Photoelectron Spectroscopy of Nitroalkane Anions
By
Christopher Lynn Adams
B.S. Adams State College, 2006

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirement for the degree ofDoctor of Philosophy
2011This thesis entitled:
Photoelectron Spectroscopy of Nitroalkane Anions
written by Christopher Lynn Adams
has been approved for the Chemical Physics program by____________________________
J. Mathias Weber____________________________
Veronica Bierbaum

Date:__________________

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

iii
Adams, Christopher L. (Ph.D. Chemical Physics)
Photoelectron Spectroscopy of Nitroalkane Anions
Thesis directed by Professor J. Mathias WeberThis thesis describes research on the electronic properties of nitroalkane anions,CH CH  NO (where0, 1, 2, and 3), using a newly built velocity-map imaging

photoelectron spectrometer. The nitroalkanes are an intriguing class of molecules that possessmany similarities owing to the dominating presence of the nitro group.

They all possess

relatively low adiabatic electron affinities, large dipole moments, and undergo similar distortions(largely localized within the nitro group) upon electron attachment. The focus of the studies
presented in this thesis is quantitatively characterizing the anion and neutral states, determining
fundamental molecular properties (i.e., adiabatic electron affinities, bond dissociation energies,
vertical detachment energies, etc.), and highlighting some of the interesting dynamics that thenitroalkane anions can exhibit using photoelectron spectroscopy.
The first two studies present the photoelectron spectra of nitromethane and nitroethane
anions. As these molecules are small enough to be tractable by a more involved theoreticalanalysis, ab initio theory and Franck-Condon simulations were utilized to simulate and interpretthe photoelectron spectra. In order for the simulations to achieve good agreement with theexperimental spectra, additional efforts were directed at characterizing the torsional potentials of
the methyl and ethyl groups and predicting the contribution of torsional motion to thephotoelectron spectra. These investigations led to new assignments for the adiabatic electron
affinities, the first observation of their dipole-bound states in photoelectron spectra, andiv
determinations of the C-NO bond dissociation energies for the nitromethane and nitroethane
anion.
The third study presents the photoelectron spectra of nitropropane and nitrobutane anions.
Each of these molecules possesses a number of stable anion and neutral conformers. Ab initio
theory and Franck-Condon simulations were employed to identify possible conformations
responsible for the dominant features in the photoelectron spectra. Finally, an anomalous feature
belonging to a separate photoelectron emission pathway was identified in the photoelectron
spectrum using spectra obtained at different photon energies and through an analysis of theangular distributions.
The nitroalkane anions possess adiabatic electron affinities that are smaller than thetransition energies of their fundamental CH stretching modes. As a consequence, excitation of
one of these modes will lead to vibrational autodetachment that is facilitated by intramolecularvibrational excitation. The last study presents vibrational autodetachment photoelectron spectraof the nitromethane anion. The spectra were analyzed using several simple models to extract
some of the important underlying information regarding the intramolecular vibrational relaxation
occurring prior to vibrational autodetachment.

v

Dedication
To my parents. For giving me the confidence to aspire, but more importantly, the humility tolearn from my mistakes.

vi

Acknowledgements
Over the last five years, graduate school has demanded a great deal of my time, energy
and focus. It wasn’t uncommon that I would gauge my commitment to work by the likelihood ofattempting to enter my house using my lab key. Fortunately, I have been surrounded by a strong
cast of friends, family, and coworkers who have helped me accomplish this work. I must first
thank my advisor, Mathias Weber.

As the third graduate student to join Mathias’ researchgroup, I was given a wonderful opportunity to contribute to the first generation of instruments inthe lab, which proved to be an invaluable personal and scientific learning experience. During
this time, I got to know Mathias as both a mentor and a friend and I am grateful for all that hehas done for me.
I have been fortunate to work with a number of outstanding coworkers over the last
several years. When I joined the lab, I got the opportunity to work with Holger Schneider.
Holger was the senior graduate student on the instrument I worked on. He was not only a great
mentor, who would go above and beyond to help, but he also became a great friend. While Jesse
Marcum and I never worked on the same instrument during my time in Mathias’ lab, he has
become a good friend and a great resource in just about any scientific discussion. I know I cancount on an engaging and active dialogue anytime I present him with a problem.

I can only

hope that he didn’t mind being my Litmus test for stupid ideas on more than one occasion.
Sydney Kaufman joined the lab two years after me. While we have not had the opportunity to domuch science together, she has become a good friend and has been a great addition to the lab. I
wish her all the luck in taming the temperamental ESI.

Recently, I have had the privilege of

passing the torch to Ben Knurr. Over the last year, Ben has almost taken full charge over the
instrument I toiled on for so many years. In this time, he has seen it through three self-guided

vii
studies! I’m glad to see I’ll be leaving it in capable hands. I wish him the best of luck and I
hope, for his sake, that it keeps behaving. I must also thank one of our undergraduates, Eric
Pozzi. Eric has been doing research with us over the last two years. I had the privilege to teachhim in a general chemistry lab and later the opportunity to work with him on a failed Raman
shifter. He has been a good friend and as he continues to graduate school at Northwestern
University, I wish him the best of luck.
I also would like to thank the people in the negative ion super group from the labs of Carl
Lineberger, Barney Ellison, and Veronica Bierbaum. Having the opportunity to be surrounded
by a great number of pioneering researchers in the field on anion chemistry has produced
numerous discussions and been a great resource. I must also thank the amazing people staffing
the electronic and machine shops of JILA. Their patience and expertise has saved us countlesshours.
Lastly, I must thank my friends and family. In my first year of graduate school, I met
Marta Maron, Meghan Dunn, and Danny Bell. Over the last several years, we became greatfriends and have shared a great deal of laughs. I wish them the best of luck in their careers after
graduate school and I hope we can stay in touch. My family has helped me and my wife somuch in the last several years. I cannot thank them enough for their love and support. Finally, I
must thank my wife. Lil, has supported me through undergraduate and now through graduate
school, she has been a wonderful mother to our child, and she has remained my best friend. I am
truly thankful that I have her in my life.

viii

Contents
1

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

2

3

References for Chapter 1 .................................................................................................. 4

Anion Photoelectron Spectroscopy ......................................................................................... 6
2.2

Selection Rules ................................................................................................................. 7

2.3

Threshold Laws ................................................................................................................ 8

2.4

Anisotropy Parameters ................................................................................................... 10

2.5Franck-Condon Factors .................................................................................................. 11

2.6

References for Chapter 1 ................................................................................................ 15

Experimental Apparatus........................................................................................................ 17
3.1

Overview ........................................................................................................................ 17

3.2

Ion Source ...................................................................................................................... 18

3.2.1

Implementation ....................................................................................................... 20

3.3

Time-of-Flight Mass Spectrometer ................................................................................ 22

3.4

Laser System .................................................................................................................. 24

3.6

Velocity Map Imaging Photoelectron Spectrometer ...................................................... 28

3.6.1

Photoelectron Image Acquisition............................................................................ 28

3.6.2

Data Analysis .......................................................................................................... 31

3.7

Using Argon Solvation for Accurate Adiabatic Electron Affinity Determinations ....... 37

3.8

References for Chapter 3 ................................................................................................ 38ix
4Photoelectron Spectroscopy of Nitromethane Anion ........................................................... 40
4.1

Introduction .................................................................................................................... 40

4.2

Computational Details .................................................................................................... 42

4.2.1

Ab Initio Calculations ............................................................................................. 42

4.2.2

Franck-Condon Analysis ........................................................................................ 43

4.3

Results and Analysis ...................................................................................................... 50

4.3.1

Interpretation of the photoelectron spectrum: the adiabatic electron affinity and thedipole-bound state ................................................................................................................. 50

5

4.3.2

Photoelectron Angular Distributions ...................................................................... 55

4.3.3

Franck-Condon Simulation ..................................................................................... 58

4.3.4

CN bond energy ...................................................................................................... 68

4.4

Summary and Conclusions ............................................................................................. 69

4.5

References for Chapter 3 ................................................................................................ 70

The Photoelectron Spectroscopy of Nitroethane Anions ...................................................... 75
5.1

Introduction .................................................................................................................... 75

5.2

Computational Details .................................................................................................... 77

5.2.1

Ab Initio Calculations ............................................................................................. 77

5.2.2

Franck-Condon Analysis ........................................................................................ 79

5.4

Results and Discussion ................................................................................................... 87

5.4.1

Experimental Results .............................................................................................. 87

x

6

5.4.2

Threshold Effects and Angular Distributions ......................................................... 93

5.4.3

Franck-Condon Simulations ................................................................................... 96

5.5

Summary ...................................................................................................................... 101

5.6

References for Chapter 5 .............................................................................................. 102

Photoelectron Spectroscopy of 1-Nitropropane and 1-Nitrobutane anions ........................ 105
6.1

Introduction .................................................................................................................. 105

6.2

Computational Details .................................................................................................. 106

6.2.1
6.4

Ab Initio Calculations ........................................................................................... 106

Results and Discussion ................................................................................................. 108

6.4.1

Experimental Photoelectron Spectra ..................................................................... 108

6.4.3

Stable Anionic Conformers................................................................................... 119

6.4.4

Franck-Condon Simulations ................................................................................. 122

6.4.5

Angular Distributions............................................................................................ 128

6.4.6

Remarks on the Anomalous Feature in Photodetachment from 1-nitropropane and1-nitrobutane anions............................................................................................................ 131

7

6.5

Summary ...................................................................................................................... 131

6.6

References for Chapter 6 .............................................................................................. 132

Vibrational Autodetachment Photoelectron Spectroscopy of Nitromethane Anion ........... 135
7.1

Overview of Vibrational Autodetachment ................................................................... 135

7.2

Vibrational Autodetachment of Nitromethane Anion: General Considerations .......... 141

xi
7.3

Photoelectron Imaging Results..................................................................................... 144

7.4

Modeling ...................................................................................................................... 146

7.4.1“No Torsion” Model ............................................................................................. 148

7.4.2

Energy Randomization Model .............................................................................. 150

7.4.3

Partial Randomization Model ............................................................................... 153

7.4.4

Application to VAD PE for the Symmetric and Antisymmetric Methylene

Stretching Modes ................................................................................................................ 156
7.4.5

8

Critical Discussion of the Modeling ..................................................................... 158

7.5Open Questions and Future Challenges ....................................................................... 160

7.6

References .................................................................................................................... 162

Bibliography ....................................................................................................................... 173

xii

List of Tables
Table 4.1 – Frequencies (cm-1) and geometry displacements. ...................................................... 45
Table 4.2 – Lowest Free and Hindered Internal Rotor States ....................................................... 49
Table 5.1 – Dipole Moments and AEA’s of Nitromethane and Nitroethane................................ 76
Table 5.2 - Frequencies (cm-1) and geometry displacements. ...................................................... 80
Table 5.3 – Lowest Hindered Internal Rotor States ...................................................................... 86

xiii

List of Figures
Figure 2.1 – A representation of two scenarios. The scenario on the left involves
photodetachment in which there is no displacement but the force constant changes. The scenario
on the right depicts a displacement without a change in the force constant. ................................ 13
Figure 3.1 – Schematic diagram of the instrument used for a photoelectron spectroscopy study.18
Figure 3.2 – Schematic diagram of the entrainment ion source. The source has the capability toentrain two gases. However, all of the studies within this thesis utilize only one entrainment
valve. ............................................................................................................................................. 21
Figure 3.3 - Overview of the IR OPO/OPA tunable light source. ................................................ 27
Figure 3.4 – Schematic diagram of velocity map imaging photoelectron spectrometer. ............. 29
Figure 3.5 – Shown on the right is a photoelectron image of S taken at 532 nm. The left half
illustrates the raw image. The right half illustrates the BASEX reconstructed equatorial slice
through the original three-dimensional photoelectron distribution. On the left side of the figureis the photoelectron spectrum generated from the reconstructed image. The transitions arelabeled to illustrate corresponding features. ................................................................................. 33
Figure 4.1 – Geometries of CH3NO2¯ and CH3NO2; α denotes the angle between the ONO plane
and the CN bond axis from DFT calculations; results from the CCSD(T) calculations are inparentheses. ................................................................................................................................... 42
Figure 4.2 – PE spectra of CH3NO2¯ at 322 meV (2600 cm-1, lower trace), 397 meV (3200 cm-1)
and from Ref. 1 (inset, abscissa is in units of eV); the circles are data points, the full lines are
five point adjacent averages to guide the eye; the arrow denotes the peak corresponding to thevibrational ground state of the neutral molecule AEA=(168 ± 6) meV; D is the signature of thedipole-bound state. ........................................................................................................................ 51
Figure 4.3 – PE image of CH3NO2¯ at 322 meV (2600 cm-1) photon energy, based on one
quadrant of the raw PE image. The left half of the figure contains the raw PE image with theintensity in the upper portion doubled to better show the dynamic range of the image without
saturating the gray scale. The right half of the figure shows the BASEX transformed image withthe “center-line artifact” from the BASEX transformed image removed. The signature belonging
to the dipole-bound state has been labeled “D.” ........................................................................... 52
Figure 4.4 – PE spectra of CH3NO2¯ (upper trace) and CH3NO2¯ ·Ar (lower trace) at 397 meV(3200 cm-1) photon energy. The dashed lines and arrows show how the vibrational progressionof the neutral molecule shifts by 63 meV toward higher binding energies upon solvation by one
Ar atom. ........................................................................................................................................ 54

xiv
Figure 4.5 – Anisotropy parameters are presented as a function of electron kinetic energy for
different photon energies. Squares, circles, triangles, and stars correspond to data taken at 285
meV (2300 cm-1), 310 meV (2500 cm-1), 322 meV (2600 cm-1), and 397 meV (3200 cm-1),
respectively. Filled symbols correspond to the valence electronic state, while the open symbols
belong to the dipole-bound state. The data points marked with larger circles belong to the hotbands at binding energies of 77 and 122 meV. Error bars have been conservatively estimated tobe ±0.1, except in cases where fits of the angular distributions resulted in larger error bars
because of poorer signal-to-noise ratios. The dashed line shows the theoretical maximum value
for the anisotropy parameter (β = 2). ............................................................................................ 57
Figure 4.6 – Comparison of the torsional Franck-Condon simulation (vertical sticks) at 100 K
temperature and the experimental results for CH3NO2¯ ·Ar, shifted by the EA of the Ar complex(full line). ...................................................................................................................................... 59
Figure 4.7 – Comparison of the experimental PE spectrum by Compton et al. (Ref 1) with a fitted
Franck-Condon simulation (see text). ........................................................................................... 61
Figure 4.8 – HOMO of CH3NO2¯ . The HOMO is mainly localized on the nitro group, but small
parts also penetrate into the rest of the molecule, notably the σ∗ antibonding orbital of the CH
group syn to the nitro group. ......................................................................................................... 63
Figure 4.9 – Franck-Condon fits to the experimental PE spectrum of CH3NO2¯ (open circles) at
397 meV (3200 cm-1) with no velocity scaling (dotted line), scaled with EK1/2 (pure s-wave, fullline), and scaled with EK3/2 (pure p-wave, dashed line); see text for details. The sequences
marked in the spectrum are based on the NO2 wagging mode ν1 with transitions n←0 (series I)
and n←1 (series II). The asterisk marks the 0←2 hot band of the NO2 wagging mode. Note thatthe top scale shows the electron kinetic energy while the bottom scale gives the electron binding
energy. ........................................................................................................................................... 67
Figure 4.10 – Thermochemical cycle for the determination of the BDECN in nitromethane anion.
....................................................................................................................................................... 69
Figure 5.1 - Geometries of CH3CH2NO2¯ and CH3CH2NO2; θ denotes the angle between the
ONO plane and the CN bond axis, which is the most salient geometry change upon electron loss.
....................................................................................................................................................... 78
Figure 5.3 - Calculated potential energy curves (circles) generated by rotation about the CN bondin the nitroethane anion (lower curve) and neutral (upper curve). The black line illustrates the fit
to each calculated potential (see text). The filled blue circles represent points calculated using arelaxed-geometry scan while the filled red circles are points generated by a fixed-geometry scan(see text). The different nitroethane anion conformers are illustrated above their respective
positions along the potential energy curve. The anion has two isoenergetic conformers separated
by a 660 cm-1 barrier and flanked by a higher energy conformer with a large barrier toxv
isomerization. The neutral has a relatively flat potential nearly coinciding with that of a freeinternal rotor.................................................................................................................................. 83
Figure 5.4 - PE spectra of CH3CH2NO2¯ at 389 meV (upper trace), 347 meV (center trace), and322 meV (lower trace) photon energy. The dashed line denotes the (0←0) transition
corresponding to an AEA of (191 ± 6) meV................................................................................. 88
Figure 5.5 – PE image obtained at a photon energy of 389 meV (3140 cm-1). The left half of theimage is the raw PE image. The right half is the BASEX reconstructed image. ......................... 89
Figure 5.6 - PE spectra of CH3CH2NO2¯ (upper curve) and CH3CH2NO2¯ ·Ar (lower curve) taken
at 389 meV photon energy. The arrows and drop lines illustrate the shift of the spectrum upon
Ar solvation by (61 ±7) meV. ....................................................................................................... 91
Figure 5.7 - PE spectrum of CH3CH2NO2¯ taken at a photon energy of 322 meV. The spectrum
is plotted as a function of electron speed to best illustrate the dipole-bound state; the inset shows
the PE spectrum as a function of binding energy. In each spectrum the dipole-bound state islabeled “D”.................................................................................................................................... 92
Figure 5.8 - Thermochemical cycle for the determination of the C-NO2 bond dissociation energy,
BDEC-NO2, in nitroethane anion. .................................................................................................... 93
Figure 5.9 - Anisotropy parameters as a function of electron kinetic energy obtained at different
photon energies where solid circles, hollow circles, and triangles represent parameters fromimages taken at photon energies of 389 meV, 347 meV, and 322 meV, respectively. The error
bars are conservatively estimated to be ± 0.1. .............................................................................. 95
Figure 5.10 - Experimental PE spectrum taken at a photon energy of 389 meV (open circles)
along with Franck-Condon simulations performed with the inclusion of the hindered-to-freeinternal rotor transitions (solid line) and in their absence (dotted line). The inset is centeredabout the (0←0) transition and includes a stick spectrum of the individual calculated transitions
to emphasize the contribution of the hindered-to-free CN rotor transitions to the peak profile. .. 97
Figure 5.11 - Experimental PE spectra taken at a photon energy of 389 meV (open circles) alongwith the Franck-Condon simulations performed without threshold scaling (dash-dotted line),scaled by E3/2 (dotted line) and scaled by E1/2 (solid line). The sequences marked in the spectrum
are based on the nitro wagging mode ν22 with transitions n←0 (series A) and n←1 (series B). 100
Figure 6.1 – Photoelectron spectrum of 1-NP anion obtained at a photon energy of 2.331 eV. 109
Figure 6.2 – Photoelectron image of 1-NP taken at a photon energy of 329 meV (2650 cm-1).
The labels “A” and “B” denote two features that are consistently observed at the same binding
energy. The label “F” denotes a prominent feature found at a fixed kinetic energy in the spectraxvi
taken at photon energies of 329 meV (2650 cm-1) and 368 meV (2968 cm-1). See text for moredetails. ......................................................................................................................................... 110
Figure 6.3 - Photoelectron spectra of 1-NP anion taken at photon energies of 393 meV (3170 cm1), 368 meV (2968 cm-1), 329 meV (2650 cm-1), and 273 meV (2200 cm-1), from top to bottom.
The spectra on the left are presented as a function of binding energy while the spectra on theright are graphed as a function of kinetic energy. The labels “A” and “B” denote two featuresthat are consistently observed at the same binding energy. The label “F” denotes a prominent
feature found at a “fixed” kinetic energy in the spectra taken at photon energies of 329 meV(2650 cm-1) and 368 meV (2968 cm-1). The dotted line in the spectra on the right indicates akinetic energy of 165 meV (see text for further details). ............................................................ 111
Figure 6.4 – Photoelectron spectra of bare 1-NP anion (upper curve) as well as argon-solvated 1NP anion (lower curve) taken at a photon energy of 393 meV. The arrows represent the shift ofthe spectrum by an energy of (56 ± 7) meV upon argon solvation. The labels “A” and “B”
denote two features that are consistently observed at the same binding energy in the bare 1-NP
anion spectra. .............................................................................................................................. 113
Figure 6.5 – Photoelectron spectra of 1-NB anion, obtained at a photon energy of 2.331 eV. .. 114
Figure 6.6 - Photoelectron spectra of 1-NB anion at photon energies of 444 meV (3580cm-1), 424
meV (3420 cm-1), 404 meV (3260 cm-1), 384 meV (3100 cm-1), 329 meV (2650 cm-1), from top
to bottom. The spectra on the left are presented as a function of binding energy while the spectraon the right are graphed as a function of kinetic energy. The labels “A” and “B” denote two
features that are consistently observed at the same binding energy. The label “F” denotes aprominent feature found at a “fixed” kinetic energy in the spectra. The dotted line in the spectraon the right indicates a kinetic energy of 165 meV (see text for further details). ...................... 116
Figure 6.7 - – Photoelectron image of 1-NB taken at a photon energy of 384 meV (3100 cm-1).
The labels “A” and “B” denote two features that are consistently observed at the same binding
energy. The label “F” denotes a prominent feature found at a fixed kinetic energy in the spectra.
See text for more details. ............................................................................................................ 117
Figure 6.8 – The photoelectron spectrum of bare 1-NB anion (top curve) and argon-solvated 1NB anion (bottom curve), taken at a photon energy of 384 meV. The arrow is used to illustrate
the shift of the 0←0 feature. A “F” is placed above the anomalous feature and an “A” is placed
above the feature corresponding to the 0←0 transition. ............................................................. 118
Figure 6.9 - Illustration of the various conformers of the 1-NP anion generated from rotations
about the CN or central CC bond, their relative energies, and their population relative to thelowest energy conformer, according to a Boltzmann distribution. H1 and H2 are consistently
defined but arbitrary designations to distinguish the hydrogens. The key at the top denotes theatoms H1 and H2.
The conformers outlined in red are excluded in the Franck-Condon
simulations, see discussion for more details. .............................................................................. 119

xvii
Figure 6.10 - Diagram of the stable conformers along with their relative energies, excluding theconformations in which the CC bond is syn to the NO2 group. Their population relative to thelowest energy conformer, according to a Boltzmann distribution, is denoted in the parentheses.
H1 and H2 are consistently defined but arbitrary designations to distinguish the hydrogens. The
key at the top denotes the atoms H1 and H2. The conformers outlined in red are excluded in theFranck-Condon simulations, see discussion for more details. .................................................... 122
Figure 6.11 – The Franck-Condon simulations of the two lowest energy conformers of 1-NP
anion, using an anion temperature of 250 K. An inset in each graph illustrates the 1-NP anion
conformer along with its relative energy. The open circles, solid lines, and bars represent theobserved, simulated curve, and the individual transitions. ......................................................... 125
Figure 6.12 - The Franck-Condon simulations of the three lowest energy conformers of 1-NB
anion, using an anion temperature of 250 K. An inset in each graph illustrates the 1-NP anion
conformer along with its relative energy. The open circles, solid lines, and bars represent theobserved, simulated curve, and the individual transitions. ......................................................... 127
Figure 6.13 – Plot of the fitted anisotropy parameters, β, as a function of kinetic energy for thefour different lower-energy photon wavelengths used in the study of 1-NP anion (gray lines) as
well as their average (dark black line). The dashed line is located at a kinetic energy of 166
meV. ............................................................................................................................................ 129
Figure 6.14 - Plot of the fitted anisotropy parameters, β, as a function of kinetic energy for thefive different lower-energy photon wavelengths used in the study of 1-NB anion (gray lines) as
well as their average (dark black line). The dashed line is located at a kinetic energy of 166
meV. ............................................................................................................................................ 130
Figure 7.1 – Schematic drawing of the diabatic anionic and neutral potential energy surfaces ofnitromethane. The anionic surface is connected to that of the neutral along the detachment
coordinate Q. In contrast, the CH stretching modes are not connecting the two surfaces andexcitation of one quantum of a CH stretching vibration will not lead to immediate detachment.
VAD will only occur after sufficient energy has been redistributed into Q. .............................. 142
Figure 7.2 – Highest occupied molecular orbital (HOMO) of the nitromethane valence anion. 143
Figure 7.3 – Comparison of BASEX transformed photoelectron images of CH3NO2¯ . The image
on the left panel is taken off-resonance at an energy of 3200 cm-1. The image on the right panel
is taken on the vibrational resonance of the totally symmetric νs(CH3) at 2777 cm-1. ............... 144
Figure 7.4 – Comparison of the experimental VAD spectrum (connected open circles) of
CH3NO2¯ on the νs(CH3) resonance (symmetric methyl stretch) with the “no torsion” model (fullline, see text). The contribution of each vibrational state was weighted to yield the best fit of themodel to the experimental data. .................................................................................................. 150

xviiiFigure 7.5 – Comparison of the experimental VAD spectrum (solid black line) of CH3NO2¯ onthe νs(CH3) resonance (symmetric methyl stretch) with the energy randomization model (solid
red line, see text). The model has been normalized to the experimental data at the 0-eV feature.
..................................................................................................................................................... 151
Figure 7.6 – Comparison of the experimental VAD spectrum (connected open circles) of
CH3NO2¯ on the νs(CH3) resonance (symmetric methyl stretch) with the density of states (fullline) in neutral CH3NO2. The DOS has been normalized to the experimental data at the 0-eVfeature. ........................................................................................................................................ 153
Figure 7.7 – Comparison of the experimental VAD spectrum (solid black line) of CH3NO2¯ onthe νs(CH3) resonance (symmetric methyl stretch) with the partial randomization model (solid
red line). The model has been normalized to the experimental data at the 0-eV feature........... 155
Figure 7.8 – Comparison of the experimental VAD photoelectron spectra (connected open
circles) belonging to the ν′s(CH3) (symmetric methylene stretch, left panel) and νa(CH3)(antisymmetric methylene stretch, right panel) modes of CH3NO2¯ with the partial
randomization model (full lines). The dashed line in both spectra is a representation of the fullenergy randomization model. The models have been normalized to the experimental data at the
0-eV feature for each spectrum. .................................................................................................. 157

1

1 Introduction
It has been almost forty years since the first laser photodetachment study of a molecularanion was performed.1 Since that time, the field of gas-phase anion research has expanded
immensely.

The relative ease with which anions can be controlled in electric fields, theincreasing sophistication of laser technology, and the advances in producing gas-phase anions
have made the molecular anion an ideal molecular laboratory that has been used with great
success to elevate our understanding of chemical phenomena.
In the field of gas-phase anion chemistry, it would be difficult to overstate the importanceof photoelectron spectroscopy. Its relatively simple premise has lent itself to constant adaptationand improvement which have allowed this technique to stay relevant as one of the best means ofinvestigating anions. Photoelectron spectroscopy provides a sensitive probe of the geometries
and the electronic structure of the anions and their neutral counterparts and can yieldfundamental properties such as the molecular adiabatic electron affinity, which in turn can beused to find bond energies (provided the other relevant thermochemical data are known).2
Beyond this, many other interesting facts can be learned about unstable neutral molecules orclusters if they are stabilized as anions. In this vein, photoelectron spectroscopy has served as a
powerful technique to study radicals,3-5 which often form stable anions, or to study transition
states on the neutral potential energy surface which are accessible from an anion or anioniccluster.6-8
In the work presented here, we employ a newly built velocity-map imaging photoelectron
spectrometer to investigate nitroalkane anions, CH CH  NO

where0, 1, 2, and 3. The

nitroalkanes are all colorless oily liquids under standard conditions which are primarily used as

2fuel additives (all of the species are known explosives), solvents, or as pharmaceuticalprecursors.

The similarities in their structures and properties stems from the dominatingpresence of the NO group.

