons are velocity mapped onto a position
sensitive phosphor screen-coupled microchannel plate detector, and subsequently imaged by a
CCD camera. This photoelectron image is analyzed first by reconstructing the two dimensional
image into a three dimensional velocity distribution using an inverse Abel transform as part ofthe BASEX program package. This three dimensional distribution is then converted to a one
dimensional electron speed distribution, and finally an electron Kinetic Energy (eKE)distribution by a Jacobian transformation. This spectrum is converted to an electron Binding
Energy (eBE) distribution by subtracting eKE from the laser photon energy, yielding the reported
photoelectron spectra. For these experiments, the energy scale is calibrated by the S–
photoelectron spectrum.7, 8
In contrast with the hemispherical analyzer, the VMI spectrometer has a spectral
resolution that is a function of eKE, namely the resolution decreases with increasing eKE. This
resolution is determined by the eKE and the Full Width at Half Maximum (FWHM) of a peak ina photoelectron spectrum due to a single transition. In this case, an atomic anion (S–) was used todetermine the resolution as a function of electron kinetic energy, as well as calibrate the energy
scale. Typically, the resolution is ~2–3% (resolution ~ FWHM/eKE). However, if the photon
energy is within ~50 meV of the binding energy of a transition, this results in low kinetic energy
92electrons, and the instrument attains a resolution of ~3 meV. This technique is commonlyreferred to as Slow Electron Velocity Map Imaging (SEVI).9 To take advantage of the varying
energy resolution of this spectrometer, a variety of photon energies are used in this experiment.
The 3rd harmonic of a nanosecond Nd:YAG laser is used directly (355 nm, 3.494 eV). In
addition, a visible light OPO is pumped by the 3rd harmonic of the Nd:YAG laser, supplying
tunable light between 400 and 600 nm. Finally, the second harmonic of the Nd:YAG laser (532
nm, 2.330 eV) is used to pump a dye laser using DCM dye and producing 647 nm light, which isthen frequency doubled to produce 323 nm (3.832 eV) light.

93§ 3.3 Theoretical Methods and Simulations
Electronic and vibrational quantum calculations were carried out using the Gaussian 09
program package.10 All ground electronic state calculations were at the B3LYP/aug-cc-pVQZ
level, which has been shown to be an effective compromise between accuracy and computational
cost for large molecules, such as the substituted aromatic compounds studied here.11 For excitedelectronic state calculations, Time Dependent Density Functional Theory (TD–DFT) was usedwith the B3LYP functional and Dunning’s correlation consistent basis set, aug-cc-pVTZ. Theenergies and geometries of the anion ground electronic states, neutral ground electronic states,
and neutral first excited electronic states were optimized for all isomers. In the case of eachisomer, the ground and excited electronic state vibrational information was calculated separately
for every anionic and neutral state used to simulate the PE spectra. In addition, the electron
affinities (EAs) and electronic term energies for the o-, m-, and p-methylphenoxyl radicals were
calculated. The calculated geometries, normal mode vectors, and the harmonic vibrationalfrequencies of the anion and the corresponding neutral are used to calculate the Franck-Condon(FC) factors for the simulated photoelectron spectra using the PESCAL program.12 In addition, aBoltzmann distribution of anion internal energy is applied (150 K or 200 K for the pulsed andcontinuous experiments, respectively). In the simulation, the FC factors are calculated using theharmonic oscillator approximation, which includes Duschinsky rotations and employs the SharpRosenstock-Chen method.13-15 The calculated FC factors are shown as purple sticks whereappropriate in the figures displayed in this chapter. These sticks are then convolved with
Gaussian functions, whose integral is normalized to be equal to the calculated transition
intensity, and whose FWHM is consistent with instrumental resolution, which is a function ofeKE for the VMI spectrometer and is a constant 10 meV for the hemispherical analyzer. The sum

94

of these Gaussian functions results in the simulated spectra shown as green traces, which havebeen scaled in each case to match the experimental peak amplitude of the origin transition. Eachsimulated spectrum has also been shifted by ~40–50 meV to match the experimental electronic
band origin.

95§ 3.4 Expectations and Error Analysis
Before scrutinizing the photoelectron spectra of the product anions generated fromreacting OH¯ (or NH2¯ in the FA source) with the methylphenol isomers, it is useful to consider
how the resulting photoelectron spectra from different deprotonated products might qualitatively
vary. For example, the EA of the corresponding neutral is a good indication of the site of
deprotonation of methylphenol. If the product anion is the result of deprotonating the alcoholgroup, the electron affinity of the neutral should be similar to that of the phenoxyl radical (EA =2.2538 ± 0.0008 eV16). If the product anion is instead the result of CH3 deprotonation, it isreasonable to expect an EA of the corresponding neutral to be near that of the benzyl radical (EA= 0.912 ± 0.006 eV17). In either case, the electron is likely to localize on the deprotonated group;
this should result in both a lengthening of the bond between the deprotonated group and thearomatic ring, as well as an elongation of the ring away from the negative charge whencompared to the equilibrium geometry of the corresponding neutral radical. Thus, upon
photodetachment, it might be expected that there will be vibrational transitions involving
excitation in the neutral ring distortion modes with vibrational frequencies of ~ 500 cm-1. In
addition, it is reasonable to predict excitation of vibrational modes in the neutral that incorporatea stretching motion along the bond between the ring and the deprotonated group, which would
have fundamental vibrational frequencies of ~ 1000 - 1500 cm-1.
For each photoelectron spectrum, a vibrational analysis is performed by comparing theobserved spectrum with the calculated photoelectron spectrum simulation. Throughout this work,
the errors in the reported peak positions are related to the statistical error in finding the peak
center, the error in the absolute energy scale, and the number of independent measurements ofparticular peaks. When reporting the position of a particular transition, such as the neutral96

vibrational frequencies or the EA, this error is combined with the error associated with the offsetof the actual transition from the peak center. This uncertainty can be near zero if a single
transition is the major contributor to the peak shape, or, if there are multiple transitions under the
peak envelope, it could be as much as the Half-Width-at-Half-Maximum (HWHM) of the peak.
Note that the peaks presented in the reported spectra are broader than the instrument resolution
and always arise from multiple transitions, so this latter error dominates the reported uncertainty.
In the error analysis for each isomer, rotational contours were also considered. In every case, theanalysis of Engelking was used18 and in all cases the shifts between peak centers of a particulartransition and the true origin were estimated to be less than 0.1 meV. Thus this effect was
accounted for in the uncertainty of the measurements contained in this work, but was determined
to have a small impact on the reported error, even in the case of the EA measurements.

97§ 3.5 Electron Affinities and Term Energies
In order to gain an overview of the full photoelectron spectra, data were collected on the
VMI PE spectrometer with a photon energy of 3.494 eV (Fig. 3.1). Following deprotonationfrom all three reactant isomers (o-, m-, and p-methylphenol), there are two progressions
observed. The first progression begins at approximately 2 eV eBE, with a second progression
appearing at ~ 3 eV eBE. Considering the prior arguments, this would suggest that the anionicproduct formed is indeed methylphenoxide for all three reactant isomers. Further, considering thephotoelectron spectroscopy spin selection rule ΔS ± ½, and the fact that the methylphenoxideanion electronic ground state has singlet spin multiplicity, these two observed progressions must
both be of doublet multiplicity. The progression starting near 2 eV in binding energy for eachisomer arises from electron detachment from the anion singlet ground electronic state to form theneutral doublet ground electronic state (labeled 𝑋̃). Each progression starting near 3 eV inbinding energy corresponds to a vibrational progression arising from transitions from the anion
singlet ground electronic state to the first excited doublet electronic state of the neutral (labeled𝐴̃) of methylphenoxyl. These assignments will be justified in the following paragraphs. While all
three isomers of methylphenol show evidence of methylphenoxide products, another progression
starting near 1 eV is observed for only the meta isomer. This will be shown to be thephotoelectron spectrum of the anion product arising from the deprotonation of the methyl group(see Section E), forming the methylenephenol radical. It is not expected, based on TD-DFT
calculations, that excited states of methylenephenol would be energetically accessible in thisstudy.

98Fig. 3.1 The 355 nm photoelectron spectra of the three methylphenoxide anion isomers. These
data were taken using the VMI photoelectron spectrometer. The 𝑋̃ refers to the ground doublet
electronic state, while the 𝐴̃ refers to the first excited doublet electronic state of thecorresponding neutral methylphenoxyl radicals. The data are reported as dots, with a line
marking the baseline at zero. The shaded boxes indicate the energy range covered in Fig. 3.2.

99

In addition to the photoelectron spectra presented in Fig. 3.1, several additional spectra
were collected on the continuous experiment and displayed in Fig. 3.2. Fig. 3.2 displays survey
photoelectron spectra of all four products of the reaction of methylphenol with hydroxide madein the Flowing Afterglow (FA) ion source and collected with the hemispherical electron energy
analyzer, utilizing 3.40814 eV photons. Note that due to the method by which hydroxide is
generated in the FA source (reaction of O– with methane gas), there is still some O– present in theion source region. This excess O– is capable of both dehydrogenating (removal of H2+) anddeprotonating the methylphenols, and a trace signal from this product channel is seen in thebottom panel of Fig. 3.2.

100

Fig 3.2 Photoelectron spectra of the products of methylphenol reacted with hydroxide in theflowing afterglow ion source obtained with a photon energy of 3.40814 eV and collected in ahemispherical energy analyzer. Note that there is a trace of O– in this source which leads to asmall signal from methylenephenoxide in the para case.

101

High resolution photoelectron spectra of the origin peaks for the transition to the neutral𝑋̃ state for each of the three methylphenoxide isomers were obtained using the SEVI technique(Fig. 3.3). In addition to measuring these portions of the spectra with increased resolution, thephotoelectron spectra were also simulated and overlaid with experimental data. Considering theexcellent agreement between the experiment and simulation, we report the EA of o-, m-, and pmethylphenoxyl as 2.1991 ± 0.0014, 2.2177 ± 0.0014, and 2.1199 ± 0.0014 eV, respectively(summarized in Table 3.2).

102

Fig. 3.3 The Slow Photoelectron Velocity-Map Imaging (SEVI) spectra of o-, m-, and pmethylphenoxide, allowing for the determination of the EAs of the corresponding
methylphenoxyls. Experimental data are reported as dots, with a line marking the baseline atzero. Peaks labeled with an asterisk possibly represent the methyl hindered rotor vibrationaltransitions discussed in the text. Simulated Franck-Condon factors (T = 150 K) are shown as
purple sticks and their convolution with the experimental resolution is shown in green. Note thex-axis only spans 220 meV, while Fig. 3.1 spanned 1.5 eV.

103

The transition to the neutral 𝐴̃ state was investigated for all three isomers making use of
3.494 eV photons with the VMI spectrometer and shown in Fig. 3.4. The photoelectron spectra
for this transition were simulated and overlaid with the experimental data, and again, these agree
very well. Therefore, we can identify the electronic band origin of these progressions at 3.228 ±
0.009, 3.180 ± 0.002, and 3.149 ± 0.009 eV electron binding energy for o-, m-, and pmethylphenoxyl, respectively. Thus the term energy between the ground and first excited doublet
electronic states of the neutral radicals are determined: 1.029 ± 0.009, 0.962 ± 0.002, and 1.029 ±0.009 eV for o-, m-, and p-methylphenoxyl, respectively (summarized in Table 3.2). This energy
is very similar to that in phenoxyl (1.06(5) eV),17 although the excited neutral state was not
explored in detail in that study. Note that for the ortho and para isomers, a conservative error barof 9 meV is used. This is based on the HWHM of the origin peak. However, the simulationsuggests that for the meta isomer, less significant transitions are symmetrically distributed about
the origin transition in peak A, suggesting that a smaller error bar of ± 0.002 eV is warranted.

104

Fig. 3.4 The 355 nm anion photoelectron spectra of o-, m-, and p-methylphenoxide, examining
the first excited doublet electronic states (𝐴̃) of the methylphenoxyl radicals. Experimental data
are shown as dots. Simulated Franck-Condon factors are shown as purple sticks and theirconvolution with the experimental resolution is shown in green.

105

Table 3.2. Summary of results. The SEVI technique was used to obtain the EAs of the three
methylphenoxyl isomers. Combining these with the results from King et al.19 and Karsili et al.20
allows for the determination of the gas phase acidities shown. The term energies determined in
the current study for the methylphenoxyl radical 𝐴̃ ← 𝑋̃ electronic transitions are also listed.
DeprotonatedMethylphenol
IsomerEA(eV)

D0 ,0K𝑜∆𝑎𝑐𝑖𝑑 𝐻0𝐾

𝑜∆𝑎𝑐𝑖𝑑 𝐻298𝐾

T0 (𝐴̃←𝑋̃)(CH3-PhO-H)(CH3-PhO-H)(CH3-PhO-H)(CH3-PhO●)(kcal/mol)a(kcal/mol)(kcal/mol)b

(eV)

cis

cis

cis

84.43 ± 0.14

347.31 ± 0.14

348.39 ± 0.25

trans

trans

trans

84.86 ± 0.14

347.74 ± 0.14

348.82 ± 0.25

1.029 ±

2.1991 ± 0.0014
0.009

0.962 ±
2.2177 ± 0.0014

86.53 ± 0.14

348.98 ± 0.14

350.08 ± 0.25
0.002

1.029 ±
2.1199 ± 0.0014

83.83 ± 0.14

348.53 ± 0.14

349.60 ± 0.25
0.009

a

Results from King et al.19 and Karsili et al.20
Obtained from the 0 K experimental results and calculated Cp values at the B3LYP/aug-ccpVQZ level. The increased error from the 0 K results is due to uncertainty in the calculated
heat capacities.
b

106

In addition to the electron affinities, photoelectron angular distributions with respect tothe laser polarization, characterized by the anisotropy parameter (β), were measured for thesephotoelectron spectra.4 Since the electron is being detached from what can be described as anoxygen p-like orbital, it is expected that we should observe a negative β. Photodetachment
forming the neutral 𝑋̃ state shows the same photoelectron anisotropy for the three isomers, β ~ –0.3, using 3.494 or 3.408 eV photons. This was also measured for the methylenephenol anion,
and is approximately the same as for the methylphenoxides, β ~ – 0.3. The photoelectron angular
distributions resulting from photodetachment to form the neutral 𝐴̃ state for all three isomers
were isotropic (β ~ 0). However, these transitions are all close in binding energy (< 400 meV) to
the photon energy used to examine them, and so an isotropic angular distribution is expected.21107

§ 3.6 Vibrational Analysis: ortho-Methylphenoxyl
The vibrational progression in the o-methylphenoxide photoelectron spectrum resulting
from the transition to the o-methylphenoxyl radical 𝑋̃ state was studied in the hemisphericalanalyzer and is shown in Fig. 3.5 and with the VMI PE spectrometer with lower resolution in
Fig. 3.6. Focusing on Fig. 3.5, it is evident that there is a progression resulting from exciting ring
distortion vibrational modes, as the dominant peaks observed are spaced by approximately 500
cm–1, as expected. All of the observed peaks are broader than the instrument resolution, implyingthat each peak is comprised of multiple overlapping transitions. Nonetheless, the partiallyresolved peak doublet labeled B and C may be fit to two Gaussian functions, and their positions
relative to the band origin are 434 ± 8 and 542 ± 8 cm–1, respectively. While all of the peaks withlarger binding energy than C are predicted to be due to multiple transitions and are beyond theability of either instrument to resolve, peaks B and C are predicted to be primarily due to only a
few transitions. Peak B is predicted to be due to a transition from the ground vibrational state of
the anion to one quanta in a ring distortion vibrational mode in the neutral (3310 = 430 ± 80 cm-1),
which has a calculated harmonic frequency of 453 cm–1. Peak C is due to two transitions, eachstarting in the ground vibrational state of the anion and going to one quanta in two different ring
distortion vibrational modes of the neutral molecule (3010 , 3110 = 540 ± 80 cm–1). These twomodes are calculated to have harmonic frequencies of 540 and 577 cm–1. The hindered methyl
rotor vibrational mode was possibly resolved for o-methylphenoxyl, seen in Fig. 3.3, indicatedwith an asterisk, and located 196 ± 8 cm-1 relative to the origin. This peak is likely due to atransition from the ground vibrational state in the anion to two quanta in the hindered methyl
rotor vibration in the neutral state, 3920 = 196 ± 16 cm-1. Neglecting anharmonicity, 3910 = 98 ± 8cm-1 is derived, which compares well with the calculated harmonic frequency of 106 cm-1.
108

However, due to the inherent complexity of hindered rotor vibrations, a harmonic approximation
is likely not valid, and so this assignment, while plausible, is only suggested.

109

Fig. 3.5. The anion photoelectron spectra of o-, m-, and p-methylphenoxide, examining the
ground doublet electronic states of the methylphenoxyls. Experimental data are displayed as
dots. The top two panels were obtained with a photon energy of 3.40814 eV using ahemispherical energy analyzer, while the bottom panel was collected using a VMI spectrometerwith a photon energy of 2.520 eV. Simulated Franck-Condon factors are shown as purple sticks
and their convolution with the experimental resolution is shown in green.

110Fig. 3.6 Photoelectron spectrum of ortho-methylphenoxide taken with a photon energy of 2.822
eV, utilizing the VMI photoelectron spectrometer. Only experimental data are shown. The peakintensities in this spectrum are consistent with those shown in the top panel of Fig. 3.4 whichwere taken using the hemispherical analyzer.

111

The vibrational progression resulting from the transition to the excited electronic 𝐴̃ stateof the o-methylphenoxyl radical can be seen in Fig. 3.4. None of the peaks observed in the
spectrum are resolution limited, which again indicates that there are multiple transitions
contributing to each peak. The photoelectron spectrum was simulated and reproduces theexperiment with excellent agreement. Peak A is assigned as the band origin, with peaks B and C
being located 483 ± 16 cm-1 and 1225 ± 16 cm-1 higher in binding energy, respectively. While
there are several predicted transitions with significant FC factors attributed to peak B, the peakintensity and position is primarily due to a transition from the anion vibrational ground state to
one quanta in a ring distortion vibrational mode in the neutral 𝐴̃ state, 3210 = 480 ± 190 cm-1,
predicted to have a harmonic frequency of 506 cm-1. Peak C is similarly broadened past the
instrument resolution, but its dominant contributor (based on FC factor intensity) is predicted tobe due to a transition from the anion vibrational ground state to a single quanta in a quasi C–O
stretching mode in the neutral 𝐴̃ state, 1510 = 1220 ± 130 cm-1, predicted to have a harmonicfrequency of 1312 cm-1.
The normal mode eigenvectors of the vibrational states which are activated upon
photodetachment of o-methylphenoxide are depicted in Table 3.3.

112

Table 3.3 The following diagrams show the primary Franck-Condon active normal modes of omethylphenoxide. Each eigenvector shown here is that of the neutral radical, and was calculated
at the B3LYP/aug-cc-pVQZ level of theory. The listed frequencies are theoretical and harmonic.

Ortho-methylphenoxyl radical (𝑋̃ state)ν33 (453 cm-1)

ν31 (540 cm-1)

ν39 (106 cm-1)

ν30 (577 cm-1)

113

Ortho-methylphenoxyl radical (𝐴̃ state)ν32 (506 cm-1)

ν15 (1312 cm-1)

114

§ 3.7 Vibrational Analysis: meta-Methylphenoxyl
The vibrational progression in the m-methylphenoxide photoelectron spectrum resulting
from detachment to the m-methylphenoxyl radical 𝑋̃ state was studied in the continuous
experiment (see Fig. 3.5) and in the pulsed experiment (Fig. 3.7). As with the ortho isomer, aprogression spaced by approximately 500 cm-1 is observed for the meta isomer, once again
suggesting that ring distortion modes are excited upon photodetachment of the anion. Assumingthat peaks B–D in Fig. 3.5 are due to only one mode, these peaks are due to transitions from theground anionic state to 1, 2, or 3 quanta of a single vibrational mode in the neutral state, withpeak locations (relative to the EA) of 564 ± 8 cm-1, 1079 ± 8 cm-1, and 1587 ± 8 cm-1 for peaks
B, C, and D, respectively. These correspond to the following transitions: 3110 = 560 ± 50 cm-1,
3120 = 1080 ± 64 cm-1, 3130 = 1590 ± 90 cm-1. The simulated spectrum (overlaid) agrees wellwith the experiment and confirms that this vibrational progression is primarily due to transitions
from the ground anionic state to excitation in a ring distortion vibrational mode in the neutral
state with a calculated harmonic frequency of 540 cm–1, agreeing with the experimentallymeasured 3110 = 560 ± 50 cm-1 transition. The hindered methyl rotor vibrational mode was also
likely measured for m-methylphenoxyl (Fig. 3.3, designated with an asterisk, located at 134 ± 8
cm-1 relative to the origin). This peak is possibly due to a transition from the ground vibrational
state in the anion to two quanta in the hindered methyl rotor, 3920 = 135 ± 16 cm-1. This allows
for a determination of the methyl hindered rotation vibrational frequency, assuming noanharmonicity, 3910 = 67 ± 8 cm-1, compared with the calculated harmonic frequency of 81 cm-1.
Again, this assignment is only suggested, due to the anharmonicity of hindered rotor vibrations.

115

Fig. 3.7 The photoelectron spectrum of meta-methylphenoxide taken on the VMI spectrometerwith a photon energy of 2.329 eV.

116

The photoelectron spectrum vibrational progression observed for the transition to theexcited 𝐴̃ state of the m-methylphenoxyl radical can be seen in Fig. 3.4, overlaid with thesimulated spectrum. Peak A is assigned as the band origin, with peaks B and C located 523 ± 8
cm-1 and 1260 ± 8 cm-1 higher in binding energy, relative to peak A. According to the simulation,
peak B has many contributing transitions, broadening it past instrument resolution, but it isprimarily due to two transitions from the anionic ground vibronic state to one quanta in either of
two ring distortion vibrational modes in the neutral 𝐴̃ state, 3110 , 3210 = 520 ± 96 cm-1, predicted
to have harmonic frequencies of 516 and 542 cm-1. These peaks cannot be resolved due to our
instrument resolution. Peak C is similarly broadened past instrument resolution and is predicted
to be primarily due to a transition from the anionic ground state to a single quanta in a quasi O–C
stretching mode in the neutral 𝐴̃ state, 1510 = 1260 ± 90 cm-1, which has a calculated harmonicfrequency of 1330 cm-1.
The normal mode eigenvectors of the vibrational states which are activated upon
photodetachment of m-methylphenoxide are depicted in Table 3.4.

117

Table 3.4 The following diagrams show the primary Franck-Condon active normal modes of mmethylphenoxide. Each eigenvector shown here is that of the neutral radical, and was calculated
at the B3LYP/aug-cc-pVQZ level of theory. The listed frequencies are theoretical and harmonic.

Meta-methylphenoxyl radical (𝑋̃ state)ν31 (540 cm-1)

ν39 (81 cm-1)

118

Meta-methylphenoxyl radical (𝐴̃ state)ν31 (542 cm-1)

ν32 (516 cm-1)

ν15 (1330 cm-1)

119

§ 3.8 Vibrational Analysis: para-Methylphenoxyl
The photoelectron spectrum of p-methylphenoxide, accessing the neutral 𝑋̃ state, wasstudied in the VMI spectrometer with a photon energy of 2.520 eV, Fig. 3.5. Peak A is assigned
as the band origin. Peaks B and C, located at 460 ± 8 cm-1 and 921 ± 8 cm-1 relative to the origin,suggest a vibrational progression arising from transitions from the ground state of the anion to
the p-methylphenoxyl ground state with one or two quanta (for peaks B and C) in a ring
distortion vibrational mode in the neutral radical, 3210 = 460 ± 40 cm-1 and 3220 = 920 ± 40 cm-1.
The simulated spectrum agrees with the measured transition, and predicts this ring distortion
harmonic frequency to be 463 cm–1. There is one notable disagreement with theory that may beseen in this spectrum, located at ~2.5 eV binding energy and labeled D. Peak D contains 3
separate peaks at 2893 ± 8, 2931 ± 8, and 3006 ± 8 cm-1, relative to peak A. These are reasonablefrequencies for C-H stretching modes in the radical, but their intensities are not recovered in thesimulation. It should be noted that due to the nature of the variable resolution of the VMI and thelocation of the peaks being close to the photon energy, the peak amplitudes exaggerate theperceived importance of these transitions. In fact, the integrated intensity of these peaks areapproximately equal to half the intensity of peak C.
The p-methylphenoxide photoelectron spectrum accessing the neutral excited 𝐴̃ state isshown in Fig. 3.4, and overlaid with the simulated spectrum. Peak A is the band origin. Peak B,
located 459 ± 8 cm-1 from the origin, corresponds to the transition from the anionic vibrational
ground state to one quanta in a ring distortion vibrational mode in the neutral 𝐴̃ state, 3210 = 460± 80 cm-1, with a predicted harmonic vibrational frequency of 461 cm–1. Peak C, located at 1239
± 8 cm-1 relative to the origin, is assigned to a transition from the anionic ground vibrational state
to one quanta in a quasi C–O stretch vibrational mode, 1510 = 1240 ± 90 cm-1, in the neutral 𝐴̃120state with a calculated fundamental vibrational frequency of 1320 cm–1. Finally peak D, locatedat 1703 ± 80 cm-1, relative to peak A, is assigned to a combination band, 1510 3210 = 1700 ± 90cm-1.
The normal mode eigenvectors of the vibrational states which are activated upon
photodetachment of p-methylphenoxide are depicted in Table 3.5.

