Photoelectron Spectroscopy of Small Organic Anions Prepared Using a Novel
Cold Ion Source Employing Entrainment of Charged Particles
by
Yu-Ju Lu
B.S., National Chiao-Tung University, 2003
M.S., National Taiwan University, 2005

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirements for the degree ofDoctor of Philosophy
Department of Chemistry and Biochemistry2014This thesis entitled:
Photoelectron Spectroscopy of Small Organic Anions Prepared Using a Novel Cold Ion Source
Employing Entrainment of Charged Particles
written by Yu-Ju Lu
has been approved for the Department of Chemistry and Biochemistry_______________________________________
W. Carl Lineberger_______________________________________
G. Barney Ellison

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

iii

Lu, Yu-Ju (Ph.D., Physical Chemistry)
Photoelectron Spectroscopy of Small Organic Anions Prepared Using a Novel Cold Ion Source
Employing Entrainment of Charged Particles
Thesis directed by Professor W. Carl Lineberger

Negative ion photoelectron spectroscopy is utilized to investigate the methyl anions (CH3–and CD3–), the propadienylidene anion (H2CCC–), and the propargylene anion (HCCCH–) preparedusing recently developed variations to the entrainment anion sources. Combining experiment andtheory allows for further insight into the electronic and vibrational structures of these molecules.
To enhance the anion-synthesis capability, multiple, pulsed entrainment valves are used
for stepwise chemistry. In addition to operation of multiple valves, pulsed plasma-entrainment
anion source is developed. The major feature of this anion source is the additional pulsed valve for
perpendicular entrainment of plasma, which is made in an electrical discharge. This anion source
provides the capability to make substantial clusters of entrained anions, e.g., OH–(Arn =0−32), as
well as to stabilize products of exothermic reactions in the primary expansion, e.g., HOCO– andH3COO–. We expect that more pulsed valves could be used for additional complex syntheses ifnecessary.
The methyl anion CH3– is formed using the pulsed plasma-entrainment source. Upon
photodetachment, the out-of-plane bending angle from the pyramidal CH3− anion to the planar CH3
radical changes significantly, leading to an extended Franck-Condon progression in the umbrellamode. The observation of the well-studied v2 bands of CH3 and CD3, coupled with the dramaticallyiv

improved electron energy resolution, enables us to directly measure the inversion splitting between
the 0+ and 0− energy levels in both CH3− and CD3−. The EAs of CH3 and CD3 are measured
precisely; using a thermochemical cycle, the methane gas-phase acidity is refined.
The photoelectron spectra of H2CCC– and HCCCH– are used to characterize the electronic
and vibrational structures of the corresponding neutral molecules. The reaction of O− with allene(H2C=C=CH2) produces nearly pure H2CCC−, which exhibits resolved vibrational progressions.
In contrast, the O− + propyne reaction produces both H2CCC− and HCCCH− products. Comparisonof the HCCCH– and H2CCC− photoelectron spectra provides information about the electronic
states of HCCCH. With the aid of calculations and simulations, we can assign and characterize theelectronic states of H2CCC and HCCCH.

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Dedication
To my parents.

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Acknowledgements
First, I would like to thank my advisor, Carl Lineberger, for the opportunity to work in thephotoelectron imaging laboratory and for his support and guidance over the past five years. Thankyou for giving me the freedom to carry out my own ideas. Additionally, I have been fortunate towork with many excellent people in the group. I want to thank Elisa Miller for teaching me about
imaging experiment. I had a great time working with you. I also thank Amanda Case for providing
helps when needed. Your leadership and scientific knowledge impressed me. Thanks to Allan
Maple de Oliveira for your friendship and help in both research and life. I wish you the best ofluck with the future projects. Many wonderful group members have helped and listened to me,
including Wilson Gichuhi, Julia Lehman, Daniel Nelson, Joshua Martin, Quanli Gu, Scott Wren,
Kristen Lemke, and Adam Merchant. You are always my friends.
I would like to thank Barney Ellison, Mathias Weber, Robert Parson, and Heather
Lewandowski for being my committee and giving comments about my thesis. Additionally,questions and discussions from the people in the Ion Super Group (Carl Lineberger, Barney
Ellison, Veronica Bierbaum, and Mathias Weber groups) are also appreciated. I also thank JILA
visiting fellows and many collaborators, including Anne McCoy, John Stanton, Mark Johnson,
David Osborn, Robert McMahon, John Maier, Lai-Sheng Wang, and John Yukich, for a lot ofsuggestions and discussions about my research. This provides me many opportunities to networkand meet these great professors and scientists. If I pursued my Ph.D degree in Taiwan, I would nothave chances to interact with them.
I am grateful to have supports from JILA staff. Krista Beck has filled out a lot of paperwork
for me; hence, I could focus on my research. Todd Asnicar and Hans Green of the machine shop