In the neutral species, the nitro group gives rise to a large dipole

moment, which has been shown using Rydberg electron transfer studies9, 10 to support a dipole
bound state for the lower nitroalkanes, nitromethane and nitroethane.

Additionally, thenitroalkanes have generated interest since the barrier to rotation for the methyl and ethyl group of
the lower order nitroalkanes is negligible, leading to sufficient motion in this mode at moderatetemperatures.11, 12 This has led to several studies that have focused on the manner in which theorientation of the nitro group, relative to the alkyl group, affects the vibrations of the moleculeand subsequently influences vibrational energy redistribution11 in these known explosives.

In

regards to the anions, all of the species adopt a similar geometry about the nitrogen as the excess
electron is largely localized on the lowest unoccupied molecular orbital centered on the -orbitalof the nitrogen atom which drives the planar geometry into a trigonal pyramidal configuration.
In turn, the localization of the electron makes the adiabatic electron affinity relatively invariant
to the size of the alkyl chain. Vibrational spectroscopy studies performed by Schneider et al.13
have indicated that the electron affinity is less than 2500 cm-1 for the different alkyl chain
lengths. This property leads to an uncommon situation in which the fundamental CH stretchingmodes have energies in excess of the adiabatic electron affinity. Excitation within the midinfrared region can lead to two distinctly different processes governing the photodetachment. If
a photon interacts with the anion at a non-resonant frequency the electron can be photodetachedimmediately as an electronic excitation of the detaching electron. But if the anion interacts witha photon at a vibrationally resonant frequency, this can lead to a vibrational transition which will
be followed by electron emission facilitated by intramolecular vibrational relaxation. Each of

3these processes will leave distinct “footprints” on the photoelectron spectrum. The former will
yield a conventional photoelectron spectrum while the latter will leave a signature of theintramolecular vibrational relaxation occurring prior to photoemission.
This thesis discusses photoelectron spectra of the nitroalkane anions obtained at photon
energies away from the influence of any vibrational frequencies (conventional photoelectron
spectroscopy) and at vibrationally resonant frequencies. Chapter 2 will provide an overview of
the theory governing photodetachment and the information that can be acquired fromphotoelectron spectroscopy as these ideas form the foundation of any analysis. Chapter 3 will
describe the instrument used to create the anions, isolate the anions of interest and acquire thephotoelectron spectra. Chapters 4, 5, and 6 will discuss conventional photoelectron spectra
obtained for the nitroalkane anions. Chapter 4 gives an account of the studies and simulations
performed on the nitromethane anion that have led to the determination of the electron affinity,
the observance of a dipole bound state and identification of much of the vibrational structure
within the spectrum.

Chapter 5 focuses on the photoelectron spectra and simulations of

nitroethane anion that also yielded the adiabatic electron affinity and the observance of the dipole
bound state. Chapter 6 concentrates on the spectra of nitropropane and nitrobutane anions aswell as the simulations performed to identify the conformers yielding the experimentally
determined electron affinity. Chapter 7 will focus on the photoelectron spectra obtained at
vibrationally resonant photon energies of the nitromethane anion and present some simple
models to describe the results.

4

1.1 References for Chapter 1
1.

M. W. Siegel, R. J. Celotta, J. L. Hall, J. Levine and R. A. Bennett,"Molecular
Photodetachment Spectrometry. I. The Electron Affinity of Nitric Oxide and theMolecular Constants of NO." Physical Review A 6 (2), 607 (1972).

2.

S. J. Blanksby and G. B. Ellison,"Bond dissociation energies of organic molecules."Accounts of Chemical Research 36 (4), 255-263 (2003).

3.

K. M. Ervin, T. M. Ramond, G. E. Davico, R. L. Schwartz, S. M. Casey and W. C.
Lineberger,"Naphthyl radical: Negative ion photoelectron spectroscopy, Franck-Condon
simulation, and thermochemistry." Journal of Physical Chemistry A 105 (48), 1082210831 (2001).

4.

T. Ichino, S. W. Wren, K. M. Vogelhuber, A. J. Gianola, W. C. Lineberger and J. F.
Stanton,"The vibronic level structure of the cyclopentadienyl radical." Journal ofChemical Physics 129 (8) (2008).

5.

P. G. Wenthold and W. C. Lineberger,"Negative ion photoelectron spectroscopy studies
of organic reactive intermediates." Accounts of Chemical Research 32 (7), 597-604(1999).

6.

P. G. Wenthold, D. A. Hrovat, W. T. Borden and W. C. Lineberger,"Transition-state
spectroscopy of cyclooctatetraene." Science 272 (5267), 1456-1459 (1996).

7.

S. N. Eustis, D. Radisic, K. H. Bowen, R. A. Bachorz, M. Haranczyk, G. K. Schenterand M. Gutowski,"Electron-driven acid-base chemistry: Proton transfer from hydrogenchloride to ammonia." Science 319 (5865), 936-939 (2008).

8.

E. Garand, J. Zhou, D. E. Manolopoulos, M. H. Alexander and D. M.
Neumark,"Nonadiabatic interactions in the Cl+H2reaction probed by CIH2- and CID2photoelectron imaging." Science 319 (5859), 72-75 (2008).

9.

R. N. Compton, H. S. Carman, C. Desfrancois, H. Abdoul-Carmine, J. P. Schermann, J.
H. Hendricks, S. A. Lyapustina and K. H. Bowen,"On the binding of electrons tonitromethane: Dipole and valence bound anions." Journal of Chemical Physics 105 (9),3472-3478 (1996).

5
10.

S. T. Stokes, K. H. Bowen, T. Sommerfeld, S. Ard, N. Mirsaleh-Kohan, J. D. Steill and
R. N. Compton,"Negative ions of nitroethane and its clusters." Journal of Chemical
Physics 129 (6) (2008).

11.

D. Gorse, D. Cavagnat, M. Pesquer and C. Lapouge,"Theoretical and Spectroscopic
Study of Asymmetric Methyl Rotor Dynamics in Gaseous Partially Deuterated
Nitromethanes." Journal of Physical Chemistry 97 (17), 4262-4269 (1993).

12.

P. Groner, R. Meyer, A. Günter, H. Kühne and H. H. Günthard,"Far Infrared Spectrumof Nitroethane." Chemical Physics 5 (1), 136-141 (1974).

13.

H. Schneider, K. M. Vogelhuber, F. Schinle, J. F. Stanton and J. M. Weber,"Vibrational
spectroscopy of nitroalkane chains using electron autodetachment and Ar
predissociation." Journal of Physical Chemistry A 112 (33), 7498-7506 (2008).

6

2 Anion Photoelectron Spectroscopy
If an anion interacts with a photon with energy in excess of the binding energy of theelectron, photoelectron emission can occur. The conservation of energy requires that the emitted
photoelectron has an energy that can be written as
Equation 2.1

  

where  represents the photoelectron’s kinetic energy,  represents the binding energy ofthe electron or ‒ more accurately ‒ the energy difference between the initial and final molecularstates, and  is the energy of the photodetaching radiation. As the neutral molecule remaining
after photoemission is much heavier than the emitted electron, it is a good approximation toneglect the kinetic energy imparted to the neutral.1 As a consequence, the photoelectrons can beanalyzed as a function of kinetic energy, at a fixed photon energy, and a photoelectron spectrum
may be constructed as a function of the electron binding energy. The photoelectron spectrum
encodes a wealth of information pertaining to the neutral or parent states, such as geometricalchanges that the molecular frame undergoes upon photoemission, the energies of the electronics
states of the neutral, the vibrational spacing within each electronic band, and the adiabatic
electron affinity of the neutral.2, 3 In addition to the kinetic energy of the emitted electron, theangular distribution for photoemission can be obtained as a function of the kinetic energy. The
photoelectron angular distribution is primarily used to extract information about the nature of thedetaching orbital of the anion. The following sections will highlight the principles that govern
the shape of a photoelectron spectrum and as a consequence, form the foundation for its analysis.

7

2.2 Selection Rules
The selection rules for photodetachment govern what electronic states of the neutral canbe accessed, the acceptable orbital angular momenta of the outgoing photoelectron, and themanner in which the overlap of the vibrational wavefunctions modulate the intensity within aparticular electronic transition. As the final state in photodetachment involves an unboundelectron, the selection rules are slightly different from other electronic spectroscopic methods.
The following rules are for single photon photodetachment involving one electron within theelectric dipole approximation.
The spin selection rules require that the total spin of the system is conserved. As theelectron will carry away its spin angular momentum the spin of the resulting neutral must differ
from that of the parent anion by ∆!1/2 upon electron loss.

This offers the perfectcomplementary tool to optical electronic spectroscopy, in which the spin selection rule strictly
forbids a change in multiplicity.

This is one example of many, in which photoelectron

spectroscopy provides a valuable means of investigating the properties of a neutral molecule thatare difficult to attain otherwise.
The orbital selection rules specify the allowed outgoing waves for the photodetachedelectron. As in electronic spectroscopy, the photon will impart one unit of angular momentum tothe electron resulting in a selection rule of ∆ℓ!1 relative to the initial orbital for the outgoingwave.4, 5 In atomic systems this leads to a situation in which detachment from a  orbital resultsin outgoing $- and %-waves. For molecular systems, in which the electronic orbital angularmomentum is no longer a good quantum number, the outgoing electron wavefunction is assumed
to transform like a member of the irreducible representation of the symmetry group of the anion

and the allowed symmetries of the outgoing electron may be determined by the direct product of

8
the irreducible representations of &' , (̂ , and &* , which represent the outgoing electron’s
wavefunction, the dipole moment operator, and the wavefunction of the parent orbital in theanion, respectively.5 The allowed symmetries of the outgoing electron will yield a direct productwhich contains the totally symmetric representation. The orbital selection rules will be revisitedlater, as the weighting of the outgoing waves is strongly modified by threshold laws.
An analysis of the transition moment integral, which leads to the selection rules presented
above, does not yield vibrational selection rules.6 Rather, it leads to the term, +&,- |&," 0, where&,1 represents the vibrational wavefunction of the neutral and &," represents the vibrational

wavefunction of the anion. This term serves to strongly modify the intensity profile of anallowed electronic transition as a function of the overlap of the vibrational wavefunction of theanion and the neutral molecule. The absolute square of this term, |+&,- |&," 0| , is called theFranck-Condon factor. Since it plays an important role in understanding any photoelectron
spectrum, a more complete discussion of its ramifications will be given below.

2.3 Threshold Laws
The intensity profile of photoelectron spectra acquired at photon energies just above
electron detachment threshold are strongly modified by threshold laws. Wigner7 first illustrated
the generality of the threshold rules, in which the longest-range interaction of two reacting
particles dictates the energy dependence of the cross-section. In photodetachment from singly
charged anions, the reacting particles are the neutral core and the detached electron, and thelongest range interaction is either the anisotropic 1/2  potential of a molecular dipole or theℓℓ 3 1/2  centrifugal potential.8

9Considering the simplest case of an atomic anion first, in which the longest range
interaction is the centrifugal potential, Wigner’s laws7 show that the near-threshold cross section
for photodetachment is given by
Equation 2.2

4  5 ℓ67/

where  is the kinetic energy of the photoelectron and ℓ is the angular momentum of theoutgoing electron. Wigner’s threshold law is exact at the limit of threshold but for larger kinetic
energies a power series expansion in  is necessary to account for the shorter-range
interactions. The dependence of the cross section on the angular momentum of the outgoing
electron is a consequence of the centrifugal barrier. An electron near threshold will have totunnel through this barrier which is larger for electrons with larger angular momentum. This, inturn, favors the production of electrons with the lowest angular momentum near threshold.8 As
was discussed earlier, photodetachment from a  orbital results in an outgoing $- or %-wave.

However, if the photodetachment is performed near threshold the %-wave will be strongly
suppressed resulting in an almost pure $-wave detachment with a cross-section at thresholdproportional to  .
7/

An extension of Wigner’s threshold law for molecules (without a dipole moment) was
derived by Reed et al.,5 who outlined a procedure to determine the leading term for the crosssection.

The procedure begins with determining the allowed symmetries of the outgoing

electrons, as outlined above for the orbital selection rules. Since the spherical waves of a given
angular momentum can be assigned to irreducible representations, the symmetry corresponding
to the spherical wave with the lowest angular momentum is expected to give the leading term in10
the cross section.

This is a very powerful approach in molecules with higher symmetry;

unfortunately, there is no simple way to accurately predict the cross section at threshold forsystems of low symmetry, as there are very few symmetry restrictions on the nature of theoutgoing wave.
Further complications exist when a dipole is present.

In this case, the long-range

interaction will be the sum of the centrifugal and dipolar potential and as a result, the leading
term of the near-threshold expansion of the cross section will be modified. The manner in which
the dipole modifies the leading term is difficult to predict, as the orientation relative to theemitted electron becomes important. In addition, near threshold the electron will depart on timescales comparable to the rotational period of the molecule, which will make the cross section
sensitive to the rotational motion perpendicular to the dipole moment.8 Analyses of the nearthreshold cross section of the nitroalkane anions discussed in this thesis are difficult, since they
all have significant dipoles and are of low symmetry.

2.4 Anisotropy Parameters
The anisotropy characteristics of the photodetachment offer insight into the symmetry of
the parent orbital in the anion. The photoelectron angular distributions for a one-photon processwith linearly polarized light are described by the function,9Equation 2.3

8, 94
<1 3 => cos 9B4;

11
where 9 is the angle between the laser polarization and the emission angle, 4 is the total

detachment cross section, = is the anisotropy parameter and varies within the limits of -1 to 2,

and > cos 9

1/23 cos 9  1 is the second-order Legendre polynomial. The anisotropy

parameter completely defines the angular distribution and is a function of the angular momentumof the contributing partial waves and the interference that occurs between them. Given the low
symmetry of the nitroalkane anions it is difficult to extract detailed information regarding theparent orbital or identify the interfering partial waves yielding a particular =. But despite theselimitations, the anisotropy parameter can still provide useful information. At very low kinetic
energies, the threshold analysis of Reed et al.5 indicates that one partial wave will dominate overthe contributions of others. Consequently, the beta parameter at low kinetic energies can be used
to get a sense of the identity of the leading term governing the threshold law. Finally, large or
abrupt changes in the beta parameter can be used to identify photoelectrons originating fromdifferent electronic states since the variation of the anisotropy parameter should be smooth for agiven parent state.

2.5 Franck-Condon Factors
The Franck-Condon factors are given by,
Equation 2.4

CDC|+&,- |&,˝ 0|where &,1 represents the vibrational wavefunction of the neutral and &,˝ represents thevibrational wavefunction of the anion. The validity of the Franck-Condon factors is based on

12several approximations. The first approximation is that the electronic transition probability is nota function of the nuclear coordinates. This is typically a good approximation since the transition
probability is a slowly varying function of the nuclear coordinates. Second, within the BornOppenheimer approximation it is expected that the electronic transition happens on a much fastertime scale than the nuclear motion. As the nuclei are effectively frozen, the most probablevibronic transition will occur between the two states in which the overlap of the vibrational
wavefunctions is greatest.
The effect of the Franck-Condon principle is most easily seen in the example ofphotoemission from a diatomic anion approximated as a harmonic oscillator, as it has only one
vibrational mode. In an effort to highlight the impact of certain parameters on the photoelectron
spectrum, several scenarios are discussed. In the first example, the shape of the neutral potential(i.e., force constant) is varied relative to the anion potential. The second example discusses theeffect of changing the equilibrium displacement of the neutral relative to the anion, and lastly,
the significance of including vibrationally excited anion states (hot bands) is considered. Figure
2.1 illustrates the effects of the first two scenarios. If the equilibrium distance of both potential
energy curves is the same, the overlap of the initial and final vibrational wavefunctions will begreatest for the transition to the ground state of the neutral since the vibrational wavefunctions
are very similar. In this scenario (i.e., no displacement), transitions to the other vibrational states(F1 G FH ˝) only occur if the force constant (i.e., shape) of the neutral curve is different from thatof the anion. If this is the case, the 2 G 0 transitions may become active in the spectrum, given

the overlap of the even wavefunctions. The
potential curve is anharmonic.

G 0 transitions can only become possible if the13
Displacement of the neutral potential energy curve leads to substantial activity intransitions to higher-lying vibrations as the anion vibrational ground state wavefunction begins toachieve the greatest overlap in the classical turning points of the excited vibrational states.
Additionally, it is important to note the diminished intensity of the 0 G 0 transition,corresponding to the adiabatic electron affinity.

If there is a substantial difference in theequilibrium distance of the anion and neutral molecules, this will lead to substantial difficulties
in identifying the adiabatic electron affinity. If the temperature of the system is sufficiently high,excited vibrational states of the anion will be populated. The excited states will create additional
progressions or hot bands that are interspersed in the progression created from the ground state.
This gives rise to great difficulty in locating the 0 G 0 transition, as it is necessary to identify thecontributions from the hot band and the ground state before an accurate assignment can be made.

Figure 2.1 – A representation of two scenarios. The scenario on the left involves photodetachment in which
there is no displacement but the force constant changes. The scenario on the right depicts a displacement
without a change in the force constant.

In a polyatomic system, the difficulty in interpreting the spectrum is compounded by thelarge number of vibrational modes. For a molecule with I atoms, which is not linear, there will

14
be 3I  6 vibrational modes. If the vibrational modes are well represented as uncoupled,harmonic oscillators the vibrational wavefunction can be written as,
Equation 2.5&,&,K L7  M &,N L  O &,PQRS LT U ,

where L represents the normal coordinate of thethnormal mode.10 If the vibrational normal

modes are preserved in the photoelectron emission, the Franck-Condon factors can be expressed
as a product of the overlap of each normal mode since the vibrational wave functions will still beorthogonal.

However, the geometry changes that occur during photoelectron emission are

typically accompanied by a change in the normal coordinates. This phenomenon was first notedby Duschinsky;11 he suggested the transformation could be described according to the following
equation,
Equation 2.6

L˝ J˝L 1 3 Y"

where Y" is the normal coordinate displacement vector in the basis of the normal coordinates of

the anion and J˝ is the Duschinsky rotation matrix which describes the mixing of the normal
coordinates.10

Additionally, Duschinsky outlined some general conclusions that are aconsequence of symmetry considerations.11

First, a common symmetry group is found todescribe both the anion and the neutral. If a normal coordinate is antisymmetric with respect tosome symmetry element, then the displacement of that mode is zero. This leads to little activity
in the nontotally symmetric modes since, with no displacement, the active transitions must be
even transitions of 2 G 0 or facilitated by anharmonicity. The second result is that only normal
modes that behave identically under all symmetry operations will mix. As a consequence,

15
normal modes belonging to the totally symmetric group will mix, as will the nontotallysymmetric normal modes belonging to the same symmetry groups.

2.6 References for Chapter 1
1.

J. H. D. Eland,"Molecular Photoelectron Spectroscopy." Journal of Physics E-Scientific
Instruments 11 (10), 969-977 (1978).

2.

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

3.

P. Chen, in Unimolecular and Bimolecular Reaction Dynamics, edited by C.-Y. Ng, T.
Baer and I. Powis (John Wiley & Sons, Chichester, 1994), pp. 371.

4.

R. Mabbs, E. R. Grumbling, K. Pichugin and A. Sanov,"Photoelectron imaging: anexperimental window into electronic structure." Chemical Society Reviews 38 (8), 21692177 (2009).

5.

K. J. Reed, A. H. Zimmerman, H. C. Andersen and J. I. Brauman,"Cross-Sections for
Photodetachment of Electrons from Negative-Ions near Threshold." Journal of Chemical
Physics 64 (4), 1368-1375 (1976).

6.

D. C. Harris, & Bertolucci, M.D., Symmetry and Spectroscopy: An Introduction toVibrational and Electronic Spectroscopy. (New York: Oxford University Press, 1978).

7.

E. P. Wigner,"On the Behavior of Cross Sections near Thresholds." Phys. Rev. 73 (9),1002-1009 (1948).

8.

J. R. Smith, J. B. Kim and W. C. Lineberger,"High-resolution thresholdphotodetachment spectroscopy of OH." Phys. Rev. A 55 (3), 2036-2043 (1997).

9.

J. Cooper and R. N. Zare,"Angular Distribution of Photoelectrons." Journal of Chemical
Physics 48 (2), 942-943 (1968).

16
10.

K. M. Ervin, J. Ho and W. C. Lineberger,"Ultraviolet Photoelectron Spectrum of NO2."
J. Phys. Chem. 92 (19), 5405-5412 (1988).

11.

F. Duschinsky,"The importance of the electron spectrum in multi atomic molecules.
concerning the Franck-Condon principle." Acta Physicochimica URSS 7 (4), 551-566(1937).

17

3 Experimental Apparatus
3.1 Overview
The experimental apparatus may be divided into four parts: ion source, time-of-flightmass spectrometer, light source, and photoelectron spectrometer. Ions are created in a pulsed
supersonic entrainment ion source and directed into the acceleration region of a time-of-flightmass spectrometer. The ions are mass-selected within the time-of-flight mass spectrometer andinjected into the interaction region of a velocity-map-imaging photoelectron spectrometer. The
mass-selected ion beam is intersected with either the tunable output of an infrared opticalparameter converter or 532 nm radiation generated from the second-harmonic of a Nd:YAG
laser. The nascent photoelectrons are then accelerated into the flight tube of the photoelectron
spectrometer and mapped onto a position sensitive imaging detector. Each image is digitized by
a CCD camera and collected until a sufficient signal-to-noise ratio is achieved for a
photoelectron image.

Each part of the instrument will be described in more detail in thefollowing sections, see Figure 3.1 below for a schematic overview of the instrument.

18

Figure 3.1 – Schematic diagram of the instrument used for a photoelectron spectroscopy study.

3.2 Ion Source
Anions are created in an electron impact plasma generated in a pulsed supersonic
expansion.

Over the last two decades, this type of ion source has proven a successful andreliable way to create molecular anions.1, 2 Central to the success of this technique, is theefficient attachment of slow electrons to many neutral molecules.1 Slow-secondary electrons areproduced from the injection of high energy electrons (~ 1 kV) into the high density region of asupersonic expansion of an inert gas, such as argon. The ionization of argon produces slowsecondary electrons that will remain in the expansion while the high-energy electrons continue
through the expansion, leaving a trail of slow-secondary electrons. This technique produces asufficient density of slow secondary electrons in the expansion and yields a neutral plasma thatcircumvents issues arising from space-charge limits associated with charged beams.2
The electron bombardment of a supersonic expansion serves as a well-suited environment
for anion formation; however, there are several approaches to introducing the anion precursors of19interest into the expansion. Perhaps the most obvious means of introducing a neutral molecule(or anion precursor) into the expansion is by expanding a gas mixture. This approach can beused, but the composition of the expansion mixture is difficult to control as many molecularprecursors readily adsorb onto the walls of the gas lines.2 A more precise means of introducing
the anion precursor and controlling the nature of the expansion is realized using supersonicentrainment.2
Supersonic entrainment is a hydrodynamic process, first characterized and extensively
studied by Campargue,3 in which a gas in the vicinity of the high density region of an expansion
efficiently penetrates the boundary of the expansion and is readily incorporated into thesupersonic flow. Practically, this is achieved by admitting the anion precursor, prior to thecreation of the supersonic expansion, into an entrainment reactor that serves as a “holding area”
which allows sufficient gas densities to build up in the vicinity of the main expansion nozzle.2
When the supersonic expansion is created, the anion precursor is entrained in the expansion
where it may cluster with the argon gas and subsequently form temporary negative ions:
Equation 3.1

Z[ • Ar 3 _ ` <Z[ • Ar Ba
The transient negative anion may undergo simple autodetachment, but more interestingly it may
be stabilized by dissociative electron attachment, evaporation of the argon atoms, or collisions
with background molecules (M):Equation 3.2<Z[ • Ar Ba ` Z • Arc 3 [ 3   dAr

20
Equation 3.3<Z[ • Ar Ba ` Z[ • Arc 3   dAr
Equation 3.4<Z[ • Ar Ba 3 M ` Z[ • Ar 3 M
While the supersonic expansion provides cooling, particularly in the rotational degrees of
freedom, numerous studies have revealed that the vibrational temperature of the anions formed
within the expansion is governed primarily by evaporation. As a consequence, the binding
energy of the most weakly bound species will dictate the upper limit for the internal energy of acluster.2, 4, 5 Cooler ion temperatures can be achieved by optimizing source conditions to favor
the production of target anions solvated by moderately large clusters of argon atoms, which havebinding energies of ~500 cm-1/Ar. This is accomplished using relatively low pressures of theanion precursors to ensure that most collisions are between molecules and argon atoms.

3.2.1 Implementation
A schematic diagram illustrating the supersonic entrainment ion source is shown in
Figure 3.2. The entrainment ion source is located within the a large chamber pumped by a 2000
L/s diffusion pump (BOC Edwards Diffstak) and a 1000 L/s turbomolecular pump (Turbovac,
1000C, Leybold) that maintain a background pressure of several 10-7 mbar. The source consistsof three independently controlled solenoid valves, an entrainment reactor, and an electron gun.

21

Figure 3.2 – Schematic diagram of the entrainment ion source. The source has the capability to entrain twogases. However, all of the studies within this thesis utilize only one entrainment valve.

The electron gun continuously produces a beam of high-energy electrons, typicallyaccelerated to a kinetic energy of 800 eV.

The beam is counter-propagating to the main

expansion and is directed by several electron optics elements to the high-density region of the
expansion. The experiment is operated at 20 Hz repetition rate. The sequence begins when thepulsed solenoid-actuated valves flanking the main expansion (Series 9, Parker Hannifin
Corporation, General Valve Division) admits the anion precursors of interest into theentrainment reactor.

Following this, a supersonic expansion of neat argon (99.9999%) is

generated. As the argon jet dictates the character of the expansion, a more precise valve isdesirable. To achieve these ends, we employ an Even-Lavie valve capable of a backing pressureof up to 100 bar (typically operated at 14 bar) and very short opening times (~25 µs). The large
backing pressure allows us to maximize cooling from the supersonic expansion while the short22
opening times make it possible to maintain a relatively low background pressure during
operation (around several 10-6 to 10-5 mbar). The neutral plasma that is generated expands freely
between the repeller and extractor of our Wiley-McLaren6 time-of-flight mass spectrometer
which is oriented perpendicularly to the expansion. The source conditions are optimized for coldanion creation through varying the amplitude of each gas pulse, the relative timing of each gas
pulse, the path and intensity of the electron beam, and the timing of the mass spectrometer’s
extraction pulse.

3.3 Time-of-Flight Mass Spectrometer
The instrument is equipped with a Wiley-McLaren6 time-of-flight mass spectrometer.
The plasma created in the ion source is injected directly into the acceleration region of the mass
spectrometer. A high voltage pulse is applied to the repeller, extractor, and ground electrodes,
accelerating the anions to a kinetic energy of ~ 3.5 keV. The beam is guided by xy-deflectors
into a differentially pumped stage pumped by a 230 L/s turbomolecular pump (Pfeiffer TMU 261
P) with an operating pressure of several 10-7 mbar. The differentially pumped stage is equippedwith an additional pair of xy-deflectors and an Einzel lens that serve to direct and focus the beamthrough the time-of-flight.

The flight tube is pumped by a 520 L/s turbomolecular pump(Pfeiffer TMU 521 YP) and kept at an operating pressure of several 10-8 mbar.
The anions will separate during the flight as a function of their mass-to-charge (d/e)
ratio.

