NEGATIVE ION PHOTOELECTRON SPECTROSCOPY OF ALKYLPEROXIDES, ALKOXIDES, AND GROUP VIII TRANSITION METAL OXIDESBy

TANYA MICHELLE RAMOND

A. B., Bryn Mawr College, 1994
M. S., University of Colorado, 1996

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirements for the degree ofDoctor of Philosophy
Department of Physics
2001

This thesis entitled:
Negative Ion Photoelectron Spectroscopy of Alkyl Peroxides, Alkoxides, and Group
VIII Transition Metal Oxides
written by Tanya Michelle Ramondhas been approved for the Department of Physics

Prof. W. Carl Lineberger

Prof. G. Barney Ellison

Prof. Chris Greene

Prof. Stephen Leone

Prof. Henry Kapteyn

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

iii
Ramond, Tanya Michelle (PhD., Physics)
Negative Ion Photoelectron Spectroscopy of Alkyl Peroxides, Alkoxides, and Group
VIII Transition Metal Oxides
Thesis directed by Professor W. Carl Lineberger
I employed negative ion photoelectron spectroscopy to investigate thestructure and energetics of three groups of anions and their corresponding neutrals:
alkyl peroxides ROO−, (R = H, D, CH3, CD3, and CH3CH2); alkoxides RO− (R=CH3,
CD3, CH3CH2, CD3CD2, (CH3)2CH, and (CH3)3C); and Group VIII transition metaloxides XO− and OXO− (X = Ni, Pd, Pt). The peroxides and the alkoxides are of greatinterest to those who study atmospheric or combustion chemistry, while the metaloxides play an important role in catalysis reactions. However, each of these groups ofmolecules displays interesting behavior that is itself a motivation for theirinvestigation.
The spectra of HOO− and DOO− are relatively straightforward to analyze andunderstand and provide a good basis from which to compare the larger alkyl
peroxides. The ROO− spectra exhibit the normal Franck-Condon behavior leading toclear assignments of the expected vibrational progressions in both the ground and firstexcited state of the neutral species. Although the molecules increase in size from
HOO to CH3CH2OO, many of the spectral characteristics such as electron affinity(EA) and prominence of the O-O stretch vibration do not appreciably change.

The

EA of HOO is revised, which becomes important as part of a newly revised
thermochemistry of HOO and HOOH.

iv
The RO− species exhibit an additional layer of complexity. Both the CH3O−
and (CH3)3CO− molecules possess relatively high C3v symmetry about the CO axis as
well as a doubly degenerate ground electronic state of the neutral RO molecule. Bothof these elements are expected to produce a Jahn-Teller effect, where in order tobreak the molecular symmetry and electronic state degeneracy, the BornOppenheimer approximation breaks down and nuclear and electronic wavefunctions
become coupled. The extent to which Jahn-Teller effects affect the RO molecule
photoelectron spectra is discussed.
Although the transition metal monoxides are diatomics and thus perhaps
presumed to be uncomplicated molecules, they are the most difficult to understand in
this thesis and the most difficult to obtain in the laboratory. The d orbitals of themetals are closely spaced together, leading to congested spectra and mixing of theproperties of orbitals that complicates analysis and ab initio calculations.
Furthermore, the high nuclear charge of the atoms involved leads to non-negligible
spin-orbit and other relativistic effects. Perhaps for these reasons there is relatively
little information in the literature on these molecules.

However, despite thecomplexities involved, comparison of all three metal molecules has allowed for aconsistent interpretation of the spectra. Assignments are made including electron
affinities, spin-orbit excited states of both anion and neutral XO molecules, otherexcited electronic states of anion and neutral, and vibrational frequencies and bondlength changes. Analysis of the OXO molecules yields electron affinities, vibrationalfrequencies, and anion to neutral geometry changes.

v

Acknowledgements

There are many people I want to thank for acting as a positive force in my trek
through graduate school. To begin with, I thank my advisor, Dr. Carl Lineberger. He
took me into his lab having had no lab experience and only one (high school!)chemistry class under my belt and gave me the chance to learn. He has great physicalinsight and I hope to carry away a small amount of that when I leave. I have also had
the good fortune to work with Dr. Barney Ellison. Barney has been a persistentambassador of organic chemistry to my resistant physics-oriented mind and throughhis lively and convincing manner of presentation may have succeeded in getting someof it to stick!
I thank Dr. Rebecca Schwartz and Prof. Gustavo Davico, who taught memuch of what I know, and especially to Gus who has kept in touch and remained
supportive even after joining the ranks of busy university professors. I have enjoyedworking with and learning from Profs. Kent Ervin and Mitchio Okumura. I havebenefited from the camaraderie of both Lineberger and Ellison group members
including Krista Beck, Todd Sanford, Sang Yun Han, Mark Taylor, Bert Callicoatt,
Felician Muntean, Andrei Sanov, Xu Zhang, Mark Nimlos, Evan Jochnowitz, Django
Andrews, and especially Stephen Blanksby who showed me how explosive chemicals
can actually yield beautiful data. Many of these people are depicted below in thewonderful drawing by a great visiting fellow, Prof. Zdenek Herman. None of theexperiments could have been possible without the superb expertise of members of thevi
JILA machine and electronics shops, most notably Hans Green and James Fung-AFat.

I am thankful for the “Peace Room presence” of Nada Djuric and Duska

Popovic, who have been great friends and colleagues.
I began friendships with the members of my physics graduate class throughcountless late-night homework sessions, TA grading assignments, and post-exam
revelry. I am happy to have stayed in touch throughout the various stages of grad
school, especially with Erin Gill, Maija Besky, Jamie Williams, Adrienne Allen, andYoriko Morita. Several people have been particularly encouraging for helping get me
through those turbulent comps years: Profs. Anna Hasenfratz, Jinx Cooper, and John
Taylor, and Mike Dubson.

I cannot forget Dr. Katy Garmany who provided a

sympathetic ear and unconditional support. I also have fond memories of working
with Profs. Peter Conti and Bob Blum and I hope they finally realize I didn’t leaveastrophysics because of anything they did! I feel lucky to have earned the friendshipof Kelsey Johnson over my grad school years. I admire her intelligence and drive,and enjoyed sharing many talks, hikes, and ultimate games.

I hope to stay closefriends with her and her husband, Remy Indebetouw, and wish them the best of luckas they leave Boulder and start their post-doc careers.
I cannot forget the unqualified love and support from the members of myfamily: Pierre and Lillian, Lisa and Jennifer, as well as all my grandparents, aunts,
uncles, and cousins.

I come from a very scientifically and academicallyaccomplished family, and they have provided much empathy along the path to a
Ph.D. in physics. Finally, I thank my sweetie, Jim Piccolini, for overcoming a(probably warranted!) suspicion of scientists and loving and believing in me.

vii

viii
Contents

Chapter
1

Introduction1

1.1

Negative ion photoelectron spectroscopy overview………………….8

1.2

Franck-Condon analysis.………………………………………….…14

1.3

Thesis overview……………………………………………………..18

Bibliography…………………………………………………………………21

2

Description of experimental methods….…………………………………….23
2.1

Introduction…………………………………………………………..23

2.2

Ion beam……………………………………………………………...23
2.2.1 Microwave discharge………………………………………...25
2.2.2 Cold cathode DC sputter source………………………….….27

2.3

Mass selection………………………………………………………..33

2.4

Ultraviolet laser system……………………………………….……...35

2.5

Electron detachment and angular distributions………………………40

2.6

Photoelectron detection….…………………………………………...44

2.7

Position sensitive detector……………………………………………47

2.8

Data processing………………………………………………………57

Bibliography…………………………………………………………..……..60

3ix
Photoelectron spectroscopy of alkyl peroxides ROO (R = H, D, CH3, CD3,−

and CH3CH2)………………………………………………………………....61
3.1

Introduction…………………………………………………………..61

3.2

Experiment………………………………………………………...…64

3.3

HOO− and DOO−…………………………………………………….65

3.4

CH3OO−……………………………………………………………...73

3.5

CD3OO−……………………………………………………………...81

3.6

CH3CH2OO−……………..…………………………………………..85

3.7

Revised thermochemistry of HOO and HOOH……….…………..…92

3.8

Conclusion………………………………………….………………..98

Bibliography………………………………………….…………………….100

4Photoelectron spectroscopy of alkoxy radicals RO− (R = CH3, CD3, CH3CH2,
CD3CD2, (CH3)2CH, and (CH3)3C )………………………………………..103
4.1

Introduction……………………………………………………...…103

4.2

Experiment…………………………………………………………107

4.3

CH3O−……………………………………………………….……..109

4.4

CD3O−……………………………………………………….……..116

4.5

CH3CH2O− and CD3CD2O−………………………………………..118

4.6(CH3)2CHO−………………………………………...……………..125

4.7(CH3)3CO−………………………………………………………....130

4.8

Vibronic coupling and Jahn-Teller effects………………………...133

4.9

Thermochemistry………………………………………………….135

4.10

x
Conclusion………………………………………………………….138

Bibliography………………………………………………………………..140

5Photoelectron spectroscopy of Group VIII transition metal oxides XO−, OXO−
(X = Ni, Pd, and Pt)………………………………….…………….…..…...144
5.1

Introduction…………………………………………………..……..144

5.2

Experiment……………………………………………………...…..146

5.3

Analysis of NiO, PdO, and PtO……………………….…………...148
5.3.1 NiO……………………………………………………….…148
5.3.2

PdO………………………………………………………....161

5.3.3

PtO………………………………………………………….169

5.4

Discussion of NiO, PdO, and PtO…………………………………..177

5.5

Analysis of ONiO, OPdO, and OPtO………………………………179
5.5.1 ONiO………………………………………………………..180
5.5.2

OPdO……………………………………………………….183

5.5.3

OPtO………………………………………………………..185

5.6

Discussion of ONiO, OPdO, and OPtO…………………………….188

5.7

Conclusion………………………………………………………….189

Bibliography……………………………………………….……………….190

Comprehensive bibliography

194

Appendix206

xi
TablesTable
3.1

Data extracted from the 364 nm photoelectron spectra (Figures 3.2-3.3) of__

HOO and DOO …………………………………………………………….67
3.2

_
2
~2
Molecular geometries for 1A′ HOO and ~X A″ and A A′ states of the neutral
HOO radical, resulting from a B3LYP/aug-cc-pVDZ DFT calculation……..71

3.3

Data extracted from the 364 nm photoelectron spectra (Figures 3.4-3.6) of___

CH3OO , CD3OO , and CH3CH2OO ………………………………………..73
3.4

The molecular geometries and harmonic vibrational frequencies (cm-1) for 1A′_
2~2
CH OO and ~X A″ and A A′ states of the neutral CH OO radicals, calculated
33

at the B3LYP/aug-cc-pVDZ level of theory…………………………………76
_

_

3.5

Negative ion photoelectron peak positions for CH3OO and CD3OO .
Uncertainties are given in parentheses…………………………...…………..78

3.6

Negative ion photoelectron peak positions for CH3CH2OO ………………..87

3.7

The molecular geometries and harmonic vibrational frequencies (cm-1) for 1A′_
2~2
CH CH OO and ~X A″ and A A′ states of the neutral CH CH OO radicals,

_

3

2

3

2

calculated at the B3LYP/aug-cc-pVDZ level of theory. All vibrationalfrequencies are unscaled……………………………………………………..89
3.8

Recalculation of ∆fHT(HOO) using data measured from the reaction HOO +NO ! NOO + OH from Howard (33) and thermochemical quantities fromthe tables of Gurvich et al. (39) using both the ∆fHT(OH) from Ref. (39) andfrom Ref. (40)………………………………………………………………..96

3.9

Recalculation of ∆fHT(HOO) using data measured from the HOO + Cl !
ClO + OH reaction from Hills and Howard (34) and thermochemicalquantities from the tables of Gurvich et al. (39) using both the ∆fHT(OH) from
Ref. (39) and from Ref. (40)…………………………………………………96

3.10

Calculation of DT(HOO-H) and ∆acidHT(HOO-H) using data measured fromthe reaction HOO + NO ! NOO + OH from Howard (33) andthermochemical quantities from the tables of Gurvich et al. (39) using boththe ∆fHT(OH) from Ref. (39) and from Ref. (38)……………………………97

xii
3.11

Calculation of DT(HOO-H) and ∆acidHT(HOO-H) using data measured fromthe HOO + Cl ! ClO + OH reaction from Hills and Howard (34) andthermochemical quantities from the tables of Gurvich et al. (39) using boththe ∆fHT(OH) from Ref. (39) and from Ref. (38)……………………………97

4.1

Peak positions, asymmetry β values, and assignments for the photoelectron
spectrum of CH3O−…………………………………………………………112

4.2

Peak positions, asymmetry (β) values, and assignments for thephotoelectron spectrum of CD3O−………………………………………….117

4.3

Peak positions, asymmetry (β) values, and assignments for the photoelectron
spectrum of CH3CH2O− and CD3CD2O−…………………………………...120

4.4

Peak positions and assignments for the photoelectron spectrum of(CH3)2CHO−……………………………………………………………..…127

4.5

Peak positions and assignments for the photoelectron spectrum of(CH3)3CO−………………………………………………………………….132

4.6

Electron Affinities (EA) in eV……………………………………………...136

4.7

Derived thermochemical data………………………………………………137

5.1

Absolute and relative positions, and assignments for peaks in thephotoelectron spectrum of NiO−…………………………………………....152

5.2

Summary of assignments for NiO and NiO−……………………………….153

5.3

Absolute and relative positions, anisotropy parameters (β), and assignmentsfor peaks in the photoelectron spectrum of PdO−…………………………..164

5.4

Summary of assignments for PdO and PdO−……………………………….164

5.5

Absolute and relative positions, anisotropy parameters (β), and assignments
for the photoelectron spectrum of PtO−…………………………………….171

5.6

Summary of assignments for PtO and PtO−………………………………...172

5.7

Anion 2Π3/2−2Π1/2 spin-orbit splittings and neutral 3Π2−3Π1 spin-orbit
splittings for the NiO, PdO, and PtO molecules……………………………174

xiii
5.8

Electron affinity and vibrational frequency measurements and literature values
for the OXO 1Σ +g ← 2A2 OXO− (X=Ni, Pd, Pt) photoelectron spectra……..182

xiv

FiguresFigure1.1

Valence 2p4 configuration of O………………………………………….……2

1.2

Valence picture of O-CH3……………………………………………………..2

1.3

Molecular orbital diagram of (1sσ)2 (1sσ*)2 (2sσ)2 (2sσ*)2 (3pσ)2 (1pπx)2(1pπy)2 (1pπx*)1 (1pπy*)1 O2 composed from 1s2 2s2 2p4O…………………...4

1.4

Schematic picture of formation of bonding 3σ and antibonding 3σ* orbitals
from the pz atomic orbitals of oxygen………………………………………....5

1.5

Schematic picture of formation of bonding 1πx and antibonding 1πx* orbitals
from the px atomic orbitals of oxygen…………………………………………5

1.6

Schematic picture of formation of bonding 1πy and antibonding 1πy* orbitals
from the py atomic orbitals of oxygen…………………………………………6

1.7

Valence orbital schematic of dioxygen O2. The single dots represent unpaired
electrons and the double dots indicate a filled lobe…………….……………..6

1.8

Valence orbital schematic of HOO. The single dots represent unpaired
electrons and the double dots indicate a filled lobe. The solid line indicates abond…………………………………………………………………………....7

1.9

Schematic picture of negative ion photoelectron spectroscopy……………….9

1.10

Photoelectron spectrum simulation of an arbitrary diatomic molecule andvarious changes to the anion bond length re upon electron photodetachment.17

2.1

Schematic description of the main parts of the negative ion photoelectron
spectrometer including anion creation and delivery, the UV laser system, andphotoelectron energy analysis and detection…………………………...……24

2.2

The microwave discharge ion source………………………………………..26

2.3

Schematic of the cold cathode DC sputter ion source……………………….28

2.4

Schematic of the flowing afterglow and the first section of ion optics before
the mass analyzer………………………………………………………….…30

xv

2.5

Diagram of the entire ion beam line from the source region to the Faraday
cup……………………………………………………………………………32

2.6

Diagram of the UV laser system……………………………………………..36

2.7

Single-mode operation of a cw laser…………………………………………37

2.8

Schematic of the electron kinetic energy analyzer……………………….….45

2.9

Diagram of the primary elements involved in the photoelectron position
sensitive detector and associated electronics………………………………...48

2.10

Multichannel plate/resistive anode encoder assembly, which comprises the
position sensitive detector…………………………………………..………..50

3.1

Generalized valence bond (GVB) diagrams of peroxide anions, ROO , theperoxyl radicals, ROO, and the peroxyl cation, ROO+……………………....63

3.2

364 nm photoelectron spectrum of HOO taken at 200 K and magic angle
laser polarization……………………………………………………………..66

3.3

364 nm photoelectron spectrum of DOO taken at 200 K and magic angle
laser polarization………………………………………………………….….69

3.4

364 nm photoelectron spectrum of CH3OO taken at 200 K and magic angle~ ←X~ transition
laser polarization. (a) Bottom panel, an expansion of the A

_

_

_

_

spectral profile……………………………………………………………….74

3.5_

364 nm photoelectron spectrum of CD3OO taken at 200 K and magic angle~ ←X~ transition
laser polarization. (a) Bottom panel, an expansion of the A
spectral profile……………………………………………………………….82

3.6_

364 nm photoelectron spectrum of CH3CH2OO taken at 200 K and magic~ ←X~
angle laser polarization. (a) Bottom panel, an expansion of the A
transition spectral profile…………………………………………………….86

4.1

Schematic of the two degenerate highest occupied molecular orbitals of X~
1
A1 CH3O−………………………………………………………………….106

4.2a) Magic angle photoelectron spectrum of CH3O− taken at 300 K sample
temperature. (b) 300 K photoelectron spectrum of CH3O− taken at 90° (heavyline) and 0° (light line) polarization angles with respect to the electronxvi

detection axis. (c) Magic angle photoelectron spectrum of CD3O− taken at
300K sample temperature, DOO− subtracted…………………………...…..110
4.3a) 200 K CH3CH2O− photoelectron spectrum taken at 90° (heavy line) and 0°(light line) polarization angles with respect to the electron detection axis. (b)The 200 K CH3CH2O− magic angle photoelectron spectrum is displayed inbold. The thin line at the baseline corresponds to a Method 2 (B3LYP/aug-cc~ state ofpVDZ) simulation of the photoelectron transition into the A
CH3CH2O as described in the text. The floated line is the simulation of thephotoelectron transition into the X~ state of CH3CH2O………………...….119

4.4

300 K CD3CD2O− photoelectron spectrum taken at 90° (heavy line) and 0°(light line) polarization angles with respect to the electron detection axis…124

4.5a) The 200 K magic angle photoelectron spectrum of (CH3)2CHO− isdisplayed in bold. The thin line corresponds to a theoretical simulation using
Method 1 (B3LYP/6-31+G*) of the photoelectron transition into the X~ stateof (CH3)2CHO. (b) The 200 K magic angle photoelectron spectrum of(CH3)2CHO− is displayed in bold. The thin line corresponds to a theoreticalsimulation using Method 3 (CIS/aug-cc-pVDZ) of the photoelectron transition~ state of (CH ) CHO…………………………………………….126
into the A
3 2

4.6

Magic angle photoelectron spectrum of (CH3)3CO− taken at 200 K sample
temperature in bold. The thin line corresponds to a Franck-Condon simulationof the photoelectron spectrum in a simple harmonic oscillator approximation
using experimental frequencies (Method A)………………………..………131

5.1

Molecular orbital diagram of the 3Σ− ground state of NiO showing the mixing
of atomic O and Ni orbitals…………………………………………………149

5.2

364 nm photoelectron spectrum of NiO− taken at 0° laser polarization angle
and 200 K flow tube temperature……………………………..…………….150

5.3

364 nm photoelectron spectrum of the onset region of PdO− taken at magic
angle polarization and 200 K flow tube temperature………….……………162

5.4

364 nm magic angle photoelectron spectrum of PdO− taken at 300 K flow tube
temperature…………………………………………………………………167

5.5

364 nm magic angle photoelectron spectrum of PtO− taken at 200 K flow tube
temperature…………………………………………………………………170

5.6

364 nm magic angle photoelectron spectrum of ONiO− at 300 K flow tube
temperature………………………………………………………...……….181

xvii

5.7

364 nm photoelectron spectrum of OPdO− taken at magic angle polarization
angle and 200 K flow tube temperature………..…………………………...184

5.8

364 nm photoelectron spectrum of OPtO− taken at magic angle polarization
angle and 200 K flow tube temperature…………………………...………..186

1
Chapter 1
Introduction

This thesis offers an examination of the structure and energetics of threegroups of molecules through negative ion photoelectron spectroscopy: alkyl peroxyls(ROO), alkoxy radicals (RO), and metal oxides (XO, OXO). All have in common anorganic (R) or inorganic (X) substituent bonded to either one or two oxygen atoms.
Oxygen has obvious importance to our everyday lives and is abundant in ouratmosphere; however, an important property is that oxygen is the second mostelectronegative element of the periodic table and thus bonds readily. It is useful tostart with a physical picture of the orbital structure of both the oxygen atom and thebonding pattern in the dioxygen molecule. A basic overview is given here although
for more details, the reader should consult Ref. (1).
Oxygen atom has a ground state configuration of 1s2 2s2 2p4. The orbitals of
the valence configuration are displayed in a schematic way in Figure 1.1, with the px
orbital pointing up, the py orbital coming out of the page, and the pz orbital pointing
to the right. The way the figure is drawn, both the py and pz electrons are unpaired,although there is no preferred direction and the lone pair may go to any of the threelobes. The ground state of the oxygen atom possesses two electrons with spins
parallel, forming a 3P state. Anything bonding to the oxygen may bond with either
the pz or py electron, both configurations being energetically equivalent.

For

example, the oxygen bonds to a methyl CH3 group to form the methoxy radical
CH3O, as shown in Figure 1.2.

Here a σ-type bond is formed along the z axis

2

O
••

x

•

z

•

Figure 1.1. Valence 2p4 configuration of O.

O - CH3

H

••
•

H
•

•

C
H

Figure 1.2. Valence picture of O-CH3.

3(represented by the solid line) between the pz oxygen lobe and the pz carbon lobe andone unpaired electron is left in the py orbital.
The homonuclear diatomic O2 molecule offers a fairly straightforward pictureof bonding, with a ground state molecular orbital diagram shown in Figure 1.3. Each
oxygen molecule has 8 electrons, for a total 16 electrons in O2, which are depicted in
the diagram. The s orbitals combine to give bonding σ and antibonding σ* orbitals
symmetric about the z axis, depending on if the individual s wavefunctions are addedor subtracted, respectively. The three orthogonal p orbitals may be combined in three
ways to make a bond. The first way the overlap of pz bonds forms the σ and σ*
orbitals depicted in Figure 1.4. The dashed lines indicate the spatial region where theatomic orbitals overlap to make the molecular orbitals. The second way is displayedin Figure 1.5. In this case, the px orbitals overlap broadside to form both πx and πx*
orbitals.

This bond is energetically equivalent to forming the πy and πy* orbitals

from the broadside overlap of the py orbitals, as shown in Figure 1.6.
The ground state configuration of O2 is (1sσ)2 (1sσ*)2 (2sσ)2 (2sσ*)2 (3pσ)2(1pπx)2 (1pπy)2 (1pπx*)1 (1pπy*)1 (see Fig. 1.3) which is commonly written as(1σg+)2 (1σu+)2 (2σg+)2 (2σu+)2 (3σg+)2 (1πu)4 (1πg)2 3Σg- . This triplet (1pπx*)1 (1pπy*)1
configuration is energetically favored over a singlet (1pπx*)2 1∆g or 1Σg+ because
Hund’s rules predict that a configuration with higher multiplicity will be lower inenergy. The O2 ground state is represented schematically in Figure 1.7.

The twodegenerate unpaired electrons are staggered, one in a πx* and one in a πy* orbital, thedouble dots symbolizing closed-shell orbitals. From this point it is not difficult to
picture the structure of HOO. The hydrogen will bond with one of the unpaired

4

3pσ2*

1pπx2* 1pπy2*2p42p4
1pπx2 1pπy2
3pσ2
2sσ2*

2s2

2s2
2sσ2
1sσ2*

1s2

1s2
1sσ2

O

O2

O

Figure 1.3. Molecular orbital diagram of (1sσ)2 (1sσ*)2 (2sσ)2 (2sσ*)2 (3pσ)2(1pπx)2 (1pπy)2 (1pπx*)1 (1pπy*)1 O2 composed from 1s2 2s2 2p4 O.

5

x

y

z

3σ

3σ*

Figure 1.4. Schematic picture of formation of bonding 3σ and antibonding 3σ*
orbitals from the pz atomic orbitals of oxygen. Dashed lines indicate region whereatomic orbitals overlap to form molecular orbitals.

1πx

1πx*

Figure 1.5. Schematic picture of formation of bonding 1πx and antibonding 1πx*orbitals from the px atomic orbitals of oxygen. Dashed lines indicate regions whereatomic orbitals overlap to form molecular orbitals.

6

1πy

1πy*

Figure 1.6. Schematic picture of formation of bonding 1πy and antibonding 1πy*orbitals from the py atomic orbitals of oxygen. Dashed lines indicate regions whereatomic orbitals overlap to form molecular orbitals.

••
•

•
•

•

••

O-O

Figure 1.7. Valence orbital schematic of dioxygen O2. The single dots represent
unpaired electrons and the double dots indicate a filled lobe. The solid line indicatesa bond.

7
electrons, as shown in Figure 1.8, with rearrangement of the orbital positions tominimize interactions between electrons. This will be further addressed in Chapter 3.
The experimental method used to probe such molecular structure in this thesis will
now be discussed.

•

••

H

•
•

•

•

••

O

O

H
••
•

••
••

HOO

Figure 1.8. Valence orbital schematic of HOO. The single dots represent unpaired
electrons and the double dots indicate a filled lobe. The solid line indicates a bond.

8
1.1

Negative ion photoelectron spectroscopy overview

Negative ion photoelectron spectroscopy (PES) is a valuable tool to examine
the fundamental structure of molecules (2).

A schematic of the experiment ispresented in Figure 1.9. Negative ions are synthesized in a flowing afterglow source,
shaped into a beam with a series of ion lenses, and then are mass-selected with a
Wien mass filter. The continuous ion beam intersects the beam of a fixed frequency
3.4 eV cw laser, which ejects the photoelectron and leaves a neutral molecule and aphotoelectron with non-zero kinetic energy (eKE).

The experiment may besummarized as follows:

AB − + hν → AB + e − (eKE )(1.1)

This process is depicted schematically in Figure 1.9. It is a direct photodetachment
process in a frozen nucleus approximation which may be thought of as proceeding via
an intermediate (AB−)* state whose lifetime is significantly less than a vibrational
period.

In this case, the nuclear geometry of the anion is effectively directlysuperimposed onto that of the neutral, neglecting any intermediate state contribution.
This is the Franck-Condon approximation. The photon imparts enough energy toexpel the extra electron from the anion and the molecule transitions into a boundinternal state of the neutral molecule. The energy imparted to the system, hν, must be
conserved. The energy that is not used to make the transition from anion to neutral,the electron binding energy (eBE), leaves the system in the form of the kinetic energy

9ν 364 nmhν
A! AB
Electron Binding Energy (eBE)ν - eKEeBE = hν
ν → AB + e − (eKE)AB− + hν

X! AB

T0

EA(AB)
Electron Affinity

X! AB ¯Figure 1.9. Schematic picture of negative ion photoelectron spectroscopy. Negativeions intercept 364 nm photons, leaving the molecule in a bound state of the neutral
molecule. The excess energy leaves in the form of the kinetic energy of the detachedelectron and a spectrum of photoelectron peaks versus electron kinetic energy (eKE)
is recorded.

10of the photoelectron; for every discrete transition AB → AB there is an electron−with a discrete amount of electron kinetic energy (eKE) expelled. The observable forthis experiment is the electron kinetic energy of the photoelectrons. They pass into ahemispherical kinetic energy analyzer and then onto a position sensitive detector. In
this way a spectrum of photoelectron peaks versus eKE is collected. The energy axis
may also be expressed in terms of electron binding energy, where eBE = hν − eKE.
A wealth of information may be extracted from the spectra, including
vibrational frequencies and anharmonicities of both anion and neutral species,
geometrical parameters (see following section), and the electron affinity (EA). The
EA is defined as the transition between the vibrationless ground states of the anion
and the neutral, denoted 000 . The EA is both a key spectroscopic value and of vital
importance for use in thermochemical cycles in order to determine bond dissociationenergies (Chapters 3 and 4) (3).
There are many advantages to negative ion PES. One is that frequently the
3.4 eV photon energy is great enough to observe transitions into excited states of theneutral molecule, which is represented in Figure 1.9 by two potential wells in theneutral state. In the case of the RO and ROO organic molecules examined in thisthesis, the separation between the anion highest occupied molecular orbital (HOMO)
and HOMO-1 orbitals, for example, is large enough that accessing only ground andfirst excited states is possible with the 3.4 eV photon energy. For the metal oxideanions, the energy levels are much closer together, which permits several low-lying
electronic states to be observed.

11
In optical spectroscopy, a direct transition from a singlet to triplet electronic
state is dipole forbidden. However, the PES technique is valuable in that splittings
between singlet and triplet electronic states may be measured directly: the spin
selection rule is ∆S = ± ½ so that a transition made from a doublet anion state canaccess both S=3 and S=1 neutral states.
Another advantage to PES is the ability to glean information from short-lived
radical species and anions. Radical molecules would not endure long enough to serve
as samples for most spectroscopic techniques as they react so readily. Furthermore,
there is much less information in the literature in general about molecular anions
compared to neutrals, and negative ion PES offers a way to observe anion structure
directly.

The anion source region of the PES can be cooled, which shifts the

population distribution in the vibrational levels of the anion ground state. Upon
cooling the sample, a peak originating from a vibrationally excited anion state (hotband) will decrease in intensity with respect to one coming from the ground state.
Therefore, comparisons of spectra taken with cooled and non-cooled anions permits
identification of hot bands (and thus anion vibrational frequencies), which also allows
an unambiguous electron affinity assignment.
Another important tool for assignment of spectral features is the ability tomeasure angular distributions of photoelectrons. In this PES configuration, a halfwave plate is inserted into the laser beam path that allows rotation of the laser
polarization.

Comparing the intensities of peaks recorded at different laser

polarization angles yields insight into the symmetry of the detached photoelectron.
Electrons detached from two different orbitals will yield transitions into different

12electronic states of the neutral. In most cases these two orbitals will possess different
symmetries that will translate into observation that the photoelectron peaks ofdifferent electronic states yield contrasting angular distributions (4).
In addition to our experiments (5-8), there are several other laboratories thatuse negative ion photoelectron spectroscopy. For example, the experiments headed
by both Kit Bowen (9) and Doreen Leopold (10) are modeled after this apparatus. In
these labs, a visible 488 nm laser line is used. This provides more laser intensity thanour 364 nm (3.4 eV) uv line and therefore avoids the need for a laser intensity buildup cavity (Chapter 2). However, there is less laser photon energy available, which
decreases the energy range of observable spectral features.

Also, the laser

polarization is fixed, preventing the measurement of photoelectron angular
distributions. Bowen uses a Branscomb hot-cathode discharge source (11), which canprovide a large amount of ions although with higher vibrational temperatures than canbe obtained with a flowing afterglow microwave discharge source (Chapter 2).
Leopold uses a magnetic sector mass analyzer instead of a Wien filter, which
provides mass resolution around 10 times better than what is available here (m/∆m ≈40).
If significantly greater photon energies than around 3.4 eV are desired, pulsed
lasers are a necessity, and the experimental architecture changes fundamentally. The
first anion photoelectron spectrum obtained using a pulsed experiment was reported
in 1986 by Mark Johnson and co-workers (12). Cluster ions were formed using
electron impact ionization of a pulsed supersonic jet that passed through a time-offlight (TOF) mass spectrometer and intersected a 532 nm Nd: YAG laser. The

13
detached photoelectrons were then analyzed in a field-free TOF apparatus. The
experiment of Daniel Neumark (13,14) employs a similar setup. Ion sources usedhave included both a pulsed molecular beam supersonic expansion crossed with abeam of electrons as well as a pulsed electrical discharge (15). Both methods are
effective means of producing a vibrationally cooled beam of anions. A TOF mass
spectrometer is used with a mass resolution of m/∆m = 250-300, and the resolution
can reach 2000 with the incorporation of a reflectron stage (14). A field-free TOF
electron energy analyzer is used.

The energy resolution therefore degrades as(eKE)3/2 from 5-10 meV resolution (similar to what is used here) at low electron
kinetic energies. This setup also allows angular distribution measurements throughrotation of the laser polarization angle.
Other laboratories use a pulsed source/laser setup. Both Wang (16) andJarrold (17) use a pulsed molecular beam laser ablation source with a time of flightmass spectrometer and a 3.49 eV Nd: YAG laser.

Like Neumark, Jarrold uses a

field-free TOF electron energy analyzer that permits angular distribution studies.
Wang, however, uses a magnetic bottle TOF, which uses a magnetic field to direct all
the photoelectrons toward the detector. This is efficient because it collects a full 2πsolid angle of photoelectrons, although it sacrifices all photoelectron angular
information. Continetti has a similar apparatus to Neumark (18,19). However, in thiscase angular distribution information is obtained not by rotating the laser polarization
angle but using a time and position sensitive detector.
Pulsed photoelectron spectroscopy has been successfully employed in the
study of molecular clusters such as Cun(CN)m− (20), (C6F6)n− and (Au-C6F6)− (21),14
and Inn and Tln (22) produced using laser vaporization sources. These were studies−−

of relatively small clusters, where n and m ≤ 20. Development of a pulsed arc cluster
ion source (PACIS) (23,24) has allowed production of clusters of several hundreds ofanions, neutrals, or positive ions. The PACIS source has been incorporated into thenegative ion photoelectron spectroscopy of Sbn− and Bin− clusters (25) and Pbn− (26).

1.2Franck-Condon analysis

Understanding the factors that determine the relative intensities ofphotoelectron peaks reveals further information about molecular structure.

The

intensity of a photoelectron transition is expressed (27) as the square of the transition
moment integral, M,
M = 〈Ψ′ µˆ Ψ〉(1.2)

where µ̂ is the dipole moment operator and Ψ is the total wavefunction of themolecule. At this point, the Born-Oppenheimer approximation allows simplificationof Eq. 1.2. This principle acknowledges the different timescales for nuclear andelectron motion in a molecule. Because the nuclei are orders of magnitude heavier
than the electrons, the Born-Oppenheimer approximation can be used to treat theelectrons as moving with respect to nuclei that are considered fixed in space. As aresult, the molecular wavefunction can be separated into an electronic part and avibrational part:15
Ψ = Ψ es Ψ v .

(1.3)

Furthermore, the electric dipole operator may be assumed to have a negligible effecton the nuclear coordinates of the molecule on the timescale of electron detachment,µˆ ≅ µˆ e . This allows Eq. 1.2 to be written as

M = Ψ e′s′ µˆ e Ψ es Ψ v′ Ψ v(1.4)

Therefore relative intensities of vibrational levels all originating and ending in thesame electronic states are determined by the square of the overlap between initial andfinal state vibrational wavefunctions,Ψ v′ Ψ v

2.

This is known as the Franck-Condon factor.

(1.5)

The validity of the approximation

depends upon the absence of resonant anion states in the photodetachment continuum.
In Chapter 4, the photoelectron spectrum of methoxy radical is discussed.
This molecule suffers from a Jahn-Teller effect that couples electronic and nuclear
motion and in this case, Eq. 1.3 is not valid. However, for the remainder of themolecules in this thesis and the majority of the molecules investigated with thisapparatus, the Born-Oppenheimer approximation is legitimate and the FranckCondon factors determine the intensities of the vibrational levels observed.