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have offered good ideas to efficiently solve instrumental problems. James Fung-A-Fat in theelectronics shop has provided his professional skills and experiences of circuit troubleshooting in
our lab. Without these technical supports, many works would not be done as fast as they were.
Thanks to the staff at JILA for all offered services.
Finally, I want to thank my family and Boulder friends. These friends have helped me getused to American life, which was challenging to me when I moved to Boulder in my first graduate
year. My mother and siblings have given their full love and support to me for pursuing my Ph.D
degree in America. To my wife, Pao-Hsing Hsu, thank you for your understanding and support
throughout this process. I love you!

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Contents
Chapter 1. Introduction………………………………………………………………….…………1
1.1 Negative Ion Photoelectron Spectroscopy………………………………….………….1
1.1.1 Principle……………………………….……………………………………………2
1.1.2 Franck-Condon Factor……………….……………………………………………5
1.1.3 Selection Rules……………………….……………………………………….…...6
1.1.4 Wigner Threshold Law……………………………………………...……….……7
1.1.5 Photoelectron Angular Distributions..……………………..…….………………8
1.2 Thesis Overview…………………………………………………………………………8
Chapter 2. Experimental Apparatus………………………………………………………….…...10
2.1 Overview………………………………………………………………………….10
2.2 Pulsed Anion Source………………………………………………………...……12
2.3 Time-of-Flight Mass Spectrometer……………………………………………….16
2.4 Velocity-Map Imaging Photoelectron Spectrometer……………………………...17
2.5 Laser System……………………………………………………………………...19
2.6 Data Analysis……………………………………………………………………...23
Chapter 3. Versatile, Pulsed Anion Source Utilizing Plasma-Entrainment: Characterization andApplications……………………………………………………………………….…27
3.1 Introduction………………………………………………………………….……27
3.2 Anion Source Details…………………………………………….…..……………29
3.3 Results and Discussion……………………………………………………………35
3.3.1 Electron Entrainment……………………………….……………………….35
3.3.2 Exotic Species: CH3–………………………………………………………...38
3.3.3 Ar-tagging: OH–(Ar)n……………………………………………………….41
3.3.4 H2-tagging: CH3O–(H2)n…………………………………………………….46
3.3.5 HOCO–……………………………………………………………………...51

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3.3.6 CH3OO–…………………………………………………………..…………55
3.4 Conclusion………………………………………………………….……………..57
Chapter 4. Photoelectron Spectroscopy of Methyl Anion, CH3– & CD3–…………………………58
4.1 Introduction……………………………………………………………………….58
4.2 Experimental Details……………………………………………………………...61
4.3 Results…………………………………………………………………………….64
4.4 Discussion ………………………………………………………………………..72
4.5 Conclusion………………………………………………………………………...77
Chapter 5. Photoelectron Spectroscopy of Propadienylidene Anion, H2CCC–…………………..79
5.1 Introduction……………………………………………………………………….79
5.2 Experimental Details……………………………………………………………...80
5.3 Results…………………………………………………………………………….82
5.4 Discussion………………………………………………………………………...86
5.5 Conclusion…………………………………………………………….…………..90
Chapter 6. Photoelectron Spectroscopy of Propargylene Anion, HCCCH–………………….…..91
6.1 Introduction……………………………………………………………………….91
6.2 Experimental Details……………………………………………………………...92
6.3 Results…………………………………………………………………………….93
6.4 Discussion………………………………………………………………………...99
6.5 Conclusion………………………………………………………….……………102
Bibliography……………………………………………………………………………………103
Appendix A: Schematic Drawings of the Parts of the Discharge Source……..…………………116
Appendix B: Expanded Sectional View of the Discharging Region…………………….………121
Appendix C: Source Conditions for OH–(Ar)n Cluster Formation.………………….……..…...122

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TablesTable 4.1. Experimental peak positions and assignments in the photoelectron spectra of CH3− andCD3−. The values in parentheses are the uncertainties described in the text………………………69
Table 4.2. Experimentally measured out-of-plane bending frequencies of CH3 and CD3 (cm-1) in
the gas phase………………………………………………………...……………………………74
Table 4.3. Experimentally determined values: adiabatic electron affinity (EA) and inversion
splitting (∆EInv.)….…………………………………………………………………….…………75
Table 5.1. Comparison between experimental and calculated detachment energies and term
energies……………….…………………………………………………..…………………..….89
Table 6.1. Comparison between experimental and calculated detachment energies and term
energies………………..………………………………………………………………..………101