23
Equation 3.5df5geDue to the finite volume of the anions in the acceleration region, ions of the same mass will be
accelerated to slightly different kinetic energies. All ions of the same mass-to-charge ratio will
arrive at the same time at one point in the flight path; this point defines the space focus of themass spectrometer. The ratio of the accelerating electric fields in the Wiley-McLaren6 setup
determines the position of the space focus.
The instrument currently has two operational modes: one is used to characterize the ion
source and perform infrared photodissociation spectroscopy studies while the other is used forphotoelectron spectroscopy. In the first mode, the anions are directed into a two-stage reflectron(R.M. Jordan Company); the space focus of the reflectron is centered on a dual microchannel
plate detector in a chevron configuration coupled to a conical anode for ion detection. The
detector output is monitored as a function of time to obtain a mass spectrum. The reflectronnearly doubles the flight path and serves as an energy focusing device which substantially
increases the resolution of the mass spectrometer, defined as
Equation 3.6

h

d
∆d

f
.
2∆f

For this mode, the resolution is R~800. Procedurally, ion source optimization is achieved byadjusting source conditions in order to maximize the intensity of large, argon solvated clusters
containing the anion of interest, ensuring that the overall temperature of the expansion is low.

24
Once the source has been optimized for the production of relatively cold ions, the second
mode is used. The beam is directed through the electrically grounded reflectron, at which point a
pulsed mass gate can be used to transmit only the ions of interest. The mass gate is operated byapplying !150 V across a deflection electrode, a voltage sufficient to deflect the ion beam into
the chamber walls. When the anion of interest is in the vicinity of the mass gate the voltage ispulsed to 0 V allowing an unimpeded flight into the photoelectron spectrometer. The massselected beam is passed through a 5 mm aperture as it crosses the threshold into thephotoelectron spectrometer chamber. The beam is then passed through an Einzel lens that serves
to focus the anion beam to several millimeters within the photodetachment region of thephotoelectron spectrometer. The mass resolution in this mode is R~150. As the photoelectron
spectrometer is oriented perpendicular to the path of the ion beam, the anions continue andimpinge on a dual microchannel plate in the chevron configuration coupled to an anode. The
detector signal can be measured on a digital oscilloscope to acquire a mass spectrum.
Alternatively, a retarding voltage may be applied to a grid directly in front of the detector suchthat photoneutrals generated in the photoelectron spectrometer may be monitored; this detection
scheme is important for maximizing the signal in the photoelectron spectrometer.

3.4 Laser System
Near-threshold photodetachment experiments are performed with the tunable output of an
optical parametric converter (LaserVision). A schematic diagram of the light source is displayedin Figure 3.3. The system consists of a Nd:YAG pump laser, an optical parametric oscillator(OPO), and an optical parametric amplifier (OPA). The Nd:YAG pump laser (Innolas Spitlight600) produces a linearly polarized, ~7 ns pulse with a typical output power of 475 mJ/ pulse at a

25wavelength of 1064 nm. The pump beam is directed by a series of steering optics and through abeam splitter; one-third of the beam is frequency-doubled in a potassium titanyl phosphate(KTP) crystal while the remaining two-thirds is directed through a delay before being used topump the OPA. After the OPA, the residual 1064 nm radiation is removed by a longpass
dichroic mirror while the 532 nm radiation is used to pump the two KTP crystals within thelinear cavity of the OPO. Conservation of energy requires,
Equation 3.72kl 3 m ,where k , l , and m are the frequencies of the pump, signal and idler, respectively. Angletuning of the KTP crystals is used to modulate phasematching conditions for the desired output
wavelengths.

The current setup allows for continuous tuning of the signal wave from 710 

885 nm, corresponding to a range between 1330  2120 nm for the idler wave. At the present

time, a polarizer is used to filter out the signal wave within the OPO cavity. After this stage, as asafety precaution the idler wave is passed through a half-waveplate such that the polarization isrotated to permit angle-tuning of the OPA crystals within the horizontal plane. The OPA
consists of four potassium titanyl phosphate (KTP) crystals; the idler and the 1064 nm radiation
are superimposed and directed through the KTP chain. Difference frequency mixing generates
mid-infrared radiation from 2200  4500 cm-1 according to,Equation 3.8

Mid-IRk  m .

26
Following the OPA, a dichroic mirror is used to remove the residual 1064 nm radiation. Of the
co-propagating vertically-polarized mid-infrared radiation and horizontally-polarized idler
radiation, a beam is chosen using a polarizer.
The vertically-polarized mid-infrared radiation is directed towards a twisted periscope
which flips the radiation to a horizontal polarization. Before the beam is admitted into thephotodetachment region, a Brewster “stack of plates” polarizer is used to ensure a clean,horizontal polarization. Following this, a lens focuses the beam into the photodetachment regionof the photoelectron spectrometer. Irises are placed at the entrance and exit of the photoelectron
spectrometer chamber. The beam is centered on each iris to ensure reproducible alignment of thelaser beam in the photoelectron spectrometer, as the focus of the imaging spectrometer issensitive to the vertical (as defined by the imaging spectrometer’s flight tube) position of thedetachment region. The optical parametric converter is calibrated by analysis of the signal wave
with a fiber optic spectrometer (Ocean Optics), which is calibrated to the emission spectrum ofan argon discharge.

As the bandwidth of the fiber optic spectrometer is 2 cm-1 and the

bandwidth of the optical parametric converter is 2 cm-1 we estimate a 3 cm-1 resolution of thesystem.

27

Figure 3.3 - Overview of the IR OPO/OPA tunable light source.

Higher-energy photoelectron studies are performed using the radiation generated from thesecond-harmonic of the Nd:YAG laser. In this case, the beam is directed by steering optics andfrequency doubled within a KTP crystal. The co-propagating 532 nm and 1064 nm radiation is
directed into a Pellin-Broca prism, where the angle of incidence is adjusted to select the
wavelength that is transmitted to the photoelectron spectrometer. The beam is then directed
through several prisms before reaching a half-waveplate used to rotate the polarization to thehorizontal plane, as is required by the photoelectron experiment. Finally, the beam is passed
through a converging lens which focuses the light into the photodetachment region of thephotoelectron spectrometer. The beam is aligned within the photoelectron spectrometer using
irises (mentioned above).

28

3.6 Velocity Map Imaging Photoelectron Spectrometer3.6.1 Photoelectron Image Acquisition
The photoelectron imaging spectrometer is based on the design of the Lineberger andSanov7, 8 groups and utilizes the velocity-map imaging technique pioneered by Eppink andParker.9, 10 The details of the photoelectron spectrometer are as follows. The spectrometer ishoused within an 8″ six-way chamber and oriented perpendicularly to the plane defined by theion and laser beams. The chamber maintains a typical operating pressure of several 10-8 mbar
and is pumped by a 520 L/s turbomolecular pump (Pfeiffer TMU 521 YP). The photoelectron
spectrometer consists of a set of imaging optics and a drift region terminated by a position
sensitive detector.

The imaging optics consist of three, 1 mm thick, oxygen-free-high-conductivity (OFHC) copper plate electrodes with diameters of 108 mm, which are evenly
spaced by 25 mm and coated with a thin surface of colloidal graphite. The three electrodes aredesignated the repeller (st , the extractor (s , and ground (su , where st v s v su . The
extractor and ground electrodes each have 20 mm diameter holes, while the repeller electrode isa solid plate. The stack of imaging optics is encapsulated by two concentric µ-metal cylindricalcups (1 mm thick) with four 25 mm apertures permitting the passage of the ion and laser beams
through the interaction region of the photoelectron spectrometer. The annealed µ-metal is a soft
magnetic material that provides shielding from stray magnetic fields. The boundary between theimaging optics and the drift region is defined by the grounded electrode. The drift region is a
150 mm field-free flight tube (as defined from the ground electrode to the face of the
microchannel plate) shielded by two concentric, 1 mm thick µ-metal cylinders. The drift regionis terminated by a position sensitive detector. A schematic representation of the spectrometer isgiven in Figure 3.4 below.

29

Figure 3.4 – Schematic diagram of velocity map imaging photoelectron spectrometer.

The mass-selected anion beam is collimated by a 5 mm aperture and focused by an Einzel
lens into the detachment region. The photodetachment region is defined by the overlapping
volume of the mass-selected anion beam with the laser beam which is located between therepeller and extractor electrodes. The overlap of the two beams is monitored by the generationof the photoneutrals as described at the end of section 3.3. The signal is optimized by adjusting
the focus of the Einzel lens and steering optics directing the beam into the photoelectron
spectrometer; additionally, the timing of the laser beam pulse is adjusted to ensure overlap withthe most intense part of the anion bunch. The simple geometry defined by the imaging optics
creates an inhomogeneous field within the photodetachment region. The field acts to focus all
nascent photoelectrons with the same initial velocity, irrespective of their initial position within
the detachment plane, to the same location on the position sensitive detector.

The large

insensitivity of the origin of the photoelectron within the detachment plane is what defines this asvelocity map imaging.9, 10 The location of the imaging-optics’ focal plane is determined by the

30
ratio of the repeller to extractor voltages, st /s . This plane must be empirically optimized tocoincide with the face of the detector. This is achieved in a brute-force optimization, in which an
initial estimate of the st /s ratio is obtained from a simulation (performed with the Simion
charged particle optics simulator, Version 7) and fine adjustments to the ratio are inferred fromthe peak width of well-defined transitions in an atomic photoelectron spectrum, typically S , asiterative adjustments are made. In the current setup, the ratio is
Equation 3.9

sts

0.721;

however, if any adjustments are made in the laser beam height, the ratio will have to be adjusted.
The photoelectrons are accelerated by the electric fields at the detachment region into thefield-free drift tube and are projected onto the front face of a position sensitive detector. The
position sensitive detector consists of two microchannel plates, with an active area of 40 mm, ina chevron configuration followed by a fiber optic P-47 type phosphor screen (Burle ElectroOptics, Inc.). The front face of the detector, i.e., the face of the first michrochannel plate, is kept
at ground. The back face of the second microchannel plate is pulsed from a voltage of 1100 V to2000 V in a 5 µs window centered on the photodetachment event; since the gain is only
appreciable at the greater voltage, the noise in the photoelectron image is greatly reduced. The
front face of the phosphor screen is held at a voltage of 5000 V. The phosphorescence isdigitized by a charge coupled device (CCD) camera (UNIQ UP-610CL, 659x694) mounted justabove the phosphor screen and housed in a shroud to eliminate any ambient light.
Each image is acquired and analyzed in real-time using the centroiding (WenImaging)program developed by Suits and coworkers.11 The centroiding algorithm performs a center-of-

31mass calculation on each event spot (caused by a single detected electron), achieving sub-pixel
resolution. This approach permits the use of a relatively inexpensive camera that is not cooled,possesses a comparatively small CCD array, and has a lower intrinsic signal-to-noise ratio. The
data coordinates of each analyzed event are stored in an output file which is updated with every
cycle of the experiment (at a rate of 20 Hz).

Each photoelectron image is typically an

accumulation of several 106 events. It should be noted that the number of photoelectrons in eachshot has to be low enough to prevent overlapping electron spots. The centroiding algorithm
partitions the image into smaller arrays to perform the center-of-mass calculation. When several
event spots are spread over the partitioned array the program produces an artifact located in theupper left corner of the images, after some time the shape of the artifact takes the form of a “jet”
pattern in one corner of the image.

This issue usually manifests itself in near-thresholdphotoelectron studies, as the intensity at very low kinetic energies is often very large. Problems
arising from this issue are alleviated by adjusting the photodetachment yield to an intensity thatsignificantly reduces the chance of nearly overlapping events.

3.6.2 Data Analysis
The acquired photoelectron image is a two-dimensional projection of the initial threedimensional photoelectron distribution. The laser is polarized in the direction of the ion beam(i.e., in the detector plane); this serves to define an axis of cylindrical symmetry for the system.
This property allows the initial three-dimensional distribution to be reconstructed by an inverse
Abel transformation as implemented by the BASEX program developed by Reisler andcoworkers.12 The centroided image is imported into BASEX. The center of the image must be
defined. This may be performed manually or by using the center-finding algorithm within the32program (discussed in more detail below). The program performs the reconstruction, returns anequatorial slice through the three-dimensional distribution, and then integrates over all emission
angles at each radius to obtain the photoelectron yield as a function of electron velocity. At thispoint, the spectrum is calibrated (see discussion below) to a well-known photoelectron spectrum
and recast as a function of the kinetic energy according to the Jacobian transformation,
Equation 3.10

8  5

8y
.
y

Additionally, the BASEX program can calculate anisotropy parameters by fitting the intensity
profile as a function of radius using
Equation 3.11

82, 9

D<1 3 =2> cos 9B

where 82, 9 is the well-known function governing a one-photon process, 9 is the angle between

the laser polarization and the emission angle, D is a constant, and = is the anisotropy parameterthat completely characterizes the angular distribution (see chapter 2 for a detailed discussion of
the anisotropy parameter). Figure 3.5 illustrates a composite of a raw photoelectron image (lefthalf) and a reconstructed image (right half) from a photoelectron study of S .

33

Figure 3.5 – Shown on the right is a photoelectron image of S taken at 532 nm. The left half illustrates theraw image. The right half illustrates the BASEX reconstructed equatorial slice through the original threedimensional photoelectron distribution. On the left side of the figure is the photoelectron spectrum generated
from the reconstructed image. The transitions are labeled to illustrate corresponding features.

Each photoelectron spectrum is calibrated to the well-known photoelectron spectra of S

or O .13, 14 Calibrations are performed with the output of the second harmonic of the Nd:YAG
laser at a photon energy of 2.331 eV. The S photoelectron spectrum is used to calibrate thelow-energy photoelectron spectra which use the output of the tunable optical parametricconverter. The S ions are created within the source by dissociative electron attachment to CS

vapor, entrained in the argon expansion. The O photoelectron spectrum is used to calibrate thehigher energy photoelectron spectrum taken at a wavelength of 532 nm; the anions are created byelectron attachment to molecular oxygen entrained in the argon expansion. The resolution of the34
spectrometer is determined by fitting the width of the features in these well-characterized spectra
as a function of kinetic energy; results indicate a resolution of ∆y5800 ! 1000 m/s for the

spectrometer.
The microchannel plates utilized in the imaging experiment suffer from several
inhomogeneities over the detector surface, leading to uneven amplification across the detector.
This introduces two complications in the reconstruction of the photoelectron image. First, thecenter of the image becomes difficult to accurately find, either by the algorithm or by eye.
Second, the inhomogeneities decrease the signal-to-noise ratio of the image.
As a consequence of the first issue, a procedure has been developed for accurately
determining the image center. The idea is to process the image, eliminate the inhomogeneities,
and make a “dummy” image that has the same center as the centroiding photoelectron image but
has an easily identifiable center. The centroided images are processed in a Java-based image
processing program (ImageJ 1.43u). The image is first subjected to an edge finding algorithm,
converted to binary, and finally a “fill holes” algorithm is used to create a large circle. The
center for this image is then easily found using the BASEX center-finding algorithm. For veryanisotropic images, it is often better to only perform the edge finding algorithm. In the event, thephotoelectron image has no sharp features it is necessary to use a time-consuming iterative
procedure, in which an estimate of the image center is given by the center finding algorithm orby eye and as many as sixteen or twenty-five images with different centers are reconstructed tofind the best center.
Regarding the second issue, the inhomogeneities decrease the signal-to-noise ratio inparticular regions of the image. Due to symmetry arguments, all of the information is contained

35
within a quadrant of the photoelectron image. If the inhomogeneities are localized within one
half of the image, it is often advantageous (i.e., increases the signal-to-noise) to construct a raw
image from one half of the centroided image prior to reconstruction. This is performed in
ImageJ by locating the center of the image according to the procedures outlined above. Then
one half of the image is mirrored along the center line, imported into BASEX and reconstructed.
As described above, in the current setup the photoelectron imaging spectrometer isoriented perpendicularly to the ion beam. This leads to several important ramifications in theoperation of the spectrometer. The first issue arises from the photodetachment of an anion
moving with ~ 3.5 keV of kinetic energy. When the electron is emitted, its velocity will be thevector sum of the anion velocity and the velocity imparted from photodetachment. As a result,the image is displaced from the center of the phosphor screen and the useable area of the
microchannel plate is reduced.

The displacement is given by the product of the velocity

imparted from the ion beam and the flight time of the photoelectron, this is limited to an
acceptable distance by increasing the magnitude of the repeller voltage (while maintaining the{ratio {| ). The most consequential effect of the imaging spectrometer’s orientation comes from}the nature of the space focus of the time-of-flight spectrometer. The space focus is typicallylocated at the photodetachment region in order to get the largest photodetachment signal.
Unfortunately, this is associated with a large uncertainty in the ion kinetic energy commensuratewith the spread of the plasma within the acceleration region of the time-of-flight mass
spectrometer. The uncertainty in the velocity of the anion beam will contribute to an uncertainty
in the velocity of the photodetached electrons along the time-of-flight axis, deteriorating theresolution of the spectrometer. The argument can be made more quantitative if we look at themathematical relations. Let us first consider a collection of anions, with the same mass, spread36
along the time-of-flight axis within the acceleration region of the mass spectrometer. These
anions will have an associated intensity profile, 8~, where z is the coordinate along the time-of-flight axis. This intensity profile will also have some finite uncertainty, ∆~. Since the time-offlight acceleration region produces a linear potential gradient between the repeller and extractor

electrodes, ∆~ 5 ∆ , where  is the initial kinetic energy of the anions in the acceleration
region of the mass spectrometer. The initial kinetic energy spread is given by
Equation 3.12∆d y ∆y ,where d , y and  represent the anion’s mass, velocity and kinetic energy, respectively. It
can be shown that this will lead to a kinetic energy spread along the time-of-flight axis of
Equation 3.13

∆

d
∆ ,
d 

where d and  represent the mass and kinetic energy of the photodetached electrons. If weconsider some typical parameters in our experiment, the plasma is freely expanded between therepeller and extractor (separated by 30 mm). For this setup, a fair estimate of ∆ is ~ 400 V.
This indicates that for an anion with a 50 amu mass, the uncertainty introduced to thephotoelectron, in the direction of the anion beam, is ~ 4 meV. This introduces a large part of theuncertainty present in every photoelectron spectrum and is particularly harmful to spectra inwhich = approaches 2 (a parallel transition). We attempt to abate the effect of this inherent
problem by lowering the magnitude of the ion acceleration voltages while maintaining the TOF
space focus, resulting in a reduction of ∆ .

37

3.7 Using Argon Solvation for Accurate Adiabatic Electron Affinity
Determinations
One of the primary goals of many photoelectron studies is to accurately determine theadiabatic electron affinity of the molecule under study. This is often complicated by small
Franck-Condon factors for the origin of the vibrational progression of the electronic ground stateof the neutral, which must be resolved for to determine the adiabatic electron affinity, or by thefinite temperature of the ions prepared for photodetachment, which leads to multiple
progressions that make the identity of the adiabatic electron affinity unclear. The FranckCondon factors obviously cannot be modified, as they are a property of the system; however, thetemperature of the ions may be reduced using several experimental approaches.
The first approach to reducing the temperature of the ions has been outlined above insection 3.3, in which the source conditions are optimized to produce the anions of interest
solvated in large argon clusters. This technique serves to moderately cool the bare anions and, assuggested from Franck-Condon simulations, yields an anion temperature of ~ 250 K. At thistemperature, especially in molecules with many energetically low-lying vibrations, the adiabatic
electron affinity remains difficult to assign as many of the progressions arising from hot bands
still have appreciable intensity.
The photoelectron spectra of the anions solvated with argon atoms provide one of themost effective means of eliminating hot bands and correctly identifying the adiabatic electron
affinity. The presence of the argon ensures that the ions have temperatures of only a few tens ofK, since the argon binding energies to most anions are only a few tens of meV. Additionally, thevery weak binding energy of the argon to the neutral molecule is expected to have a negligible
effect on the neutral vibrational modes. The relatively cool argon-solvated anion will yield a38photoelectron spectrum that is shifted relative to the spectrum of the unsolvated anion by theargon solvation energy, but more importantly the hot bands will be strongly suppressed oreliminated. This often allows for an unambiguous assignment of the features that correspond tothe

G 0 transitions in the photoelectron spectrum.

3.8 References for Chapter 3
1.

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

2.

W. H. Robertson, J. A. Kelley and M. A. Johnson,"A pulsed supersonic entrainment
reactor for the rational preparation of cold ionic complexes." Review of Scientific
Instruments 71 (12), 4431-4434 (2000).

3.

R. Campargue,"Aerodynamic Separation Effect on Gas and Isotope Mixtures Induced by
Invasion of Free Jet Shock Wave Structure." Journal of Chemical Physics 52 (4), 1795(1970).

4.

C. E. Klots,"Evaporative Cooling." Journal of Chemical Physics 83 (11), 5854-5860(1985).

5.

C. E. Klots,"The evaporative ensemble." Zeitschrift für Physik D Atoms, Molecules andClusters 5 (1), 83-89 (1987).

6.

W. C. Wiley and I. H. McLaren,"Time-of-Flight Mass Spectrometer with Improved
Resolution." Review of Scientific Instruments 26 (12), 1150-1157 (1955).

7.

A. Sanov and W. C. Lineberger,"Dynamics of cluster anions: a detailed look atcondensed-phase interactions." Physchemcomm, 165-177 (2002).

8.

A. Sanov and W. C. Lineberger,"Cluster anions: Structure, interactions, and dynamics inthe sub-nanoscale regime." Physical Chemistry Chemical Physics 6 (9), 2018-2032(2004).

39

9.

A. Eppink and D. H. Parker,"Velocity map imaging of ions and electrons using
electrostatic lenses: Application in photoelectron and photofragment ion imaging ofmolecular oxygen." Review of Scientific Instruments 68 (9), 3477-3484 (1997).

10.

D. H. Parker and A. Eppink,"Photoelectron and photofragment velocity map imaging ofstate-selected molecular oxygen dissociation/ionization dynamics." Journal of Chemical
Physics 107 (7), 2357-2362 (1997).

11.

W. Li, S. D. Chambreau, S. A. Lahankar and A. G. Suits,"Megapixel ion imaging withstandard video." Review of Scientific Instruments 76 (6) (2005).

12.

V. Dribinski, A. Ossadtchi, V. A. Mandelshtam and H. Reisler,"Reconstruction of abeltransformable images: The Gaussian basis-set expansion abel transform method." Review
of Scientific Instruments 73 (7), 2634-2642 (2002).

13.

H. Hotop and W. C. Lineberger,"Binding-Energies in Atomic Negative-Ions .2." Journalof Physical and Chemical Reference Data 14 (3), 731-750 (1985).

14.

K. M. Ervin, I. Anusiewicz, P. Skurski, J. Simons and W. C. Lineberger,"The Only
Stable State of O2- Is the X 2Πg Ground State and It (Still!) Has an Adiabatic Electron
Detachment Energy of 0.45 eV." The Journal of Physical Chemistry A 107 (41), 85218529 (2003).

40

4 Photoelectron Spectroscopy of Nitromethane Anion
This chapter has been reproduced in part with permission from Adams, C.L., Schneider, H.,
Ervin, K.M., and Weber, J.M., “Low-energy photoelectron imaging spectroscopy ofnitromethane anions: Electron affinity, vibrational features, anisotropies, and the dipole-boundstate.” Journal of Chemical Physics, 2009. 130: 074307. Copyright 2009, American Institute ofPhysics.

4.1 Introduction
The interaction of nitromethane (CH NO ) with low-energy electrons and the propertiesof the CH NO anion have been the subject of many studies, both experimentally1-10 andtheoretically.11-13 One remarkable feature of CH NO is that it has a sufficiently large dipole

moment ((3.46 D) to bind an excess electron not only in a valence-like state but also in astate more accurately described as a dipole-bound state.1-3, 11, 12 Interestingly, the signature of thedipole-bound state has not been observed in photoelectron (PE) spectroscopy. The binding
energy of the valence-like state is small1 relative to several of the fundamental vibrationaltransitions in the anion, making it an ideal molecular system for the investigation of theinteraction of vibrational excitation and electron emission.4, 10
The adiabatic electron affinity (AEA) of CH NO has been determined to be (260 ! 80)
meV by Compton et al.1 by using the last discernible peaks on a very extended and congested
vibrational progression as bracketing features. The error bars were intended to cover the rangeof possible origin assignments of a vibrational progression with (80 ! 9) meV or (645 ! 56)
cm-1 spacing, yielding possible electron affinities of about 180, 260, or 340 meV. In light of the41interest in the interaction of CH NO with low-energy electrons and investigation of vibrationelectron interaction, a more accurate and precise determination of the AEA is desirable.
The extended and congested vibrational progressions that made earlier AEA
measurements difficult is based on the large differences in the geometry of the nitro groupbetween the anion and the neutral molecule. In the neutral molecule, the CN bond lies in the

ONO plane, while the nitro group is strongly angled away from the CN bond in the anion
geometry (see Figure 4.1). The vibrational spacing found in the PE spectrum reported byCompton et al.1 is consistent with both the NO bending mode and the NO wagging mode of theneutral molecule.
In this chapter, low-energy PE imaging results for CH NO and cold CH NO • Ar arepresented along with simulations of the PE spectrum.

The feature corresponding to thevibrational ground state in the PE spectrum is unambiguously determined by comparison of thebare (and relatively warm) CH NO with that of cold CH NO • Ar, resulting in a value for the

AEA of CH NO of (168 ! 6) meV. A weak signature in the PE spectrum is tentatively
assigned as the dipole-bound state. Finally, the angular distributions of the observed features are
discussed in the context of the threshold laws for photodetachment and assignments of thevibrational structure in the spectra are made, based on the simulations.

42

4.2 Computational Details
4.2.1 Ab Initio Calculations
Computational chemistry calculations were performed using the GAUSSIAN03
program.14 Geometries and frequencies for (2A′) CH3NO2¯ and (1A′) CH3NO2 were calculatedusing density functional theory (DFT) with the Becke-3-Lee-Yang-Parr (B3LYP) hybrid
functional15, 16 and the 6-311++G(2df,2p) basis set.17, 18 Geometries were also calculated using
coupled-cluster theory with single and double excitations and perturbative inclusion of triples(CCSD(T)) (Ref. 19) with the aug-cc-PVTZ basis set.20 The anion and neutral geometries for thetwo levels of theory are shown in Figure 4.1. The most significant geometry deviation between
the DFT and coupled-cluster calculations is in the angle α between the plane of the NO2 groupand the CN bond.

Figure 4.1 – Geometries of CH3NO2¯ and CH3NO2; α denotes the angle between the ONO plane and the CNbond axis from DFT calculations; results from the CCSD(T) calculations are in parentheses.

Calculated DFT frequencies are provided in Table 4.1.

The calculated electron affinity is

EA0(CH3NO2) = 0.162 eV at the CCSD(T)/aug-cc-pVTZ level with vibrational zero-point energy
corrections from the B3LYP/6-311++G(2df,2p) harmonic frequencies.

43
4.2.3 Franck-Condon Analysis
Nitromethane has fifteen vibrational modes. Fourteen of these modes are well describedas harmonic oscillators; for these modes, the calculated vibrational normal modes from the DFT
calculations were employed to simulate the Franck-Condon profiles using the PESCAL program
developed by Ervin and coworkers.21, 22 The remaining mode corresponds to the torsional
vibration for internal rotation about the CN bond. This mode was treated independently and later
incorporated into the final Franck-Condon simulations; this will be discussed in more detailbelow. The Franck-Condon factors were calculated in the independent harmonic oscillator
approximation with full treatment of Duschinsky rotation23 of the normal mode eigenvectors
between the anion and neutral, using the Sharp-Rosenstock-Chen recursion method.21, 24, 25
The vibrational frequencies for neutral CH NO , excluding the torsion, were taken fromthe experimental Infrared/Raman fundamental frequencies26 and substituted for the calculated
values. The three CH stretch fundamental frequencies for the anions were taken from infrared
spectroscopy experiments.4, 10

The other anion frequencies were estimated by scaling the

calculated B3LYP/6-311++G(2df,2p) harmonic values by the ratio of the experimental
fundamental to the calculated harmonic frequency of the neutral for the similar normal mode.
These frequencies are given in Table 4.1.
Both neutral and anionic CH NO possess C symmetry with nine totally symmetricvibrational modes, which have nonzero normal coordinate displacement between the anion andneutral, and six nontotally symmetric modes. Nontotally symmetric modes undergo no normal
coordinate displacement upon electron emission and are considered Franck-Condon “inactive,”but can have some activity for 2 G 0 transitions because of the overlap of the even wave
functions with different harmonic frequencies.