121

Table 3.5 The following diagrams show the primary Franck-Condon active normal modes of pmethylphenoxide. Each eigenvector shown here is that of the neutral molecule, and was
calculated at the B3LYP/aug-cc-pVQZ level of theory. The listed frequencies are theoretical andharmonic.

Para-methylphenoxyl radical (𝑋̃ state)ν32 (463 cm-1)

Para-methylphenoxyl radical (𝐴̃ state)ν32 (461 cm-1)

ν15 (1320 cm-1)

122

§ 3.9 CH3 Deprotonated meta-Methylphenol
The meta isomer of methylphenol behaves differently from the other isomers in its
chemical reactivity. While the other isomers only show evidence of deprotonation at the alcoholgroup, the meta isomer displays deprotonation at the methyl group as well. The photoelectron
spectrum of methyl deprotonated m-methylphenol is shown in Fig. 3.8, with the simulated
spectrum overlaid. While the agreement between the experiment and the simulation is clear,
there is more broadening than predicted. The dominant peaks in the spectrum are spaced by ~500
cm-1, again indicating that there is significant vibrational activity in ring distortion vibrationalmodes in the neutral radical. Peak A, the band origin, arises from the transition from the groundvibronic state of the anion to the ground vibronic state of the neutral. As such, the EA of mmethylenephenol is 1.024 ± 0.008 eV. Peak B is located 513 ± 8 cm–1 higher in binding energy
relative to peak A. The simulation predicts that peak B is mainly due to vibrational transitions
from the anionic ground state to one quanta in either of two modes, 3110 , 3010 = 510 ± 110 cm-1,
with predicted harmonic frequencies of 525 and 547 cm–1. However, due to our resolution, we
could not resolve these. Our calculations show that the CH2 and OH groups lie in the plane of
the ring for both anion and neutral states, and thus excitations of CH2 umbrella modes or OH
internal rotations are neither expected nor observed.
The normal mode eigenvectors of the vibrational states which are activated upon
photodetachment of the m-methylenephenol anion are depicted in Table 3.6.

123Fig. 3.8. Photoelectron spectrum of methyl deprotonated meta-methylphenol, taken with aphoton energy of 3.408 eV and collected with a hemispherical energy analyzer. Experimental
data are plotted with dots, the calculated Franck-Condon factors are shown as purple sticks, andthe stick spectra convolved with the experimental resolution is shown in green.

124

Table 3.6 The following diagrams show the primary Franck-Condon active normal modes of mmethylenephenol. Each eigenvector shown here is that of the neutral molecule, and was
calculated at the B3LYP/aug-cc-pVQZ level of theory. The listed frequencies are theoretical andharmonic.

Meta-methylenephenol (𝑋̃ state)ν30 (547 cm-1)

ν31 (525 cm-1)

A summary of all the vibrational analysis results may be found in Table 3.7.

125

Table 3.7 The experimentally measured frequencies of the Franck-Condon active normal modes
for the doublet ground (𝑋̃) and first excited (𝐴̃) states of the three methylphenoxyl radicals. The
calculated (B3LYP/aug-cc-pVQZ) harmonic frequencies are listed in parentheses below themeasured frequency. All values are in cm-1.
o-Methylphenoxyl Radical

m-Methylphenoxyl Radical

p-Methylphenoxyl Radical

̃ state𝑿̃ state𝑿̃ state𝑿

ν30 = 540 ± 80(577)

ν31 = 560 ± 50(540)

ν32 = 460 ± 40(461)

ν31 = 540 ± 80(540)

ν39 = 66 ± 8(81)

ν33 = 430 ± 80(453)
ν39 = 98 ± 8(106)
̃ state𝑨̃ state𝑨̃ state𝑨ν15 = 1220 ± 130(1312)

ν15 = 1260 ± 90(1330)

ν15 = 1240 ± 90(1320)

ν32 = 480 ± 190(506)

ν31 = 520 ± 96(542)

ν32 = 460 ± 80(461)

ν32 = 520 ± 96
(516)

126

§ 3.10 Photon Energy Dependence of the para-Methylphenoxide Photoelectron Spectra
The photoelectron spectrum of p-methylphenoxide was first obtained using the fixed
frequency (364 nm, 3.408 eV) spectrometer. The simulated spectrum did not reproduce therelative intensity of the first three peaks in the experimental photoelectron spectrum as well as
we would normally expect; additionally, empirically adjusting bond lengths and angles did not
improve the comparison. Consequently, the VMI instrument, which utilizes a tunable lightsource, was employed to reinvestigate the photoelectron spectrum of p-methylphenoxide at anumber of photon energies above and below 3.408 eV, and these spectra are shown in Fig. 3.9.
The photon energies employed spanned from 3.494 eV (panel A) to 2.520 eV (panel D). The
experimental photoelectron spectra show substantial variations over this range of photon
energies. An additional two photon energies were used during this investigation and are reported
in Fig. 3.10.

127

Fig. 3.9 Photoelectron spectra of p-methylphenoxide taken at a variety of photon energies,
displaying a photon energy dependence as discussed further in the text. Data in panels A, C, andD (h = 3.494 eV, 2.920 eV, and 2.520 eV) are obtained with a VMI spectrometer, while panel B(h = 3.408 eV) was collected with a hemispherical energy analyzer. Experimental data areshown as dots, while calculated Franck-Condon factors are shown as purple sticks and theirconvolution with experimental resolution shown in green. The highlighted peaks emphasize theintensity dependence on photon energy and are attributed to autodetachment, which is discussed
further in the text.
128

Fig. 3.10 Photoelectron spectra of para-methylphenoxide obtained with the VMI spectrometerwith photon energies of 2.329 eV and 2.637 eV for the top and bottom panels respectively. Note
how the changing resolution can cause peaks close to the photon energy to appear more
dominant due to their narrowing FWHM and corresponding increase in amplitude to conserve
the transition integrated intensity.

129

An important assumption for all of the calculated photoelectron spectra simulations
reported here is that the electronic contribution to the overall photodetachment cross section isessentially constant over the photon energy range corresponding to the width of the vibronicenvelope; in this limit, the relative intensities of the vibronic transitions are well-represented by
the calculated Franck-Condon factors (§ 1.1). This assumption means that the simulated
photoelectron spectrum should not depend on photon energy. Thus the photoelectron spectra
simulations (solid green lines) plotted in all four panels of Fig. 3.9 differ only in the convolutionof the Franck-Condon factors to reflect the changing experimental resolution with photoelectron
kinetic energy. The experimental data in Fig. 3.9, however, show a significant photon energy
dependence of the p-methylphenoxide photoelectron spectrum, which cannot be justified by thechanging experimental resolution. While there is good agreement between the experiment andthe photoelectron simulation at the lowest photon energy (panel D), theory and experiment fall
further out of agreement as the photon energy increases. As the photon energy is increased, there
are several peaks which change intensity relative to the origin peak, and there is an appearance ofa broad continuum under the spectrum in the top panel. For example, the highlighted peak in Fig.
3.9 changes intensity relative to its neighboring peaks with different photon energies. Thisbehavior arises from electron autodetachment, a multistep process whereby the anion absorbs aphoton to an excited anionic state which lies above the electron detachment threshold, andsubsequently transfers to the neutral ground electronic state, ejecting an electron in the process.
TD-DFT calculations predict eight optically accessible anion excited electronic states between
1.9 and 3.3 eV above the ground anion state, and thus it is outside the scope of this work tofurther pursue a theoretical modeling of these spectra. In the current study, m-methylphenoxide
also displayed evidence of autodetachment, but at a greater photon energy (3.832 eV obtained130via doubling the output of a dye laser utilizing DCM dye) than p-methylphenoxide. See Fig.
3.11. Observing autodetachment from o-methylphenoxide was not pursued in this work, but
likely exists as well. This will be discussed in greater detail in the following section.

131

Fig. 3.11 The photoelectron spectrum of meta-methylphenoxide taken on the VMI spectrometerwith photon energies of 3.494 eV and 3.832 eV. Only experimental data are shown. The
highlighted region demonstrates the photon energy dependence of the peak intensities
characteristic of autodetachment.

132§ 3.11 Discussion
The photoelectron spectra of methyl deprotonated m-methylphenol and o-, m-, andp-methylphenoxide have been presented. In the following discussion, the electron affinities andactive vibrational modes of the corresponding neutral radicals will be compared with othersubstituted aromatic radicals, e.g. halobenzyl, phenoxyl, and benzyl radicals. In addition, theautodetachment behavior of the methylphenoxides studied in this work will be discussed andcompared with previous work on phenoxide. Finally, we combine the experimental EAs with themethylphenol O–H bond dissociation energies19, 20 to derive the gas phase acidity of the three
methylphenols.
The electron affinities of the three methylphenoxyl isomers are all close to 2.2 eV, while
the meta isomer has the largest EA, and the para has the smallest (see Table 3.2). This ordering
can be readily rationalized using resonance stabilization and aromaticity concepts: the metaisomer does not benefit from resonance stabilization of the neutral radical, whereas the ortho andpara isomers do. In the neutral meta form, the electron donating group (CH3) cannot donate
charge to the strongly electron withdrawing O radical group, hence raising its energy relative tothat of the other neutral isomers. For all three isomers, the anionic form is closed shell and would
be minimally perturbed by resonance stabilization since there is a formal negative charge on O.
Thus, it is the neutral resonance stabilization (or lack thereof in the meta isomer) that dictates therelative EA of the meta isomer compared to the ortho and para isomers. The relative EAs of theortho and para isomers are more difficult to predict and explain the observed trend. Both benefit
from resonance, and the ortho isomer has the additional complication of the O…CH3 interaction.
Regardless, it is experimentally determined that the para isomer has the lowest EA, while meta
has the highest. This same trend is seen in the halobenzyls, which are also doubly substituted

133

benzenes, but with an electron withdrawing group (CH2) and another very weak electron
withdrawing group (the halogens, by induction).22
There is only one prior experimental measurement of the EA of any of themethylphenoxyls in the literature. McMahon and Kebarle23 used gas phase proton transfer
equilibria to determine an EA for p-methylphenoxyl of ~2.17 eV. This result is consistent with
our more accurate measurement of 2.1199±0.0014 eV. In addition to the previous experimental
work, there was also a theoretical study by Angel and Ervin24 which used CBS-QB3 calculations
to obtain the EA of m-methylphenoxyl to be 2.19 eV. This calculation also agrees well with ourexperimental measurement of 2.2177±0.0014 eV.
The vibrational progressions seen in the photoelectron spectra for all of these molecules
show the clear signature of both active ring distortion normal modes (for transitions to theneutral 𝑋̃ and 𝐴̃ states) and active quasi C–O stretching normal modes (for transitions to theneutral 𝐴̃ state). A summary of the measured (and calculated) vibrational frequencies for all three
methylphenoxyl radicals is found in Table 3.7. While many transitions comprise the spectrapresented in this work, the transitions shown in this table account for all significant vibrationalmodes which compose the observed progressions for each isomer. As mentioned previously, thisbehavior can be explained with a simple physical picture. For the methylphenoxide anions,adding the extra electron elongates the bond between the oxygen and the aromatic ring, as wellas elongating the bonds within the ring away from the localized negative charge on the oxygen,compared to the equilibrium structure of the neutral radical. Upon photodetachment, one would
therefore expect the excitation of ring distortion vibrations, which are observed, and C–Ostretches. However, there is little evidence for any significant activity in C–O stretching modes
for the transition to the neutral 𝑋̃ state, and B3LYP calculations also predict little change in the134

C–O bond lengths. A similar physical picture can be obtained for the methyl deprotonated mmethylphenol photoelectron spectrum. For the photoelectron spectra of methylphenoxide
accessing the neutral 𝐴̃ state, both ring distortion and C-O stretching modes are excited. The
neutral 𝐴̃ state is formed by removing an electron from the aromatic  system in the anion,
resulting in the * neutral excited 𝐴̃ state. The antibonding  system in the benzene ringelongates the C-O bond in the 𝐴̃ neutral state relative to the anion. Thus, upon photodetachmentfrom the anion to form the neutral excited 𝐴̃ state, one would expect this C-O stretching vibration
to be active.
Beyond the direct comparison with previous studies of the methylphenoxyl radicals,
similar substituted aromatic compounds should be considered. Perhaps the most obvious of thisgroup would be phenoxyl and benzyl. The phenoxyl radical has an EA of 2.2538 ± 0.0008 eV.16
There is a relatively small decrease (< 130 meV) in the EA of phenoxyl upon the addition of amethyl group, forming the methylphenoxyl radicals studies here. Methyl is a very weak electron
donating group and so it is expected to have only a minor effect on the EA. It was observed thatphenoxide had the same ring distortion vibrational modes excited upon photodetachment as areseen here for all three of the methylphenoxides.
In the case of deprotonation from the methyl group, a comparison with benzyl isappropriate. The EA of benzyl is 0.912 ± 0.006 eV.17 Benzyl shows a minor increase (~0.1 eV)
in its EA upon attaching a meta hydroxyl group, forming methylenephenol radical. This isperhaps surprising considering OH is a strongly electron donating group in aromatic systems,
and therefore might lead one to conclude that it should largely disrupt the electronic structure of
the system and thus the EA. However, there are two reasons why this is actually not the case.
First, the only isomer of methyl deprotonated methylphenol observed in this work is the meta135

isomer. Meta isomers in aromatic systems are unique for their lack of resonance structurescompared to that of ortho and para isomers. Hence, the impact of the electron donating effect of
the OH in the meta isomer is diminished. Second, the primary effect of adding an electron
donating group to the aromatic ring would be to localize the negative charge in the CH2
carbanion. However, the charge is already localized on the CH2 group, hence the change inoverall behavior would be minimal. Both m-methylenephenol and benzyl show similar ring
distortion vibrational progressions in their photoelectron spectra, further confirming the low levelof perturbation.
In addition to comparing the EAs and vibronic structure of methylphenoxyl radicals, themethylphenoxide anion autodetachment behavior can be compared and discussed. Gunion et al.17
and more recently Liu et al.25 found autodetachment in the photoelectron spectroscopy ofphenoxide. Specifically, Liu et al. identified a dipole bound anion electronic excited state as thestate responsible for facilitating autodetachment. Hydroxyphenoxide, a close analog ofmethylphenoxide, has also been recently studied.26, 27 Autodetachment was observed in thephotoelectron spectrum of o-hydroxyphenoxide, again attributed to a dipole bound anion
electronic excited state.26 In the current study, the m- and p-methylphenoxide anions both display
signatures of electron autodetachment. Considering the many similarities between the
methylphenoxides, one might also expect to observe autodetachment in the ortho isomer, but thiswas not pursued experimentally in this work. For m-methylphenoxide (see Supplementary
Material for more information), there is a higher onset energy (3.494 eV< hνonset ≤ 3.832 eV) for
observing autodetachment as compared to the para isomer (2.520 eV< hνonset ≤ 2.637 eV). Theseonsets and the anion state(s) involved could not be confirmed by our TD-DFT anion excited statecalculations, primarily due to the abundance of available predicted excited electronic states that136

could be responsible for this phenomenon. The TD-DFT calculations suggest a similar number of
optically accessible excited states for all three anionic isomers. However, theoreticallyinvestigating this further is beyond the scope of this work.
Having measured the electron affinities of the methylphenoxyl radicals, these results are
combined with the measured bond dissociation energies19, 20 (Table 3.2) to determine the gas
phase acidities using a thermodynamic cycle.3 This cycle provides a direct experimental measureof the 0 K enthalpy with a small (± 0.14 kcal/mol) uncertainty for the deprotonation reaction: HA→ H+ + A–. The dominant source of uncertainty arises from the error associated with the H–A
bond strength, as seen in Table 3.2. We may further combine this 0 K result with the calculated
constant pressure heat capacities of the species involved in the acid reaction in order to obtain theenthalpy at 298 K:30∆𝑎𝑐𝑖𝑑 𝐻298
𝐾 (CH3 PhOH)
298

0 (CH
= ∆𝑎𝑐𝑖𝑑 𝐻0𝐾
3 PhOH) + ∫0[𝐶𝑝 (CH3 PhO− ) + 𝐶𝑝 (H + ) − 𝐶𝑝 (CH3 PhOH)] 𝑑𝑇0This results in ∆𝑎𝑐𝑖𝑑 𝐻298
𝐾 = 348.39 ± 0.25, 348.82 ± 0.25, 350.08 ± 0.25, and 349.60 ± 0.25

kcal/mol for cis-ortho-, trans-ortho-, m-, and p-methylphenol, using a conservative estimate of
the error associated with calculating the Cp of these various molecules.3 These results are also
found in Table 3.2. Note that King et al.19 were able to differentiate between cis-ortho and transortho isomers of methylphenol, where cis and trans refer to the direction of the hydrogen of the
OH group with respect to the CH3 group.
The acidities of the three methylphenol isomers (four isomers, if the cis-, and transconformers of the ortho isomer are considered) have been studied several times in the past. In
general, the reported error bars have been in the range of 1-3 kcal/mol. McMahon and Kebarle23
137

initially measured the acidity at 298 K of o-methylphenol as 346.2 kcal/mol. Two other studies(McMahon et al. 23 and Angel et al.24) examined m-methylphenol and determined the gas phase
acidity at 298 K to be 347.3 and 348.2 ± 1.2 kcal/mol, respectively. Finally, p-methylphenol wasstudied by McMahon23 and determined the gas phase acidity at 298 K to be 347.7 kcal/mol. The
current study presented here agrees well with the previous studies, and has greatly improved onthese measurements with an error of ± 0.25 kcal/mol. In addition to a direct comparison, one
could also compare the acidity of phenol at 298 K (348.0 ± 1.0 kcal/mol, reported by Angel and
Ervin,24 or the 0 K acidity derived from the works of Kim et al.16 and King et al.28: 347.43 ± 0.12
kcal/mol) to that of the methylphenols. The addition of a methyl group to a phenol minimallyaffects the acidity by increasing the room temperature reaction enthalpy by only ~ 1 kcal/mol. It
is interesting, though, that because of the increased accuracy in the current study, one is able toobserve that the trend in acidity increases going from ortho to para isomers. This order is due tothe combination of the D0 and the EA measured for each isomer; the trend is not mirrored in theD0 and EA energetic ordering, where increasing D0 and EA goes as para, ortho, meta instead.
This order is also different than that observed for the aqueous acidity, which places the trend
ortho, para, meta for increasing acidity,29 likely due to solvation effects.

138§ 3.12 Conclusions
The EAs of o-, m-, and p-methylphenoxide were measured: 2.1991±0.0014,
2.2177±0.0014, and 2.1199±0.0014 eV, respectively. The EA of m-methylenephenol was alsomeasured: 1.024 ± 0.008 eV. Using the methylphenol (O-H) bond strengths reported by King etal.19 and revised by Karsili et al.20 a thermodynamic cycle was constructed and the acidities of
the three methylphenols were obtained to a substantially degree of accuracy than previously0reported, with ∆𝑎𝑐𝑖𝑑 𝐻298
𝐾 = 348.39 ± 0.25, 348.82 ± 0.25, 350.08 ± 0.25, and 349.60 ± 0.25

kcal/mol for cis-ortho-, trans-ortho-, m-, and p-methylphenol. The term energies for the neutral𝐴̃ ← 𝑋̃ doublet  to doublet * transition in the three isomers of methylphenoxyl were measured:1.029 ± 0.009, 0.962 ± 0.002, and 1.029±0.009 eV, for ortho, meta, and para isomers,
respectively. In all reported photoelectron spectra, ring distortion vibrational normal modes wereactivated upon photodetachment, leading to vibrational progressions spaced by ~500 cm-1. For
the photoelectron detachment to the neutral 𝐴̃ states, C–O stretching modes were also excited,leading to additional vibrational transitions with associated frequencies of ~ 1300 cm-1. Electron
autodetachment was observed in both m- and p-methylphenoxide photoelectron spectra, although
the para isomer had an earlier onset photon energy for observing autodetachment, despite having
similar calculated anion excited electronic states.

139§ 3.13 References

1) B. Ruscic, A. F. Wagner, L. B. Harding, R. L. Asher, D. Feller, D. A. Dixon, K. A. Peterson,
Y. Song, X. Qian, C.-Y. Ng, J. Liu, W. Chen and D. W. Schwenke, J. Phys. Chem. A 106, 27272747 (2002).
2) C. T. Wickham‐ Jones, K. M. Ervin, G. B. Ellison and W. C. Lineberger, J. Chem. Phys. 91,
2762-2763 (1989).
3) K. M. Ervin and V. F. DeTuri, J. Phys. Chem. A 106, 9947-9956 (2002).
4) J. Cooper and R. N. Zare, J. Chem. Phys. 48, 942-943 (1968).
5) Y.-J. Lu, J. H. Lehman and W. C. Lineberger, J. Chem. Phys. 142, 044201 (2015).
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8) C. Blondel, W. Chaibi, C. Delsart, C. Drag, F. Goldfarb and S. Kröger, Eur. Phys. J. D 33,
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Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
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K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C.
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142

CHAPTER IV
PHOTOELECTRON SPECTROSCOPY AND THERMOCHEMISTRY OF o-, m-, AND pMETHYLENEPHENOXIDE ANIONS
Portions of this chapter will be submitted for publication in The Journal of Chemical Physics
under the same title, by Daniel J. Nelson, Wilson K. Gichuhi, Charles M. Nichols, Julia H.
Lehman, Veronica M. Bierbaum, and W. Carl Lineberger§ 4.1 Introduction and BackgroundAs was discussed in § 1.2 and § 1.4, distonic radical ions and diradicals are of interest to
many fields of chemistry, affecting such varied fields from pharmacology1 to combustion
chemistry.2 This chapter will focus on three distonic radical anions and the diradicals which
result upon electron photodetachment:
o-methylenephenoxidem-methylenephenoxidep-methylenephenoxide

Due to their frequently unstable and short lived nature, as well as their complicatedelectronic structure,3-12 they are challenging to investigate both experimentally and theoretically.
Collecting Photoelectron (PE) spectra of these distonic anions will prove to be no exception.
While the ortho and para isomers will be shown to display some signal that is interpretable as
143

direct photodetachment, the meta isomer shows no conclusive evidence of the same. All threeisomers display photoelectron signal that cannot be easily interpreted, and the implications of
this will be discussed.
In addition to anion PE spectroscopy, the thermodynamic properties of themethylenephenoxide isomers are studied using a Flowing Afterglow – Selected Ion Flow Tube
(FA-SIFT) mass spectrometer in conjunction with the acid bracketing procedure,13 thus
providing information on the acidities of the methylenephenol radicals. Together with theElectron Affinities (EAs) provided by PE spectroscopy, acidities from the FA-SIFT machine,
and a thermodynamic cycle, bond dissociation energies are also measured.
A note to future readers of this document: Unlike the other chapters in this dissertation,
the analysis and interpretation of the data presented here is not yet fully complete. As such, any
reader who seeks the final analysis is directed to the paper that will be published under the same
title as this chapter in The Journal of Chemical Physics.