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FiguresFigure 1.1. A schematic representation of the photodetachment process of anion, AB-, along thereaction coordinate of the internuclear distance between A and B. The corresponding spectrum of
the eBE and eKE distribution is shown on the left. EA is the electron affinity, VDE means vertical
detachment energy, ωe presents vibrational frequency, ∆R indicates geometry change ordisplacement, and ħω is the photon energy……………………...……………………………...….4
Figure 2.1. Overview of the experimental apparatus. Red represents the ion source, green
represents the TOF MS region, brown represents the VMI, and blue represents the laser system...11
Figure 2.2. Schematic drawing of (a) the pulsed plasma-entrainment ion source and (b) the neutralentrainment ion source. In this thesis, both designs use two pulsed valves but have the capability
to entrain additional gases with extra pulsed valves………………………………………………13
Figure 2.3. Schematic layout of mid-IR generation utilizing a Coherent Infinity Nd:YAG laser anda QuantaRay PDL-1 dye laser. The dye cells (yellow) in the day laser are pumped by the 532-nm
light (green), which laser polarization is flipped to be vertical using a half-wave plate. The residual
1064-nm light (red) with horizontal laser polarization is aligned through the MgO:LiNbO3 crystal,and is mixed with the dye laser output (orange) to generate a mid-IR light with horizontal laser
polarization. The delay stage is used to simultaneously get both lights to the non-linear crystal,i.e., the same traveling distance. The power of the mid-IR light is optimized by rotating the phasematching angle of the MgO:LiNbO3 crystal………………………………...……………………22
Figure 2.4. Photoelectron spectrum of S– at 532 nm. The spectrum is reconstructed from thequadrant-symmetrized photoelectron image. The peaks are labeled a through f, and the peak widths
are also shown. The corresponding known transitions are presented in the inset…………….…...26
Figure 3.1. Schematic arrangement of the pulsed anion source. The source is capable of entraining
ions from a side expansion………………………………………………………………………..31
Figure 3.2. Time trace of a -2kV discharge voltage for OH– production. Through two 10kΩ
resistors, the voltage drops about 1500 V, which corresponds to the peak discharge current of 75
mA. The inset represents the circuit diagram, in which the discharge electrodes are under vacuum
condition in the source chamber……………………………………………………………….....34
Figure 3.3. Comparison between TOF mass spectra from experiments using neat Ar (black) and
1% O2/Ar (red) for the main expansion. Neat Ar gas is used for both experiments in the side
discharge source. Entrainment of electrons is demonstrated from the additional formation of theO– and O2– anions………………………………………………………………………………...37
Figure 3.4. Time-of-flight mass spectrum from experiments using (a) 3% CH4/Ar and (b) neat CH4
in the side discharge source. Neat Ar gas is used as the main expansion for both experiments. The
chemical identity of the mass-selected anions is confirmed by comparing their photoelectron
spectra with other known anion photoelectron spectra.…………………………………………..39

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Figure 3.5. The 1064-nm photoelectron spectrum of CH3– showing an extensive progression in theout-of-plane bending (umbrella) mode of CH3…………..……………………………………….40
Figure 3.6. Composite mass spectra of OH–(Ar)n=0-32. The insets show the color-coded 5x (redcurve) and 20x (blue curve) magnifications of the selected mass spectral region to better view thecluster distribution and magic numbers of n = 12 and 18…………………………………………42
Figure 3.7. Photoelectron spectra of OH–(Ar)n=0, 1, 2, 3, 7, 12, and 18 at three different wavelengths: 459(top), 532 (middle), and 602 (bottom) nm………………………………………………………..45
Figure 3.8. Comparison of the CH3O–(H2)n=0-3 TOF mass spectra from experiments using neat Ar(black) and 25% H2/Ar (red) for the main expansion. The CH3O– is made in the side dischargesource………………………………………………………………………………………….....48
Figure 3.9. Photoelectron spectra of CH3O–, CH3O–(Ar), and CH3O–(H2)n=1-3 using 532- and 459nm light. The color-coded traces correspond to the indicated anion complexes.... ……………….50
Figure 3.10. Photoelectron spectra of HOCO– (a) at three different wavelengths: 532 (red), 602(purple) and 764 (black) nm, and (b) comparison between the 764 nm spectrum (this work) andthe 775 nm spectrum (blue) from Johnson et al. The diamond and star symbols denote the EA ofcis– and trans–HOCO–, respectively, from the assignments in the work of Johnson et al………..54
Figure 3.11. Comparison of the CH3OO– spectra obtained using the wavelengths of 363.8 (blue)and 532 (black) nm. For the blue spectrum from Blanksby et al., CH3OO– is formed via a proton
abstraction reaction following addition of CH3OOH downstream of OH– in a flowing afterglow
ion source at JILA. For the black spectrum (this work), we perform the association reaction of O2
with CH3– in an Ar pulsed jet to generate CH3OO–……………………………………………….56
Figure 4.1. Mass spectrum for the production of CH3– and CD3– by introducing ions formed from(a) a neat methane or (b) a neat methane-d4 pulsed discharge, respectively, into a neat Ar
expansion………………………………………………………………………………………...62
Figure 4.2. Schematic potential energy surfaces for the electronic ground states of CH3 (green) andCH3– (black) as a function of the out-of-plane bending angle. The red and blue arrows indicate a
non-zero Franck-Condon overlap for v″=0+ (even wavefunction, red dashed line) and 0– (oddwavefunction, blue dashed line) in the anionic ground state to even and odd quanta of the neutral
state, respectively. The 0+−0– inversion splitting of the CH3– is labeled ∆……………………….66
Figure 4.3. Photoelectron spectra of CH3– at three different photodetachment wavelengths: 763(black), 1064 (red), and 3236 (blue) nm…………………………………………………………68
Figure 4.4. Mid-IR photoelectron spectra of (a) CH3– and (b) CD3–. The CD3– spectrum is rebinned
to 2.5 pixels to improve the signal-to-noise ratio without changing the spectral appearance……71
Figure 5.1. Ground-state electronic configuration of propadienylidene anion, H2CCC–, andcorresponding singlet or triplet states formed by removal of an α or a β electron…………….…...83