All

G 0 transitions are possible with

44
nonharmonic potentials; for example, the torsional mode. The calculated geometry changes
 ” displacement vector in
between the anion and neutral are given in Table 4.1, presented as the Y
the anion normal coordinate frame.21, 25 The signs of the Cartesian displacement vectors arearbitrary in the harmonic oscillator approximation, but need to be defined self-consistently for
the Duschinsky rotation calculations. In this simulation, the direction of the normal coordinates
for each mode was chosen such that the B rotational constant of the molecule decreases (Iyycomponent of the moment of inertia increases) for a positive displacement.

45

Table 4.1 – Frequencies (cm-1) and geometry displacements.

CH NO

K″ /(g/mol)1/2Angstroma

Mode

Assign.b

B3LYPcScaledd

B3LYPcExp.b

B3LYPcfite

1

w(NO2)

411.7

403f

616.1

603

0.68020.65

21391.51369

1402.91380−0.0428−0.14

1201.71172

1431.91397

0.32550.34

4

δs(CΗ3)vs(NO2)r┴(CH3)1089.61084

1136.71131

0.1267

0.03

5

v(CN)

844.1

836

926.9

918

−0.2745−0.31

6

δ(ΝΟ2)

583.8

579

3

7

vs(CH3)2887.5662.5

657

0.08542777

g3081.62974−0.0345g

3168.43045−0.0034

1479.61434

0.0122

8

vs'(CH3)3054.92922

9

δs'(CΗ3)

1461.2

1416
h

11.6

F.R.

h

0

10

torsion

204.2

H.R.

11

r(NO2)

446.2

440

481.5

475

0

12

r||(CH3)

1058.51044

1111.4

1096

0

13

va(NO2)

1280.5

1252

1619

1583

0

14

δa(CΗ3)1472.51416

1466.61410

03108.1f

3200.5

3080

0

15
a

CH NOva(CH3)2965

Geometry displacements in terms of anion normal coordinate vectors.

bExperimental fundamental frequencies (Ref. 26). Assignment labels in column 2 are takenfrom Ref. 26.

c

B3LYP/6-311++G(2df,2p) harmonic frequencies.

dExcept as noted, scaled theoretical values as described in the text.

eOptimized displacements to fit PE spectrum of Compton et al. (Ref. 1).

f

The experimental value from the present work is (380±56) cm-1 as derived from the hotbands (see text).

g

Experimental fundamental frequencies (Refs. 3 and 5).

hH.R. threefold hindered rotor with 953 cm-1 barrier from B3LYP/6-311++G(2df,2p)
calculation. F.R., free rotor.

46
4.2.3.1 Franck-Condon Simulations: Internal Rotor
The internal rotation of the methyl and NO groups about the CN bond is a hinderedmotion in the anion but is essentially a barrier free motion in neutral CH NO . The torsional
potentials are calculated at the B3LYP/6-311++G(2df,2p) level by scans of the HCNX dihedral
angle, where X is a dummy atom placed 90° to the CN bond and defining the bisector of the NO

group, while all other distances and angles are allowed to relax. For the anion, the potential
energy is well approximated by a cosine function,
Equation 4.1

sH 1  cos ,
2

s
with

3 and a barrier of sH

represented bystudy, of sH

953 cm-1. For the neutral, the potential is sixfold symmetric,

6 with a barrier, determined by a high resolution Fourier-transform infrared

2.11 cm-1.26 For the present study, with our resolution, it is appropriate to treat

neutral CH NO as a free internal rotor.

The free internal rotor of neutral CH NO is modeled as a particle-in-a-ring to obtain theenergy levels and corresponding wavefunctions. In this treatment, the Hamiltonian is given by,
Equation 4.2  
,
28  where 8 is the reduced moment of inertia. The reduced moments of inertia were obtained as

47
Equation 4.3

8

8P M 8TN
8P 3 8TNwith the CH and NO group moments of inertia obtained from the calculated geometries

relative to the CN bond axis; this is the simplest approximation for the rotor moment of inertia as
described by East and Radom.27 The energy eigenvalues for the free internal rotor are

Equation 4.4

.t.




28

 D ;



0, !1, !2, !3, …

where  is the quantum number of the free internal rotor and D is the torsional constantcalculated as D

5.621 cm-1 and the corresponding normalized, free internal rotor

wavefunctions are given by,
Equation 4.5

&

1

√2;

_ * .

The Hamiltonian of the hindered internal rotor of CH NO is
Equation 4.6

  
3 s
28  where s is the torsional potential and DD  3 s,

5.609 cm-1 . A truncated basis set of free rotor

wavefunctions is applied to the hindered internal rotor Hamiltonian, as outlined by Ingham andStrickler28, as well as Spangler and Pratt29, 30. The resulting Hamiltonian matrix,

48
Equation 4.7

 D 3sHsH
 ,  ,! ,2
4

may be diagonalized to yield the hindered-rotor wavefunctions and the corresponding energy
eigenstates. For the present study, a basis set from 20 to 

320 was found to converge;

the energies of the free and hindered internal rotor states may be found in Table 4.2 below.
The Franck-Condon factors for the torsional vibration were simulated in Mathematica(Wolfram Research Inc.). A program was written to generate the eigenvectors of the anionic andneutral CH NO , according to the procedure outlined above, and project the wavefunctions of
the anion onto the neutral free internal rotor wavefunctions. The Franck-Condon overlap integral
is given by
Equation 4.8

CDC, 

 



H

&a & %



;



20, 19, … ,19, 20

where & is the wavefunction of the  neutral internal rotor, & is therotor wavefunction of the anion, and  is the Boltzmann weighting of thehindered internal
state in the anion

for a temperature specified in the program. Since the hindered internal rotor states are simply
expansions in the orthogonal free internal rotor wavefunctions this may be simplified toEquation 4.9&a & H&a  

1¡ H1 &1  M ,1 .

49
The transition intensities are imported into PESCAL22 and convolved with a Gaussian to matchthe experimental resolution.

This approach effectively treats the torsional mode as anindependent harmonic oscillator with no Duschinsky mixing with other modes.

Table 4.2 – Lowest Free and Hindered Internal Rotor States

Quantum Number
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Free Internal Rotor Energy[cm-1]
0.0
5.6
22.5
50.6
89.9
140.5
202.4
275.4
359.7
455.3
562.1
680.1
809.4
949.9
1101.71264.71439.01624.51821.22029.22248.4

Hindered Rotor Energy[cm-1]
106.4
106.4
106.4
312.3
312.3
312.3
503.1
503.2
503.2
676.3
676.3
677.6
821.0
831.2
831.2
936.9
936.9
984.5
1019.51093.81093.850

4.3 Results and Analysis
4.3.1 Interpretation of the photoelectron spectrum: the adiabatic electron affinity
and the dipole-bound state
The uncertainty in the AEA determination of Compton et al.1 was due to the shape of the
PE spectrum, which is dominated by an extended vibrational progression, peaking at a vertical
detachment energy of 0.9 eV. The AEA determination was done by using the last discernible
peaks at the low binding energy of the spectrum.
Figure 4.2 shows the PE spectra of CH NO at 397 meV (3200 cm-1) and 322 meV(2600 cm-1) photon energy, and a sample PE image is presented in Figure 4.3. This corresponds
to the low binding energy side of Compton’s et al. spectrum (see Figure 4.2). The spectrum isdominated by a progression of intense peaks with spaced by (76 ! 7) meV or (613 ! 56) cm-1.
The lower binding energy range shows a series of smaller features with less regular spacing. It
should be noted that all photon energies in this study were chosen carefully to avoid theinfluence of known vibrational autodetachment resonances of the CH NO anion.4, 10

photoelectron yield [arb. units]51

0.0 0.5 1.0 1.5 2.0

D

0

100

200

300

400

electron binding energy [meV]Figure 4.2 – PE spectra of CH3NO2¯ at 322 meV (2600 cm-1, lower trace), 397 meV (3200 cm-1) and from Ref. 1(inset, abscissa is in units of eV); the circles are data points, the full lines are five point adjacent averages toguide the eye; the arrow denotes the peak corresponding to the vibrational ground state of the neutral
molecule AEA=(168 ± 6) meV; D is the signature of the dipole-bound state.

At this point, three intense features may be assigned to the main vibrational progression,
with the vibrational 0-0 origin transition at the nominal peak position of 170 ! 5 meV

and the features toward lower binding energies are assigned as hot bands. The energy spacing of
613 ! 56 cm-1 is in agreement with the value of 645 ! 70 meV or 80 ! 9 meV reported

52
by Compton et al.1 and corresponds to the 603 cm-1 frequency for the NO2 wag mode of gasphase CH NO reported by Gorse et al.26 The comparison of the spectrum of bare CH NO andthat of CH NO M Ar (see Figure 4.4) corroborates this assignment. The series of intense peaks

spaced by 76 meV is shifted by (63 ! 7) meV toward higher binding energies upon Ar solvation.

The hot bands at lower binding energies are suppressed for CH NO M Ar because of the much
lower internal energy of the argon complex. The PE spectrum of CH NO M Ar (not shown) is

shifted by an additional (61 ! 7) meV relative to that of CH NO M Ar. This very regularbehavior in the Ar-induced shifts confirms our identification of the hot bands in the spectrum ofbare CH NO . Therefore, we determine the AEA of CH NO to be (168 ! 6) meV (after
corrections for torsional and rotational contour shifts discussed below).

D

Figure 4.3 – PE image of CH3NO2¯ at 322 meV (2600 cm-1) photon energy, based on one quadrant of the raw
PE image. The left half of the figure contains the raw PE image with the intensity in the upper portion
doubled to better show the dynamic range of the image without saturating the gray scale. The right half ofthe figure shows the BASEX transformed image with the “center-line artifact” from the BASEX transformed
image removed. The signature belonging to the dipole-bound state has been labeled “D.”

53Comparing the geometries of anionic and neutral CH NO , the most prominent geometry

difference between the two charge states is the position of the NO group relative to the CN
bond; this difference will likely lead to significant activity in this mode and result in an extended
vibrational progression in the PE spectrum. Based on this assumption, we can tentatively assign

the most intense peaks in the experimental PE spectrum of CH NO at 397 meV (3200 cm-1)
photon energy as the first three members of a progression of the neutral NO wagging mode F7

H(see Table 4.1 above for the labeling of the modes) with quantum numbers

0, 1, and 2. InH
7

addition, we can identify a peak at (122 ! 5) meV binding energy as a hot band of the anionicNO wagging mode F7 , i.e., corresponding to the transition  7
 

H

0 G  7 

peak at (77 ! 5) as a hot band in the same mode but corresponding to the  7H

1, and a0 G  7

 

2 transition. We therefore arrive at an experimental frequency for the anionic NO2 wagging

mode of (380 ! 56) cm-1. A band at (195 ! 5) meV binding energy is the signature of thetransition  7

H1 G  7 1.

The higher order members, i.e.,  7 ¢ 1, of thisHprogression are very weak but observable as shoulders on the higher energy sides of the intense
neutral progression. We note that other low-frequency modes may contribute to the observed hot
bands. A more thorough discussion on the assignment of the observed vibrational features basedon the Franck-Condon simulations will be presented below (see Section 4.3.3).

photoelectron yield [arb. units]54

0

50 100 150 200 250 300 350 400

binding energy [meV]Figure 4.4 – PE spectra of CH3NO2¯ (upper trace) and CH3NO2¯ ·Ar (lower trace) at 397 meV (3200 cm-1)
photon energy. The dashed lines and arrows show how the vibrational progression of the neutral molecule
shifts by 63 meV toward higher binding energies upon solvation by one Ar atom.

The PE spectra of bare CH NO show a weak feature at a binding energy of (8 ! 8)
meV. This feature does not correspond to any of the possible anion vibrational modes with any
appreciable Franck-Condon intensity (see below), which makes it highly unlikely to be a hot
band. Compton et al.1 determined the binding energy of the dipole-bound state of CH NO

55
populated in Rydberg electron transfer experiments to be (12 ! 3) meV, in agreement with thefeature at low binding energies to the dipole-bound state of CH NO , which has not been

observed previously by PE spectroscopy. The signature of the dipole-bound state is absent in the
PE spectrum of CH NO M Ar (as can be seen in Figure 4.4). This behavior is in agreement withprevious observations that have determined that the presence of an argon atom will only lead tothe formation of the valence-bound state, as solvation should lead to a greater stabilization of thevalence state relative to the dipole-bound state.2 The existence of the dipole-bound state at
relatively long lifetimes (i.e., ~ 25 µs) provides evidence that the dipole bound state ismetastable.2 As will be discussed below, the anisotropy of the photodetached electrons from thisfeature (taken at several photon energies) does not follow the same trend as those from clearlyvalence-bound hot bands, hinting at the involvement of a different electronic state.

4.3.2 Photoelectron Angular Distributions
Anisotropy parameters were determined as a function of electron kinetic energy forseveral photon energies: 285 meV (2300 cm-1), 310 meV (2500 cm-1), 322 meV (2600 cm-1), 397
meV (3200 cm-1).

The anisotropy parameters were fitted at known vibrational features,

including hot bands, in the PE spectrum; additionally, the anisotropy parameter was determined
for the feature at (8 ! 8) meV, tentatively assigned as the dipole-bound state, for eachwavelength.
At the low kinetic energies, accessed by the photon energies used in this study, Wigner’s
threshold law31 governs the relative weighting of the allowed partial waves. This stipulates thatthe relative cross section 4ℓ of a partial wave with given orbital angular momentum ℓ behaves as56
Equation 4.10

4ℓ 5 

ℓ67/

As $ waves (i.e., ℓ0) are isotropic (=0), and dominate at low kinetic energies over thecontributions of partial waves with higher orbital angular momentum, we expect the anisotropy
parameter to progressively deviate from =0 as  increases. For a given electronic final

state, the variation of = with energy is expected to be monotonic and smooth. Inspecting theexperimental results for CH NO (see Figure 4.5) we observe that the values of the anisotropy
parameter for the valence states (including hot bands) follow a consistent trend, increasing from= £ 0 (outgoing s-wave) at low  to = £ 1.25 for  £ 325 meV. This is consistent with thecontribution from the $ partial wave dominating at low kinetic energies, while higher angularmomenta play an increasing role as  increases. We caution that Wigner’s threshold law

strictly only holds for electron-molecule interaction potentials that fall off faster than 1/2  .
Interactions with polar molecules, especially those with a dipole moment sufficiently large tosupport a bound state, can behave stepwise and oscillatory.32

57

anisotropy parameter β

2.5

2.0

1.5

1.0

0.5

0.0
0

50

100

150

200

250

300

350

400

electron kinetic energy [meV]Figure 4.5 – Anisotropy parameters are presented as a function of electron kinetic energy for different photon
energies. Squares, circles, triangles, and stars correspond to data taken at 285 meV (2300 cm-1), 310 meV(2500 cm-1), 322 meV (2600 cm-1), and 397 meV (3200 cm-1), respectively. Filled symbols correspond to thevalence electronic state, while the open symbols belong to the dipole-bound state. The data points marked
with larger circles belong to the hot bands at binding energies of 77 and 122 meV. Error bars have beenconservatively estimated to be ±0.1, except in cases where fits of the angular distributions resulted in larger
error bars because of poorer signal-to-noise ratios. The dashed line shows the theoretical maximum value forthe anisotropy parameter (ββ = 2).
2

In contrast to the valence state (including the hot bands), photodetachment from thedipole-bound state results in = £ 2 for all data points corresponding to this state for all photon
energies (see Figure 4.5). This dramatic difference confirms that the electrons in this feature
originate from a different electronic ground state of the anion. Moreover, the large = value isconsistent with a nearly pure  character for the outgoing wave, similar to photodetachment from

58
H

ions,33, 34 resulting from photodetachment of an $-like orbital. The dipole-bound state is

expected to be spatially extended and $-like in shape. This is consistent with our observations

along the lines of this simple argument. We note, however, that an outgoing -wave is not the
only possibility to generate a = £ 2 anisotropy parameter.35, 36

4.3.3 Franck-Condon Simulation
In the photodetachment transition from CH NO to CH NO , the largest geometry
displacement is along the NO2 wagging normal coordinate with the known frequency26 of 603
cm-1 in the neutral. As discussed above, this mode accounts for the major Franck-Condon active
progression in the PE spectrum according to the simulation based on the DFT calculations. Fourother modes, listed as modes 2-5 in Table 4.1 above, also show substantial Franck-Condon
activity, primarily for their 1←0 and 2←0 transitions. Together these five modes account forover 90% of the Franck-Condon profile intensities. Combination bands involving modes 3-5
with frequencies of 1100-1300 cm-1 tend to overlap with the transitions from the NO2 wagging
mode at moderate resolution, causing shifts in the observed peaks; this near degeneracy accounts
for the observed spacing of 645 cm-1 reported by Compton et al.1

photoelectron yield [arb. units]

59

-400

-200

0

200

400
-1

relative binding energy [cm ]Figure 4.6 – Comparison of the torsional Franck-Condon simulation (vertical sticks) at 100 K temperatureand the experimental results for CH3NO2¯ ·Ar, shifted by the EA of the Ar complex (full line).

As a consequence of the differences in geometry between anionic and neutral CH NO ,the character of the internal rotation of the methyl group changes completely upon electron
detachment. In the anion, the methyl rotor is strongly hindered, while it is essentially a free rotor
in the neutral. This leads to each vibrational band being accompanied by combination bands
involving the hindered-to-free internal rotor transitions. Figure 4.6 shows that the simulated
spectrum for CH NO at 100 K in comparison with the experimental PE spectrum will be
virtually free of hot bands, as the presence of the weakly bound argon demands. While the60
individual transitions are not resolved in the experimental spectrum, the shape of the 0←0 bandis clearly asymmetric with a steeper low binding energy slope. On the basis of our FranckCondon calculations, we argue that this asymmetry is mainly due to the hindered-to-free methyl
rotor transitions. A stick spectrum of the Franck-Condon transitions for the torsional modes isshown in Figure 4.6. Because the upper state is nearly a free rotor and the lower state hinderedrotor ground-state wavefunction includes components of a series of free-rotor basis set functions
that overlap with the free-rotor wave functions, the simulated spectrum is qualitatively
reminiscent of rotational structure. In the independent oscillator approximation, this structure isrepeated for every vibrational transition in the nitromethane spectrum.

At the available

experimental resolution, the individual torsional transitions are not resolved but they contributeto the broadening of the observed transitions.

photoelectron yield [arb. units]

61

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

electron binding energy [eV]

Figure 4.7 – Comparison of the experimental PE spectrum by Compton et al. (Ref 1) with a fitted FranckCondon simulation (see text).

Rotational contours were calculated as described previously37, 38 using DFT geometries
and treating the anion and neutral as asymmetric tops. At the experimental resolution, therotational contours are only slight asymmetric and the peak shift is -1.2 meV at 250 K. The
contour due to the internal torsional transitions shifts the peaks an additional -1.0 meV.
Inclusion of the shifts, at a temperature of 250 K, gives a net shift of (2.2 ! 1) meV. This
shift has been accounted for in the electron affinity values reported above.
Because the low-energy imaging PE spectra exhibit only the first few transitions of theFranck-Condon envelope, Franck-Condon fits where the geometry displacements are optimized

62
are not well constrained by the present experiments.

In addition, variations in thephotodetachment cross sections are expected owing to threshold effects for imaging spectra
taken at low photon energies. Therefore, we first compare the Franck-Condon simulations to theCH NO PE spectrum of Compton et al.,1 which was taken at a photon energy of 2.54 eV andexhibits the entire Franck-Condon band profile. Figure 4.7 compares the PE spectrum withFranck-Condon simulations using normal coordinates from the DFT calculations, frequencies
given in Table 4.1, and the origin assignment as obtained from our imaging experiments. The
peaks are convolved by a Gaussian function with a FWHM of 30 meV as reported for the
instrumental resolution.1 An anion vibrational temperature of 300 K is assumed. The raw
calculated displacements already gave a very good match with the experimental spectrum, but
the displacements for the first five modes, the electron affinity, and a scaling factor were
optimized to provide a least-squares fit to the experiment over the electron binding energy rangeof 0  1.3 eV. The optimized displacements are given in Table 4.1. The changes from thecalculated displacements are minor and should be considered as qualitative values because
individual transitions for the five optimized modes are not resolved in the experiment. The
optimized electron affinity is 168 meV, in excellent agreement with the imaging result of(168 ! 6) meV.

As shown in Figure 4.7, the simulation provides an excellent fit to theexperimental PE spectrum, especially considering that anharmonicity effects are not included,
even though the progression of the NO2 wagging mode extends to fairly high vibrational levels.
This agreement confirms that the PE spectrum of Compton et al.1 is consistent with our origin
assignment from low-energy PE imaging. As indicated by Compton et al., however, theirspectrum alone is insufficient for a definitive assignment of the origin. Indeed, we could also
produce reasonable Franck-Condon fits with assignments of EA £ 20, 95, or 245 meV.

63
Therefore, the higher resolution and lower temperature imaging experiments are essential for
determining a precise electron affinity.

Figure 4.8 – HOMO of CH3NO2¯ . The HOMO is mainly localized on the nitro group, but small parts also
penetrate into the rest of the molecule, notably the σ∗ antibonding orbital of the CH group syn to the nitro
group.

For the present purposes, we now have a validated set of Franck-Condon intensity
parameters constrained to match the full experimental PE spectrum.1 These may be used tocompare Franck-Condon simulations with the present experiments.

However, because theimaging spectra are taken at near-threshold photon energies, it is also necessary to consider the
dependence of the photodetachment cross section on PE velocity, which can modify the FranckCondon intensities. As discussed earlier (see Chapter 1), there is no simple or general functional
form for the velocity dependence for molecular anion photodetachment intensities, as it depends
on the details of the interference of outgoing electron waves from molecular orbitals arising from64
a mixture of atomic orbitals on different centers. Nevertheless, some general considerations
apply (neglecting the influence of the dipole moment of the neutral molecule): (1) First, theleading term for the velocity dependence is proportional to  . This electron velocity factor
7/

appears in the coefficient of the expressions for atomic negative ion photodetachment crosssections given by Massey,39 for molecular photodetachment by Reed et al.,40 and was cited by
Ervin and co-workers37, 38 as appropriate for scaling Franck-Condon factors in PE spectroscopy
simulations.

Because 

7/

gives a relatively slow variation at moderate electron kinetic

energies,  , this factor usually has a very minor effect on the Franck-Condon simulation forconventional negative ion PE spectroscopy, i.e., for photon energies in the 2-4 eV range andelectron kinetic energies greater than 0.3 eV over the energy range of typical vibronic transitions.
The 

7/

scaling factor is exact for pure $-wave detachment and will predominate over higher-

order terms very near threshold. However, scaling proportional to 

7/

is incorrect in generalwhen detachment involves higher-order electron waves with possible interference effects. Pure-wave detachment (or higher-order spherical harmonics) with /

dependence (or higher

powers) can become relevant at slightly higher kinetic energies, until the detachment behavior is
no longer governed by Wigner’s law.

Detailed evaluation of the photodetachment matrix

elements as a function of photon energy and averaging over molecular orientations is requiredfor complete treatment.40-42 (2) Second, using the approach outlined by Reed et al.,40 which is
discussed in more detail in Chapter 2, a comparison of the overall group symmetry of the highest
occupied molecular orbital (HOMO) from which the electron is detached to atomic orbital
symmetry can be used to predict the dominant near-threshold electron detachment symmetry.
For the present case, the HOMO of CH NO is an §1 orbital centered on the NO2 groupcomposed predominantly of -orbitals pointing out of the ONO plane with the same phase on the65
O atom and the opposite phase on the N atom. This orbital is illustrated in Figure 4.8; in thepseudoatomic limit the HOMO exhibits %-like symmetry with two angular nodes (one in the
ONO plane and one passing between the N atom and the two O atoms). That would imply thatthe detachment would result in a -wave as a first-order approximation. However, the HOMO
also includes other atomic orbital components with smaller coefficients, which will complicate
this picture. (3) Third, the angular distribution of the photodetachment relative to the electricfield vector of the laser provides complementary information regarding the symmetry of thephotodetached electron wave.

In the present case, the detachment at the lowest electron

velocities exhibits isotropic $-wave behavior (= £ 0) as shown in Figure 4.5, which impliesℓ

0, but increases with increasing photoelectron velocities suggesting that higher-order terms

become important.
Experimental evidence shows that near-threshold low-electron-velocity detachmenttransitions in imaging experiments are indeed suppressed compared to Franck-Condon
intensities. For example, Surber and Sanov43 noted the diminution of near-threshold transitions
compared to Franck-Condon intensities in the CS imaging PE spectrum. This effect is also
evident in the present experiments as may be seen by comparing the relative intensities of the0←0 and 1←0 NO2 wagging transitions in the spectra in Figure 4.2 taken at several different
photon energies. The 1←0 transition has a lower relative intensity in the spectrum taken atlower photon energy where it is closer to threshold.
The imaging spectrum at 397 meV (3200 cm-1) photon energy is compared toFranck-Condon simulations in Figure 4.9. Three simulations are shown: one without electron
velocity scaling (simple Franck-Condon approximation), one scaled by 

7/

(pure $-wave

66
detachment), and one scaled by /(pure -wave detachment). Comparing the simulation

without velocity scaling, it is obvious that transitions at low electron velocity (high electron
binding energy) are strongly suppressed. The simulations that are scaled by 

7/

or 

/

bettermatch the relative intensities of the observed spectrum, but neither is an exact fit over the wholeenergy range.

That behavior suggests that a more sophisticated calculation of thephotodetachment cross sections as function of photoelectron velocity, including multiple terms,
would be required to match the experimental intensities.

Nevertheless, the overall match

between the experimental spectrum and the simulations is sufficient to verify the assignments ofthe transitions discussed above.

67

electron kinetic energy [meV]300

photoelectron yield [arb. units]

400

200

100

I

0

2
1

II
2

0
1
0

*
0
0

100

200

300

400

electron binding energy [meV]Figure 4.9 – Franck-Condon fits to the experimental PE spectrum of CH3NO2¯ (open circles) at 397 meV(3200 cm-1) with no velocity scaling (dotted line), scaled with EK1/2 (pure s-wave, full line), and scaled with
EK3/2 (pure p-wave, dashed line); see text for details. The sequences marked in the spectrum are based on the
NO2 wagging mode ν1 with transitions n←0 (series I) and n←1 (series II). The asterisk marks the 0←2 hotband of the NO2 wagging mode. Note that the top scale shows the electron kinetic energy while the bottom
scale gives the electron binding energy.

The intensity of the hot band assigned in Figure 4.9 as the 0 G 2 ( £ 77 meV)transition to the NO2 wagging mode is about a factor of four higher than predicted by the FranckCondon simulations. The simulations in Figure 4.9 use an anion temperature of 250 K, whichroughly reproduces the relative intensities of the 0 G 0 origin and nearby 1 G 1 sequence bandof the wagging mode. At this temperature, the observed intensity of the 0 G 2 band is too high68
relative to the 0 G 1 and 0 G 0 transitions to be explained by a Boltzmann distribution.