144§ 4.2 Experimental Specifics
Samples of o-, m-, and p-methylphenol were obtained from Sigma-Aldrich Inc. (≥ 98%pure) and used without further purification. Selectively deuterated samples of two methylphenol
isomers, 3,4,5,6-d4,OD-o-methylphenol and methyl-d3-m-methylphenol, were purchased fromCDN Isotopes, Inc. (≥ 98% pure) and used without further purification. Selectively deuterated pmethylphenol, 2,3,5,6-d4,OD-p-methylphenol, was purchased from Sigma-Aldrich Inc. (≥ 98%pure) and used as received. Acids for the acid bracketing experiments (Instrument C, see below)
were all purchased from Sigma-Aldrich Inc. with the exception of phenol which was purchased
through Mallinckrodt Pharmaceuticals Inc. The acids and their given purities are as follows: ptrifluoromethylphenol (≥ 97%), 3-mercaptopropionic acid (≥ 99%), o-chlorophenol (≥ 99%), pfluorophenol (≥ 99%), propionic acid (≥ 99.5%), acetic acid (≥ 99%), phenol (≥ 99%),
2,2,3,3,3-pentafluoro-1-propanol (≥ 97%), 2,2,2-trifluoroethanol (≥ 99.5%). All of these acids
were used without further purification. The low vapor pressure of many of these compounds
necessitated that they had to be gently heated (~40 ̊C).
Three distinct experimental setups are used. Instrument A is a continuous photoelectron
spectrometer (§ 2.2). Instrument B is the pulsed photoelectron spectrometer discussed in § 2.7.
Instrument C is a Flowing Afterglow-Selected Ion Flow Tube (FA-SIFT) mass spectrometer.14
All three of these instruments have been discussed in detail previously, so only a brief
description follows.14-19
In Instrument A, the ions of interest are generated in a flowing-afterglow He (~ 0.5 Torr)
plasma. Molecular oxygen is added to this plasma, which generates O–. This radical anion is a𝑜20+strong base (∆𝑎𝑐𝑖𝑑 𝐻298
𝐾 (HO)=382.60 ± 0.07 kcal/mol ), and is capable of abstracting either Hfrom a hydrocarbon reactant, resulting in an anion and hydroxyl radical, or it may remove both a145hydrogen atom and a proton, resulting in an anion and a water molecule. In the present
experiment, the hydrocarbons in question are the three methylphenol isomers. Eachmethylphenol isomer is added through a gas inlet immediately after the O• – radical anions are
generated. The products from this reaction remain in ~0.5 Torr of He for several milliseconds,
thermally equilibrating with the walls of the chamber, 300 K, before being extracted through a 1
mm orifice into a differentially pumped region, ~10-6 Torr.
The reactants and products undergo more than a single collision, and hence the nascentproduct distributions arising from the reaction of O– and the methylphenols (i.e. the products of asingle collision between an O– and methylphenol molecule) are not necessarily the products
observed in the PE spectra. This is due to the long time (several milliseconds, on average) thereactants and products are in a collisional environment before entering a collision freeenvironment. The O– anion can in general react at many sites on any given hydrocarbon, giving
rise to many possible products. These products stay in a ~0.5 Torr atmosphere of He after theircreation, allowing for subsequent collisions with other products or reactants allowing for thesystem to proceed toward chemical equilibrium. In the extreme case that the system is allowed tocome fully to equilibrium, then one should expect to primarily observe the mostthermodynamically favorable products (methylenephenoxide + water) with only trace amounts ofother possibilities. While this experiment is likely not at chemical equilibrium, it is also likelythat the nascent product distribution is not preserved.
After being produced and extracted into the differentially pumped region, the anionicproducts are accelerated to 735 eV, mass selected with a Wein velocity filter (m/Δm ~ 60),
before being slowed to 35 eV and intersected with a fixed frequency argon ion laser(hν = 3.40814 eV), thereby photodetaching electrons from the mass-selected ions of interest. A
146

small solid angle of these electrons is gathered in a direction perpendicular to both the ion andlaser beams, and their kinetic energy is measured using a hemispherical electron energy analyzer.
The laser radiation is linearly polarized at the magic angle (54.7°) with respect to theplane in which the photoelectrons are detected. Using this particular angle ensures that thegathered photoelectrons will be directly proportional to the anion’s PE cross-section.21 The laser
has an output power of ~1 Watt at 364 nm, which is coupled into the chamber via a build-up
cavity locked to the laser cavity, increasing the circulated in-cavity power to ~100 Watts.
The collected PE spectra are calibrated to the PE spectra of O– and O2–.22, 23 This provides
an absolute linear energy scale and corrects for nonlinear distortion with a minor (0.7%)
compression factor. The calibration spectrum of O– also allows for a measurement of the
instrument’s resolution, 10 meV. This is obtained by measuring the full-width-half-maximum(FWHM) of the O(3P2) + e– ← O– (2P3/2) transition. As this peak results from a single atomic
transition, its FWHM is a good approximation of the instrument’s resolution.
In Instrument B,19 the anions of interest are synthesized in a dual pulsed valve plasma
entrainment source utilizing the same reaction of methylphenol with O–. This source uses two
General Valves placed perpendicular to each other. The first is the primary supersonic expansion(10 psig, ~1% methylphenol in Ar), while the other valve is designated as the side valve (35 psig,
30% O2 in Ar). During daily operation, the side valve tension is adjusted such that it is
responsible for ~10% of the total pressure rise in the source vacuum chamber. The side valve hasa pulsed parallel plate discharge at the valve exit (ΔV ~ –2000 V).17 This generates a plasma
which is entrained in the primary supersonic expansion. The dominant anion generated therein,O–, then undergoes reactions with the methylphenol isomer of interest contained in the main
expansion, generating products which are collisionally cooled with Ar. While not necessary to be147

the case, the most thermodynamically favorable products from the reaction of O– withmethylphenol were found to be the dominant product in the ortho and para cases.
Following their initial formation, the anions are steered into a Wiley-McLaren TOF mass
spectrometer by a pulsed extraction plate, where the ions are temporally separated by their massto-charge ratio (m/z) and spatially focused into the center of a Velocity Map Imaging (VMI)interaction region.18 A nanosecond laser pulse is timed to intercept the anion m/z of interest,
photodetaching electrons. The light sources used to obtain the spectra reported here are thesecond and third harmonic of a Nd:YAG laser, with photon energies of 2.330 and 3.494 eV,
respectively. The three dimensional distribution of photodetached electrons are velocity mapped
onto a two dimensional position sensitive phosphor screen-coupled microchannel plate detector,
and then imaged by a CCD camera. This photoelectron image is reconstructed into the original
three dimensional velocity distribution utilizing an inverse Abel transform. This is thenconverted to a one dimensional electron speed distribution. Both steps are performed using the
BASEX program.24 Finally an electron Kinetic Energy (eKE) distribution is generated by meansof a Jacobian transformation. This spectrum is converted to an electron Binding Energy (eBE)
distribution by subtracting the eKE from the laser photon energy (eBE = h – eKE), yielding thereported photoelectron spectra. For these experiments, the energy scale is calibrated by the S–
photoelectron spectrum.25, 26
The VMI spectrometer has a spectral resolution that is a function of eKE; the resolution
decreases with increasing eKE. This resolution is determined by the eKE and the FWHM of apeak in a photoelectron spectrum due to a single transition. In all cases shown in this work, thephotoelectron spectrum of S– was used to determine the resolution as a function of eKE, as wellas calibrate the energy scale. Typically, the resolution is ~2–3% (resolution ~ FWHM/eKE). 25, 26
148

Instrument C produces ions in a similar fashion to Instrument A.14 The ions are created ina Flowing Afterglow He plasma, though for this instrument the plasma is made by electron
emission via a heated filament. The O– is generated via N2O in the presence of the flowing
afterglow plasma. The methylphenol sample is added downstream of O– formation, and the same
reaction discussed with Instrument A takes place. The products and reactants similarly remain in
a ~ 0.5 Torr atmosphere of He for several milliseconds before being extracted through a 1 mm
orifice into a differentially pumped region, ~10-6 Torr. The anionic products are then mass
selected with a quadrupole mass filter and subsequently entrained in a reaction flow tube with alaminar flow of He (~ 0.5 Torr). There are gas inlets at regular intervals along this flow tube
where neutral gas reagents may be added so as to study the thermochemistry and kinetics of gas
phase ion-neutral reactions; however in this study, only the thermochemistry is examined.
Specifically, neutral acids with known gas phase acidities are reacted with the anions of interestin order to perform acid bracketing. All anions are then extracted into a differentially pumped
chamber, ~10-7 Torr, mass analyzed with a triple quadrupole mass analyzer and detected with aChanneltron.
Acid bracketing is a procedure that can be summarized as follows.13 The mass selected
methylenephenoxides are reacted with neutral acids of known deprotonation enthalpy (reference
acids) in the laminar flowing helium. As the anions are Lewis bases, a collision with a neutral
acid will likely result in a proton transfer from the acid to the base if the proton affinity of thebase is larger than the deprotonation enthalpy of the acid. This yields a decrease in the anion
abundance, which is monitored for a variety of reference acids. A rapid proton transfer reaction
with a reference acid indicates an exothermic reaction and is indicated by a “+” in Table 4.3. The
deprotonation enthalpy of the conjugate acid of the methylenephenoxides (i.e. the149methylenephenols) must therefore be between the values of the two reference acids wherein the
proton transfer reaction changed from endothermic to exothermic.
While performing experiments with Instrument C, it became necessary to inject CO2
clustered with the ions of interest into the flow tube region, as this produced greater signal. This
procedure should not interrupt the chemistry observed since the clusters are injected at 30-70 V
which is more than sufficient to break the cluster into the parent ion and CO2, whereupon theparent ion is free to react unencumbered in the flow tube.
Throughout this work, errors in reported peak positions in the photoelectron spectra,
typically on the order of 1 meV or less, are due to several factors: the statistical error in fitting
the peaks to Gaussian functions to obtain the peak center, the uncertainty in the absolute energy
scale, and the number of independent measurements. When reporting the energy difference
between peaks as a particular transition, the previous errors are taken into account and are
combined with the uncertainty associated with the offset of the actual transition compared to thepeak center. This uncertainty can be smaller or comparable to the aforementioned errors if only asingle vibronic transition is the major contributor to the peak. However, if there are multiple
transitions under the peak envelope, which can sometimes manifest itself as a non-Gaussian peakor a peak possessing a FWHM wider than would be predicted by the experimental resolution atthat eKE, it could be as much as the Half-Width-at-Half-Maximum (HWHM) of the peak. The
peaks presented in all reported spectra are broader than the instrument resolution and always
arise from multiple transitions with significant transition intensity. Hence, this latter error
dominates the reported uncertainty.

150

§ 4.3 Theoretical Methods and Simulations
The ROCBS-QB3 composite quantum chemistry method has been used to calculate theadiabatic EAs of all the diradicals studied here. This method has been shown to work well withheavily spin-contaminated systems, as well as for open shell species.12 The B3LYP/aug-ccpVTZ density functional theory model chemistry27 was used to calculate the normal modes of all
the anions and neutrals studied here, which are subsequently used to compute Franck–Condon𝑜(FC) factors. This level of theory was also used to determine ∆𝑎𝑐𝑖𝑑 𝐻298
𝐾 of the protonated formof all the anions of interest. All of these electronic structure, normal mode, and thermochemical
calculations were carried out using the Gaussian 09 program package,28 the FC factors were
calculated using the PESCAL program,29 and the FC factors were converted into simulations of
the spectra with home–built code.
To simulate the experimental spectra, PESCAL calculates the positions of all the possible
vibrational transitions and the Franck-Condon factor associated with each transition. This
information, combined with a 200 K Maxwell-Boltzmann distribution of energy (for Instrument
B, 300 K for Instrument A) in the initial anionic states, provides a stick spectrum. These sticks
are then convolved with Gaussian functions whose integral area is equal to the calculatedtransition intensity (FC factor) and whose FWHM is commensurate with the instrument
resolution to yield the final simulated spectra. PESCAL utilizes Duschinsky rotation matrices
and displacement vectors to match the initial state normal mode coordinates to the final statenormal mode coordinates such that the FC integrals may be evaluated via the Sharp-RosenstockChen method.30-33 These Franck-Condon based simulated photoelectron spectra make use of theharmonic oscillator approximation, with no anharmonic corrections or cross-mode couplings.

151

§ 4.4 Results: Overview
The reaction of O– with methylphenol could produce a number of different anionicproducts in theory. Deuteration studies were employed to verify which products were dominant
and would be further studied using photoelectron spectroscopy. Fig. 4.1 displays mass spectracollected with Instrument C, showing the products from the reaction of selectively deuterated
methylphenols with O–. In each case, the primary product is shown to be deprotonation from thehydroxyl group, generating methylphenoxide. This product has been previously studied and its
PE spectrum collected.34 Photodetachment from methylphenoxide produces signal that spansfrom approximately 2.1 to 2.7 eV. The next most abundant product evident in the mass spectra isa loss of both H and D from the starting methylphenol. The acidities associated with thehydrogens attached to the aromatic ring in the methylphenols has been previously calculated,34
and abstraction from these sites is either thermo–neutral or endothermic. This is in contrast to theacidity of the hydrogens found on the methyl groups of the methylphenols, for which it isexothermic to abstract those hydrogens with either O–, OH, or OH–. Therefore, the anionicproduct from the loss of both H and D is expected to be an isomer of methylenephenoxide.

152

Fig. 4.1 Mass spectra collected on the FA-SIFT instrument of selectively deuterated cresols reacted with
O–. In each panel, the parent cresol structure is shown and a vertical line indicates the parent
mass. In each case, deprotonation from the O site is the largest observed product channel, while
the next largest channel is H + D loss from the hydroxyl and methyl groups. More details may befound in the text.

153Electronic structure calculations as well as FC simulations were carried out for otherpossible structural isomers with the same mass as methylenephenoxide (m/z = 106), and werecompared to the experimental PE spectra. This was in an effort to rule out possible protontransfer, ring opening, ring rearrangement, or other reactions giving rise to any of the features
seen in the spectra. No photoelectron signatures belonging to isomers other thanmethylenephenoxide were positively identified in the spectra. However, even in combination
with the deuteration study (Fig. 4.1), this cannot definitively rule out the existence of such anions
in the experiment.
Prior to examining the spectra, it is useful to consider what one might expect from thephotoelectron spectra of the methylenephenoxide anions. There are three observations we might
expect to make: the EAs of the neutral isomers, the normal mode vibrations excited upon
photodetachment, and the possibility of observing excited electronic states. The PE spectra of themethylphenoxide anions have previously been studied34 and it was found that the EAs of themethylphenoxyl isomers are approximately 2 eV. This was primarily due to the fact that thenegative charge in the anion was localized on the oxygen. In the case of the methylenephenoxideanions, the functional groups on the aromatic ring are O and CH2, as opposed to O and CH3 formethylphenoxide. CH2 is electron withdrawing and can be expected to attract charge density
away from the O and onto the aromatic ring in the anion. In the extreme case that the charge
density is totally localized on the aromatic carbon ring, as in benzyne,35 then the EA will be ~1
eV. Thus, an electron withdrawing group such as CH2 is expected to reduce the EA of
methylenephenoxyl as compared to methylphenoxyl. In addition, the isomers of themethylphenoxides all had ring distortion vibrational progressions in the ground electronic groundstate which were excited upon photodetachment, spaced by ~ 500 cm-1. This is also expected to154be observed in photodetachment from methylenephenoxides, with the caveat that due to the moredelocalized nature of the negative charge in the anion, a more diverse selection of vibrationalmodes may become excited upon photodetachment. Finally, with a sufficiently high photon
energy, we might expect to access both the ground and the lowest lying excited electronic statesof the neutrals. Considering the PE spin selection rule ΔS = ± ½, detachment from the doublet
ground state of the anion would result in either singlet or triplet multiplicity for the electronic
states of the neutral. The singlet-triplet splitting for the methylenephenoxyls is calculated to beon the order of 1.5–2.0 eV. With an EA estimated to be between approximately 1 and 2 eV, aphoton energy of 3.494 eV might be sufficient to observe the origin of the triplet excited state of
the neutral.
Fig. 4.2 shows the photoelectron spectra of the three methylenephenoxide anions,obtained utilizing Instrument B with a photon energy of 3.494 eV. For each isomer, aconsiderable amount of activity may be seen spanning the spectra from ~1 eV through to thephoton energy, 3.494 eV. The activity can largely be split into three regions, based on eBE: A) 1
– 2 eV; B) 2 – 2.7 eV; C) 2.7 – 3.494 eV, as denoted by the shaded regions in Fig. 4.2. The
intensity of these spectra have been scaled for the reader’s convenience, however, the overallsignal of the meta isomer was approximately 10x lower than for the other two isomers. This wasdue to a decrease in ion signal in the mass spectrum, when compared to the remaining two
isomers, and not from a difference in the integrated photoelectron cross section. For ortho andpara isomers, the largest integrated intensity belongs to the lowest eBE peaks, spanningapproximately 1 – 2 eV, or region A. The onset of these peaks being near 1 eV is consistent withthe predicted EA of the methylenephenoxyl diradical. These progressions are labeled S0 on theirrespective spectra. While there is signal above the noise level in region A of the middle panel155

(meta) of Fig. 4.2, the integrated intensity is far lower than for the other spectra presented.
Assigning this region was not possible and will be explained in the discussion section. The
transition from the anionic electronic ground state to the first electronically excited triplet state of
the neutral is also identified for the ortho and para isomers. These transitions are labeled T1 in
Fig. 4.2 and comprise region C of the spectral region considered.

156

Fig. 4.2 Photoelectron spectra taken on the VMI photoelectron spectrometer with a photon
energy of 3.494 eV of the three isomers of methylenephenoxide. The spectra are highlighted into
three separate regions, A, B, and C, to make discussion easier. The PE spectra of the ortho andpara isomers accessing the ground singlet and first excited triplet electronic states of the neutral
are labeled. The meta isomer and the other unlabeled portions of PE spectra are explained in themain text, while the vibrational structure of the labeled electronic states are further investigatedin the following figures. Note that the meta photoelectron signal is approximately 10x smaller
than that from ortho and para isomers, but is shown on the same scale for ease of viewing.

157

This leaves region B of the three spectra presented in Fig. 4.2 initially unassigned, and thereasons for this will be examined both in § 4.7 and in § 4.9.
Photoelectron angular distributions with respect to laser polarization were measured forthe photoelectron spectra shown in Fig. 4.2. The angular distributions are characterized by theanisotropy parameter (β). The electron in each case must be detached from what may accurately
be described as a hybrid oxygen p-like molecular orbital and a conjugated aromatic π system; itis therefore expected that we should observe a negative β when observing detachment to theneutral singlet ground state utilizing a photon energy of 3.494 eV.21, 36 Photodetachment forming
the neutral S0 state displays the same photoelectron anisotropy for the ortho and para isomers, β~ – 0.15, using a photon energy of 3.494 eV. This anisotropy parameter stayed approximately
constant at ~ – 0.15 for the progression also seen beginning at ~2 eV binding energy in all threeisomers as well. The photoelectron angular distribution for electrons with binding energies
greater than 3 eV for all three isomers were isotropic (β ~ 0). However, these transitions are all
close in binding energy (< ~ 500 meV) to the photon energy used to examine them, thus thephotoelectrons have low kinetic energies, and so an isotropic angular distribution is expected.36

158

§ 4.5 Results: Photodetachment to the ground S0 State of o-, and p-Methylenephenoxyl
In order to obtain accurate EAs for the ortho and para isomers, higher resolution spectraof region A in Fig. 4.2 are necessary. Recall that the resolution of the VMI spectrometer
improves as the eKE is decreased. While the transition to S0 might appear broad in Fig. 4.2,
higher resolution can be achieved by either changing the photon energy used in Instrument B, orutilizing the constant energy resolution inherent to Instrument A. This latter option was chosen
and the results are shown in Fig. 4.3, where the experimental data are shown as black and red
traces, while the theoretical results are shown as the purple sticks (FCFs) and the green curves(convolution). The higher resolution data reveal structured spectra, including clearly definedpeaks that can be attributed to transitions from the anion to the neutral S0 state. The origin peakis approximately 1 eV, as expected for the EA of methylenephenoxyl radicals. The PE spectra
for both the ortho and para isomers displayed in Fig. 4.3 show a series of peaks spaced byintervals of approximately 500 cm-1, indicating the expected excitation of ring distortion
vibrational modes.

159

Fig. 4.3 Photoelectron spectra of the o- and p-methylenephenoxide anions obtained using Instrument A
utilizing a photon energy of 3.408 eV are shown in the black and red traces, respectively. Instrument A
has a constant spectral resolution of 10 meV. The calculated transitions and their corresponding Franck–
Condon intensities are represented by purple sticks, while their convolution with experimental resolution
are shown in green.

160

In order to aid in further analyzing the spectra in Fig. 4.3, a comparison between theory
and experiment is instructive. For both isomers, there is very good agreement between theexperimental results and the theoretical predictions, further confirming the assignment of thesephotoelectron spectra as detachment from o- and p-methylenephenoxide to form the S0 state of
the corresponding neutral diradical. For o-methylenephenoxide (top panel), peak A is predicted
to be due primarily to the transition from the anion vibronic ground state to the neutral vibronic
ground state, i.e. the EA. Note that while the origin transition possesses the largest predictedtransition intensity to contribute to the peak, many other transitions also contribute. These are
primarily transitions from the anion ground electronic state with one quanta populating a FC
active vibrational mode to the neutral ground electronic state with one quanta populating theanalogous vibrational mode which was populated in the anion, commonly known as sequencebands. The center of peak A is measured to be 1.217 ± 0.001 eV, leading to a measurement of
the EA = 1.217 ± 0.012. Similarly, peak A for the para isomer (lower panel, Fig. 4.3) ispredicted to be dominated by the origin transition (EA), with its peak center located at 1.096 ±0.001 eV, and thus the EA is measured to be 1.096 ± 0.007 eV. For both ortho and para isomers,
the width of peak A is significantly broader than the experimental resolution and the transitions
that make up peak A are distributed non-symmetrically about the peak center, giving rise to the
non-Gaussian shape. Thus, the peak HWHM is used in the error analysis and is the dominant
contributor to the error on the EA.
For the o-methylenephenoxide isomer photoelectron spectrum (top panel, Fig. 4.3), Peaks
B, C, and D are located 571 ± 10, 1571 ± 10, and 2105 ± 10 cm-1 relative the center of peak A.
Peak B is predicted to have arisen primarily from a transition to an excited ring distortion
vibrational mode, as was expected. The peak spacing relative to the EA yields an experimental161

measure of the neutral o-methylenephenoxyl radical 2810 = 570 ± 180 cm-1, as compared with thepredicted value of 576 cm-1. Peak C is predicted to arise from the transition from the groundvibronic state of the anion to one quanta of either of two vibrational modes of the neutral, whichhave approximately equal FCFs. Both of these neutral vibrational modes combine ring distortion
motion with C–O stretch motion, and have calculated harmonic frequencies of ν8 = 1625 and ν9 =1586. Due to experimental resolution and congestion, they were not experimentally separated,leading to a measurement of both 910 , 810 = 1570 ± 270 cm-1. Peak D is a combination bandarising from transitions from the ground vibronic state of the anion to combinations of ν9 and ν8
with ν28, thus allowing for the measurement, 810 2810 , 910 2810 = 2100 ± 140 cm-1. These results are
summarized in Table 4.1, while the eigenvectors for these modes may be found in Table 4.2.
The vibrational excitations found within the electronic transition from the ground doublet
state of the p–methylenephenoxide anion to the electronic ground singlet state of thecorresponding neutral (bottom panel of Fig. 4.3) is now examined. Peaks B and C have peak
centers 452 ± 10 and 899 ± 10 cm-1 higher in binding energy relative to peak A. The intensitiesof peaks B and C are primarily attributed to excitations of one ring distortional vibrational modewith a predicted harmonic frequency of 464 cm-1 and assigned ν30. Thus, the transitions
involving this mode may be measured: 3010 = 450 ± 80 cm-1 and 3020 = 900 ± 140 cm-1. Peaks
higher in binding energy than peak C are due to many transitions, including combination bandsof a variety of ring distortion vibrational modes (~ 500 cm-1) as well as modes which incorporatering distortion motion with C–O stretch motion (~ 1500 cm-1). Since these peaks are not clearly
dominated by only one or two transitions, further assignments are not straightforward.

162

Table 4.1 The experimentally measured frequencies of the Franck-Condon active normal modes
for the singlet ground (S0) and first triplet excited (T1) states of o- and p-methylenephenoxyl
diradicals. The calculated (B3LYP/aug-cc-pVTZ) harmonic frequencies are listed in parentheses
below the measured frequency. All values are in cm-1. Details describing the motion of these
normal modes are described in the text.
o-Methylenephenoxyl

p- Methylenephenoxyl

S0 state

S0 state

ν28 = 570 ± 180 cm-1
(577 cm-1)

ν30 = 450 ± 80 cm-1
(464 cm-1)

ν9 = 1570 ± 270 cm-1
(1586 cm-1)
ν8 = 1570 ± 270 cm-1
(1625 cm-1)
T1 state
ν31 = 450 ± 160 cm(456 cm-1)

T1 state-1N/A

ν28 = 450 ± 160 cm-1
(529 cm-1)
ν7 = 1470 ± 170 cm-1
(1530 cm-1)

163

Table 4.2 The following diagrams display the primary Franck-Condon active normal modes
discussed in the text for each product studied. Each eigenvector shown here is that of the neutral
radical, and calculations were carried out at the B3LYP/aug-cc-pVTZ level of theory/basis set.