16
To gain a qualitative sense of the mechanics of Franck-Condon overlap
integrals, consider the case of an arbitrary diatomic that has only one geometricalparameter, the internuclear spacing, or re . Furthermore, assume that both the groundanion and neutral states may be approximated as harmonic oscillators and all anion
population is in the vibrational ground state. In the case of zero bond length change
between the anion and neutral molecules, the potential wells are exactly aligned, one
directly above the other.

The resulting photoelectron spectrum from detaching anelectron from the anion looks like a single peak, as shown in Figure 1.10a. Because
the wavefunctions of the harmonic oscillator are orthogonal, the only Franck-Condon
factor with a non-zero integral is the 0-0 transition (1-1, 2-2, etc. are also non-zero,but they will have the same energy as the 0-0 transition in the harmonic oscillator
approximation).
Now consider the case where the two potential wells are slightly offset so thatthe neutral bond length is, say, 0.07 Å shorter or longer than that of the anion. Thismay be thought of as a slight perturbation away from case a, and is reflected in thecorresponding spectrum shown in Figure 1.10b. Here the 0-0 transition is still themost intense, but now there is non-zero Franck-Condon overlap with excited
vibrational states of the neutral, producing a monotonically decaying peak profile.
If the two potential wells are separated farther, say by 0.12 Å, the vibrational
progression becomes more extended, and the 0-0 transition is no longer the mostintense (Figure 1.10c).

For a relatively large ∆re of 0.20 Å, the vibrational

progression extends further (Figure 1.10d). The peak profile begins to follow aPhotoelectron Counts

17a) ∆re=0.01Å

c) ∆re=0.12 Åb) ∆re=0.07 Åd) ∆re=0.20 Å

3.0

2.0

1.0

0.0

3.0

2.0

1.0

0.0

Electron Binding Energy (eV)Figure 1.10. Photoelectron spectrum simulation of an arbitrary diatomic moleculeand various changes to the anion bond length re upon electron photodetachment. a)
No change in re yields non-zero Franck-Condon intensity only in the 0-0 transition.
b) A small ∆re yields a monotonically decaying profile of peaks. c) A large ∆reshows an extended progression where the 0-0 peak is no longer the most intense peak.
d) For large enough ∆re , the peak profile is even more extended, takes on a gaussianenvelope, and the Franck-Condon factor for the 0-0 transition is small tonondetectable.

18
gaussian envelope. More significantly, the overlap between the vibrational groundstates of the anion and neutral is relatively small and may not be observable above thenoise of the experiment.
In this way, the photoelectron spectrum offers insight into the geometricalalteration an anion suffers upon electron detachment. Each of the peak profiles
shown above is observed in the various molecules in this thesis. For a polyatomic
molecule, more than one geometrical parameter (bond length or bond angle) willchange upon detaching an anion electron.

However, if the detached electron

originates in an orbital that is essentially localized to one or two atoms in themolecule, as will be seen in chapters 3 and 4, the changes in geometry may be limited
to a change in a single bond length or bond angle. In the case of a diatomic molecule,a previous knowledge of the neutral bond length allows an estimation of the anion
bond length based on the ∆re needed to reproduce the intensities of the peaks in the
spectrum.

1.3

Thesis overview

The remainder of this thesis is organized into four chapters. Chapter 2 offersa general overview of the negative ion photoelectron spectrometer apparatus used toobtain the data presented here, with references to sources of more specificinformation. In addition, a detailed section is provided regarding the new position
sensitive detector and accompanying electronics and software incorporated during thesummer and fall of 2000.

19
Chapters 3-5 present data and analysis of three different groups of molecules
beginning with the most straightforward spectra to understand and ending with themost complex. Chapter 3 covers the photoelectron spectroscopy of alkyl peroxideanions ROO−, (R = H, D, CH3, CD3, and CH3CH2). Such molecules are of greatinterest to those studying atmospheric chemistry and combustion as they play key
roles as reaction intermediates.

The spectra of HOO− and DOO− are relatively

straightforward to analyze and understand and provide a good basis from which tocompare the larger alkyl peroxides. Furthermore, with a few exceptions explained in
the chapter, the ROO− spectra exhibit the normal Franck-Condon behavior leading toclear assignments of the expected vibrational progressions in both the ground and firstexcited state of the neutral species. Although the molecules increase in size from
HOO to CH3CH2OO, many of the spectral characteristics such as the electron affinity
and the prominence of the O-O stretch vibration do not appreciably change. Finally,
the EA of HOO is revised, which becomes important as part of newly revised
thermochemistry of HOO and HOOH.
In Chapter 4, the photoelectron spectroscopy of alkoxide anion RO− species(R=CH3, CD3, CH3CH2, CD3CD2, (CH3)2CH, and (CH3)3C) is presented. Thesemolecules are also important in atmospheric chemistry, although they are intriguingin their own right due to their inherent spectroscopic complexities. Both the CH3O−
and (CH3)3CO− molecules possess relatively high (C3v) symmetry about the CO axis
as well as a doubly degenerate ground electronic state of the neutral RO molecule.
Both of these elements are expected to produce a Jahn-Teller effect where in order tobreak the molecular symmetry and state degeneracy, the Born-Oppenheimer

20
approximation breaks down and nuclear and electronic wavefunctions are coupled.
The extent to which Jahn-Teller effects affect the RO photoelectron spectra isillustrated throughout the chapter.
Finally, Chapter 5 examines the structure of Group VIII transition metaloxides XO− and OXO− (X = Ni, Pd, Pt). Although the XO molecules are small
diatomics, they are the most difficult to understand in this thesis and the most difficult
to obtain in the laboratory. The d orbitals of the metals are closely spaced together,leading to congested spectra and mixing of the properties of orbitals, which
complicates analysis and ab initio calculations. Furthermore, the size of the atoms
involved leads to non-negligible spin-orbit and other relativistic effects. Perhaps forthese reasons there is relatively little information in the literature on these moleculesthat are so prevalent in catalysis processes and surface science. However, despite thecomplexities involved, comparison of all three metal molecules has allowed for aconsistent interpretation of the spectra. Assignments are made including electron
affinities, spin-orbit excited states of both anion and neutral XO molecules, otherexcited electronic states of anion and neutral, and vibrational frequencies and bondlength changes. Analysis of the OXO molecules yields electron affinities, vibrationalfrequencies, and anion to neutral geometry changes.

21
Bibliography

1.

P. W. Atkins, "Physical Chemistry.", 2nd ed. W. H. Freeman and Company,
San Francisco, 1978.

2.

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

3.

J. C. Rienstra-Kiracofe, G. S. Tschumper, H. F. Schaefer III, S. Nandi, and G.
B. Ellison, Chem. Revs., (2001, in press).

4.

R. L. Schwartz, G. E. Davico, T. M. Ramond, and W. C. Lineberger, J. Phys.
Chem. A 103, 8213-8221 (1999).

5.

G. E. Davico, R. L. Schwartz, T. M. Ramond, and W. C. Lineberger, J. Phys.
Chem. A 103, 6167-6172 (1999).

6.

G. E. Davico, R. L. Schwartz, T. M. Ramond, and W. C. Lineberger, J. Am.
Chem. Soc. 121, 6047-6054 (1999).

7.

G. E. Davico, T. M. Ramond, and W. C. Lineberger, J. Chem. Phys. 113,8852-8853 (2000).

8.

T. M. Ramond, G. E. Davico, R. L. Schwartz, and W. C. Lineberger, J. Chem.
Phys. 112, 1158-1169 (2000).

9.

J. V. Coe, J. T. Snodgrass, C. B. Freidhoff, K. M. Mc Hugh, and K. H.
Bowen, J. Chem. Phys. 84, 618 (1986).

10.

A. A. Bengali, S. M. Casey, C.-L. Cheng, J. P. Dick, P. T. Fenn, P. W.
Villalta, and D. G. Leopold, J. Am. Chem. Soc. 114, 5257-5268 (1992).

11.

L. M. Branscomb and S. J. Smith, Phys. Rev. 98, 1028 (1955).

12.

L. A. Posey, M. J. Deluca, and M. A. Johnson, Chem. Phys. Lett. 131, 170174 (1986).

13.

R. B. Metz, A. Weaver, S. E. Bradforth, T. N. Kitsopoulos, and D. M.
Neumark, J. Phys. Chem. 94, 1377-1388. (1990).

14.

22
C. Xu, G. R. Burton, T. R. Taylor, and D. M. Neumark, J. Chem. Phys. 107,3428 (1997).

15.

D. L. Osborn, D. J. Leahy, D. R. Cyr, and D. M. Neumark, J. Phys. Chem.
104, 5026 (1996).

16.

L.-S. Wang, H.-S. Cheng, and J. Fan, J. Chem. Phys. 102, 9480 (1995).

17.

V. D. Moravec and C. C. Jarrold, J. Chem. Phys. 108, 1804 (1998).

18.

V. Zengin, B. J. Persson, K. M. Strong, and R. E. Continetti, J. Chem. Phys.
105, 9740 (1996).

19.

K. A. Hanold, C. R. Sherwood, M. C. Garner, and R. E. Continetti, Rev. Sci.
Instrum. 66, 5507 (1995).

20.

Y. Negishi, T. Yasuike, F. Hayakawa, M. Kizawa, S. Yabushita, and A.
Nakajima, J. Chem. Phys. 113, 1725-1731 (2000).

21.

A. Nakajima, T. Taguwa, K. Hoshino, T. Sugioka, T. Naganuma, F. Ono, K.
Watanabe, K. Nakao, Y. Konishi, R. Kishi, and K. Kaya, Chem. Phys. Lett.
214, 22-26 (1993).

22.

M. Gausa, G. Gantefor, H. O. Lutz, and K. H. Meiwes-Broer, Int. J. Mass
Spectrom. 102, 227-237 (1990).

23.

H. R. Siekmann, C. Luder, J. Faehrmann, H. O. Lutz, and K. H. MeiwesBroer, Z. Phys. D 20, 417-420 (1991).

24.

H. R. Siekmann, E. Holub-Krappe, B. Wrenger, C. Pettenkofer, and K. H.
Meiwes-Broer, Z. Phys. B 90, 201-206 (1993).

25.

M. Gausa, R. Kaschner, G. Seifert, J. H. Faehrmann, H. O. Lutz, and K. H.
Meiwes-Broer, J. Chem. Phys. 104, 9719-9728. (1996).

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27.

D. C. Harris and M. D. Bertolucci, "Symmetry and Spectroscopy." Dover
Publications, New York, 1989.

23
Chapter 2
Description of experimental methods

2.1

Introduction
There are three primary sections to the anion photoelectron spectrometer, as

shown in Figure 2.1. The first involves creation of the anions followed by the
manipulation of the anion beam and mass selection. The second is the 364 nm cw
laser whose beam intersects the anion beam perpendicularly to eject an electron. The
third is the electron energy analysis and detection system. Detached electrons leave
the molecule in all directions, but those traveling perpendicular to the anion and laser
beams are kinetic energy-selected through a hemispherical energy analyzer andsubsequently focused onto a position sensitive detector.

The apparatus measures

photoelectron intensity as a function of electron kinetic energy to produce a
photoelectron spectrum.

2.2

Ion beam

The formation of a beam of anions begins with ion synthesis. There are two
primary ion sources employed in this apparatus that may be used to create them: amicrowave discharge in a flowing afterglow and a cold cathode sputtering source.

LaserUV LaserExB
Mass Filtere–Electron Detector

Figure 2.1. Schematic description of the main parts of the negative ion photoelectron
spectrometer including anion creation and delivery, the UV laser system, andphotoelectron energy analysis and detection.

Electron Detection

Energy Analyzer

Deceleration

AB− + hν → AB + e−

Photoelectron
Spectrometer

Ion Source

Acceleration

Ion Delivery

24

25
2.2.1 Microwave discharge
The microwave discharge ion source was used in the creation of both thealkoxide anions and the peroxide anions, as it is generally used for creation of organic
anions. A diagram of the apparatus is shown in Figure 2.2. To begin with, He buffer
gas is injected through a microwave discharge as shown. The gas has been purified
upon passage through a liquid nitrogen-cooled molecular sieve trap. A Tylan flow
controller sets the helium flow at typically 6-7 standard liters per minute, although
this number may be varied depending on the system studied.

The microwave

discharge takes place in a quartz cylinder around which a microwave resonant cavity
fits. The cavity is powered by a 2.45 GHz microwave generator that ionizes andheats the gas into a plasma upon ignition with a Tesla coil. The cavity may be tuned
with a ceramic tuning rod. The neutral plasma contains metastable He as well as He+,
He2+, and free electrons. A small amount of O2 (7-10 cm3 minute-1) is also injected
into the discharge at the same point as the He through a variable leak valve. In the
microwave cavity, the O2 collides with the free electrons to produce metastable O2−
that then dissociates into O and O−.

The products are carried out of the discharge

with the He buffer gas into the afterglow region. At this point they react withreagents that can be introduced into the system through one or two inlets at varying
positions.

For example, the reaction used to create the RO− species (andequivalently, ROO−) went as follows. Methane gas passes through a leak valve and isinjected through an inlet positioned just after the discharge to form OH−:

CH4 + O− → CH3 + OH−(2.1)

Microwave Discharge Source

Ceramic
tuning rod

Flow Tube(0.5 Torr)

Figure 2.2. The microwave discharge ion source. He and O2 gas are injected
into a 2.48 GHz microwave resonator which produces O- and O2- anions.
These anions can react with a variety of precursor gases introduced by inlets
located further downstream to create the desired negative ion.

Inlets for
Reagents

He/O2

Quartzdischarge
cylinderMicrowave cavity
ion reactor26

27
The second inlet is positioned further downstream. One end is attached to a test tubeof liquid ROH. The vapor pressure is high enough to ensure a sufficient amount ofsample present in the gas phase. The ROH gas travels into the flow tube through aneedle valve and loses a proton to OH− to form RO−:

ROH + OH− → RO− + HOH(2.2)

In order to maximize the production of the desired anion, gas flow rates, absolute andrelative inlet positions, and source temperature may be varied.

2.2.2

Cold cathode DC sputter source

In order to efficiently create metal anions, a sputter source is employed. A
diagram of the apparatus is shown in Figure 2.3.
The sputter cathode is held at the end of an approximately 30 cm longstainless steel rod of 1/8″ outer diameter. The cathode is located on the axis of theflow tube. The rod is surrounded by a glass cylinder of ¼″ outer diameter that serves
to electrically insulate it from the flow tube. On the end inside the chamber, a metal
holder is attached with set screws onto the rod. In turn, the other end of the metal
holder supports the metal cathode rod. High purity (≥ 99.95 %) metal cathode rods
(Goodfellow) of about 2 mm diameter and 5 cm length Ni, Pd, or Pt were used.
Around the holder a glass shield is fit in order to prevent arcing between the sample

28

Cold Cathode DC Sputter source

H2O coils

He, ArH2O inFlow
tube

H2O out

100 kΩ
Electrical Ballast

4H

HolderSample rod

-

+
3 kV

Figure 2.3. Schematic of the cold cathode DC sputter ion source. A sample metal is
attached to a stainless steel water-cooled holder that is biased at a few thousand voltsand is electrically isolated from the grounded flow tube. Argon gas is leaked into thesystem, is accelerated to the rod, and anions are sputtered off upon impact.

29
and the chamber walls. An open loop water cooling system is employed to reduce thetemperature of the sample. The stainless steel rod is hollow and inside it fits asmaller stainless steel rod. Water flows in through the smaller rod to the tip inside thechamber, then returns between the concentric rods and exits the system as shown in
the figure. In addition, the entire source region is wrapped in copper tubing throughwhich cooling water flows. The stainless steel rod is negatively biased through avariable power supply in series with an electrical ballast consisting of a 100 kΩ
resistor and a 4H inductor. The ballast is designed to add resistive elements to limit
current surges in the event of arcing between the cathode and the grounded chamber
walls. Typical bias voltages for the cathode are roughly -1500 V and the discharge
current is about 15 mA. Helium buffer gas prepared as described above is seededwith typically 10-20 % argon and flows into the chamber near the sample. The highbias of the cathode creates a strong electric field with respect to the grounded walls of
the chamber that is sufficient to ionize the neutral Ar. The Ar+ is then accelerated by
the electric field and collides with the cathode. Neutrals and charged atoms, as well
as dimers, trimers, etc. of the sample metal, are sputtered off the rod. O2 may beintroduced in the same manner as described above to form metal monoxides,
dioxides, etc.
Once created via microwave discharge or sputtering, the anions proceed
through the flow tube as shown in Figure 2.4 and are thermalized via 104-105
collisions with the helium gas. In the flow tube, most of the He is pumped away by a
Roots pump operating at 300 l/s which maintains the region at a pressure of
approximately 0.5 Torr. The flow tube is surrounded by a dewar in which liquid

30

Roots
Blower(330 l/s)
L2
L1

10-4 Torr
L4

L3

L5

L6

Dewar
Ion
SourceFlow Tube (0.5 Torr)

N

D1

A

D2

D3 (10º)-5

10 Torr

Diff Pump(2000 l/s)

Flowing Afterglow and Ion Optics
Figure 2.4. Schematic of the flowing afterglow and the first section of ion optics
before the mass analyzer. “N” stands for nosecone, “D” indicates a deflector, “A”
indicates an aperture, and “L” indicates an einzel electrostatic lens. Ions created in
the source region undergo collisional relaxation in the flow tube filled with helium
buffer gas. The ions are gently extracted into a differentially pumped region and areshaped into a beam and accelerated with ion electrostatic optics.

31
nitrogen (LN2) may be introduced. The helium buffer gas thus collides with theliquid nitrogen-cooled walls and thermalizes to LN2 temperature and subsequently the
sample is cooled through collisions with the cold helium.

In the case of the

microwave discharge source without the liquid nitrogen, the sample is thermalized toroom temperature and with the LN2, the sample is cooled to around 200 K. However,the sputter source creates much hotter ions than the microwave discharge andcollisions with the helium thermalize the ions to higher final temperatures (< 500K).
Cooling of the flow tube is a highly valuable tool in photoelectron spectrum
analysis. Comparison of a spectrum taken at different sample temperatures is adefinitive way to identify peaks that depend on the temperature of the sample, i.e. hotbands. For example, the electron affinity (EA) is the 000 transition from anion toneutral. The same transition originating from a vibrationally excited state in the anion
would appear to the lower binding energy side of the electron affinity. It is notalways obvious which peak is the EA or if there is a hot band. If upon cooling thesample to 200 K the intensity of the lower binding energy peak diminishes withrespect to the other peak, one may assign the EA with confidence.
The anions are gently extracted through a roughly 1 mm aperture in a
nosecone at the end of the flow tube (labeled ‘N’ in Figure 2.4) with a small positive
voltage (< +1 V). They then pass into a differentially pumped region where they areaccelerated, focused into a beam, and deflected through a system of electrostatic
lenses. These are indicated schematically as the letters L, D, and Q in Figures 2.4 and2.5. The first lens is an einzel lens, denoted by an ‘L’. It is a biased flat plate with ahole in the center that serves to accelerate and focus the beam. The ‘D’ elements are32
Source00.5
Nosecone
L1-3
D1

10-4

A1

L3-4
D2
L4-6

10° bend

D3

Pressure (Torr)

735

Q1
A2
D4

A3
Q2

Wien Velocity
Filter10-7

Q3
A3
Deceleration
Lens
Stack

Beam Energy (eV)

10-6

Q4
D5
38

10-8

•

hυ

M

M
Faraday
Cup

Magnetic
ShieldingInteraction
VolumeFigure 2.5. Diagram of the entire ion beam line from the source region to the
Faraday cup. “D” indicates a deflector ion lens, “A” indicates an aperture,“Q” signifies a quadrupole electrostatic lens element, “L” indicates an einzel
electrostatic lens, and “M” indicates interaction cavity mirrors. The ion beam
is extracted from the source region, accelerated, mass selected in a Wien
velocity filter, and decelerated into the region of interaction with the laser
beam.

33
deflectors, formed by two horizontal bars and two vertical bars, which further
accelerate and steer the beam.

By the time they reach the last einzel lens, L6, theions have been accelerated to 735 eV kinetic energy. A 6″ diffusion pump operating
at a rate of 2000 l/s maintains the chamber at about 10-4 Torr at the nosecone toaround 10-6 Torr at the opposite end. The acceleration to 735 eV is done gradually
through this region so as to avoid collisional excitation in these higher pressure areas.
At the third deflector, D3, there is a 10° bend in the beam direction. The anions are
guided through the bend while any remaining neutrals collide with the walls and are
pumped away. The ions pass into the next chamber pumped by a second diffusion
pump (8″) that maintains the pressure at 10-6 Torr. Here, there is a quadrupole lens,
Q1, which is formed by four rods positioned symmetrically about an axis throughwhich the ion beam travels. The quadrupoles serve to further deflect and shape thebeam. The beam is navigated through a plate with an aperture A2. At this point theanions that miss the aperture collide with the plate and their current is measured on anelectrometer.

2.3

Mass selection

Products of the ion source region will include many more species than the one
intended for photodetachment. In order to secure an ion beam of an unambiguous
identity, a Wien mass filter is placed in the beam path. The filter works withperpendicular electric and magnetic fields (E and B) that produce an electric force (FE= qE, where q is the charge) on the ions normal to the beam direction, and a magnetic

34
force (FM = qvB, where v is the velocity of the anion) in the opposite direction fromthe electric force. The magnetic force is related to the mass of the ion, since v =(2KE/m)1/2 where KE is the kinetic energy of the ion (735 eV) and m is the mass.
When the two forces are equal for the desired mass they cancel and the selected ion
species passes out of the mass filter undeflected.

This happens for mass m=

2KE(B/E)2. All other ions not of this mass will suffer a net force up or down and thus
be deflected out of the beam. If the electric force is kept constant, the magnetic force
can be changed to mass-select different species. If the magnetic force is variedmonotonically, a mass spectrum of ion species abundance versus ion mass can begenerated in order to investigate the products created in the flow tube chemistry.
The Wien mass filter gives a mass resolution of m/∆m approximately 40.
Therefore for higher mass species one cannot resolve between M and M-1 species, forexample. However, an advantage to this mass selection apparatus is that if theelectric and magnetic fields are turned off, one regains the full ion beam, which canbe useful for diagnostic purposes.
Once the ions of desired mass have left the Wien filter, they are guided into
the interaction region, which is pumped down to 10-8 Torr by two ion pumps. In thisregion deceleration lenses slow the beam to 38 eV to maximize the interaction timewith the laser. At the center of the interaction region the ion beam crosses the laser
beam and then continues on to terminate in a Faraday cup. The current in the Faraday
cup is read on an electrometer and is recorded in the lab book. The magnitude of theion current depends on the species investigated and the ion source employed.
Currents of one nanoamp of O− are possible while for both the RO− and ROO−

35
systems, beams of several tens of picoamps are more typical. For sputtered systems,
smaller currents of order a few picoamps are common.

2.4

Ultraviolet laser system

The near uv laser system used in the photoelectron spectroscopy apparatus
consists of a fixed frequency 364 nm argon ion continuous wave (cw) laser whose
output is steered through a buildup cavity where it is maintained in resonance via anelectronic servo system. A diagram of the laser system is shown in Figure 2.6. Avery detailed treatment is given in Ref. (1); however, a more conceptual overview ispresented here.
There are two main parts to the laser system: the laser itself and a separate
power buildup cavity where the laser intersects the ion beam. We start by discussing
the laser itself. We use a Spectra Physics Argon Ion model 2045 operating at a single
364 nm near-uv line. The laser consists of four primary elements: an end mirror, an
output coupler, the Ar gas, and an etalon. The back mirror has been adapted from theusual stationary mirror into a broadband UV flat reflector that is mounted on a
piezoelectric transducer stage. The position of the stage can be translated along thelaser cavity axis to lengthen or shorten the cavity. The 364 nm output coupler ismounted at a fixed position at the other end of the laser cavity. The two mirrors
define a length L of the cavity. Only when the cavity length equals an integer numberof half-wavelengths, L = nλ/2, where n is an integer, can radiation of wavelength λL
be sustained inside the cavity; all other wavelengths will dissipate (2,3).

36Interaction
Spot
R1
M

Build-up Cavity

AOM

M

R2

•

Telescope

λ/2

M
M Piezo

Back mirror
Argon Ion Laser

M
Piezo

EtalonOutput coupler

UV Laser System

Figure 2.6. Diagram of the UV laser system. In the drawing, “M” represents mirror
elements, and “R” indicates highly reflecting cavity mirrors. The system consists
primarily of an argon ion laser and an external intensity build-up cavity. The backmirror of the laser and the front mirror of the build up cavity are mounted on
piezoelectric stages. Together with an acousto-optic modulator (AOM) and various
electronics, these stages form a servoamplifier system designed to amplify the laser
output intensity inside the build-up cavity.

37

Ar++ gain profile

Gain

Etalon gain profile
Longitudinal modes

2L
n

2L
n +1

2L
n+2

λ

Gain
Single-line
resonance

λFigure 2.7. Single-mode operation of a cw laser. The Doppler-broadened gain
profile of the Ar++ gas overlaps with the comb of allowed longitudinal modes for alaser with cavity length L. An etalon is incorporated into the laser cavity to select asingle resonance line.

38
Equivalently, only wavelengths where λL = 2L/n will resonate in the laser cavity.
These resonant wavelengths, or longitudinal modes, are indicated in Figure 2.7.
The laser cavity is filled with Ar gas. An intense discharge in the laser
capillary tube doubly ionizes the gas. The output coupler and back mirror optics
select a single lasing transition of Ar++ that is doppler-broadened due to the hightemperatures during operation. This results in a specific gain profile for the medium,
which is superimposed upon the longitudinal modes in Figure 2.7. The region where
the two overlap determines the lasing modes for the cavity.

The number ofresonating modes can be reduced to one using a around 1 cm thick frequencyrestricting etalon placed between the two end mirrors. The gain profile of the etalon
is such to be able to select a single mode to resonate, as illustrated in the figure.
The second main part of the laser system is a buildup cavity. The argon ion
laser typically outputs 1 W of single frequency power at 364 nm. This alone is notenough to detach workable intensities of photoelectrons from picoamp ion beam
currents and therefore prohibits working intracavity. Thus the laser beam intensity is
amplified in an external buildup cavity. The idea is to match the buildup cavity
length to the laser output wavelength using the same idea as for laser cavity
resonance. The buildup cavity has two highly reflecting mirrors: R1 = 99.6 % and
R2 = 99.8% in Figure 2.6. R1 and R2 define a cavity length LBC.

In order for thelaser output wavelength, λL, to resonate and thus be amplified in the cavity, one musthave λL = 2 LBC/n .
The length of the cavities L and LBC can change slightly for various reasons:thermal fluctuations as well as acoustic noise due to the Roots pump or building

39
noise, etc. To respond to all these length fluctuations in order to maintain thecondition λL = 2LBC/n, an electronic servoamplifier system has been incorporated into
the laser setup.

There are three main elements to the servo system, which are allconnected through various electronics. One element is the back mirror of the laserthat can change the laser cavity length L and thus λL. Another is the front mirror in
the buildup cavity that is mounted on piezos that change the cavity length LBC. The
third element is an acousto-optic modulator (AOM) placed in the beam path before
the interaction region.
The AOM is an integral part of the servo system used to lock the laser
radiation to the buildup cavity. It is comprised of a transducer connected to a blockof quartz glass and vibrates back and forth to produce a standing wave in the medium.
This wave acts as a Bragg diffraction grating for the incoming laser beam (364 nm is
about 800 THz). The diffraction produces a beam that has been Doppler shifted by anamount equal to the acoustic frequency in the glass.

Therefore the first orderdiffraction of the beam is shifted by a constant bias, between 55-85 MHz. The output
first order beam is different from the input beam by this amount, and if the transducer
frequency is varied by a small amount on top of the beam frequency, a small dither(around 1 MHz), or frequency jitter about the laser frequency + bias is superimposedon the beam.
The output of the AOM can therefore be thought of as {800 THz + (55-85)
MHz} ± 1 MHz. This frequency modification serves two purposes. First, the 55-85
MHz constant frequency shift is employed to frequency isolate the two resonancecavities. This way, none of the beam reflected from the buildup cavity will return to40
the laser cavity and produce interference since before and after the AOM the beam
has different frequencies by an amount shifted out of the resonance range of the laser
cavity. Second, the frequency dither (± 1 MHz) serves to vary the frequency ofradiation entering the interaction chamber by a small amount about the exact
resonance frequency. The reflected light off the cavity front mirror is monitored by
the servoamplifier electronics. Assume the frequency is exactly on resonance. As thefrequency is dithered slightly away from this resonance frequency, the system will
begin to come out of resonance, resulting in an increase in light reflected by the front
mirror. The electronic response will come in some combination of adjusting thecavity lengths L or LBC or varying the AOM frequency accordingly so that thebuildup cavity resonance is recovered. When this is done properly, all cavity lengths
are maintained such that there is a permanent state of resonance in the buildup cavity.
This is known as “locking” the laser.

When locking is achieved, laser power isincreased by a factor of 150-200.

2.5

Electron detachment and angular distributions

In the center of the laser buildup cavity, the ion and laser beams intersect. At
this point, the laser beam is approximately ten times narrower (about 0.1 mm) than
the ion beam (around 1 mm). Photoelectrons are detached and are ejected in all
directions in the chamber. The eKE analyzer is located below the interaction region.
Thus the direction of photoelectron detection is fixed in space. We cannot rotate thedetector about the interaction point to monitor the angular distribution of

41
photoelectrons.

However, we effectively do the same thing by varying thepolarization of the laser interacting with the molecules.

This is done with a half-

waveplate (λ/2) placed in the beam path after the AOM and before the interaction
region in Figure 2.6.

Here 0° is defined as the electron detection direction,

perpendicular to the laser beam/ion beam plane. The waveplate is calibrated using
the angular distribution of O−, whose photoelectrons are heavily peaked in thehorizontal direction.
In the ion beam delivery, no effort is made to spatially orient the molecules,
therefore in the case of molecules with non-zero dipole moments, they cross theinteraction point with a random spatial distribution of dipole moments.
waveplate selects an orientation of the laser electric field.

The

One may imagine,

therefore, that molecules with electric dipoles oriented parallel to the laser electricfield will preferentially absorb the radiation while molecules whose dipole moment isperpendicular to the polarization direction will not absorb. Thus to sample the entire
angular distribution of photoelectrons one has only to rotate the laser polarization
through 90°. Of course, there is a cos θ effect since the proportion of molecules
which absorb the radiation vary with cos θ , where θ is the angle between the laser
polarization and the dipole moment of the molecule. However, it has been workedout (4) that in the dipole approximation the angular distribution of photoelectrons
follows the equationdσ σ tot(1 + β P2 (cos θ )) ,=d Ω 4π(2.3)

42where P2( cos θ ) is the second Legendre polynomial

P2 (cosθ ) =1(3cos2 θ − 1) .
2

(2.4)

The β factor is the asymmetry parameter and ranges between –1 ≤ β ≤ 2. The angular
distribution of a peak in the photoelectron spectrum of a molecule is directly related
to the symmetry character of the orbital from which the photoelectrons originate.
This is most easily demonstrated starting with the case of atomic photoelectron
detachment. If a photon detaches an electron from an s-orbital, it imparts one unit ofangular momentum to the system, and that photoelectron departs in the form of a p
partial wave. A p partial wave has an angular distribution of cos2θ. Such behavior
for dσ/dΩ is achieved if β has a value of 2. If a photon detaches an electron from a porbital, the single quantum of angular momentum can add or subtract, yielding eithers-waves or d-waves. However, if the photon energy is close to the threshold for
detachment, the s-wave channel will be preferred. An s partial wave is isotropic, andrequires that β = 0. As the photon energy increases, mixing between s and d partial
waves occurs, and the angular distribution moves to sin2θ, achieved when β = −1.
For molecules, the physics is not as clear-cut. In some cases, the symmetry ofmolecular orbitals can be reasonably approximated by atomic orbitals; however, inothers (for example if the orbital is involved in bonding), the orbitals can mix
together characters from two or more atomic orbital components.

It is difficult tomake generalizations about the angular distributions from detaching electrons in43molecular orbitals. The exception to this is that a very positive β (>1) may still beconsidered the signature of detaching from a σ-like orbital.
Thus measuring the asymmetry parameter can lend insight into the symmetryof the detached electron.

This is particularly useful for identifying different

electronic states. Frequently, transitions to the ground and excited states of theneutral require detaching different types of electrons. In these cases, peaks belonging
to the different electronic states will exhibit contrasting angular distributions and arereadily identified as such.
In this experiment, it is the large contrasts in β which provide us with clues forspectrum analysis. Thus we limit ourselves to only sampling β at two different
polarization angles, and β is calculated through the relationβ=

I 0 − I90
1
2

(2.5)

I 0 + I90

where I0 is the photoelectron intensity of the peak at 0° laser polarization angle andI90 is the intensity at 90° laser polarization.

We also record spectra when thepolarization angle is set to the ‘magic angle’, θ MA , which is defined as

P2 (cosθ MA ) =1(3cos2 θ MA − 1) ≡ 0 .
2

(2.6)

44
This condition is fulfilled for an angle θ MA ≅ 54.7°. At this angle, the photoelectron
angular distribution is independent of laser polarization angle and is proportional to
the total cross section for detachment, σ tot . More importantly, recording a spectrum
at either polarization extremes (0° or 90°) frequently results in electronic states withlittle or no electron detachment (for all electronic transitions without an isotropic
angular distribution). Taking a spectrum at θ MA ensures that some amount of signal
is recorded from all detectable transitions.

2.6

Photoelectron detection

A diagram of the photoelectron kinetic energy analyzer is displayed in Figure
2.8. The diagram is oriented with the ion beam in the plane of the paper and the laser
beam directed into the page. The detached electrons leave the interaction spot in
different directions at different kinetic energies. Only those which travel through an
acceptance cone of 5° half angle directly downward enter the detection apparatus. It
is comprised of an input stack of electrostatic lenses (V1-3 and VHC) leading to ahemispherical electron energy analyzer, then an output stack of lenses (VHC and V4-6)
and finally a position sensitive detector. Lenses V1 and V2 serve to accelerate theelectrons by a factor of roughly 5-10. The voltage on V3 is computer-controlled. It isa function related to the electron kinetic energy being measured at the moment. Thisfunction is determined during the tungsten calibration discussed in the data analysis
section. The V3 voltages are selected to be those which best focus the electrons onto
the center of the hemispherical analyzer opening at a given electron kinetic energy.

45

Position
Sensitive
Detector

Channel Electron
Multiplier

Deceleration Lens Stack
Ions

Faraday
Cup

VIR

V6

•

V1

V5

V2

V4

Magnification

5-10×

Zoom
Lens

V3

6×

VHC

Hemispherical
Energy Analyzer

Photoelectron Kinetic Energy Analyzer

Figure 2.8. Schematic of the electron kinetic energy analyzer. The laser beam is
directed into the page. The desired kinetic energies are selected using ahemispherical analyzer together with acceleration and positioning voltages (denoted“V”) and the photoelectrons are measured on a position sensitive detector.

46
VHC is the Herzog corrector voltage that is employed to compensate for fringe fields
surrounding the entrance and exit to the charged hemispherical analyzer.
The hemispherical analyzer serves to select a single electron kinetic energy. It
is composed of two hemispheres of OFHC copper, each with a constant voltage
applied to it. The two hemisphere voltages define a central transmission energy, TE,such that only electrons with kinetic energy equal to TE (plus or minus some small
spread in energies) will successfully travel through the analyzer. Electrons withhigher or lower energies will collide with the walls of the hemisphere.