However, if the anion vibrational temperature is increased to 400 K to match the 0 G 2 hot band

intensity, then the intensities of theG 2 transitions for

1, 2, and 3 are too strong comparedto the experiment. The source of this discrepancy is uncertain; however, it is possible that whensufficient energy is pooled into the low-lying modes of nitromethane, i.e., the wagging mode, theinternal rotor, etc. the vibrational modes are not sufficiently modeled as independent harmonic
oscillators resulting in a poor recovery of the spectrum within the limitations of our simulations.
The disappearance of the peak in the cold CH NO • Ar spectrum (Figure 4.4) and the fact thatits anisotropy follows the same trend as the main progression (Figure 4.5) imply that this feature
is indeed due to hot bands and is unrelated to the dipole-bound state (peak D in Figure 4.2).
Finally, we note that the two vibrational hot bands predicted to be nearest the dipole-bounddetachment peak at 8 ! 8 meV are 0 G 3 NO2 wag transition at 

combination of 0 G 1 in the NO2 wag and 0 G 1 in the CN stretch at 

19.1 meV and a

15.4 meV. Even

allowing for frequency shifts (and ignoring the distinctive anisotropy of peak D), the calculated
Franck-Condon intensities of these hot bands are each less than 0.3% of the 0 G 0 origin peak,much too small to account for the observed feature.

4.3.4 CN bond energy
As was outlined by Sanov et al.,44 the AEA may be used to establish the bonddissociation energy of the C-N bond in CH NO : BDEC-N

0.477 eV, using the following

thermodynamic quantities in the thermochemical cycle illustrated in Figure 4.10. The presence

69
of the excess electron serves to destabilize the CN bond in nitromethane making the bond almost
five times weaker than in neutral CH NO .

Figure 4.10 – Thermochemical cycle for the determination of the BDECN in nitromethane anion.

4.4 Summary and Conclusions
We have studied the photoelectron energy spectra and angular distributions for lowenergy photodetachment of nitromethane anions. We have determined a more accurate andprecise value of the AEA of nitromethane (168 ! 6) meV, and we were able to unambiguously
assign many of the vibrational features in the PE spectrum. The most intense peaks observed in
the PE spectrum are due to progressions of the neutral NO2 wagging mode. We observe a longlived (¢ 25 (s) dipole-bound state of the nitromethane anion, clearly identifiable through its
anisotropy, which is very different from the valence-state based vibrational features.

We

postulate the existence of bound-to-free internal rotor transitions of the internal methyl rotor,
which are unresolved in the present PE spectra, but are the cause for an asymmetry in the70prominent vibrational features and contribute strongly to the spectral congestion of the PE
spectra. Future high-resolution PE spectroscopy experiments at even lower photon energies may
be able to resolve these features.

4.5 References for Chapter 3
1.

R. N. Compton, H. S. Carman, C. Desfrancois, H. Abdoul-Carmine, J. P. Schermann, J.
H. Hendricks, S. A. Lyapustina and K. H. Bowen,"On the binding of electrons tonitromethane: Dipole and valence bound anions." Journal of Chemical Physics 105 (9),3472-3478 (1996).

2.

F. Lecomte, S. Carles, C. Desfrancois and M. A. Johnson,"Dipole bound and valence
state coupling in argon-solvated nitromethane anions." Journal of Chemical Physics 113(24), 10973-10977 (2000).

3.

L. Suess, R. Parthasarathy and F. B. Dunning,"Rydberg electron transfer to CH3NO2:
Lifetimes and characteristics of the product CH3NO2- ions." Journal of Chemical Physics
119 (18), 9532-9537 (2003).

4.

J. M. Weber, W. H. Robertson and M. A. Johnson,"Argon predissociation and electron
autodetachment spectroscopy of size-selected CH3NO2-·Arn clusters." Journal ofChemical Physics 115 (23), 10718-10723 (2001).

5.

A. Modelli and M. Venuti,"Empty level structure and dissociative electron attachment in
gas-phase nitro derivatives." International Journal of Mass Spectrometry 205 (1-3), 7-16
(2001).

6.

I. C. Walker and M. A. D. Fluendy,"Spectroscopy and dynamics of nitromethane(CH3NO2) and its anionic states." International Journal of Mass Spectrometry 205 (1-3),
171-182 (2001).

7.

W. Sailer, A. Pelc, S. Matejcik, E. Illenberger, P. Scheier and T. D. Mark,"Dissociativeelectron attachment study to nitromethane." Journal of Chemical Physics 117 (17), 79897994 (2002).

71
8.

C. Q. Jiao, C. A. DeJoseph and A. Garscadden,"Formation of positive and negative ionsin CH3NO2." Journal of Physical Chemistry A 107 (42), 9040-9044 (2003).

9.

E. Alizadeh, F. F. da Silva, F. Zappa, A. Mauracher, A. Probst, S. Denifl, A. Bacher, T.
D. Mark, P. Limao-Vieira and P. Scheier,"Dissociative electron attachment tonitromethane." International Journal of Mass Spectrometry 271 (1-3), 15-21 (2008).

10.

H. Schneider, K. M. Vogelhuber, F. Schinle, J. F. Stanton and J. M. Weber,"Vibrational
spectroscopy of nitroalkane chains using electron autodetachment and Ar
predissociation." Journal of Physical Chemistry A 112 (33), 7498-7506 (2008).

11.

G. L. Gutsev and R. J. Bartlett,"A theoretical study of the valence- and dipole-boundstates of the nitromethane anion." Journal of Chemical Physics 105 (19), 8785-8792(1996).

12.

T. Sommerfeld,"Coupling between dipole-bound and valence states: the nitromethane
anion." Physical Chemistry Chemical Physics 4 (12), 2511-2516 (2002).

13.

G. L. Gutsev, P. Jena and R. J. Bartlett,"Thermodynamical stability of CH3ONO andCH3ONO-: A coupled-cluster and Hartree-Fock-density-functional-theory study." Journalof Chemical Physics 110 (1), 403-411 (1999).

14.

M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, J. A. Montgomery, J. Vreven, T. , K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.
Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.
Hratchian, 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, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S.
Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J.
Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P.
M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian
03, Wallingford CT, 2004.

15.

A. D. Becke,"Density-Functional Thermochemistry .3. The Role of Exact Exchange."
Journal of Chemical Physics 98 (7), 5648-5652 (1993).

72
16.

C. T. Lee, W. T. Yang and R. G. Parr,"Development of the Colle-Salvetti CorrelationEnergy Formula into a Functional of the Electron-Density." Physical Review B 37 (2),785-789 (1988).

17.

A. D. McLean and G. S. Chandler,"Contracted Gaussian-Basis Sets for Molecular
Calculations .1. 2nd Row Atoms, Z=11-18." Journal of Chemical Physics 72 (10), 56395648 (1980).

18.

R. Krishnan, J. S. Binkley, R. Seeger and J. A. Pople,"Self-Consistent Molecular-Orbital
Methods .20. Basis Set for Correlated Wave-Functions." Journal of Chemical Physics 72(1), 650-654 (1980).

19.

J. A. Pople, M. Head-Gordon and K. Raghavachari,"Quadratic Configuration-Interaction- a General Technique for Determining Electron Correlation Energies." Journal ofChemical Physics 87 (10), 5968-5975 (1987).

20.

J. T. H. Dunning,"Gaussian basis sets for use in correlated molecular calculations. I. The
atoms boron through neon and hydrogen." The Journal of Chemical Physics 90 (2), 10071023 (1989).

21.

K. M. Ervin, T. M. Ramond, G. E. Davico, R. L. Schwartz, S. M. Casey and W. C.
Lineberger,"Naphthyl radical: Negative ion photoelectron spectroscopy, Franck-Condon
simulation, and thermochemistry." Journal of Physical Chemistry A 105 (48), 1082210831 (2001).

22.

K. M. Ervin, PESCAL, University of Nevada, Reno, 2008.

23.

F. Duschinsky,"The importance of the electron spectrum in multi atomic molecules.
concerning the Franck-Condon principle." Acta Physicochimica URSS 7 (4), 551-566(1937).

24.

T. E. Sharp and H. M. Rosenstock,"Franck-Condon Factors for Polyatomic Molecules."Journal of Chemical Physics 41 (11), 3453-& (1964).

25.

P. Chen, in Unimolecular and Bimolecular Reaction Dynamics, edited by C.-Y. Ng, T.
Baer and I. Powis (John Wiley & Sons, Chichester, 1994), pp. 371.

73
26.

D. Gorse, D. Cavagnat, M. Pesquer and C. Lapouge,"Theoretical and Spectroscopic
Study of Asymmetric Methyl Rotor Dynamics in Gaseous Partially Deuterated
Nitromethanes." Journal of Physical Chemistry 97 (17), 4262-4269 (1993).

27.

A. L. L. East and L. Radom,"Ab initio statistical thermodynamical models for thecomputation of third-law entropies." Journal of Chemical Physics 106 (16), 6655-6674(1997).

28.

K. C. Ingham and Strickle.Sj,"Hindered Rotation in Ground and Excited Electronic
States of Ortho-Xylene." Journal of Chemical Physics 53 (11), 4313-& (1970).

29.

L. H. Spangler and D. W. Pratt,"Laser-Induced Phosphorescence Spectroscopy inSupersonic Jets - the Lowest Triplet-States of Glyoxal, Methylglyoxal, and Biacetyl."
Journal of Chemical Physics 84 (9), 4789-4796 (1986).

30.

L. H. Spangler,"Structural information from methyl internal rotation spectroscopy."
Annual Review of Physical Chemistry 48, 481-510 (1997).

31.

E. P. Wigner,"On the Behavior of Cross Sections near Thresholds." Physical Review 73(9), 1002-1009 (1948).

32.

P. C. Engelking,"Strong Electron-Dipole Coupling in Photodetachment of Molecular
Negative-Ions - Anomalous Rotational Thresholds." Physical Review A 26 (2), 740-745(1982).

33.

J. Cooper and R. N. Zare,"Angular Distribution of Photoelectrons." Journal of Chemical
Physics 48 (2), 942-943 (1968).

34.

J. L. Hall and M. W. Siegel,"Angular Dependence of the Laser Photodetachment of the
Negative Ions of Carbon, Ozygen, and Hydrogen." Journal of Chemical Physics 48 (2),943 (1968).

35.

C. H. Greene,"Manifestation of Dynamically Unfavored Transitions in the AngularDistribution of Photofragments." Physical Review Letters 44 (13), 869-871 (1980).

36.

C. Pan, A. F. Starace and C. H. Greene,"Parallels between High Doubly-Excited State
Spectra in H- and Li- Photodetachment." Journal of Physics B-Atomic Molecular andOptical Physics 27 (8), L137-L142 (1994).

74

37.

K. M. Ervin, J. Ho and W. C. Lineberger,"Ultraviolet Photoelectron Spectrum of NO2."
Journal of Physical Chemistry 92 (19), 5405-5412 (1988).

38.

C. T. Wickham-Jones, K. M. Ervin, G. B. Ellison and W. C. Lineberger,"NH2 ElectronAffinity." Journal of Chemical Physics 91 (4), 2762-2763 (1989).

39.

H. W. W. Massey, Negative Ions. (Cambridge Press, Cambridge, 1976).

40.

K. J. Reed, A. H. Zimmerman, H. C. Andersen and J. I. Brauman,"Cross-Sections for
Photodetachment of Electrons from Negative-Ions near Threshold." Journal of Chemical
Physics 64 (4), 1368-1375 (1976).

41.

W. B. Clodius, R. M. Stehman and S. B. Woo,"Zero-Core-Contribution Calculation of a
Polyatomic Photodetachment Cross-Section - NO2." Physical Review A 28 (2), 760-765(1983).

42.

R. M. Stehman and S. B. Woo,"Zero-Core-Contribution Calculation of Photodetachment
Cross-Sections of O2- and S2." Physical Review A 23 (6), 2866-2876 (1981).

43.

E. Surber and A. Sanov,"Photoelectron imaging spectroscopy of molecular and cluster
anions: CS2- and OCS-(H2O)1,2." Journal of Chemical Physics 116 (14), 5921-5924(2002).

44.

D. J. Goebbert, D. Khuseynov and A. Sanov,"Photodissociation of nitromethane cluster
anions." Journal of Chemical Physics 133 (8) (2010).

75

5 The Photoelectron Spectroscopy of Nitroethane Anions
This chapter has been reproduced in part with permission from Adams, C.L., and Weber, J.M.,“Photoelectron imaging spectroscopy of nitroethane anions.” Journal of Chemical Physics, 2011.
134: 244301. Copyright 2011, American Institute of Physics.

5.1 Introduction
Weakly bound states of excess electrons in molecular systems have received muchattention because they can be supported by a number of different mechanisms, with diffuse
dipole-bound states1-7 and more localized but still weakly bound valence anions being the twomain motifs. Some molecules can support both of these types of anion states.2, 6 In a joint study
using photoelectron (PE) spectroscopy experiments by the Bowen group in concert with Rydberg
electron transfer experiments by the Schermann and Compton groups4 it was shown that
nitromethane is capable of forming both a dipole-bound state and a valence anion state. It wasargued that formation of the valence anion in attachment of free electrons is facilitated bycapture into the dipole-bound state and subsequent relaxation into the valence state.1, 2, 4 A
recent combined experimental and theoretical study by the groups of Bowen, Compton andSommerfeld showed that the same processes are at play in nitroethane.4
These two small nitroalkanes are very similar in several respects, for example in terms of
their dipole moments and of their adiabatic and vertical electron binding energies (see Table 5.1).
In particular, the adiabatic electron binding energies in both systems have been shown to belower than many of the fundamental vibrations of the anion, making these molecule model
systems for studying the interaction of vibrational excitation and electron emission.8, 9

76

Table 5.1 – Dipole Moments and AEA’s of Nitromethane and Nitroethane

Dipole Moments (Debye)

AEA [meV]this workrefs. 4 and 5

VDE [eV]

CH3NO2

3.4610168 ± 6300 ± 200

0.9 ± 0.1

CH3CH2NO2

3.4a

191 ± 6260 ± 800.9 ± 0.1

asee refs 11 and 12 for discussion on the dipole moment of nitroethane

One important cornerstone in the descriptions of such systems is an accurate
determination of the adiabatic electron affinity.

A PE spectrum of nitroethane anion wasrecorded by Bowen and coworkers several years ago.4 The spectrum revealed an extended,unresolved vibrational progression, affording a determination of the vertical detachment energyof 0.92 ! 0.1) eV and an estimate for the AEA at (0.3 ! 0.2) eV.

The relatively large

uncertainty in the latter value is a caused by the combination of a substantial geometry difference
between the molecular geometries in the anionic and neutral charge states and a large spectralcongestion due to a large number of Franck-Condon active modes, making the transition between
the vibrational ground states of the anion and the neutral molecule (0 G 0 transition) difficult to
pinpoint. In the structurally related nitromethane anion,9, 13 the largest geometry difference
between the anion and the neutral is the angel between the NO plane and the CN bond, where
the accommodation of the excess electron (in an orbital of mainly ; a character) changes thegeometry of the nitro group towards a trigonal pyramidal configuration. The vibrational modeconnecting these two geometry motifs is the nitro wagging mode, which leads to a similarly
strong progression in the PE spectrum of nitromethane.9

77
In this chapter, PE spectra of bare and Ar-solvated nitroethane anions are presented.
Using argon solvation and Franck-Condon simulations, the AEA of nitroethane is determined tobe (191 ! 6) meV. This is used to determine the C-NO bond dissociation energy in nitroethane.
Finally, several of the active modes in the PE spectrum are assigned with the aid of the FranckCondon simulations and the dipole-bound state is tentatively assigned to a weak feature at(27 ! 5) meV.

5.2 Computational Details
5.2.1 Ab Initio Calculations
In order to understand the PE spectrum of nitroethane, information on the molecular
structures and the vibrational frequencies of the anion and the neutral molecule is needed. In thisstudy, we performed calculations using the GAUSSIAN 03 program suite.14 We used density
functional theory with the Becke-3-Lee-Yang-Parr (B3LYP) hybrid functional15, 16 and the 6311++G(2df,2p) basis set to obtain geometries and vibrational frequencies for CH CH NO andCH CH NO (calculated frequencies are listed in Table 5.2 below). This method was chosen
since it has been shown in our previous study on nitromethane9 to be computationally efficientand provide reliable frequencies. The calculated electron affinity and vertical detachment energy(VDE) of nitroethane, after accounting for vibrational zero-point energy corrections, are
EACalc0.45 eV and VDECal

1.20 eV.

It should be mentioned that recent calculations on

the nitromethane anion (which is very similar in its electronic structure, owing to the localizationof the excess electron on the NO group), indicated that at this level of theory one would expect

78
an overestimate of the adiabatic electron affinity and that multiconfigurational wavefunctions arenecessary to achieve sufficient convergence with experimental results.13

Figure 5.1 - Geometries of CH3CH2NO2¯ and CH3CH2NO2; θ denotes the angle between the ONO plane andthe CN bond axis, which is the most salient geometry change upon electron loss.

Two conformers of the nitroethane anion were found, in agreement with an earlier study
by Schneider et al.8 The conformations were calculated to be separated by 660 cm-1 after zeropoint vibrational energy corrections were taken into account. In the lower energy conformer, theNO2 is syn to one of the CH bonds in the CH2 group, while the NO2 is syn to the CC bond in thehigher energy conformer. A careful and detailed analysis of the infrared spectra of nitroethaneby Schneider et al.,8 performed using the same source and similar source conditions, indicatedthat only the lower energy conformer was significantly populated in the anion beam.
Additionally, exploratory Franck-Condon simulations performed on the higher energy conformer
indicated the higher energy conformer was not compatible with the PE spectra. Therefore, thefocus of the study will be on the lower energy conformer.

79
5.2.2 Franck-Condon Analysis
The Franck-Condon profiles were simulated using the PESCAL program developed by
Ervin and coworkers.17

With the exception of the torsional vibration about the CN bond(discussed separately), the vibrational normal modes from the calculations were used to generate
the Franck-Condon factors within the independent harmonic oscillator approximation with thefull treatment of Duschinsky rotation using the Sharp-Rosenstock-Chen recursion method.18, 19
The calculated vibrational frequencies of neutral CH CH NO , excluding the torsion,
were replaced with experimentally determined vibrational frequencies from a gas-phase IR
study20 of CH CH NO . Of the twenty-four vibrational modes of CH CH NO , five have been
experimentally determined;8 the remaining frequencies were determined by scaling the
calculated frequencies by the ratio of the experimental fundamental to the calculated harmonicfrequency of the neutral for the similar mode. The frequencies may be found in Table 5.2 below.
Symmetry considerations often play an important role in the simplification of theinterpretation of a PE spectrum.

Wherein, if a normal coordinate of a harmonic mode isantisymmetric to some symmetry element within the common point group of the anion andneutral, that mode will have a zero normal coordinate displacement and be considered FranckCondon “inactive”, contributing very little to the PE spectrum. Unfortunately, the nitroethane
anion and neutral share C7 symmetry and all modes must be considered active (nonzero normal
coordinate displacement).

 ", were defined along the anion
The geometry displacements, Y

normal coordinate frame and calculated within the FCFGAUS program21 developed by Ervinand coworkers, they are displayed in Table 5.2 below.

80
Table 5.2 - Frequencies (cm-1) and geometry displacements.

Modea

CH3CH2NO2⁻

B3LYP

ScaledExpt.bB3LYP

H.I.R.e24

K"/(g/mol)1/2 Åc

CH3CH2NO2

F.I.R.e

B3LYP
N/Ae

23

210.6

205.6

220.8

221

0f

22

408.4

400.3

594.8

583

-0.8211

21

802.6

793

814.7

805

-0.156420

1111.91096.11152.41136-0.0685191294.51258.11300.61264

0.0365181474.61455.51490.31471-0.0059173055.92940d3117.82972-0.0953163075.4

2913d

3119

3001

0.1666

15

276.2

272.9

288.4

285

-0.0156

14

481.5

478

507.7

504

0.1193

13

594.3

625.5

636.6

670

0.0349

12

783.1

776.7

883.2

876

-0.3418

11

992.1

984.7

1003.5996

0.1051

101088.71070.51120.81102-0.386191356

1327.81356.11328

0.002681211.91178.41405.91367-0.237271387.41350.71434

1396-0.071961470

1439.61478.61448

0.036951507.31472.81505.51471

0.0393

41234.51204.1

1615.91576

0.04073004.6

2846

d

3054

2926

-0.1831d3082.52957-0.1958

3141.93019

0.0568322870.72769

13109.22979daModes are labeled in agreement with Groner et al.20bExperimental fundamental frequencies.20cGeometry displacements are defined in terms of the anion normal coordinate vectors.

dExperimental fundamental frequencies.8e

The character of this mode changes from a hindered internal rotor (H.I.R.) in the anion to a free internal
rotor (F.I.R.) in the neutral. The calculated K” value for this mode is -2.1095, but the CN torsion has beentreated separately (see text).
f

The calculated K” value for this mode is -0.2337, but it has not been used in the simulations (see text).

81

5.2.2.1 Treatment of Internal Rotors
The GAUSSIAN 0314 output gives harmonic modes for all twenty-four vibrational
degrees of freedom. While this is a fair approximation for the twenty-two modes that do notcorrespond to internal rotations the potential energy curve for the CC and CN internal rotors must
be modeled with a periodic potential. Since, at the present time, PESCAL17 cannot treat internal
rotors it was necessary to treat these modes independently.
The torsional vibrational about the CN bond is a hindered internal rotor in CH CH NO

but is nearly a free rotor in CH CH NO . This leads to substantial activity in the PES due to thehindered-to-free rotor transitions, as has been seen in a previous study of PE study ofnitromethane anion. On the other hand, the potential energy curve of the CC rotor undergoesvery little change upon electron emission which should lead to very little activity in this mode in
the PE spectrum; based on this, we exclude this mode in the Franck-Condon simulations.
The torsional potentials were generated at the B3LYP/6-311++G(2d,p) level by scanningthe dihedral angle, HCNX, where X is a vector perpendicular to the plane of the NO group and H
is one of the H atoms in the methylene group. This was done for the anion, as well as theneutral. In the neutral molecule, all other bond angles and distances were allowed to relax during
the scan. This yielded a two-fold symmetric potential with a barrier of 35 cm-1. For the anion, as
mentioned earlier, rotation about this bond will lead to a higher energy conformer, where theNO group is syn to the CC bond. The part of the anion potential energy curve around the low-energy isomer was calculated by scanning the HCNX dihedral angle and allowing the remaining

bond lengths and angles to relax at each HCNX dihedral angle. The barrier to isomerisation andthe potential energy curve in the region of the higher energy conformer was generated byscanning the HCNX dihedral angle while the rest of the geometry was restricted to that of the82lower-energy conformer. This procedure was necessary to prevent the molecular geometry fromreverting to the lower energy conformer during geometry optimization. The potential energy
curves along with the geometries at selected positions are illustrated in Figure 5.3.
Each curve was fitted with the following expansion in cosine terms,
Equation 5.1cs
where d6 for the neutral and d§ 3  V¬ M Cos<­ M B ,¬¡7

10 for the anion. The fits to the potential energy curve arerepresented in Figure 5.3. The reduced moments of inertia for torsion are defined as
Equation 5.2

8 T8®¯P ®¯N M 8°±N,
8®¯P ®¯N 3 8°±N

where the geometries of the ethyl and nitro groups were obtained from their lowest energy
structures. Additionally, 8 T was tabulated for all values of  to ensure that the variation of the
reduced moment of inertia was negligible; the results indicated a deviation of less than five
percent which is acceptable within the goals of this study. The calculated values for the reduced
moment of inertia (lowest energy structures) for the nitroethane anion and neutral are8 T anion

3.32 · 10 ³U kg·m2 and 8 T neutral

3.16 · 10 ³U kg·m2 .

83

Figure 5.2 - Calculated potential energy curves (circles) generated by rotation about the CN bond in thenitroethane anion (lower curve) and neutral (upper curve). The black line illustrates the fit to each calculated
potential (see text). The filled blue circles represent points calculated using a relaxed-geometry scan while thefilled red circles are points generated by a fixed-geometry scan (see text). The different nitroethane anion
conformers are illustrated above their respective positions along the potential energy curve. The anion has
two isoenergetic conformers separated by a 660 cm-1 barrier and flanked by a higher energy conformer witha large barrier to isomerization. The neutral has a relatively flat potential nearly coinciding with that of afree internal rotor.

84
As was described in detail earlier in Chapter 4 (and according to the approach outline by
Spangler)22 , the Hamiltonian for the hindered internal rotor is
Equation 5.3  
3 s
28  D
3 s, 

where sis the torsional potential (Equation 5.1) and D anion

CH CH NO and D neutral

0.844 cm-1 for

0.901 cm-1 for CH CH NO . As before, a truncated basis

set of free rotor wavefunctions,
Equation 5.4

&

1

√2;

_ * ,

is applied to the hindered internal rotor Hamiltionian resulting in the following matrix elementsin the Hamiltionian matrix,
Equation 5.5c D 3 §, 3 ·
¬¡7

V¬
.
M
2 ,!¬The matrix is diagonalized to yield the hindered-rotor wavefunctions and corresponding energy
eigenstates. A basis set from 20 to 

20 was found to converge; the energies for theanion and neutral hindered rotors are listed in Table 5.3.

85
The Franck-Condon factors for the torsional vibration were simulated in Mathematica(Wolfram Research Inc.) using a newly written program that was capable of simulating hinderedto-hindered rotor transitions. The Franck-Condon overlap integral is given by,
Equation 5.6

CDCd, 

¸ 



H


a & %¹
;
&c


d

0, 1, … , 40

where &c is the wavefunction of the d neutral hindered rotor, & is therotor wavefunction of the anion, and  is the Boltzmann weighting of thehindered internal
state in the anion

for a temperature specified in the program. Given the anion and neutral hindered internal
wavefunctions are composed of orthogonal wavefunctions this may be simplified toEquation 5.7
a & 
&c


H



1¡ Ha1 &1 H º" &" "¡ H M º" M ,"where &1  and &"  are free internal rotor basis wavefunctions. The resulting transition
intensities were imported into PESCAL and convolved with a Gaussian to match theexperimental resolution.

86

Table 5.3 – Lowest Hindered Internal Rotor States

Quantum
Number
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24

Neutral Hindered Rotor Energy[cm-1]
7.2
7.2
17.4
17.5
23.3
24.0
28.5
30.9
34.1
39.5
40.8
49.5
49.8
61.1
61.2
74.5
74.5
89.7
89.7
106.8
106.8
125.7
125.7
146.3
146.3

Anion Hindered Rotor Energy[cm-1]
47.1
47.2
140.9
141.5
233.8
236.1
329.0
332.4
426.9
429.9
514.1
529.1
586.5
607.1
635.4
702.8
705.6
744.9
781.8
791.5
873.4
896.3
959.9
1043.41052.1

87

5.4 Results and Discussion
5.4.1 Experimental Results
The spectrum by Bowen and coworkers4 was taken at a photon energy of 2.540 eV. Thelong progression is identified as the excitation into the ground electronic state of CH CH NO

and extends from about 0  2 eV in binding energy, with a vertical detachment energy of 0.92
eV. Without vibrationally resolved features, it was necessary to estimate the adiabatic electron
affinity based on the location of a discernible threshold, necessitating generous error bars.

photoelectron yield

88

0

50

100

150

200

250

300

350

electron binding energy [meV]Figure 5.3 - PE spectra of CH3CH2NO2¯ at 389 meV (upper trace), 347 meV (center trace), and 322 meV(lower trace) photon energy. The dashed line denotes the (0←←0) transition corresponding to an AEA of (191
± 6) meV.

The PE spectra of CH CH NO shown in Figure 5.3 above were obtained using photon
energies of 380 meV (3140 cm-1), 347 meV (2800 cm-1), and 322 meV (2600 cm-1). The PE
image obtained at 380 meV (3140 cm-1) is shown in Figure 5.4. Care was taken to ensure that

89
none of the photon energies used were in the vicinity of any vibrational autodetachment
resonances of CH CH NO .8

The spectrum at the highest photon energy reveals threeprominent bands, with the first at (193 ! 5) meV binding energy. Interspersed between the
bands are many weaker transitions without immediately recognizable patterns. The low-binding
energy side of the spectrum reveals a broad, spectrally congested tail that extends to a binding
energy of about 75 meV.