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Figure 5.2. Photoelectron spectra of H2CCC– anion taken with photon energies of 3.809 (black),
4.164 (red), and 4.704 (blue) eV. The labels denote all origin transitions except the B̃ 1B1 state….85
Figure 5.3. Composite photoelectron spectrum of H2CCC– anion using NIPES, VMI, and SEVI
techniques. The three techniques are color-coded in black, blue, and cyan, respectively. The labels
indicate the origins of different electronic states of H2CCC radical except the B̃ 1B1 state……......87
Figure 6.1. Photoelectron spectra of the product anions from O– + allene (black), and O– + propyne(red) acquired with the VMI photoelectron spectrometer. Spectra were taken with ħω = 2.352 eV....
…………………………………………………………………………………………………....95
Figure 6.2. Photoelectron spectra of the product anions from O– + allene (black) and O– + propyne(red) reactions. Spectra in red were acquired with ħω = 4.660 eV, and in black with ħω = 4.704
eV………………………………………………………………………………………………...97
Figure 6.3. Photoelectron spectra of the product anions from O– + allene (black) and O– + propyne(red) reactions. Both spectra were taken with ħω = 5.158 eV……………………………………..98
Figure A.1 Schematic arrangement of the pulsed plasma-entrainment anion source employing twosolenoid valves. Part 1 is a custom-made conical nozzle coupling with a General Valve body, part2 is a cylindrical MACOR plate, part 3 is an electrode (stainless steel or aluminum) pulsed tonegative high voltage, part 4 is a thin MACOR insulator between two electrodes, and part 5 is athin electrode (stainless steel) attached to a commercial General Valve faceplate held at earth
ground. These components are expanded below……………………………………………….116
Figure A.2 Picture of the setup showing how both pulsed valves are mounted in the sourcechamber. This picture also displays how the stack of insulators and electrodes is held together. Allcomponents of the discharge device have four bolt holes and are fastened onto the faceplate of theside solenoid valve using two nylon screws…………………………………………………......117

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Chapter 1
Introduction
1.1 Negative Ion Photoelectron Spectroscopy
Photoelectron spectroscopy involves the measurement of the kinetic energy of electronsejected from atoms or molecules following photoabsorption, analogous to the century-old
photoelectric effect. Photoelectron spectroscopy is used to determine the binding energies ofelectrons in a neutral molecule, yielding information of the cationic states. Likewise, photoelectron
spectroscopy of a negative ion yields the electron affinity and other information of thecorresponding neutral molecule. Negative ion photoelectron spectroscopy was first implemented
in JILA at the University of Colorado,1 and has been widely applied to investigate fundamental
problems in spectroscopy2-4 and chemical dynamics.5-12 The great advantage of negative ion
photoelectron spectroscopy is that the anion precursors can be separated, manipulated, and massanalyzed through time-of-flight mass spectrometry and other ion-based experimental techniques,
resulting in unambiguous selection of the anion. Upon photodetachment, the negative ion
photoelectron spectroscopy technique directly measures the electron affinity (EA) of the neutral
species (see Section 1.1.1). The adiabatic EA is defined as the energy difference between theneutral molecule and the corresponding anion, both in their ground states. Useful thermochemicaland spectroscopic properties of the neutral molecule can be further derived from negative ion
photoelectron spectroscopy. Employing a thermochemical cycle, the measured EA, together with
the ionization energy of the hydrogen atom, determines either the deprotonation enthalpy,∆acidH(RH), or the bond dissociation energy, D(RH), if the other is already known.13 Negative ion
photoelectron spectroscopy is also able to measure transitions between neutral electronic states ofdifferent spin multiplicity, which is spin-forbidden (optically dark) from neutral molecules in2