Without

acceleration or deceleration of electrons into the analyzer, only one electron kinetic
energy, TE, will be measured. To pass an electron of a different kinetic energy, thatelectron can be accelerated (or decelerated) to the transmission energy and passed
through and recorded. Thus in order to scan through a range of electron kinetic
energies, the input lens acceleration voltages step through values to scan through thedesired spectrum energy limits. A more detailed description of hemispherical energy
analyzers may be found in reference (5).
The kinetic energy-selected electrons pass through the analyzer with about 2
eV transmission energy. They must be accelerated to the detector for maximum
detection efficiency at around 300 eV. VHC and V4 accelerate the electrons by afactor of 6 with no magnification. The final voltages (V4,V5,V6) serve as a zoom
lens, accelerating the electrons to 300 eV and magnifying their image by a factor of 5
to maximize use of the face of the detector. The typical energy resolution of thisapparatus is 5-10 meV.

47
Finally, a channel electron multiplier (CEM) electron detector is positioned
above the interaction spot. This is a Amptektron MD-502 ceramic CEM housed in acylinder 3.81 cm in length and 1.905 cm diameter, with a 3.6 mm acceptanceaperture. The CEM signal is passed out of the vacuum chamber, into amplifierelectronics, and then directly to a digital counter for diagnostic purposes.

2.7

Position sensitive detector

During the summer of 2000, the position sensitive detector was upgraded to ahigher gain device with five microchannel plates. This detector replaced the previous
one with only two microchannel plates (6).

The new detector is a Quantar

Technology 3318A microchannel plate/resistive anode encoder (MCP/RAE) sensor
including 10-bit digitized output (2412B).

This hardware is installed inside theinteraction chamber at ultra high vacuum. Outside the chamber it interfaces with a2401 Preamp module, a 2401B Position Analyzer, and a 2412B Histogramming
Memory Buffer (see Figure 2.9).
The MCP/RAE sensor is comprised of five wafer-type microchannel plates(MCPs) followed by a resistive anode. A MCP is an approximately 1mm thick platecomprised of a honeycomb of electron multiplying channels. In model 3318A, theMCPs have an 18 mm diameter active area. Since each channel is about 10 µm in
diameter, this gives approximately 106 channels per plate. Each channel operates
along the same principles as an electron multiplier (5). The channels are lined with aseries of electrodes in alternating positions on either side of the channel. The

48

VacuumInterface

5 Microchannel
PlatesHV Power Supply
3400V

Voltage
Divider

Oscilloscope
Resistive B
AnodeA

VacuumInterlock

Host ComputerC
D

Preamp
A

A

A

B
C
D

B
C
D

B
C
D

X Y

Histogramming
Memoryshielding

DIO 6533

Position Analyzer

Position sensitive detector and supporting electronics

Figure 2.9. Diagram of the primary elements involved in the photoelectron position
sensitive detector and associated electronics.

49
electrons multiply in a cascading effect: an incident electron hits an electrode in thechannel, secondary electrons are produced and collide with the electrode on the
opposite side, etc., until they exit the MCP.

The incident electrons must hit the first

MCP with enough energy to initiate the cascade process, thus the need forphotoelectron acceleration in the output lens stack to 300 eV. Each of the MCPs isbiased at successively higher voltages so as to propagate the avalanche of electrons
through the MCPs. As the electron avalanche proceeds through 5 MCPs the cloud
expands and overflows into neighboring microchannels due to space-charge
repulsion. However, the channels are small enough such that the final electron cloud
produced is resolved as a single localized event when it hits the resistive anode. A
Bertan 205B-05R high voltage power supply operating at 3400 V connects to a
voltage divider network that supplies the five leads into the chamber. Multiplication
through 5 MCPs provides 5 x 107 gain in signal, up a factor of 10 from the previous
detector with only 2 MCPs.
Once the electrons exit the MCPs, they collide with a resistive anode (seeFigure 2.9). This is a metal plate in a “warped” rectangular shape with a circulareffective detection area. Electronic leads are connected to each of the four corners(A, B, C, and D). When the electron cloud hits the anode in a specific position, acurrent is measured at each of the leads that is proportional to the center of gravity ofthe electron charge pulse. After signal processing in the position analyzer, the X andY outputs are calculated as

X=

B+C
A+ B
,Y=
A+ B +C + D
A+ B +C + D

(2.7)

50

0.635″

Ceramic Baseplate

Electronic
Leads

1.50″

Resistive AnodeMicrochannel
PlatesFront Mounting
Ring

MCP/RAE assembly, side view
Figure 2.10. Multichannel plate/resistive anode encoder assembly, which comprises
the position sensitive detector. The detector consists of five multichannel plates
placed directly before a resistive anode encoder. The current incident on the plates is
amplified and the position information regarding where the emerging electron cloud
collides with the resistive anode is passed to signal processing electronics.

51
The detector yields a FWHM for each resolvable pulse as 1/400 of the sensor
diameter. Furthermore, the output is given in a 10-bit digitized form in both X and Y
axes.
The MCP/RAE components are held in place on a spring-loaded assemblythat is screwed into a 1.5″ diameter ceramic baseplate containing all electrical leads(see Figure 2.10).

The 001/SE extra electrically-isolated front ring allows for

mounting the assembly from the front as opposed to the rear baseplate. This housing
is placed at the very end of the output lens stack such that the first MCP is flush withthe end of the output lens stack. The sensor leads are capacitively decoupled andshielded, which is necessary to prevent arcing due to the high voltages applied. The
four leads exit the chamber and enter a preamp that amplifies the signals and converts
the low-level pulses to high-level shaped bipolar pulses that can be digested by the
position analyzer.
The position analyzer accepts the output from the preamp module andconverts this into both analog and digital forms of position information as well asother signal event information described below.

The position analyzer outputs

analog X and Y position. These signals connect directly into an oscilloscope for realtime monitoring of electron flux. Dials on the front panel of the position analyzer
allow one to mask out a section of the detector. This is useful to avoid processing“hot spots” into the data, or spots on the detector that produce false events. Hot spots
are created when the plates are permanently damaged by arcing to a localized positionon the detector. Given the high voltages at which the microchannel plates are biased,

52
the MCP power supply is interlocked to cut the power in the event that the pressure
goes above a certain level if, say, an ion pump shuts off.
The analyzer also outputs various digital pulses related to an electron
detection event. When no histogramming memory is installed in the analyzer, these
outputs may be connected directly to a computer for the purposes of handshaking anddata transfer.

The X and Y positions are output in digital format on a 50-pin

Amphenol ribbon-type connector on the rear of the position analyzer box. Pins 1-10
output the 10 bits of Y axis position information while pins 11-20 output the available
10 bits of X axis position information.

Pin 21 of the connector outputs a digital

STROBE pulse while pin 22 outputs digital RATE information. The digital STROBE
pulse is a TTL high-to-low edge produced upon the completion of analog to digitalconversion of a single detection event. This pulse communicates with an external
computer to signal that position information is available to be read (“read-me”
signal). The digital RATE pulse is a TTL low-to-high edge produced for every event
incoming to the analyzer box with the only criterion being that the event is timeseparated from a previous event by 400 nsec. Thus some of the events counted as a
RATE pulse may be rejected by the analyzer electronics for not meeting various
digital pulse criteria. While a STROBE pulse indicates an event that has met thesecriteria and thus has been fully processed by the analyzer electronics, the RATE pulse
indicates an incoming event to the analyzer. A comparison of the two pulses candetermine the ratio of input to output events from the analyzer. The RATE signal
may be observed on the front panel meter.

53
Incorporated inside the position analyzer box is a 2412A Histogramming
Buffer Memory card.

This serves as an alternative method of interfacing theaccumulated data with a computer.

Instead of transferring every detected event

directly to the computer as it is processed in the analyzer, the events are accumulated
within the data buffer and then transferred all at once to the computer.

Thissignificantly decreases and simplifies the amount of handshaking and interaction withthe host computer, freeing it for other activities.
The output of the position analyzer is read directly into the histogramming
memory (HM). Once the HM receives a strobe pulse from the analyzer, it reads the
10-bit X axis information and the 10-bit Y axis information. In other words, eachsingle processed event correlates to a 20-bit word of position information. 10 bits of
X axis information means that the X axis is interpreted as having 210 = 1024 divisions
to it. The 10 bits are binary, read from right to left as most significant bit (MSB) toleast significant bit (LSB). This corresponds to position information in the following
way. The first bit, the MSB, divides the axis in two equal sections. A ‘1’ value for
the MSB corresponds to residing in the right half of that division, say, whereas a ‘0’value corresponds to a location in the left half. The next most significant bit divides
the total axis into four equal sections, thus dividing the right half into two. A ‘1’ forthis second bit indicates a location in the right half of that division, i.e. the right-mostquarter. This is repeated into finer and finer divisions for the full 10 bits.

For

example, 3 bits divides an axis into 23 =8 divisions. The word 001 indicates a
position anywhere within the 5th division from the left.
resolution requires more bits of information.

Obviously, better position

54
While the detector and position analyzer divide the X and Y axes into 10-bitsof position information each, the 2412B Histogramming Memory can only process 12
bits total. The user has discretion to appropriate those bits in any manner deemed
desirable (i.e., 12 bits in X and 0 bits in Y, or 4 bits in X and 8 bits in Y). In
photoelectron spectroscopy, the X axis is the energy dispersion axis and the Y axis issimply a spatial representation of where the electron hits the detector. In the end, we
will sum over all y positions to obtain the number of events versus x position (related
to electron kinetic energy). Therefore, much more resolution is necessary in the X
axis than the Y axis and so 8 bits are assigned to the X axis and the remaining 4 areassigned to the Y axis. This is an improvement over the 7-bit resolution in the energy
axis available from the previous detector.
The HM divides the effective detector area into a grid of 28 = 256 divisions in
X and 24=16 divisions in Y, or 212=4096 boxes total. As each event arrives from theposition analyzer, the HM increments a tally of the number of hits for the appropriate
square of the grid. At the end of the accumulation time interval designated by thehost computer, the HM outputs the integrated number of hits for each grid division.
This occurs as a succession of 212=4096 digital numbers (one for each grid element)of the HM digital output: each read is a digital number corresponding to the total
number of hits recorded within a grid division during the time interval.
The HM buffer space is divided into two “pages”. The HM switches back andforth between the two for writing and reading data. For example, the HM will write
data to page 1, transfer the data to page 2 and read from page two, write to page 1,

55
etc. To avoid writing over data, every read is destructive. In other words, reading thedata out to the computer simultaneously erases the buffer page.
When taking a spectrum, timing is crucial.

All the timing functions arecontrolled from a LabVIEW 6i program entitled PESDAT running on a MS Windows
NT platform on a Gateway 2000.
Data is sampled over the desired spectrum energy range in small overlapping
divisions, called segments. For example, a single spectrum may be divided into 100
segments. A segment is defined by its center electron kinetic energy (eKE). Oncethe center eKEs are calculated by PESDAT for each segment, the necessary V3
voltages are calculated from the optimized V3 voltages determined from the tungsten
calibration. PESDAT then sends a timer value, the time interval over which toaccumulate data for each segment. This is set to 0.5 seconds. When the data taking
begins, PESDAT sends the eKE and V3 voltage for the first segment, which isprocessed by a voltage controller box to determine the necessary voltages for the
input lens stack, and these voltages are applied. Then PESDAT sends a signal to the
HM to start data accumulation. When the 0.5 seconds are completed as determinedby a timer within the HM, the data is read. Then a new center eKE and V3 value issent and a start accumulation is sent again for the next segment. This is repeated foreach of the number of segments per scan, the data is unpacked, processed, anddisplayed in PESDAT, and the data accumulation restarts at the first segment.
The model 2412B HM interfaces with a host computer hardware interface that
follows HP 98622A GPIO protocol. In this protocol, there is a 16-bit Data In line, 16
bits of Data Out, and I/O, PCTL, and PFLG data transfer control lines. Each of these56
digital lines travels over a 50-wire ribbon connector between the HM (in the Position
Analyzer box) and the host computer hardware interface. 12 of the 16 bits of theData In line are used to transfer HM data from the HM to the computer. 16-bit
command words are sent from the computer to the HM via the Data Out line. The I/Oline is either TTL high or low depending on whether the commands are coming fromthe host computer to the HM on the Data Out line (high) or if data is coming from the
HM to the computer (low). The PCTL line indicates the moment when data transfer
in either direction may begin. The computer initiates a transfer by sending a TTL
high-to-low edge on the PCTL line. The line stays low until the HM acknowledges
the command by sending back a high-to-low edge on the PFLG line. At this point thecomputer resets the PCTL back to a high level.

Thus the PFLG line is used as

ready/busy line where if the line is low the HM is busy and not able to accept acommand.
In order to set the HM timer, the computer sends a 16-bit word to the HM on
the Data Out line. This word will have a 0 in the 15th bit (LSB) and specify the timeinterval in milliseconds using the remaining 15 bits (0-14).

To start the data

accumulation and signal the beginning of the preset time interval, a start
accumulation command is sent. This is a 16-bit word with both LSB and MSB set to1.

When a start accumulation command is sent, the HM buffer pages areautomatically switched.
Thus data are accumulated and transferred to the computer during a scan. The
standard PC hardware is not set up for the digital data transfer and handshaking
necessary for this operation.

Therefore, the HM interfaces with a National

57
Instruments DIO 6533 (PCI-DIO-32HS) DAQ card. This is a 32-bit parallel digital
I/O interface for use with a PC computer for digital input and output, patterngeneration, and handshaking. It connects to the computer via a standard ribbon cable.
The card translates all the commands and information sent from the computer to HM
and reverse.

The DIO 6533 device is directly compatible with the LabVIEW

software program.

2.8

Data processing

In addition to initiating data transfer and dialogue with the HM and electron
detector devices, the PESDAT program also provides a user interface for dataacquisition, display, and writing and reading data from files.

Unprocessed data isstored in text files with a .SEG extension. In this raw format, the first line of the file isthe center eKE at which the data was taken, and the following 256 lines have a single
integer each representing the number of accumulated counts at that energy channel
position in the detector. Line number 256+2=258 is the center eKE for the second
segment followed by 256 lines of integers, etc., all the way through the full number ofsegments in the scan.
For analysis, the data have to be converted into an absolute energy-calibrated
form of eKE vs. counts across the spectrum limits. To begin with, it is necessary toknow by how much the segments overlap so they can be combined. The energy
spacing between the centers of each segment is known by taking the difference incenter eKEs of adjacent segments. What is then needed is the energy width that each

58
detector channel represents. This is calculated as a quantity called the magnification,or meV/channel.

Once the magnification is determined, the segments may beoverlapped correctly and combined. Then the absolute energy scale must be fixed.
Both these tasks are accomplished by taking a calibration photoelectron spectrum ona daily basis. Usually the calibration anion is O− for several reasons. It is an atomand therefore has no vibrational or rotational structure and may be accurately fit to agaussian curve; it is relatively easy to make and detach; and its EA is known to muchbetter accuracy than can be resolved on our instrument. The O− scan determines themagnification, and it measures the position of the EA peak, which then can becompared to the true position to calculate the necessary energy scale shift.
A second calibration is performed every time a new argon laser tube is
installed. This will slightly change the laser alignment and thus the exact spot ofintersection between the laser and ion beams, affecting the angle at which thephotoelectrons enter the kinetic energy analyzer. This calibration is done using thephotoelectron spectrum of tungsten anion, W−. The PES of W− contains 12 energy
accessible transitions spread over a majority of the 3.4 eV available eKE range of the
instrument. The W− calibration has two purposes. The first is that for each of thesepeaks, photoelectron intensity is recorded over a range of V3 voltages and an optimal
voltage is chosen. A table of optimal V3 voltage versus eKE is created from which
optimal V3 voltages for any center eKE can be interpolated. The second purpose isfor an energy compression measurement. The energy scale becomes compressed bya small amount (about 0.5%) at lower eKEs. This may be measured by recording the

59absolute position of all the W− peaks and then comparing to the true line positions astabulated by Charlotte Moore (7). This line determines the compression factor γ.
The last correction to account for is a conversion of energy from the lab frameto the center of mass of the anion frame. Because the reaction AB − + hν → AB + e −happens in the center of mass (CM) frame of the anions AB − , and the photoelectron
kinetic energies (eKE) are measured in the laboratory frame, the kinetic energies must
be converted into the CM frame. This is explained in detail elsewhere (1). In brief,
the final electron kinetic energy is calculated by the following relation 1
1  
eKE = hν −  EAref + γ (Ω ref − Ωlab ) + Wme −  ,
 M ref M 

 


(2.8)

where EAref is the electron affinity of the reference ion (usually O−), γ is the energy
compression factor, Ωref is the measured eKE of the reference ion, Ωlab is themeasured eKE of the molecule of interest in the lab frame, W is the ion beam energy(38 eV), me is the mass of the electron, Mref is the mass of the reference ion, and M is
the mass of the molecule of interest. The true eKE is calculated with respect to theeKE of the reference ion multiplied by the compression factor (the second term in thebrackets above); this is then corrected into the center of mass frame of the moleculesby the third term in the brackets above.

60

Bibliography

1.

J. Ho, Doctoral dissertation, University of Colorado, 1991.

2.

J. T. Verdeyen, "Laser Electronics", 3rd. ed. Prentice Hall, Englewood Cliffs,
NJ, 1995.

3.

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

4.

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

5.

J. H. Moore, C. C. Davis, and M. A. Coplan, "Building Scientific Apparatus",
2nd ed. Addison-Wesley, Redwood City, CA, 1989.

6.

C. S. Feigerle, Doctoral dissertation, University of Colorado, 1983.

7.

C. E. Moore, "Atomic Energy Levels." Nat. Bur. Stand., U. S. Government
Printing Office, Washington DC, 1958.

61
Chapter 3
Photoelectron spectroscopy of alkyl peroxides ROO− (R = H, D, CH3, CD3, andCH3CH2)

3.1

Introduction
Alkyl peroxyl radicals (ROO) are generally the first oxidation products ofsaturated hydrocarbons in the troposphere (1-3). Atmospheric hydrocarbons reactwith strong oxidizers such as OH, O3, or NO3. The OH radical reacts with saturatedhydrocarbons by H atom abstraction (2). The nascent alkyl radical, R, combines
rapidly with atmospheric O2 in a three-body reaction to produce the corresponding
alkyl peroxyl radical (1).
RH + OH → R + H2O(3.1)

R + O2 + M → ROO + M(3.2)

Atmospheric peroxyl radicals react quickly with NO to generate NO2, which is
photodissociated by sunlight to produce NO and O (3P). The latter is primarily
responsible for the formation of tropospheric ozone (reactions 3.3-3.5) (4,5). Given
that NO and hydrocarbons are both by-products of combustion, reactions 3.1-3.5 forma crucial part of the chemistry responsible for the formation of photochemical smog
in industrialized areas.
ROO + NO → RO + NO2(3.3)

NO2 + sunlight → NO + O (3P)

(3.4)

62
O2 + O (3P) + M → O3 + M(3.5)

Alkyl peroxides are also known to play an important role in the chemistry of
combustion (6). Under some conditions, particularly at low combustion temperatures(< 700 K) hydrocarbon fuels can undergo a series of facile radical reactions involving
the intermediacy of alkyl peroxyl radicals. In an internal combustion engine suchchemistry may result in accelerated ignition and indeed premature ignition. Thisphenomenon is referred to as auto-ignition or “engine knock” and has practicalramifications for engine wear and efficiency (7).
This chapter describes the measurement of the negative ion photoelectron_

spectra of ROO anions (R=H, D, CH3, CD3, and CH3CH2).
__

ROO + !ω364nm → ROO + e (KE)

(3.6)

Analysis of the kinetic energy of the scattered photoelectrons (KE) enables
determination of the electron affinity (EA) of the corresponding peroxyl radical,
EA(ROO) as well as molecular vibrational frequencies and anharmonicities andangular distributions.
There have been numerous studies of electronic properties of alkyl peroxyl
radicals (1,3,8,9). We can use generalized valence bond (GVB) diagrams (9-11) in_

Figure 3.1 to represent the peroxide anions, ROO , the peroxyl radicals, ROO, and theperoxyl cations, ROO+.

63
••
•• ••

R

••

••
•• •
••
_

R

••
• ••

••

••

R

••

••
•• ••
••

ROO

ROO

ROO*

1
~
X A′ ≅ -1 eV

2~
X A″ ≅ 0 eV

2~
A A′ ≅ 1 eV

R

•

••
• •
••

ROO*
2
~
B A″ ≅ 5 eV

R

••

••

ROO+
3
~
X A″ ≅ 11 eV_

Figure 3.1. Generalized valence bond (GVB) diagrams of peroxide anions, ROO ,the peroxyl radicals, ROO, and the peroxyl cation, ROO+.
2
The ground electronic state of alkyl peroxyl radicals is ROO ~X A″. These radicals
2
display two characteristic electronic excited states: a low-lying ROO ~A A′ state is2
present at about 1 eV and there is a high-lying, dissociative state ROO ~B A″ at a term
_ ~value of roughly 5 eV. The binding energy (9) of the alkyl peroxide anions, ROO X
1A′, is about 1 eV; ionization energies (12) of peroxyl radicals are high (IP(ROO) ≅11 eV).
The photoelectron spectra of ROO− molecules are in general the moststraightforward presented in this thesis. This is true for several reasons.

The basic

physical picture is fairly intuitive as it is well-represented by the GVB diagrams
presented in Fig. 3.1, which depict the highest occupied molecular orbital (HOMO)_

and HOMO-1 of ROO to be localized on the terminal oxygen. Detachment ofelectrons from these orbitals is expected to excite RO-O stretching modes in forming
both the ground state and first excited states of the neutral, as will be shown below.
Furthermore, the spectra of the triatomic molecules HOO− and DOO− may beconsidered prototypical photoelectron spectra.

In this case, the hydrogen atom

64
provides a minimum amount of interaction with an OO− diatomic, and thephotophysics involves primarily the two oxygens.

Furthermore, many of thephotoelectron spectrum features of HOO− are quite similar to those of OO− (13,14), as
will be seen below. The spectra exhibit typical Franck-Condon behavior. All thesereasons yield a straightforward spectrum with an uncomplicated analysis.

It istherefore important to be able to understand the spectra of HOO− and DOO− for the
purpose of comparison to the ROO− molecules with larger alkyl groups (CH3,
CH3CH2…) where the interaction between R and OO would be expected to increase.

3.2

Experiment

DOO− anions were created in a microwave discharge source by introducing
the vapor above a mixture of HOOH and D2SO4 in D2O. Enough HOOH remained in
the solution to enable the generation of HOO− anions using the same sample. This
simple procedure was not adequate to create the larger ROO− anions (R ≠ H, D).
Instead, these were created through the reaction of the parent hydroperoxide, ROOH,
with hydroxide ions as described in the previous chapter.
Electronic structure calculations were carried out using the GAUSSIAN98
program package (15) to aid in interpretation of the photoelectron spectra. The Becke
3LYP hybrid density functional method (16) was employed in conjunction with acorrelation consistent Dunning basis set of double ζ quality (aug-cc-pVDZ) (17) to
calculate optimized molecular structures and harmonic frequencies for the anion and65
the ground and first excited states of the neutral. The results of these calculations arecompared to the measured values in the discussion below.

3.3

HOO− and DOO−

The magic angle photoelectron spectrum of HOO− taken at 200 K sample
temperature is shown in Figure 3.2. Comparison between 300 K (not shown) and 200
K spectra confirms that there are no hot bands in this spectrum. The peak marked
with a star does not originate from HOO−, but rather is contamination from OO−
resulting from incomplete rejection of m/z = 32.
The spectrum clearly shows two different Franck-Condon envelopes
suggesting transitions into two different electronic states of the neutral molecule.
Peaks a1 through a7 have an interpeak spacing of about 1100 cm-1, peaks b1 throughb7 have an approximate 900 cm-1 separation, while the spacing between peaks a7 and~b1 is about 740 cm-1. Because of this, peak b1 is determined to be the 000 of the X~
HOO− to A HOO transition at 1.951 ± 0.004 eV. It is not immediately obvious
which peak should be assigned the electron affinity peak. Because the FranckCondon profile has a shape that is symmetric about peak a4, the EA may have too
small an intensity to detect, falling to the lower binding energy side of peak a1.
~
However, the term energy of the A state is known to great precision (7029 cm-1)(18), which is the spacing between peaks a1 and b1 (7025 cm-1). Therefore, theelectron affinity may be assigned with confidence as peak a1, yielding EA(HOO) =

66

-

T0

b2

HOO
Photoelectron Counts

b3

a4
a5

a3

b4
b1
a6

EA
a2

b5
b7
2.6

a7

b6

2.4

2.2

2.0

a1 *
1.8

1.6

1.4

1.2

1.0

Electron Binding Energy (eV)_Figure 3.2. 364 nm photoelectron spectrum of HOO taken at 200 K and magic angle
laser polarization.

67
Table 3.1. Data extracted from the 364 nm photoelectron spectra (Figures 3.2-3.3) of__

HOO and DOO . The data treatments utilized to extract these values and theirassociated uncertainties (given in parentheses) are discussed in the text.
EA (eV) T0 (eV)
xO-O
νO-O
ωO-O
β

2
HOO ~
X A″

1.078(6)

2
HOO ~
A A′
2
DOO ~
X A″
2
DOO ~
A A′

0.872(7)1.077(5)0.874(7)(cm-1)(cm-1)(cm-1)-0.7(1)1125(6)11(1)1102(10)-0.5(1)921(4)

5(1)910(7)

1138(4)13(1)1112(7)955(6)

13(2)929(10)~ HOO - A
~ HOO) = 0.872 ± 0.007 eV (see1.078 ± 0.006 eV and a term energy of T0(X
Table 3.1). This EA agrees precisely with the photoelectron spectroscopy value given
in Oakes et al. of 1.078 ± 0.017 eV (19), although it significantly improves the error
bars.

The photoelectron spectrum of HOO− has been reported recently by Clifford

et al. with an EA of 1.089 ± 0.006 eV (9) although an addition error was made in therotational correction to the raw EA value. The value reported here is the most current
and most accurate measurement available.
Peaks a1 through a7 represent transitions into six quanta of a single~
vibrational mode in the X state of HOO which will be labeled mode a. Likewise,
peaks b2 through b7 represent a single progression of six quanta of vibrational~
excitation in the A state of a mode labeled b. Because of the number of wellresolved peaks in this spectrum, the measured energy spacings between all thesepeaks with respect to the appropriate 000 peak (a2-a1, a3-a1, a4-a1, etc.), (G(v) –68
G(0)) where v = 1-6, of modes a and b were fitted to Eq. 3.7 (20) using a weighted
least squares approach:G(v) – G(0) = ω(v + 1/2) – x(v + 1/2)2 – 1/2ω + 1/4x.

(3.7)

Fitting the data to Eq. 3.7 gives the harmonic frequency of mode a, ωa = 1125 ± 6
cm-1 and an anharmonicity term, xa = 11 ± 1 cm-1. Using these parameters we canextract an accurate value for the 1 ← 0 vibrational transition of mode a, νa = 1102 ±10 cm-1. Doing the same for mode b yields ωb = 921 ± 4 cm-1, xb = 5 ± 1 cm-1, and νb= 910 ± 7 cm-1.
The 90° laser polarization angle spectrum of DOO− taken at 200 K sample
temperature is shown in Figure 3.3. Inspection of the spectrum shows no obvious
difference from that of HOO−. The same analysis discussed above is applied to
DOO−. The electron affinity and term energy are EA(DOO) = 1.077 ± 0.005 eV and~T0(~
X DOO - A DOO) = 0.874 ± 0.007 eV (see Table 3.1), respectively, which are thesame, within error, as for HOO. This is what would be expected since deuteration of
the molecule does not affect its electronic structure. However, it does affect its massand thus some vibrational frequencies.

If a normal mode includes motion of ahydrogen atom, its frequency would decrease upon substitution of H for D. Again,using Eq. 3.7 for DOO, the frequency measurements for mode a are ωa = 1138 ± 4
cm-1, xa = 13 ± 1 cm-1, and νa = 1112 ± 7 cm-1 and for mode b are ωb = 955 ± 6 cm-1,
xb = 13 ± 2 cm-1, and νb = 929 ± 10 cm-1. The frequencies and anharmonicities forboth states in HOO and DOO are reported in Table 3.1. What is important to note is

69

DOO-

T0

b2

a4 a3

Photoelectron Counts

b3
b1

a5

2.4

a2

a7

b5

2.6

EA

a6

b4

a1

2.2

2.0

1.8

1.6

1.4

1.2

1.0

Electron Binding Energy (eV)_Figure 3.3. 364 nm photoelectron spectrum of DOO taken at 200 K and magic angle
laser polarization.

70
that the normal mode fundamentals in both electronic states do not change
significantly upon deuteration, as would be expected from the representation given in
Fig. 3.1. Thus deuterating the HOO molecule and comparing the spectra of the two
isotopomers allows confirmation that the vibrational activity in the photoelectron
spectra of HOO− and DOO− does not involve hydrogen motion. The only possible
vibrational mode candidate then is ν 3 , the O-O stretching mode. There is little
evidence from the photoelectron spectra of any significant mixing of the O-O stretch
and the HOO bend, as has been previously suggested (21).
~The value of ν 3 (1102 ± 10 cm-1) for X HOO agrees nicely with the IR
spectroscopic value of 1097.6 cm-1 (22), in addition to other more longstanding
values (21,23,24). It also agrees well with the fundamental frequency found for OO−~(25), 1073 ± 50 cm-1. Furthermore, the ν 3 value for the A state (910 ± 7 cm-1) is inagreement with prior findings of Holstein et al. (926.5 ± 2.0 cm-1) (26). The 929 ± 10~
cm-1 value of ν 3 in the A state of DOO matches the 930 cm-1 frequency reported in
the photoelectron spectroscopy study of Oakes et al. (19). However, theirmeasurement of the DOO− ν 3 does not agree with the value of the ground state ν 3
mode of DOO− presented here. It does agree with McKellar (1120.22 (13) cm-1) (27)and Smith and Andrews (1123.2 cm-1) (23), and it is likely to be a more reliablemeasurement than the earlier measurement of Oakes et al. (19) due to a cooler ion
source and improved resolution.
An important feature to notice about the spectra in Figures 3.2 and 3.3 is how
extended the vibrational progressions are in both ground and excited states of the

71
neutral molecule. This is a direct indication of a relatively large change in moleculargeometry incurred upon detaching an anion electron. The amount that the moleculargeometry changes is a direct consequence of the degree to which the detachedelectrons participate in bonding with other atoms in the molecule. If the detachedelectron came from a non-bonding orbital, for example, no change in geometry would
occur and therefore no vibrational activity would be expected. In the case of HOO,
the ab initio electronic structure calculations tabulated in Table 3.2 show an O-O_
2~2
Table 3.2. Molecular geometries for 1A′ HOO and ~X A″ and A A′ states of theneutral HOO radical, resulting from a B3LYP/aug-cc-pVDZ DFT calculation.
X" 1A′ HOO−
X" 2A″ HOO
A" 2A′ HOO

rOH (Å)

0.965

0.980

0.976

rOO (Å)

1.513

1.328

1.389

∠HOO

99.3°

105.4°

103.6°

bond length shortening of 0.185 Å between the anion ground state and the neutral
ground state and an increase in the HOO bond angle by 6º. This is a relatively
substantial change. However, in the case of the O-H bond, the change is only 0.01 Å.
This supports the observation that in the spectrum of HOO− there are six quanta ofvibrational excitation in the ground state assigned to pure oxygen stretching alongwith the finding of no hydrogen participation in the vibrational activity in the
spectrum.

All the evidence points to the fact that the detached electron does not

significantly participate in bonding with the hydrogen, while it plays a large role in72
the O-O bond. The change in geometry is somewhat less pronounced in the A" ← X"
transition than the X" ← X" transition (∆rO-O = 0.124 Å versus 0.185 Å and ∆∠HOO ofapproximately 4º versus 6º). This agrees with the observed structure in that theX" state has a slightly more extended profile than the A" state.
As was concluded above, the photoelectron detachment is a process fairly−

localized to an orbital on the terminal oxygen. In the case of OO , the orbital would
be a pure π orbital. Detachment of this electron produces an angular distribution of−

approximately –1 (14). The value for peaks a1 through a7 in the HOO spectrum is– 0.7 ± 0.1. This supports the idea that the HOMO orbital in the anion is not severely
perturbed by the hydrogen presence. The average value for the peaks involved in the~transition into the A state of HOO is slightly lower, – 0.5 ± 0.1. It is the HOMO-1
a' orbital which is primarily involved in the bonding with the hydrogen and therefore
retains σ character which is absent from the HOMO a" orbital. It is this addition of σ~~character to the primarily π character found in the X ← X peaks which lowers the~~average β value from – 0.7 to – 0.5 in the X ← A peaks.
In summary, because of the observation of the O-O stretch, lack ofobservation of hydrogen activity, and measurement of approximately the same β−

value, we conclude that electron detachment from HOO is similar to detachment−from OO .

−This supports the assertion that, in HOO , the hydrogen atom may be−considered a small perturbation to the OO system.

73
3.4

_

CH3OO

_

The 200 K magic angle photoelectron spectrum of CH3OO is presented in_

Figure 3.4 and the critical data extracted from this spectrum (and those of CD3OO_

and CH3CH2OO ) are listed in Table 3.3. Like the photoelectron spectra of HOO_

Table 3.3. Data extracted from the 364 nm photoelectron spectra (Figures 3.4-3.6) of___

CH3OO , CD3OO , and CH3CH2OO . The data treatments utilized to extract thesevalues and their associated uncertainties (given in parentheses) are discussed in the
text. The fundamental frequencies (ν) are listed without mode assignments. These
are also discussed in the text.
EA (eV)~
CH3OO X 2A″

1.161(5)~CH3OO A 2A′~
CD3OO X 2A″~
CH3CH2OO A 2A′

β-0.7(2)0.914(5)ω (cm-1)

1147(3)xνν(cm-1)(cm-1)(cm-1)

12(1)1124(5)482(9)

1005(10)910(10)1123(7)440(7)

975(10)840(10)1089(16)234(9)

900(15)

178(10)-0.2(2)

1.154(4)~CD3OO A 2A′~
CH3CH2OO X 2A″

T0 (eV)1142(5)9(1)0.913(4)1.186(4)-0.2(2)0.938(4)-0.2(2)1096(10)4(2)and DOO− discussed above, there are two peak profiles present in the spectrum,indicative of transitions to two electronic states of the peroxyl radical. The first bandat lower electron binding energy represents a transition to the ground state of the2
neutral radical, CH3OO ~X A", while the second band at higher eBE represents a

74

Photoelectron Counts

CH3OO- c1

T0

c2
c3

a4

a3

a5
c6

c5
c4

a2

a6
b4
b5
a7
b6

2.6

2.4

2.2

2.0

EA
b3

a1

b2
b1

1.8

1.6

1.4

1.2

1.0

Electron binding energy (eV)Photoelectron Counts

CH3OO-

c1
c2
c3
c5
c4
c6

2.5

2.4

2.3

2.2

2.1

2.0

Electron binding energy (eV)_Figure 3.4. 364 nm photoelectron spectrum of CH3OO taken at 200 K and magic~
angle laser polarization. (a) Bottom panel, an expansion of the ~A ←X transition
spectral profile.

75
2
transition to the first excited state of the neutral radical, CH3OO ~A A′. Peak c1_
0~(Figure 3.4) can be unambiguously assigned as the 00 of the ~A CH3OO ← X CH3OO

transition. If feature a1 is assigned as the electron affinity, this yields an EA(CH3OO)~= 1.161 ± 0.005 eV and a term energy T0(~
X CH3OO - A CH3OO) of 0.914 ± 0.005 eV(or 7370 ± 40 cm-1). The values are slightly larger than for HOO; however to firstorder, both molecules have the same EA and T0. This measurement of the term
energy T0(CH3OO) is in excellent agreement with the recent cavity ring-down
measurement of Pushkarsky et al. (28) of 7382.8 ± 0.5 cm-1 as well as thelongstanding value of Hunziker and Wendt (29) of 7375 ± 6 cm-1.
As was seen in HOO, the extended progression of peaks corresponding to the_~
CH3OO ~X ← CH3OO X state transition indicates a significant geometrical change

between the ground state anion and neutral structures.