Figure 5.4 – PE image obtained at a photon energy of 389 meV (3140 cm-1). The left half of the image is the
raw PE image. The right half is the BASEX reconstructed image.

The spectrum of CH CH NO M Ar contains two prominent bands, shown in Figure 5.5
below. The features on the low-energy side of the first peak and many of the interspersedfeatures seen in the spectrum of bare CH CH NO have been strongly suppressed or eliminated.
Their absence from the spectrum indicates that many of these features are contributions from hotbands which are suppressed in the spectrum of CH CH NO M Ar as a result of the much lower

90
internal energy in this anion. The first two prominent peaks in the spectra of both Ar-solvatedand bare nitroethane anion are spaced by (73 ! 7) meV or (595 ! 56) cm-1. This spacing is invery good agreement with the experimentally determined energy of the nitro wagging mode (»in Table 5.2 above) in neutral nitromethane of 583 cm-1.20 The prominent features are shifted tohigher binding energies by (61 ! 7) meV which is in good agreement with the shift observed innitromethane (63 ! 7) meV,9 and we attribute this shift to the solvation energy of theCH CH NO M Ar complex. A PE spectrum of CH CH NO M Ar2 exhibited an additional shiftof (55 ! 7 meV compared to the CH CH NO M Ar spectrum. The Ar-induced shift in the PE

spectra allows us to identify the contributions from hot bands and assign the band at (193 ! 5)

meV binding energy to the transition between the ground states of the anion and neutral
molecule (0←0 transition). As a consequence, we determine the adiabatic electron affintiy ofnitroethane to be (191 ! 6) meV after correcting for torsional band contour shifts, as discussed
below. Better assignments of the vibrational features require the aid of the Franck-Condon
simulations which is discussed below.

photoelectron yield [arb. units]

91

0

50

100

150

200

250

300

350

electron binding energy [meV]Figure 5.5 - PE spectra of CH3CH2NO2¯ (upper curve) and CH3CH2NO2¯ ·Ar (lower curve) taken at 389 meVphoton energy. The arrows and drop lines illustrate the shift of the spectrum upon Ar solvation by (61 ±7)
meV.

The PE spectrum of nitroethane shows a very weak feature at (27 ! 5) meV binding
energy. The intensity of this feature depends on source conditions, and it can be seen best in
photoelectron velocity distributions (see Figure 5.6 below), where it is remarkably sharp andwell separated from the broad background of hot bands at higher binding energies (i.e., lower
velocities). The broad, spectrally congested tail observed at higher binding energies in thephotoelectron velocity distribution is due to a multitude of overlapping hot-band transitions. At
a binding energy of 27 meV, there are no hot bands with appreciable intensity, making it unlikelythat this feature is due to a hot band. Bowen and coworkers determined the binding energy of

92
the dipole bound state of nitroethane to be ~ 25 meV.4 In agreement with their experimental
binding energy value, we assign the feature observed at (27 ! 5) meV in our PE spectra to be thesignature of the dipole-bound anion state of nitroethane. This is the first observation of dipole-

photoelectron yield [arb. units]intensity [arb. units]

bound state of nitroethane using PE spectroscopy.

D
0

50

100

150

200

250

300

binding energy [meV]

D
0

5

1x10

5

2x10

5

3x10

speed [m/s]

Figure 5.6 - PE spectrum of CH3CH2NO2¯ taken at a photon energy of 322 meV. The spectrum is plotted as afunction of electron speed to best illustrate the dipole-bound state; the inset shows the PE spectrum as afunction of binding energy. In each spectrum the dipole-bound state is labeled “D”.

Based on our measurement of the adiabatic electron affinity, the adiabatic C-NO bonddissociation energy, BDE® °±N0.539 ! 0.012 eV can be determined from otherthermodynamic quantities23, 24 in the thermochemical cycle shown in Figure 5.7 below. The

93
effect of the excess electron in the antibonding orbital of the nitroethane molecule serves todestabilize the CN by ~ 2 eV. The strong similarities in the electronic structure of nitromethaneand nitroethane anions, in which the electron is largely localized on the NO group, result inalmost identical C-NO bond energies with the C-NO bond in nitromethane anion, as recently
calculated by Sanov et al.25

Figure 5.7 - Thermochemical cycle for the determination of the C-NO2 bond dissociation energy, BDEC-NO2, in
nitroethane anion.

5.4.2 Threshold Effects and Angular Distributions
As outlined in Chapter 2, the approach by Reed et al.26 utilizes symmetry arguments toextend Wigner’s Threshold laws27 to near-threshold photodetachment of molecular anions,offering insight into the dominant symmetry-allowed partial waves. However, two issues arise
in applying this to nitroethane: the anion and neutral share no common symmetry and the dipole

94
moment of CH CH NO is supercritical (i.e., capable of supporting a dipole-bound state; thedipole moment is (3.4 ! 0.2) D). Wigner’s threshold law for photodetachment is founded on theidea that the longest range potential will dictate the behavior of the cross-section at threshold.28
In photodetachment, in the absence of a dipole moment, the longest range potential that exists isthat of the centrifugal barrier and will therefore dictate the threshold behavior. However, if adipole is present the long-range term that dictates the cross-section will be the sum of the twopotentials. In past experiments on OH it has been shown that the dipole moment was importantin determining the threshold behavior and led to a modification in the leading term in thethreshold law.28 Characterizing the contribution of the dipole is impractical for such a large
molecule as nitroethane anion and is well beyond the scope of this study. However, inspectionof the angular distributions should yield some insight into the partial waves contributing above
threshold, giving us a sense of the leading term contributing to the cross-section.
Analysis of the angular distributions in photodetachment from CH CH NO at different
kinetic energies (see Figure 5.8 below) reveals an isotropic distribution (=0) at low kinetic

energies and an increasing anisotropy parameter as the kinetic energy increases. This behavior
indicates that $-wave detachment dominates at low kinetic energies but contributions fromhigher-order partial waves become significant at larger kinetic energies.

95

2.0

anisotropy parameter, β

1.5

1.0

0.5

0.0

-0.5

-1.0
0

20

40

60

80

100

120

140

160

180

200

220

electron kinetic energy [meV]Figure 5.8 - Anisotropy parameters as a function of electron kinetic energy obtained at different photon
energies where solid circles, hollow circles, and triangles represent parameters from images taken at photon
energies of 389 meV, 347 meV, and 322 meV, respectively. The error bars are conservatively estimated to be± 0.1.

The dominance of the $-wave contribution at low kinetic energy implies that the leading

term in the threshold law is proportional to  . However, the increase of = at higher kinetic
7/

energies indicates that higher angular momenta also play a role. As a direct result of threshold
effects, the relative intensity of hot bands increases at lower photon energies. This has been
observed in several other studies29, 30 including a previous study on nitromethane anion. The
inclusion of threshold effects will be necessary to accurately model the PE spectrum in theFranck-Condon simulation.

96
We note that the signature of the dipole-bound state can have a significantly different
anisotropy parameter than signatures of valence anion states with comparable kinetic energies.9
However, the signal-to-noise ratio in our PE images is insufficient to obtain a reliable anisotropy
parameter for this feature.

5.4.3 Franck-Condon Simulations
The PE spectra revealed three prominent peaks with weaker transitions interspersed
between them, superimposed on a congested, broad background of unresolved transitions. In
order to assign the most important contributions to the PE spectrum, Franck-Condon simulations
were performed and compared to the PE spectrum recorded at 389 meV photon energy since thisspectrum captures the most transitions in the present study. The simulations revealed 136transitions within at least 1% of the intensity of the 0 G 0 transition in this small spectral
window, in addition to more numerous, even weaker transitions, explaining the spectralcongestion.
Our tentative assignment of two transitions at (193 ! 5) meV and (267 ! 5) meV

belonging to the (0 G 0) and (1 G 0) transitions of the nitro wagging mode » (see above) are

confirmed by the Franck-Condon simulations. The prominent peak at (366 ! 5) meV can be

assigned to overlapping features of the (1 G 0) transition of the symmetric NO2 stretching mode,»¼ , and the combination band arising from the (1 G 0) transition of »7 and the (1 G 0)transition of » .

Many of the weaker features observed in the spectrum are identified as originating fromhot bands, consistent with their suppression upon argon solvation. The main contributions from97
hot bands are based on the (0 G 1) transition of the CCN bending mode »7½ and on the (0 G 1)

photoelectron yield [arb. units]

photoelectron yield [arb. units]transition of the nitro wagging mode » .

180

190

200

210

electron binding energy [meV]

0

100

200

300

electron binding energy [meV]Figure 5.9 - Experimental PE spectrum taken at a photon energy of 389 meV (open circles) along withFranck-Condon simulations performed with the inclusion of the hindered-to-free internal rotor transitions(solid line) and in their absence (dotted line). The inset is centered about the (0←0) transition and includes a
stick spectrum of the individual calculated transitions to emphasize the contribution of the hindered-to-free
CN rotor transitions to the peak profile.

In principle, Franck-Condon simulations can be performed without including the CN
torsion. However, as a result of its exclusion the simulations underestimate the width of many ofthe prominent features and fail to recover the broad background of unresolved transitions.
Broadening the simulated spectral resolution instead of accounting for CN torsion does not leadto a better description of the spectrum. In order to accurately simulate the spectrum it was

98
necessary to include the transitions involving the changing character of the CN torsion from ahindered to a nearly free internal rotor. Incorporating these transitions leads to combination
bands of the CN torsional transitions with each vibrational feature observable in the spectrum.
These combination bands are not individually resolved in our PE spectrum, due to the large
reduced moment of inertia for the CN torsion. Instead, these hindered-to-free torsion transitions
superimpose a profile of each individual vibrational transition. One important consequence isthat the peak of the profile of each (non-torsional) transition is shifted by 31.9 ! 1 meV
towards higher binding energies and broadened asymmetrically (see Figure 5.9 above). This
serves to broaden the peak of transition to the observed width while the wings of the torsional
profile add to the broad background. As mentioned above, the individual transitions are too
closely spaced to be resolved in our experiment; however, the success of the fit upon inclusion of
the internal rotation strongly suggests that the torsional transitions are responsible for much of
the broadening and congestion in the spectrum. The shift in the profile of each transition must be
accounted for in the determination of the adiabatic electron affinity from the experimentallyobserved 0 G 0 transition and has been included in the adiabatic electron affinity values reported
in the present work. It should be noted, given the uncertainty in the barrier height for internalrotation in the neutral, that the simulation was also performed for a hindered-to-free internal
rotor transition; the results indicated that the peak profile, at this level of uncertainty, was stableto small changes in the barrier height.
As discussed above, the intensities of the features may be written as
Equation 5.8

8,- ,,"   5 CDC,- ,," · ¾ ,

99
where CDC,- ,," are the Franck-Condon factors for a particular transition and ¾  describes themodification of the photodetachment cross section by threshold effects. Following a pure $-wave threshold law in absence of a dipole moment, ¾  is proportional to  , while
7/¾  5 /for pure -wave detachment. Simulations were performed for these two thresholdlaws and without taking threshold effects into account. As Figure 5.10 shows, the unscaled
simulation overestimates the bands at higher binding energy (i.e., low kinetic energy) while the-wave simulation underestimates them. The best fit was achieved by using an $-wave thresholdlaw but this fit still slightly misrepresents the intensities of the low kinetic energy bands.
Interestingly, at low kinetic energy the best fit would be afforded by a function that rises fasterthan  . This behavior mirrors observations by Lineberger et al.28 for OH that indicated the7/

presence of a dipole potential modified the leading term of the threshold law such that the crosssection rises faster than Wigner’s law27 (in the absence of a dipole moment) would predict.

100

0

1

2

photoelectron yield [arb. units]

A

0

1

B

0

100

200

300

electron binding energy [meV]Figure 5.10 - Experimental PE spectra taken at a photon energy of 389 meV (open circles) along with theFranck-Condon simulations performed without threshold scaling (dash-dotted line), scaled by E3/2 (dottedline) and scaled by E1/2 (solid line). The sequences marked in the spectrum are based on the nitro wagging
mode ν22 with transitions n←0 (series A) and n←1 (series B).

The spectrum was found to be well represented at a simulation temperature of 250 K,
similar to the previous PE study conducted in our laboratory on nitromethane anion.9 Thistemperature recovers the transitions based on the CCN bending (1←0) and the nitro wagging(1←0) hot bands. However, the (2←0) transitions for both of these modes were underestimatedby the simulations.

Interestingly, a similar effect has been observed in the PE spectra ofnitromethane anion. In the present case, these features could be fitted at a temperature of 350 K
but the resulting fit is not consistent with the rest of the PE spectrum. Given the similarities of

101
nitromethane and nitroethane anion (i.e., namely that the excess electron, localized on the NO2
group, leads to almost identical distortions about the nitrogen) it is likely that when sufficientinternal energy is pooled in the rotor and wagging modes of the molecule, the potential energy
surface is poorly recovered within the independent harmonic oscillator approximation, thus
leading to an inadequate recovery of the spectrum within the approximations used for theFranck-Condon analysis. We should note that the higher simulation temperature cannot accountfor the feature assigned to the dipole-bound state.

5.5 Summary
Near-threshold PE imaging spectroscopy has been used to obtain the PE spectrum andangular distributions of Ar-solvated and bare nitroethane anions. The adiabatic electron affinity
is determined to be (191 ! 6) meV, providing the most precise and accurate value to date. The
adiabatic electron affinity was utilized in a thermochemical cycle to establish a C-NO bonddissociation energy, BDE® °±N0.539 ! 0.012 eV, for the nitroethane anion. We were able

to characterize many of the vibrational features observed in the PE spectrum. The most intensefeatures are due to progression of the nitro wagging mode of neutral nitroethane. In close
analogy to the case of nitromethane,9 hindered-to-free internal rotor transitions in the CN
torsional coordinate were identified to be the origin of much of the spectral congestion and of
torsional broadening of vibrational features.

Angular distributions indicate that $-wave

detachment dominates at low kinetic energies but contributions from higher-order partial waves
become significant at higher kinetic energies. We observe the dipole-bound anion state ofnitroethane as a very weak feature at (27 ! 5) meV, consistent with earlier Rydberg electrontransfer studies.4

102

5.6 References for Chapter 5
1.

G. L. Gutsev and R. J. Bartlett,"A theoretical study of the valence- and dipole-boundstates of the nitromethane anion." Journal of Chemical Physics 105 (19), 8785-8792(1996).

2.

T. Sommerfeld,"Coupling between dipole-bound and valence states: the nitromethane
anion." Physical Chemistry Chemical Physics 4 (12), 2511-2516 (2002).

3.

M. Gutowski, P. Skurski, A. I. Boldyrev, J. Simons and K. D. Jordan,"Contribution ofelectron correlation to the stability of dipole-bound anionic states." Physical Review A 54(3), 1906 (1996).

4.

S. T. Stokes, K. H. Bowen, T. Sommerfeld, S. Ard, N. Mirsaleh-Kohan, J. D. Steill and
R. N. Compton,"Negative ions of nitroethane and its clusters." Journal of Chemical
Physics 129 (6) (2008).

5.

R. N. Compton, H. S. Carman, C. Desfrancois, H. Abdoul-Carmine, J. P. Schermann, J.
H. Hendricks, S. A. Lyapustina and K. H. Bowen,"On the binding of electrons tonitromethane: Dipole and valence bound anions." Journal of Chemical Physics 105 (9),3472-3478 (1996).

6.

F. Lecomte, S. Carles, C. Desfrancois and M. A. Johnson,"Dipole bound and valence
state coupling in argon-solvated nitromethane anions." Journal of Chemical Physics 113(24), 10973-10977 (2000).

7.

K. Yokoyama, G. W. Leach, J. B. Kim and W. C. Lineberger,"Autodetachment
spectroscopy and dynamics of dipole bound states of negative ions: 2A1-2B1 transitions of
H2CCC-." Journal of Chemical Physics 105 (24), 10696-10705 (1996).

8.

H. Schneider, K. M. Vogelhuber, F. Schinle, J. F. Stanton and J. M. Weber,"Vibrational
spectroscopy of nitroalkane chains using electron autodetachment and Ar
predissociation." Journal of Physical Chemistry A 112 (33), 7498-7506 (2008).

9.

C. L. Adams, H. Schneider, K. M. Ervin and J. M. Weber,"Low-energy photoelectron
imaging spectroscopy of nitromethane anions: Electron affinity, vibrational features,
anisotropies and the dipole-bound state." Journal of Chemical Physics 130, 074307074301 - 074310 (2009).

103
10.

W. M. Haynes, CRC Handbook of Chemistry and Physics, 91 ed. (CRC Press, Boca
Raton, 2010).

11.

P. Groner, R. Meyer, A. Günter, H. Kühne and H. H. Günthard,"Far Infrared Spectrumof Nitroethane." Chemical Physics 5 (1), 136-141 (1974).

12.

J. Ekkers, A. Bauder and H. H. Günthard,"Microwave spectrum and internal rotation of
nitroethane." Chemical Physics Letters 22 (2), 249-252 (1973).

13.

J. N. Bull, R. Maclagan and P. W. Harland,"On the Electron Affinity of Nitromethane(CH3NO2)." Journal of Physical Chemistry A 114 (10), 3622-3629 (2010).

14.

M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, J. A. Montgomery, J. Vreven, T. , K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.
Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.
Hratchian, 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, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S.
Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J.
Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P.
M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian
03, Wallingford CT, 2004.

15.

A. D. Becke,"Density-Functional Thermochemistry .3. The Role of Exact Exchange."
Journal of Chemical Physics 98 (7), 5648-5652 (1993).

16.

C. T. Lee, W. T. Yang and R. G. Parr,"Development of the Colle-Salvetti CorrelationEnergy Formula into a Functional of the Electron-Density." Physical Review B 37 (2),785-789 (1988).

17.

K. M. Ervin, PESCAL, University of Nevada, Reno, 2008.

18.

T. E. Sharp and H. M. Rosenstock,"Franck-Condon Factors for Polyatomic Molecules."Journal of Chemical Physics 41 (11), 3453-& (1964).

104
19.

P. Chen, in Unimolecular and Bimolecular Reaction Dynamics, edited by C.-Y. Ng, T.
Baer and I. Powis (John Wiley & Sons, Chichester, 1994), pp. 371.

20.

P. Groner, R. Meyer and H. H. Günthard,"Matrix and gas infrared spectra of nitroethane
isotopic species." Chemical Physics 11 (1), 63-85 (1975).

21.

K. M. Ervin, FCFGaus03, University of Reno, Nevada, 2008.

22.

L. H. Spangler,"Structural information from methyl internal rotation spectroscopy."
Annual Review of Physical Chemistry 48, 481-510 (1997).

23.

K. M. Ervin, J. Ho and W. C. Lineberger,"Ultraviolet Photoelectron Spectrum of NO2."
Journal of Physical Chemistry 92 (19), 5405-5412 (1988).

24.

S. J. Blanksby and G. B. Ellison,"Bond dissociation energies of organic molecules."Accounts of Chemical Research 36 (4), 255-263 (2003).

25.

D. J. Goebbert, D. Khuseynov and A. Sanov,"Photodissociation of nitromethane cluster
anions." Journal of Chemical Physics 133 (8) (2010).

26.

K. J. Reed, A. H. Zimmerman, H. C. Andersen and J. I. Brauman,"Cross-Sections for
Photodetachment of Electrons from Negative-Ions near Threshold." Journal of Chemical
Physics 64 (4), 1368-1375 (1976).

27.

E. P. Wigner,"On the Behavior of Cross Sections near Thresholds." Physical Review 73(9), 1002-1009 (1948).

28.

J. R. Smith, J. B. Kim and W. C. Lineberger,"High-resolution thresholdphotodetachment spectroscopy of OH-." Physical Review A 55 (3), 2036-2043 (1997).

29.

H. K. Gerardi, K. J. Breen, T. L. Guasco, G. H. Weddle, G. H. Gardenier, J. E. Laaserand M. A. Johnson,"Survey of Ar-Tagged Predissociation and Vibrationally Mediated
Photodetachment Spectroscopies of the Vinylidene Anion, C2H2-." Journal of Physical
Chemistry A 114 (3), 1592-1601 (2010).

30.

E. Surber and A. Sanov,"Photoelectron imaging spectroscopy of molecular and cluster
anions: CS2- and OCS-(H2O)1,2." Journal of Chemical Physics 116 (14), 5921-5924(2002).

105

6 Photoelectron Spectroscopy of 1-Nitropropane and 1-Nitrobutane
anions
6.1 Introduction
In a macroscopic system, the emission of an electron from hot, vibrationally excited
matter is characterized as thermionic emission.1, 2

As one moves to smaller systems, theemission becomes less statistical, necessitating a molecular description in which sufficientvibrational excitation facilitates a breakdown of the Born-Oppenheimer leading to vibrational
autodetachment.3 The nitroalkane anions are an intriguing group of molecules that possess
sufficiently small adiabatic electron affinities (AEA) to undergo vibrational autodetachment
following the excitation of a fundamental CH stretching mode.4, 5 A photoelectron spectroscopy
study used to monitor vibrational autodetachment from the simplest of these nitroalkane anions,
nitromethane anion, revealed substantial non-statistical contributions to the photoelectron
spectrum.3 An expansion of this study to the larger nitroalkane anions is expected to tend
towards electron emission modeled more accurately as a statistical process.

A step-wise

approach (adding one CH at a time) can offer some insight into the boundaries of eachtheoretical description of the electron emission. An important property in the description withineither theoretical description is the AEA. This has been determined for the smaller nitroalkanes,
nitromethane and nitroethane.6, 7 The focus of this chapter will be to determine the AEA of thelarger nitroalkanes, 1-nitropropane (1-NP) and 1-nitrobutane (1-NB).
At present, there only a few studies performed on 1-NP and 1-NB. The infrared spectrumof 1-NP and 1-NB anions has been determined by Schneider et al.5 and several studies infrared

106
studies of neutral 1-NP have been performed in the liquid phase.8, 9 Each of these molecules is
expected to have several stable conformers in the anion as well as the neutral.5, 9 In this chapter,
we will present photoelectron spectra for bare and argon solvated 1-NP and 1-NB anions which
are used to determine the AEA of each neutral molecule. This will be followed by a discussionof the various stable conformers found using ab initio theory and their relative populations in theanion beam. Franck-Condon simulations are presented for the stable anion conformations in an
attempt to identify the conformer that accounts for the majority of the Franck-Condon profile
intensity.

Finally, an anomalous feature appearing at a fixed kinetic energy in the barephotoelectron spectrum of both 1-NP and 1-NB anions is discussed.

6.2 Computational Details
6.2.1 Ab Initio Calculations
In both CH3 (CH2 ) NO and CH3 (CH2 ) NO , a number of stable conformers, as well as
their enantiomers, can be generated by internal rotations about the CN bond or various CC
bonds.

It is possible that in our experiment several of the conformers are present as thetemperature of our ions is estimated to be approximately 250 K, as suggested by previous studies
on nitromethane and nitroethane anions under very similar source conditions.6, 7

Densityfunctional theory (DFT) calculations were performed using the Becke-3-Lee-Yang-Parr(B3LYP) hybrid functional10, 11 using the GAUSSIAN 0312 software package. The 6311++G(2df,2p)13, 14 basis set was used for the anion and neutral of 1-NP while computations forthe larger 1-NB utilized the 6-311+G(2d,p) basis set for the anion and neutral. Various stableconformers were found for both anionic species. The stability of several conformers, with

107
respect to isomerization from internal rotation about the CN bond, was established by carryingout relaxed scans, employing a 6-311++G(2d,p) basis set for 1-NP anions and a 6-311+G(2d,p)basis set for 1-NB anions. Each scan was performed by scanning the HCNX dihedral angle,
where X is a vector perpendicular to the plane of the NO2 group and H is one of the H atoms on
the adjacent carbon, while allowing all other distances and angles to relax.
Franck-Condon profiles for 1-NP and 1-NB anions were simulated using the PESCAL
program, developed by Ervin and coworkers.15

All vibrational degrees of freedom

corresponding to the hindered internal rotations were excluded from the Franck-Condon
simulation as they are poorly represented in the harmonic oscillator approximation.

The

remaining vibrational degrees of freedom were treated as independent harmonic oscillators andused to generate Franck-Condon factors. Duschinsky rotation16 was taken into account using the
Sharp-Rosenstock-Chen17, 18 recursion method as implemented by the FCFGAUS03 program,also developed by Ervin and coworkers.19 The spectra were simulated as a function of electron
velocity, and later converted to spectra as a function of kinetic or binding energy, with a velocity
resolution of 5000 m/s.

108

6.4 Results and Discussion
6.4.1 Experimental Photoelectron Spectra

6.4.1.1 1-Nitropropane Anion
Several photon energies were used to capture the photoelectron spectrum of 1-NP anion:
273 meV (2200 cm-1), 329 meV (2650 cm-1), 368 meV (2968 cm-1), 393 meV (3170 cm-1) and
2.331 eV (532 nm). The higher photon energy PE spectrum was chosen to obtain the entire
vibrational progression belonging to the electronic ground state of the neutral, while the spectra
obtained at lower photon energies (and correspondingly high resolution) were used to identify
any important vibrational bands and to measure the adiabatic electron affinity. It should be notedthat the mid-infrared photon energies were chosen to avoid contributions from vibrational
autodetachment resonances of the 1-NP anion.5
The spectrum taken at 2.331 eV, shown in Figure 6.1, reveals an unresolved vibrational
progression extending to ~1.5 eV. The maximum of the spectrum, and therefore its vertical
detachment energy, is found at (0.92 ! 0.05) eV. The origin of the vibrational progression
cannot be determined from this spectrum. Some vibrational structure may be seen in the highbinding energy region of the spectrum (which corresponds to relatively low kinetic energy where
the resolution is best for the spectrometer); the features are roughly spaced by (420 ! 150) cm1, but in light of anharmonicity effects at high vibrational quantum numbers and the possibility

that several vibrations could be contributing to the observed pattern, better resolution is needed
to determine what vibrational modes are active.

photoelectron yield [arb. units]

109

0
0.0

0.4

0.8

1.2

1.6

2.0

electron binding energy [meV]Figure 6.1 – Photoelectron spectrum of 1-NP anion obtained at a photon energy of 2.331 eV.

The low-energy photoelectron spectra are presented together in Figure 6.3 as functions ofelectron kinetic energy and electron binding energy. At binding energies above ~ 100 meV, allspectra exhibit a largely unresolved plateau. The first sharp peak common to all spectra (labeled“A” in Figure 6.3) is observed at a binding energy of (223 ! 6) meV with a second peak (“B”)

observed consistently at (296 ! 5 meV). The spacing between the first two features is (73 !
7) meV or (589 ! 56) cm-1.

110

Figure 6.2 – Photoelectron image of 1-NP taken at a photon energy of 329 meV (2650 cm-1). The labels “A”
and “B” denote two features that are consistently observed at the same binding energy. The label “F”
denotes a prominent feature found at a fixed kinetic energy in the spectra taken at photon energies of 329
meV (2650 cm-1) and 368 meV (2968 cm-1). See text for more details.

Curiously, an additional prominent feature (“F”) appears at electron binding energies
below 220 meV in the photoelectron spectra taken at 329 meV and 368 meV. The electron
binding energy determined for this anomalous feature varies, but it shows a kinetic energy of(165 ! 5) meV in both spectra, i.e., independent of photon energy. Feature “F” is nearlycoincident with feature “A” in the PE spectrum taken at 393 meV photon energy.