Ortho- Methylenephenoxyl radical (S0 state)ν9 (1586 cm-1)

ν28 (577 cm-1)

ν8 (1625 cm-1)

164

Ortho- Methylenephenoxyl radical (T1 state)ν28 (529 cm-1)

ν31 (456 cm-1)

ν7 (1530 cm-1)

165

Para-Methylenephenoxyl radical (S0 state)ν30 (464 cm-1)

166

§ 4.6 Results: Photodetachment to the T1 State of o-, and p-Methylenephenoxyl
The singlet – triplet splittings for the ortho and para isomers are also measured. Fig. 4.4
displays region C of the PE spectra accessing the excited neutral states of the ortho and para
isomers, collected utilizing Instrument B with a photon energy of 3.494 eV. The experimental
data are colored black and red, while theoretical modeling is shown in green and purple. Peak A
in the ortho spectrum is located at 2.752 ± 0.001 eV. This is approximately 1.5 eV above the EA.
Theoretical calculations (ROCBS–QB3) predict the first electronically excited triplet state to be
1.498 eV above the ground state, while the first electronically excited singlet state is over 3 eVhigher in energy than the ground state. Given the experimental location of this peak in the
spectrum, it is assigned to arise from the transition from the anion vibronic ground state to thevibrational ground state of the neutral first electronically excited triplet state. This leads to ameasurement of the T1 electronic band origin, 2.752 ± 0.015 eV electron binding energy.
Combining this value with the EA allows for a measurement of ΔEST(o-methylenephenoxyl) =1.535 ± 0.019 eV, agreeing very well with the theoretically predicted energy difference (1.495
eV). As for the para isomer, due to the fact that there is a predicted π/2 radian angle change in
the CH2 group, a similar simulation is not possible to aid our analysis. Since all of the peaks inthis spectrum for the para isomer are far wider than the experimental resolution (by a factor of 2
or even 3) and considering the presence of a broad underlying continuum, without a simulated
spectrum to provide more guidance, only a lower bound may be placed on the origin transition
for this progression: ≥ 2.9 eV. This provides an experimental bound on the ΔEST ≥ 1.8 eV,compared to the theoretical prediction of ΔEST = 1.962 eV.

167

Fig. 4.4 Photoelectron spectra accessing the first triplet electronic excited states of the o- and pmethylenephenoxyl diradicals are shown in black and red traces, respectively. These spectra werecollected on the VMI PE spectrometer utilizing a photon energy of 3.494 eV. The spectrum of the ortho
isomer was successfully simulated and shown as the convolution (green trace) of the predicted transition
positions and their respective Franck–Condon factors (purple sticks). The para isomer simulation proved
to be unsuitable at the same level of theory due to a π/2 radian angle change between the anion and theneutral of the CH2 group.

168

The vibrational transitions found within the electronic transition from the ground doublet
state of the o-methylenephenoxide anion to the first electronic excited triplet state of thecorresponding neutral (top panel of Fig. 4.4) may also be analyzed. Fig. 4.4 displays thephotoelectron spectrum of the o- methylenephenoxide anion accessing the neutral triplet state.
These data were obtained with Instrument B and a photon energy of 3.494 eV. Experimental
data are presented as red or black curves while the simulated spectrum is represented by a green
trace. Peaks B and C are located 455 ± 10 and 1471 ± 10 cm-1 higher in binding energy relative
to peak A which is dominated by the origin transition. Theory predicts peak B to be dominatedby excitation of two ring distortion vibrational modes with harmonic frequencies calculated to be
456 and 529 cm-1, ν31 and ν28, respectively. Due to their proximity in frequency to each other,
these two modes cannot be independently measured, and therefore we assign both transitions
3110 , 2810 = 450 ± 160 cm-1. Peak C is predicted to be due to one vibrational excitation of a
combination ring distortion, C–O stretch mode with a harmonic frequency predicted to be 1530
cm-1 and is determined to arise from excitation of ν7. This allows for an experimental
measurement of the transition 710 = 1470 ± 170 cm-1.
The para isomer also shows evidence of an excited electronic state vibrational
progression, however no modeling of this progression is shown. There is a predicted large
geometry change in the CH2 group between the neutral electronic ground state and the firstelectronic excited state; the CH2 rotates π/2 radians with respect the ring plane. Several attempts
to simulate the photoelectron spectrum and compensate for this effect were made, but none
qualitatively reproduced the experimental data and displayed non-physical behavior. These
efforts included excluding vibrational transitions in the simulations which involved CH2 motion,169however, as all modes include this motion to varying degrees, this approach failed to produce
meaningful results.

170

§ 4.7 Results: Unassigned Features
There is little that can be concluded from region A in the PE spectrum of mmethylenephenoxide, found in Fig. 4.2. There are large disparities between the experimental
results and the numerous calculations attempted, unlike the results for o- and pmethylenephenoxide. In addition, each considered isomer was calculated at several levels oftheory, and these displayed large differences in the predicted EAs of the same molecule. For
example, calculating the EA of m-methylenephenoxyl using ROCBS-QB3 yields an EA of 0.777
eV, however that same structure calculated with B3LYP/aug-cc-pVTZ returned a value of 2.170
eV. Other possible isomers were considered, from ring-opened structures to seven memberedring structures, but were discounted due to the lack of agreement at both a qualitative andquantitative level between the experimental and calculated EA and overall photoelectron
spectrum. The methods and basis sets were also varied; CBS-APNO, wB97XD/aug-cc-pVTZ,
and CCSD/aug-cc-pVDZ were all computed, but this did not reveal any insight into the problem.
The only constant throughout these calculations was their agreement that the ground state of them-methylenephenoxyl diradical was the triplet state, contrary to the other two isomers. The
reasons for these theoretical failings are explored in § 4.9.
In order to further attempt to understand the PE spectrum of m-methylenephenoxide, anadditional spectrum was collected at a higher photon energy than 3.494 eV. Fig. 4.5 displays thephotoelectron spectrum of m- methylenephenoxide obtained with two photon energies, 3.494 and3.832 eV (generated via doubling the output of a 532 nm pumped DCM dye laser), collected on
instrument B. As can be seen, there is a dramatic difference between the two spectra which is
attributed to electron autodetachment, and cannot be explained by changing instrument
resolution nor threshold effects.
171Region B in Fig. 4.2 is now examined in greater detail and with higher resolution. Fig.
4.6 displays the PE spectra of all three methylenephenoxide isomers recorded with the VMI PE
spectrometer utilizing a photon energy of 2.330 eV. These spectra provide a higher resolution
examination of the progressions beginning at ~2 eV binding energy for each isomer. As can beseen, no theoretical modelling of these spectra is shown. This omission was the result of a failureof theory to accurately simulate or predict these progressions. When attempting to simulate these
spectra excited electronic states of the neutral diradicals were considered. Additionally,
sequential photon absorption processes were also simulated, e.g. the anion absorbs a photon andtransitions to a repulsive electronic state, ejecting a hydrogen or methylene group and then the
resulting anion subsequently absorbs a second photon which undergoes photodetachment asnormal. These processes were discounted as no bond simulated had a predicted dissociationenergy below 3.8 eV, and thus, with even the highest energy photon used in this work, 3.494 eV,
we would not have been able to access such a process. Peak A in the PE spectra for the ortho,
meta, and para isomers is located at 2.118 ± 0.001, 2.227 ± 0.001, and 2.177 ± 0.001 eV binding
energy. Assuming these peaks represent the origin transitions, they do not match any known EA
value of any likely organic molecule of the appropriate mass.34, 37-40 This was further confirmed
by also obtaining the PE spectra of methylphenoxide with a photon energy of 2.330 eV andoverlaying these data upon the spectra found in Fig. 4.6. See Fig. 4.7. Additionally, examining
the energetic spacing between peak A in Fig. 4.6 and peaks at higher binding energy in eachisomer reveals energy differences of ~500 cm-1, suggesting ring distortion vibrational excitation.
This is examined further in § 4.9.

172

Fig. 4.5 Photoelectron spectra of m-methylenephenoxide anions obtained on instrument B withtwo different photon energies, 3.494 and 3.832 eV. As can be observed, there is a photon energy
dependence displayed between the two spectra. This again arises from autodetachment. Moredetails in the text.

173

Fig. 4.6 Photoelectron Spectra of the three isomers of the methylenephenoxide anion, collectedon the VMI PE spectrometer with a photon energy of 2.330 eV. No simulated spectra are shownas none succeeded in reproducing these data. These spectra were compared to the known PE
spectra of methylphenoxide, phenoxide, and benzyl anion, and quantitatively disagree. See Fig.
4.7 for a comparison to methylphenoxide at the same photon energy.

174

Fig. 4.7 Photoelectron spectra obtained with a photon energy of 2.330 eV on instrument B.
These spectra were obtained by simply alternating the laser timing to either intersect m/z = 106(methylenephenoxide, black curves) or 107 (methylphenoxide, red curves). For any given
isomer, the red and black spectra may share several similarities, however, they quantitatively are
not the same.

175§ 4.8 Results: Thermochemistry
The acid bracketing technique was used to identify the acidity of the protonated forms ofthe three anions of interest in this paper, namely o-, m-, and p-methylenephenol. See Table 4.3.
These results were gathered with Instrument C and are combined with the other thermodynamic𝑜results in Table 4.4. Using these data, experimental values for the acidity (∆𝑎𝑐𝑖𝑑 𝐻298
𝐾 ) of the o-,m-, and p-methylenephenol radicals were found to be 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3
kcal/mol respectively. These were theoretically (B3LYP/aug-cc-pVTZ) predicted to be 341.3,
347.0, and 340.4 kcal/mol, respectively. This modelling was performed assuming that the
protonation site of the distonic radical anions in the experiment is at O, and the close agreementwith the experimental results corroborate this assumption. Protonation on the CH2 site was alsomodeled, but those predictions fell outside the experimental error bars by >10 kcal/mol, and were
so discounted.
The experimental determination of both the acidities, as well as the EAs of o- and pmethylenephenoxyl enables a determination of the O-H bond strength in the methylenephenol
radical isomers in the gas phase, via a thermochemical cycle.41 The experimental values for theacidities had to be adjusted to their 0 K values using the calculated heat capacities at constantpressure, Cp.34 The dissociation energies were derived to be 48 ± 5 and 45 ± 5 kcal/mol for oand p-methylenephenol, respectively.

176

Table 4.3 Summary of acid bracketing results. A “+” denotes an exothermic proton transfer
occurred, while a “–” denotes its absence. All acidities taken from the NIST Chemistry
Reference Database.42

omethylene–
phenoxide

mmethylene–
phenoxide

pmethylene–
phenoxide337.0 ± 2.2

+

+

+

332.4 ± 1.9

339.4 ± 2.2

+

+

+

o-Chlorophenol

337.1 ± 1.9

343.4 ± 2.4

̶

+

̶

p-Fluorophenol

339.9 ± 1.9

346.8 ± 2.2

̶

+

̶

Propionic acid340.4 ± 2.0

347.4 ± 2.2

̶

+

̶

Acetic acid341.1 ± 2.0

348.1 ± 2.2

̶

+

̶

Phenol342.3 ± 2.0

350.0 ± 2.0

̶

̶

̶

2,2,3,3,3-Pentafluoro-1propanol

348.8 ± 6.0

355.4 ± 6.1

̶

̶

̶

Reference Acid∆𝒂𝒄𝒊𝒅 𝑮𝒐𝟐𝟗𝟖 𝑲 ∆𝒂𝒄𝒊𝒅 𝑯𝒐𝟐𝟗𝟖 𝑲/kcal mol-1 /kcal mol-1

p-Trifluoromethylphenol

330.1 ± 1.9

3-Mercapto–propionic
acid

177

Table 4.4 Summary of Results. Measured values are displayed in plain text while calculated
values are in italics.

a

EA / eVSinglet-Triplet
splitting / eV𝑜a∆𝑎𝑐𝑖𝑑 𝐻298
𝐾 /kcal mol-1

BDEa /
kcal mol-1

1.217 ± 0.012

1.535 ± 0.019

341.4 ± 4.3

48 ± 51.296b

1.498b341.3b58b1.096 ± 0.007≥ 1.8

341.4 ± 4.3

45 ± 51.178b

1.962b340.4b56bRefers to O-H bond of the corresponding methylenephenol radical
Calculated value: ROCBS-QB3 for all but the acidities, which used B3LYP/aug-cc-pVTZb178

§ 4.9 Discussion
In general, the ortho and para isomers display good agreement with theory in regions A
and C of Fig. 4.2, and may be mostly understood with simple physical pictures, such as
resonance and molecular orbital theory. On the contrary, the data obtained on the meta case has
proven highly resistant to the calculations that were attempted. Region B (Fig. 4.2) is similar
among the three isomers but is unexplained thus far. Regardless, several observations may bemade. First, all three isomers display a progression starting at ~ 2 eV binding energy, whichsuggests that the observed electrons were strongly associated with an oxygen prior tophotodetachment. Second, each of these progressions display regularly placed peaks with aspacing of ~ 500 cm-1. This is indicative of ring distortion vibrational mode excitations upon
photodetachment. Third, in each case the highest intensity peak is not at the start of the
progression, see Fig. 4.2, suggesting a large geometry change between the anion and neutral
species. These spectra were also compared with PE spectra of other likely aromatic compounds,
such as phenoxide37 or the methylphenoxides.34 While the spectra are not in quantitative
agreement, both in peak positions and relative peak intensities, there are striking similarities tothe overall spectra. See Fig. 4.7. The similarities among these spectra, along with the generalobservations listed above, suggest that the anion responsible for region B contains a ring
structure and that the negative charge is localized on the oxygen.
Given the above experimental observations, a range of structural isomers with m/z = 106
were tested with the aid of calculations. Single reference electronic structure calculations failed
to predict the origin of the progressions in region B, and thus we conclude that these
progressions do not arise from single photon detachment (or direct detachment) from179

methylenephenoxide. It is possible for multi-photon events to occur in this spectrometer,
although this was not tested directly using power dependence measurements.
The electron affinities of the two methylenephenoxyl diradicals measured in this work
can be compared to the deprotonated methylphenols, studied previously, in order to gain somephysical insight.34 The EAs for the o-, m-, and p-methylphenoxyl radicals are 2.1991 ± 0.0014,
2.2177 ± 0.0014, and 2.1199 ± 0.0014 eV, respectively. Qualitatively, the two measured
methylenephenoxyl diradicals have an EA of ~1 eV, while the methylphenoxyl radicals all have
EAs of ~2 eV. The main difference between these two groups of molecules is that the
methylphenoxyls have a CH3 functional group, which is slightly electron donating, whereas the
methylenephenoxyl diradicals have a •CH2 group, which is electron withdrawing. Using this as astarting point, one can think of the CH3 group repelling the excess charge in the anion onto theO, while the •CH2 group attracts the excess charge away from the O, and into the ring. Removing
an electron strongly localized on a functionalized oxygen on a ring generally takes ~1.5-2 eV toaccomplish, whereas removing an electron delocalized about an aromatic ring requires ~1 eV.35,
37

This model accounts for the ~1 eV difference in EAs between the methylenephenoxyls andmethylphenoxyls. This physical picture would also lead one to expect more ring distortion upon
photodetachment for the methylenephenoxide anions compared to the methylphenoxides, andindeed, the longer, more congested vibronic progressions found within the PE spectra of themethylenephenoxide anions support this expectation.
The singlet-triplet splittings in the o- and p- methylenephenoxyl diradicals are now
considered. The ground state has been identified as an open shell singlet electronic state for bothisomers. An experimental determination of the singlet-triplet splittings for each has been
identified and found to be 1.535 ± 0.019 and ≥ 1.8 eV, in agreement with the calculated values of180

1.498 and 1.962 eV, respectively. This was an initially surprising result. One might consider thecase of the benzyne diradical, studied previously,35 to be an appropriate analog ofmethylenephenoxide and use this to guide their intuition. In that system, as the two radical sites
were brought closer together, i.e. starting with para-, then meta-, and finally ortho-benzyne, thesinglet-triplet energy difference monotonically increased from 0.167 ± 0.016 to 1.628 ± 0.013
eV. This makes intuitive sense. One would expect the coupling between the two radical electrons
would increase as they come into closer spatial proximity with one another, thus translating thesinglet and triplet states further apart in energy. In the methylenephenoxyl diradicals, this does
not appear to be the case. Indeed, the opposite trend is observed; the para isomer displays thelargest observed splitting, ≥ 1.8 eV. However, it is important to consider the influence ofresonance on this analysis. In the neutral singlet state of methylenephenoxyl, the radical electronon the •CH2 functional group can interact with the conjugated π system of the aromatic ring, andlikewise for the radical electron localized about the O. This has a stabilizing effect in the neutral
ground state, as the •CH2 group may form a quasi-double bond with the ring, drawing charge
density from it. However, in the triplet state the radical electrons are forced into orbitals in theplane of the benzene ring, and are so unable to conjugate with the aromatic π system. Thus thesystem has lost the stabilizing effects it benefitted from in the neutral ground electronic singlet
state, putting the triplet state higher in energy than what might have otherwise been expected.
While this does explain why the para isomer has a much larger ΔEST as compared to p-benzyne
by an order of magnitude, it does not necessarily explain why p- methylenephenoxyl has a larger
singlet-triplet splitting than o- methylenephenoxyl, since they should both benefit fromapproximately the same resonance effects. Likely, the added complication of the O…CH2
interaction in the ortho isomer and/or the increased symmetry of the para case is significant. As

181

to the meta isomer, the spectrum remains unassigned for the reasons previously given, and thus
no measurement of the singlet – triplet splitting can be performed. Since calculations have
proven unreliable for this isomer, a calculation of ground and excited states, though performed,
will not be presented here. Whether the ground state is a singlet or triplet state must remain an
open question for the moment.
Utilizing these same resonance ideas discussed above, the vibrational structures observed
can also be explained. As opposed to the methylphenoxide anions, which showed primarily only
ring distortion modes (~500 cm-1) being activated upon photodetachment, themethylenephenoxide anions display both ring distortion, and C–O, C–C stretching/ring
distortion modes (~1600 cm-1) becoming excited upon photodetachment. This is again due to thefact that •CH2 is an electron withdrawing group, attracting the excess charge away from the O
and onto the ring in the anion, as well as onto the •CH2 itself. In the anionic form, these ions will
distort the ring away from a benzene-like structure. For example, in the p-methylenephenoxideanion the CCC angle about the α-carbon to the O is 115° while the CCC angle about the αcarbon to the •CH2 group is 114°, according to calculations. If this structure were an ideal
benzene ring, these angles would be 120°. Upon photodetachment to form the neutral diradical
however, these angles have equilibrium values of 117° and 116°, respectively. Thus upon
photodetachment, the ring becomes less strained and approaches a more benzene-likeequilibrium structure. This is qualitatively true for the ortho isomer as well. In addition, the C–O
and C–CH2 bonds each constrict by ~0.05 Å upon photodetachment giving rise to C-O, and C-C
stretching-like modes which were not as prevalent at all in the PE spectra of themethylphenoxides accessing the ground neutral state. This makes intuitive sense; in the
deprotonated case, an electron donating methyl group attached to the ring would not cause the182electron to be localized as much about the α-carbon of the O, and so less distortion of the C-O
bond is expected. In the case of the methylenephenoxides the opposite is true, as the electron
would be drawn more towards the ring and to the •CH2 group.
For those spectra wherein we claim good agreement between experimental data andtheoretical simulations (see Figs. 4.3 and 4.4), there remain several disparities which should be
addressed. First, in all cases, there is more broadening than can be explained by experimental
resolution or predicted spectral congestion. One might consider if rotational broadening could
explain this, but the predicted changes in the rotational constants for all the isomers consideredhere are < 0.01 cm-1, making this an unlikely contribution to the observed broadening.43 Such an
extensive broadening in larger aromatic, and/or frequently diradical, compounds have beenfrequently observed previously.3, 8, 44 Second, at higher binding energies, the predicted peak
positions tend to trend higher in energy compared to the experiment, but this is an expectedfeature of anharmonicity. Third, in the course of studying these species, several more spectrathan shown in the main text here were taken with other photon energies, and a photon energy
dependence of the meta PE spectra were observed and are found in Fig. 4.5. This phenomenon is
attributed to electron autodetachment, which has been a hallmark in similar aromatic species.34,
37-39

This hypothesis is corroborated by time–dependent density functional theory (TD-DFT)

excited state calculations which predict many (>10) optically accessible anionic excitedelectronic states within 3 eV of the ground electronic state. The broadening observed in these
spectra varies with photon energy, and autodetachment is the likely cause of the majority of thedisagreement with our simulations, which do not take such phenomena into account.
The thermochemistry of these molecules is now considered. The relatively weak bondstrengths of the methylenephenol radical isomers may be found in Table 4.4. This property may
183also be explained with similar resonance ideas as were introduced earlier. The electron
withdrawing nature of the •CH2 group removes electron density, and therefore bond strength,from the O-H bond, explaining these weak BDEs. Additionally, the methylenephenoxide distonic
radical anions are relatively non-reactive, as evidenced by the low proton affinities. As a generalrule, one might expect such species to be reactive and quite basic.45 Indeed, distonic radical
anions and cations are often found to be reactive intermediates in organic mechanisms.46, 47
However, these data suggest that this group would be long lived, and reasonably stable. In thiscase, this is likely due to the electron withdrawing nature of the •CH2 group as well, strongly
drawing the negative charge onto the aromatic ring, and onto the •CH2 itself, thus stabilizing theanion. Relatively nonreactive distonic radical anions are not without precedent, but could prove
to be interesting to some in the scientific community.48, 49

184

§ 4.10 Conclusions
Anion photoelectron (PE) spectroscopy was performed and the photoelectron spectra of
the distonic radical o-, m-, and p-methylenephenoxide anions have been reported. The EAs of theo-, and p- methylenephenoxyl diradicals were measured to be 1.217 ± 0.012 and 1.096 ± 0.007
eV, respectively. Upon photodetachment, these distonic radical anions were shown to haveFranck Condon active vibrational ring distortion modes with measured frequencies of 570 ± 180
and 450 ± 80 cm-1 in the ortho and para isomers accessing the neutral diradical singlet groundelectronic states. C-O/ring distortion vibrational modes were also activated in the para isomer
accessing the same electronic state of the neutral diradical. These transitions could not be
separately measured and therefore the only value for the frequency of these modes is 1570 ± 270
cm-1. Photodetachment to the first electronically excited triplet state was also investigated andsimilar vibrational modes were found to be Franck–Condon active for the ortho isomer, while
the para isomer proved more difficult to interpret. The ortho isomer showed excitation of ring
distortion vibrational modes with a measured frequency of 450 ± 160 cm-1 and excitation of a CO/ring distortion vibrational mode with a frequency of 1470 ± 170 cm-1. The thermochemistry ofthese molecules were also investigated using Flowing Afterglow-Selected Ion Flow Tube mass
spectrometry with the acid bracketing technique, and thus the proton affinity of the o-, m-, and pmethylenephenol radicals were determined to be 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3
kcal/mol respectively. Construction of a thermodynamic cycle allowed for an experimental
determination of the weak bond dissociation energy of the O-H bond for the ortho and para
isomers, 49 ± 5, and 47 ± 5 kcal/mol respectively. Most of the trends and behaviors observed
here can be explained by resonance and the effects of electron withdrawing groups in aromatic
chemistry.

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189

CHAPTER V
ANION PHOTOELECTRON SPECTROSCOPY OF DEPROTONATED INDOLE AND
INDOLINE
Portions of this chapter have been submitted for publication to The Journal of Chemical Physics
under the same title, by, Daniel J. Nelson, Allan M. Oliveira, and W. Carl Lineberger§ 5.1 Introduction and BackgroundAs was discussed in § 1.5, the biologically relevant bicyclic aromatic molecules indole
and indoline are of consequence to the fields of biochemistry and biophysics due to their role as
chromophores in biological systems.
The anion photoelectron spectra of indolide is presented in this study. These data provide
the electron affinity of the associated neutral, indolyl, and those vibrational modes which are
excited upon electron photodetachment of indolide, aided by quantum chemical calculations.
Combining the newly measured EA of indolyl with the past measured acidity allows for thedetermination of the bond dissociation energy.1
Structure of Indolide

The PE spectroscopy of deprotonated indoline was also collected, however the anion
photoelectron spectrum of deprotonated indoline consisted of a featureless broad band extending190

from ~1.3 eV to 1.7 eV electron Binding Energy (eBE). The congested nature of the spectrum islikely due to the presence of multiple isomers of deprotonated indoline, including ring opened
structures. As such, the results and discussion of the deprotonated indoline data are held separate
from the indolide data and examined last, for the sake of clarity.