By contrast, the second~
Franck-Condon envelope of peaks corresponding to the ~A ← X transition has a0narrower profile, with the 00 (c1) peak being the most intense and subsequent peaks
rapidly decreasing in intensity. This indicates a smaller relative change between the
_
~
~
~
~
X CH OO and A CH OO geometries than that observed in the X CH OO ← X
3

3

3

_

CH3OO transition. This is a somewhat larger contrast than observed in HOO, where
both neutral states exhibited comparable and large changes in geometry. The anion
and neutral structures predicted by both the qualitative GVB expression (Fig. 3.1) andB3LYP/aug-cc-pVDZ ab initio calculations (Table 3.4) support this explanation. In~
the ~X ← X transition the O-O bond length is calculated to change by about 0.16 Å and76
Table 3.4. The molecular geometries and harmonic vibrational frequencies (cm-1) for_1
2~2
A′ CH OO and ~X A″ and A A′ states of the neutral CH OO radicals, calculated at
33

the B3LYP/aug-cc-pVDZ level of theory. Vibrational frequencies given inparentheses represent the values for the d3-isotopomers. All vibrational frequencies
are unscaled.
_
CH3OO
CH3OO
CH OO
3

Molecular Geometries (Å, º)
1
C (~X , A′)2CS (~X , A″)
1.452
1.317
111.2

2
CS (~
A , A′)
1.440
1.385
107.6

2938
(2176)
2874(2057)
1463(1147)
1390(1053)
1188(1012)
1074
(945)
806
(764)
428
(387)

3166
(2353)
3052(2182)
1451(1203)
1414(1070)
1211(1045)
1149
(985)
910
(825)
491
(444)

3157
(2342)
3025(2164)
1475(1087)
1419(1069)
1157(1007)
1015
(950)
917
(836)
370
(343)

2885
(2139)
1394(1015)
1145
(886)
271
(207)

3152
(2342)
1440(1040)
1111
(853)
129
(101)

3104
(2306)
1432(1037)
1150
(884)
247
(192)

S

re(H3C-OO)
1.387
re(H3CO-O)
1.476
106.2
∠(H3C-O-O)
Vibrational Frequencies (cm-1)
a′
modes
ω1
ω2
ω3
ω4
ω5
ω6
ω7
ω8
a″
modes
ω9
ω10
ω11
ω12

77
o

the C-O-O bond angle by 5 . These are the same geometry changes as found in the~
HOO molecule. In contrast, as was also the case in HOO, the ~A ← X transition
calculations predict a smaller change in both the O-O bond length of roughly 0.1 Åoand in the C-O-O bond angle of about 2 .
In the Franck-Condon profile for detachment to the ground state, CH3OO ~X
2A", there are two vibrational progressions. The most prominent progression is0

denoted by peaks a1-a7 in Figure 3.4. The peak a1 is assigned as the 00 transition,
while peaks a2-a7 correspond to a progression of six quanta (vp = 1-6) of an activemode which we label p. The measured energy spacings (Table 3.5) between thepeaks in this progression are fit to Eq. 3.7 as described above. This gives ωp = 1147 ±3 cm-1 and an anharmonicity xp = 12 ± 1 cm-1, which yields νp = 1124 ± 5 cm-1 (seeTable 3.3).
In contrast to the spectrum of HOO−, in addition to the primary progression aless intense although clearly resolved second progression is identified in peaks b1-b6.
These peaks appear to the higher binding energy side of the a peaks and may be
identified as a combination band with this series. We assign b1-b6 as a combinationof the progression due to mode p, discussed above, plus one quantum of a mode wehave labeled q. Taking a weighted average of these energy spacings (Table 3.5) we
obtain the fundamental of mode q to be νq = 482 ± 9 cm-1.
The fundamental of the primary progression, νp = 1124 ± 5 cm-1, correlates
well with other fundamental O-O vibrational transitions in peroxyl radicals: (i) for

78
_

_

Table 3.5. Negative ion photoelectron peak positions for CH3OO and CD3OO ,uncertainties are given in parentheses. Peak positions are given in wavenumbers
0(cm-1) relative to 00 transitions (peaks a1 and c1). Assignments are given for the mostprominent transitions labeled as indicated in Figures 3.4 and 3.5.
For CH3OO the νp = 1124(5) cm-1 (O-O stretch), νq = 482(9) cm-1 (C-O-O bend),
fundamentals are:νs = 910(10) cm-1 (see text), νt = 1005(10) cm-1 (see text)

For CD3OO theνp = 1123(7) cm-1 (O-O stretch), νq = 440(7) cm-1 (C-O-O bend),

fundamentals are:νs = 840(10) cm-1 (see text), νt = 975(10) cm-1 (see text)

2
~
X A″

2
~
A A′

CH3OO

CD3OO

a1

00

0

0

0

b1

q0

1

515(15)470(15)a2p0

11125(15)1130(10)

b2

1 1
0 0

pq

1600(15)1565(10)

a3

p0

2

2225(15)2235(10)

b3

p0q0

2 1

2700(20)2660(10)

a4

p0

3

3290(15)3310(10)

b4

p0q0

3 1

3750(15)3750(10)

a5

p0

4

4365(15)4380(10)

b5

p0q0

4 1

4810(20)4820(15)

a6

p0

5

5390(15)5425(10)

b6

p0q0

5 1

5870(15)5850(20)

a7

p0

6

6390(15)6490(25)

c1

00

0

0

0

c2

s0

1

910(10)840(10)c3t011005(10)975(10)

c4

s

2
0

1790(10)1690(15)

c5

s0t0

1 1

1910(10)1800(10)c6

202040(15)1950(10)t

79
HOO, 1102 ± 10 cm-1, as measured in the photoelectron spectrum discussed above,(ii) the ν3 = 1097.63 cm-1 value measured by Tuckett and co-workers (18,30), (iii) for
matrix isolated CH3OO, ν6 = 1109 cm-1 measured by Nandi et al.(31) and, (iv) for
CH3OO the ab initio harmonic frequency, ω6 = 1149 cm-1, calculated at theB3LYP/aug-cc-pVDZ level of theory (Table 3.4). From both the GVB diagrams(Fig. 3.1) and the ab initio geometries (Table 3.4), one anticipates strong activity in_
the RO-O stretching mode upon detachment of the peroxide anion, ROO , to the ~Xstate of the neutral. Based on these considerations and the good agreement between
fundamental frequencies, we assign mode p to the CH3O-O stretching mode ν6.
Similarly, the fundamental frequency νq = 482 ± 9 cm-1 correlates well with both the
CH3-O-O bending fundamental ν8 = 492 cm-1 measured by Nandi et al.(31) for matrix
isolated CH3OO and the harmonic frequency, ω8 = 491 cm-1, calculated at theB3LYP/aug-cc-pVDZ level of theory which corresponds to a H3C-O-O bending mode
(Table 3.4). Theoretical geometries (Table 3.4) predict activity in the H3C-O-O_
bending mode upon detachment of the peroxide anion, CH3OO , to the ~X state of theneutral radical. These considerations lead us to assign mode q to the H3C-O-O_

bending mode ν8. This assignment is further supported by the spectrum of CD3OO(which will be further discussed in the subsequent section), where both νq and ωq are
shifted to lower frequency.
_~
Examination of the Franck-Condon envelope for the CH3OO ~A ← CH3OO X

transition (see expansion Figure 3.4a) reveals a progression of two active modes of0similar intensity. Peak c1 is assigned to the 00 transition, while the peaks c2 and c4

80
correspond to one and two quanta, respectively, of the mode which we designate s.
The positions of peaks c1 and c2, given in Table 3.5, fix the fundamental of mode s tobe νs = 910 ± 10 cm-1. The second progression in the excited state is represented by
the peaks c3 and c6, which correspond to one and two quanta, respectively, of themode that we have assigned as t. The positions of peaks c1 and c2, given in Table
3.5, fix the fundamental of mode t to be νt = 1005 ± 10 cm-1. The peak c5 in Figure
3.4 represents a combination band of one quantum of s and one quantum of t.
~
The ~A ← X transition peaks show activity in two modes (s and t) of similarintensity and frequency (910 and 1005 cm-1). The fundamental frequency νs = 910 ±10 cm-1 is comparable to the frequencies: (i) the O-O stretch fundamental, 910 ± 7
cm-1, measured in the photoelectron spectrum of HOO discussed above, (ii) ν3 =929.068 cm-1, which is the fundamental of the O-O stretching mode of the ~A state of
HOO measured by Tuckett and co-workers (18,30), and (iii) the vibrational transition
at 896 ± 9 cm-1 measured for the ~A state of CH3OO by Hunziker and Wendt (29). The
B3LYP/aug-cc-pVDZ calculated harmonic mode ω7 = 917 cm-1 (Table 3.4) is similar
in magnitude to νs and corresponds to a “symmetric” C-O-O stretching mode.
However, the magnitude of νt = 1005 cm-1 is not significantly different from that of νs(910 cm-1). The former frequency is close in magnitude to the calculated harmonicfrequency ω6 = 1015 cm-1 which corresponds to a C-O-O “anti-symmetric” stretchingmode. The GVB diagrams (Fig 3.1) and ab initio calculations both predict some~
activity in the CH3O-O stretching vibration for the ~A ← X transition. For this reason
it is not clear which of these two frequencies, if either, could be strictly assigned to an81
O-O stretch. Furthermore, the ab initio calculations suggest that the ~A state vibrationsin CH3OO are unlike the local modes observed for the ground state in that the normal
modes involve a significant amount of mixing of local modes. Examination of the_

CD3OO spectrum should clarify these issues.
_

The asymmetry parameters β for both electronic transitions in CH3OO (as__~well as CD3OO and CH3CH2OO ) are reported in Table 3.3. The ~X ← X transition~
carries an average β of -0.7 ± 0.2 while the ~A ← X transition has a β of -0.2 ± 0.2.
These values are similar to the corresponding β values observed for HOO and thesame conclusions may be made for CH3OO. Both β values are negative, suggesting
detachment from a π-like orbital. The detachment into the ~X state is close to a β of -1,
which (at a electron kinetic energy of roughly 1 eV) implies that the detected
electrons are ejected from an essentially pure π orbital. Detachment into the ~A stateyields a β closer to 0, indicating that the orbital from which the electrons are detachedpossess some of the σ character of the O-C bond.

3.5

CD3OO

_

_

The 200 K magic angle photoelectron spectrum of CD3OO is shown in
Figure 3.5. An initial comparison does not show any great difference from that of

82

Photoelectron Counts

CD3OO- c1

T0

c3
c2

a4

c6 c5
c4

a3

a5

a2 EA

a6
b4 b3 b2
a7 b6 b5

2.6

2.4

2.2

2.0

1.8

1.6

a1
b1

1.4

1.2

1.0

Photoelectron Counts

Electron binding energy (eV)c1

CD3OOc3
c2
c6 c5
c4

2.5

2.4

2.3

2.2

2.1

2.0

Electron binding energy (eV)_Figure 3.5. 364 nm photoelectron spectrum of CD3OO taken at 200 K and magic~
angle laser polarization. (a) Bottom panel, an expansion of the ~A ←X transition

spectral profile.

83
_
2
~2
CH3OO in Figure 3.4. Transitions from the anion to both CD3OO ~X A" and A A'
0states are observed, with peak c1 assigned as the ~A 00 transition. Feature a1 again

corresponds to the electron affinity, with EA(CD3OO) = 1.154 ± 0.004 eV, slightly
less than CH3OO. This yields a term energy, T0 = 0.913 ± 0.004 eV (7365 ± 30cm-1), which agrees with values of Pushkarsky et al. (7372.6 ± 0.5 cm-1) (28) andHunziker and Wendt (29).
The same two progressions observed in the neutral ~X state of CH3OO also
appear in CD3OO. As before, the positions of the a peaks (Table 3.5) allow us todetermine a harmonic frequency, ωp = 1142 ± 5 cm-1, and an anharmonicity, xp = 9 ±1 cm-1. Using these parameters we can precisely extract a value for the fundamentalof mode p, νp = 1123 ± 7 cm-1. The b series of peaks is again a combination of the
progression due to mode p plus one quantum of mode q. Taking a weighted averageof these energy spacings (Table 3.5) we obtain the fundamental of mode q to be νq =440 ± 7 cm-1.
The harmonic (ωp) and fundamental (νp) frequencies of mode p extracted__

from the CD3OO spectrum are very similar to those obtained from the CH3OO

spectrum and both isotopomers also have similar anharmonicity in this mode. The_

magnitude of this value is also similar to that observed in the DOO photoelectron
spectrum. The fact that ωp and fundamental νp do not change appreciably upondeuteration confirms the assignment of this mode to a localized O-O stretch because amode that involves hydrogen motion would have been expected to shift to a lower

84
frequency upon deuteration. This rationale is also consistent with the matrix work of
Nandi et al. (31) where the fundamental of the O-O stretch mode does not decrease
upon deuteration, shifting from 1109 ± 12 cm-1 to 1144 ± 12 cm-1 in CH3OO andCD3OO. In contrast, here the frequency νq is shifted 42 cm-1 lower upon deuteration,from 482 cm-1 to 440 cm-1, which is nearly a 10% shift. This implies that mode q
does involve hydrogen motion and supports the assignment of mode q to the C-O-O
bend. Although the hydrogen atoms in this mode may not be moving with respect tothe carbon, bending the carbon toward oxygen requires hydrogen motion. Therefore
one would anticipate such a shift to lower frequency. This shift is analogous to thatobserved in the matrix experiments (31).
~In the ~A ← X transition (see Figure 3.5a) there are two active modes close infrequency and intensity, as was also observed in the photoelectron spectrum of_

CH3OO . One quantum of the first mode is observed at the peak c2 that appears 840
0cm-1 from the 00 transition (c1), with a second quantum at 1690 cm-1 (c4). The
second active mode appears at 975 cm-1 (c3), with a second quantum at 1950 cm-1(c6). The peak c5 at 1800 cm-1 corresponds to a combination band of one quantum of_each of these modes. By analogy with the CH3OO spectrum, we assign these twomodes as s and t with fundamentals, νs = 840 ± 10 cm-1 and νt = 975 ± 10 cm-1. The
value of νs is red-shifted by about 70 cm-1 compared with the corresponding
frequency for CH3OO. This indicates a significant hydrogen contribution to the
motion of mode s, and therefore this mode cannot be assigned as a pure O-O stretch.
~
By analogy to the ~X ← X transition assignments, and by comparison with the ab initio

85
frequency motions discussed for CH3OO, there is likely some C-O-O bend involvedin the motion. The fundamental frequency νt has also shifted upon deuteration (30cm-1) although somewhat less than in the case of νs. This suggests that H/D motion isa small component of mode t and thus it should be closer to a localized O-O stretch~
than mode s. Thus, in contrast to the ~X ← X state, there is evidence for mixing~
between local modes in the two normal modes observed in the ~A ← X transition.
Based on their intensity, both modes likely have a large component of O-O stretchwith contributions from motion involving the hydrogen atoms, likely the C-O-O
bend.

3.6

CH3CH2OO__

The 200 K magic angle photoelectron spectrum of CH3CH2OO is displayed__in Figure 3.6. The spectrum is more congested than that of HOO or CH3OO , aswould be expected for a larger molecule with a greater number of modes. However,
the overall structure remains remarkably similar. One sees the same Franck-Condon
peak intensity envelopes representing transitions into two electronic states of the0~
neutral. The peak c1 is assigned to the 00 feature of the ~A ← X transition. Peak a1 is~
the origin of the ~X ← X transition and thus gives the electron affinity,
EA(CH3CH2OO) = 1.186 ± 0.004 eV and the term energy T0 = 0.938 ± 0.004 eV

86-

CH3CH2O2
Photoelectron Counts

c2

T0

c1

a4

d2

a3

a5

d3 c3
d1
HB

a6

b5 b4

b3

b6
a7

a2 EA
b2

a1
b1

bb4

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

Electron binding energy (eV)Photoelectron Counts

CH3CH2OO-

d3

c1
c2

c3

d2
d1
HB

2.6

2.5

2.4

2.3

2.2

2.1

2.0

Electron binding energy (eV)_Figure 3.6. 364 nm photoelectron spectrum of CH3CH2OO taken at 200 K and~magic angle laser polarization. (a) Bottom panel, an expansion of the ~A ←X transition

spectral profile.

87_

Table 3.6. Negative ion photoelectron peak positions for CH3CH2OO . Peak0
positions are given in wavenumbers (cm-1) relative to 00 transitions (peaks a1 and c1).
Assignments are given for the most prominent transitions labeled as indicated in
Figure 3.6.
For CH3CH2OO the νp = 1089(16) cm-1 (O-O stretch), νu = 234(9) cm-1 (bend).
fundamentals are:
2
~
X A″

2
~
A A′

νs = 900(15) cm-1 (O-O stretch), νv = 178(10) cm-1 (bend).
a1

00

0

0

b1

u0

1

240(15)a2p0

11100(10)

b2

p0u0

1 1

1305(15)

a3

p0

2

2180(15)

b3

p0u0

2 1

2315(15)

a4

p0

3

3245(15)

b4

p0u0

3 1

3490(10)

bb4

3 2
0 0

pu

3695(10)

a5

p0

4

4305(15)

b5

4 1
0 0

pu

4545(15)

a6

p0

5

5330(15)

b6

p0u0

5 1

5540(10)

a7

p0

6

6435(10)HB

See text for details-345(15)

c1

00

0

0

d1

v0

1

190(10)c2s0

1

900(15)

d2

s0v0

1 1

1050(15)

c3

s0

2

1785(15)

d3

s0v0

2 1

1910(25)

88(7565 ± 30 cm-1), in agreement with the earlier findings of Hunziker and Wendt(7593 ± 6 cm-1) (29). Again, these values are essentially the same as for HOO and__CH3OO. The same primary ground state progression observed in HOO and CH3OO

appears in peaks a1-a5, and we assign the active mode responsible as p. Fitting theenergy spacings (Table 3.6) to Eq. 3.7 as previously described gives the harmonicfrequency, ωp = 1096 ± 10 cm-1, and a small anharmonicity, xp = 4 ± 2 cm-1. Using
these parameters we can extract the fundamental frequency of mode p to be νp = 1089
± 16 cm-1.

A second progression, b1-b6, is assigned as a combination of the

progression due to mode p plus one quantum of a mode we have labeled u. Taking aweighted average of these energy spacings (Table 3.6) we obtain the fundamental ofmode u to be νu = 234 ± 9 cm-1. This frequency is small enough such that one would
expect to observe a combination band involving two quanta of mode u on top of the pprogression. This is supported by the observation of peak bb4; however, any othermembers of such a progression are obscured by noise in the data.
The similarity of ωp and νp between HOO, DOO, CH3OO, CD3OO, andCH3CH2OO is evidence that this frequency corresponds to a local O-O stretchingmode, as increasing the mass of the alkyl substituent has a minimal effect on themeasured frequency.

The fundamental frequency νp = 1089 ± 10 cm-1 forCH3CH2OO also correlates well with the O-O stretching frequencies: (i) the 1102 ±10 cm-1 HOO value measured above, (ii) ν3 = 1097.63 cm-1 measured (18,30) for
HOO, (iii) ν15 = 1112 cm-1 measured (32) for matrix isolated CH3CH2OO, and (iv) ω8= 1199 cm-1, the calculated harmonic frequency that corresponds to a localized O-O

89

Table 3.7. The molecular geometries and harmonic vibrational frequencies (cm-1) for_1
2~2
A′ CH CH OO and ~X A″ and A A′ states of the neutral CH CH OO radicals,
3

2

3

2

calculated at the B3LYP/aug-cc-pVDZ level of theory. All vibrational frequencies
are unscaled.
_
CH3CH2OO
CH3CH2OO
CH CH OO
3

2

Molecular Geometries (Å, º)
1
C (~X , A′)
S

re(CH3H2C-OO)
1.388
1.477
re(CH3H2CO-O)
re(H3C-CH2OO)
1.532
106.9
∠(CH3H2C-O-O)
Vibrational Frequencies (cm-1)
a′
modes
3065
ω1
3002
ω2
2878
ω3
1485
ω4
1464
ω5
1354
ω6
1325
ω7
1181
ω8
1045
ω9
908
ω10
816
ω11
465
ω12
291
ω13
a″
modes
3075
ω14
2872
ω15
1441
ω16
1220
ω17
1163
ω18
789
ω19
220
ω20
122
ω21

2
CS (~
X , A″)
1.467
1.316
1.515
111.6

2
CS (~
A , A′)
1.453
1.383
1.517
108.1

3122
3067
3047
1479
1469
1396
1354
1199
1125
1016
831
501
305

3123
3048
3032
1500
1474
1391
1361
1111
1018
999
841
427
279

3137
3116
1456
1259
1134
794
215
71

3136
3079
1452
1259
1167
825
225
120

90
stretching mode (Table 3.7). Based on these considerations, we assign mode p to theCH3CH2O-O stretching mode ν8.
The assignment of mode u is not assisted by matrix frequency data, as such
low frequencies are not observable in that experiment. However, analogy with the
CH3OO and CD3OO radicals would identify u as a bending mode. Furthermore, thedownward shift in the fundamental frequency of 248 cm-1 from νq = 482 ± 9 cm-1 forCH3OO to νq = 234 ± 9 cm-1 for CH3CH2OO supports this assignment. Moving thecarbon towards the oxygen would involve movement of more atoms for CH3CH2OO
than for CH3OO. Calculations of the harmonic frequencies predict two a′ modes inthis region: ω12 = 501 cm-1 which corresponds to the C-O-O bending mode (cf.
CH3OO), and ω13 = 305 cm-1 which corresponds to a C-C-O bending mode. Thus theactive mode u may be assigned as a bending mode, although precisely which atoms
are involved remains undetermined.
~
In the ~A ← X transition, the largest progression is an active mode denoted by0

the series of peaks c1-c3. Peak c1 is assigned to the 00 transition, while the peaks c2
and c3 correspond to one and two quanta, respectively, of the mode which we
designate s. The energy spacing of the peaks c1 and c2 gives the fundamental
frequency of this progression, νs = 900 ± 15 cm-1. In addition, there is evidence of asecond active mode in the less intense d series of peaks. These features appear to thehigher binding energy side of the s progression and are thus are assigned as the sprogression plus one quantum of mode v. A weighted average of these energy
spacings gives the fundamental of mode v as νv = 178 ± 10 cm-1. Finally, the peak

91labeled ‘HB’ (Figure 3.6) to the lower binding energy side of peak c1 is assigned as avibrational hot band, as its intensity significantly decreases when the sample is cooled
from 300 K to 200 K.
By analogy with HOO and CH3OO, and from examination of the GVB
diagrams (Fig. 3.1), activity in the O-O stretch is expected for detachment to the 2A′
electronic state. Thus mode s is assigned as the O-O stretch. The fundamental
frequency, νs = 900 ± 15 cm-1, is comparable to the ~A state vibrational transition forCH3CH2OO measured to be 918 ± 9 cm-1 by Hunziker and Wendt (29), as well as the~
HOO photoelectron spectrum measurement (Table 3.1). In contrast to the ~A← X
transition of CH3OO and CD3OO, the second series of peaks corresponds to a lessintense active mode with a small fundamental frequency, νv = 178 ± 10 cm-1. Thus
mode v is not expected to involve any significant O-O stretching character. The low
frequency is more consistent with a bending motion. The lowest calculated harmonicfrequency of a′ symmetry (Table 3.7) is ω13 = 279 cm-1 that corresponds to a C-O-O
bending mode.
_

The β parameters for the two CH3CH2OO electronic transitions are given in~~~
Table 3.3. The ~X ← X and the A ← X transitions have the same asymmetry, with anegative value of -0.2 ± 0.2. This finding is consistent with the other peroxidesystems studied where negative β parameters are also observed for both transitions(9). However, the lower β value in the X" state compared with HOO and CH3OO

92
implies that the HOMO orbital is no longer essentially a pure π orbital and characterfrom σ bonds is mixed in, lowering the magnitude of β from about –1 to –0.2.

3.7

Revised thermochemistry of HOO and HOOH

The electron affinity (EA) of HOO has another importance, as it provides themeans to calculate important thermochemical quantities. Specifically, it can be used
in a thermochemical cycle along with the ionization potential of hydrogen, IP(H), to
determine the gas phase acidity of HOO-H, ∆acidH0(HOO-H), provided the bonddissociation energy of HOO-H, D0(HOO-H), has been measured*:∆acidH0(HOO-H) = D0(HOO-H) – EA(HOO) + IP(H).

(3.8)

D0(HOO-H) may be in turn determined from the heats of formation of HOO, H, andHOOH:
D0(HOO-H) = ∆fH0(HOO) + ∆fH0(H) − ∆fH0(HOOH)(3.9)

Because both ∆fH0(H) and ∆fH0(HOOH) are known precisely, measurement of∆fH0(HOO) gives both D0(HOO-H) and ∆acidH0(HOO-H) from equations 3.9 and 3.8,
respectively.
The value of ∆fH0(HOO) has been essentially unchallenged for the past 20
years. Two different papers have measured ∆fH0(HOO), one by Howard in 1980 (33)* The value of D0(HOO-H) is not directly measured, but is obtained by applying a thermal correction
to the measured value D298.15(HOO-H).

93
and one by Hills and Howard in 1984 (34). In the first paper, the rate constants forboth forward and reverse reactions of
HOO + NO #OH + NO2

are measured to determine the heat of reaction, ∆Ηrxn.

(3.10)
Then ∆fH298(HOO) isdetermined by∆fH298(HOO) = ∆fH298(OH) + ∆fH298(NO2) − ∆fH298(NO) − ∆Ηrxn (3.11)
where ∆fH298(NO2) and ∆fH298(NO) are known precisely and ∆fH298(OH) has beenlast determined in 1969 (35). In this paper, ∆fH298(HOO) = 2.5 ± 0.6 kcal mol-1. The
second determination of ∆fH298(HOO) studied the reaction
OH + ClO #HOO + Cl .

(3.12)

Using the heat of reaction, ∆fH298(HOO) is determined by∆fH298(HOO) = ∆fH298(OH) + ∆fH298(ClO) − ∆fH298(Cl) + ∆Ηrxn.

(3.13)

Because ∆fH298(ClO) and ∆fH298(Cl) are known precisely, again we see that
∆fH298(HOO) is truly anchored in the value of ∆fH298(OH).

In this paper,∆fH298(HOO) = 3.0 ± 0.4 kcal mol-1. The value that was chosen for “Chemical
Kinetics and Photochemical Data for Use in Stratospheric Modeling” tables (36) was
2.8 ± 0.5 kcal mol-1. Therefore, there is some confusion as to the origin of thetabulated value in addition to the issues involved in measuring ∆fHT(OH).

94
The accepted measurement of ∆fH298(OH) comes from a spectroscopic
method of Carlone and Dalby from 1969 (35). They determined D0(OH X" ) from therelationD0(OH X" 2Π) = P1(1)( A" ← X" OH) + D0( A" 2Σ+ OH) – T0 (3P ← 1D O). (3.14)
The first and last terms are known to great precision, while the value of D0(OH A"Σ ) depends on using a Birge-Sponer extrapolation for A" 2 Σ + OH, which can be2 +complex. However, the alternative positive ion cycle method has been consideredmore suspect.

In this method, the appearance potential of OH+ from HOH,

AE(HO+/H2O), is measured, which is acknowledged as a difficult number to extract.
Its value andD0(HO-H) = AE(HO+/H2O) - IP(HO)(3.15)

give ∆fH0(OH) if IP(HO) is known. Recent ZEKE measurements of IP(OH) areunambiguous, IP(OH) = 104 989 ± 2 cm-1 or 13.016 98 ± 0.000 25 eV (37). However,recently Ruscic et al. (38) have precisely re-measured AE(HO+/H2O) and recovered
the same number from the original measurement. Furthermore, Ruscic et al. havedone a careful analysis of their data and have appealed to the highest levels ofelectronic structure theory to calculate the energetics of OH(2Π) → O(3P2)+ H(2S0).
These calculations clearly favor the recent AE(HO+/H2O) measurements of Ruscic etal. over the spectroscopic method.
As a result, the heat of formation of OH has been revised downward from∆fH0(OH) = 9.3 ± 0.1 kcal mol-1 and ∆fH298(OH) = 9.4 ± 0.1 kcal mol-1 to ∆fH0(OH) =95
8.86 ± 0.07 kcal mol-1 and ∆fH298(OH) = 8.92 ± 0.07 kcal mol-1. This significant shiftof 0.5 kcal mol-1 in ∆fHT(OH), which is outside of the original error bars, places thethermochemistry of HOO at risk. The implications of this are far-reaching. The OH
and HOO radicals are key combustion intermediates in the oxidation of all fuels.
Equally important, four of the most potent oxidizers in the troposphere are O3, NO3,
OH, and HO2. Essentially all volatile organic compounds (VOC’s) are oxidized by
one of these species. The OH and HOO radicals oxidize literally thousands of VOC’sin the atmosphere. Large numbers of rate constants, such as k1(OH + VOC →products), have been measured. It is common to estimate k-1(OH + VOC → products)from the thermochemistry. If ∆fH298(OH) moves, then large numbers of rate constants
have to be revised and all models for VOC oxidation have to be re-considered. Thisimplies that the values of ∆fH0(OH) and ∆fH0(HOO) are of crucial importance.
We use the data reported in both Howard and Hills and Howard andthermochemical values from the tables of Gurvich et al. (39), which represent themost accurate and current thermochemical tables, to compute ∆fHT(HOO). Then we
repeat the calculation but instead using the most current values of ∆fHT(HOO) to date(40) (see Tables 3.8 and 3.9). As can be seen from the tables, the values with andwithout the most recent ∆fHT(OH) (40) are consistently lower by 0.5 kcal mol-1, as
would be expected. The values in Tables 3.8 and 3.9 are used in Eq. 3.9 to determineDT(HOO-H) (Table 3.10). Finally, the newly revised value of EA(HOO) reported
here is used in Eq. 3.8 to subsequently determine ∆acidHT(HOO-H) (Table 3.11). The
details of how they were computed are presented in the Appendix.

96
Τable 3.8. Recalculation of ∆fHT(HOO) using data measured from the reaction HOO+ NO # NOO + OH from Howard (33) and thermochemical quantities from thetables of Gurvich et al. (39) using both the ∆fHT(OH) from Ref. (39) and from Ref.
(40). All values are in kcal mol-1. Details of the calculations are given in the
Appendix.
Old value of ∆fHT(OH)
New value of ∆fHT(OH)aΤ (Κ)
0

3.15 ± 0.43

2.66 ± 0.44

298.152.45 ± 0.43

1.96 ± 0.44400

2.25 ± 0.43

1.76 ± 0.44500

2.09 ± 0.43

1.60 ± 0.44700

1.85 ± 0.43

1.36 ± 0.44

1000

1.62 ± 0.43

1.13 ± 0.44

aRef. (40)Τable 3.9. Recalculation of ∆fHT(HOO) using data measured from the HOO + Cl
# ClO + OH reaction from Hills and Howard (34) and thermochemical quantities
from the tables of Gurvich et al. (39) using both the ∆fHT(OH) from Ref. (39) andfrom Ref. (40). All values are in kcal mol-1. Details of the calculations are given in
the Appendix.
Old value of ∆fHT(OH)
New value of ∆fHT(OH)aΤ (Κ)
0

4.06 ± 0.15

3.58 ± 0.16

298.153.36 ± 0.15

2.88 ± 0.16

400

3.17 ± 0.15

2.68 ± 0.16500

3.01 ± 0.15

2.52 ± 0.16700

2.77 ± 0.15

2.28 ± 0.16

1000

2.54 ± 0.15

2.05 ± 0.16

aRef. (40)97Τable 3.10. Calculation of DT(HOO-H) and ∆acidHT(HOO-H) using data measured
from the reaction HOO + NO # NOO + OH from Howard (33) and thermochemicalquantities from the tables of Gurvich et al. (39) using both the ∆fHT(OH) from Ref.
(39) and from Ref. (38). All values are in kcal mol-1. Details of the calculations are
given in the Appendix.
DT(HOO-H)
Τ (Κ)

Old value of ∆fHT(OH)

New value of ∆fHT(OH)a085.82 ± 0.43

85.33 ± 0.44

298.1587.03 ± 0.43

86.54 ± 0.44

0374.55 ± 0.45

374.06 ± 0.46

298.15375.76 ± 0.45

375.27 ± 0.46

∆acidHT(HOO-H)

aRef. (38)Τable 3.11. Calculation of DT(HOO-H) and ∆acidHT(HOO-H) using data measured
from the HOO + Cl # ClO + OH reaction from Hills and Howard (34) andthermochemical quantities from the tables of Gurvich et al. (39) using both the∆fHT(OH) from Ref. (39) and from Ref. (38). All values are in kcal mol-1. Details of
the calculations are given in the Appendix.
DT(HOO-H)
Τ (Κ)

Old value of ∆fHT(OH)

New value of ∆fHT(OH)a086.73 ± 0.16

86.25 ± 0.17

298.1587.94 ± 0.16

87.46 ± 0.17

0375.46 ± 0.21

374.98 ± 0.22

298.15376.67 ± 0.21

376.19 ± 0.22

∆acidHT(HOO-H)

aRef. (38)98
One method for checking the ∆fHT(HOO) values newly computed above is tomeasure the heat of reaction for
HOO− + HCCH #HOOH + HCC− .

(3.16)

This is a similar approach to what was done in Eqs. 3.12 and 3.10 but this reaction
does not involve OH. Instead, the measured heat of reaction yields∆rxnH298 = ∆acidH298(HCCH) − ∆acidH298(HOO-H).

(3.17)

Because ∆acidH298(HCCH) is known precisely, this experiment would determine∆acidH298(HOOH). Then, using Eqs. 3.8 and 3.9, the value of ∆fH298(HOO) and then∆fH0(HOO) may be extracted. The error in this value would be determined solely by
the experimental error of ∆rxnH298 and avoids the issues of ∆fHT(OH) discussed above.

3.8

Conclusion

The 364 nm photoelectron spectra of ROO− molecules (R=H, D, CH3, CD3,
and CH3CH2) were presented and analyzed. The HOO− spectrum was found to be
similar to that of OO−. Going from H to CH3 and CH3CH2 results in greater mixingof the R substituent with the OO−. This is supported by the observation of activity in
the ROO bending mode, the mixing of local vibrational modes in the A" state of
CH3OO, and the increased mixing of the HOMO and HOMO-1 anion orbitals with σ

99character. However, there are similarities in all these photoelectron spectra as well.
The energetics are the same: the electron affinities and term energies, to first order,
all hover around 1 eV.

Furthermore, even in the largest molecule analyzed,

CH3CH2OO, all the peroxides exhibit similar structure of their photoelectron spectra.
The O-O stretching vibration is the most intense feature in the spectra, and theFranck-Condon peak profiles to first order do not change.

Finally, the new

determination of the electron affinity of HOO, along with improved values of∆fHT(OH) allow for revised thermochemistry of HOO and HOOH.

100

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36.

W. B. De More, S. P. Sander, D. M. Golden, R. F. Hampson, M. J. Kurylo, C.
J. Howard, A. R. Ravishankara, C. E. Kolb, and M. J. Molina, "Chemical
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37.

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38.

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40.

B. Ruscic, D. Feller, D. A. Dixon, K. A. Peterson, L. B. Harding, R. L. Asher,and A. F. Wagner, J. Phys. Chem. A 105, 1-4 (2001). The values reported inthis citation have since been revised slightly by the authors. The most current
values are: ∆fH0(OH) = 8.86 ± 0.07 kcal mol-1 and ∆fH298(OH) = 8.92 ± 0.07
kcal mol-1.