optical spectroscopy.14 For example, from electron detachment from a doublet anion, the termenergy difference between the singlet and triplet states of the neutral molecule, or singlet−triplet
splitting, can be determined directly (see Section 1.1.3).
In conventional negative ion photoelectron spectroscopy, an anion beam is intersected witha fixed frequency laser beam, leading to an electron ejection from an anion.15 To obtain thephotoelectron spectrum, the kinetic energy of photodetached electrons is measured with ahemispherical analyzer. The energy resolution of the conventional spectrometer is about 4 meVunder optimal conditions (typically about 10 meV) and is constant over a 2.5-eV energy range.
Recently, photoelectron velocity-map imaging (VMI) detectors have been used to obtain thephotoelectron spectrum, where the velocity and angular distribution of the photodetached electrons
can be simultaneously acquired.16 The typical spectral resolution is approximately 6 meV for anelectron kinetic energy of 0.3 eV. If, instead, slow electrons are selectively detected, high energy
resolution of a few wavenumbers can be achieved; this is called SEVI (slow electron velocity-map
imaging).3 However, since electrons are detached very close to threshold, SEVI spectra are limitedby the threshold effect (see Section 1.1.4), which influences the intensity of the observed signals.

1.1.1 Principle
Upon photodetachment from anions, we obtain the electron kinetic energy (eKE)distribution of the photodetached electrons, represented in Fig. 1.1. Since the electron is at least
1800 times lighter than the neutral species, nearly all of the kinetic energy is given to the electron.
As a result, according to energy conservation, the electron binding energy (eBE) is calculated bysubtracting the eKE from the photon energy:

3eBE = ħω - eKE(1.1)

where ħω (blue arrow) is the photon energy of the laser. If an anion has significant Franck-Condon
overlap with the ground state of the neutral molecule, its photoelectron spectrum shows theadiabatic EA and vertical detachment energy, VDE (maximum intensity of the vibrational
progression). Measurement of the eKE distribution also gives information about the internalenergy of the neutral molecule. From the spectrum we determine additional spectroscopic
properties, such as vibrational frequencies and anharmonicities. Using higher photon energy,
electronic term energies can be measured. Negative ion photoelectron spectroscopy can accessneutral electronic excited states that would otherwise involve forbidden transitions from the neutral
ground state, enabling characterization of excited states that are inaccessible using otherspectroscopic techniques. Geometry information can be obtained from the vibrational progression
in the photoelectron spectrum following the Franck-Condon principle (see Section 1.1.2). The
principle states that the nuclear configuration of the molecule has no significant change during thephotodetachment process since the electron moves much faster than any nuclear motion. Thus, inthis important approximation, immediately following photodetachment, the anion and neutral
geometries are identical. As such, a long vibrational progression is indicative of a large equilibriumgeometry difference between anion and neutral species, while insignificant geometry difference
between the species results in a single peak in the photoelectron spectrum at the adiabatic EA.

4Figure 1.1 A schematic representation of the photodetachment process of anion, AB-, along thereaction coordinate of the internuclear distance between A and B. The corresponding spectrum of
the eBE and eKE distribution is shown on the left. EA is the electron affinity, VDE means vertical
detachment energy, ωe presents vibrational frequency, ∆R indicates geometry change ordisplacement, and ħω is the photon energy.

5

1.1.2 Franck-Condon Factor
The photodetachment intensities of various transitions are quantitatively described by theFranck-Condon factor. In the Born-Oppenheimer approximation, the nuclei do not move when theelectron is photodetached. Thus, the total wavefunction can be written as: Ψ =    , where ,  , and  represent the electronic, vibrational, and rotational wavefunctions, respectively.
The  and  depend on nuclear and electron coordinates, but the  only depends on the nuclear
coordinate.
The transition probability that a molecule absorbs a photon depends on the transition
moment of the molecule. The transition dipole moment, , is defined as: = Ψ | ̂ |Ψ , Ψ =     ̂ = ̂  + ̂ (1.2)

where ̂ is the operator; ̂  and ̂  are the electronic and nuclei dipole operators, respectively.
The transition moment becomes: = " " " ( ̂  + ̂  )      (1.3)= " " "  ̂         + " " "  ̂        (1.4)= "   " "  ̂       + "  ̂    " "     (1.5)

The last term in Eq. 1.5 is zero due to orthogonality of the two electronic wavefunctions. In theCondon approximation, the electronic transition moment is independent of the nuclear coordinate.
Thus, = "   " "  ̂      
Further, the transition probability can be written as:

(1.6)

6


 = "    " "  ̂      



(1.7)

The first term in the Eq. 1.7 is so called Franck-Condon factor (FCF), which is the square of theoverlap of the initial and final vibrational states: FCF= "    .
In the case of polyatomic molecules, using anharmonic oscillator and symmetry arguments,∆v = 0, ±1, ±2… yields a non-zero Franck-Condon factor in totally symmetric vibrational modes.
In contrast, for non-totally symmetric modes, only ∆v = 0, ± 2, ± 4… have significant FranckCondon factors.