191§ 5.2 Experimental Specifics
The experimental apparatus has been discussed in detail previously2, 3 (§ 2.7) and will only
be explained briefly here, with special attention as to the generation of the anions of interest.
Indole (99% pure) was purchased from Sigma Aldrich Inc. and used without further purification.
Crystalline indole has a relatively low vapor pressure and was heated to ~70 °C to obtain adequate
indolide production.
The anion of interest is formed in a dual pulsed valve ion source.3 (§ 2.8) These valves(Parker-Hannifin General Valve, Series 9) are oriented such that the gas expansions areperpendicular to each other. One of the valves is the primary supersonic expansion (10 psig, ~1%
indole or indoline, in Ar). The other valve, designated the side valve (35 psig, 1% O2, 30% H2, inAr), produces about 5 % of the total gas load. The side valve has discharge plates placed
immediately in front of the valve exit, producing a plasma containing hydroxide. The plasma is
subsequently entrained into the supersonic expansion of the primary valve. This allows for
deprotonation of indole by OH–.4 The hydroxide anion will be shown to only generate indolide
upon reaction with indole in this experimental setup. The resultant product and byproducts arecollisionally cooled by the Ar atoms in the main supersonic expansion.
The entrained anions are then extracted into a Time-Of-Flight (TOF) Wiley-McLarenmass spectrometer. (§ 2.9) The anions are separated by their mass-to-charge ratio (m/z) andspatially focused at the interaction region of a Velocity Map Imaging (VMI) photoelectron
spectrometer. (§ 2.10) An appropriately timed nanosecond pulse from the photodetachment laser
then intersects the anions of a chosen mass. The VMI first stage is pulsed to the operating voltage,velocity mapping the photoelectrons onto the plane of a Micro-Channel Plate (MCP) coupled to aphosphor screen and subsequently imaged with a CCD camera. The resulting image gives direct
192

measurement of the two-dimensional velocity distribution. The MEVELER (Maximum Entropy
Velocity Legendre Reconstruction) program5 was used to reconstruct the full three-dimensional
velocity distribution of the photoelectrons. This three dimensional distribution is transformed into
the one dimensional speed distribution, and then into the electron Kinetic Energy (eKE)distribution, by means of a Jacobian transform. Finally, the eKE is transformed into the electron
Binding Energy (eBE) by subtracting the eKE distribution from the photon energy.
The VMI has an energy resolution that depends upon eKE, with the best resolution
obtained for the lowest photoelectron kinetic energy. For the experiments reported here, theenergy resolution was determined to follow the relation
Resolution(eKE) ≅ (0.003 + 0.03 eKE) eV(1)

The constant term is the limiting resolution of the apparatus, in this case 3 meV. The energy scalein the overview spectrum shown in Fig. 5.1 was calibrated with the known photoelectron
spectrum of S–.6-8 By employing multiple laser wavelengths, we can obtain composite
photoelectron spectra spanning several eV, with resolution near the instrumental limit throughout
the full range. This procedure is known as Slow Electron Velocity–Map Imaging (SEVI)
photoelectron spectroscopy, pioneered by Neumark.9 The data in Figs. 5.2-4 were obtained at anumber of laser wavelengths, chosen so as to take advantage of this higher resolution approach.
The photon energies used to obtain these spectra were calibrated with an ATOS λ-meter (LRL–005).
All of the spectra shown here are obtained making use of a seeded Nd:YAG pulsed
nanosecond laser. The data found in Fig. 5.1 were obtained utilizing the 3rd harmonic of the
Nd:YAG lasing transition (3.494 eV). The spectra shown in Figs. 5.2 and 5.3 were collected193

utilizing photon energies generated by a visible light optical parametric oscillator (OPO) pumpedby the 3rd harmonic of the aforementioned Nd:YAG laser.
Throughout this work, uncertainty in the reported peak positions are functions of thestatistical error in finding the peak center, the error in the absolute energy scale, and the numberof independent measurements of particular peaks. When reporting the energy associated with aparticular transition associated with a peak, the uncertainty in that peak position is combined withthe error associated with the offset of the actual transition from the peak center. This aggregatedadditional uncertainty can be near zero if only a single vibronic transition is the major contributor
to the peak shape. If, however, multiple unresolved vibronic transitions contribute significantly tothe overall envelope, the uncertainty in location of any individual transition could be as large as
the Half-Width-at-Half-Maximum (HWHM) of the peak. Shifts caused by asymmetry in therotational envelope about each vibrational transition are taken into account. using the peak shapeanalysis of Engelking and coworkers.10 This rotational correction did not have a significant effectfor transitions reported here. In general, the peaks presented in these spectra are comprised ofmultiple transitions, and the uncertainty associated with these contributions is the dominantcomponent of the reported uncertainties of transition energies.

194§ 5.3 Theoretical Methods and Simulations
Optimized geometries, electronic structure, and harmonic vibrational normal mode
analyses were carried out utilizing the Gaussian 09 program suite.11 These calculations were
performed at the B3LYP/aug-cc-pVTZ level of theory/basis set. This combination has been foundto be an effective compromise between accuracy and computational cost.12 Several excited statecalculations were also carried out and made use of Time Dependent Density Functional Theory(TD–DFT) employing the same hybrid functional, B3LYP, and Dunning basis set, aug-cc-pVTZ.
All reported EAs are calculated from the Zero–Point Energy (ZPE) corrected energy difference
between the optimized electronic ground neutral state and the optimized electronic ground anionicstate.
The optimized geometries and harmonic normal mode analyses were utilized to simulate
the photoelectron spectra collected in this experiment and aid in their interpretation. These
spectra were simulated assuming that only the Franck–Condon Factors (FCFs, § 1.1) influence agiven photodetachment transition intensity; i.e. the electronic component of the photodetachment
cross-section is constant across a given spectrum. Thus, the simulated spectra do not capture
behavior such as electron autodetachment. The FCFs were computed via the Sharp–Rosenstock–
Chen method, making use of Duschinsky rotations.13-15 These factors, associated with specificvibrational transitions, along with an assumed 200 K Boltzmann distribution of internal energy inthe anions, were computed with the PESCAL program.16 While these simulations assume fullthermal equilibration among the vibrational degrees of freedom of indolide, such is notnecessarily the case. These intensities associated with specific transitions are represented as red
sticks in Figs. 5.1–5.4, where the length of any stick is equal to the calculated intensity of thattransition, and it is located at the calculated transition electron binding energy. These transition
195

intensities are converted into simulations of the experimental spectra by convolving eachtransition with Gaussian functions whose FWHM is commensurate with the instrument
resolution, which is a function of eKE, and whose integrated area is equal to that of the calculatedtransition intensity via home-built programs. The simulations presented are shifted such that thecalculated EA matched the observed value. They are also scaled in intensity such that the origin
transition of the simulated spectra is in agreement with the peak attributed to the origin transitionin the experimental data. This procedure results in the green curves shown in Figs. 5.1–5.4.
It is important to underscore several subtleties involved in a Sharp–Rosenstock–Chen
Franck–Condon analysis of polyatomic photoelectron spectra of species that are as large as
indole. This analysis first requires identifying and matching the molecular motion of particularvibrations in the anion (in the ground electronic state, for any case shown here) to the molecularmotion of vibrations of the neutral in some electronic state. The eigenvectors representing thesemotions may be found in § 5.6, Table 5.3. Hence, a vibrational normal mode designated ν35 in theanion might be associated, for instance, with ν39 in the neutral molecule. While the molecularmotion of a particular vibration will be relatively invariant to the level of theory used to computeit, the frequency associated with that eigenvector may shift by ~50 cm–1. This shift might change
what is labeled as ν30 at one level of theory to be labeled, for example, ν33 in a different level oftheory, but the molecular motions will remain largely unchanged and therefore would be matchedto the same mode in the anion. This relabeling of normal modes depending on the level of theory
utilized is not unique to the neutral and will also affect the anion. The simulations shown here rely
only on the molecular motions to calculate the FCFs; thus while the ordering scheme used here tolabel the vibrational modes might be sensitive to the level of theory one might use, thesimulations of the PE spectra are not.

196

Assigning any given peak to a specific transition or transitions is a challenging task for alarge molecule, such as indole, and one must rely heavily upon theoretical simulations.
Considering the low symmetry of indolyl (Cs), the ordering convention employed here for the
calculated vibrational normal modes are simply organized first by the allowed symmetries of thevibrational modes (𝐴′ or 𝐴′′ ) and then by frequency as seen in Table 5.1. Table 5.1 also provides
each Franck–Condon factor associated with the transition from the ground vibrational state in theanion to one quanta of the appropriate mode (and zero quanta in all other normal modes) in theneutral radical. The FCFs have been normalized such that the FCF for the origin transition fromthe anionic ground vibronic state to the neutral ground vibronic state is unity. This table provides
the basis for the vibrational analysis found within this work. Considering the density ofvibrational states of both the anion and neutral molecules studied here, computing a given
vibrational transition energy and comparing this against the position of the peaks comprising thecollected PE spectra cannot result in any firm assignments. Consequently, the computed Franck–
Condon factors must be utilized in order to narrow the field of potential vibrational transition
assignments to only one or two transitions for a given peak. Even this can only be done if one or
two calculated FCFs dominate the predicted intensity of the peak in question. These assignments
are further governed by the inherent limitations of relying on the appropriateness of the harmonic
oscillator approximation. As such, several assignments made in this work must only be suggested,rather than concretely stated. It is within the limits of these constraints that vibrational transitions
are assigned to peaks.

197

Table 5.1 Calculated Vibrational Analysis of Indolyl
𝐴′ Calc. Indolyl Harmonic
Transition Energies (cm-1)
3221
110
3196
210
1
3193
30
3189
410
1
3175
50
1
3167
60
1
1629
70
1
1602
80
1495
910
1
1466
100
1460
1110
1
1373
120
1
1346
130
1303
1410
1
1233
150
1202
1610
1
1169
170
1
1162
180
1
1096
190
1
1026
200
966
2110
1
899
220
854
2310
1
770
240
1
587
250
545
2610
1
408
270

𝐴˶ Calc. Indolyl Harmonic
Transition Energies (cm-1)
997
2810
963
2910
1
922
300
886
3110
1
783
320
1
774
330
1
744
340
1
575
350
543
3610
1
412
370
241
3810
1
202
390

FCF
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.10
<0.01
0.24
0.07
0.08
0.03
<0.01
0.03
0.07
<0.01
0.06
0.02
<0.01
0.06
0.11
<0.01
0.02
0.08
<0.01
<0.01

FCF
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01

Calculated photoelectron harmonic transition energies and their associated FCFs. Note that thenotation of 𝑋01 implies that all vibrational modes other than vibrational mode X received noquanta of vibrational excitation. The Franck–Condon factors have been normalized such that thevibronic origin transition FCF is unity. Those vibrations which have been positively identified inthis work are shown in bold red font. Note that while the difference in frequency for many
modes may be small (~10 cm-1), the FCF can vary by a factor of up to 1,000.

198

§ 5.4 Results: Overview
Prior to examining the data, it is useful to hypothesize the likely behaviors one might
expect to observe upon photodetachment of deprotonated indole. In the case of indole, it is likelythat deprotonation will occur at the N–H site as this likely has a much greater acidity than any
other site on the molecule. This would result in the formation of an anion which might be
expected to have a similar PE spectrum to that of pyrrolide, based on the structural similarity ofthe molecules. The pyrrolyl radical was shown to have an EA of 2.145 ± 0.010 eV and the PE
spectrum of pyrrolide displayed excitation of ring distortion vibrational modes upon
photodetachment.17 Assuming that the EA of indolyl would be close to the EA of pyrrolyl is
consistent with our calculation of the EA of indolyl, 2.357 eV. If, however, a ring carbon site isdeprotonated, a photoelectron spectrum similar to cyclopentadienide might be expected.18
Although vibrational structure similar to pyrrolyl was observed upon photodetachment of
cyclopentadienide (ring distortion vibrational modes), the EA of cyclopentadienyl radical wasmeasured as 1.808 ± 0.006 eV,18 a value which is significantly lower than that of the pyrrolyl
radical. This analysis implies that if the product of deprotonation from a C–H site on indole waspresent, we would observe a photoelectron spectrum with an origin ~ 0.5 eV eBE below thecalculated indolyl electron affinity.
In order to examine the above hypotheses, an overview photoelectron spectrum of indolide
obtained with a photon energy of 3.494 eV was collected and is shown in Fig. 5.1. The
experimental data are shown as black dots, the calculated Franck Condon factors appear as red
sticks, and the full theoretical simulation of the photoelectron spectrum of the indolyl ground stateis shown as a green curve. The agreement between theory and experiment is qualitatively correct,
but the ground state simulation does not appear to account for the weak, broad feature around199

3.2 eV binding energy. As can be seen, the origin of this band is comprised of an intense peak,with a center near 2.4 eV eBE. This result is consistent with the calculation of the EA of indolyl,
2.357 eV, and confirms the formation of the indolide anion. The shaded region was obtained inhigh resolution (SEVI) and is shown in Fig. 5.2. No photoelectron signals were observed at lower
binding energies, which indicates that indole anions arising from carbon site deprotonation are notpresent in detectable quantities. Thus we conclude that the only observed isomer of deprotonatedindole is indolide.

200

Fig. 5.1 The photoelectron spectrum of indolide obtained with a photon energy of 3.494 eV. The
calculated transitions and their associated intensities are represented by red sticks, while theconvolution of these sticks with Gaussian functions whose FWHM is commensurate with the
instrument resolution is shown in green. This simulation assumed a temperature of 200 K. The
shaded region was obtained in high resolution and shown in Fig. 5.2.

201

The photoelectron spectra presented in this work showed no definitive evidence of thepresence of an excited electronic state of indolyl. This result is consistent with the calculated term
energy of the first excited electronic state of indolyl being ~1.5 eV, and thus would not be
accessible with the 3.494 eV photon energy available.

202§ 5.5 Results: Electron Affinity of Indolyl and Photoelectron Angular Distributions of Indolide
In order to obtain the EA of indolyl with a greater degree of accuracy and precision, theSEVI technique was employed. Fig. 5.2 displays the SEVI PE spectrum of the origin peak of the
progression shown in Fig. 5.1 (grey shaded area). Again, the experimental data are shown as
black dots, while the green trace and the red sticks represent the simulation and individual
calculated harmonic vibrational transitions and their respective intensities. The calculated origin
transition, i.e. the transition from the ground electronic and vibrational anionic state to the groundelectronic and vibrational state of the neutral radical (000 ), has been made bold in the figure. The
experimental peak is asymmetric indicating that while the peak primarily arises from the origin
transition, other vibrational transitions also contribute. The simulation supports this; while theorigin transition dominates the simulated spectrum, there are a number of less significant redshifted transitions from the origin transition. These additional transitions in the 200 K simulation
are the result of sequence-band transitions involving vibrationally excited anions. Whileobservable in the simulations, they do not have a significant effect on the determination of thelocation of the origin peak, when the fit is limited to an energy range corresponding to roughly theupper 60% of the origin peak, as seen in Fig. 5.2. This procedure allows for a direct measurementof the position of the intense peak, and hence the EA of indolyl, 2.4315 ± 0.0017 eV.

203

Fig. 5.2 The Slow Electron Velocity–Map Imaging (SEVI) PE spectrum of the origin transition ofindolide acquired with a photon energy of 2.4500 eV. The simulated spectrum is shown in green,with the individual transitions and their respective intensities shown as red sticks. The 000transition stick is shown in bold. This simulation assumed a temperature of 200 K.

204

In addition to the electron affinities, photoelectron angular distributions with respect to thelaser polarization, characterized by the anisotropy parameter (β), were measured for thesephotoelectron spectra. Since the electron detached from indolide to form the ground electronic
state of indolyl can be described as populating a nitrogen p-like orbital, it is expected that weshould observe an isotropic angular distribution (β ~ 0) for low kinetic energy photoelectrons
which shifts to an anisotropic distribution characterized by negative values of β at higher kinetic
energies.19, 20 This is, in fact, what is observed. In the case of low kinetic energy photoelectrons,
such as those found in Figs. 5.2 and 5.3, an isotropic angular distribution of the origin transition isobserved. When utilizing a photon energy of 3.494 eV (Fig. 5.1), giving rise to photoelectron
kinetic energies higher than in the PE spectra found in Figs. 5.2 and 5.3, an anisotropic angular
distribution of the origin transition was observed, characterized by β ~ –0.25.

205

§ 5.6 Results: Vibrational Analysis of Indolyl
Considering the large number of vibrational modes predicted to be activated upon
photodetachment of indolide, several spectra were obtained at three different photon energies in
an attempt to obtain higher resolution spectra of these vibrational transitions. See Fig. 5.3.
Experimental data are shown in black, while theoretical modeling is shown as red sticks andgreen curves. Several observations are immediately apparent. First, in all three spectra there
appears to be large photoelectron signal at near zero eKE (the highest reported eBE in each case).
This arises from the high resolution of the instrument at very low eKE (high eBE) combined withmany transitions accessible near the photodetachment threshold for all three photon energies
employed. Second, the intensity ratios of all peaks relative to the origin transition appear to bequalitatively different from the same ratios in Fig. 5.1, where the photon energy utilized washigher, 3.494 eV. This is evidence of a photon energy dependence in the PE spectrum ofindolide. This is discussed in section 5.7. In addition to these phenomena, there are several sharp
peaks identified in the figure, and a vibrational analysis may be performed as follows.

206

Fig. 5.3 Photoelectron spectra of indolide obtained with three separate photon energies, 2.518 eV(upper panel), 2.588 eV (middle panel), and 2.678 eV (lower panel). This demonstrates the highresolution of the VMI spectrometer at low electron kinetic energies, and takes advantage of thisfact. Using this feature, several peaks may be identified and assigned to specific transitions. The
sharp rise at near-zero kinetic energy (the highest reported eBE in each panel) is due to thehighest resolution of the instrument being within 30 meV of the photon energy. See the text, andTable 5.2 for more information.
207

The vibrational analysis begins with the spectrum on the top panel of Fig. 5.3 with two
peaks labelled A and B. The peak centers are located energetically at 521 ± 15 and 577 ± 15 cm-1,
respectively, above the origin peak center. The simulation shown in green overlaid on the datashows one transition with significant intensity in this region, the 2510 transition, with a transitionenergy calculated as 587 cm-1 relative to the origin transition. Hence, peak B may be assigned to
the 2510 transition, with a measured transition energy of 577 ± 15 cm-1. Peak A remains
unassigned, though it is possible to suggest either 2610 or 2710 , based on symmetry and thepredicted harmonic frequencies of these vibrational modes. The middle panel of Fig. 5.3 shows
four additional peaks labelled C–F, with peak centers located at 722 ± 15, 870 ± 15, 1071 ± 15,1139 ± 15 cm-1, respectively, relative to the origin transition. Several transitions overlap the
spectral region containing peak C, however, only one has the proper symmetry and an appreciable
FCF, 2410 . Hence this is the transition assigned to give rise to peak C with a measured transitionenergy, relative to the origin transition, of 2410 = 720 ± 50 cm-1. Peak D is predicted to primarily
arise from two separate transitions, 2210 and 2110 . Considering the proximity of the predictedharmonic vibrational frequencies of these normal modes, the transitions cannot be individually
measured. In addition, we cannot verify the presence of both transitions independently, and soeither one or both may be contributing to this peak. Hence the transitions are measured to be2210 𝑎𝑛𝑑/𝑜𝑟 2110 = 870 ± 80 cm-1. Similarly, peaks E and F, located respectively at 1071 ± 15
and 1139 ± 15 cm-1 relative to the origin transition peak arise from at least two of the threedifferent transitions with significant predicted FCFs in this region of the photoelectron spectrum,
1610 , 1810 , and 1910 . The bottom panel of Fig. 5.3 identifies peaks G and H, located 1314 ± 15 and1407 ± 15 cm-1 higher in binding energy than the origin peak. Peak G arises primarily from the1210 transition, and thus a transition energy may be determined, 1310 ± 35 cm-1. Peak H is208

predicted to be dominated by two separate transitions with calculated harmonic frequencies
inseparably close to each other, 1110 and 1010 . Similar to the situation for peak D, there is noexperimental way to prove the contribution of either transition to the peak H line shape, though
the 1110 transition is theoretically predicted to have a FCF ~3.4 times larger than the 1010
transition. Thus the transitions associated with Peak H are assigned as 1110 𝑎𝑛𝑑/𝑜𝑟 1010 = 1400 ±70 cm-1. These results are summarized in Table 5.2.
All of the above assigned vibrational transitions are associated with vibrational normal
modes which are of 𝐴′ symmetry and can be described as ring distortion motions. The
eigenvectors associated with these normal modes may be found in Table 5.3.

209

Table 5.2 Summary of Experimental Vibrational Assignments
Peak
A
B
C
D
E
F
G

Position Relative to 000 (cm-1)
521 ± 15577 ± 15722 ± 15870 ± 151071 ± 151139 ± 151314 ± 15

H1407 ± 15

Assignment (cm-1)
2510 = 580 ± 252410 = 720 ± 50
1
220 and/or 2110 = 870 ± 80
1210 = 1310 ± 35
1
110 and/or 1010 = 1400 ±70

Vibrational analysis of the indolide photoelectron spectrum found in Fig. 5.3. Some assignments
could not be made to a satisfactory level of confidence. Several suggested assignments arepresented in the main text for these unassigned peaks.

210

Table 5.3 The following diagrams show the vibrational normal modes with associated modelabels and calculated frequencies discussed in the main text for indolyl in the ground electronic
state. Each eigenvector shown here is that of the neutral radical, and was calculated at theB3LYP/aug-cc-pVTZ level of theory.

Indolyl Radical (modes of 𝐴′ symmetry)ν1 (3221 cm-1)

ν2 (3196 cm-1)

ν3 (3193 cm-1)

ν4 (3189 cm-1)

211

ν5 (3175 cm-1)

ν6 (3167 cm-1)

ν7 (1629 cm-1)

ν8 (1602 cm-1)

212

ν9 (1495 cm-1)

ν10 (1466 cm-1)

ν11 (1460 cm-1)

ν12 (1373 cm-1)

213

ν14 (1303 cm-1)
ν13 (1346 cm )-1

ν16 (1202 cm-1)

ν15 (1233 cm-1)

214

ν18 (1162 cm-1)

ν17 (1169 cm-1)

ν20 (1026 cm-1)

ν19 (1096 cm-1)

215

ν22 (899 cm-1)
ν21 (966 cm-1)

ν24 (770 cm-1)
ν23 (854 cm-1)

216

ν26 (545 cm-1)
ν25 (587 cm-1)

ν27 (408 cm-1)

217

Indolyl Radical (modes of 𝐴" symmetry)ν28 (997 cm-1)

ν29 (963 cm-1)

ν30 (922 cm-1)

ν31 (886 cm-1)

218

ν33 (774 cm-1)

ν32 (783 cm )-1

ν35 (575 cm-1)

ν34 (744 cm-1)

219

ν37 (412 cm-1)

ν36 (543 cm-1)

ν38 (241 cm-1)

ν36 (202 cm-1)

220

§ 5.7 Discussion
The EA of indolyl was measured to be 2.4315 ± 0.0017 eV. This value may be compared
to previous studies of indolyl in the literature. The EA of the indolyl radical has been measured
twice previously.21, 22 In the case of Taft et al.21 this is accomplished by means of applying athermochemical cycle to their proton transfer equilibria data, and thus the EA was determined tobe 2.52 ± 0.20 eV. This value was later updated in 2010 by McKay et al.22, utilizing anion
photoelectron spectroscopy to find an EA of 2.31 ± 0.15 eV via photoelectron spectroscopy. This
present work is in agreement with these two past studies and greatly improves the accuracy of themeasurement of the EA of the indolyl radical.
Several structural analogs of indolyl may also be examined and compared to the measured
EA of indolyl: pyrrolyl, phenyl, cyclopentadienyl, and cyclopentyl radical.17, 23, 24 One may thinkof pyrrolide as an analog of indolide, while phenide or cyclopentadienide may represent indole
deprotonated at a carbon site on the molecule. The pyrrolyl radical was shown to have an EA of
2.145 ± 0.010 eV, while the EAs of phenyl and cyclopentadienyl are 1.096 ± 0.006 eV and 1.808± 0.006 eV, respectively.17, 23, 24 Both analogs of carbon site deprotonation display a lower EA
than pyrrolyl radical, wherein the excess charge on the anion is concentrated about the nitrogenatom. This is consistent with the EA of indolide, 2.4315 ± 0.0017 eV and demonstrates that the
EA of pyrrolyl radical is not greatly perturbed by the addition of a fused aromatic ring to thestructure, resulting in indolyl. This is expected; the pyrrolyl ring is aromatic and the addition ofanother aromatic ring, which is neither strongly electron donating or withdrawing, to the
conjugated π system will not significantly disrupt the qualitative aspects of the aromatic electronic
structure.