103
Chapter 4
Photoelectron spectroscopy of alkoxy radicals RO− (R = CH3, CD3,
CH3CH2, CD3CD2, (CH3)2CH, and (CH3)3C )4.1

Introduction

There is great motivation to understand the structure and properties of alkoxy
radicals due to their prevalence in combustion processes, interstellar and atmosphericchemistry, and as intermediaries in hydrocarbon reactions. However, the theoreticalchallenges posed by their inherent structure also offer great motivation for theirinvestigation.
Methoxy radical in particular is one of the smallest organics, which makes it
well-suited to study via both experiment and calculation. Consequently, there has
been no shortage of experimental work on this ubiquitous radical. Some of the earlier
research involved laser magnetic resonance (LMR) (1), microwave studies (2) andnegative ion photoelectron spectroscopy (3), as well as numerous fluorescence~ to X~ electronic transitions (4-9); however,spectroscopic examinations of the A

analysis of much of this data was complicated by the overlap and congestion due toroom temperature samples as well as poor resolution. Furthermore, there was no
indication of any Jahn-Teller effects that would be expected as a result of both theelectronic degeneracy of the2

E ground state and the high-symmetry C3v

configuration. Supersonic expansion of the sample in laser-induced fluorescence104(LIF) experiments significantly clarified the spectral features (10-14), permitting thefirst direct observation of Jahn-Teller activity in methoxy radical (15). However, it
was only within the past several years that the advent of improved resolution
techniques in conjunction with supersonic expansions allowed the biggest step toward
understanding the spectroscopy of methoxy radical (16,17).

Specifically, therotationally resolved stimulated emission pumping (SEP) studies have allowedassignment of vibronic symmetry to the CH3O spectroscopic features because therotational profile of a transition is a signature of the vibronic symmetry of the states
involved (18,19).
Such information has been a breakthrough for advancing the theoreticalunderstanding of CH3O and its Jahn-Teller activity.

Early theoretical works havecomputed geometries (20,21), and Jahn-Teller stabilization energies and distortions(22). It is only recently that full treatments with the ability to predict spectroscopic
features have started to appear (23). Specificially, in a recent review, Barckholtz andMiller (24) presented the first theoretical treatment of the Jahn-Teller problem
including a multimode analysis of both linear and quadratic Jahn-Teller coupling aswell as spin-orbit coupling.

They were able to incorporate Temps' rotationallyresolved SEP data into their model, which in turn calculated the Jahn-Teller
parameters that fully characterize the Jahn-Teller activity of the radical.
There have been considerably fewer studies involving CH3CH2O,(CH3)2CHO, and (CH3)3CO. Negative ion photoelectron spectra of the anions haveestablished electron affinities (EAs) for these three species, although the resolution of
the spectra was not sufficient to yield significant spectral detail (25,26). Like CH3O,

105
the majority of the investigations of CH3CH2O have employed fluorescence
spectroscopy (5,9,10,27-29). Most recently, LIF of jet-cooled CH3CH2O has allowed
spectroscopic identification of several vibrational modes (30). Various ab initio
calculations have been performed on CH3CH2O (31,32) and CH3CH2O− (20);however, there appears to be some discrepancy as to the symmetry of the groundelectronic state of the radical (25,27,29).

Fluorescence excitation spectra of(CH3)2CHO were recorded to determine rate constants for reaction with various gases(5,33). In addition, Foster et al. (10) obtained rotationally resolved spectra for jetcooled CH3O, CH3CH2O, as well as (CH3)2CHO. Based on the rotational structure of
the spectra, they assign the ground state of ethoxy radical to be 2A″ with a low-lying
2

A' state, whereas in iso-propoxy radical, the rotational signature of the fluorescencetransitions suggests that the symmetry switches to a 2A' ground state with a 2A″ firstexcited state.

Aside from the EA determination in the PES study (25), ourunderstanding of tert-butoxy extends to a structure calculation of the anion andradical (6,20) and measurement of the electron photodetachment cross section of theanion (6).
The previous chapter examined the structure of ROO species. In that case, we
observed a suite of molecules that increased in complexity with larger R group while
retaining much of the same underlying structure seen in the smallest HOO molecule.
One might assume that the RO molecules would follow the same behavior;furthermore, because there is one less oxygen involved in bonding, they should yield
simpler molecular structure.

However, this is not the case due to geometricalconsiderations. Methoxide is a closed-shell anion with C3v symmetry. The highest

106
occupied molecular orbitals involve the two nonbonding essentially atomic p orbitals
highly localized on the oxygen atom (11,15,21,22). The 3.408 eV laser photon can
photodetach an electron from either the px or the py orbital, resulting in twoenergetically equivalent configurations for the radical.

A schematic of thesedegenerate orbitals is shown in Figure 4.1. The degenerate electronic state coupled

Figure 4.1. Schematic of the two degenerate highest occupied molecular orbitals of
X~ 1A1 CH3O−.

with the C3v symmetry of the radical causes a Jahn-Teller effect in CH3O. Replacing
one of the hydrogens in CH3O with a methyl group to yield CH3CH2O breaks the C3v
symmetry of CH3O. This should only be a small perturbation away from the C3v
configuration, and thus the resultant splitting in energy of the previously degeneratep-like orbitals is expected to be small. Substituting two hydrogens in CH3O for
methyl groups gives (CH3)2CHO. This can be thought of as an even larger departurethan in CH3CH2O away from the methoxy radical C3v structure and thus perturbing

107
the orbital energies even further. However, if we replace each hydrogen in CH3O
with a methyl group to obtain (CH3)3CO, we return to the C3v symmetry of CH3O andaccordingly we would expect Jahn-Teller effects to be manifested in this radical. In
this paper we attempt to characterize these highest occupied molecular orbitals and~ ← X~ photelectron spectra as the number
their interactions via the X~ ← X~ and A

of methyl groups substituted for hydrogens increases.

We present anion

photoelectron spectra of the alkoxides CH3O-, CH3CH2O-, (CH3)2CHO-, and(CH3)3CO-, as well as CD3O- and CD3CD2O- using a photon energy of 3.408 eV for
different laser polarization angles and different sample temperatures. In addition, ab
initio calculations for ethoxy and iso-propoxy anions and radicals are presented inorder to guide the assignment of the spectral peaks.

4.2

Experiment

Details on ion synthesis were discussed in Chapter 2.

The CD3O

photoelectron spectrum contains contamination from DOO− peaks. The flow tube
conditions described above also allow for the synthesis of DOO− through the reactionof CD3OD with OO−. Because both DOO− and CD3O have m/z = 34, the DOO−
could not be eliminated via mass selection. A separate pure DOO− photoelectron
spectrum was subsequently obtained and was subtracted from the CD3O− spectrum.
Ab initio calculations of both CH3CH2O and (CH3)2CHO anions and radicals
were implemented using the GAUSSIAN94 program package (34). In CH3CH2O,
two sets of calculations were employed. The first, referred to as Method 1, used the

108
density functional theory B3LYP method with a 6-31+G* basis set, while the second,
Method 2, used the B3LYP method with an aug-cc-pVDZ basis set. In (CH3)2CHO,
Method 2 was employed for the anion and ground state of the radical, but no potential
minimum for the first excited state was found. Therefore a third set of calculations
was executed using a HF method with an aug-cc-pVDZ basis set for the anion andground state of the (CH3)2CHO radical, and the CIS method with the aug-cc-pVDZ
basis set for the first excited state of (CH3)2CHO. This is referred to as Method 3.
The GAUSSIAN94 calculation output files, which include optimized geometries,
normal modes, and vibrational frequencies, were then used in the CDECK suite ofprograms (35). This package calculates Franck-Condon factors for the possible anion
to neutral transitions using a harmonic oscillator approximation, and outputs a stick
photoelectron spectrum simulation. This stick spectrum is then convoluted with a 10
meV Gaussian function to produce a simulated spectrum that can be directlycompared to the data. If the correspondence between theory and experiment is good,
the theoretical output may be used as a guide for understanding and assigning theexperimental spectral features. The calculated vibrational frequencies are not scaled
unless otherwise stated.
A different procedure, referred to as Method A, was used to simulate thephotoelectron spectrum of (CH3)3CO−. In this case, the anion and neutral potential
energy surfaces were assumed to be harmonic. Experimental frequencies were input,
assuming the same frequency in the anion and neutral species for each mode. Thechange in geometry between the anion and neutral was adjusted in order to reproduce
the intensities of the peaks in the experimental spectrum. Finally, the simulated peaks

109
were convoluted with a full-width at half-maximum (FWHM) of 10 meV. No hotbands were simulated.

4.3

CH3O−

A detailed theoretical description of Jahn-Teller effects, particularly withrespect to CH3O radical, has recently been given by Barckholtz and Miller (24). A
brief overview is given here. As mentioned above, the ground electronic state of
CH3O has a two-fold degeneracy in its C3v geometry configuration. This degeneracy
leads to a Jahn-Teller phenomenon: in order to minimize energy, the radical will
distort, thus breaking the C3v symmetry and lifting the degeneracy. CH3O has six
vibrational modes. Three are symmetric a1 modes: ν1 CH stretch, ν2 umbrella, and ν3
CO stretch; and three are asymmetric e modes: ν4 CH stretch, ν5 HCH scissors, andν6 methyl rock, which involves movement of the oxygen off the C3v axis toward oneof the hydrogens. The symmetric modes by definition do not provide the distortion
necessary to remove the degeneracy and lower the energy, and thus only theasymmetric e vibrational modes can be Jahn-Teller active. The methoxy radical is
known to exhibit a “dynamic” Jahn-Teller effect, where the zero-point energies of thevibrations are much greater than the stabilization energy of the Jahn-Teller distortion(24). Therefore the distortion is not large enough to provide a permanent departurefrom C3v to Cs symmetry and must be treated as a vibronic coupling problem where
the asymmetric vibrational e modes couple to the degenerate E electronic states.

110(a) CH3Oa-dc

Photoelectron counts

lk

h

j i

g f

e

b
a

(b) CH3O

-

dc

lk

h

j i

(c) CD3O

gf

e

b

a

-

dc
j i h

2.4

2.2

2.0

g

1.8

1.6

1.4

Electron binding energy (eV)Figure 4.2. a) Magic angle photoelectron spectrum of CH3O− taken at 300K sample
temperature. (b) 300K photoelectron spectrum of CH3O− taken at 90° (heavy line)
and 0° (light line) polarization angles with respect to the electron detection axis. (c)Magic angle photoelectron spectrum of CD3O− taken at 300K sample temperature,
DOO− subtracted. See text for details.

111
The room temperature magic angle, and 0° and 90° photoelectron spectra for
CH3O− are presented in Figure 4.2a and b, respectively. Peak a is assigned as the 000transition that gives EA(CH3O) = 1.572 ± 0.004 eV. The magic angle spectrum
shows what appears at first glance to be a regular Franck-Condon progression of
doublets in peaks a, c and d, g and h, i and j, and k and l. Closer examination of thespacings, however, reveals that this is not the case. The interpeak spacings of c and d,g and h, i and j, and k and l are 255 cm-1, 155 cm-1, 365 cm-1, and 355 cm-1,
respectively. Furthermore, the spacing between peaks a and c, c and g, g and i, and i
and k are 1210 cm-1, 1530 cm-1, 1195 cm-1, and 1400 cm-1, respectively. In addition,
there are the relatively weak peaks b and e whose photoelectron angular distributions
differ markedly from the other peaks in the spectrum. Peaks a, c, d, f, and h show
similar behavior in the 90° and 0° polarization scans in that their intensities areheavily weighted in the 90° direction (Figure 4.2b).

This is reflected in theasymmetry parameter values (β of about −1) for these peaks listed in Table 4.1. The
opposite is true for peaks b and e, whose β values are closer to +1. (These values aretaken from closeup spectra of the region, which are not shown). Peaks g, i, j, k, and ldisplay essentially isotropic angular distributions and β is around 0 as given in Table
4.1.
The position and intensity of the magic angle spectrum peaks agree well witha recently published CH3O− photoelectron spectrum by Osborn and co-workers (36).
In contrast to the original photoelectron spectra taken of CH3O− under significantly
poorer resolution (3), none of the major features in the spectra in Figure 4.2a are112
Table 4.1.
Peak positions, asymmetry β values, and assignments for thephotoelectron spectrum of CH3O−. Errors on β values are ± 0.1. Assignments are
made to SEP peak positions (cm-1) and vibronic symmetries given in Ref. (19). See
text for details.
Peak Position (cm-1)
Assignment
β
a

0

-0.9

0 000 (E)b

630 ± 10≥ 1.2

652 610 (A1)c1210 ± 10-0.91198 610 (E)d1465 ± 10-0.81490 510 (E)e2015 ± 30≥ 1.02018 210 610 (A1)f

2450 ± 90-0.82432 210310 (E)2441 510 610 (E)2488 630 (E)g2740 ± 10-0.22730 520 (E)2750 ?
2760 210510 (A1)h2895 ± 10-0.72862 [520 / 410 (E)]i3935 ± 10

-0.1

-

j

4300 ± 10

-0.3

-

k

5335 ± 30

-0.1

-

l

5690 ± 15-0.1-

spaced such that they could be assigned to any of the three a1 symmetric vibrationalmodes: ν1 = 2869 cm-1, ν2 = 1375 cm-1, ν3 = 1062 cm-1 (14). This leaves only the
doubly degenerate e vibrational modes ν4, ν5, and ν6; the fact that all the prominent

113
spectral features can be attributed to asymmetric modes testifies to the presence ofsignificant Jahn-Teller effects in CH3O.
Due to the dynamic nature of the Jahn-Teller effect in CH3O, ab initio
calculations using programs such as GAUSSIAN94 that are based on the BornOppenheimer approximation are not feasible as they do not account for the vibronic
coupling of electronic and vibrational energy levels.

For this reason, and tounderstand more about the photoelectron spectrum of CH3O−, we compare our peak
positions with those observed by Temps (19) who has catalogued and assigned
vibronic symmetry to lines observed in rotationally resolved SEP/LIF studies of
CH3O and CD3O.
We are able to use the information provided by the β parameters as discussed
above, in addition to Temps' results, to make assignments (see Table 4.1). The 000transition, peak a, necessarily carries an E vibronic symmetry assignment. This isbecause the degenerate E symmetry of the electronic state couples with the totallysymmetric vibrationless level to give E vibronic symmetry, E ⊗ a1 = Evibronic. Peaksb, c, and d are assigned to the 610 (A1), the 610 (E), and the 510 (E) vibronic lines,
respectively. Peaks a, c, and d (β of about -1) are all assigned to E vibronic symmetry
while peak b (β ≥ 1.2) has A symmetry; this suggests a strong correlation between
asymmetry parameters and the vibronic symmetry assignment given to the final state
involved in the anion to neutral transition. Furthermore, the A vibronic peak has anintensity significantly smaller than that of its neighboring peaks of E symmetry. With
this in mind, peak e is assigned to the 210610 (A1) transition at 2018 cm-1, as its β value
and relative intensity are similar to that of peak b.

114
A similar phenomenon was observed (37) in the photoelectron spectrum of
C2H−.

In this case, the spectrum was dominated by peaks assigned to Franck-

Condon-allowed transitions. However, structure on a scale approximately 50 times
smaller than the origin transition was observed, which was attributed to non-FranckCondon transitions into odd quanta of asymmetric modes in the ground state of theradical. In addition, the non-Franck-Condon peaks displayed asymmetry parameterswith opposite sign as the peaks corresponding to activity of symmetric modes of theneutral.

These normally forbidden transitions gain intensity via an electronic state

mixing between the ground state of the neutral and an excited electronic state. The~ 2A ↔ X~ 2E
same mechanism is proposed to explain anomalous intensities in the A
1

excitation spectrum of methoxy radical (17,38).

In the LIF studies, for peaks

involving modes of e vibrational symmetry, normally forbidden parallel transitions
are observed with intensities of comparable magnitude to the allowed perpendicular~ statetransitions. Here the electronic state mixing is suggested to be between the A

and a closely lying excited E electronic state. In the case of the CH3O− photoelectron
spectrum, the ground E electronic state of the radical may mix with an excitedelectronic state although the first excited state lies several eV above the ground state.
Peak f, which falls at 2450 cm-1, has the same β value as peaks a, c, and d. It
may correspond to either the 210 310 (E) line at 2432 cm-1, the 510610 (E) line at 2441
cm-1, or the 000 (E) line at 2488 cm-1. Due to its breadth, peak f may obtain intensityfrom any or all of these lines. At 2740 cm-1 above the origin lies peak g. Its β value
is closer to zero than the previous large-intensity peaks mentioned with significant
intensity. This suggests that peak g is a mix of peaks with E and peaks with A1

115
vibronic symmetry. The possibilities are the 520 (E) line at 2730 cm-1, a line ofundetermined symmetry at 2750 cm-1, and the 210510 (A1) line at 2760 cm-1. Peak hfalls into the same category as peaks a, c, d, and f at 2895 cm-1.

Its assignment iseither the 2862 cm-1 line (this line was given a tentative vibronic label of [520 / 410(E)](19), which is distinguished by its brackets in Table 4.1), or the 410 (E) transitionat 2889 cm-1. Lines with excitation energies greater than 3000 cm-1 are not assigned.
The higher the transition lies in the X~ 2E potential well, the more vibronic mixing
there is, rendering it too difficult to determine vibronic symmetry past this energy(19).
It should be emphasized that the photoelectron spectrum of CH3O− isdominated by the Jahn-Teller-active (asymmetric) vibrational modes and thenormally expected features due to vibrations solely of symmetric modes are absent.
Peaks e, f, and g carry assignments involving combination bands of symmetric andasymmetric vibrational modes. Thus we do see activity of the symmetric modes in
the CH3O− photoelectron spectrum, although only when coupled with motion fromthe asymmetric modes.
The spin-orbit splitting of the ground state of methoxy radical is 61.5 ± 1.0
cm-1 (about 7 meV) (1,39), which is less than the resolution of our experiment.
Nevertheless, the spin-orbit splitting contributes to the widths of the peaks observed.
The FWHM of peaks a, c, and d in the cold magic angle spectrum of CH3O− are 172,
158, and 102 cm-1 respectively.

As energy increases, the spin-orbit splitting isquenched, as expected (22,24). Both spin-orbit and Jahn-Teller interactions compete
for orbital angular momentum and as the Jahn-Teller energy interaction increases at

116higher energies in the potential well, the spin-orbit splitting should necessarily
decrease.

4.4

CD3O−

The room temperature deuterated methoxide spectrum with DOO− subtracted(see section 4.2) is shown in Figure 4.2c. Peak a, the 000 transition, falls at an energyof 1.559 ± 0.004 eV.
At first glance, we see the same coarse structure in CD3O− as in CH3O− in thatthe intensity pattern and the relative spacings are the same. The peaks appear to have
uniformly shifted to smaller binding energy; in fact, if a one-to-one correspondence isassumed between peaks a, c, d, g, h, i, and j in both spectra, which is not intuitively
unreasonable, all of these peaks shift their position in an essentially uniform manner
relative to peak a by between 21 and 26 %. This agrees with the findings in the
recent photoelectron spectrum of CD3O− (36). A vibrational mode that involves alarge amount of hydrogen movement would decrease in frequency upon substitutinghydrogen with deuterium and thus a photoelectron peak would shift to lower binding
energy.

A mode that involves little or no hydrogen movement will not shift. The

CO stretch, ν3, is the only mode of CD3O that falls into this category. Therefore theevidence indicates that we are not seeing any activity in the symmetric ν3 mode.
We should be able to use the information provided by the β parametersignature of the peaks to further corroborate the apparent one-to-one correspondence
and make assignments. Assignments for CD3O are made in the same manner as

117
discussed in CH3O, i.e., matching our lines to those of Temps LIF/SEP studies of
CD3O (19) taking into account both the position and the β parameter/symmetry of
the peak as reported in Table 4.2. In the case of peaks a, c, d, and h, the β parameter
Table 4.2. Peak positions, asymmetry (β) values, and assignments for thephotoelectron spectrum of CD3O−. Errors on β values are ± 0.1. Assignments are
made to SEP peak positions (cm-1) and vibronic symmetries given in Ref. (19).
Peak
Position (cm-1)
Assignment
β
a

0

-0.9

0 000 (E)c

920 ± 15-0.9

885 610 (E)d1160 ± 15-0.91172 510 (E)g2035 ± 15-0.61995 520 (E)h2275 ± 20-0.8? [ 502 / 410 (E)]i2920 ± 15

-0.4

-

j

3180 ± 20--

values for the peaks in both the CH3O− and CD3O− spectra are essentially identical;
this provides additional confirmation that peaks a, c, d, and h in the CH3O− andCD3O− spectra can be attributed to the same set of transitions. Peaks c and d areassigned to 610 (E) and 510 (E), respectively, as is seen in peaks c and d in CH3O−.
Peak g, however, has a β of −0.2 in CH3O− but a β of −0.6 in CD3O−, which is asignificant change in asymmetry parameter. It is likely that the three vibronic linesthat were assigned to peak g in the CH3O− spectrum shifted, but by different amounts.
The shifts could happen in such a way that the 520 (E) line no longer overlaps with an
A line, and therefore its β looks more like peaks c and d. Peak g is therefore assigned

118
to the 520 (E) line of CD3O− at 1995 cm-1. This further supports the assignments forpeak g in CH3O-.
Peaks h in both spectra have similar asymmetry values, and therefore they are
confirmed to be the same peak, either the [ 520 / 410 (E)] line or the 410 (E) line assignedin CH3O−. However, the 410 (E) assigned position does not match peak h in CD3O−,
so both peaks h obtain the [ 520 / 410 (E)] assignment. The Temps assignments for
CD3O− terminate around 2200 cm-1 (19), so we are not able to confirm the assignmentof the CD3O− peak h as the [ 520 / 410 (E)] line. Peak i in CD3O− has changed its valueof β significantly, so it is possible that the peak i in CH3O− is composed of several
different lines which shift by different amounts upon deuteration, as has been
discussed above for peak g.

4.5

CH3CH2O− and CD3CD2O−

The 90° and 0° polarization photoelectron spectra at 200 K of CH3CH2O− areshown in Figure 4.3a. No structure in the CH3CH2O− photoelectron spectra could be
attributed to hot bands. Peak a is assigned as the EA at 1.712 ± 0.004 eV.
Examination of the spectra shows that there are two groups of peaks with
contrasting β parameters (see Table 4.3). The first group, which includes peaks a ande possesses a β < 1, while peaks b-d and f-i have opposite angular distributions, β > 1.
The existence of the two groups of β values indicates that they correspond to different

119

(a)

b

CH3CH2O-

Photoelectron countsfkj i h

2.3

g

d

a
c

kj i h g f e d c

(b)

2.4

e

2.2

2.1

2.0

1.9

1.8

1.7

1.6

Electron binding energy (eV)Figure 4.3. a) 200K CH3CH2O− photoelectron spectrum taken at 90° (heavyline) and 0° (light line) polarization angles with respect to the electron
detection axis. (b) The 200K CH3CH2O− magic angle photoelectron spectrum
is displayed in bold. The thin line at the baseline corresponds to a Method 2~(B3LYP/aug-cc-pVDZ) simulation of the photoelectron transition into the A
state of CH3CH2O as described in the text. The floated line is the simulationof the photoelectron transition into the X~ state of CH3CH2O.

120
Table 4.3. Peak positions, asymmetry (β) values, and assignments for thephotoelectron spectrum of CH3CH2O− and CD3CD2O−. Errors on β values are ± 0.1.
CH3CH2O−
CD3CD2O−
Peak

Position (cm-1)

β

Peak

Position (cm-1)

β

a

0

-0.8

a

0

-0.7

b

355 ± 100.9b275 ± 100.9c830 ± 100.9c660 ± 201.2

d1040 ± 100.7d805 ± 100.6e1295 ± 10-0.4e1090 ± 100.2

f1490 ± 100.8

f1380 ± 150.8g

1730 ± 100.7g1915 ± 150.5h2110 ± 150.9h2135 ± 100.1

i2365 ± 150.6i2360 ± 15

0.6

j2575 ± 100.1

k2740 ± 15-0.1

electronic states. This is strong evidence supporting the presence of a low-lying firstexcited state of ethoxy radical. Peaks a and e correspond to transitions from theground state of CH3CH2O− anion into the ground state of neutral CH3CH2O, while
peaks b-d, and f-i represent transitions into the first excited state of the neutral.
Therefore, the spacing between peaks a and b, 355 ± 10 cm-1, determines the term
energy of the first excited state. To our knowledge, this quantity has never before
been directly measured, although there has been a laser fluorescence study ofCH3CH2O that did not observe any excited states within 300 cm-1 of the ground state(27). The direct observation of the ground to first excited state spacing in the radical

121
supports the substitution argument previously mentioned. The substitution of one
hydrogen of CH3O with a methyl group perturbs the C3v geometry and thus slightly
lifts the electronic degeneracy present in methoxy radical.
The spacing between peaks a and e is 1295 cm-1 and the position of two
quanta falls at 2590 cm-1, which is in the vicinity of peaks j and k. The fact that the βvalues for peaks j and k are the same within error, however, indicates that the second
quantum transition is contributing to both these peaks, and therefore they receive thesame assignment in the ground state of the radical. The spacings between peaks band d match that of peaks d and g, about 685 cm-1. Furthermore, the intensity pattern
between peaks b, d, and g matches that of a typical Franck-Condon progression andtherefore these peaks are assigned as a vibrational progression in the first excited stateof the radical. The relationship between the remaining peaks, aside from whichelectronic state in the radical they belong to, is unclear.
Comparison of the observed spectrum with information provided by ab initio
calculations of CH3CH2O and CH3CH2O- can help clarify and confirm the above
observations. Method 2 (B3LYP/aug-cc-pVDZ) calculates the anion to be a 1A'
state, while the radical has a 2A" ground state with a 2A' first excited electronic statepredicted to lie 360 cm-1 higher. This spacing is in good agreement with the observed
difference between peaks a and b.
The CDECK simulation of the photoelectron spectrum of CH3CH2O− using
the results from Method 2 is presented in Figure 4.3b along with the magic angle
photoelectron spectrum taken at 200 K sample temperature. The simulation at thebaseline represents the first excited state results while the floated plot represents the

122ground state simulation.

The positions and intensities of the 000 peaks in thesimulations (peaks a and b) are fixed to match the experimental peaks (in bold).
Method 2 predicts the most active mode in the X~

2A″ ← X~

1

A'

photoelectron spectrum to be ν9 at 1303 cm-1, a symmetric scissor motion of the nonmethyl hydrogens.

As is shown in Figure 4.3b, the simulation has excellent

agreement for the intensity of peak e with respect to peak a, as well as the intensitiesof peaks j/k with respect to peak a. These peaks are therefore given the assignment of
910 and 9 20 , respectively.
The agreement between the excited state calculation and experiment is not as
conclusive, however.

For example, the predicted intensity of peak c is highly

overestimated. Furthermore, peaks c and d are predicted to form a progression in asingle mode, and from examining their intensities in the experimental spectrum, thisis not obviously true. This would also conflict with the implication that peaks d and g
form a vibrational progression.

Peak f lines up well with the simulation peakcorresponding to the fundamental of ν7, the CO stretch predicted at 1118 cm-1.
However, it is unclear which simulated peak to match to peak g. For features tohigher binding energy of peak g (and including peak g), the agreement with thesimulation is ambiguous.

In short, the less-than-ideal match between theexperimental intensities and spacings and those derived by theoretical means does nothelp us to assign the remaining peaks. This lack of agreement is not completely
unexpected, however.

If the ground and first excited states of CH3CH2O are

separated by only 355 cm-1, the two states should couple together strongly with theeffect of perturbing vibrational levels away from a harmonic oscillator

123
approximation, and the intensities of the transitions may be affected as well.
Therefore, peaks d and g are tentatively assigned as the first and second quanta in a
~ state whose motion is undetermined, while peak f
vibrational progression in the A

may correspond to the fundamental of ν7.
The 0° and 90° polarization spectra of CD3CD2O− are shown in Figure 4.4.
Peaks a and b have similar relative angular distributions and intensities as peaks a andb in CH3CH2O−, and the spacing has decreased to 275 ± 10 cm-1 (see Table 4.3). As
in CH3CH2O−, peaks a and b are assigned to the transitions into the ground and firstexcited state of the radical, respectively. Peak a is assigned as the EA at 1.699 ±0.004 eV.
In contrast to the CH3CH2O− photoelectron spectrum, there is no peak that
may be grouped with peak a as having similar β parameter. The mode assigned in the
ground state of the ethoxy radical, ν9 at 1303 cm-1, is predicted by Method 2 to shift
to 965 cm-1 upon deuteration. There is no match for such a peak in the CD3CD2O−
experimental spectrum.

Method 2 also calculates that the mode (7 tentatively

assigned to the CH3CH2O− photoelectron spectrum at 1118 cm-1 would shift to 987
cm-1. This may correspond to peak e in the CD3CD2O− spectrum.
Comparison of the CH3CH2O− and CD3CD2O− photoelectron spectra suggeststhat many of the features of the CH3CH2O− spectrum have been crowded together due
to the shifting expected upon deuteration. There are fewer features in the CD3CD2O−
spectrum, and for peaks to the higher binding energy of peak d, the line shapes are124

Photoelectron counts

CD3CD2O

b

-

d
f
i

2.4

2.3

2.2

2.1

2.0

h

g

1.9

a

e

c

1.8

1.7

1.6

Electron binding energy (eV)Figure 4.4. 300K CD3CD2O− photoelectron spectrum taken at 90° (heavy line) and 0°(light line) polarization angles with respect to the electron detection axis.

125such that they appear to be formed from several peaks. This blending would also
affect the intensities of the observed features and their asymmetry parameters. Unlikein the CH3O− photoelectron spectrum, we are not able to make a visualcorrespondence between peaks in the deuterated and non-deuterated spectra. As was
the case in CH3CH2O−, aside from the few assigned transitions, the simulated
spectrum did not accurately reproduce either the relative spacings or the intensities ofthe spectral features in a consistent manner. Therefore, the congestion observed in
the photoelectron spectrum of CD3CD2O− hinders any possible corroboration withstructure in the spectrum of CH3CH2O−.
In the CH3CH2O− photoelectron spectrum no signature of dissociation of thespecies is observed. Photodissociation of CH3CH2O− has been observed with a 2.331
eV photon with a low branching ratio of dissociative to stable products (40).

4.6(CH3)2CHO−

The magic angle spectrum for (CH3)2CHO− at 200 K sample temperature isdisplayed in Figure 4.5. The β parameters for all the peaks in the (CH3)2CHO−
spectrum are essentially identical, spreading over a small range between 0.1 and 0.5.
For this reason the polarization spectra are not shown.
The observed peaks and their positions in the (CH3)2CHO− photoelectron
spectrum are given in Table 4.4. The electron affinity for (CH3)2CHO, corresponding
to the position of peak a, is 1.847 ± 0.004 eV. The Method 1 calculation predicts ananion with a 1A' ground state and Cs symmetry. The neutral ground state is calculated

126

(a)

a

-

(CH3)2CHO

Photoelectron counts

f

e

hg

d

cb

a

(b)
f

e

hg

2.4

2.3

2.2

2.1

d

2.0

cb

1.9

1.8

1.7

Electron binding energy (eV)Figure 4.5. a) The 200 K magic angle photoelectron spectrum of (CH3)2CHO− isdisplayed in bold. The thin line corresponds to a theoretical simulation using Method
1 (B3LYP/6-31+G*) of the photoelectron transition into the X~ state of (CH3)2CHO.
(b) The 200K magic angle photoelectron spectrum of (CH3)2CHO− is displayed inbold. The thin line corresponds to a theoretical simulation using Method 3 (CIS/aug~ state of (CH ) CHO.
cc-pVDZ) of the photoelectron transition into the A
3 2

127
Table 4.4. Peak positions and assignments for the photoelectron spectrum of(CH3)2CHO−.
Assignment
Peak
Position (cm-1)
hot band-330 ± 25

3'10

a

0

000 X~ 2A’ ← X~ 1A’b375 ± 25310c

500 ± 15510d925 ± 50

See text.

e1175 ± 15~ 2A″ ← X~ 1A′ or 141
000 A
0

f

1270 ± 20~ 2A″ ← X~ 1A′ or 141
000 A
0

g

1545 ± 15~ ← X~ 41
X~ ← X~ 3101410 + A
0h1740 ± 40~ ← X~ 51
X~ ← X~ 5101410 + A
0as a 2A' state with Cs geometry. A peak measured from the room temperature
photoelectron spectrum (not shown) is assigned as the hot band transition 3'10 , whereν’3 is the CCC bend predicted at 360 cm-1 in the anion. The CDECK simulation of
the photoelectron transition into the ground state radical using results from Method 1
is displayed in Figure 4.5a. The spectrum simulation reproduces both the spacing andintensity of many of the spectral features relatively well. Peak b corresponds to one
quantum of vibration in ν3, predicted at 360 cm-1, which is the CCC bending motionin the radical. Peak c is assigned to one quantum of ν5, a CCCO symmetric bending
mode, which Method 1 calculates at 452 cm-1. Peak d is the result of severaltransitions. The one with the greatest intensity is the fundamental of ν10, a rocking
motion of all hydrogens, predicted to fall at 1054 cm-1. In addition there is a smaller

128
peak to lower binding energy that is primarily composed of overtones andcombination bands of ν3 and ν5.
The simulation predicts the most active vibration to be ν14, the CO stretch, tolie at 1262 cm-1 above the 000 transition. This is a better match with the position ofpeak f (1270 cm-1) than peak e (1175 cm-1). However, because peak e is the mostintense peak in the spectrum after peak a, the simulated peak may also be assigned topeak e. Therefore it is not clear if it is peak e or peak f that corresponds to 1410 . The
observation of the CO stretch is expected because detaching an electron localized on
the oxygen should affect the CO bond and thus activate the CO stretch, parallel to the
O-O stretch activity observed in the ROO molecules.

However, unlike theCH3CH2O− spectrum, we are not able to use peaks with contrasting β values todistinguish between different electronic states. What is clear, however, is that thepeak that is not assigned to 1410 is absent from the photoelectron spectrum simulationof the transition into the ground state of (CH3)2CHO. Thus either peak e or f is the 000transition from the ground state anion into the first excited state of the neutral. The~ state is therefore assigned as the average of the positions ofterm energy of the A

peak e and f, 1225 ± 65 cm-1. This spacing is believable in light of the substitution
argument applied above. Specifically, replacing two of the hydrogens in CH3O withmethyl groups can be considered a perturbation that lifts the degeneracy of the CH3O
X~ 2E state to a greater degree than in CH3CH2O.

The assignment of transitions into the first excited state of (CH3)2CHO can befurther supported by ab initio calculations.

Method 3 (HF/CIS/aug-cc-pVDZ)

predicts that the anion has a 1A' ground state with Cs geometry and that the neutral

129
has a 2A' ground state with Cs geometry, in agreement with Method 1. The CIS
calculation additionally predicts that the first excited state is a 2A" state with Cs
geometry, 2011 cm-1 above the X~

2

A' state.

This agrees fairly well with ourmeasurement of the term energy of the excited state. The CDECK simulation of thephotoelectron spectrum using the results from the Method 3 calculation is displayedin Figure 4.5b. In this figure, the intensity and position of the 000 simulation peak are
fixed to match peak f, although choosing between peaks e and f is arbitrary, as
discussed above. It is clear that both peaks g and h are composed of transitions into
~ electronic states of the radical. Peak g is assigned to the overtone
both the X~ and A

combination 3101410 in the ground state radical plus 410 in the first excited state, the~ ← X~ transition. Similarly,
CCC bend, predicted to fall 371 cm-1 above the 000 A~ state,peak h results from the overtone of 5101410 ground state modes plus 510 in the A

the CCCO symmetric bend, predicted at 516 cm-1. Beyond peak h the convolution offeatures from transitions into both ground state and first excited states in the radical is
too complex to make unambiguous assignments.
It is interesting that our calculations predict a 2A" ground state with a 2A' firstexcited state in ethoxy radical, while in iso-propoxy radical, the two states arereversed. This is in agreement with the rotational spectroscopy of Foster et al. (10).
They postulate that this is because in CH3CH2O, the non-bonding a' orbital that lies in
the Cs plane is closer to the methyl group; in (CH3)2CHO, the a' orbital is closest to
the non-methyl hydrogen and it is the out-of-plane a" orbital that has the maximum
opportunity for interaction with the two methyl groups. The ordering of the states

130
observed here implies that the proximity of a lone-pair orbital to a methyl groupserves to lower its energy relative to the other lone-pair orbital.