1.1.3 Selection Rules
The selection rules for photodetachment are different from those in optical electronic
spectroscopy. In addition to the Franck-Condon factors considered in Section 1.1.2, the spin
angular momentum needs to be conserved. Since an electron is removed in the photodetachment
process, the selection rule for electron spin is ∆S= ± ½, which is significantly different fromtraditional electronic spectroscopy. For example, removing an electron from a doublet anion canpossibly yield either a singlet or a triplet neutral molecule, allowing for determination of thesinglet−triplet splitting. Of course, in addition to the spin angular momentum, the angularmomentum of the system also needs to be conserved. Since the incident photon has an angularmomentum of 1ħ, photodetachment leads to a change of one unit of angular momentum, ∆l = ±1.
If an electron is detached from an s orbital (l = 0), the photoelectron leaves as p-type outgoingwave (l = 1). In terms of the electronic selection rule, single-photon detachment allows thetransitions involving the removal of a single electron from an orbital, without changing otherelectrons.

7

1.1.4 Wigner Threshold Law
Another factor influencing the photodetachment intensity arises from the strong energy
dependence of the near-threshold anion photodetachment cross section. While thresholdphotoionization of a neutral molecule is dominated by the long-range coulomb interaction between
the ejected electron and the cation core, the longest range interaction in negative ionphotodetachment arises from the centrifugal potential  ∝l ( l + 1)  associated with the angularr2 momentum of the outgoing electron. The energy dependence of threshold processes in which apair of final state particles is produced the dominant long range interaction being via the centrifugal
potential was derived in a classic paper by Wigner, and is now known as the Wigner Threshold
Law. This effect is extremely important for photodetached electrons with low kinetic energy,
which greatly affects the intensity in a SEVI spectrum. The photodetachment cross section nearthreshold is given by17,18
!" ∝ ($%&)

'(/

(1.8)

where eKE and l are the kinetic energy and the angular momentum of the ejected photoelectron,
respectively. The values of the angular momentum, l, are 0 (s-wave), 1 (p-wave), and 2 (d-wave).
The higher the angular momentum is for the departing electron, then the stronger the long-range
interaction (or higher centrifugal barrier). At a photon energy very close to the threshold, onlytransitions leading to the ejection of s-wave photoelectrons from a p-type orbital have significant
intensities, where the cross section scales as eKE1/2.

8

1.1.5 Photoelectron Angular Distributions
In addition to the electron velocity distribution, the reconstructed images show thedifferential cross section, which provides the photoelectron angular distribution with respect to thecylindrical symmetry axis, defined by the laser polarization direction. For linearly polarized lightand single-photon detachment, the photoelectron angular distribution is described by the
relationship19
I(θ ) =dσσ=
[1 + β P2 (cosθ )]
dΩ 4π

(1.9)

where θ is the angle between the velocity vector of the ejected electron and the laser polarization
vector, P2(cosθ) is the second-order Legendre polynomial, and σ is the total cross section, and Ω
is the solid angle. The anisotropy parameter, β, is determined by fitting the photoelectron angular
distribution with Eq. 1.9. The β value varies from β = -1 (dσ/dΩ ~ sin2θ, a perpendicular transition)to β = 2 (dσ/dΩ ~ cos2θ, a parallel transition) and depends on the orbital angular momentum, l, ofthe ejected electron. For atoms, β = 2 corresponds to an outgoing p-wave electron, arising fromelectron detachment from an atomic s-orbital at any energy; β = 0 corresponds to thresholddetachment from a p orbital, and β = -1 can stem from a linear combination of s and d waves thatcomes from above threshold photodetachment of a bound p-electron. For molecules, β can provide
information to distinguish between transitions to different electronic states.20