221

Upon electron photodetachment from indolide, several vibrational modes are excited; theeigenvectors associated with these vibrational modes may be found in the supplementary material.
All of the modes that are activated upon photodetachment share two characteristics. First they are
all of 𝐴′ symmetry, and so preserve the only plane of symmetry of the molecular point group, Cs,
which is also the plane of the molecule itself. Second, all of the observed vibrational modes
describe molecular motion that can be characterized as ring distortions. This behavior has been
observed in aromatic anion PE spectroscopy previously.17, 18, 23, 25 Both of these observations are
intuitively understandable. Indolide will have the excess charge density primarily localized about
the nitrogen group. This increased charge density will cause the bonds along the fused aromaticring structure to lengthen from an idealized pyrrole–like structure, while preserving the Cs
symmetry. Upon photodetachment, the new equilibrium geometry will be closer to that of pyrrole,causing vibrational motion to be activated which incorporates displacements along the bonds
which make up the fused aromatic ring, resulting in both vibrations which do not break thesymmetry of the molecule and which might be described as ring distortion motion.
The intensities of peaks seen in the photoelectron spectrum of indolide change as afunction of photon energy, as can be observed by comparison of the PE spectrum displayed inFig. 5.1, obtained with a photon energy of 3.494 eV, with the PE spectra displayed in Fig. 5.3,
that were obtained with several photon energies ~ 1 eV lower than 3.494 eV. For example, in thebottom panel of Fig. 5.3 one may note that the ratio of the intensity of peak H to the origin
transition peak is different from the same ratio in Fig. 5.1. This ratio in Fig. 5.3 is far lower thanwhat is found in Fig. 5.1. This variability in peak intensity ratios cannot be explained bythreshold effects. This variation must be due to electron autodetachment in competition withdirect photodetachment, a behavior that has been observed in other aromatic systems.23, 25, 26 This222

conclusion is borne out in excited state quantum chemical calculations (TD–DFT) of the indolide
anion which predict 9 optically accessible electronic states with term energies ranging from 2.5 –3.6 eV above the ground vibronic state energy. Due to the density of the available states and theinherent complexity of autodetachment, it is beyond the scope of this work to further investigate
this phenomenon theoretically.
The thermodynamic implications of the improved measurement of the EA of indolyl
measured in this work are now considered. By means of a thermochemical cycle,1 we may
combine the measured EA value found in this work with the previous measurement of the acidity(∆𝑎𝑐𝑖𝑑 Ho298 K) of indole. Meot–Ner et al.27 previously reported the acidity as ∆𝑎𝑐𝑖𝑑 Ho600 K (N–H) =352 ± 2 kcal/mol. This enthalpy may be converted to the more conventional enthalpy at 298 K bymeans of the calculated constant–pressure heat capacities of the species involved in the standard
acid reaction: HA(g) → H+(g) + A–(g).
𝑇∆𝑎𝑐𝑖𝑑 𝐻𝑇𝑜𝑓 (HA) = ∆𝑎𝑐𝑖𝑑 𝐻𝑇𝑜𝑖 (HA) + ∫𝑇 𝑓[𝐶𝑝 (A− ) + 𝐶𝑝 (𝐻+ ) − 𝐶𝑝 (HA)] 𝑑𝑇𝑖(2)Where Ti and Tf are the initial and final temperatures. Utilizing eqn. 2 allows for thedetermination of the temperature adjusted previous measurement of the acidity, ∆𝑎𝑐𝑖𝑑 Ho298 K(N-H)= 351 ± 2.25 kcal/mol. Thus, one may combine the EA of indolyl, measured in this work, withthe temperature corrected gas phase acidity of indole (Meot–Ner et al.) to derive the bonddissociation energy of indole, D0(N–H) = 93.3 ± 2.5 kcal/mol. The value of D0(N–H) wasinvestigated recently by Nix et al.28 who reported an upper bound of 91.2 kcal/mol for this bondstrength. This value, coupled with the present measurement of the bond dissociation energy,
shows that D0(N–H) lies within the narrow range 90.8 ≤ D0(N–H) ≤ 91.2 kcal/mol.

223

§ 5.8 Deprotonated Indoline
As was stated in § 5.1, the case of deprotonated indoline proved less than revealing and assuch shall be discussed separately here.
Deprotonated indoline was generated in the same ion source detailed in § 5.2, changing
the main valve to expand a mixture of ~1% indoline in Ar with a backing pressure of 10 psig,
while the side valve was changed to discharge a gas mixture at 35 psig backing pressure with 2%
NF3 in Ar. This generates fluoride ions29 which are entrained in the main expansion, which canthen react with the neutral indoline.
The photoelectron spectrum presented in Fig. 5.4 is obtained with the laser radiation froma dye laser making use of the LDS 698 dye, pumped by the 2nd harmonic (2.330 eV) of the
Nd:YAG laser.
Prior to examining the data, it is useful to hypothesize the likely behaviors one might
expect to observe upon photodetachment of deprotonated indoline. In the case of N–H
deprotonated indoline, there is little guidance from previously studied anions. It is possible to
make several predictions based on the electronic structure calculations performed here. Assuming
the deprotonation occurs at the N–H site, the EA of the corresponding radical was calculated to be
1.291 eV. Alternatively, if deprotonation occurs at one of the sp3 hybridized carbon centers, thenthe EA of the corresponding radical was calculated to be considerably lower, < 0.5 eV. For
comparison, the EA of the cyclopentyl radical was measured through a thermochemical cycle andshown to be a negative value.24
There are fewer conclusions that may be drawn from the photoelectron spectrum of
deprotonated indoline shown in Fig. 5.4. The experimental data are shown in black, while the224simulation of the molecule shown in the figure is represented by both the green curve and the red
sticks. This PE spectrum is qualitatively different from the simulation of the molecule displayedin the figure. Regardless, several observations and conclusions may be made. First, the calculated
EA for the N–H deprotonated indoline is 1.291 eV and is in reasonable agreement with the data
which displays the PE spectrum commencing at ~1.3 eV. Second, deprotonated indoline has amass of 118 amu, and the ratio of the ion intensity of m/z =119 to m/z = 118 in the mass spectrum
was 8%. Considering the abundance of 13C (~1%), this confirms that the PE spectrum obtained
from the m/z = 118 products contain 8 carbon atoms, and thus is a product of indoline. Third, thePE angular distribution of deprotonated indoline was determined to be characterized by a slightly
positive value of β ~ 0.1. However, considering the fact that we suspect multiple isomers to becontributing to the PE spectrum shown in Fig. 5.4, the physical meaning of this anisotropy is notinterpretable. Finally, several ring–opened structures were calculated and were shown to haveplausible values of electron affinities, 1.2 – 1.45 eV, while deprotonation from the sp3 hybridized
carbon atoms resulted in EA values < 0.5 eV and can thus be ruled out as contributing to thisspectrum. It is important to note that, although it is likely that N–H deprotonated indoline isformed in the experiment, it is possible that the observed photoelectron spectrum derives solely
from ring-opened structures and/or isomers of C8H8N–. This is due to our inability to resolve
individual transitions in the spectrum.

225

Fig. 5.4 The photoelectron spectrum of the anionic product(s) of F– + indoline with a mass tocharge ratio of 118. This spectrum was obtained with a photon energy of 1.759 eV. Note that at abinding energy of 1.4 eV, with this photon energy, the VMI spectrometer has a spectral resolutionof 11 meV. The simulation shown in green is that of the anion shown in the figure. Moreinformation may be found in the text.

226

Attempts were made to simulate the harmonic FC PE spectra of the ring-opened
structures; however, this did not offer any new insights as to the interpretation of the data.
Considering the available bonds within the fused rings of the molecule, a N–C bond is the most
likely to be broken. The ring-opened structures all display large geometry changes upon
photodetachment, and thus result in two major obstacles with respect to simulating PE spectra.
First, large geometry changes imply that the PE spectrum will activate many vibrational normal
modes with a high probability of populating multiple quanta in these modes. This results in a PE
spectrum which is highly congested and experimentally unresolvable with a broad envelope.
Second, the simulations which we are able to perform are explicitly reliant on the fact that theharmonic oscillator approximation is appropriate for the system being simulated. The harmonic
oscillator approximation would not be at all appropriate for these ring-opened systems, as theresultant potential energy surfaces attributed to the alkyl chains would almost certainly be highly
anharmonic. Hence, very little of actual consequence may be concluded from a harmonic
simulation of the PE spectra. Performing anharmonic calculations or more sophisticatedsimulations would likely elucidate these data more clearly, but are beyond the scope of this work.
While no obvious analogs of N–H deprotonated indoline have been studied to date, thecyclopentyl radical may be considered an analog of indoline deprotonated at either of the sp3
hybridized carbon sites on the molecule. The cyclopentyl radical does not bind electrons, and so
one may safely hypothesize that the EA of the radical corresponding to indoline deprotonated at a
sp3 hybridized carbon site would be either quite low or negative. This is corroborated by theory
which predicts the EAs of these neutral deprotonated indoline isomers to be < 0.5 eV. Therefore,227

it is concluded that the PE spectrum displayed in Fig. 5.4 does not arise from indoline
deprotonated at a sp3 carbon site, considering that the spectrum shown originates at ~1.3 eV.

228§ 5.9 Conclusions
The anion photoelectron spectra of deprotonated indole and indoline have been collectedutilizing several photon energies in order to take advantage of the high resolution associated withslow electrons in this apparatus. The SEVI photoelectron spectrum of indolide show the EA ofindolyl to be 2.4315 ± 0.0017 eV. Indolide photodetachment activates numerous ring distortion
vibrations upon photodetachment, which have 𝐴′ symmetry. Previous work studying the gas
phase acidity of indole27 allows for a new independent measure of D0(N–H) = 93.3 ± 2.5
kcal/mol, which compares well with the previously measured bound on the bond dissociationenergy.28 Combining the conclusions of this experiment with the previously reported bound on
the dissociation energy implies that the value of D0(N–H) falls within the range: 90.8 ≤ D0(N–H)≤ 91.2 kcal/mol. The photoelectron spectrum of deprotonated indoline exhibited far more spectralcongestion than was anticipated, apparently a result of the presence of multiple isomers of
deprotonated indoline, although specific identification was not possible. The relative intensitiesof vibrations excited in the photoelectron spectrum of indolide displayed a strong photon energy
dependence, the result of electron autodetachment.

229

§ 5.10 References
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5) B. Dick, Phys. Chem. Chem. Phys. 16, 570-580 (2014).
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9) D. M. Neumark, J. Phys. Chem. A 112, 13287-13301 (2008).
10) M. J. Travers, D. C. Cowles, E. P. Clifford, G. B. Ellison and P. C. Engelking, J. Chem. Phys.
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11) M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C.
Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Farkas, J. B. Foresman, J. V. Ortiz, J.
Cioslowski and D. J. Fox, Gaussian 09, Revision B.01, 2009, see http://gaussian.com/
12) A. D. Becke, J. Chem. Phys. 98, 5648-5652 (1993).
230

13) T. E. Sharp and H. M. Rosenstock, J. Chem. Phys. 41, 3453-3463 (1964).
14) E. B. Wilson, Jr., Phys. Rev. 45, 706-714 (1934).
15) P. Chen, Unimolecular and Bimolecular Reactions Dynamics (John Wiley & Sons,
Chichester, 1994) 371.
16) K. M. Ervin, PESCAL, Fortran Program for Franck-Condon Analysis of Molecular
Photoelectron Spectra, 2010, see http://wolfweb.unr.edu/~ervin/pes/17) A. J. Gianola, T. Ichino, R. L. Hoenigman, S. Kato, V. M. Bierbaum and W. C. Lineberger, J.
Phys. Chem. A 108, 10326-10335 (2004).
18) T. Ichino, S. W. Wren, K. M. Vogelhuber, A. J. Gianola, W. C. Lineberger and J. F. Stanton,
J. Chem. Phys. 129, 084310 (2008).
19) J. Cooper and R. N. Zare, J. Chem. Phys. 48, 942-943 (1968).
20) A. Sanov, Annu. Rev. Phys. Chem. 65, 341-363 (2014).
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Sci. Instrum. 81, 123101 (2010).
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24) C. H. DePuy, S. Gronert, S. E. Barlow, V. M. Bierbaum and R. Damrauer, J. Amer. Chem.
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232

CHAPTER VI
STATE–RESOLVED VELOCITY MAP IMAGING OF SURFACE–SCATTERED
MOLECULAR FLUXPortions of this chapter have been published in the journal Physical Chemistry Chemical Physics(PCCP) under the same title by, J. R. Roscioli, D. J. Bell, D. J. Nelson, and D. J. Nesbitt§ 6.1 Introduction and Background
Unlike the preceding chapters in this dissertation, this final chapter presents an entirely
different study focusing on dynamical interactions of molecules with surfaces and does not
utilize photoelectron spectroscopy at all. As such, more detail and background shall be given to
introduce new concepts before detailing the results and conclusions of the study.
The field of surface chemistry has experienced a surge of research in recent years, withparticular focus on scattering,1-10 surface composition,11, 12 dynamics13, 14 and reactivity.15-17 The
primary goal of these activities has been to develop, with ever increasing clarity, a coherent
picture of the inelastic and reactive events which govern much of the heterogeneous chemistrythat we observe in nature. These processes are ubiquitous, finding relevance in ice crystal
formation and dust grain chemistry in deep space,18-20 gas-aerosol21, 22 and gas-liquid6, 7interactions of atmospheric interest, as well as metal-surface chemistry such asoxidation/reduction and energy transfer at gold, silver, and other metal interfaces.4, 23 In light ofthis importance, a variety of techniques have been employed to explore the physical nature ofreactions at interfaces. Reflected-angle infrared spectroscopy (RAIRS),24 sum frequency
generation (SFG),11, 25 and surface photoelectron spectroscopy,26 for example, have served ashighly selective probes of local interfacial structure and surface interactions. Time-of-flightmass spectrometry,5, 13 quantum-state resolved spectroscopic methods,1-4, 9, 17, 27 and pressure-233

based sticking coefficient measurements,8 have been successful at measuring the dynamics andinteractions that dominate gas-surface collisions.
These combined classes of experimental methods have yielded important insight into thecollision dynamics exhibited by a wide variety of atomic, non-polar (e.g. N2, CO2, etc.), andpolar (e.g. HCl, NO, etc.) projectiles, scattering from both insulating and metallic solids andliquids. From a broad perspective, experiments and simulations over the past 30 years haverevealed that there are at least two components to molecular flux following non-reactive
scattering – one which has come into thermal equilibrium with the surface before desorbing(trapping or thermal-desorption, TD), and one which has inelastically scattered from the surface
in one or a few collisions (impulsive scattering, IS).1, 5, 27 The IS component is especiallyinteresting, as it retains a “memory” of the projectile’s incident energy content, impact geometry,and velocity distribution.28 These parameters map via the collision event into a set of final
quantum state and velocity distributions, with a transformation function that depends sensitively
upon a variety of surface properties, such as roughness, local surface mass, temperature,electronic structure, and gas-surface interaction potentials. The experimental ability to control a
projectile’s incident parameters via supersonic molecular beams and chopped effusive sources,
along with the ability to precisely measure much of the final energy and velocity distribution of
the scattered flux, has shed light upon the nature of the transformation function and the dominantfactors that govern it.
Experiments have built upon the TD/IS model framework to elucidate details about
particularly chemically relevant surface interactions. For example, Kay and coworkers used
King and Wells-based trapping probability measurements combined with incident angle
variations to uncover information on novel binding mechanisms at the water-vapor/water-ice

234interface.8 Likewise, two-laser pump-probe methods have been combined with state-resolved
resonance-enhanced multiphoton ionization (REMPI) measurements by Huang et. al., to reveal
highly non-adiabatic energy transfer events by which vibrationally excited NO relaxes when
scattering from a metal surface (Au(111)), but not from an insulating surface (LiF).4 Molecularbeam chopping schemes have been used in combination with mass spectrometry in the work of
Nathanson et. al. and Saecker et. al. to explore gas-liquid scattering dynamics that reveal
possible binding and proton exchange mechanisms at the interface.5, 6
While these studies and many others have provided a strong methodological foundation
for future experimental research as well as established directions for theoretical exploration,much work still lies ahead in the surface-scattering field. For example, the correlation between
internal quantum states and translational degrees of freedom in the scattered flux is challenging
to obtain and generally not well understood, although recent state-resolved Dopplerimetry
studies of gas-liquid collisions have made crucial first steps toward this goal.29 Additional key
insights should in principle be available from experiments which combine the sensitivity ofmass-resolved time-of-flight methods with the detailed dynamical information from quantumstate resolved techniques. In this context, velocity map imaging (VMI) represents a recent andpowerful advancement in measurement of 2D and 3D velocity distributions, which has helped
establish new paradigms in a wide arena of chemical processes including photoionization,
photolysis, and photodetachment.30, 31 A critical feature of the VMI technique is the use ofelectrostatic lenses that both accelerate and focus a spatially extended group of ions to create ahigh resolution velocity map. Most importantly, each pixel in this map corresponds to aparticular velocity vector projected onto the image plane, independent of spatial origin for theionization process. Consequently, velocity map imaging methods offer access to a powerful new235tool in surface scattering experiments, based on a full accounting of translational and internal
quantum state energy content before and after a collision event.
The central focus of this study is to introduce such an experimental tool, which combinesi) supersonic molecular beams, ii) state-resolved resonance enhanced multiphoton ionization, andiii) velocity-map imaging, and in the process offers a powerful new glimpse into the quantum
state resolved dynamics of surface scattering events. As an initial test system, we apply thesemethods to quantum state resolved HCl scattering from Au(111) supported on atomically flat
mica. In particular, these VMI methods reveal that the incident hyperthermal HCl clearly
exhibits branching between trapping-desorption (TD) and impulsive scattering (IS) trajectories,
which are now easily resolved and cleanly identified in the velocity maps. This resolution invelocity space allows for unambiguous analysis of the IS component without a model-dependent
deconvolution from a TD fraction. Coupled with the quantum-state resolution inherent to
REMPI-based ionization,9, 32, 33 analysis of the images probes correlations between rotational andtranslational distributions of surface-scattered molecules, and thereby yielding fundamentallynew dynamical insights into the scattering of this prototypical HCl/Au(111) system.

236§ 6.2 Experimental Setup and Apparatus
The apparatus used in this study is a modified velocity-map imager, as shown in Fig. 6.1,
with important changes in both the repeller plate electrode as well as the molecular beamorientation. The important issues in achieving such a capability are a) placing a surface within
the imager, b) probing the scattered molecules such that we observe the full velocity distribution,c) incorporating temperature control of the surface and d) designing the imager to minimize
electrostatic charging of its insulating components.

237

hREMPI
Au

To
To TP
TP

Fig. 6.1 Experimental setup of the velocity-map imager (VMI) adapted to probe surface
scattering dynamics. A pulsed supersonic jet of 1% HCl in 150 Torr of H2 impinges upon an
Au(111) surface at an incident angle of 75º (relative to the surface normal) after skimming. The
surface is embedded in a 75 x 25 x 6 mm copper heater mount which also serves as a repeller
plate as shown in the right inset. The VMI is mounted within a grounded flight tube at 5 x 10-9Torr (maintained by two 250 L/s turbomolecular pumps, TP). Ions impact a double-layer MCP
coupled to a phosphor screen that is monitored by a 690 kpixel CCD and a photomultiplier tube.
The flight tube resides in an outer chamber (base pressure 5 x 10-8 Torr) pumped by a 1400 L/sturbomolecular pump. The UV REMPI laser is introduced perpendicular to the molecular beam
within < 0.5 mm of the gold surface (right inset).
238

Many possible layouts could allow one to introduce a molecular beam to a surface within
a VMI, but of utmost importance is that the electric fields which define the focusing potential
between the repeller plate and first lens are unaffected by the presence of the surface. This
restriction precludes the possibility of placing the interface at some angle between the electrodes,
as this would lead to distorted velocity maps. Instead, the surface is embedded in a heater mountwhich also serves as the repeller plate (as shown in Fig. 6.1) of the imager. The typical repeller
plate of the VMI is therefore replaced with a free-standing, gold-plated copper structure
containing a groove in which the sample rests. The mount is heated by an embedded cylindrical
cartridge heater, capable of heating the surface to 600 K. The aluminum electrostatic lenses of
the VMI are 1.5 mm thick and 70 mm in diameter. The distance between the repeller plate (i.e.,the heater mount) and the first lens is 15 mm. Typical voltages applied to the repeller and firstlens are +2000 V and +1600 V, respectively.
The surface used in this experiment is a 100 nm thick layer of Au vapor-deposited on an
atomically-flat 20 x 20 mm mica substrate (Phasis, Inc.), with typical Au(111) domain sizes of
0.5 - 1 m2 in area, verified with Atomic Force Microscopy (AFM). The sample is held in placeusing two copper clamps at the ends of the surface. These scattering experiments are all
performed with the Au sample heated to 500 ± 5 K, where the temperature is measured using atype K thermocouple mounted on the copper heating block. In conjunction with the basepressure of ~ 5 x 10-9 Torr within the VMI flight tube and the heated substrate conditions, thegold remains clean over the course of a typical 6-hour data run, as confirmed by water contact
angle measurements taken before and after an experiment. Over the course of the 4 datasets thatcomprise the data presented here, 3 separate Au(111)/mica surfaces were used.

239

In order to create undistorted velocity maps, the repeller plate (i.e. the heater mount)cannot be tilted to allow access to a molecular beam. Instead, the molecular beam must be
introduced at an angle relative to the VMI axis. In this study, the HCl molecules impinge at anear-grazing 75º angle relative to the surface normal, thereby largely confining the incident
velocity vector component within the surface plane. The VMI is mounted on the bottom flangeof an 89 mm ID stainless steel flight tube, which is pumped using two 250 L/s turbomolecular
pumps to yield a base pressure of < 5 x 10-9 Torr. The flight tube has 3 small apertures in it, one(12 mm dia.) for the entrance of the pulsed molecular beam, and two (9.5 mm dia.) for the laser
entrance and exit. Two aluminum struts on the outside of the flight tube are used to mount thepulsed valve. A 2 mm diameter skimmer is displaced 40 mm from the orifice using four
aluminum posts attached to the pulsed valve face. The valve itself is a PZT design based uponthat of Proch and Trickle,34 with a typical pulse width of 250 s and orifice diameter of 0.5 mm.
The supersonic jet is formed from 150 Torr of a 1% mixture of HCl in H2. These conditions
produce beam kinetic energies of 21 ± 5 kcal/mol. The ionization laser fires within the first 90
s of the gas pulse, in order to eliminate contributions from thermalized background species due
to collisions in the chamber.
The detection scheme is based upon state-resolved 2 + 1 resonantly enhanced
multiphoton ionization (REMPI) of HCl.32, 33 To ensure a complete picture of the velocity
distribution, the REMPI ionization region must be intersected by all (or nearly all) possible
velocity vectors in the scattered flux. Two possible scenarios afford this condition: One is asmall gas-surface collision area probed by a much larger ionization region located directly overthe impact point. Alternatively, a large impact area can be probed by a very small ionization
region located close to the surface. Because the REMPI scheme is two-photon based (where the240third photon is a saturating ionization pulse), large electric fields and a tight laser focus are
critical to efficient ionization. The first scenario of a spatially extended ionization region would
reduce the ion yield too severely, thus rendering this configuration unfeasible. The second
scenario, however, is quite feasible, and in particular can be attained using a skimmed molecularbeam traveling 12.5 cm from the orifice before forming a ~17 (width, w) x 66 (length, l) mm
elliptical collision area on the gold surface. The UV REMPI laser passes within d = 370 m ofthe Au, where the suitable choice of𝑑𝑤

 0.03 ensures that the ionization pulse probes a nearlycomplete velocity distribution of the scattered molecular flux.
In this experimental layout, the ionization region is defined by the extent of the laser
focus rather than the spatial extent of the gas pulse. The importance of this is illustrated byconsidering what possible range of ionization loci within a VMI apparatus map onto points in theimaging plane reflective of the true velocity distribution. Only those ions created within an area
approximately 1/3 of the diameter of the first electrostatic lens aperture focus correctly at theimaging plane. Those ions generated outside of this area are mapped incorrectly to points on theimaging plane that are not representative of their velocity vectors. In the case of the VMI
dimensions used here, the region in the apparatus that produces a well-focused image is ~3 mmin diameter. As a consequence, the ionization region must be restricted to < 3 mm in length,
which is achieved by expanding the beam width from 2 mm to 19 mm in a Galilean telescope,
followed by tight focusing with a f = 40 cm lens. Spherical aberration at the focus is minimized
using plano-convex lenses with their convex sides facing the collimated part of the laser beam.
In the limit of Gaussian beam focusing without aberration or astigmatism, this setup produces afocal region that is ~350 m in length and ~7 m in radius. By moving the focus toward andaway from the center of the VMI and monitoring the velocity map of a room temperature241

distribution of HCl molecules, we find that the ionization region is ~1 mm in length. This isroughly three times larger than the expected value of 350 m, likely due to the non-Gaussiantransverse profile of the frequency-tripled dye laser beam.