4.7(CH3)3CO−

The magic angle photoelectron spectrum of (CH3)3CO− at 200 K is displayedin Figure 4.6. As was the case for (CH3)2CHO−, there is essentially no variation in βparameter over the span of the spectrum, as it ranges between 0.1 and -0.3.
Peak b is measured at 435 cm-1 above the 000 transition. This is the same
spacing between peaks b and c, peaks d and e, and peaks e and f. This suggests thatpeaks b and c form a vibrational progression in one mode, and peaks e and f form thesame vibrational progression in combination with peak d. Peak d falls at 1205 cm-1
above peak a, and peak g lies at twice that amount, implying that peaks d and g forma second vibrational progression.
Because (CH3)3CO is expected to exhibit Jahn-Teller effects due to its C3v
geometry and concomitant degenerate X~ 2E electronic ground state (6), we wereunable to carry out our own calculations on (CH3)3CO using ab initio methods basedon the Born-Oppenheimer approximation. However, recently Barckholtz and Miller
have outlined a method for ab initio calculations that includes coupling between
electronic and vibrational wave functions and is therefore able to compute
spectroscopically observable parameters of Jahn-Teller species (41). A calculationfor (CH3)3CO using this above method (42) corroborates the assignments proposed
above (see Table 4.5). It predicts the symmetric a1 CCC umbrella mode ν6 at 389

131

Photoelectron counts(CH3)3CO

a

-

d

b

e
g

2.5

2.4

2.3

2.2

f

c

2.1

2.0

1.9

1.8

Electron binding energy (eV)Figure 4.6. Magic angle photoelectron spectrum of (CH3)3CO− taken at 200K sample
temperature in bold. The thin line corresponds to a Franck-Condon simulation of thephotoelectron spectrum in a simple harmonic oscillator approximation using
experimental frequencies (Method A). See text for details.

132
Table 4.5. Peak positions and assignments for the photoelectron spectrum of(CH3)3CO−.
Peak
Position (cm-1) Assignment
hot band-440 ± 25

6'10

a

0

000

b

435 ± 15610c855 ± 25620

d1205 ± 15

1810e1625 ± 45

1810 610f

2080 ± 30

1810 620g2410 ± 30

1820

cm-1 and the a1 mode ν18 at 1234 cm-1 which is the CO stretching motion. In theanion, mode ν′6 corresponds to the same CCC umbrella motion as the radical mode
ν6, and is predicted at the slightly lower frequency of 381 cm-1.

A peak wasmeasured from the room temperature photoelectron spectrum (not shown) at 440
cm-1, which we assign as a hot band resulting from excitation from 6'10 .

Theobservation of the CO stretch is expected for the same reason discussed in(CH3)2CHO.
Figure 4.6 displays a spectral simulation created using Method A.

Theexperimental frequencies of 435 and 1205 cm-1 are input to produce the displayed
curve. The agreement is surprisingly accurate considering the relative crudeness ofthe simulation for such a theoretically complicated species.
discussed further below.

This point will be

133

4.8

Vibronic coupling and Jahn-Teller effects

As was stated above in the discussion of ethoxy radical, the majority of thevibrational features in the CH3CH2O− photoelectron spectrum exhibit irregularfrequency and intensity patterns that the reasonably high-level calculations andsimulations fail to accurately reproduce. This has been attributed to the vibronic
coupling between the ground and first excited states in the radical, separated by only
355 cm-1. In the photoelectron spectrum of (CH3)2CHO−, however, regularly spaced
series of peaks are observed that, with the help of theoretical simulations, can be
explained and assigned. In (CH3)2CHO, the excitation to the first excited state (1225
cm-1) is approximately three times greater than in CH3CH2O; therefore the degree of~ states of (CH ) CHO would be expected to becoupling between the X~ and A
3 2

smaller. In addition, however, one must consider the magnitude of the frequencies
involved with respect to the energy splitting between the interacting electronic states.
In (CH3)2CHO, the majority of the assigned peaks--b, c, d, and possibly e--fall at~ state begins. Therefore, vibronic couplingenergies less than the energy where the A

effects would be expected only for peaks at greater binding energies than peaks e or f.
In ethoxy radical this is not the case. The splitting between the electronic states of theneutral is small enough that all of the observed features are subject to vibronic
coupling effects.

134
The existence of the signature of vibronic coupling upon the frequency andspacings of the photoelectron features can be explained, and this may be the reason
for the variation of β in the CH3CH2O- spectrum and not in (CH3)2CHO-.
The relative ordering of the a' and a" orbital energy is switched between
CH3CH2O and (CH3)2CHO, but both orbitals show the same pπ character. Therefore
there is no reason to first order why transitions to the ground and first excited states,
corresponding to detaching electrons from the a' and a" orbitals, would exhibit
different angular distributions, which is the case for (CH3)2CHO, but not forCH3CH2O. Thus the different angular distribution behavior for the two species must
be originating from another source. In the CH3O− photoelectron spectrum, electronic
state coupling is proposed to explain intensity in peaks that have different angular
distributions than the other peaks in the spectrum. It is possible that the same kind of
mixing can explain the angular distribution variation for peaks in the CH3CH2O−~ states of the radical, or
spectrum through coupling either between the X~ and A

between either of these states and a third electronic state.
The relationship of (CH3)3CO to CH3O with respect to the prominence of
Jahn-Teller effects in the photoelectron spectra is similar to that between CH3CH2O
and (CH3)2CHO with respect to vibronic coupling effects. The CH3O− photoelectron
spectrum is dominated by activity in the e asymmetric modes falling at positions
determined by the Jahn-Teller coupling parameters of the radical. In (CH3)3CO,
however, the major structure is assigned to harmonic progressions in symmetric a1
vibrational modes. Furthermore, the method used for simulation of the spectrum in
Figure 4.6 only allows for optimization of a limited number of free parameters strictly

135
within the Born-Oppenheimer approximation and nevertheless provides a surprisingly
accurate simulation of the photoelectron spectrum. This does not mean, however,that there is no Jahn-Teller activity in the (CH3)3CO radical. A simulation of the
Jahn-Teller active modes in the (CH3)3CO spectrum indicates that the most prominent
Jahn-Teller transitions have an intensity of about 10 % of the origin transition (42).
Although (CH3)3CO is expected to be Jahn-Teller-active, the activity is predicted tobe small, which is what we observe in our spectrum.

The greater amount ofvibrational activity in the larger (CH3)3CO radical, as well as the resolution of ourexperiment, produces a baseline of intensity under which any Jahn-Teller transitions
may lie.

This may include transitions into pure e vibrational modes as well as any

combinations of symmetric and asymmetric modes. The fact that the gross structureof the (CH3)3CO− photoelectron spectrum may be assigned to regularly-spacedtransitions into symmetric vibrational modes characterizes this species more closelywith (CH3)2CHO than CH3O. Furthermore, the asymmetry parameter behavior of(CH3)2CHO and (CH3)3CO is similar because β is constant across the spectrum. In
light of these parallels, it may prove more fruitful to think of (CH3)2CHO as ageometrical perturbation away from the (CH3)3CO structure, where one methyl grouphas been substituted by a hydrogen.

4.9

Thermochemistry

The EAs for each neutral in this study are reported in Table 4.6. The EAs havebeen rotationally corrected and the error bars account for energy compression,

136
rotational broadening, and measurement fitting error.

The table lists previous

photoelectron spectrum determinations of the EA for comparison. In each case,agreement with the prior studies is good and the error bars have improved. In the
Table 4.6. Electron Affinities (EA) in eV. Values have been rotationally corrected.
See text for details.
Species

EA(eV)

Literature Value

Reference

CH3O

1.572 ± 0.004

1.568 ± 0.005

(36)

CD3O

1.559 ± 0.004

1.551 ± 0.005(36)

CH3CH2O

1.712 ± 0.004

1.715 ± 0.007(26)

CD3CD2O

1.699 ± 0.004

1.703 ± 0.007(26)(CH3)2CHO

1.847 ± 0.004

1.839 ± 0.029(25)(CH3)3CO

1.909 ± 0.004

1.912 +−00..029
054

(25)

case of (CH3)2CHO and (CH3)3CO, the error bars have been improved dramatically,
by a factor of at least seven.
As was outlined in Chapter 3 and Eq. 3.9, the RO EAs from this work may be
used together with literature values of the gas-phase acidity (GPA) of ROH (26,4349) and the ionization potential of the hydrogen atom in a thermochemical cycle todetermine the bond dissociation energy, D300(RO-H) (50), where R = CH3, CD3,
CH3CH2, CD3CD2, (CH3)2CH, and (CH3)3C. Because the ionization potential of thehydrogen atom and the RO EAs were measured at 0 K, and the GPAs of ROH were
measured at 300 K, a thermal correction to the bond dissociation energy wasincorporated into the values calculated in Table 4.7 (50) (This correction has been
illustrated in the Appendix).

The GPAs chosen were the directly determined

137
experimental values with the best error bars, where available, and those used for R =CH3, (CH3)2CH, and (CH3)3C are not significantly different than those measured inTable 4.7. Derived thermochemical data. Values were computed with literature
values for ∆acidH300(ROH) and electron affinities from this work as outlined in Ref.
(50). Numbers in parentheses are offered as alternative values originating fromindependent determinations. All values are in kJ/mol.
R∆acidH300(ROH)

D300(RO-H)∆fH300(RO)∆fH300(RO−)∆fH300(RO+)

CH3

1600 ± 2a

440 ± 321 ± 4-138 ± 2

1060 ± 10

CD31597 ± 8b

436 ± 9---CH3CH21586 ± 5c

441 ± 6-12 ± 7-185 ± 5980 ± 15

CD3CD2

1582 ± 9d

436 ± 10---

(CH3)2CH

1578 ± 3e445 ± 4-46 ± 5-231 ± 4840 ± 10447 ± 4-83 ± 5-274 ± 4810 ± 10(1576 ± 4c)(CH3)3C

1575 ± 3e(1573 ± 3c)aRef. (43)
Ref. (44)cRef. (48)dRef. (26)eRef. (45-47,49)b

DeTuri et al. (48). The calculated bond dissociation energy values (see Table 4.7)
agree well with those of DeTuri et al. (48), who have used the EAs reported in thiswork with their GPAs to calculate bond dissociation energies. We find that within
the given error bars, the energy needed for the reaction ROH → RO + H is relatively
independent of R, where R is an aliphatic hydrocarbon.
In addition, literature values for ∆fH300(H) and ∆fH300(H+) (51), ∆fH300(ROH)(52), and IP0(RO)(53-55)

allow calculation of the heats of formation for the

undeuterated RO, RO−, and RO+ species according to the methods outlined in Ref.
(50) and in Eq. 3.9. The ∆fH300 (RO) and ∆fH300(RO−) values show a stabilization

138
effect with an increase in the size of R. This effect is greater in the anion than in theradical, a result of inductive effects in the anion. The differences in the amount ofstabilization in the anion with respect to the neutral may be examined most precisely
by looking at the differences in the EAs. The EA changes by the same amount whengoing from CH3O to CH3CH2O as when going from CH3CH2O to (CH3)2CHO, but
the difference between the EAs of (CH3)2CHO and (CH3)3CO is only half thatamount. This indicates that the degree of perturbation of the system decreases whensubstituting the last H in CH3O for a methyl group.
It is also interesting to note that the same decrease in EA, 0.013 eV, isobserved upon deuteration for both CH3O and CH3CH2O.

This implies that

deuteration decreases the zero point energy by a greater amount in the radical than in
the anion.
The heat of formation of the cation also displays a stabilization effect withincreasing size of R. It should be noted that ∆fH300(RO+) is highly dependent on thevalue of IP0(RO), and in the case of (CH3)2CHO and (CH3)3CO, the IP0 values used
are the only available measurements and have not been confirmed recently.

4.10

Conclusion

Photoelectron spectra of alkoxides were presented and analyzed. Jahn-Teller
effects dominate the CH3O− and CD3O spectra, with peaks of contrasting β value
carrying different vibronic symmetry assignments. The splitting between the ground~ 2A' states of CH CH O is observed directly at 355 ± 10
X~ 2A" and first excited A
3
2

139
cm-1.

This close spacing causes vibronic coupling between the two states that

perturbs vibrational levels; as a result, the ability of theoretical calculations toeffectively explain the vibrational structure of CH3CH2O is hindered.

Both the(CH3)2CHO− and (CH3)3CO− spectra show good agreement with theoretical~ 2A" states
predictions. The splitting between the ground X~ 2A' and first excited Aof (CH3)2CHO is determined to be 1225 ± 65 cm-1. No vibronic coupling is observed
directly for (CH3)2CHO, as is true for Jahn-Teller activity in (CH3)3CO. There is noevidence of photodissociation of the species in any of the spectra. Finally, theelectron affinities of all six neutrals (RO) were measured, showing good agreementwith previous measurements, and in some cases to much greater accuracy. Values ofD300(RO-H), as well as the heat of formation for the alkoxide neutrals, anions, andcations are tabulated. We also see that although the ROO peroxides are larger
molecules than the alkoxy RO radicals, it is the RO spectra that are more difficult toanalyze and understand. This is due to the inherently bent ROO shape that avoids anyof the Jahn-Teller degeneracy issues associated with a high symmetry C3v geometricalconfiguration. The final chapter examines the spectra of diatomic metal oxides,
which are the smallest molecules considered in this thesis, although they have also
proven the most challenging to understand, and metal dioxides.

140

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144

Chapter 5
Photoelectron spectroscopy of Group VIII transition metal oxides XO−,OXO− (X = Ni, Pd, and Pt)

5.1

Introduction

Metal oxides play a central role in a variety of fields including hightemperature materials, catalysis, semiconductors, and electrode materials (1-8). Thus,
understanding their properties has both technological and fundamental importance.
Despite this importance, it is remarkable how relatively little information exists about
the basic structure and properties of Ni, Pd, and Pt metal oxides, with the majority of
the literature concentrating on the lighter metal oxides. The open d, p, and s shells
give rise to an enormous complexity in the energy level structure that provides adaunting challenge to theorists and experimentalists alike.
For NiO, several experimental spectroscopic studies (9-16) have identified a3 −Σ ground state, measured the vibrational frequency, and measured the electron

affinity (15,16), among other observations. An essentially Ni+O− ionic picture of theNiO ground state has emerged (11,17). This ionic framework is desirable because it
allows the electronic properties of the molecule to be treated, to first order, as those of
the atomic oxygen, which is well-understood. The experimental work has been
supported by various theoretical studies of NiO ground and excited electronic states(17-23).

145
Much less is known for PdO and PdO−. Several theoretical investigations(17,24-29) have concluded that either the 3Σ− or the 3Π is the PdO ground state, bothlying close in energy.

Theoretical studies have found relativistic effects lessimportant for PdO than PtO (26,29).

PdO is calculated to be quite ionic (17),although covalent bonds are formed.

The photoelectron spectrum of PdO− wasreported recently (30) where the negative ion was created in a laser ablation source.
For PtO, more spectroscopic data are available than for PdO although there
are no anion photoelectron spectra. The electronic ground state of PtO was reported
to be 1Σ+ in early spectroscopic studies (31-33). Later, it became clear that the groundstate is a 3Σ− state with a low-lying, related state only 936 cm-1 above the ground state(34). Scullman and co-workers (35,36) reported transitions to several uv-vis states ofgaseous PtO, but have no information on the states lying 0-2 eV above the groundstate. Theoretical efforts have primarily focused on the consequences of relativistic
effects on the molecular geometry and other physical properties (26,37).
Information on the metal dioxide species ONiO, OPdO, and OPtO is scarce.
Experimental data exist for ONiO and OPtO (14,15,38), and a few theoretical studies
have calculated molecular structures and electronic properties (28,39,40). Several
photoelectron spectroscopic studies have been carried out on similar metal molecules,
such as CuO− (41,42), RhO− (43), PdC2H− and PdC2HN− (44), PdCO− and PdCN−(45).
One of the main difficulties associated with analysis of these metal oxide
species lies in their inherent electronic structure (46). The valence configurations of
the metal atoms include many open-shell d orbitals, which are spaced closely spaced

146
in energy and also close in energy to the valence s orbitals. Upon bonding withoxygen, the result is a molecule with a high density of valence energy levels, often
rendering electronic spectra quite crowded and thus difficult to sort out. Furthermore,
the proximity of energy states leads to mixing of the orbital characters, which resultsin spectroscopic properties that bear little resemblance to a simple zero-order picture.
All these reasons render theoretical calculations difficult, even without the additional
complication of relativistic effects. These include radial contraction of the s and pelectrons along with the radial expansion of the outer d electrons, as well as spin-orbit
splitting (47). The magnitude of these effects varies as Z2, and therefore becomesdominant by the 6th row metal oxides.
The 364 nm photoelectron spectra of XO− and OXO− molecules are presented(X=Ni, Pd, and Pt). The electron affinities (EAs) for all six molecules are identified,
as well as vibrational frequencies and bond length changes.

Several low-lying

electronic states of the neutral and anion XO molecules are assigned, which allow formeasurements of anion and neutral spin-orbit splittings.

While the assignments

reported here differ from some of the earlier photoelectron findings, the differences
arise primarily from the better energy resolution available in the present apparatus.
Vibrational frequencies for the OXO molecules are measured, as is the bond lengthchange of OPtO− upon electron detachment.

The measurements obtained here

provide a much improved although still incomplete description of the low-lying statesof these metal oxide molecules.

5.2

Experiment

147

As was discussed in detail in Chapter 2, the metal oxide anions were producedin the sputter source connected to the flowing afterglow.

Metal oxide ions areproduced downstream from the sputter source by adding a small amount of oxygen
into the flow tube. Platinum is the easiest metal to sputter, producing ion currents ofabout 80 pA for PtO−. NiO− was by far the most difficult to obtain, with our best
efforts producing < 10 pA. Typical metal dioxide currents, however, were roughly
100 pA. The hot anions produced in the sputter source undergo collisional relaxation
with the He buffer gas in the flow tube. However, spectra were also collected withthe flow tube cooled by liquid nitrogen, which substantially lowers the anion
vibrational temperature.

In this way, spectra were collected at both 300 K (nocooling) and 200 K (liquid nitrogen-cooled) flow tube temperatures.
To assist in the interpretation of the spectra, we carried out ab initio
calculations for the electronic ground states of the OPtO anion and neutral species.
The calculations employed the hybrid density functional theory (DFT) method
B3LYP, and the SDD effective core potential/basis set was used for Pt while the augcc-pVTZ basis set was used for oxygen, as implemented in the GAUSSIAN 98 suiteof programs (48). In addition, a harmonic-oscillator Franck-Condon program wasused to simulate the vibrational progressions in the photoelectron spectra (49).
Vibrational frequencies, origin peak position, and geometry differences between
anion and neutral are directly determined from this simulation, without recourse to ab
initio calculations.

148
5.3

Analysis of NiO, PdO, and PtO

5.3.1

NiO

An orbital diagram of the ground state of NiO is presented in Fig. 5.1.
Although this is oversimplified and cannot be interpreted too literally (46), it canserve as a zero-order picture of the molecular orbitals to aid in analysis of the NiO−
photoelectron spectrum. The orbitals are formed from the 3d8 and 4s2 orbitals of Ni
with the 2p4 orbitals of O. The result is bonding σ2π4 orbitals, nonbonding δ4orbitals, and weakly antibonding dσ2 and π2 orbitals. There are also nonbonding 4sσ2
orbitals formed from the valence 4s orbitals of Ni. There is a consensus in theliterature that the ground state of NiO is a σ2π4δ4π2σ2 3Σ− state (9-11,17-20,50) with
substantial ionic character. To form the anion NiO−, the extra electron fills one of theantibonding π orbitals to make a 2Π3/2 state. In the 3.408 eV photoelectron spectrumof NiO−, we expect to observe several low-lying neutral NiO states throughdetachment of any one of the δ4π3σ2 electrons. In general, detachment of sσ electrons
is expected to produce intense photoelectron peaks due to the fact that their diffuse
spatial distributions yield a large cross-section for detachment. However, because thedσ and sσ orbitals are expected to be close in energy, they can mix and it is notunreasonable in these molecules to expect a large intensity from dσ electron
detachment as well.
The 364 nm photoelectron spectrum of NiO− taken under 200 K flow tube
conditions and 0° polarization direction is displayed in Fig. 5.2. The spectrum shows

149

X! NiO 3Σ−
4s

sσ
dσ
π
2p

δ

3d

π
σ

Ni 3d8 4s2

NiO

O 2p4

Figure 5.1. Molecular orbital diagram of the 3Σ− ground state of NiO showing themixing of atomic O and Ni orbitals.

150

N

-

NiO

O P

D

Photoelectron Counts

MLK J
W
HG F E

VU

S
T

C B A

R
Q
I

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

Electron Binding Energy (eV)Figure 5.2. 364 nm photoelectron spectrum of NiO− taken at 0° laser polarization
angle and 200 K flow tube temperature.

151
many sharply resolved features, labeled A through W, with a large variation in
intensity. There is a high density of peaks within about 1.5 eV, indicating transitions
into several low-lying electronic states in neutral NiO.

The β parameters were

measured for every peak in the spectrum. With two exceptions, all the peaks in the
spectrum yield β values of > 1. The exceptions are peaks I and Q, which both have
an isotropic angular distribution, i.e., β = 0.
There have been two recent photoelectron spectroscopy studies of NiO−, one
by Wu and Wang (15) and one by Moravec and Jarrold (16). Both studies employ
pulsed laser ablation ion sources coupled with time-of-flight electron kinetic energy
analysis. The energy resolution is not constant over the spectrum energy range, butvaries between roughly 10 and 50 meV.

For metal oxides, NiO in particular withsuch a great density of peaks, the resolution results in blending several peaks
together, eliminating many of the features observed here or changing their appearance
or relative intensities. The result is that a full assignment of the spectrum can be very
difficult. The spectra reported here are generally consistent with the earlier data
(15,16), but the higher resolution permits observation of many new features thatnecessitate alternative interpretations. The differences in assignments are discussed
below.
Peak N is the most intense peak in the spectrum and is located 0.485 eV above
peak D.

Peak P falls nearby at 0.536 eV above peak D. As was stated above, σelectron-detachment is expected to yield the highest intensity peaks. In this case, σelectron detachment from δ4σ2π3 NiO− gives a 3Π state. In a fluorescence detection
experiment, Friedman-Hill and Field (11) measure the 3Π2 state falling at 0.484 eV

152

Table 5.1. Absolute and relative positions, and assignments for peaks in thephotoelectron spectrum of NiO−.
Peak
Position (eV)Relative Position
Assignment
-1
(cm )
A
-910 ± 15
X! 3Σ− ← A! 'B
-650 ± 10
X! 3Σ− ← A!
C
-275 ± 10
X! 3Σ− ← X! ' 2Π1/2
D
1.455 ± 0.004
E
190 ± 10
X! 3Σ− (v=1) ← A!
F
550 ± 20
X! 3Σ− (v=1) ← X! ' 2Π1/2
G825 ± 10
X! 3Σ− (v=1) ← X! 2Π3/2
H1055 ± 20
X! 3Σ− (v=2) ← A!
I

1.768 ± 0.004−−
3

1.991 ± 0.004-885 ± 10-635 ± 10-500 ± 10-230 ± 100125 ± 10405 ± 10Q

2.103 ± 0.004

−

−

R
S
T

2.404 ± 0.004

0575 ± 101130 ± 10

3UVW
2.706 ± 0.004
a
See text for details.

-640 ± 10-265 ± 10
0

1

J
K
L
M
N
O
P

1.940 ± 0.004

Π2 ← A! 'Π2 ← A!
3
Π1 ← A! '3
Π1 ← A! and 3Π2 ← X! ' 2Π1/2 a
3
Π2 ← X! 2Π3/2
3
Π1 ← X! ' 2Π1/2
3
Π1 ← X! 2Π3/2
3

Φ4, 3∆3, or 3Π2 ← X! 2Π3/2 a3
Φ3, 3∆2, or 3Π1 ← X! 2Π3/2 a3
Φ2, 3∆1, or 3Π0 ← X! 2Π3/2 aΠ ← A! 2Π3/2
1
Π ← X! ' 2Π1/2
1
Π ← X! 2Π3/2

153
Table 5.2. Summary of assignments for NiO and NiO−. All values are from thispaper unless otherwise stated.
re (Å)
Relative
Te (eV) ν(cm-1)
Energy(eV)−2-1.455(4)01.67(1)
NiO
X! Π3/2a-1.470(3)-1.46(2)b0.034
X! ' 2Π1/2
0.081
A!
0.113
A! '
NiO

3 −

Σ

0

0

825
840a
800b
839.69c*825.74d825.7e1.627g?

0.313

1.67(1)

3

Π2

0.485
0.484c1.67(1)

3

Π1

0.536
0.533c0.532a1.67(1)?

0.648

1.67(1)

3

0.949
1.020
1.089

1.67(1)1.67(1)1.67(1)

1.251
ΠRef. (16) (Anion photoelectron spectroscopy).
b
Ref. (15) (Anion photoelectron spectroscopy).
cRef. (11) (LIF).
d
Ref. (9) (Matrix IR).
eRef. (14) (Matrix IR).
f
See text for details.
gRefs. (10,12,13).
* ωe1.67(1)

Φ4, 3∆3, or 3Π2 f
Φ3, 3∆2, or 3Π1 f
3
Φ2, 3∆1, or 3Π0 f
3

1a

excitation energy above the ground state. Likewise, they measure the 3Π1 state at
0.533 eV. Both these measurements coincide with the spacing between peaks N and

154
P and peak D and securely fix the assignment of the 3Π2 and 3Π1 and 3Σ− states,
respectively (see Tables 5.1 and 5.2). Therefore, peak D is assigned as the electron
affinity transition, with EA(NiO) = 1.455 ± 0.004 eV. This agrees with the value of
Wu and Wang (15) at 1.46(2) eV and qualitatively agrees with that of Moravec andJarrold (16) at 1.470(3) eV. Moravec and Jarrold also find the 3Π1 electronic state at
0.532 eV excitation energy, in agreement with our assignment. Wu and Wang assign
this band to the 3Π NiO electronic state. However, they put the 3Π1 state at 0.49 eV
excitation energy and the 3Π0 state at 0.55 eV excitation energy, in conflict with theFriedman-Hill and Field (11) measurement cited above. This results in assigning
their peak at 0.43 eV excitation energy as the 3Π2 state, as opposed to the anion
excited state assignment offered below. The spacing between the 3Π2 and 3Π1 peaks,
405 ± 10 cm-1, gives the neutral NiO spin-orbit splitting (see Table 5.7). We do not
observe the 3Π0 ← 2Π3/2 transition, which could be attributed to the spin-orbit valuechanging by more than ½.
Peak G, at a third of the intensity of the electron affinity peak, is 825 cm-1 tothe higher binding energy side of peak D (Table 5.1). This value coincides with thevibrational frequency values measured in Argon matrix-isolated NiO (ν = 825.74
cm-1 from Ref. (9) and 825.7 cm-1 from Ref. (14)) and ω = 839.69 cm-1 measured inRef. (11), and the photoelectron spectrum measurements of ν = 840 cm-1 (16) and ν =800 cm-1 (15). Peak G, therefore, is assigned as the first quantum of the NiO stretch.
Using a Franck-Condon fitting routine (49), a 0.04 ± 0.01 Å change in bond length isfit to the v=0 and v=1 peaks assuming a 1.627 Å measured neutral bond length(10,12,13). Because the detached electron comes from an antibonding orbital, the155anion bond length is expected to be larger than the neutral bond length. This results
in a Ni-O− separation of 1.67 ± 0.01 Å. As will be discussed below, peak G is the
only manifestation of vibrational activity in the NiO− spectrum.

Therefore, theremaining peaks in the spectrum must be assigned to individual electronic states of
NiO where the neutral bond length is nearly equal to the anion bond length of 1.67 Å(see Table 5.2).
The intensity of the (v=1) 3Σ−← (v′=0) 2Π3/2 peak (G) relative to the origin
peak is determined by the magnitude of the 〈1| 0〉 2 Franck-Condon factor as
explained in Eq. 1.5,
I (1, 0)
= 〈1| 0〉 2 .
I (0, 0)

(5.1)

Assuming a harmonic-oscillator approximation means that 〈1| 0〉 2 = 〈0 |1〉 2 .
The intensity of the (v=0) 3Σ−← (v′=1) 2Π3/2 peak relative to the origin peak depends
on both the 〈0 |1〉 2 factor as well as the relative populations of the anion v=0 and v=1
states:I (0,1) N (v = 1)=
〈1| 0〉 2 .
I (0, 0) N (v = 0)

(5.2)

Given the fact that the intensity of peaks A-C is comparable to that of peak G, thismeans that if peaks A-C originate from vibrational hot bands, thenI (1, 0) I (0,1).
"
I (0, 0) I (0, 0)(5.3)

Equating Eqs. 5.1 and 5.2 implies that the population needed in the anion v′=1 state
would have to be on the same order as that in the v=0 state, which is not at all likely
in our ion source. Thus the intensity of peaks A-C is much too great to suggest they

156
originate from vibrational hot bands, as was assigned in Refs. (15) and (16). Thesepeaks must result from transitions from electronically excited NiO−. Furthermore,
Peak C is 275 cm-1 from the EA peak, which is simply too small a frequency to be a
NiO− vibration. NiO has considerable ionic character and therefore the anion may beconsidered Ni−O− (11). Therefore the spin-orbit splitting would be expected to be on
the order of the O− spin-orbit splitting, which is about 180 cm-1 (51) as opposed to theNi− spin-orbit splitting, which is around 1500 cm-1 (52).

The 275 cm-1 splitting

observed here (Table 5.7) is in the range of what would be expected for the 2Π3/2 −2

Π1/2 energy spacing of NiO−. Thus peak C is assigned as the

3 −

Σ ←

X! ' 2Π1/2transition.
Peaks A and B, therefore, must result from transitions originating in anotheranion excited electronic state. Peak B is 650 cm-1 from the electron affinity peak
(Table 5.1) and is assigned as an unspecified A! state in the anion. Peak A is 260
cm-1 to the lower binding energy side of the 3Σ− ← A! transition, which is similar to
the anion 2Π3/2−2Π1/2 spacing. Thus it is possible that peak A is the spin-orbit excitedcounterpart of this 3Σ− ← A! peak. Peak A is assigned as an unspecified A! ' state in
the anion. One possible orbital configuration of this anion excited electronic state A!
is (Fig. 5.1) the excitation of two dσ2 electrons to sσ2 electrons, which would produce
a 2Π3/2 state with a δ4π3sσ2 configuration. This excited electronic state should have a2

Π3/2 − 2Π1/2 energy spacing on the same order as that of the anion ground state

splitting, as is what we observe here.

Other possibilities include the spin-orbit

157
components of dσ1 sσ1 2,4Π states. With the information available to us, a definite
assignment is not possible.
Peak E is 190 cm-1 from the EA peak, which is too small to be the vibrationalfrequency of NiO. Instead, it is assigned to 3Σ− (v=1) ← A! (Table 5.1). Peak F is825 cm-1 from peak C, which is the fundamental of the NiO vibration assigned above,and thus is attributed to the X! 3Σ− (v=1) ← X! ' 2Π1/2 transition. Peak H is 1055 cm-1
from peak D. This is most likely the X! 3Σ− (v=2) ← A! transition (825 x 2- 650 =1000), although because peak H is somewhat ragged, there is some room for error in
the interpretation.
Peak I, at a 0.313 eV term energy (Table 5.1), has an isotropic angular
distribution (β = 0), in contrast to peaks A-H and J-P (β >1). Thus it must constitute a
separate electronic state. The relatively low intensity suggests detachment from a δorbital. Walch and Goddard (18) calculate a 1∆ as the first excited state, althoughlater calculations (53) place this state at much higher energies. Our data do not
permit assignment of this state.
Peak K is 635 cm-1 to the lower binding energy side of the NiO 3Π2 peak (N).
This is the NiO− 2Π3/2 − A! spacing. Peak K is about half the intensity of peak N and,along the same line of reasoning outlined above, is too great to originate from avibrational hot band and so it must come from an excited electronic state. Peak K is
assigned as the 3Π2 ← A! transition. Peak J falls 250 cm-1 to the lower binding energy
side of peak K, which is the anion A! − A! ' spacing. Peak J is assigned as the 3Π2 ←A! ' transition (Table 5.1).

158
Peak M is 635 cm-1 from the NiO 3Π1 peak (P). Because this is the NiO− 2Π3/2− A! spacing, peak M receives the assignment 3Π1 ← A! . However, peak M is about¾ the intensity of the NiO 3Π1 peak, compared with the relative 3Π2 ← A! and 3Π2←2Π3/2 intensities. Peak M also sits 230 cm-1 to the lower binding energy side of the3

Π2 peak (N) and there is a shoulder to the right of peak M. This shoulder is the 3Π2← X! ' 2Π1/2 transition. The two lines are close enough that their intensities both add
into peak M, accounting for its additional intensity. Peak L falls 270 cm-1 from peakM (3Π1 ← A! ) and 905 cm-1 from peak P (3Π1 ← X! 2Π3/2). Peak L receives theassignment 3Π1 ← A! ' . Peak O is 280 cm-1 from the 3Π1 ← X! 2Π3/2 peak and thus isassigned as 3Π1 ← X! ' 2Π1/2 (Table 5.1).
Peak Q appears 0.648 eV above the EA transition. This peak has a contrasting
isotropic (β = 0) angular distribution from peaks J through P, and thus arises from adifferent electronic state.
3

Furthermore, the intensity is much less than peaks in theΠ state, ruling out detachment of a σ electron to form this peak. As was the case forpeak I, the relatively small intensity suggests detachment from a δ orbital. A possible
assignment is the 3Φ state that Bauschlicher et al. (17) calculate at 0.720 eV,
produced from detaching a non-bonding δ electron.

The fact that the detachedelectron would be non-bonding correlates with the lack of vibrational structure
observed. Moravec and Jarrold (16) assign this peak as the 3Φ3 state, and they find asecond peak at 0.77 eV excitation energy that they ascribe to the 3Φ2 state. This latterpeak is absent in our spectrum, and casts doubt on the 3Φ assignment. The 1∆ state
seems more likely, but without further theoretical comparison, peak Q is unassigned.

159
Peak R is measured at 0.949 eV excitation energy, with peak S at 1.020 eV
and T at 1.089 eV. Both Moravec and Jarrold (16) and Wu and Wang (15) observe
this group of peaks, and assign them as a vibrational progression in a 1∆ NiO state.
Although they are equally spaced, these three peaks are not likely to be a vibrational
progression, however. Such an extended vibrational progression would necessitate asubstantial change in bond length (> 0.1 Å).

Even though a relative lack ofvibrational activity is observed in the rest of the spectrum (only the ground state has
any, with only a single quantum of excitation observed), this would not be
impossible. However, such a bond length change would be expected to result inpossibly another peak between peaks T and U, and would produce significant hotband excitation, neither of which we observe. These peaks are therefore assigned astransitions into a triplet NiO state.
The intensity of peaks R-T is smaller than most of the other peaks in the
spectrum, which rules out σ electron detachment for the formation of these peaks.
Bauschlicher et al. (17) calculate a 3∆ state at 1.02 eV excitation energy that would
arise from π electron detachment. Another possibility is the 3Φ state created bydetaching a δ electron. Bauschlicher et al. (17) predict this to lie at 0.720 eV above
the 3Σ− state, and Bridgeman (23) calculates this state at 0.768 eV excitation energy.
Finally, Walch and Goddard (18) predict a 3Π state arising from a δ3π3σ2
configuration to lie approximately 1 eV above the ground state. Because more data
are needed to choose between these potential assignments, peaks R, S, and T may be
assigned to any of these three triplet states (Tables 5.1 and 5.2).