1.2 Thesis Overview
This thesis discusses development and application of novel pulsed anion sources to obtain
the photoelectron spectra of alkyl and radical anions taken with different photon energies to9improve the spectral resolution. In Chapter 2, the experimental apparatus, described in more detail,consists of the pulsed anion sources, the Wiley-McLaren time-of-flight mass spectrometer, theVMI detector, and the laser system. Various pulsed anion sources used in this thesis are presented.
The laser systems employed here are described along with descriptions of data analysis. Chapter
3 details the design and characterization of a novel, cold anion source. Applications of this anion
source are also demonstrated. In Chapter 4, photoelectron spectroscopy of methyl anion is
discussed. The EAs of CH3 and CD3 are measured accurately; using a thermochemical cycle, themethane gas-phase acidity is refined. The observation of the well-studied v2 (umbrella) bands of
CH3 and CD3, coupled with the dramatically improved electron energy resolution, enables the
direct measurement of the inversion splitting between the 0+ and 0− energy levels in both CH3− andCD3−. Chapters 5 and 6 focus on the photoelectron spectra of H2CCC and HCCCH anions,
respectively, to obtain information about the high-lying electronic excited states of thecorresponding neutral molecules.

10

Chapter 2
Experimental Apparatus
2.1 Overview
An overview of the experimental apparatus is illustrated in Fig. 2.1. The detailed
description of the apparatus is available in previous publications.21-23 The apparatus consists offour main sections: an anion source, a linear time-of-flight mass spectrometer (TOF MS),24 avelocity-map imaging (VMI) photoelectron spectrometer,25,26 and a tunable laser system. In thisthesis, two types of pulsed anion sources are developed and employed for anion formation, bothof which offer significant improvements over the original anion source.27 Specifically, the anions
formed in Chapters 3 and 4 are generated using a pulsed, perpendicular, plasma-entrainment anion
source. In Chapters 5 and 6, the anion syntheses occur via an ion-molecule reaction using a pulsed,
coaxial, supersonic neutral-entrainment reactor. Following anion formation, anions are accelerated
to ~3 keV, and mass-separated using a Wiley-McLaren time-of-flight mass spectrometer before
entering the laser-ion interaction region. Electrons are then photodetached from the anions withradiation from a tunable laser system, allowing broad control of the kinetic energies of the detachedelectrons. Subsequently, a velocity-map imaging detector is utilized to obtain photoelectron
velocity and angular images. The photoelectron images are reconstructed and analyzed to convert
into photoelectron energy spectra. The experimental apparatus and data analysis will be further
described in the following sections.

11

Figure 2.1 Overview of the experimental apparatus. Red represents the ion source, green
represents the TOF MS region, brown represents the VMI, and blue represents the laser system.

12

2.2 Pulsed Anion Source
Two recently developed variations to the entrainment anion sources are employed throughout
this thesis. Typically, this laboratory has generated a pulsed, anion beam by having a gas (or gas
mixture) expand through a single, pulsed valve. The expanding gas undergoes interactions with abeam of electrons and produces slow, secondary electrons that attach to the molecules, forming
anions.27 Other common methods to produce anions include an electrical discharge source or
sputtering,28-31 laser vaporization,32-35 and electrospray ionization (ESI).36-39 However, thesemethods tend to be more “black magic”, which makes it problematic to carry out complex
syntheses of vibrationally cold anionic species. In order to enhance the anion-synthesis capability,
we have employed multiple, pulsed entrainment valves to enable stepwise chemical anion
synthesis that has proven exceptionally effective in continuous flowing afterglow experiments.4043

In addition to operation of multiple valves, different entrainments schemes are employed toincorporate plasma containing anions and/or electrons into the primary expansion. Currently, ourrealization of both arrangements (schematically shown in Fig. 2.2) utilize two pulsed valves withseparate timings and pulse widths that are controlled by commercial (General Valve) Iota-1 valve
drivers and a digital delay generator. When operating the two pulsed valves at a repetition rate of
80 Hz, the pressure within the source chamber increases to ~5x10-5 Torr from 1x10-7 Torr withouta gas load when being pumped by a 10” diffusion pump. Additional pulsed valves could be
employed for multistep syntheses if necessary.

13

Figure 2.2 Schematic drawing of (a) the pulsed plasma-entrainment ion source and (b) the neutralentrainment ion source. In this thesis, both designs use two pulsed valves but have the capability
to entrain additional gases using additional pulsed valves.