111

photoelectron yield

A

B

F

F

0

100

200

300

400

electron binding energy [meV]

0

100

200

300

400

electron kinetic energy [meV]Figure 6.3 - Photoelectron spectra of 1-NP anion taken at photon energies of 393 meV (3170 cm-1), 368 meV(2968 cm-1), 329 meV (2650 cm-1), and 273 meV (2200 cm-1), from top to bottom. The spectra on the left arepresented as a function of binding energy while the spectra on the right are graphed as a function of kinetic
energy. The labels “A” and “B” denote two features that are consistently observed at the same binding
energy. The label “F” denotes a prominent feature found at a “fixed” kinetic energy in the spectra taken at
photon energies of 329 meV (2650 cm-1) and 368 meV (2968 cm-1). The dotted line in the spectra on the rightindicates a kinetic energy of 165 meV (see text for further details).

The spectrum of CH3 (CH2 ) NO M Ar obtained at a photon energy of 393 meV showed
only two prominent features, as may be seen in Figure 6.4. The features on the low binding
energy side of the bare CH3 (CH2 ) NO photoelectron spectrum have been strongly suppressed.
We attribute the residual signal in that region to the feature observed at fixed kinetic energy,
which also seems to have been suppressed in this case. The spacing of the two prominent
features corresponds to the spacing of the two prominent features at in the bare PE spectrum.

112
Vibrational spectroscopy on liquid and solid 1-NP revealed bands at 580 cm-1 and at 602 cm-1
,present in both phases, that have been identified as the out-of-plane bending vibration of thenitro group (or nitro wagging mode) for two different conformations.9 This energy range iscompatible with the spacing of the two prominent features at an energy of (589 ! 56) cm-1. Inview of the prior observation of this mode as a prominent Franck-Condon active mode in the PE
spectra of smaller nitroalkane anions,6, 20 we assign feature “A” to the 0 G 0 transition and “B”

to the 1 G 0 transition of the nitro wagging mode in 1-NP. Features “A” and “B” are shifted by(56 ! 7) meV upon Ar solvation; we attribute this shift to the solvation energy of argon. It is ingood agreement with the corresponding shifts observed in the PE spectra of nitromethane andnitroethane anions upon argon solvation.6, 7 Identification of the Ar induced shift of the spectralfeatures and the suppression of the hot bands in the PE spectrum of the Ar solvated anion
corroborates the assignment of the feature located at (223 ! 6) meV to the (0 G 0) band.

113

B

photoelectron yield

A

B

A

0

50

100

150

200

250

300

350

electron binding energy [meV]Figure 6.4 – Photoelectron spectra of bare 1-NP anion (upper curve) as well as argon-solvated 1-NP anion(lower curve) taken at a photon energy of 393 meV. The arrows represent the shift of the spectrum by anenergy of (56 ± 7) meV upon argon solvation. The labels “A” and “B” denote two features that areconsistently observed at the same binding energy in the bare 1-NP anion spectra.

6.4.1.2 1-Nitrobutane Anion
A similar protocol as described for 1-NP was followed for 1-NB anion with several
photon energies chosen to capture the low binding energy portion as well as the entire the entire
vibrational progression belonging to the electronic ground state of the neutral: 329 meV (2650
cm-1), 384 meV (3100 cm-1), 404 meV (3260 cm-1), 424 meV (3420 cm-1), 444 meV (3580cm-1),

114
and 2.331 eV (532 nm). Again, all mid-IR photon energies were chosen to avoid influencesfrom known vibrational autodetachment resonances.5
The photoelectron spectrum obtained at 2.331 eV, shown below in Figure 6.5, reveals an
extended, vibrationally unresolved progression, similar to the case of 1-NP anion and nitroethane
anion.21 The vertical detachment energy is found at a binding energy of (0.88 ! 0.05) eV. At
high binding energies, there are some irregular spaced features suggestive of vibrational
structure. As in 1-NP anion, the (0 G 0) transition cannot be unambiguously determined fromphotooelectron yield [arb. units]

the spectrum taken at high photon energy.

0
0.0

0.4

0.8

1.2

1.6

2.0

electron binding energy [meV]Figure 6.5 – Photoelectron spectra of 1-NB anion, obtained at a photon energy of 2.331 eV.

The low-energy, bare 1-NB anion photoelectron spectra are presented in Figure 6.6 as afunction of both electron kinetic energy and electron binding energy. The spectra are remarkably

115
similar to the spectra of 1-NP anion with two prominent features at constant binding energies andan anomalous feature that appears at constant kinetic energy, all superimposed on a vibrationallyunresolved background that extends down to approximately 125 meV binding energy. In the
spectra of 1-NB anion the anomalous feature is clearly observed at all photon energies, always at
a constant kinetic energy of (165 ! 5) meV (the same kinetic energy as the anomalous featureobserved in 1-NP anion), as shown by the dotted line in Figure 6.6. The other two features arespaced by an energy of (74 ! 15) meV or (598 ! 120) cm-1 with the first feature located at abinding energy of (240 ! 6) meV. Again, based on the similarity of the photoelectron spectraof all of the 1-nitroalkane anions,6 we assign the first two prominent bands to a vibration that islargely characterized by the nitro wagging mode in neutral 1-NB. The subject of the anomalousfeature will be discussed below.

116

B
A F

photoelectron yield

F

F

F

0

100

200

300

400

electron binding energy [meV]

0

100

200

300

400

electron kinetic energy [meV]Figure 6.6 - Photoelectron spectra of 1-NB anion at photon energies of 444 meV (3580cm-1), 424 meV (3420
cm-1), 404 meV (3260 cm-1), 384 meV (3100 cm-1), 329 meV (2650 cm-1), from top to bottom. The spectra on
the left are presented as a function of binding energy while the spectra on the right are graphed as a functionof kinetic energy. The labels “A” and “B” denote two features that are consistently observed at the same
binding energy. The label “F” denotes a prominent feature found at a “fixed” kinetic energy in the spectra.
The dotted line in the spectra on the right indicates a kinetic energy of 165 meV (see text for further details).

117

Figure 6.7 - – Photoelectron image of 1-NB taken at a photon energy of 384 meV (3100 cm-1). The labels “A”
and “B” denote two features that are consistently observed at the same binding energy. The label “F”
denotes a prominent feature found at a fixed kinetic energy in the spectra. See text for more details.

A PE spectrum of CH3 (CH2 ) NO M Ar was taken at a photon energy of 384 meV (3100
cm-1), shown along with the bare 1-NB anion spectrum in Figure 6.8. The spectrum shows astrong suppression of the hot bands and only one prominent feature. The question is which of
the features observed in the spectrum of the bare anion corresponds to the (0 G 0) transition.
The argon solvation energy has been observed to be around 60 meV for the smaller 1-nitroalkane
anions,6 and it is reasonable to assume that this continues to be the case for 1-NB. In addition,
the anomalous feature observed in the spectrum of 1-NP was suppressed upon Ar solvation.
Based on these observations, we assign the feature observed at (240 ! 6) meV binding energy

118
in the spectrum of bare 1-NB anion to the (0 G 0) transition. It is shifted by (57 ! 7) meV
upon Ar solvation.

photoelectron yield

B

A
F

A

0

50

100 150 200 250 300 350

electron binding enregy [meV]Figure 6.8 – The photoelectron spectrum of bare 1-NB anion (top curve) and argon-solvated 1-NB anion(bottom curve), taken at a photon energy of 384 meV. The arrow is used to illustrate the shift of the 0←0
feature. A “F” is placed above the anomalous feature and an “A” is placed above the feature corresponding
to the 0←0 transition.

119
6.4.3 Stable Anionic Conformers

6.4.3.1 1-Nitropropane Anion
In order to perform FC simulations, we first have to determine which of the conformersof the anions under study are likely to be populated in the experiment. In 1-NP, five stableconformers were found as shown in Figure 6.9.

Figure 6.9 - Illustration of the various conformers of the 1-NP anion generated from rotations about the CN
or central CC bond, their relative energies, and their population relative to the lowest energy conformer,
according to a Boltzmann distribution. H1 and H2 are consistently defined but arbitrary designations to
distinguish the hydrogens. The key at the top denotes the atoms H1 and H2. The conformers outlined in red
are excluded in the Franck-Condon simulations, see discussion for more details.

120
The conformers can be characterized by the CCCN dihedral angle and further

distinguished based upon the bond that is syn to the NO2 group (C1H or C1C2). The relative
populations, according to a Boltzmann distribution, are given for each conformation in Figure
6.9. These suggest that only the two lowest lying conformers are likely to be significantly
populated in the beam. Additionally, calculations of the different conformers indicate that theexcess electron in nitroalkane anions is stabilized by delocalization into the σa orbital of the CH

or CC bond that is syn to the nitro group. For the conformer in which the CH bond is syn to thenitro group, this leads to a large red-shift in the CH stretching vibrational mode largely describedby the motion of the destabilized CH bond. This effect has been both simulated and observed in
the IR spectra of all the nitroalkane anions.5 Importantly, calculations suggest that this shift will
not be observed for any of the CH vibrational modes for the conformers in which the CC bond is
syn to the nitro group. The spectra obtained by Schneider et al.5 did not identify all of the bands
in the IR spectrum of 1-NP anion, but any conformers with the CC bond syn to the nitro groupwould have a distinct vibrational band corresponding to the unshifted CH stretch.

Upon

sequential argon solvation the infrared spectrum remained relatively unchanged. The presenceof the argon ensures that the temperature of the target cluster is low. The contribution of the twohigher-energy conformers in which the CC bond is syn to the nitro group should be suppressed.
Since no significant change was observed in the IR spectrum upon argon solvation, we find it
safe to say that the two highest energy conformers are not present. The decision to exclude theconformer at a relative energy of 512 cm-1 was based upon relaxed scans (described in detail in
section 6.2.1). These scans revealed a relatively small, ~100 cm-1 barrier to isomerisation to thelowest-energy conformer. We expect that within the ion source, upon electron attachment
molecules in which the NCCC dihedral angle is initially 60° will readily isomerize to the lowest

121energy conformation. On these grounds in addition to the low predicted population, we exclude
this conformation from further analysis.

6.4.3.2 1-Nitrobutane Anion
The 1-NB anion has twelve stable conformations generated by internal rotations. Five of
the higher-lying conformers, corresponding to the conformations in which the NO2 group is syn
to the CC bond, are not considered as they are not consistent with the infrared spectrum of 1-NB.
The remaining seven conformations are illustrated in Figure 6.10 along with their relative
energies and relative populations, according to a Boltzmann distribution.
Of the seven remaining conformers, the three highest energy conformers are excluded
simply on the basis of their negligible populations according to the Boltzmann analysis. Relaxedscans (described in more detail in Section 6.2.1) indicated that the conformer at a relative energyof 410 cm-1 possesses a relatively small barrier of ~ 115 cm-1 to isomerisation to the lowest
energy conformer. Based upon this we exclude this conformation as we expect this conformer
will readily isomerize to the lowest-energy conformer.

122

Figure 6.10 - Diagram of the stable conformers along with their relative energies, excluding theconformations in which the CC bond is syn to the NO2 group. Their population relative to the lowest energy
conformer, according to a Boltzmann distribution, is denoted in the parentheses. H1 and H2 are consistently
defined but arbitrary designations to distinguish the hydrogens. The key at the top denotes the atoms H1 andH2. The conformers outlined in red are excluded in the Franck-Condon simulations, see discussion for moredetails.

6.4.4 Franck-Condon Simulations
Franck-Condon simulations were performed for the conformers of 1-NP and 1-NB anions(discussed above) that were assumed to potentially make significant contributions to thephotoelectron spectra of the anions. The photoelectron spectra for both species, at low photon
energies, have several features superimposed on a vibrationally unresolved and congested

123background. To recover every detail of the spectrum would be exhaustive, if possible, and is notnecessary to explain the experimental spectra. The purpose of these simulations is to identify
which conformers best represent the experimental photoelectron spectra.

Given that thephotoelectron spectra of nitromethane and nitroethane anions were well represented with little tono adjustments of the displacements determined with similar calculations we assume the direct
output of our simulations is sufficient for a first-order simulation.6, 7 It should be mentioned thatthe entire band of the photoelectron spectrum was not fitted as the great number of active modes
becomes too computationally expensive.
Calculations were also performed to identify the stable neutral conformers for eachmolecule. The number of neutral conformers decreases, relative to its anionic counterpart, since
there is only one stable orientation for the nitro group about the CN bond.

Two stableconformers were found for 1-NP and five were found for 1-NB. All of the stable neutral
conformers, for each molecule, are separated by large barriers to internal rotation about the CCbonds. As the neutral conformers are well separated, each stable anion conformer will besimulated only with the neutral conformer with a matching geometry for the alkane chain.

6.4.4.1 1-Nitropropane Anion
Simulations were performed, using an anion temperature of 250 K, for the two calculated
lowest energy conformers of 1-NP anion (see Section 6.2.1 for discussion), and compared to theexperimental photoelectron spectrum of 1-NP anion taken at a photon energy of 368 meV (2968
cm-1). The simulations were performed without the inclusion of the first four energetically lowlying vibrations as they all had significant character of the torsional modes. The nitro wagging

124
mode was adjusted to 589 cm-1 as this was the value extracted from the experimental PES. A
threshold scaling factor of  ,22, 23 where  represents the electron’s kinetic energy was used7/

as it has been found to be necessary in previous near-threshold photoelectron spectroscopy
studies of the smaller nitroalkane anions.6 The simulations are shown in Figure 6.11 below
along with the experimental photoelectron spectrum.
The Franck-Condon simulation of the lowest-energy conformer yields a very congested
spectrum with little Franck-Condon intensity at the 0 G 0 transition or at lower binding energies.
The simulation suggests that the unexpectedly intense and congested higher binding energy sideof the experimental spectrum could be, in part, due to the presence of this conformer. However,
the torsional mode around the CN bond axis has been observed in nitromethane and nitroethane
anion to contribute significantly to spectral congestion and we expect this to be of similar
importance here.
The simulation of the higher-energy conformer achieves a much better agreement withthe prominent features of the experimental PES, as it has a substantial overlap with theexperimentally identified 0 G 0 transition and the intensities of the two largest features areintense enough to be clearly observable through the congested background. Additionally, if thenumerous combination bands with the torsional modes (not included in this simulation) were
accounted for, it appears that the active transitions would serve as a reasonable manifold which
could approach the shape of the observed PES. These simulations are a simplistic representationof a very complex and congested spectrum. We conclude that the simulations suggest that theconformer calculated to be at higher energy is responsible for the two prominent features in thephotoelectron spectrum, and therefore is the conformer that gives rise to an AEA of (223 ! 6)
meV.

125

-1

photoelectron yield [arb. units]

180 cm

-1

0 cm

0

50

100

150

200

250

300

350

electron binding energy [meV]Figure 6.11 – The Franck-Condon simulations of the two lowest energy conformers of 1-NP anion, using ananion temperature of 250 K. An inset in each graph illustrates the 1-NP anion conformer along with its
relative energy. The open circles, solid lines, and bars represent the observed, simulated curve, and theindividual transitions.

126

6.4.4.2 1-Nitrobutane Anion
Franck-Condon simulations were performed using an anion temperature of 250 K for thethree conformers discussed in Section 6.4.3.2. The nitro wagging mode was adjusted to theexperimentally observed value of 598 cm-1 and the five lowest energy vibrational modes were
excluded from the analysis as they are all strongly characterized by torsional motion. Just likefor 1-NP, the simulations were all scaled by  . The simulation for each conformer is shown7/

in Figure 10 alongside the experimental photoelectron spectrum obtained at a photon energy of
384 meV.
The Franck-Condon simulations of the 1-NB conformers results in both the lowest andhighest energy conformers yielding congested spectra with little to no Franck-Condon intensity
for the experimentally identified 0 G 0 transition. The conformer second lowest in energy best

represents the experimental spectrum as it has a substantial in both the 0 G 0 transition and the1 G 0 transition of the nitro wagging mode.

As was the case in 1-NP, the assignment of theconformer responsible for the prominent features in the photoelectron spectrum is by no means
definitive, given the simplifications employed in the analysis. With this in mind, the simulations
indicate that the higher-energy conformer is responsible for the two prominent features in thephotoelectron spectrum, and therefore is the conformer that gives rise to an AEA of (240 ! 6).

photoelectron yield [arb. units]

127

300 cm-1

256 cm

-1

0 cm

0

50

100

-1

150

200

250

300

350

electron binding energy [meV]Figure 6.12 - The Franck-Condon simulations of the three lowest energy conformers of 1-NB anion, using ananion temperature of 250 K. An inset in each graph illustrates the 1-NP anion conformer along with its
relative energy. The open circles, solid lines, and bars represent the observed, simulated curve, and theindividual transitions.

128
6.4.5 Angular Distributions
The photoelectron angular distribution for single-photon detachment with linearlypolarized light is described by the function24, 25
Equation 6.1

89

4<1 3 => cos 9B,
4;where 9 is defined by the angle between the laser polarization and the emission angle, 4 is thetotal photodetachment cross section, > cos 9 is the second-order Legendre polynomial, and =is the anisotropy parameter which varies from -1 to 2. The angular dependence is defined by =and is found by fitting Equation 6.1 to the reconstructed image over a desired range of radii.
While the anisotropy parameter β does not lend itself to a straightforward interpretation,in the case of photodetachment from anions, it carries the signature of the electronic nature of thedetachment transition,22 and features belonging to the same electronic transition usually exhibit asmooth behavior in β. Therefore, the anisotropy parameter β can hint at anomalous behavior in
the electronic transition that leads to observed features in experimental PE spectra.

129

anisotropy parameter, β

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0
0

50

100

150

200

electron kinetic energy [meV]Figure 6.13 – Plot of the fitted anisotropy parameters, β, as a function of kinetic energy for the four different
lower-energy photon wavelengths used in the study of 1-NP anion (gray lines) as well as their average (darkblack line). The dashed line is located at a kinetic energy of 166 meV.

130

2.0

anisotropy parameter, β

1.5

1.0

0.5

0.0

-0.5

-1.0
0

50

100

150

200

electron kinetic energy [meV]Figure 6.14 - Plot of the fitted anisotropy parameters, β, as a function of kinetic energy for the five different
lower-energy photon wavelengths used in the study of 1-NB anion (gray lines) as well as their average (darkblack line). The dashed line is located at a kinetic energy of 166 meV.

The plot of the anisotropy parameters is very similar for both 1-NP and 1-NB anions (seeFigure 6.13 and Figure 6.14) and remains relatively stable with respect to the photodetachingenergy. The beta parameter is insensitive to the two well-behaved peaks that appear at constantbinding energy in all of the spectra. The angular distribution is nearly isotropic out to a kinetic
energy of 50 meV at which point both plots exhibit a steady increase in the anisotropy parameter.
At an energy of 166 meV, the anisotropy parameter exhibits a sharp decrease. This sharp
deviation of the anisotropy parameter at a kinetic energy of 166 meV, which is coincident withthe anomalous feature seen at fixed kinetic energies in all of the plots, strongly suggests that thisfeature is due to a different electronic detachment transition.

131
6.4.6 Remarks on the Anomalous Feature in Photodetachment from 1nitropropane and 1-nitrobutane anions
The observation of an anomalous feature in photoelectron imaging of 1-nitropropane and1-nitrobutane is puzzling. Naturally, the first suspicion one would have is that this feature is due
to an experimental artifact, but we saw no such feature in the spectrum of nitromethane anion,
which was often used to check the validity of the PE spectra during the present study. After anumber of careful tests recording the known PE spectrum of nitromethane and reproducing theexperiments on 1-NP and 1-NB on several different days, we are convinced that the anomalousfeature in both spectra is not due to an experimental artifact. At the present time, we feel
confident that this feature belongs to a detachment process distinctly different from that of direct
photodetachment but our understanding of the origin of this feature is fairly rudimentary. As
such, we will postpone further discussion of this feature until we are able to obtain a moredetailed understanding of its origin.

6.5 Summary
We have studied the photoelectron spectra and angular distributions of the 1-NP and 1NB anion at several photon energies. The relatively high-energy photoelectron studies were able
to capture the entire vibrational progression corresponding to the electronic ground state of theneutral. From these, vertical detachment energies of (0.92 ! .05 eV and (0.88 ! .05 eV were
established for 1-NP anion and 1-NB anion, respectively. We have determined a new value forthe adiabatic electron affinity of 1-NP at (223 ! 6) meV and (240 ! 6 meV for 1-NB. We
utilized ab initio calculations to establish the stable conformers of each anion and employed
Franck-Condon simulations to identify what conformations could be responsible for the

132empirically determined AEA’s. Finally, an anomalous feature appearing at a constant kinetic
energy was shown to have significantly different anisotropy parameter, indicating that it
originates from a different electronic transition.

6.6 References for Chapter 6
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2.

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3.

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4.

J. M. Weber, W. H. Robertson and M. A. Johnson,"Argon predissociation and electron
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5.

H. Schneider, K. M. Vogelhuber, F. Schinle, J. F. Stanton and J. M. Weber,"Vibrational
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6.

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7.

C. L. Adams and J. M. Weber,"Photoelectron imaging spectroscopy of nitroethane
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8.

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133

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M. I. Shilina, V. M. Senyavin, G. M. Kuramshina, V. V. Smirnov and Y. A.
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11.

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13.

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A. D. McLean and G. S. Chandler,"Contracted Gaussian-Basis Sets for Molecular
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134
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Baer and I. Powis (John Wiley & Sons, Chichester, 1994), pp. 371.

19.

K. M. Ervin, FCFGaus03, University of Reno, Nevada, 2008.

20.

R. N. Compton, H. S. Carman, C. Desfrancois, H. Abdoul-Carmine, J. P. Schermann, J.
H. Hendricks, S. A. Lyapustina and K. H. Bowen,"On the binding of electrons tonitromethane: Dipole and valence bound anions." Journal of Chemical Physics 105 (9),3472-3478 (1996).

21.

S. T. Stokes, K. H. Bowen, T. Sommerfeld, S. Ard, N. Mirsaleh-Kohan, J. D. Steill and
R. N. Compton,"Negative ions of nitroethane and its clusters." Journal of Chemical
Physics 129 (6) (2008).

22.

K. J. Reed, A. H. Zimmerman, H. C. Andersen and J. I. Brauman,"Cross-Sections for
Photodetachment of Electrons from Negative-Ions near Threshold." Journal of Chemical
Physics 64 (4), 1368-1375 (1976).

23.

E. P. Wigner,"On the Behavior of Cross Sections near Thresholds." Physical Review 73(9), 1002-1009 (1948).

24.

C. N. Yang,"On the Angular Distribution in Nuclear Reactions and Coincidence
Measurements." Phys. Rev. 74 (7), 764-772 (1948).

25.

J. Cooper and R. N. Zare,"Angular Distribution of Photoelectrons." Journal of Chemical
Physics 48 (2), 942-943 (1968).

135

7 Vibrational Autodetachment Photoelectron Spectroscopy of
Nitromethane Anion
This chapter has been reproduced in part with permission from Adams, C.L., Schneider, H., andWeber, J.M., “Vibrational Autodetachment-Intramolecular Vibrational Relaxation Translated
Into Electronic Motion.” Journal of Physical Chemistry A, 2010. dx.doi.org/10.1021/jp105910s.
Copyright 2010 American Chemical Society.

7.1 Overview of Vibrational Autodetachment
All chemical reactions are governed by the nuclear dynamics of molecules, in otherwords, their patterns of vibrational motion, and any predictive theoretical treatment of chemical
reactions needs to describe these motions. Therefore, the way in which vibrational energy flows
through and is redistributed in molecules after excitation has significant impact on theunderstanding of chemical reactions (see, e.g., refs 1 and 2 ). Moreover, energy flow throughnanoscale systems is presently of much interest as technology utilizes progressively smaller
electronic devices which can approach the destruction limit of energy density.3, 4Consequently, there has been much work on intramolecular vibrational relaxation (IVR)
processes during the last several decades. Much progress has been made, which has been
documented in a number of excellent reviews of the field.5-24 “Standard models” of IVR are in
existence, which have been used with great success in the modeling of IVR in relatively small
molecules.7, 10, 25-28 However, the extension to larger systems is still a challenging and fertile
area for experimental and theoretical investigation (see, e.g., refs 2, 15, 17, 23, and 29-44).

136
It is important to make a distinction between the relatively simple normal mode
description of molecular vibration and the “true” eigenstates of the full molecular Hamiltonian.
While providing a reasonable approximation at low vibrational energies, the “zero-order” normal
mode description is based on the harmonic approximation and therefore neglects the coupling
between different modes. In contrast, the eigenstates of the full Hamiltonian are the result of allforces in the molecule, including anharmonic contributions. They can be expanded in terms of
the “zero-order” basis.
One important experimental access to studying IVR has been optical excitation. Since
the transition dipole matrix for vibrational transitions from the ground state is effectively
diagonal in the zero-order basis, only one of the zero-order basis states will carry significant
transition strength from the ground state and couple efficiently to the exciting radiation field.
This is the zero-order bright state (ZOBS), which is not a stationary state (since it is not aneigenstate of the system). In a one-photon excitation with narrow bandwidth (i.e., relatively longpulses or cw radiation), the spectrum consists of a series of molecular eigenstates. Since thesemolecular eigenstates can be represented in the zero-order basis, the intensity of the transition toa given eigenstate reflects the contribution of the ZOBS to that eigenstate.
Often, IVR processes are analyzed in terms of a tier model where the energy in the ZOBS
is first transferred by strong, low-order anharmonic coupling to a “tier” of energetically close
dark states (often called doorway states), from which the energy flows into states further
removed (see, e.g., ref 7 for a review). The density of states becomes successively greater witheach new tier. Another approach to describe energy flow in molecular systems is in a state spacepicture, representing normal mode basis states as lattice points in multidimensional quantum
number space, where eigenstates typically delocalize over the lattice.45, 46 In the condensed

137
phase, the energy eventually dissipates out of the molecule into the “bath” states of the solvent.
In the case of a molecule in the gas phase under single collision conditions, the only pathways
for relaxation are radiative cooling, electron emission, and dissociation, all of which can beneglected for nanosecond time scales and sufficiently low excitation energies. Therefore, thesum of rovibrational energy in the molecule is conserved and one can view the lowest frequency
modes of the molecule (e.g., skeletal modes of a long alkane chain) as “bath modes”, if themolecule is large enough.
There is continued interest in IVR as new theoretical methods are developed and need to
be tested with ever larger molecular species.38, 40, 41, 47, 48 While IVR processes are in general
qualitatively understood and the theory on small molecules has reached a reasonable level ofpredictive power, the complexity of anharmonic coupling in larger molecules makes thesesystems formidably difficult to understand.
Experiments at low vibrational energies seem like an obvious starting point, as theprobability of accidental resonances between zero-order modes will be small.

Moreover,quantum effects and “non-statistical” processes in IVR will be most important at low energies,
while high energies often afford a description on the basis of statistical models. Many groups
have obtained IVR time scales from the line width analysis of IR excitation (see, e.g., refs 7, 1517, 25, and 49-62) and fluorescence (e.g., refs 63-67) resolution, these experiments primarily
probe the time scales corresponding to the coupling of the various “tiers”. The multiple timescales involved can be extracted from hierarchical analysis of these frequency-resolved spectra.
Another class of experiments on IVR on the ground state electronic surface employed pumpprobe ultrafast spectroscopy, both in molecules and clusters (e.g., 31, 32, 68, 34, and 69-72), and138
in liquids (e.g., refs 3, 24, 72-75). In these cases, the coupling time scales can be obtained frommulti-exponential behavior of the molecular response.
One intriguing group of targets is anions which possess low adiabatic electron affinities
in which a vibrational mode represents a ZOBS embedded in a continuum of dark states (theelectron emission continuum). Vibrational autodetachment (VAD) can occur if the electron
binding energy of the excess electron in an anion is comparable to or less than the energies ofsome of its vibrational modes.
The conceptual and theoretical foundation for auto-ionization in neutral molecules andautodetachment was laid as early as 1966 by Berry.76 Theoretical studies of autodetachmentfrom anions by Simons and co-workers77, 78 followed some 15 years later, prompted by earlystudies of Brauman and co-workers79-83 and Beauchamp and co-workers84-86 on vibrational
autodetachment from multiphoton excitation of anions. The VAD approach has not been widelyused for spectroscopic purposes.