160
Peaks U, V, and W are a fairly intense ensemble of peaks with peak W 1.251
eV above the electron affinity peak (Table 5.1). One might expect that these peaks
come from a neutral triplet state, like the δ3π3σ2 3Π assignment in Moravec andJarrold (16) or the δ3π3σ2 3Φ assignment by Wu and Wang (15). However, this is not
the case. The intensity of peak W with respect to others in this spectrum suggests thatit arises from detachment of a σ electron, not a δ electron whose cross section for
detachment should be relatively small. Peaks V and U are spaced just as the hotbands seen for peaks D and N. Specifically, peak V is 265 cm-1 from peak W, whichis the same anion 2Π3/2−2Π1/2 splitting previously assigned, and peak U is 640 cm-1
from peak W, which is the anion 2Π3/2− A! energy difference (see Table 5.1). The 265
cm-1 and 640 cm-1 energy spacing is repeated in several places in the NiO−
photoelectron spectrum presented here.

However, the associated assignment ofelectronic (not vibrational) hot bands is truly anchored in peaks U and V, where only
simple inspection of their intensities is needed to rule out a vibrational assignment.
Therefore we assign peak W as the 1Π ← 2Π3/2 transition formed from detaching a σelectron. No spin-orbit splitting in the neutral state is observed, consistent with thesinglet assignment. Peak W is assigned as the 1Π ← 2Π1/2 state while peak V
corresponds to the 1Π ← A! transition.
A summary of results for the photoelectron spectrum of NiO− is given in
Table 5.2. In this spectrum we have identified transitions from four electronic statesof the anion, the ground X! 2Π3/2 state and its spin-orbit excited X! ′ 2Π1/2 state at 275
cm-1 (Table 5.7), plus two additional unspecified A! and A! ′ electronic states at higher

161
excitation energies. In neutral NiO, the ground 3Σ− is observed and the electron
affinity is 1.455 eV. The 3Π and 1Π states are assigned as well as another triplet state,either a 3Φ4,3,2, a 3∆3,2,1, or a 3Π2,1,0 state. The 3Π2 and 3Π1 spin-orbit spacing is 405
cm-1 (Table 5.7). A vibrational frequency of 825 cm-1 was found for the NiO groundstate, as well as a bond length change of 0.04 Å, yielding an anion bond length of
1.67 Å.

5.3.2

PdO

The 364 nm photoelectron spectrum of PdO− recorded at 200 K flow tube
temperature and magic angle polarization is shown in Fig. 5.3. This is an expanded
spectrum focusing on the lower binding energy range of the spectrum, although a fullscan was also taken at 200 K flow tube temperature (not shown). There are several
peaks of various intensities within a relatively small energy range, as was the case for
NiO−, although the density of peaks is considerably less than for NiO−.
The orbital configuration for PdO− may be treated as similar to that of NiO−
(see Fig. 5.1), with a σ2 π4 δ4 π3 σ22

Π3/2 ground state. Those calculating PdOelectronic states agree that the δ4 π2 σ2 3Σ− and δ4 π3 σ1 3Π states lie close in energy(17,24-28), but are inconclusive as to which is the ground state. Peak A is small
compared to others in the spectrum so it does not originate from the σ electron
detachment that would produce the 3Π state. Rather, the peak is produced from πelectron detachment, resulting in the 3Σ− state. Thus the 3Σ− ← 2Π3/2 transition is162

Photoelectron Counts

C

-

PdO

B

D

E

2.2

A

2.1

2.0

1.9

1.8

1.7

1.6

Electron Binding Energy (eV)Figure 5.3. 364 nm photoelectron spectrum of the onset region of PdO− taken at
magic angle polarization and 200 K flow tube temperature.

1.5

163
assigned to peak A (see Tables 5.3 and 5.4) and corresponds to the electron affinity,
EA(PdO) = 1.672 ± 0.004 eV.

Klopcic et al. (30) have recently reported a

photoelectron spectrum of PdO− using a laser ablation ion source and a time of flightelectron energy analyzer with roughly 5-50 meV energy resolution. The peaks intheir photoelectron spectrum qualitatively match ours with one exception. Theyobserve a peak at 1.570 eV binding energy that they take to be the electron affinity.
This peak is absent in our spectrum (see Fig. 5.3) and for reasons outlined below, we
differ in our assignment of the EA.
Peak B is located 0.095 eV above the EA transition (Table 5.3). It has a β of
0.8, which contrasts with that of peak A (0.4), and confirms that peaks A and B
represent two different PdO electronic states. Peak B, therefore, is assigned as 3Π ←2

Π3/2 . The approximate 100 meV energy difference between the 3Π and 3Σ− states inneutral PdO is consistent with calculations (17,24-28) that predict the two states to beclose in energy.

Furthermore, peak B is the second most-intense peak in the

spectrum; thus it is expected to result from the σ electron detachment necessary to
produce a 3Π state in PdO.
Peaks B, C, and D form a trio of intense peaks. They all have similar angular
distributions (β around 1) and are evenly spaced (about 0.1 eV between B and C and
C and D). They do not constitute a vibrational progression, however, for several
reasons. First, peaks B-D would make a relatively extensive vibrational progression.
If this were the case, we would expect to see vibrational hot bands in the spectrum
and we do not. Second, and more importantly, efforts to simulate these peaks as avibrational progression using a Franck-Condon fitting routine (49) failed. The bond

164
Table 5.3. Absolute and relative positions, anisotropy parameters (β), andassignments for peaks in the photoelectron spectrum of PdO−.
Assignment
Peak
Position (eV)Relativeβ-1
Position (cm )3 −A′-335 ± 20
Σ ← X! ′ 2Π1/2
A
0
0.4 (2) 3Σ− ← X! 2Π3/2
1.672 ± 0.004
B′
B
C′
C
D′
D
E′
E
F′
F

1.767 ± 0.004-340 ± 1501.867 ± 0.004-330 ± 1001.970 ± 0.004-345 ± 30
02.141 ± 0.004-390 ± 3002.236 ± 0.004-350 ± 150G′G2.454 ± 0.004
a
See text for details.

-350 ± 20030.8 (1)

3 −

Σ
Π2
3
Π1
3
Π0
1
∆, 1Σ, or 3Φ
1
Π3
Φ or 3Π
30

Π2 ← X! ′ 2Π1/2
Π2 ← X! 2Π3/2

Π1 ← X! ′ 2Π1/2
3
Π1 ← X! 2Π3/2
3

1.5 (1)Π0 ← X! ′ 2Π1/2
3
Π0 ← X! 2Π3/2
3

1.1 (1)∆, 1Σ, or 3Φ ← X! 2Π1/2 a
1∆, 1Σ, or 3Φ ← X! 2Π3/2 a1

1.1 (1)

Π ← X! ′ 2Π1/2
1
Π ← X! 2Π3/2
1

1.2 (1)

Φ or 3Π ← X! 2Π1/2 a
3
Φ or 3Π ← X! 2Π3/2 a3-0.2 (2)Table 5.4. Summary of assignments for PdO and PdO−.
Relative Energy Te (eV)(eV)
2
-1.672(4)
0
PdO−
Π3/2
2
0.041
Π1/2
PdO

3

0
0.095
0.195
0.298
0.469
0.564
0.782

165
length change necessary to approach simulating the height of peak C results in a large
v=3 peak that should appear between peaks D and E; no such peak appears in our
spectrum (see Fig. 5.3). So, while the approximately 100 meV separation between
peaks B, C, and D could equally well have represented either the PdO vibrationalfrequency or the spin-orbit splitting, the modeling rules out the former. Thus B, C,
and D are assigned to the 3Π2,1,0 ← 2Π3/2, respectively (Table 5.3). The neutral PdO3

Π2−3Π1 spin-orbit splitting is measured to be 805 ± 10 cm-1 (Table 5.7).
Peak E is located at 0.469 eV excitation energy (Table 5.3). Its intensity

eliminates an assignment as the 1Π state, which would result from detaching a σelectron and would have a relatively large cross-section for detachment. A possible
assignment is the 3Φ or 1∆ states formed by detaching a δ electron.

Alternatively,

Schwerdtfeger et al. (24) predict a 1Σ+ state at 0.48 eV excitation energy. The data do
not allow a selection between any of these possibilities. The small peaks to the higher
binding energy side of peaks C and D are left unassigned.
Peaks F and G are the last two peaks visible in the full scan of thephotoelectron spectrum of PdO− at 200 K flow tube temperature (not shown),
although they are out of the range of the close-up scan displayed in Fig. 5.3. They
are, however, visible in the 300 K flow tube spectrum in Fig. 5.4, discussed below.
Peak F, at 0.564 eV above the 3Σ−← 2Π3/2 (Table 5.3), has a comparable intensity to
the 3Π2,1,0 ← 2Π3/2 peaks and a similar β ( approximately 1). The height of the peak
as well as its angular distribution points to σ electron detachment in its formation.
Therefore peak F is assigned to the 1Π ← 2Π3/2 transition.

166
Peak G at 0.782 eV excitation energy has an essentially isotropic angular
distribution and is a relatively small feature in the spectrum. This suggests δ electron
detachment and possible final states include 3Φ and 1Φ or a 3Π or 1Π, depending on
which δ electron is detached. Hund’s rules would suggest that the triplet states would
lie lower in energy than the singlet states, and therefore a 3Φ or 3Π state is expected
for the assignment for peak G. However, without additional ab initio investigation,
the identity of this peak cannot be further specified.
The 364 nm photoelectron spectrum of PdO− taken at 300 K flow tube
temperature and magic angle polarization is displayed in Fig. 5.4. A comparison of
Figs. 5.3 and 5.4 shows that every feature A-E in the 200 K flow tube spectrum
doubles at a position of lower binding energy and at lower intensity. Comparing Fig.
5.4 and the full scan spectrum taken at 200 K flow tube temperature (not shown)
reveals the same for peaks F and G. These “new” peaks are labeled A′-G′ in Fig. 5.4.
They are obviously temperature dependent and must result from an anion excitedstate.

The intensities of peaks A′-G′ are too great with respect to their unprimed

counterparts to result from a vibrationally excited anion state especially due to thecomplete lack of transitions into vibrationally excited neutral PdO. Furthermore, thespacing between primed and unprimed peaks is 330 ± 10 cm-1 (choosing the spacing
between peaks C and C′ with the smallest error—see Table 5.3), which is too small
for the stretching vibration. Thus the primed peaks originate from an electronically
excited anion state, either the 2Π1/2 spin-orbit excited state or another low-lying
excited electronic state.

167

C

Photoelectron Counts

PdO-

C'

D

F

B'
F'

D'

G
G'

2.6

2.5

2.4

B

A A'

E E'
2.3

2.2

2.1

2.0

1.9

1.8

1.7

1.6

1.5

Electron Binding Energy (eV)Figure 5.4. 364 nm magic angle photoelectron spectrum of PdO− taken at 300 K
flow tube temperature.

1.4

168
The spin-orbit excited state is the most likely explanation. The ionic nature of
the Ni−O bond allowed a first-order estimate for the anion spin-orbit splitting (close
to the splitting of O−). That line of reasoning is not guaranteed to be applicable to
PdO. However, a different argument is proposed for why the 330 cm-1 spacing
between primed and unprimed peaks is expected as a spin-orbit splitting in PdO−.
The electron affinity of Pd− is around 0.5 eV. This is relatively low, testifying to thelow electronegativity of Pd.

Thus in the PdO− molecule, the extra electron would be

expected to be more localized on the oxygen atom and so the spin-orbit splitting of
PdO− would be closer to the value of atomic O− (approximately 200 cm-1) than thePd− value (around 3000 cm-1 (52)). The primed peaks are therefore assigned astransitions from the anion spin-orbit excited 2Π1/2 state.
In the photoelectron spectra of both NiO− and PdO− we identify anion 2Π3/2
and 2Π1/2 transitions into both 3Π and 1Π neutral states. The neutral spin-orbit
splittings, about 400 cm-1 for NiO and around 800 cm-1 for PdO (Table 5.7), correlate
well with the trends found for CuO (279 cm-1) (41) and AgO (269 cm-1) (54,55) for
two reasons. First, the CuO and AgO splittings are closer to the atomic oxygen spinorbit splitting (hundreds of cm-1) than that of the metal atom (thousands of cm-1), as is
the case for NiO and PdO. Second, there is not a significant difference in themagnitude of the splitting going from CuO to AgO; in NiO and PdO, although theneutral spin-orbit splitting does double, the values still are closer to the O− splitting
than those of the metal anion.
The assignments extracted from the photoelectron spectrum of PdO aresummarized in Table 5.4. Both the 2Π3/2 and 2Π1/2 PdO− states as well as PdO 3Π

169
and 1Π states are identified. The ground state of PdO was found to be 3Σ−, as in NiO,
and EA(PdO) = 1.672 eV. Two other excited PdO states were observed, one being
either a 1∆, 1Σ, or 3Φ state and one being either a 3Π or 3Φ state. No vibrational
activity was observed in the spectrum, unlike in the NiO− spectrum. The anion 2Π3/2− 2Π1/2 splitting is 330 cm-1 while the neutral 3Π2 - 3Π1 splitting is 805 cm-1 (seeTable 5.7).

5.3.3

PtO

The 200 K flow tube temperature photoelectron spectrum of δ4π3σ2 PtO−
recorded at magic angle laser polarization is displayed in Fig. 5.5. The spectrum
shows two intense peaks surrounded by multiple smaller features.

As was thereasoning in the photoelectron spectra of NiO− and PdO−, detaching a σ electron is
expected to have a large cross section and therefore yield an intense photoelectron
peak. As the orbital configuration for PtO may be treated as parallel to that of NiO
and PdO (Fig. 5.1), this corresponds to production of a PtO Π state. Thus peaks F
and K are transitions into Π electronic states. In analogy to the assignment in NiO−
and PdO−, peak F is assigned as the transition into the δ4π3σ1 3Π2 state.
It is natural to suppose that peaks G and H are transitions into the 3Π1 and 3Π0
neutral states; however, this is not the case.

Instead, peaks E, F, G, H, and I

correspond to vibrational levels of the 3Π2 ← 2Π3/2 transition. Because peak E, which
is 770 cm-1 from peak F, has an intensity that varies with the anion temperature, it is

170

K

PtO-

Photoelectron Counts

F

G

L
O

3.2

N
3.0

M

I

E

H

D
C

J
2.8

2.6

2.4

2.2

B
2.0

A
1.8

Electron Binding Energy (eV)Figure 5.5. 364 nm magic angle photoelectron spectrum of PtO− taken at 200 K flow
tube temperature.

171
Table 5.5. Absolute and relative positions, anisotropy parameters (β), andassignments for the photoelectron spectrum of PtO−.
Position
Assignment a
Peak Absolute RelativeβRelative to
Position
Position-1
Peak F (cm-1)
(eV)
(cm )
Π 2 ← 2 Π1/ 2

0-3510 (10)0.4 (2)3B855 (15)-2655 (10)0.3 (2)(v=1) 3 Π 2 ← 2 Π1/ 2

C-770 (15)-1695 (10)0.0 (2)0-930(10)0.0 (1)3 −+Σ0 ← (v=1) 2 Π 3 / 2
3 −+Σ0 ← 2 Π 3 / 2-770 (10)-770(10)0.9 (1)3Π 2 ← (v=1) 2 Π 3 / 2000.9 (1)3

Π 2 and 3 Σ1− ← 2 Π 3 / 2G

845 (10)845 (10)

1.1 (1)(v=1) 3 Π 2 ← 2 Π 3 / 2H1655 (10)1655 (10)

1.1 (1)(v=2) 3 Π 2 ← 2 Π 3 / 2I2450 (10)2450 (10)−(v=3) 3 Π 2 ← 2 Π 3 / 2J-765 (10)2815 (10)0.4 (2)3

Π1 ← (v=1) 2 Π 3 / 203580 (10)

1.1 (1)3

Π1 ← 2 Π 3 / 2L

780 (10)4360 (10)

1.2 (1)(v=1) 3 Π1 ← 2 Π 3 / 2

M1565 (10)5145 (10)−(v=2) 3 Π1 ← 2 Π 3 / 2

1.0 (2)−0.2 (2)−AD

1.851 (4)2.172 (4)

E
F

K

N

2.287 (4)2.731 (4)2.982 (4)O
3.157 (4)aAll states are v=0 unless otherwise specified.

172
Table 5.6. Summary of assignments for PtO and PtO−.
Relative
Te (eV)ν(cm-1)
Energy(eV)2−-2.172
0
770
PtO
Π3/2
2
0.321
Π1/2
PtO

3

Σ0−+

3

Π 2 and 3 Σ1−

0

re(Å)
rPtO−

0
0.115

845

rPtO− − 0.05(1)b0.116a3

0.559
780
rPtO− − 0.035(10)b
Π1
?
0.810
?
0.985
3 −
3 −
a
Measurement of Σ0+ − Σ1 splitting from Refs. (34) and (35).
b

See text for details.

assigned to the v′=1 transition from the anion (see Tables 5.5 and 5.6). Peaks G, H,
and I are transitions into the v=1,2, and 3 levels in 3Π2 PtO, respectively.

This

conclusion is verified in several ways. First, the angular distributions for peaks E-H
are equal (β around 1.0), indicating they all arise from detachment out of the same
orbital.

Second, the angular distributions of peaks A through D have contrastingvalues (β < 1 ) and thus are not assigned with peaks E-I.

In addition, if peaks G and

H were actually 3Π1 and 3Π0 states rather than the v=1 and v=2 of the 3Π2 state, thenthere is no observed vibrational excitation of PtO, rendering the 〈 2 | 0〉 2 and 〈1| 0〉 2
Franck-Condon factors both < 0.01.

The intensity of the (0,1) hot band relative tothe (0,0) origin is given by
I (0,1) N (v = 1)
N (v = 1)=〈1| 0〉 2 ≤ 0.01.
I (0, 0) N (v = 0)
N (v = 0)(5.4)

As the observed intensity ratio between peaks E and F is about 0.1, this implies

173
N (v = 1)> 10 ,N (v = 0)(5.5)

which is an unreasonable population in the excited state. Thus we are forced toconclude that peaks F-I form a vibrational progression in 3Π2 PtO.
To further support this conclusion, the intensities and positions of peaks E-I
are well reproduced using a harmonic oscillator-approximation Franck-Condon fitting
routine (49). The anion has a 770 cm-1 fundamental frequency, and the neutral
fundamental frequency is 845 cm-1 (Table 5.5).

These values are used in thesimulation to determine a Pt—O bond length shortening of 0.05 ± 0.01 Å uponelectron detachment from PtO−, which is larger than the ∆re measured in NiO.

The

detached electron is expected to be in an antibonding orbital (see Fig. 5.1), as issupported by the fact that the anion frequency is smaller than the neutral frequency.
Therefore the removal of the electron shortens the PtO bond length.
Because in the photoelectron spectra of NiO− and PdO− we observe transitions
into both the neutral 3Π2 and 3Π1 electronic states, we expect the same for PtO−.
Peaks J, K, L, and M are therefore attributed to 3Π1 ← 2Π3/2 transitions. Like the 3Π2← (v′=1) 2Π3/2 peak, peak J has an intensity dependent on the source temperature andthus is assigned as the v′=1 transition from the anion.

Peaks L and M represent

transitions into the v=1 and 2 levels in the neutral, respectively. Peaks K and L bothhave angular distributions of approximately 1.1. Like peaks E-I, both the intensities
and positions of peaks J-M are quite accurately reproduced using a Franck-Condon
fitting routine. In this case, the anion fundamental frequency is 765 cm-1 (identical tothat measured in the 3Π2 ← 2Π3/2 transition) while the neutral fundamental is 780 cm-1

174
(Table 5.5). Upon electron detachment the bond length shortens by an amount fit tobe 0.035 ± 0.010 Å. Again, the anion frequency is smaller than the neutral frequency,
although there is a smaller change (15 cm-1) than in the 3Π2 ← 2Π3/2 case (75 cm-1).
This agrees with the smaller bond length change found, as well as the fact that theobserved vibrational progression is less extensive than in the 3Π2 ← 2Π3/2 case. The
PtO 3Π2−3Π1 spacing, 3580 ± 10 cm-1, gives the neutral spin-orbit splitting (see Table
5.7).
Table 5.7. Anion 2Π3/2−2Π1/2 spin-orbit splittings and neutral3Π2−3Π1 spin-orbit splittings for the NiO, PdO, and PtO molecules.
23
Π3/2−2Π1/2
Π2−3Π1-1
NiO
275 ± 10 cm
405 ± 10 cm-1
PdO
330 ± 10 cm-1
805 ± 10 cm-1
PtO
3510 ± 10 cm-1
3580 ± 10 cm-1

As was the case for the PdO− photoelectron spectrum, there are peaks to thelower binding energy side of the 3Π2 ← 2Π3/2 peak with significantly smaller intensity
and contrasting angular distribution (< 1). Peak D is sharp and well-defined, while
peaks A-C are broad and less intense. The identity of the PtO 3 Σ0−+ ground state iswell established (26,33-36), which is the same 3 Σ − ground state of NiO and PdO.
The first excited state 3 Σ1− of PtO was observed 936 cm-1 above the ground state(34,35). These two PtO states are related to each other. The lower energy one is the0+ state while the other is the 1 state in Hund’s coupling case c (56), which together
correspond to the 3 Σ − ground state of PtO (35,36). Peak D is located 930 ± 10 cm-1
to the lower binding energy side of the 3Π2 ← 2Π3/2 peak (Table 5.5), which matches

175
the observed 3 Σ1− − 3 Σ0−+ splitting.

Peak F therefore is coincident with the second

half of the ground state, the 3 Σ1− peak.

Peak D is assigned as the electron affinity,

EA(PtO) = 2.172 ± 0.004 eV.
The intensity of peaks A-C relative to the 3Π2 ← 2Π3/2 peak varies with thesource temperature. These peaks originate from excited states in the anion. Peak C islocated 770 cm-1 to the lower binding energy side of the 3 Σ0−+ ← 2Π3/2 peak (Table5.5), which is the anion 2Π3/2 fundamental frequency. It also shares the same isotropic
angular distribution as the electron affinity peak. Peak C therefore is assigned as thetransition from the v=1 anion level to the 3 Σ0−+ PtO state. Peak A is located 3510 ±10 cm-1 from the 3Π2 ← 2Π3/2 peak, and peak B is the same distance from the (v=1)
3

Π2 ← 2Π3/2 peak. Peaks A and B, therefore, are assigned as transitions from theanion spin-orbit excited 2Π1/2 states into the v = 0 and 1 levels of neutral 3Π2 PtO,
respectively. By analogy, one would expect to see the (v=2) 3Π2 ← 2Π1/2 transition
and at first glance peak C would be the candidate. This is not completely the case,however. The (v=2) 3Π2 ← 2Π1/2 transition would be expected to have a smaller
intensity than peaks A and B, which is not true for peak C. Also, the spacing between
peak C and H is 3350 cm-1, 160 cm-1 short of the required spacing. However, there isa small shoulder to the lower binding energy side of peak C that fulfills the required
interpeak spacings and is of the expected intensity. Regardless, the (v=2) 3Π2 ←2

Π1/2 transition should have a small enough intensity (in analogy to the relative

intensities of the v = 0, 1, and 2 3Π2 ← 2Π3/2 peaks) that it blends into the structure ofpeak C.

176
Frum and co-workers (36) observed a 3Π state with a δ3π3σ2 configuration.
They measured a 3 Π 2 − 3 Σ0−+ splitting of 8935 cm-1 (1.108 eV). Therefore a peakcorresponding to this 3 Π 2 state (when peak D is the EA) should be at 1.108 + 2.172= 3.28 eV. In addition, a weaker transition to the 3 Π 0 level might appear at 3.19 eV.
However, the sensitivity of the electron kinetic energy analyzer drops off for binding
energies greater than about 3.2 eV. The fact that we do not see either of thesetransitions is a consequence of the instrument cutoff and therefore remains consistent
with the electron affinity 3 Σ0−+ assignment to peak D at 2.172 eV.
The electron affinity assignment given here is the one that allows for the mostconsistent analysis of the peaks based on literature values. We agree with the 936
cm-1 experimental 3 Σ1− − 3 Σ0−+ splitting of Sassenberg and Scullman (34,35) with thespacing between peaks D and F, and observe a repetition of that spacing between
peaks E and C. Furthermore, we are consistent with the measurements of Frum et al.
(36) discussed above. If peaks A, B, or C were assigned as the electron affinity, we
would disagree with either Frum et al. or Sassenberg and Scullman by substantial
amounts (> 100 cm-1). In addition, with peak A assigned as the 3Π2 ← 2Π1/2 hot bandtransition, the anion spin-orbit splitting of 3510 cm-1 is of comparable magnitude as
the spin-orbit splitting in the neutral PtO, as measured between peaks F and K, 3580
cm-1, which would be expected. Finally, the 0.115 eV splitting between the 3 Σ − and3Π 2 states in PtO is nearly equal to that in PdO (0.095 eV). The relative intensities

177of the 3 Σ − ← 2Π3/2 and 3 Π 2 ← 2Π3/2 transitions are also the same between the
spectra of PtO− and PdO− as are the relative angular distributions.
Peak N, at 0.810 eV excitation energy (Table 5.5), remains unassigned. PeakO sits at 0.985 eV excitation energy and has an isotropic angular distribution. This issimilar to peak G in the photoelectron spectrum of PdO− (Fig. 5.4) because bothpeaks are around 1 eV above the ground state and both show isotropic angular
distributions. Consequently, both should receive the same assignment, although it is
not clear what that assignment should be.
As summarized in Table 5.6, like the spectra of NiO− and PdO−, thephotoelectron spectrum of PtO− shows transitions from both the anion 2Π3/2 and 2Π1/2
states, and a 3 Σ − neutral ground state. PtO 3Π2 and 3Π1 states were also identified,and EA(PtO) = 2.172 eV. An anion vibrational frequency of 770 cm-1 was measured,as well as a neutral PtO stretch of 845 cm-1 in the 3Π2 state and 780 cm-1 in the 3Π1state. The bond length is measured to shorten by 0.05 Å for the 3Π2 ← 2Π3/2
transition while it shortens by 0.035 Å for the 3Π1 ← 2Π3/2 transition.
2

The anion

Π3/2 - 2Π1/2 splitting is 3510 cm-1 while the neutral 3Π2 - 3Π1 splitting is 3580 cm-1

(Table 5.7).

5.4

Discussion of NiO, PdO, and PtOSpectroscopic studies of NiO by Friedman-Hill and Field (11) show that the
ground state of NiO is strongly ionic and imply that the anion is formed by adding an178essentially nonbonding electron to Ni, giving a zero-order Ni – O− structure.

Forboth PdO and NiO, the observed spin orbit splittings in both the π3σ 3Π2 neutral andthe π3σ2 2Π anion are closer to the O− splitting than the metal spin orbit splitting (seeTable 5.7). This observation makes a very strong case that both NiO and PdO arestrongly ionic, with PdO somewhat less ionic than NiO, based upon its larger spinorbit splitting. However, the PtO− spin-orbit splitting of around 3500 cm-1 is moreaccurately described as roughly halfway between the atomic anion spin-orbit
splittings of O− (180 cm-1 (51)) and Pt− (around 9500 cm-1 (57)). Thus the PtO bondpossesses a significantly higher covalent character than either NiO or PdO.
A single peak in the photoelectron spectrum of NiO– is assigned as vibrational
excitation in the neutral ground state. Transitions into the several remaining NiOelectronic states display no vibrational excitation and thus originate from detachingessentially nonbonding electrons. For electron detachment from PdO–, no vibrational
excitation is observed indicating detachment of only nonbonding electrons. In
contrast, in the spectrum of PtO–, not only are two extended vibrational progressions
observed, but one of those originates from a larger ∆re than in the NiO– case. Theseobservations are a direct indication of the important relativistic effects on the valenceorbitals of PtO. The antibonding orbitals from which the anion electrons are detachedhave become more antibonding than in NiO and PdO. Furthermore, the relativistic
contraction of the metal atom s orbitals is expected to have two important effects.
One is that the reduction in size allows better mixing with the oxygen p orbitals,
rendering the sσ2 orbitals no longer non-bonding. Second, the relativistic expansionof the metal d orbitals in conjunction with this contraction of the metal s orbitals

179
allows for better mixing of the characters of the sσ2 and the HOMO dσ2 orbitals.
The shift of the metal 4s contribution from non-bonding to anti-bonding sσ2 orbitals
and the mixing of the sσ2 and dσ2 orbitals implies an increased influence of the metalon the HOMO electron properties, testifying to the significant increase in covalentcharacter of the PtO bond with respect to the NiO and PdO bonds.
As explained above, the covalent nature of the PtO bond means that the spin
orbit splitting in both the anion π3σ2 states and the neutral

π3σ states should

approach the mean of the splittings for the individual atoms. It is for this reason thatthe 3Π2 − 3Π1 splitting increases from 405 cm-1 in NiO, to 805 cm-1 in PdO, and to3580 cm-1 in PtO (see Table 5.7). Thus, peak K must correspond to the transition to
the 3Π2 state of PtO, rather than the 1Π state. The photoelectron spectra of NiO− andPdO− exhibit strong transitions to the π3σ 3Π2 and 3Π1 electronic states. If peak K
were assigned as the 1Π state, it would leave unanswered the question of where is the3

Π1 state, which must be both intense and accessible with 3.408 eV photons. Theseobservations are supported by previous theoretical findings.

Chung et al. (26)performed gradient-corrected density functional calculations incorporating relativistic
effects on NiO, PdO, and PtO. They find a change in bonding mechanism going from
NiO to PtO molecules. Despite the fact that PtO has many more protons, neutrons,
and electrons than PdO, the strong relativistic contraction in PtO results in a bondlength calculated to be shorter than that of PdO due to an enhanced covalence of thePt—O bond.

5.5

Analysis of ONiO, OPdO, and OPtO

180

5.5.1

ONiO

The 364 nm magic angle photoelectron spectrum of ONiO− at 300 K flow tube
temperature is show in Fig. 5.6. The spectrum shows relatively few features within
3.408 eV of available photon energy, resulting from the dropoff of energy analyzer
sensitivity for electron kinetic energies less than about 0.2 electron kinetic energy(eKE) (electron binding energies greater than approximately 3.2 eV). Peaks A and C
appear to be part of a Franck-Condon progression of peaks, with the much more
intense peak (A) being the origin. It is therefore assigned as the electron affinity at
3.043 ± 0.004 eV (see Table 5.8).

Recently, Wu and Wang (15) reported thephotoelectron spectra of both NiO2− and ONiO−. Our result agrees nicely with theirmeasurement of EA(ONiO) = 3.05(1) eV. Furthermore, Gutsev et al. (40) calculate
an electron affinity of EA(ONiO) = 3.25 eV, agreeing with the measurement
presented here.
Peak C is separated from the electron affinity peak by 745 cm-1. This agrees
with Wang et al. (15), who measure a vibrational frequency of 750 cm-1. Peak C isassigned as the fundamental in the symmetric stretch frequency of ONiO.
Peaks B and D are small features, each located to the higher binding energy
side of the X! v=0 and 1 peaks by 250 cm-1 each. Peaks B and D retain the same

181

Photoelectron Counts

A

ONiO-

C

B

D

a
b
3.20

3.15

3.10

3.05

3.00

Electron Binding Energy (eV)2.95

Figure 5.6. 364 nm magic angle photoelectron spectrum of ONiO− at 300 K flow
tube temperature.

2.90

182

Table 5.8. Electron affinity and vibrational frequency measurements and literature
values for the OXO 1Σ +g ← 2A2 OXO− (X=Ni, Pd, Pt) photoelectron spectra.
OXO

EA (OXO)(eV)aLiterature
(eV)

ONiO−
ONiO

3.043 ± 0.004

3.05 ± 0.01b
3.25c

OPdO

3.086 ± 0.004

Symmetric
Stretch ν(cm-1)
715 ± 15745 ± 10

Literature rX-O (Å)(cm-1)
750 ± 30b819c

680 ± 10

730d

OPtO−
760 ± 15
OPtO
2.677 ± 0.004 2.72 ± 0.20e 895 ± 10aThis work.
b
Photoelectron spectrum measurement: Ref (15).
c
DFT calculation: Ref. (40).
d
B3LYP calculation: Ref. (28).
eThermodynamic cycle determination: Ref. (38).
f
B3LYP calculation, unscaled, this paper.

870f
980f

rPt-O−
rPt-O− − 0.07(1)relative intensities in the cool sample ONiO photoelectron spectrum (not shown),
ruling out vibrational hot bands as a possible assignment. Peaks a and b to the lower
binding energy side of peak A vanish upon cooling the sample and are assigned as hot
bands. Peak b is 715 cm-1 from peak A, which is similar to the 750 cm-1 stretch
frequency assigned to the neutral. Peak b, therefore, is assigned as the symmetricstretch v′=1 transition from the anion. Peak a, at 425 cm-1 from peak A, could be an
excitation of an anion bending mode, or alternatively may be an excitation from theanion spin-orbit excited state.

Without further data to choose between these

possibilities, peak a remains unassigned.
It is expected that ONiO has a linear structure (58). Gutsev et al. (40)calculate a linear 1Σ +g ground state of ONiO with an 819 cm-1 symmetric stretch

183
frequency, which confirms our vibrational assignment (see Table 5.8).

They also

calculate a slightly bent 2A2 anion ground state with a 170° ONiO bond angle.

Thus

the calculations find both ONiO and ONiO− are essentially linear molecules, and it isreasonable to assume the same geometry for OPdO and OPtO molecules and theiranions. It is possible that peaks B and D therefore originate from transitions into twoquanta of a neutral low-frequency antisymmetric bending mode, which would be
Franck-Condon-allowed but not intense. Alternatively, Gutsev et al. (40) calculate alow-lying 3A2 ONiO state at only 560 cm-1 excitation energy. It is possible that peaks
B and D may be attributed to this state.

5.5.2

OPdO

The 200 K flow tube temperature 364 nm magic angle photoelectron spectrumof OPdO− is shown in Fig. 5.7. The spectrum is remarkably similar to that of ONiO−in Fig. 5.6. Peak A is assigned as the electron affinity at 3.086 ± 0.004 eV. Peak B
is 680 cm-1 from the electron affinity peak, and is assigned as one quantum in thesymmetric stretch vibration. There are no known experimental data with which tocompare these values.

As stated above, Gutsev et al. (40) calculates a linear

1 +Σ g ground state for ONiO and this is assumed to be true for OPdO and OPtO

(28,39). Bare et al. (28) calculate a linear OPdO 1Σg state with 730 cm-1 symmetricstretch frequency, which agrees with the value reported here.
Peak a falls at 340 cm-1 to the lower binding energy side of the electron
affinity. This peak retains the same relative intensity in both the 300 K (not shown)

184

OPdO-

Photoelectron Counts

A

B
a

3.25

3.20

3.15

3.10

3.05

3.00

2.95

Electron Binding Energy (eV)Figure 5.7. 364 nm photoelectron spectrum of OPdO− taken at magic angle
polarization angle and 200 K flow tube temperature.

2.90

185

and 200 K flow tube temperature spectra, thus it cannot be a vibrational hot band.
This suggests that peak a originates from a spin-orbit excited state in the anion.