For all rotational levels from J = 0 through 12, the tripled output of a Nd:YAG-pumped

pulsed dye laser operating at 10 Hz is used to probe the Q-branch of the V 1Σ+← X 1Σ+ REMPItransition, accessing the v’ = 12 level in the V 1Σ+ state near 237 nm.33 The list of line positions
is presented in Table 6.1. Initial attempts at observing vibrationally-excited HCl in the scattered
flux yielded no appreciable population within the signal-to-background limit of this experimental
setup. The laser power is varied using a set of reflective neutral density filters (Thorlabs, Inc.) toestablish that the ionization efficiency is indeed quadratic with respect to laser power. To
minimize space-charge effects as well as facilitate operation under single-ion counting
conditions, we adjust the laser intensity to yield on-resonant signals of < 30 ions/laser shot.
Typical laser powers used are thus dependent upon the rotational state being probed, i.e.  10
J/pulse for low J states (i.e. J = 0 - 3) with larger number densities, and up to  100 J/pulse forthe highest J states significantly populated (J = 10 - 12) and much lower densities. To averageover small but measurable Doppler shifts in the image, the laser is scanned by 0.009 nm (~ 1.6
cm-1) over the REMPI transition during image acquisition.

242

Table 6.1 Line positions and state assignments of (2+1) REMPI transitions. All transitions are V
1 +Σ ← X 1Σ+ Q-branch transitions, accessing the v’ = 12 vibronic level in the V 1Σ+ state.

J"
0
1
2
3
4
5
6
7
8
9
10
11
12

 (nm)
235.997236.033
236.104
236.213236.356236.533
236.751
237.000
237.290237.599237.935
238.287
238.641243

In order to reliably measure the rotational state populations of scattered HCl, the ionsproduced by the REMPI laser are counted using centroiding software, with the resulting ion
count divided by the square of the laser power and the stagnation pressure behind the pulsed
valve orifice (~150 Torr). A weakly power- and J-dependent effective linestrength, S(J), mustalso be taken into account. This is determined from velocity maps of a distribution of room
temperature HCl molecules for J = 0 - 12, which are obtained by delaying the laser pulse relative
to the gas pulse such that the ionization step occurs ~1 ms after the pulse valve fires. This
ensures that the distribution of HCl molecules which originated from a supersonic jet have hadtime to thermally equilibrate with the VMI drift tube walls at 300 K. The resulting HClrotational state populations can be least-squares fit to a room temperature Boltzmann
distribution, where the modest residual deviations are used to extract small but finite correction
factors due to multiple detection pathways. These factors are primarily a consequence of an
H+/Cl+/HCl+ photofragmentation branching ratio:

BR (36) I (36)
I (1)  I (35)  I (36)where BR(m) and I(m) refer to the branching ratio and the intensity measured in the mass
spectrum at mass m, respectively. Room-temperature data which takes into account themeasured BR(36) branching ratio (rather than using the empirical linestrength factor) exhibits thesame rotational state distribution, only with slightly more point-to-point scatter.
The velocity calibration is determined from a velocity map of the H+ (i.e. m/z = 1) branch
produced during the HCl REMPI process. The energetics of this process are well established,35
and produces several sharp rings in the velocity map. The calibration is confirmed by similarly
obtaining the velocity map of O2 photolysis at 224.999 nm,30 and found to be within 1% of thatobtained based upon the map of proton formation from HCl.
244

Ions produced by the REMPI step are detected on a double-microchannel plate (MCP,
chevron) coupled to a fast phosphor screen (41 ± 1 ns decay constant), which is in turnmonitored by a 690 kpixel CCD and a photomultiplier tube. Because the VMI also acts as amass spectrometer for any ion produced during the REMPI step, the MCP is gated over 100 ns
from 1300 V to 1700 V using a high-voltage pulse generator (DEI) to detect only m/z = 36 ions.
The ion impacts are counted on-the-fly using centroiding software (DaVis), and then analyzed
with home-built computer programs.
For each rotational state, a scattering distribution is obtained consisting of 1800 laser
shots. Because there is a small non-resonant ionization signal from HCl, off-resonantbackground images are also acquired and subtracted from the on-resonant images. The typicalacquisition time to obtain each image is ~3 minutes. The velocity map for every J-state probedin this study reflects the sum of 4 images taken on separate days. Due to the broad range inenergies sampled by the scattered flux result in a relatively sparse velocity map, which can thenbe improved by smoothing with a rolling Gaussian average described by a standard deviation of = 4 pixels.
As one particularly useful experimental detail, we have used acetal copolymer static
dissipative posts and spacers (Pomalux SD-A) within the VMI to reduce electrostatic charging
and stray field generation. This material resembles Teflon in strength and durability, but exhibits
moderate surface and bulk resistivities of 109 - 1011 /square and 109 - 1011 -cm, respectively.
This results in an effective resistance between accelerating plates of ~8 G.

245

§ 6.3 Collected Velocity Maps
Sample final J-state dependent velocity maps produced by a high kinetic energy, 21 ± 5
kcal/mol, beam (1% HCl balance H2) scattering from a clean Au(111)/mica supported surface arepresented in the right side of Fig. 6.2. The coordinate system used in analysis of these maps isshown schematically in Fig. 6.2(a) and is defined as follows. The scattering plane is represented
by the trajectory of a molecule traveling along the cylindrical axis of the incident molecularbeam and undergoing perfect specular reflection at the surface. The velocity component parallel
to both the scattering plane and the surface is designated vin-plane, while vout-of-plane is thecomponent perpendicular to the scattering plane and also parallel to the surface. The remaining
velocity vector, vz, is directed along the surface normal. The incident (non-scattered) molecularbeam is present in all of the raw data as a very sharp, narrow feature near vin-plane ~ 2100 m/s andvout-of-plane ~ 0 m/s. Because this feature carries no information about the scattering event, we fit
it to a sum of 2D Gaussian functions and subtract from the raw data to produce images without
incident beam contamination. We note that this beam subtraction protocol is quite generally
applicable to a wide range of scattering energies, but in the present case is substantially aided bycomplete resolution of the incident beam feature in the velocity map. The 2-D Gaussianfunctions used to describe the incident beam are fit simultaneously with the scattered data to thecomplete set of J = 2 - 12 velocity maps, with overall amplitude and shifts in the central location
allowed to vary from state to state. Incident beam contamination in the J = 0 and 1 maps are also
fit and removed with a similar sum of 2D Gaussians. However, due to complications as a resultof much higher incident beam intensities in these J states, this is performed independently fromthe J = 2 - 12 data set.

246

(c)
(a)

J=11

vz
vin-plane
vout-of-plane

J=7
(b)

J=3

Fig. 6.2 Velocity map images of scattered HCl from a 1% HCl/H2 incident supersonic jet with
Einc = 21(5) kcal/mol. The coordinate system used for analysis is shown in panel (a), where thescattering plane contains the incident and specular-scattering velocity vectors. (b) Velocity mapof HCl integrated over all rotational states, revealing i) a thermal desorption feature near vin-plane= vout-of-plane = 0 m/s, and ii) an impulsive scattering component near vin-plane ~ 1500 m/s. (c)State-resolved velocity maps for HCl molecules in J = 3, 7, and 11 reveal both TD and IS
components, with the position of the IS component changing systematically with J.

247

Such an analysis permits a variety of different visual representations of the velocity map
data. For example, an integrated velocity map, which simply reflects the sum over all quantum
states if the HCl ionization step were not rotationally resolved, is shown in Fig. 6.2(b). Thisvelocity map clearly reveals a bimodal distribution, with one component peaking around thevelocity origin (vin-plane  vout-of-plane  0) and an additional strongly forward scattering featurenear vin-plane  1500 m/s. Exploration of the quantum state-resolved velocity maps, a few samplesof which are shown in Fig. 6.2(c), reveal that the low-energy feature remains relatively
unchanged between J-states, while the higher velocity component is strongly dependent uponrotational energy content. One interpretation of these sets of velocity maps would be in supportof microscopic branching, with the low energy peak centered around the velocity origin due totrapping desorption (TD) and the forward-scattered peak due to impulsive scattering (IS) events.
Such dependence highlights a major advantage of the REMPI/VMI technique, i.e. providing firstglimpses of correlated quantum state and final velocity distributions in the scattering dynamics.

248

§ 6.4 Velocity Map Analysis
Projections of the velocity map images onto the vin-plane- and vout-of-plane- axes permit amuch more detailed view of the velocity distributions parallel and perpendicular to the scattering
plane. The profile along the vin-plane direction for molecules in J = 2 - 11, shown in Fig. 6.3(a), is
obtained by integrating over vout-of-plane at each value of vin-plane. The profiles have beennormalized in order to facilitate a qualitative comparison. Immediately apparent is anasymmetry which favors the forward-scattering (vin-plane > 0) direction, signifying the presence ofa non-thermal, impulsive scattering component in the flux. Interestingly, some of the forwardscattered molecules access in-plane velocities up to +2000 m/s, implying that they retain most of
their initial momentum vector. In addition, flux in the backward-scattering region of the profile
extends to -1400 m/s, i.e., well outside of the thermal regime. For comparison, a Gaussianfunction associated with a 500 K Boltzmann distribution is shown as a dashed red line in Fig.
6.3(a). Specifically, 2.5  for a 500 K distribution corresponds to +/- 850 m/s, which fromGaussian statistics represent the velocity boundaries containing 98.8% of thermally-desorbing
flux. This indicates the important presence of impulsive backscattering trajectories, in spite of
the near-grazing incident angle of 75º. Together, these results suggest that impulsively scatteredmolecules span the remarkably broad in-plane velocity range of –1400 < vin-plane < +2000 m/s,
despite scattering from wide regions of a nominally atomically-flat Au(111) surface.

249

vout-of-plane(a)vin-plane

(c)

(b)

Fig. 6.3 (a) Integrated velocity distributions parallel to the scattering plane (along vin-plane) for
scattered HCl molecules in J = 2 - 12 originating from a 21 ± 5 kcal/mol incident beam. The
distribution is clearly forward scattered, with a J-state dependence in the high-vin-plane region. (b)The perpendicular component of scattered flux in J = 2 - 12 is symmetric about vout-of-plane = 0,
but has a hyperthermal component. Fits to the out-of-plane velocity profile of only thosemolecules with vin-plane > 850 m/s, shown in (c) for J = 2 - 12, yields a temperature of 1128 ± 34
K, illustrating that these impulsively-scattered molecules contribute to the hyperthermal
component of the profiles in (b). Red curves in (a) and (b) correspond to a Tsurf = 500 K
distribution for comparison. Top right inset illustrates the vin-plane and vout-of-plane coordinates.

250

The velocity distribution perpendicular to the scattering plane for molecules in J = 2 - 12
is obtained by integrating over all vin-plane for each value of vout-of-plane, and presented in Fig.
6.3(b). The profile in this dimension is symmetric about vout-of-plane = 0 m/s, as expected given
the symmetry axis of the experiment. This projection is nearly Gaussian, but its full-width at
half-maximum (FWHM) of over 1000 m/s for all J states reflects the presence of significantly
hyperthermal contributions (a 500 K Gaussian distribution is shown as a red dashed line). As asimple first model, we performed a two-temperature fit (sum of two Gaussian functions) to thedata, with one temperature fixed at 500 K, and found the second component to be 1582 ± 34 K.
Spatial isolation in a velocity map again provides a powerful means for probing the internal andtranslational energy content of the hyperthermal fraction, without deconvolution from a thermal
component. This capability is explicitly illustrated in Fig. 6.3(c), where the vout-of-plane
distribution is now obtained by integrating only over the forward-scattering region from vin-plane =850 - 3000 m/s (i.e. outside 99% of the 500 K thermal regime) for J = 2 - 12. The profiles arewell fit to a Gaussian lineshape, yielding an effective temperature of Tout-of-plane = 1128 ± 34 K.
Of particular interest in the data series of Fig. 6.3(a) is a reduction in the highest
translational energy component of the scattered HCl with increasing J state. Such a trend may be
the result of conservation of energy, where increased rotational excitation when scattering comesat the cost of translational motion. We can quantify this possibility further by fitting theimpulsively-scattered component of the dataset using a 3-dimensional model that, in sphericalcoordinates, is described by:

P (s, , )  A  (e( s s 0 ) 2

s2)  Cos p (   0 )  Cos q ( )where, s is the speed of the impulsively-scattered HCl, whose most probable value is s0 and has astandard deviation of s. The angular components of the flux are described by cosine251distributions in  and  that are raised to a power, with the distribution in  centered on a final
scattering angle 0. This three-dimensional model is then integrated in Cartesian coordinates
over the vz direction to create a 2-D velocity map. As a result of parameter correlation, 0 is held
fixed at the specular 75º angle, a constraint which has been shown to be justified in the case of
HCl scattering from MgO(100) and Au(111), though for less steep range of incidence angles (~0º – 45º).36, 37 The corresponding advantage of such a specular assumption is that it permits a 3D
velocity map to be characterized from 2D velocity data, and therefore a full accounting of energy
transfer in the HCl/Au(111) impulsive scattering dynamics. Based on this model, the kinetic
energy associated with the extracted speed is plotted as a function of J-state in Fig. 6.4(a) (redcircles), along with the rotational energy content (Erot = BHCl J (J+1), blue circles), and the sumof the two energy components (black squares). This plot reveals that the sum of the twocomponents is constant across J = 2 - 12, implying that the reduction in final translational energy
content is balanced by a corresponding increase in rotational energy content. Such a trend isconsistent with a dynamical constraint such as conservation of energy, and predicts that the
anticorrelated rotational excitation of the scattered HCl comes at the “cost” of translationalenergy during the scattering event. This result is consistent with previous experimental studies
of NO scattering from Ag(111),38, 39 where, within a range of Erot similar to that studied here (0 <Erot < 1600 cm-1), the translational energy content is similarly counterbalanced with Erot.
Interestingly, this anticorrelation breaks down in NO/Ag(111) at even higher rotational excitation(i.e., Erot > 1600 cm-1), ostensibly due to a roll off in efficiency of energy transfer to surface
phonon modes with increasing J. However, such an energy regime is higher than that studied
here.

252

5000
4500

(a)

4000

-1

Energy (cm )

3500
3000
2500
Erot

2000

<Etrans>(IS)1500

Erot + <Etrans>(IS) = <Etot>

1000
500
0
2

4

6

8

10

12

10

12

J
5000
4500

(b)

4000

-1

Energy (cm )

3500
3000
2500

Erot

2000

<Etin-plane>

1500<Eout-of-plane>
Erot + <Ein-plane>+2<Eout-of-plane> ~ <Etot>

1000
500
0
2

4

6

8

J

Fig. 6.4 Tracking the energy content in various degrees of freedom as a function of J-state. In(a), red circles correspond to total translational energy extracted from fits of the 2-D velocity
maps to a 3-D model as described in the text. The sum of this energy with the rotational energy(blue circles), shown as black squares, indicates that rotational excitation is occurring at the costof translational energy. In (b), deconstructing the translational energy into <Ein-plane> (redtriangles) and <Eout-of-plane> (red diamonds) reveals that most of the energy content is contained in
the in-plane coordinate. In the first-order picture where <Ez> ~ <Eout-of-plane>, the total energy
content (black squares) is very similar to that obtained using the 3-D model in (a).
253

One point worth noting from this 3D model analysis is that the predicted total energy
content of ~4300 cm-1 is roughly 40% less than the incident energy of ~7300 cm-1. This isqualitatively consistent with trends observed in scattering NO from Ag(111), where NO
molecules with low Erot were found to lose ~30% of their total energy to the surface at anincident angle of 60º (although we note the energy loss was found to decrease with inc).39 Bothof these observations differ somewhat from previous HCl/Au(111) studies,9 where thepredominant fraction of incident energy was reported to be largely retained (i.e., Etrans + Erot ~Einc) in the scattered HCl molecules. One possible explanation could be our current lack of direct
VMI information on the velocity distribution normal to the scattering plane (i.e. vz,), and acorresponding failure of our 3D model to quantitatively recapitulate this distribution. This canbe remedied by incorporating “DC slicing” methods in our VMI apparatus,40 a capability which
is currently under development.
We can rigorously measure the average in- and out-of-plane kinetic energies, <Ein-plane>and <Eout-of-plane>, for the impulsively-scattered HCl, and thereby gauge relative contributions ofthese two velocity components to the total energy content. Here, <Eout-of-plane> and <Ein-plane> arecalculated according to:

254

P

voutof  plane

Eoutof  plane  outof  plane Evoutof  planevPvoutof  planevoutof  planeP

vin  plane

Ein  plane  in  plane Evin  planevPvin  planevin  plane

To implement these calculations, Pvin  plane and Pvoutof  plane are required for the IS channel, which we
obtain from Gaussian fits to the integrated velocity component distributions in Fig. 6.3(a) and(b). Specifically, the vin-plane traces (Fig. 6.3(a)) are already partially resolved into IS and TD
components and thus are simply fit to a sum of two Gaussians, one to the TD component
centered at vin-plane = 0 and the other capturing the IS flux evident in the vicinity of vin-plane ~ 1500
m/s. For the out-of-plane velocity distributions (Fig. 6.3(b)), the traces are also fit to a sum of
two Gaussians, with both centered on vout-of-plane = 0 m/s. One of the Gaussians represents the TD
fraction at a fixed temperature of 500 K, while the other represents Pvoutof  plane , which is used in
the calculation of <Eout-of-plane>.
The corresponding results are plotted in Fig. 6.4(b). <Ein-plane> (red triangles) is thelargest energetic contributor to the IS component, containing 2500 ± 500 cm-1 of translationalenergy, while <Eout-of-plane> (red diamonds) is significantly smaller, ~500 cm-1. The larger energy
content in the scattering plane is consistent with forward scattering at grazing angles, while its
systematic decrease with J suggests that most of the rotational excitation is coming at the cost of255

the in-plane velocity component. On the other hand, the much smaller and narrower out-of-planekinetic energy content is consistent with a highly directional jet cooled incident beam scatteringfrom an atomically-flat Au surface. Once again, the sum of Erot + <Ein-plane> + <Eout-of-plane> isremarkably flat with respect to final J state. In particular, there is an anticorrelation between Erotand <Ein-plane>, which provides evidence for an energetic dynamical constraint between these twodegrees of freedom.
We can take this analysis one step further. Though <Enormal> is not measured directly inthe present studies, one would expect for such glancing incidence angles qualitatively similar
velocity distributions along the vz and vout-of-plane directions and therefore similarly modest energy
content. The corresponding J-dependent results for Etot in this approximation of <Eout-of-plane> ~<Ez> are shown in Fig. 4(b) (black squares). Interestingly, these results are very nearly identicalto those obtained from the 3-D velocity fit model, indicating that these two model treatments areat least self-consistent. However, these total energy values are still significantly short of theenergy content of the incident beam, which implies substantial energy transfer (40%) to thesurface. Indeed, to account for the entire incident beam energy, <Ez> would need to be 5-foldhigher (~3000 cm-1), i.e., nearly comparable to <Ein-plane>, which seems a surprising scenario
given the near-grazing incident angle. However, there is some indication of a trend in thisdirection. Studies by Sau et al41 and Brako et al42 for thermal rare gas atoms scattering fromAg(111) surfaces, for example, illustrate that the scattering can be sub-specular as the incident
angle approaches grazing and as the surface temperature is increased. Specifically, for scattering
Ar from Ag(111), it was found experimentally41 that the angular distributions peaked at ~55º for
an incident angle of 65º, consistent with energy transfer from incident beam into the vz
coordinate. Thus, though unlikely, we cannot rule out that scattered HCl at glancing incidence is256

gaining energy normal to the surface, giving rise to larger total energy content and therefore
consistent with a lower efficiency of energy transfer into the surface. To resolve this issue
further, DC slice experimental efforts with vz resolution are clearly needed that will allow for a
direct 3D velocity map reconstruction of the scattered HCl.40
The systematically anticorrelated reduction in <Ein-plane> with increasing Erot is especiallyinteresting and implies that an energy conservation rule connects these two degrees of freedom.
Previous studies of the HCl/Au(111) system at less grazing incident angles have shown that the
final rotational energy content after scattering from the surface depends primarily upon incident
translational energy content normal to the surface, (i.e. Ez,inc) rather than the total beam energy.
Based upon these trends, it was concluded that rotational excitation occurred primarily bycollisional exchange with the normal component of the incident energy. In contrast, we find herea very clear anticorrelation between the final values <Ein-plane> and <Erot>, with the majority of
the rotational excitation arising at a cost to vin-plane. One possible explanation is motivated by the
sub-specular scattering angles in Ar/Ag(111) studies,41 where at grazing angles the Ar atoms
gained energy in the vz coordinate. If <Ez> excitation occurs by redirecting energy from the<Ein-plane> coordinate, and such a process also induces rotational excitation, then the trend in Fig.
6.4(b) could in principle be observed. A second possibility is that at the grazing incident angles
explored in this study, the HCl may have a greater propensity to “skip along the surface” during
a nominally impulsive scattering interaction. Multiple consecutive collision dynamics could
yield increased probability for collisions with step edges between the Au(111) sample domains,thereby enhancing momentum exchange from vin-plane to vz and rotational excitation.

257

§ 6.5 Rotational State Distributions and Energy Maps
The rotational populations and velocity profiles extracted from the velocity maps provide
an initial assessment of how the internal energy is distributed in the scattered flux. Rotational
Boltzmann analyses of these high incident energy HCl molecules scattered from the 500 K
Au(111)/mica surface are presented in Fig. 6.5. The distribution extracted from the raw images(after correcting for power, stagnation pressure, degeneracy, and linestrength) is presented inlight grey, while the data for which the incident beam has been removed, as described above, areshown in black. First, we note that scattering of this high-energy HCl produces a slightly
rotationally hyperthermal distribution. Specifically, the data are consistent with single
temperature fit of Trot = 574 ± 12 K, which is clearly hotter than the Au surface. This is insupport of our previous conclusion from the velocity maps that the scattered flux exhibits morethan simple thermal desorption (TD) dynamics, for which the temperature might be expected tomatch the surface temperature. Interestingly, there is little evidence in the data in support of adual temperature TD + IS behavior, such as has been commonly observed in quantum state
resolved hyperthermal scattering from insulating liquids.1, 2 One reason for this could be that theAu(111) surface is already quite hot, making it more difficult to deconvolute from such a 2temperature Boltzmann analysis the relatively small differences between IS and TD components.
The power of the VMI method is that these TD and IS rotational distributions may neverthelessbe distinguishable by probing different locations in the velocity map, which will be addressed
below.

258

5

75º

HCl

4
3

2

ln(counts/power /Torr/S(J)/(2J+1))62574(12)K
1
0
-1
0

200

400

600

800

1000

1200

1400

1600

-1

Erot (cm )Fig. 6.5 Rotational distribution of molecules scattered from a 1% HCl/H2 beam at 21 ± 5
kcal/mol. Raw populations are shown in grey, whereas the distribution derived from images inwhich the incident beam has been subtracted is shown in black. A linear fit to the data (reddashed line) reveals a hyperthermal distribution, indicating the presence of a hyperthermal
component.

259

The complete quantum-state resolution of the REMPI process provides access torotational state populations that can be further used to correlate rotational and translationalenergy content. Specifically, we can create an intensity-weighted rotational energy “map” bycalculating <Erot> at each (vin-plane,vout-of-plane) pixel, with <Erot> determined from the expression:
12

I

vin  plane,voutof  plane, J

Erot (vin plane, voutof  plane)  J 0 12

I
J 0

 EJ

vin  plane,voutof  plane, Jand plotting it on the same axes as a normal velocity map (Fig. 6.6, center panel). Black areas in
the rotational energy map correspond to pixels where there are too few events to reliably define<Erot>, which we take to be less than one ion count in each J-state.

260

4

2

ln(counts/power /Torr/S(J)/(2J+1))

(b)

1

0

(c)

3
2
1

481(6) K

2

594(14) K

2ln(counts/power /Torr/S(J)/(2J+1))

3

-1

-2

0
-1
-2
-3

0

200

400

600

800

0

1000 1200 1400 1600

200

400

600

2

1000 1200 1400 1600

Erot (cm )(a)1ln(counts/power /Torr/S(J)/(2J+1))

800

-1

-1

Erot (cm )0-1

527(10) K
-2

-3

-4
0

200

400

600

800

1000 1200 1400 1600
-1Erot (cm )Fig. 6.6 Center: average rotational energy of a scattered HCl beam (Einc = 21 ± 5 kcal/mol) at
every value of vin-plane and vout-of-plane. Color scale is in units of cm-1. Black pixels contain too
few statistics to provide a reliable rotational energy (i.e., at least one ion count in each rotational
state). The rotational distribution is shown in (a) for the “TD” region of the map, while (b) and(c) show the distributions in different vin-plane regions of the IS-scattered component, where (b)corresponds to HCl with 850 < vin-plane < 1400 m/s, and (c) to vin-plane > 1400 m/s.
261

Visual inspection of the rotational energy map in Fig. 6.6 reveals qualitative differences
within different velocity regions. At vin-plane = vout-of-plane = 0 m/s, the map exhibits a rotationaltemperature of ~350 cm-1, which would be expected for thermally-desorbing molecules leaving
the surface at a temperature of ~500 K. In most of the remaining area of the map, the energy
content is hyperthermal, with rotational energy reaching equivalent temperatures of >720 K(>500 cm-1). This area of the map is the same region that exhibits translationally hot vin-plane andvout-of-plane components, confirming that flux in this region is hyperthermal in all observed degrees
of freedom. Of particular interest here is the reduction of <Erot> with increasing vin-plane, which isa consequence of the same effect that was manifested earlier in Fig. 6.4, where the highly
forward-scattered flux is sacrificing rotational excitation for translational energy content.
Interestingly, within the same region, this effect is not observed in the out-of-plane coordinate,
for which the rotational energy appears independent of vout-of-plane. This again supports theobservation that rotational excitation is correlated with loss of incident translational energy(mostly along vin-plane), rather than from interactions responsible for translational energy
perpendicular to the scattering plane.
The central image in Fig. 6.6 represents a compilation of the entire dataset into one
rotational energy map. However, the information in this map still contains the actual rotational
distribution as a function of vin-plane and vout-of-plane. Figs. 6.6(a), (b), and (c) illustrate the utility of
the VMI/REMPI technique to provide this additional and highly correlated information. In Fig.
6.6(a), for example, the subset of rotational data extracted from the nominally “TD” region isshown to exhibit a temperature of 527 ± 10 K. This is significantly cooler than the total velocity
map unresolved distribution (574 ± 12 K) and notably closer to that of the Au(111) surface (500
K). That this temperature is still slightly hyperthermal is not at all surprising, as there are IS-

262

type scattering events appearing in this region as well. Indeed, Fig. 6.6(b) illustrates that thesection of IS-type molecules with vin-plane closest to the TD region (i.e., 850 < vin-plane < 1400 m/s)
are now substantially hyperthermal with temperatures of 594 ± 14 K. However, it is interesting
to note that impulsive collisions do not all necessarily result in a hyperthermal distribution. Thisis explicitly shown in Fig. 6.6(c), where the subset of the most forward-scattered molecules (vinplane > 1400 m/s) is well characterized by a subthermal temperature of 481 ± 6 K.