One of the first spectroscopic studies investigating

rovibrational autodetachment was performed by Lineberger and coworkers,87 where the excess
electron from NH was ejected after rovibrational excitation of the anion. They also studied thevibrational autodetachment spectrum of enolate via excitation of the dipole-bound state ofCH CHO .88 VAD has been employed with a (partial) focus on IVR a few years ago by Johnson
and coworkers, in a study of bare and argon solvated nitromethane anions.89 In addition, thesame group applied VAD more recently to the water hexamer anion,90 where they studied how
different isomers of this cluster ion are responsible for different features in the photoelectron
spectrum as the detachment laser is tuned through OH stretching resonances belonging to aparticular isomer. This approach has been recently extended within our group to include many

139
more nitroalkane species where the excess electron has a lower binding energy than the energyof the CH stretching modes.91
The coupling of vibrational energy to electronic motion manifests itself in macroscopic
systems, e.g., as thermionic emission, described by the Richardson equation.92 In a molecular
description, VAD constitutes a breakdown of the Born-Oppenheimer approximation, since it is aclear example of the coupling of electronic and nuclear motion. Its inverse process, vibrationmediated electron attachment, has long been studied by many groups and falls into a similar
category (see, e.g., refs 93 and 94).
A simplifying assumption allowing a qualitative discussion of the underlying process isthat VAD is primarily promoted along some reaction coordinate L describing the concomitantchange in molecular geometry. L will, in general, not be a normal coordinate (except for adiatomic) but is likely a linear combination of several normal coordinates. In a photoelectron
spectroscopy experiment, the normal modes belonging to these coordinates will have the mostpronounced vibrational progressions, signifying the main geometry difference between anion andneutral. In a series of pioneering theoretical articles on VAD, Simons77, 78 described several
scenarios according to the behavior of the anionic and neutral potential energy curves along L.
He distinguished these scenarios based on whether or not the potential curves cross along L, and,if so, whether crossing would occur at the limits of L (i.e., some internuclear separation
approaching zero or infinity) or at some intermediate configuration. Moreover, the rate ofvariation of the energy difference between the anionic and the neutral potential energy curves
along L is an important point, as is the energy difference between the anionic parent vibrational
state and the neutral final state, i.e., the kinetic energy of the emitted electron. The latter is

expected to be the dominant contribution to the electron emission rate constant, since the energy

140
difference appears in a “resonance denominator” in the expression governing the emission
probability.
It should be pointed out that Franck-Condon factors are not governing the VAD process
in the same way as in “regular” (i.e., nonresonant) photodetachment. Instead of the overlap
between the vibrational wavefunction of the anion prior to excitation and of the neutral final
state (nonresonant photodetachment), it is the overlap of the vibrational wavefunction of theanion and the neutral final state after vibrational excitation of the anion that will play a role.
This overlap will critically depend on the evolution of the anion vibrational wavefunction during
IVR and will therefore be time dependent. When VAD occurs after targeted excitation of a
specific vibrational mode in the anion by infrared radiation, it encodes information on the IVR
processes that take place between excitation and electron emission.
It should be noted that in the case where the anionic and neutral diabatic curves cross(which is the relevant scenario here), one can describe the transition of the excess electron fromthe bound to the continuum state as a Feshbach or shape resonance.77, 78, 95 In this case, thefactors governing the detachment rate from a vibrational state accessing the intersection are theoverlap of the anionic and neutral vibrational wavefunctions at the “seam” of the diabatic
potential energy curves and an electronic coupling term containing the resonance description,
which does not necessarily involve large nonadiabatic terms. In this case, the description of thedetachment process contains a mixture of electronic and nonadiabatic coupling. If the curves do
not cross, there is no electronic coupling and the nonadiabatic coupling terms involved generatequalitatively different rate expressions.96

141
In the remainder of this chapter we will discuss some general considerations regarding
vibrational autodetachment in CH NO and present the photoelectron spectra of CH NO
obtained on vibrational resonances that lead to VAD. Then we will discuss the results in thecontext of several simple models before closing with a summary of open questions and an
outlook toward future work.

7.2 Vibrational Autodetachment of Nitromethane Anion: General
Considerations
As was discussed earlier in Chapter 4, the main geometry difference between the anion
and the neutral geometries of CH NO lies in angle between the ONO plane and the CN bondaxis; the angle is nearly coplanar in the neutral molecule but is ~ 30° in the anion.
Correspondingly, the NO “wagging” mode, ¿NO  (note that we use the same labels for thevibrational modes of CH NO as Gorse et al.97), in which the displacement is largely describedby this angle, has an extended vibrational progression which accounts for the majority of theFranck-Condon activity in the photoelectron spectrum of CH NO .98, 99 The remainder of theactivity in the photoelectron spectrum is almost completely accounted for by four other normal
modes. These are the CH umbrella mode,  CH ; the NO symmetric stretch, F NO ; theCH rocking mode, 2À CH ; and the CN stretch, FCN. Given that these modes all undergo asubstantial displacement upon photodetachment, it is expected that a linear combination of thecorresponding normal coordinates will largely define the detachment reaction coordinate, L, inwhich the ¿NO  mode is expected to the most prominent. The strong geometry difference

results in a crossing of the diabatic anionic and neutral potential curves along L.89, 95, 98

142
In contrast to the modes largely characterized by displacement within the nitro group, theCH stretching normal coordinates do not seem to be important contributors to L.89, 99 In a
greatly simplified picture, one could summarize the reaction coordinate L as one coordinate anda generalized CH stretching coordinate as a second coordinate orthogonal to L. The resulting
potential energy surfaces of the anion and the neutral in these two coordinates is depicted in
Figure 7.1.

Figure 7.1 – Schematic drawing of the diabatic anionic and neutral potential energy surfaces of nitromethane.
The anionic surface is connected to that of the neutral along the detachment coordinate Q. In contrast, theCH stretching modes are not connecting the two surfaces and excitation of one quantum of a CH stretchingvibration will not lead to immediate detachment. VAD will only occur after sufficient energy has beenredistributed into Q.

A very interesting aspect of nitromethane is that it can bind an excess electron in two
fundamentally different ways. In one form of CH NO , the excess electron is contained in avalence orbital (see Figure 7.2); the other is a state where the excess electron is bound in the
dipole field of the molecule, where the average distance of the electron from the molecular frame
is typically on the order of several 10 Å (see e.g., refs 95, 98, 100). The molecular geometry
belonging to the dipole-bound state is practically the same as for the neutral. In the context of143vibrational autodetachment, the dipole-bound state can be seen as a “doorway” between thevalence anion and the neutral. The actual coupling between the two states has been explored bySommerfeld using multi-reference configuration interaction.95 He describes the valence anion in
the geometry of the neutral, i.e., in the “crossing region” of the diabatic curves, as a shaperesonance with a width of 0.25 eV, corresponding to a lifetime of a few femtoseconds.

Figure 7.2 – Highest occupied molecular orbital (HOMO) of the nitromethane valence anion.

Upon absorption of an IR photon in a CH stretching vibrational mode, the absorbed
energy is redistributed in the molecule. If enough energy is transferred to vibrational modes that
promote electron emission, the excess electron leaves89 (see Figure 7.1). The absorption bandsof the CH stretching vibrations in the hydrocarbon serve as zero-order bright states, while thenitro group serves as a localized energy acceptor.
Nitromethane has an adiabatic electron affinity of (168 ! 6) meV or ~ 1355 cm-1,99 well

below the excitation energies for CH stretching modes and is therefore an excellent model system
to study VAD.

144

7.3 Photoelectron Imaging Results
Figure 7.3 shows a comparison of photoelectron images obtained off-resonance (3200
cm-1, a photon energy that is free of any contributions from vibrational autodetachment) and on
the VAD resonance belonging to the totally symmetric F CH .

There are substantial

differences between the two images. We observe that all anisotropy is lost and that new bands
appear in the VAD image, together with an accentuation of the inner part of the image, whichcorresponds to low-energy electrons.

Figure 7.3 – Comparison of BASEX transformed photoelectron images of CH3NO2¯ . The image on the left
panel is taken off-resonance at an energy of 3200 cm-1. The image on the right panel is taken on thevibrational resonance of the totally symmetric νs(CH3) at 2777 cm-1.

We note that the VAD photoelectron spectra contain some contributions from direct
photoemission. In order to extract the VAD photoelectron yield, one must subtract the offresonance photoelectron spectrum from the VAD photoelectron spectrum. The contribution
from direct photoemission may be subtracted if one considers the photoelectron spectrum taken
at a vibrational resonance to be the composite of a direct detachment process as well as avibrational autodetachment process,145
Equation 7.1

8Á,,  

Â{Ã,, M 8{Ã,,   3 ÂÃÃ,, M 8ÃÃ,,  

where 8 ,   is the intensity profile at a photon energy, ÄF , as a function of electron kinetic

energy,  , and the additional subscripts of Å, ÆÆ, and s§Æ represent the measured, direct

detachment and vibrational autodetachment spectra, respectively. The terms Â, represent therelative weighting of the two profiles.

From previous studies of direct detachment of

nitromethane anion99 (See Chapter 4), we observed that the photoelectron spectrum is wellcharacterized by,
Equation 7.2

8ÃÃ,,  

CDC  M ÇÈ  

where CDC are the simulated Franck-Condon factors and ÇÈ is a threshold function necessary torecover the experimental spectra. A comparison of the Franck-Condon simulations and the
direct off-resonance photoelectron imaging spectroscopy results seems to indicate the threshold
effects involve mainly $-wave detachment with a small admixture of -wave detachment given
byEquation 7.3

ÇÈ  

 3 5 É 10  M 
7/

/

where the first term contains the $-wave contribution and the second a small -wave term. Since
we now have an approximation for the threshold function, we may reconstruct the intensity
profile of the direct-detachment, 8ÃÃ,,   at the photon energy coincident with the vibrational

resonance. The last detail necessary to construct 8{Ã,, is the relative weighting of the direct-

146
detachment to the total photoelectron detachment. This can be found using the vibrational
autodetachment IR spectra obtained by Schneider et al., where the IR spectra sits on a smoothbackground of photoelectrons from direct-detachment. Equation 7.1 may now be rearranged togive 8{Ã,, .
We note that the weakness of this analysis lies primarily in the threshold function, which
is likely too simple to describe the actual detachment cross-section (see Chapter 4 for a morethorough discussion on the threshold function).99 As a precaution, photon energies as close as
possible (v 5 meV) to the vibrational resonances were used to avoid any problems arising fromthe threshold function. In addition, this approach assumes that there is no interference between
the direct detachment channel and the VAD channel.

7.4 Modeling
In order to describe the process of VAD in the present case, we start with several
assumptions. Ignoring rotational degrees of freedom, we describe the vibrational states of theanion upon vibrational excitation in a normal mode basis within the harmonic approximation,i.e., as 15-tuple of quantum numbers ( 7 ,, O ,

7½ ).

If the molecule is in its vibrational groundstate prior to excitation, then there is one quantum in the ZOBS upon excitation, while all otherquantum numbers are zero. As IVR sets in, vibrational energy is transferred to other vibrational
states in the molecule through low-order anharmonic coupling. As soon as sufficient energy iscoupled into vibrational modes that connect the anionic and neutral potential energy surfaces,
electron emission can occur. If we assume that the main difference between the anion and theneutral geometries is the angle between the CN bond axis and the ONO plane, the main “emission

147
active” vibrational mode is the NO “wagging” mode, ¿NO , and its normal coordinate is thedetachment coordinate L. This mode has the largest Franck-Condon factors in the photoelectron
spectra99 and we assume that electron emission will occur on a time scale of a few femtoseconds

as soon as sufficient energy is pooled in this mode. On the basis of this simple idea, thepopulation of vibrational states in the neutral will provide a rough map of how energy was
distributed in the anion just prior to electron loss.
While many of the vibrational modes of the neutral and the anion are similar, the methyl
torsion changes quite drastically upon electron loss.99 In the anion, the interaction between the
CH group syn to the NO leads to structural stabilization of the methyl group, making the methyl
torsion a hindered internal rotation. In contrast, the calculated potential for methyl torsion in
neutral nitromethane has no significant barrier (~2 cm-1), resulting in a free internal rotor. A
detailed treatment of the internal rotors can be found in Chapter 4 along with their energy levels.
There are altogether 390 vibrational and torsional states of neutral CH NO within thefirst 200 meV of the ground state. Even if we assume only relatively low-order coupling to thehindered rotor states with || ¢ 8, we still count 326 states in this energy range (we will give ajustification for this “cutoff” below). This means that it will not be meaningful to try to fit the
VAD spectrum to individual final states of the neutral, since the number of states makes anunambiguous assignment without restrictions through other assumptions and modelingimpossible. However, the VAD spectra can be modeled in different ways, and we will discuss
several of them below using the VAD photoelectron spectrum of CH NO at 2777 cm-1 as a casestudy.

We chose this state for particular attention because the relative intensity of thecontribution of VAD to direct detachment is highest for this state. The models discussed below

148
are based on a separation of the free internal rotor states, and the other vibrational degrees of
freedom. The intensity distribution of the VAD spectrum 8{Ã  can be written as
Equation 7.4

8{Ã 

ÇÈ  M Ë,Ë, M 8H   Ë, 

where y and  denote the vibrational and free internal rotor quantum states, respectively. The
coefficientsË, represent the intensities of the final states with vibrational quantum number y,torsional quantum number , and the corresponding energies Ë, . The experimental response

function, 8H   Ë, , is represented by a Gauss function corresponding to the experimental
resolution. The sum over all vibrational and internal rotor states is weighted with a threshold lawÇÈ  (see Equation 7.3) determined from the off-resonance behavior at 3200 cm-1. Eachmodel described below will make some assumptions about the coefficientsthe resulting model curves with the experimental VAD spectrum.

Ë, , and we compare

While this represents areasonably objective route for the lower energy range up to ~ 120 meV, we stress that someambiguity remains for higher energies due to the high density of states there.

7.4.1 “No Torsion” Model
A very simple approach to the problem is to assume that torsional modes do not play arole at all. This means that the coefficientsË, are zero for || ¢ 0.

This assumption could in

principle be justified by the argument that the y CH  vibrational mode and indeed all CH
stretching modes are so much higher in energy than the torsion in the anion that coupling
between these modes will be rather ineffective, since it would be of high order. However,

149
several examples from the IVR literature indicate that methyl torsion plays an important role in
IVR for other, related systems.6, 7, 26-28, 101 Figure 7.4 shows a fit of this model without internal
rotor excitation of neutral CH NO to the VAD spectrum. While some states at higher binding
energies are rather well represented by this model, the lower energy region of the spectrum is notrecovered by the simulation, and neither is the feature at 100 meV. Moreover, the valleys
between most of the features are too deep, suggesting that the zero-torsion states are too“insular” to represent the more congested structure of the VAD spectrum, 8 states are above 120
meV. Therefore, the good agreement at high energies can simply be an effect of a sufficientnumber of states in this region (even without internal rotor excitation) to reach a good
description of the spectrum.

VAD yield [arb. units]

150

0

50

100

150

energy left in neutral [meV]Figure 7.4 – Comparison of the experimental VAD spectrum (connected open circles) of CH3NO2¯ on theνs(CH3) resonance (symmetric methyl stretch) with the “no torsion” model (full line, see text). The
contribution of each vibrational state was weighted to yield the best fit of the model to the experimental data.

7.4.2 Energy Randomization Model
If the energy is completely randomized in the molecule before electron emission occurs,
one should be able to describe the VAD yield with the density of states (DOS) in the neutral
molecule. Since an anharmonic calculation for the DOS of the neutral molecule is not available
to us at the time of this writing, we represent the DOS within the harmonic approximation, using
the experimentally known values for the vibrational fundamental transitions99 and the free151internal rotor levels described above. In other words,

Ë,

1 for all y and  and theË, are thestatistical weights of the individual states. Note that each internal rotor energy level except the

VAD yield [arb. units]

ground state is doubly degenerate, since  and  have the same energy.

0

50

100

150

energy left in neutral [meV]Figure 7.5 – Comparison of the experimental VAD spectrum (solid black line) of CH3NO2¯ on the νs(CH3)
resonance (symmetric methyl stretch) with the energy randomization model (solid red line, see text). The
model has been normalized to the experimental data at the 0-eV feature.

Figure 7.5 shows a comparison of the VAD photoelectron yield on the resonancebelonging to the symmetric methyl stretch, y CH , with the DOS in neutral CH NO . One canclearly see at lower energies that the experimental VAD spectrum shows a close resemblance to152
the DOS. At higher energies (above 50 meV), the simulation recovers many of the peak
positions but the intensities are either over- or underestimated.
The feature located in the lower energy region (up to 50 meV) of the spectrum isparticularly interesting, since it is due to pure free internal rotor excitations without contributions
from other vibrational modes. Figure 7.6 shows this feature in more detail. The model curve isbased only on the statistical weight of each internal rotor energy level, where the ground state is
nondegenerate, while each excited energy level is doubly degenerate. The agreement with theexperimental curve is nearly quantitative. The main difference is that the contribution of ||8overestimates the experimental curve. As a result, we will keep the statistical weight as a basis
for the internal rotor distribution in the model described below for low || but ignore all stateswith || ¢ 8, i.e.,Ë,

0 for || ¢ 8.

VAD yield [arb. units]153

J = 0-3

-10

0

4

5

10

6

7

20

30

8

40

50

energy left in neutral [meV]Figure 7.6 – Comparison of the experimental VAD spectrum (connected open circles) of CH3NO2¯ on theνs(CH3) resonance (symmetric methyl stretch) with the density of states (full line) in neutral CH3NO2. The
DOS has been normalized to the experimental data at the 0-eV feature.

7.4.3 Partial Randomization Model
The idea of using the DOS without any modification as a model for the VAD photoelectron
spectrum as described above is fundamentally flawed, since it does not take into account any
differences in the coupling between the anionic and neutral potential surfaces for different final
vibrational states. Since we cannot assume that this coupling is uniform, it makes sense to allow
intensity variations for each vibrational state to obtain a better fit. Moreover, if only partial
randomization occurs prior to electron emission, fitting the intensities of the vibrational modes

154
can also take these differences into account. Although we cannot distinguish between these two
effects, it is appropriate to speak of a “partial randomization model”, as will become apparent
below.
Since we vary the contribution of each vibrational final state while keeping the internal
rotor intensity distribution the same for each vibrational state, this corresponds to writing thecoefficientsË, as a productEquation 7.5
Ë,

with Ì

1 for all || Í 8, ÌºË M Ì

0 otherwise, but varying the coefficients ºË to obtain the best

agreement with the experimental VAD curve.

VAD yield [arb. units]

155

0

50

100

150

energy left in neutral [meV]Figure 7.7 – Comparison of the experimental VAD spectrum (solid black line) of CH3NO2¯ on the νs(CH3)
resonance (symmetric methyl stretch) with the partial randomization model (solid red line). The model has
been normalized to the experimental data at the 0-eV feature.

Figure 7.7 shows the result of this simulation.

The agreement of the energy

randomization model with the experimental data is excellent, with the exception of the feature at
100 meV.
This “missing” feature is interesting in that it seems to be caused by a special final statethat is particularly well coupled to one or more partner states in the anion. There are only twolevels in the neutral that are candidates for this feature. One is a combination band of the nitro
wagging mode ( 1 symmetry) and the free internal rotor state with ||

6 (100 meV). The otheris a combination band of the NO bending mode ( 1 symmetry) and the free internal rotor state

156
with ||5 (99 meV).

At this time, we refrain from speculation concerning the special

significance of these states.

7.4.4 Application to VAD PE for the Symmetric and Antisymmetric Methylene
Stretching Modes
We will now compare the results of the energy randomization model with the VAD
photoelectron spectra of the other two CH stretching modes in CH NO , the symmetric y1 CH 
and antisymmetric yÎ CH  methylene stretching modes. Figure 7.8 shows the results for these

two modes. Each of the experimental VAD spectra can be fitted by the partial randomization
model, although the quality of the simulations of these spectra is not as high for the y CH 
mode. This is mainly due to the lower VAD cross section of these modes, compared to the crosssections for direct detachment. The subtraction of the direct detachment contribution results inan error in the VAD spectra, which grows with the relative importance of direct detachment,even though the data quality of the actual photoelectron spectra (which contain both the VAD
and direct detachment contributions) is similar for all ZOBS under study. This is especially true
for the yÎ CH  mode, where the VAD contribution is in fact smaller than that from direct
detachment. Nevertheless, we can use the partial randomization model to describe the VAD
spectra.

VAD yield [arb. units]

VAD yield [arb. units]

157

0

50

100

150

energy left in neutral [meV]

0

50

100

150

energy left in neutral [meV]Figure 7.8 – Comparison of the experimental VAD photoelectron spectra (connected open circles) belonging
to the ν′s(CH3) (symmetric methylene stretch, left panel) and νa(CH3) (antisymmetric methylene stretch, rightpanel) modes of CH3NO2¯ with the partial randomization model (full lines). The dashed line in both spectra
is a representation of the full energy randomization model. The models have been normalized to theexperimental data at the 0-eV feature for each spectrum.

Interestingly, the intense peak at 100 meV energy, which showed very clearly in they CH  VAD spectrum, does not appear. It is important to note that the intensity distribution in

the neutral final states is different for each ZOBS studied here; i.e., the coefficients ºË depend on

the ZOBS. Since the coupling efficiency between anion and neutral states should not depend on
which ZOBS has been excited, the difference between the intensity distributions is most likely
due to different populations in the parent anion vibrational states prior to photoemission,
justifying the term “partial randomization model”.

158
7.4.5 Critical Discussion of the Modeling
Despite our partial success in modeling the VAD spectra from nitromethane anions, it
seems appropriate to discuss the limitations common to the model discussed above. The model
description so far is purely empirical. It has no predictive character, and we can only extract
very limited information on the dynamics of IVR and VAD processes involved. It is based on arather crude idea of how VAD occurs, and it cannot distinguish between effects based on the
coupling between the anionic and neutral vibrational states and the energy distribution in theanion prior to electron loss.

It treats threshold effects in electron emission only in an

approximate, averaged way, and it contains no vibration-rotation coupling.
Moreover, while the lower energy states up to ~ 120 meV are reasonably well separated
to allow unambiguous assignment of vibrational features, the description of higher energy states
is difficult because of the high DOS, even if the internal rotor substructure of each vibrationalband is fixed. As an example, even though the combination band of the ¿NO  mode with the2NO  mode at 134 meV and the fundamental of the 2Ï CH  mode at 136 meV can barely be

distinguished on the basis of the photoelectron band contours, the 2À CH  fundamental and the

combination band of the NO  and 2NO  modes are both at 140 meV and cannot be

distinguished in the experimental spectrum. Assignments of the vibrational features at higher
energies and their relative intensities therefore carry some ambiguity.
Without losing sight of the limitations of the description, we can nevertheless extract
some interesting information from the VAD photoelectron spectra and the ideas discussed above.
Obviously, VAD cannot be described merely on the basis of the density of the available states of
the neutral. Also, as expected, the clear differences of the off-resonance and on-resonancephotoelectron spectra indicate that the Franck-Condon factors that dictate the profile of the159
direct- detachment photoelectron spectrum do not play the most crucial role in the description of
the VAD spectra.
The intensity distribution in the free internal rotor states is rather interesting. It seems not
to be subject to any restriction by symmetry, at least if one argues that the parent anion is in its
vibrational ground state prior to infrared excitation. However, this is probably not the case, since
the ion temperature is of the order of 250 K, based on Franck-Condon simulations of the offresonance photoelectron spectrum under typical experimental conditions.99 The lowest threehindered rotor states for the anion are nearly triply degenerate. As a result, we have to assume
significant population of states with torsional excitation. Moreover, if the process of populating
detaching states is sufficiently slow, vibration-rotation coupling can relax the symmetry
requirements. Our discussion implies that the main role of the methyl torsion is in the IVR
process leading up to VAD. We note, however, that the role of the methyl torsion in the VAD
process itself is unclear, especially in light of its dramatic change upon electron loss.
The difference between the intensity distributions in VAD spectra coming from different
ZOBS excitations indicates that even if there is some degree of randomization among the low ||torsional degrees of freedom, there is no complete randomization in the population of thevibrational states. This is a significant observation, since it highlights the fact that purely
statistical theoretical approaches will likely fail to describe VAD time scales.

Even more

importantly, it shows that there is a clear dependence of the VAD process in the CH NO onwhich ZOBS is excited in the beginning of the process.
From the relative intensities of the vibrational modes in the VAD photoelectron spectra,
we can try to infer which modes are of particular importance as populated final states. The states

160
that are populated higher than average for all ZOBS are the fundamentals of 2NO , ¿NO ,

2Ï CH  and the higher-order states 2NO  3 ¿NO  and 2 É ¿NO . The fundamental ofCH  is important for the VAD spectra belonging to both y1 CH  and yÎ CH  but isenergetically inaccessible via y CH  as a ZOBS.

As mentioned above, there is a feature in the y CH  VAD spectrum that isunderestimated or unrecovered in the models.
[¿NO  3 ||6] or the [NO  3 ||This feature can be traced to either the5] combination bands. Other such “special

states” have not been observed. Ideally, one would like to be able to map the final states of theneutral onto the anionic parent states. However, the VAD coupling efficiency for the parent
anion states would have to be characterized on the basis of suitable theoretical approach, such asthat of Simons,77, 102, 103 before this is possible.

7.5 Open Questions and Future Challenges
In summary, we have presented one of the first VAD photoelectron spectroscopy studies
of a polyatomic molecular anion. This approach opens up new and exciting avenues into theinvestigation of VAD and the underlying IVR problem of how the energy deposited into a ZOBS
by IR excitation is transferred into the vibrational modes facilitating electron emission. There
are many open questions that require a better theoretical treatment.
As mentioned above, one important unresolved aspect is the description of the coupling
between the anionic and neutral potential energy curve as a function of the anionic vibrational
state. This could potentially be achieved by applying the strategy outlined by Simons77, 103, 104 to
the specific molecules in question. The results from such a calculation would be instrumental to161
map the neutral final state distribution observed in a VAD photoelectron spectrum onto theanionic precursor vibrational states. Are the normal modes that are not involved in the VAD
reaction coordinate L spectators during electron loss? What is the role of the methyl torsion in
the IVR?
Another, separate problem is a good description of IVR in the parent anion. It will be
problematic to tackle this problem by molecular dynamics simulations, since the quantum
mechanical character of the methyl rotor and of the CH stretching ZOBS will probably not bewell described that way. A lattice model for the vibrational state space would probably do morejustice to this situation. For such an approach, one needs cubic and–if affordable–quartic force
constants for the molecular system under study, which is feasible for small molecules (e.g.,CH NO ) but very expensive for larger systems. In this context, the mutual interaction between
the torsional motion and the other vibrational degrees of freedom is of particular importancebecause of the obvious impact of the methyl torsion on the VAD spectra.
While the partial randomization model described above seems to recover most of thedetails of the VAD spectra, it is an open question whether it recovers even in broad strokes thephysics involved in the IVR/VAD process. If so, what is the special significance of the spectralfeatures that deviate from the partial randomization model? Do the anionic precursor states thatmap onto these neutral states play the roles of doorway states in the IVR?
Future experimental developments could include ultrafast pump-probe spectroscopy,
where a mid-infrared pump pulse 7 excites a CH stretching mode and the near-infrared probe

pulse  detaches the electron. The resulting photoelectron spectra would show the temporal

evolution from a regular photoelectron spectrum ( before 7) through developing

162
vibrational hot bands ( after 7 but before VAD) to a VAD spectrum ( after VAD). In
this way, one could determine time scales for both IVR (monitoring the vibrational hot bands)
and VAD separately.

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