5.5.3

OPtO

The magic angle, 200 K flow tube temperature 364 nm photoelectron
spectrum of OPtO− is displayed in Fig. 5.8. Peaks a, b, and c display temperaturedependence whereas peak A, which has a much stronger intensity than peaks a-c,does not. Peak A is assigned as the 0-0 transition from the anion, yielding EA(OPtO)= 2.677 ± 0.004 eV. The photoelectron spectrum of OPtO− contrasts with that of
ONiO− and OPdO− in that the lower electron affinity of OPtO (2.7 eV versus 3.1
eV—see Table 5.8) allows for a larger number of accessible features within the 3.408
eV photon energy. The EA is in agreement with the value reported by Rudnyi andco-workers (2.72 ± 0.20 eV) (38), although the error bars are improved by almost 2
orders of magnitude. Peaks A through E represent transitions into four quanta of asingle vibrational mode in the ground electronic state of OPtO with the fundamental
frequency of this symmetric stretching mode measured to be 895 cm-1.
As stated above, peaks a, b, and c are clear evidence of the presence of hot
bands. From the position of peak a (v′=1) and b (v′=2) with respect to the electron
affinity the vibrational frequency of OPtO− is obtained, 760 cm-1. However, theseparation between peaks b and c is larger than the one between a and b by greater

186

Photoelectron Counts

OPtO-

B

C

A
c

x10

D

b

a

E

3.2

3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

Electron Binding Energy (eV)Figure 5.8. 364 nm photoelectron spectrum of OPtO− taken at magic angle
polarization angle and 200 K flow tube temperature.

2.3

187
than 100 cm-1. Peak c is broad enough to suggest that it is likely composed of boththe v′=3 transition from the anion as well as the spin-orbit excited state of the anion.
Because the OPtO photoelectron spectrum consists of an extended vibrational
progression with several resolved peaks, it was simulated using a Franck-Condon
empirical fitting routine (49). In this way we were able to fit the change in moleculargeometry to the spectrum features. Assuming a linear molecule symmetric about thePt atom, the Pt—O bond length was found to change by 0.07 ± 0.01 Å whentransitioning from anion to neutral OPtO. This is a substantial bond length change,
which agrees with the broad Franck-Condon profile observed in this photoelectron
spectrum.
Ab initio frequencies and molecular geometries were also calculated for OPtO
and OPtO−.

The Pt—O bond length in the anion was found to be 1.778 Å while thatof the neutral was shorter at 1.707 Å. This yields a bond contraction of 0.071 Å, inexcellent agreement with the empirical finding described above. Furthermore, ashorter bond in neutral OPtO indicates that the detached electron originated from anantibonding HOMO in the anion. Table 5.8 shows the calculated frequency values
that agree very well with the experimental values, when scaled by an empirical 0.93
factor. In agreement with previous calculations (28), the molecular geometry of both
OPtO− and OPtO ground states is predicted to be linear. Consequently, only one
totally symmetric mode (the symmetric stretching mode) could be active within theFranck-Condon approximation in the 1Σg ground state of OPtO, in agreement with theexperimental results.

188
5.6

Discussion of ONiO, OPdO, and OPtO

The spectra of ONiO− and OPdO− display quickly decaying peak profiles.
Extrapolating from the ratio of heights of the electron affinity peak to the neutral v=1
symmetric stretch vibration in Figs. 5.6 and 5.7, even with greater laser photon
energy, one would expect to see essentially one more peak in the vibrational
progression. This contrasts with the extended profile observed in OPtO−, where theorigin peak is not the most intense. As was the case for PtO discussed above withrespect to NiO and PdO, we have evidence for significantly greater vibrational
activity in OPtO with respect to ONiO and OPdO. In the OPtO case, detaching theanion electron produces a greater bond length change than in ONiO or OPdO due to a
HOMO electron that is more antibonding. This is a result of an increased covalentnature of the O—Pt bond as was discussed for PtO.
Spectra were also taken at different polarization angles (not shown) for all
three OXO molecules. A value of 0.6 was obtained for the anisotropy parameter (β)
across the different peaks of the spectra of both OPtO− and OPdO−. This value isconsistent with electron detachment from a molecular orbital of π symmetry andagrees with values obtained for similar systems where a π electron is detached, as in
the case of WO2− (59). The calculations agree with this picture indicating that theextra electron in the anion is occupying a π* molecular orbital. This result also agrees
with the fact that the anion shows a smaller stretching frequency that the neutral,suggesting that the extra electron in the anion is occupying an antibonding orbital. In

189contrast, however, the spectrum of ONiO− yields a β value of –0.2, which may beconsidered an isotropic angular distribution.

5.7

Conclusion

The 364 nm photoelectron spectra of XO– and OXO– anions (X=Ni, Pd, andPt) were recorded and analyzed. Electron affinities (3Σ− ← 2Π3/2) were identified for
all three XO– spectra. Anion 2Π3/2−2Π3/2 spin-orbit splittings were identified as well
as neutral 3Π2−3Π1 spin-orbit splittings for NiO, PdO, and PtO. Several of theassignments for NiO and PdO were corrected from existing literature values. A small
amount of vibrational activity was observed in the spectra of NiO– and significantly
more was observed in PtO–, and bond length changes were deduced. Molecular spinorbit splittings attest to the large covalent character of the PtO bond with respect to
the NiO and PdO bonds.

Electron affinities and symmetric stretch vibrationalfrequencies were extracted from the spectra of the OXO– molecules, as well as an OX bond length change in OPtO.

190

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Appendix

A.1

Howard recalculation

This appendix illustrates precisely how the quantities in Tables 3.8-3.11 were
calculated. Table 3.8 shows the results of recalculation of ∆fHT(HOO) using datafrom Howard (1) and thermochemical quantities from the tables of Gurvich et al. (2)using both the ∆fH(OH) from Ref. (2) and from Ref. (3). In the Howard paper, therate constants for the forward and reverse reaction of
HO2 + NO !

OH + NO2(A.1)

Were measured to yield a temperature-dependent equilibrium constant Kequi = (0.116± 0.026)exp{(7145 ± 255)/RT} (1). If R = 1.9864776 cal/mol K and T = 700K, RT= 1.9864776 * 700 = 1390.5343 cal/mol, since the experiment took place at 700K.
(Note: I keep an arbitrary number of digits in all numbers until the final values, atwhich point I round) Then Kequi = 0.116 * exp(7145/1390.5343) = 19.769643.
To determine the error in Kequi, ∆Kequi :1. First, determine uncertainty in exp{(7145 ± 255)/RT}
Let y = exp α/RT, where α = 7145
∆y = |dy/dα| δα= (1/RT) * (exp α/RT) * 255= (255/1390.5343) * exp 7145/ 1390.5343= 31.2535

207
so exp{(7145 ± 255)/RT} evaluated at T=700 becomes
170.428 ± 31.2535 (not rounding yet)
2. Multiply this with 0.116 ± 0.026(0.116 ± 0.026)* (170.428 ± 31.2535)= 19.769643 ± 19.769643 *{(0.026/0.116)2 + (31.2535/170.428)2)}1/2= 19.769643 ± 19.769643 * 0.289598= 19.769643 ± 5.725
3. Kequi = 20 ± 6 (or 19.8 ± 5.7)

I want to determine the value of ∆rxnH700 ,∆rxnH700 = -RT ln Kequi + T ∆rxnS.
First, calculate -RT ln Kequi, then the error ∆RT ln Kequi.
-RT ln Kequi = - 1390.5343 cal/mol * ln (19.769643) = -4149.5596 cal/mol= - 17361.757 J/mol∆ ln Kequi = |d ln Kequi / d Kequi | δKequi= 1/ Kequi * δKequi= 1/19.769643 * 5.725= 0.289585∆RT ln Kequi = 1390.5343 cal/mol * 0.289585= 402.6778 cal/mol= 1684.80423 J/mol-RT ln Kequi = -17361.757 ± 1684.804 J/mol

(A.2)

208

Now calculate T ∆rxnS:∆rxnS700 = S700 (OH) + S700 (NO2) - S700 (HO2) - S700 (NO)(A.3)

From the Gurvich thermochemical tables (2):S700 (OH) = 209.002 J/K mol
S700 (NO2) = 276.471 J/K mol
S700 (HO2) = 262.271 J/K mol
S700 (NO) = 236.755 J/K mol
Here I am neglecting any error in the entropy as it will be small. (Speaking ofentropy, if anyone reads this far into the thesis, let me know and I’ll buy you a caseof beer!)∆rxnS700 = 209.002 + 276.471 - 262.271 - 236.755 = -13.553 J/K mol
T ∆rxnS700 = -13.553 J/K mol * 700K = -9487.1 J/mol
Now I can calculate ∆rxnH700:∆rxnH700 = (- 17361.757 ± 1684.804 J/mol) + -9487.1 J/mol= - 26.848857 ± 1.684804 kJ/mol= - 26.85 ± 1.68 kJ/mol= - 6.417 ± 0.4027 kcal/mol

The next step is to determine the heat of formation of HOO:
∆fH700(HO2) = ∆fH700(OH) + ∆fH700(NO2) - ∆rxnH700 - ∆fH700(NO)

(A.4)

209
From the Gurvich tables (2):∆fH0(NO2) = 37.000 ± 0.5 kJ/mol∆fH0(NO) = 90.773 ± 0.43 kJ/mol∆fH0(OH) = 39.110 ± 0.21 kJ/mol
Here I am assuming that the uncertainty will not change with temperature. To use
these values in Eq. A.4, they must be converted from 0 to 700 K. The value of∆fH700(NO2) can be calculated using the following:

700∆fH700(NO2) = ∆fH0(NO2) + ∫ dT {Cp(NO2) – Cp (O2) – ½ Cp (N2)}0H°(700) - H°(0) (NO2) = 27.635 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (N2) = 20.607 kJ/mol
so that ∆fH700(NO2) = 37.000 + 27.635 – 21.180 – 0.5 * 20.607= 33.152 ± 0.5 kJ/mol
Repeat to find ∆fH700(NO):∆fH700(NO) = ∆fH0(NO) + ∫ dT {Cp(NO) – 0.5 Cp (O2) – 0.5 Cp (N2)}H°(700) - H°(0) (NO) = 21.489 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (N2) = 20.607 kJ/mol
so that ∆fH700(NO) = 90.773 + 21.489 – 0.5 * 21.180 – 0.5 * 20.607= 91.369 ± 0.43 kJ/mol

(A.5)

210
Repeat once more to find ∆fH700(OH):∆fH700(OH) = ∆fH0(OH) + ∫ dT {Cp(OH) – 0.5 Cp (O2) – 0.5 Cp (H2)}
H°(700) - H°(0) (OH) = 20.701 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
so that ∆fH700(OH) = 39.110 + 20.701 – 0.5 * 21.180 – 0.5 * 20.216= 39.113 ± 0.21 kJ/mol (9.35 kcal/mol)∆fH700(HO2) = ∆fH700(OH) + ∆fH700(NO2) - ∆rxnH700 - ∆fH700(NO)(A.4)

This makes ∆fH700(HO2) = 39.113 kJ/mol + 33.152 kJ/mol – (-26.849 kJ/mol) –91.369 kJ/mol= 7.745 kJ/mol ± {(0.21)2 + (0.5)2 + (1.68)2 + (0.43)2}1/2= 7.745 ± 1.817 kJ/mol∆fH700(HO2) = 1.85 ± 0.43 kcal/mol

Now I want to convert ∆fH700(HO2) to other temperatures aside from 700K. This isdone through heat capacity corrections:700∆fH298.15(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0298+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0

The integral of the heat capacity is tabulated in Ref. (2):

(A.6)

211
H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol∆fH298.15(HO2) = 7.745– {25.873 – 21.180 – ½ 20.216} + {10.003 – 8.680 – ½ 8.468}= 7.745 – {-5.415} + {-2.911}= 10.249 ± 1.817 kJ/mol∆fH298.15(HO2) = 2.45 ± 0.43 kcal/mol

700∆fH0(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol∆fH0(HO2) = 7.745 – {25.873 – 21.180 – ½ 20.216}= 7.745 – {-5.415}= 13.160 ± 1.817 kJ/mol∆fH0(HO2) = 3.15 ± 0.43 kcal/mol

(A.7)

212

700

∆fH400(HO2) = ∆fH700(HO2) – ∫dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}

0

400

+ ∫

dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}

(A.8)

0

H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
H°(400) - H°(0) (HO2) = 13.676 kJ/mol
H°(400) - H°(0) (O2) = 11.706 kJ/mol
H°(400) - H°(0) (H2) = 11.426 kJ/mol∆fH400(HO2) = 7.745 – {25.873 – 21.180 – ½ 20.216} + {13.676 – 11.706 – ½
11.426}= 7.745 – {5.415} + {-3.743}= 9.417 ± 1.817 kJ/mol∆fH400(HO2) = 2.25 ± 0.43 kcal/mol

700∆fH500(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0500+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}
0

(A.9)

213

H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
H°(500) - H°(0) (HO2) = 17.527 kJ/mol
H°(500) - H°(0) (O2) = 14.765 kJ/mol
H°(500) - H°(0) (H2) = 14.350 kJ/mol∆fH500(HO2) = 7.745 – {25.873 – 21.180 – ½ 20.216} + {(17.527 – 14.765 – ½
14.350}= 7.745 – {-5.415} + {-4.413}= 8.747 ± 1.817 kJ/mol∆fH500(HO2) = 2.09 ± 0.43 kcal/mol

700∆fH1000(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}
0

1000

+

∫

dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
H°(1000) - H°(0) (HO2) = 39.585 kJ/mol(A.10)

214
H°(1000) - H°(0) (O2) = 31.387 kJ/mol
H°(1000) - H°(0) (H2) = 29.147 kJ/mol∆fH1000(HO2) = 7.745 – {25.873 – 21.180 – ½ 20.216} + {39.585 – 31.387 – ½
29.147}= 7.745 – {-5.415} + {-6.3755}= 6.785 ± 1.817 kJ/mol∆fH1000(HO2) = 1.62 ± 0.43 kcal/mol

Now that I have calculated values of ∆fHT(HO2) for T = 0, 298, 400, 500, 700, and
1000K, using thermochemical tables (2) and the Kequi of Ref. (1) as the only source of
information, I want to repeat the process, but using newly measured ∆fH0(OH)supplied by B. Ruscic (3). This value is∆fH0(OH) = 8.86 ± 0.07 kcal/mol = 37.07 ± 0.29 kJ/mol

which must be converted to 700 K to be used in Eq. A.4.

700

∆fH700(OH) = ∆fH0(OH) + ∫dT {Cp(OH) – 0.5 Cp (O2) – 0.5 Cp (H2)} (A.11)

0H°(700) - H°(0) (OH) = 20.701 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol

215
H°(700) - H°(0) (H2) = 20.216 kJ/mol

so that ∆fH700(OH) = 37.07 + 20.701 – 0.5 * 21.180 – 0.5 * 20.216= 37.073 ± 0.29 kJ/mol (8.86 kcal/mol)Using Eq. A.4, this makes ∆fH700(HO2) = 37.073 kJ/mol + 33.152 kJ/mol – (-26.849
kJ/mol) – 91.369 kJ/mol= 5.705 kJ/mol ± {(0.29)2 + (0.5)2 + (1.68)2 + (0.43)2}1/2= 5.705 ± 1.828 kJ/mol∆fH700(HO2) = 1.36 ± 0.44 kcal/mol

Now I repeat Eqs. A.6-A.10 but using this different value of ∆fH700(HO2) to convert
to different temperatures.

700∆fH298.15(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0298.15+∫dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol(A.12)

216
H°(298) - H°(0) (H2) = 8.468 kJ/mol∆fH298.15(HO2) = 5.705 – (25.873 – 21.180 – ½ 20.216) + (10.003 – 8.680 – ½ 8.468)= 5.705 – (-5.415) + (-2.911)= 8.209 ± 1.828 kJ/mol∆fH298.15(HO2) = 1.96 ± 0.44 kcal/mol

700∆fH0(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}(A.13)

0H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol∆fH0(HO2) = 5.705 – {25.873 – 21.180 – ½ 20.216}= 5.705 – {-5.415}= 11.12 ± 1.828 kJ/mol∆fH0(HO2) = 2.66 ± 0.44 kcal/mol

700∆fH400(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0400+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0(A.14)

217

H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
H°(400) - H°(0) (HO2) = 13.676 kJ/mol
H°(400) - H°(0) (O2) = 11.706 kJ/mol
H°(400) - H°(0) (H2) = 11.426 kJ/mol∆fH400(HO2) = 5.705 – (25.873 – 21.180 – ½ 20.216) + (13.676 – 11.706 – ½ 11.426)= 5.705 – (-5.415) + (-3.743)= 7.377 ± 1.828 kJ/mol∆fH400(HO2) = 1.76 ± 0.44 kcal/mol

700∆fH500(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0500+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
H°(500) - H°(0) (HO2) = 17.527 kJ/mol
H°(500) - H°(0) (O2) = 14.765 kJ/mol(A.15)

218
H°(500) - H°(0) (H2) = 14.350 kJ/mol∆fH500(HO2) = 5.705 – (25.873 – 21.180 – ½ 20.216) + (17.527 – 14.765– ½ 14.350)= 5.705 – (-5.415) + (-4.413)= 6.707 ± 1.828 kJ/mol∆fH500(HO2) = 1.60 ± 0.44 kcal/mol

700∆fH1000(HO2) = ∆fH700(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}
0

1000

+

∫

dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}(A.16)

0H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol
H°(1000) - H°(0) (HO2) = 39.585 kJ/mol
H°(1000) - H°(0) (O2) = 31.387 kJ/mol
H°(1000) - H°(0) (H2) = 29.147 kJ/mol∆fH1000(HO2) = 5.705 – (25.873 – 21.180 – ½ 20.216) + (39.585 – 31.387 – ½
29.147)= 5.705 – (-5.415) + (-6.3755)

219= 4.745 ± 1.828 kJ/mol∆fH1000(HO2) = 1.13 ± 0.44 kcal/mol

Now I can determine D0(HOO-H) and D298.15(HOO-H)through the equation
D0(HOO-H) = ∆fH0(HOO) + ∆fH0(H) − ∆fH0(HOOH)(A.17)

From the thermochemical tables(2):∆fH0(H) = 216.035 ± 0.006 kJ/mol = 51.634 ± 0.001 kcal/mol∆fH298.15(H) = 217.998 ± 0.006 kJ/mol = 52.103 ± 0.001 kcal/mol∆fH0(HOOH) = -129.89 ± 0.22 kJ/mol = -31.04 ± 0.05 kcal/mol∆fH298.15(HOOH) = -135.88 ± 0.22 kJ/mol = -32.48 ± 0.05 kcal/mol

so with the value of ∆fH0(OH) from the tables(2),D0(HOO-H) = 3.15 + 51.634 – (-31.04)= 85.82 kcal/mol ± ((0.43)2+(0.001)2+(0.05)2)1/2= 85.82 ± 0.43 kcal/mol

D298.15(HOO-H) = 2.45 + 52.103 – (-32.48)= 87.03 kcal/mol ± ((0.43)2+(0.001)2+(0.05)2)1/2

220= 87.03 ± 0.43 kcal/mol

And with the new value of ∆fH0(OH) (3),D0(HOO-H) = 2.66 + 51.634 – (-31.04)= 85.33 kcal/mol ± ((0.44)2+(0.001)2+(0.05)2)1/2= 85.33 ± 0.44 kcal/mol

D298.15(HOO-H) = 1.96 + 52.103 – (-32.48)= 86.54 kcal/mol ± ((0.44)2+(0.001)2+(0.05)2)1/2= 86.54 ± 0.44 kcal/mol

Finally, now I can determine ∆acidH0(HOO-H) and ∆acidH298.15(HOO-H).

∆acidH0(HOO-H) = D0(HOO-H) – EA(HOO) + IP (H)(A.18)∆acidH298.15(HOO-H) = D298.15(HOO-H) – EA(HOO) + IP(H)- thermal
correction(A.19)The thermal correction occurs because the EA and IP are both measured at 0 K.

221thermal correction =298.15∫dT {Cp(HO2) – Cp (HO2-) + Cp (H) - Cp (H+)}

0

From the thermochemical tables (2):H°(298.15) - H°(0) (HO2) = 10.003 kJ/mol
H°(298.15) - H°(0) (HO2-) = 10.017 kJ/mol
H°(298.15) - H°(0) (H) = 6.197 kJ/mol
H°(298.15) - H°(0) (H+) = 6.197 kJ/mol

thermal correction = 10.003 - 10.017 + 6.197 - 6.197 kJ/mol= -0.014 kJ/mol= -0.00335 kcal/mol

From Table 3.1,
EA(HOO) = 1.078 ± 0.006 eV = 24.8593 ± 0.13836 kcal/mol
IP(H) = 313.587 kcal/mol

so with the value of ∆fH0(OH) from the tables (2),∆acidH0(HOO-H) = 85.82 - 24.8593 + 313.587= 374.548 kcal/mol ± ((0.43)2+(0.13836)2+(0.0)2)1/2(A.20)

222= 374.55 ± 0.45 kcal/mol∆acidH298.15(HOO-H) = 87.03 - 24.8593 + 313.587 – (-0.00335)= 375.761 kcal/mol ± ((0.43)2+(0.13836)2+(0.0)2)1/2= 375.76 ± 0.45 kcal/mol

And with the new value of ∆fH0(OH) (3),∆acidH0(HOO-H) = 85.33 - 24.8593 + 313.587= 374.058 kcal/mol ± ((0.44)2+(0.13836)2+(0.0)2)1/2= 374.06 ± 0.46 kcal/mol∆acidH298.15(HOO-H) = 86.54 - 24.8593 + 313.587 – (-0.00335)= 375.271 kcal/mol ± ((0.44)2+(0.13836)2+(0.0)2)1/2= 375.27 ± 0.46 kcal/mol

A.2

Hills and Howard recalculation

Table 3.9 shows the results of recalculation of ∆fHT(HOO) using data from
Hills and Howard (4) and thermochemical quantities from the tables of Gurvich et al.
(2) using both the ∆fH(OH) from Ref. (2) and from Ref. (3). Here I repeat all of theabove calculations but instead starting from reaction rates from Hills and Howard223
instead. In the Hills and Howard paper, the rate constants for the forward and reverse
reaction of OH + ClO.

Kequi = k1a/k22
k1a = 0.86 k1 = 0.86 * (8.0 ± 1.4) x 10-12 exp{(235 ± 46/T)} cm3 molecule-1 s-1= 6.88 ± 1.2 x 10-12 exp{(235 ± 46/T)} cm3 molecule-1 s-1
k1a (298.15) = 6.88 ± 1.2 x 10-12 exp{0.7882 ± 0.1543} cm3 molecule-1 s-1= 15.1321 x 10-12 cm3 molecule-1 s-1
compute the error ∆ in k1a (298.15)(6.88 ± 1.2 x 10-12) * exp{0.7882 ± 0.1543}= (6.88 ± 1.2 x 10-12) {exp (0.7882) ± exp (0.7882) * 0.1543}= (6.88 ± 1.2 x 10-12) (2.19943 ± 0.33937)= 15.1321 x 10-12 ± 15.1321 x 10-12 * { (1.2/6.88)2 + (0.33937/2.
19943)2}1/2= 15.1321 ± 3.524 x 10-12 cm3 molecule-1 s-1
k22 = 9.1 x 10-12 cm3 molecule-1 s-1
Kequi (298.15) = 15.1321 ± 3.524/9.1 = 1.6629 ± 0.387 (1.66 ± 0.39)

Again I want to calculate∆rxnH298.15 = -RT ln Kequi + T ∆rxnS

Using Gurvich data (2):(A.20)

224

S298(OH) = 183.737 J/mol K
S298(ClO) = 225.101 J/mol K
S298(HO2) = 229.102 J/mol K
S298(Cl) = 165.189 J/mol K∆fH298(OH) = 39.349 ± 0.21 kJ/mol∆fH298(ClO) = 101.627 ± 0.11 kJ/mol∆fH298(Cl) = 121.302 ± 0.008 kJ/mol∆rxnS298 = S298(HO2) + S298(Cl) - S298(OH) - S298(ClO)= 229.102 + 165.189 – 183.737 – 225.101 J/mol K= - 14.547 J/mol K

T ∆rxnS298 = - 4.337 kJ/mol

R = 1.9864776 cal/mol K = 8.3114223 J/mol K
and T = 298.15 K
RT = 2.478 kJ/mol

Determining the error in ln Kequi∆ ln Kequi = |d ln Kequi / d Kequi | δKequi(A.21)

225= 1/ Kequi * δKequi= 1/1.6629 * 0.387= 0.23273-RT ln K = - 2.478 kJ/mol * ln 1.6629= - 1.26022 kJ/mol∆RT ln Kequi = 2.478 kJ/mol * 0.23273= 0.5767 kJ/mol-RT ln Kequi = -1.26022 ± 0.5767 kJ/mol

Plugging into Eq. A.20 gives∆rxnH298 = -RT ln K + T S298rxn= (-1.26022 ± 0.5767) + (- 4.337)= - 5.59722 ± 0.5767 kJ/mol

Then ∆rxnH298 yields ∆fH298(HO2)∆fH298(HO2) = ∆fH298(OH) + ∆rxnH298 - ∆fH298(Cl) + ∆fH298(ClO)(A.22)= 39.349 + (- 5.59722) – 121.302 + 101.627 ± {(0.21)2 + (0.5767) 2 +(0.008) 2 + (0.11) 2}1/2= 14.0768 ± 0.624 kJ/mol∆fH298(HO2) = 3.36 ± 0.15 kcal/mol

226
Again I want to convert this value to various different temperatures (0, 400, 500, 700,
and 1000 K) through heat capacity corrections.

298∆fH0(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}(A.23)

0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol∆fH0(HO2) = 14.0768 – (10.003 – 8.680 – ½ 8.468)= 14.0768 – (-2.911)= 16.988 ± 0.624 kJ/mol∆fH0(HO2) = 4.06 ± 0.15 kcal/mol

298∆fH400(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0400+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol(A.24)

227
H°(400) - H°(0) (HO2) = 13.676 kJ/mol
H°(400) - H°(0) (O2) = 11.706 kJ/mol
H°(400) - H°(0) (H2) = 11.426 kJ/mol∆fH400(HO2) = 14.0768 – (10.003 – 8.680 – ½ 8.468) + (13.676 – 11.706 – ½ 11.426)= 14.0768 – (-2.911) + (-3.743)= 13.245 ± 0.624 kJ/mol∆fH400(HO2) = 3.17 ± 0.15 kcal/mol

298∆fH500(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}]0500+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}(A.25)

0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol
H°(500) - H°(0) (HO2) = 17.527 kJ/mol
H°(500) - H°(0) (O2) = 14.765 kJ/mol
H°(500) - H°(0) (H2) = 14.350 kJ/mol∆fH500(HO2) = 14.0768 – (10.003 – 8.680 – ½ 8.468) + (17.527 – 14.765 – ½ 14.350)

228= 14.0768 – (-2.911) + (-4.413)= 12.575 ± 0.624 kJ/mol∆fH500(HO2) = 3.01 ± 0.15 kcal/mol

298∆fH700(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0700+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}(A.26)

0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol
H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol∆fH700(HO2) = 14.0768 –(10.003 – 8.680 – ½ 8.468) + (25.873 – 21.180 – ½ 20.216)= 14.0768 – (-2.911) + (-5.415)= 11.573 ± 0.624 kJ/mol∆fH700(HO2) = 2.77 ± 0.15 kcal/mol

229

298

∆fH1000(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}
0

1000

+

∫

dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}(A.27)

0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol
H°(1000) - H°(0) (HO2) = 39.585 kJ/mol
H°(1000) - H°(0) (O2) = 31.387 kJ/mol
H°(1000) - H°(0) (H2) = 29.147 kJ/mol∆fH1000(HO2) = 14.0768 – (10.003 – 8.680 – ½ 8.468) + (39.585 – 31.387 – ½
29.147)= 14.0768 – (-2.911) + (-6.3755)= 10.612 ± 0.624 kJ/mol∆fH1000(HO2) = 2.54 ± 0.15 kcal/mol

Now I want to repeat the calculation of ∆fHT(HO2) using the newly measured∆fH298(OH) supplied by B. Ruscic (3). This value is

230∆fH298(OH) = 8.92 ± 0.07 kcal/mol= 37.32 ± 0.29 kJ/mol

Recompute ∆fH298(HO2)∆fH298(HO2) = ∆fH298(OH) + ∆rxnH298 - ∆fH298(Cl) + ∆fH298(ClO)= 37.32 + (- 5.59722) – 121.302 + 101.627 ± {(0.29)2 + (0.5767) 2 +(0.008) 2 + (0.11) 2}1/2= 12.0478 ± 0.655 kJ/mol∆fH298(HO2) = 2.88 ± 0.16 kcal/mol

Now once more I convert ∆fH298(HO2) to other temperatures∆fH0(HO2) = ∆fH298(HO2)298– ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol∆fH0(HO2) = 12.0478 – (10.003 – 8.680 – ½ 8.468)= 12.0478 – (-2.911)(A.28)

231= 14.959 ± 0.655 kJ/mol∆fH0(HO2) = 3.58 ± 0.16 kcal/mol

298∆fH400(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0400+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}(A.29)

0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol
H°(400) - H°(0) (HO2) = 13.676 kJ/mol
H°(400) - H°(0) (O2) = 11.706 kJ/mol
H°(400) - H°(0) (H2) = 11.426 kJ/mol∆fH400(HO2) = 12.0478 – (10.003 – 8.680 – ½ 8.468) + (13.676 – 11.706 – ½ 11.426)= 12.0478 – (-2.911) + (-3.743)= 11.216 ± 0.655 kJ/mol∆fH400(HO2) = 2.68 ± 0.16 kcal/mol

232
298

∆fH500(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0500+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}(A.30)

0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol
H°(500) - H°(0) (HO2) = 17.527 kJ/mol
H°(500) - H°(0) (O2) = 14.765 kJ/mol
H°(500) - H°(0) (H2) = 14.350 kJ/mol∆fH500(HO2) = 12.0478 – (10.003 – 8.680 – ½ 8.468) + (17.527 – 14.765 – ½ 14.350)= 12.0478 – (-2.911) + (-4.413)= 10.546 ± 0.655 kJ/mol∆fH500(HO2) = 2.52 ± 0.16 kcal/mol

298∆fH700(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0700+ ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0(A.31)

233
H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol
H°(700) - H°(0) (HO2) = 25.873 kJ/mol
H°(700) - H°(0) (O2) = 21.180 kJ/mol
H°(700) - H°(0) (H2) = 20.216 kJ/mol∆fH700(HO2) = 12.0478 – (10.003 – 8.680 – ½ 8.468) + (25.873 – 21.180 – ½ 20.216)= 12.0478 – (-2.911) + (-5.415)= 9.544 ± 0.655 kJ/mol∆fH700(HO2) = 2.28 ± 0.16 kcal/mol

298∆fH1000(HO2) = ∆fH298(HO2) – ∫ dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}]
0

1000

+

∫

dT {Cp(HO2) – Cp (O2) – 0.5 Cp (H2)}0H°(298) - H°(0) (HO2) = 10.003 kJ/mol
H°(298) - H°(0) (O2) = 8.680 kJ/mol
H°(298) - H°(0) (H2) = 8.468 kJ/mol
H°(1000) - H°(0) (HO2) = 39.585 kJ/mol
H°(1000) - H°(0) (O2) = 31.387 kJ/mol(A.32)

234
H°(1000) - H°(0) (H2) = 29.147 kJ/mol∆fH1000(HO2) = 12.0478 – (10.003 – 8.680 – ½ 8.468) + (39.585 – 31.387 – ½
29.147)= 12.0478 – (-2.911) + (-6.3755)= 8.583 ± 0.655 kJ/mol∆fH1000(HO2) = 2.05 ± 0.16 kcal/mol

Now I can determine D0(HOO-H) and D298(HOO-H) through the equation
D0(HOO-H) = ∆fH0(HOO) + ∆fH0(H) − ∆fH0(HOOH)(A.33)

From the thermochemical tables (2):∆fH0(H) = 216.035 ± 0.006 kJ/mol = 51.634 ± 0.001 kcal/mol∆fH298.15(H) = 217.998 ± 0.006 kJ/mol = 52.103 ± 0.001 kcal/mol∆fH0(HOOH) = -129.89 ± 0.22 kJ/mol = -31.04 ± 0.05 kcal/mol∆fH298.15(HOOH) = -135.88 ± 0.22 kJ/mol = -32.48 ± 0.05 kcal/mol

so using the value of ∆fH0(OH) from these tables,

235
D0(HOO-H) = 4.06 + 51.634 – (-31.04)= 86.734 kcal/mol ± ((0.15)2+(0.001)2+(0.05)2)1/2= 86.73 ± 0.16 kcal/mol

D298.15(HOO-H) = 3.36 + 52.103 – (-32.48)= 87.943 kcal/mol ± ((0.15)2+(0.001)2+(0.05)2)1/2= 87.94 ± 0.16 kcal/mol

And with the new value of ∆fH0(OH) (3),D0(HOO-H) = 3.58 + 51.634 – (-31.04)= 86.254 kcal/mol ± ((0.16)2+(0.001)2+(0.05)2)1/2= 86.25 ± 0.17 kcal/mol

D298.15(HOO-H) = 2.88 + 52.103 – (-32.48)= 87.463 kcal/mol ± ((0.16)2+(0.001)2+(0.05)2)1/2= 87.46 ± 0.17 kcal/mol

And finally, now I can determine ∆acidH0(HOO-H) and ∆acidH298(HOO-H) using Eqs.
A.18-20:

From the thermochemical tables:236

H°(298.15) - H°(0) (HO2) = 10.003 kJ/mol
H°(298.15) - H°(0) (HO2-) = 10.017 kJ/mol
H°(298.15) - H°(0) (H) = 6.197 kJ/mol
H°(298.15) - H°(0) (H+) = 6.197 kJ/mol

thermal correction = 10.003 - 10.017 + 6.197 - 6.197 kJ/mol= -0.014 kJ/mol= -0.00335 kcal/mol

From Table 3.1, EA(HOO) = 1.078 ± 0.006 eV = 24.8593 ± 0.13836 kcal/mol
IP(H) = 313.587 kcal/mol

so using the value of ∆fH0(OH) from the tables,∆acidH0(HOO-H) = 86.734 - 24.8593 + 313.587= 375.462 kcal/mol ± ((0.158)2+(0.13836)2+(0.0)2)1/2= 375.46 ± 0.21 kcal/mol∆acidH298.15(HOO-H) = 87.943 - 24.8593 + 313.587 – (-0.00335)= 376.674 kcal/mol ± ((0.158)2+(0.13836)2+(0.0)2)1/2= 376.67 ± 0.21 kcal/mol

237
And with the new value of ∆fH0(OH) (3),∆acidH0(HOO-H) = 86.254 - 24.8593 + 313.587= 374.982 kcal/mol ± ((0.168)2+(0.13836)2+(0.0)2)1/2= 374.98 ± 0.22 kcal/mol∆acidH298.15(HOO-H) = 87.463 - 24.8593 + 313.587 – (-0.00335)= 376.194 kcal/mol ± ((0.168)2+(0.13836)2+(0.0)2)1/2= 376.19 ± 0.22 kcal/mol

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Bibliography
1.

C. J. Howard, J. Am. Chem. Soc. 102, 6937-6941 (1980).

2.

Thermodynamic Properties of Individual Substances, Vol. 1, edited by L. V.
Gurvich, I. V. Veyts, and C. B. Alcock (Hemisphere Pub. Corp., New York,1989).

3.

B. Ruscic, D. Feller, D. A. Dixon, K. A. Peterson, L. B. Harding, R. L. Asher,and A. F. Wagner, J. Phys. Chem. A 105, 1-4 (2001). The values reported inthis citation have since been revised slightly by the authors. The most current
values are: ∆fH0(OH) = 8.86 ± 0.07 kcal mol-1 and ∆fH298(OH) = 8.92 ± 0.07
kcal mol-1.

4.

A. J. Hills and C. J. Howard, J. Chem. Phys. 81, 4458 (1984).