Of course, thisobservation is also entirely consistent with the anticorrelation between Erot and Ein-plane evident in
Fig. 6.4(b), as well as a simple physical picture for conversion from incident translation into
scattered rotational energy.
As a final comment, although these velocity-map images represent initial steps toward amore complete picture of the impulsive scattering dynamics, further understanding will require
knowledge of the scattering distribution in the vz direction. This can be achieved using DCslicing methods developed by Suits and coworkers, where the ions are slowed before impact with
the MCP,40, 43 and gated to select those within a certain range of vz. Future experimental work
will include probing both non-reactive and reactive scattering trajectories from more complicatedsurfaces, such as self-assembled monolayers (SAMs),13 thin films, and low vapor pressureliquids.5 Of particular interest will be the possibility of reactions at -OH/-OD and -NH2/-ND2
terminated SAM interfaces, where a new glimpse into proton-exchange mechanisms at interfaces
may be revealed through the unique combination of full quantum state, H/D isotope, and 3D
velocity map distributions in the scattered flux.44263

§ 6.6 Conclusions
We have presented the results from a new velocity-map imaging instrument which yields
velocity and quantum-state distributions of molecules scattered from a surface. A cold HClbeam is created via a pulsed supersonic expansion, which impinges upon a 500 K Au(111)/mica
surface embedded in the repeller plate of a VMI. The REMPI-based ionization occurs within 0.5
mm of the surface, probing >95% of the velocity distribution of the scattered flux. The resulting
images clearly show forward-scattering trajectories that are both rotationally and translationally
hyperthermal, as well as back-scattering trajectories with velocity components significantly
outside the thermal 500 K regime. The ability of a velocity map to spatially separate thehyperthermal, impulsive scattering molecules from the thermally-desorbing component allows us
to analyze the IS distribution in isolation, as well as explore detailed correlations between
different degrees of freedom. In particular, we find that highly forward-scattered molecules arecharacterized by lower rotational energy content, but in a way that is balanced by acorresponding increase in translational energy. Such a trend is consistent with the presence of a
dynamical constraint reminiscent of energy conservation in the collision process, yielding a totalenergy independent of the final rotational J state. However, our best estimates of the resulting
total energies are 40% below that of the incident beam, which implies that simple energy
conservation is not the only factor, and that there is substantial energy transfer into the surface.
Indeed, the density of information provided by such a combination of i) molecular beam
scattering, ii) full quantum state resolution, and iii) velocity map imaging is much higher thanpreviously possible, and promises myriad new opportunities for exploring gas-surface, gas-liquidand gas-self assembled monolayer collision dynamics at unprecedented levels of detail.

264

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275) Y. Xie, H. Su and W. B. Tzeng, Chem. Phys. Lett. 394, 182-187 (2004).
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277) J. Yang, J. Li and Y. Mo, J. Chem. Phys. 125, 174313 (2006).
278) A. Yariv, Quantum Electronics (John Wiley & Sons, New York, 1988), 3rd edn.
279) A. M. Zolot, W. W. Harper, B. G. Perkins, P. J. Dagdigian and D. J. Nesbitt, J. Chem. Phys.
125, 021101 (2006).

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APPENDIXFORTRAN PROGRAMS FOR PHOTOELECTRON DATA ANALYSIS
Contained in this appendix are the annotated forms of the FORTRAN programs I wrote
to perform data analysis upon the photoelectron spectra collected in this dissertation. Hopefully,
these may be useful to others for similar purposes.

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PROGRAM VARIABLE_SMOOTHINGBY DAN NELSON, 5/5/2016
THIS PROGRAM TAKES A PHOTOELECTRON SPECTRUM
AND SMOOTHS IT TO SOME % OF THE EXPERIMENTALRESOLUTION, AND ASSUMES THAT THE RESOLUTION
IS A FUNCTION OF ELECTRON KINETIC ENERGY. INPUTDATA NEEDS TO BE IN ASCENDING ORDER.
MODIFIED 2/15/17 TO INCLUDE TERMINATING BOUNDARYCONDITIONSIMPLICIT DOUBLE PRECISION (A-H,O-Z)
DIMENSION EBE_IN(10000)
DIMENSION QINT_IN(10000)
DIMENSION EBE_OUT(10000)
DIMENSION QINT_OUT(10000)
OPEN(6,FILE='PES_SMOOTH_INPUT.TXT',STATUS='UNKNOWN')
OPEN(7,FILE='SMOOTHED_DATA.TXT',STATUS='UNKNOWN')
OPEN(8,FILE='SMOOTH_DEBUG.TXT',STATUS='UNKNOWN')
READ(6,*) SMOOTH,QM,QB,NPOINTSWRITE(8,*) SMOOTH,QM,QB,NPOINTSCZERO ARRAYS

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DO 11 I=1,10000
EBE_IN(I)=0.0D0
QINT_IN(I)=0.0D0
EBE_OUT(I)=0.0D0
QINT_OUT(I)=0.0D0
CONTINUE284

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DO 112 I=1,NPOINTSREAD(6,*) EBE_IN(I),QINT_IN(I)CONTINUEWRITE(8,*) EBE_IN(1),QINT_IN(1)WRITE(8,*) EBE_IN(NPOINTS),QINT_IN(NPOINTS)
DO 23 I=1,NPOINTSCOUNT=0.0D0
SUM=0.0D0
RES=(EBE_IN(I)*QM+QB)/2.0D0
IF(EBE_IN(I)-SMOOTH*RES.LE.EBE_IN(1)) THEN
DO 55 K=1,NPOINTSIF(EBE_IN(K).LE.(EBE_IN(I)+SMOOTH*RES))

THEN
*

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*

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IF(EBE_IN(K).GE.(EBE_IN(I)-*SMOOTH*RES).AND.EBE_IN(K).LE.(EBE_IN(I)+SMOOTH*RES)) THEN
SUM=SUM+QINT_IN(K)
COUNT=COUNT+1.0D0
WRITE(8,*) SUM, COUNTEND IF
END IFCONTINUE
END IF
IF(EBE_IN(I)-SMOOTH*RES.LE.
EBE_IN(1).OR.EBE_IN(I)+SMOOTH*RES.GE.EBE_IN(NPOINTS)) THEN
DO 34 J=1,NPOINTSIF(EBE_IN(J).GE.(EBE_IN(I)-SMOOTH*RES).AND.EBE_IN(J).LE.(EBE_IN(I)+SMOOTH*RES)) THEN
SUM=SUM+QINT_IN(J)COUNT=COUNT+1.0D0
END IFCONTINUE
END IF
DO 66 L=1,NPOINTSIF(EBE_IN(L).GE.(EBE_IN(I)-SMOOTH*RES))

THEN
*
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IF(EBE_IN(L).GE.(EBE_IN(I)-SMOOTH*RES).AND.EBE_IN(L).LE.(EBE_IN(I)+SMOOTH*RES)) THEN
SUM=SUM+QINT_IN(L)
COUNT=COUNT+1.0D0
WRITE(8,*) SUM,COUNTEND IF
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END IFCONTINUE
EBE_OUT(I)=EBE_IN(I)QINT_OUT(I)=SUM/COUNT
WRITE(8,*) EBE_OUT(I),QINT_OUT(I)CONTINUE78

DO 78 I=1,NPOINTSWRITE(7,*) EBE_OUT(I),QINT_OUT(I)CONTINUE66

CLOSE(6)
CLOSE(7)
CLOSE(8)END PROGRAM

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PROGRAM VARIABLE_FWHM_CONVOLUTION
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BY DANIEL NELSON, 3/25/2016
THIS PROGRAM READS IN A STICK SPECTRUM (LINE POSITIONS AND
INTENSITIES)AND THEN MAPS THEM ONTO A CONTINUOUS ELECTRON
BINDING ENERGY SPECTRUM WHILE CONVOLVING THEM WITHGAUSSIANS WHOSE FWHM VARIES LINEARLY
WITH EBE, AND WHOSE AREA EQUALS THE LINE INTENSITYIMPLICIT DOUBLE PRECISION (A-H,O-Z)

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DECLARE NEEDED ARRAYS AND CONSTANTSPARAMETER(PI=3.14159265359D0)
OUTPUT RESOLTION DETERMINES THE STEPSIZE OF THE OUTPUT
SPECTRUM IN MEVPARAMETER(OUTPUT_RESOLUTION=0.00025D0)
PARAMETER(EBE_RANGE=4.0D0)
PARAMETER(N_OUTPUT=EBE_RANGE/OUTPUT_RESOLUTION+1)
DIMENSION STICKCENTER(50000)
DIMENSION STICKCENTER_EKE(50000)
DIMENSION XLINE_INTENSITY(50000)
DIMENSION FWHM(50000)
DIMENSION CONVOLVED_INTENSITY(N_OUTPUT)DIMENSION EBE_OUTPUT(N_OUTPUT)
FIRST ZERO OUT THE ARRAYS
DO 111 I=1,50000

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STICKCENTER(I)=0.0D0
XLINE_INTENSITY(I)=0.0D0
FWHM(I)=0.0D0
STICKCENTER_EKE(I)=0.0D0
CONTINUE
DO 98 I=1,N_OUTPUTCONVOLVED_INTENSITY(I)=0.0D0
EBE_OUTPUT(I)=0.0D0
CONTINUECALCULATE OUTPUT EBE AXIS
DO 112 I=1,N_OUTPUT

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EBE_OUTPUT(I)=(I-1)*OUTPUT_RESOLUTIONCONTINUE
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NEXT OPEN AND READ IN STICK CENTERS, INTENSITIES,AND NUMBER OF TRANSITIONS AS WELL AS OUTPUT FILE
AND DIAGNOSTIC FILE
OPEN(6,FILE='VARIABLE_INFO.TXT',STATUS='UNKNOWN')
OPEN(7,FILE='CONVOLVED_OUTPUT.TXT',STATUS='UNKNOWN')
OPEN(8,FILE='CONVOLVED_DIAG.TXT',STATUS='UNKNOWN')FIRST READ IN HV AND M AND B
READ(6,*) QM,B,N_MAX
WRITE(8,*) QM,B
WRITE(8,*) EBE_OUTPUT(1),EBE_OUTPUT(N_OUTPUT)
WRITE(8,*) STICKCENTER(1),XLINE_INTENSITY(1)NOW READ IN THE STICK SPECTRUM
DO 113 I=1,N_MAX

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READ(6,*) STICKCENTER(I),XLINE_INTENSITY(I)CONTINUEWRITE(8,*) STICKCENTER(1),XLINE_INTENSITY(1)NOW CONVERT THE EBE OF THE STICKS TO EKENOW CALCULATE THE FWHM OF EACH TRANSITION
DO 74 I=1,N_MAX

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FWHM(I)=QM*STICKCENTER(I)+B
CONTINUEWRITE(8,*) FWHM(1),FWHM(N_MAX)NOW THE MAIN EVENT- CONVOLUTE!
DO 79 I=1,N_MAX
DO 89 J=1,N_OUTPUT

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CONVOLVED_INTENSITY(J)=CONVOLVED_INTENSITY(J)+2.0D0**SQRT(LOG(2.0D0)/PI)*XLINE_INTENSITY(I)/FWHM(I)*EXP(*
4.0D0*LOG(2.0D0)*(EBE_OUTPUT(J)*STICKCENTER(I))**2/(FWHM(I)**2))
CONTINUE
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CONTINUE
NOW WRITE THE OUTPUT SPECTRUM
DO 23 I=1,N_OUTPUT
WRITE(7,*) EBE_OUTPUT(I),CONVOLVED_INTENSITY(I)CONTINUE
CLOSE(6)
CLOSE(7)
CLOSE(8)END PROGRAM

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The next program I wrote with Monika Grutter Kasumaj to simulate the rovibronic
spectrum of benzene. While we did not end up utilizing this to analyze an experiment, it is a
useful example of how to simulate a relatively complex spectrum.
PROGRAM BENZENE_ROVIBRONICCCCALCULATES ROVIBRONIC TRANSITIONS OF S1 - S0 IN BENZENETHIS VERSION FOR 6_0^1 TRANSITIONIMPLICIT DOUBLE PRECISION (A-H,O-Z)C

G: GROUND STATE E: EXCITED STATE

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IF JGMAX IS CHANGED, THE NUMBER OF TRANSITIONSCALCULATED CHANGES, THEREFORE THE NUMBER ITRANS
THAT IS SENT TO GAUSSIAN_BENZENE_BROADER.F
NEEDS TO CHANGE TOO.
PARAMETER(JGMAX=160)
PARAMETER(JEMAX=JGMAX+1)
PARAMETER(JGDIM=(JGMAX+1)**2)
PARAMETER(JEDIM=2*(JEMAX+1)**2)
DIMENSION G(JGDIM)DIMENSION E(JEDIM)DIMENSION BOLTZ(JGDIM)DIMENSION FAVOR(JGDIM)DIMENSION SPIN(JGDIM)
PARAMETER (MAXTRANS=400000)
DIMENSION TRANS(MAXTRANS)DIMENSION XINT(MAXTRANS)PRINT *,JGMAX,JGDIM,JEMAX,JEDIMC

MOLECULAR PARAMETERS IN S0 GROUND STATE / CM-1
BG=0.18977D0
CG=BG/2.0D0

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EXCITED STATE S1 6^1 / CM-1 (ZC=C_0'ZETA')
BE=0.18179D0
CE=0.09087D0
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ZC=0.05256D0CTEMPJ AND TE ARE THE ADJUSTABLE PARAMETERS HERE.
TE=38606.0D0
TEMPJ=300.0D0
TEMPK=TEMPJ

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FILE 6 IS THE OUTPUT STICK SPECTRUM,
WHICH CORRESPONDS TO THE INPUT INTOGAUSSIAN_BENZENE_BROADER.F
OPEN(6,FILE='25KS.DAT',STATUS='UNKNOWN')
OPEN(8,FILE='GROUND.DAT',STATUS='UNKNOWN')
OPEN(9,FILE='EXCITED.DAT',STATUS='UNKNOWN')
OPEN(10,FILE='NOFAVOR.DAT',STATUS='UNKNOWN')
OPEN(12,FILE='NSSW.DAT',STATUS='UNKNOWN')
BOLTZMANN CONSTANTSQKB=1.3806504D-23
PLANCK=6.62606896D-34
C0=299792458
C0G=29.9792458

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INITIALIZINGDO 111 I=1,MAXTRANS

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TRANS(I)=0.0D0
XINT(I)=0.0D0
CONTINUECGROUND STATE LEVELS
DO 1 J=0,JGMAX
DO 2 K=-J,J2
1

G((J+1)**2-(J-K))=0.0D0
BOLTZ((J+1)**2-(J-K))=0.0D0
FAVOR((J+1)**2-(J-K))=0.0D0
SPIN((J+1)**2-(J-K))=0.0D0
CONTINUE
CONTINUE

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QSUM=0.0D0
DO 3 J=0,JGMAX
DO 4 K=-J,J
G((J+1)**2-(J-K))=BG*J*(J+1)+(CG-BG)*K**2CC

BOLTZ((J+1)**2-(J-K))=*
EXP(-(PLANCK*C0*100*G((J+1)**2-(J-K)))/(QKB*TEMP))BOLTZ((J+1)**2-(J-K))=*
EXP(-(PLANCK*C0*100*(BG*J*(J+1)))/(QKB*TEMPJ))**
EXP(-(PLANCK*C0*100*((CG-BG)*K**2))/(QKB*TEMPK))QSUM=QSUM+BOLTZ((J+1)**2-(J-K))C

WRITE (8,*) G((J+1)**2-(J-K)),J,K
IF(K.EQ.J) THEN

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PRINT *,BOLTZ((J+1)**2-(J-K)),J,K,QSUM
END IFCONTINUE
CONTINUECFAVOR LOWER K OVER HIGHER K STATESDO 333 J=0,JGMAXDO 444 K=-J,J
IF(J.EQ.0) THEN
FAVOR((J+1)**2-(J-K))=1.0D0
ELSE
FAVOR((J+1)**2-(J-K))=1.0D0
END IF
WRITE (10,*) FAVOR((J+1)**2-(J-K)),J,K

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CONTINUE
CONTINUECINCLUDE NUCLEAR SPIN STATISTICAL WEIGHTS292

DO 876 J=0,JGMAX
DO 864 K=-J,J
KK=ABS(K)IF(MOD(KK,6).EQ.0) THEN
IF(KK.EQ.0) THEN
IF(MOD(J,2).EQ.0) THEN
SPIN((J+1)**2-(J-K))=7.0D0
ELSE
SPIN((J+1)**2-(J-K))=3.0D0
END IF
ELSE
SPIN((J+1)**2-(J-K))=10.0D0
END IF
END IF
IF(MOD(KK,6).EQ.1) THEN
SPIN((J+1)**2-(J-K))=11.0D0
END IF
IF(MOD(KK,6).EQ.2) THEN
SPIN((J+1)**2-(J-K))=9.0D0
END IF
IF(MOD(KK,6).EQ.3) THEN
SPIN((J+1)**2-(J-K))=14.0D0
END IF
IF(MOD(KK,6).EQ.4) THEN
SPIN((J+1)**2-(J-K))=9.0D0
END IF
IF(MOD(KK,6).EQ.5) THEN

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END IFC

WRITE (12,*) SPIN((J+1)**2-(J-K)),J,K

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CONTINUE
CONTINUECEXCITED STATE LEVELS
DO 5 I=1,JEDIM

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E(I)=0.0D0
CONTINUE
DO 6 J=0,JEMAXDO 7 K=-J,J
DO 8 L=1,-1,-2
IF(L.GE.0) THEN
IL=1
ELSE
IL=0
END IF+

E(2*((J+1)**2-(J-K))-IL)=
BE*J*(J+1)+(CE-BE)*K**2+2*ZC*K*L

876WRITE (9,*) E(2*((J+1)**2-(J-K))-IL),J,K,L
CONTINUE
CONTINUE
CONTINUECCROVIBRONIC TRANSITIONS(SYMM. SEL. RULES: DELTAK=+-1 AND SAME DIRECTION AS L)
ITRANS=0
DO 10 J=0,JGMAXDO 11 K=-J,J
DO 12 JE=0,JEMAXDO 13 KE=-JE,JE
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ADD FACTOR FOR (K,KE) (0,1) AND (1,0) TRANSITIONS
GS=1.0D0
IF(K.EQ.0) THEN
IF(ABS(KE).EQ.1) THEN
GS=2.0D0
END IF
END IF
IF(ABS(K).EQ.1) THEN
IF(KE.EQ.0) THEN
GS=1.0D0
END IF
END IFCC

SELECT SPECIFIC STATES HERE (DIAGNOSTICS)
IF(1.EQ.1) THEN
IF(J.EQ.3) THENCDELTAK=K'-K''=+1 AND L=+1
IF(KE.EQ.K+1.AND.L.EQ.1) THEN
IL=1

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Q-BRANCH
IF(JE.EQ.J) THENC

PRINT *,J,K,JE,KE,L,'UPPER'ITRANS=ITRANS+1
TRANS(ITRANS)=TE+E(2*((JE+1)**2-(JE-KE))-IL)-G((J+1)**2*(J-K))XINT(ITRANS)=(2*J+1)*BOLTZ((J+1)**2-(J-K))/QSUM**((J+1+K)*(J-K)*1.0/(2*J*(J+1)))**SPIN((J+1)**2-(J-K))
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**FAVOR((J+1)**2-(J-K))*GS*1.0

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PRINT *,(J+1+K)*(J-K)*(2*J+1)*1.0/(4*J*(J+1))
PRINT *,J,K
END IFC

P-BRANCHESIF(JE+1.EQ.J) THENITRANS=ITRANS+1
TRANS(ITRANS)=TE+E(2*((JE+1)**2-(JE-KE))-IL)-G((J+1)**2*(J-K))XINT(ITRANS)=(2*J+1)*BOLTZ((J+1)**2-(J-K))/QSUM**((J-1-K)*(J-K)*1.0/(2*J*(2*J+1)))**SPIN((J+1)**2-(J-K))**FAVOR((J+1)**2-(J-K))*GS*1.0
END IFC

R-BRANCHESIF(JE-1.EQ.J) THENITRANS=ITRANS+1
TRANS(ITRANS)=TE+E(2*((JE+1)**2-(JE-KE))-IL)-G((J+1)**2*(J-K))XINT(ITRANS)=(2*J+1)*BOLTZ((J+1)**2-(J-K))/QSUM**((J+2+K)*(J+1+K)*1.0/(2*(J+1)*(2*J+1)))**SPIN((J+1)**2-(J-K))**FAVOR((J+1)**2-(J-K))*GS*1.0
END IF
END IFC

DELTAK=K'-K''=-1 AND L=-1
IF(KE+1.EQ.K.AND.L.EQ.-1) THEN
IL=0

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Q-BRANCH
IF(JE.EQ.J) THEN

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PRINT *,J,K,JE,KE,L,'LOWER'ITRANS=ITRANS+1
TRANS(ITRANS)=TE+E(2*((JE+1)**2-(JE-KE))-IL)-G((J+1)**2*(J-K))XINT(ITRANS)=(2*J+1)*BOLTZ((J+1)**2-(J-K))/QSUM**((J+1-K)*(J+K)*1.0/(2*J*(J+1)))**SPIN((J+1)**2-(J-K))**FAVOR((J+1)**2-(J-K))*GS*1.0

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PRINT *,(J+1+K)*(J-K)*(2*J+1)*1.0/(4*J*(J+1))
PRINT *,J,K
END IFC

P-BRANCHESIF(JE+1.EQ.J) THENITRANS=ITRANS+1
TRANS(ITRANS)=TE+E(2*((JE+1)**2-(JE-KE))-IL)-G((J+1)**2*(J-K))XINT(ITRANS)=(2*J+1)*BOLTZ((J+1)**2-(J-K))/QSUM**((J-1+K)*(J+K)*1.0/(2*J*(2*J+1)))**SPIN((J+1)**2-(J-K))**FAVOR((J+1)**2-(J-K))*GS*1.0
END IFC

R-BRANCHESIF(JE-1.EQ.J) THENITRANS=ITRANS+1
TRANS(ITRANS)=TE+E(2*((JE+1)**2-(JE-KE))-IL)-G((J+1)**2*(J-K))XINT(ITRANS)=(2*J+1)*BOLTZ((J+1)**2-(J-K))/QSUM**((J+2-K)*(J+1-K)*1.0/(2*(J+1)*(2*J+1)))**SPIN((J+1)**2-(J-K))**FAVOR((J+1)**2-(J-K))*GS*1.0
END IF
END IF

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END IF
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CONTINUE
CONTINUE
CONTINUE
CONTINUE
CONTINUEC'ITRANS' CORRESPONDS TO 'NB' IN GAUSSIAN_BENZENE_BROADER.F

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PRINT *,ITRANS
DO 44 I=1,ITRANS
IF(XTRANS(ITRANS).GE.SOMEVALUE) THEN

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WRITE(6,*) TRANS(I),0.0D0
WRITE(6,*) TRANS(I),XINT(I)WRITE(6,*) TRANS(I),0.0D0
END IFCONTINUE
CLOSE(12)CLOSE(10)CLOSE(9)CLOSE(8)CLOSE(6)END PROGRAM

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