14

Figure 2.2(a) displays schematically the arrangement of the pulsed, plasma-entrainment anion
source. The design, construction, and performance of the pulsed, plasma-entrainment anion source
are more fully described in Chapter 3. The major feature of this source is the additional pulsed
valves for perpendicular entrainment of plasmas. The plasma is created by choked-flow expansionof an appropriate gas through an electrical discharge. The discharge electrodes are mounted onto
the faceplate of the side pulsed valve that is oriented perpendicular to the main valve. The plasma
from the side discharge valve, which operates under choked-flow conditions, interacts with themain supersonic expansion of Ar (backing pressure of 3.7 atm). The Ar acts as a carrier gas andprovides additional collisional cooling. The principle of the supersonic entrainment technique wascharacterized and extensively studied by Campargue, in which a background gas outside of theshock-wave structure of the jet will be drawn into the free-jet zone of silence far from theorifice.44,45 These complexities arises from the background gas colliding with the dense expansion,
producing shock fronts and regions inaccessible for background gas entrainment. However,
employing short, pulsed gas expansions into high vacuum essentially eliminates these shock
effects, and the Campargue analysis does not apply. Specifically, there is no impediment tointroducing a low-density gas (or plasma) into the barrel of the main, dense, cold expansion. It isalso possible to entrain other plasmas, electrons, radicals and/or solvents at various points further
downstream from the main Ar expansion. Therefore, this technique provides the capability to makesubstantial clusters of entrained anions, e.g., OH–(Arn =0−32), as well as to stabilize products of
exothermic reactions in the primary expansion, e.g., HOCO– and H3COO–. Since many anions
formed in this anion source are demonstrated throughout this thesis, the experimental conditions
will be described in each respective chapter.

15

Figure 2.2(b) presents a schematic drawing of one realization of the neutral-entrainment anion
source also used in this thesis work. The anion source is an entrainment reactor, consisting of acentral and two side pulsed General Valves with separate timings and pulse widths. Robertson etal. first developed and built a pulsed supersonic-entrainment reactor using the supersonicentrainment principle, originally by Campargue,44,45 to make cold anionic clusters.46 Weber then
combined the supersonic entrainment technique with laser vaporization to prepare metal
containing cluster anions.47 Following their design of the entrainment reactor, we synthesize theanion of interest by entraining N2O to produce O– with aid of an electron beam. For example, O–
reacts with allene and propyne to yield H2CCC– and mixtures of H2CCC– and HCCCH–,
respectively, via a H2+ abstraction reaction; this is similar to the flowing afterglow ion source,which has great control over rational anion syntheses. In order to produce the O– reactant, one of
the side valves entrains neat N2O gas into the main expansion, which contains the other reactant
embedded in Ar. This entrained, main expansion interacts with a continuous beam of 1 keVelectrons, which are emitted from a thoriated tungsten filament in a homebuilt electron gun.
Collisions between the gas expansion and the electron beam yield slow secondary electrons thatform the O– reactant through dissociative electron attachment of N2O. To increase the number ofanions generated, the electron beam is collimated towards the main valve with a ring magnet.
Subsequently, the O– reacts with propyne or allene to form the m/z = 38 anion. If the N2O
entrainment valve is turned off, the m/z = 38 signal disappears. Additional experimental details areprovided in Chapter 5 and 6.

16

2.3 Time-of-Flight Mass Spectrometer
The apparatus is equipped with a Wiley-McLaren TOF MS for mass selection. After drifting
20 cm in the supersonic expansion, anions are perpendicularly repelled by a high-voltage pulse
constituting the first electric field of a 2-m long Wiley-McLaren time-of flight mass
spectrometer.24 Because the anions being repelled have a significant extent (~5 mm) in the
direction of this field, rather a finite volume and not from a point source, the length of the flightpath in the mass spectrometer is not identical for all anions. Therefore, we use a potential
distribution across the repelling field. This ensures that the ions traveling a longer distance havegreater kinetic energies, and the ions with the same mass reach the anion-laser interaction region
at the same time, independent of their starting positions. In this way, the amplitude of the repelling
pulse is adjusted for spatial focusing (Wiley-McLaren condition) with a typical value of about- 700 V across an approximately three-inch gap. The timing of the repelling plate pulse with respect
to the time of the opening of the pulsed ion source valve is optimized to maximize ion signals inthe detection region. Upon entering the TOF MS, the ions pass through a 3-mm diameter orificeof the grounded electrode that also acts as a differential pumping stage between the source chamber
and the rest of the instrument. The ions then enter the acceleration stack, where they are accelerated
to +2300 V by 10 evenly spaced electrodes and are given a final kinetic energy of ~3 keV. While
the Wiley- McLaren configuration allows high mass resolution, an important side effect is asignificant energy spread arising from the finite extent of the initial ion packet in the direction of
the extraction field.
The accelerated ions are steered by vertical and horizontal deflectors and spatially focused bya cylindrical einzel lens. Employment of a pulsed potential switch rapidly re-references the ionsfrom +2300 V to earth ground when passing through the potential switch. The purpose of this17

process is to keep the detector region with the imaging system at earth ground without floating thedetector region at the existing 2.3-kV beam potential.48 Using an inline microchannel plates,(MCP), detector, the time dependence of the ion signal is measured on a digital oscilloscope toacquire a mass spectrum of the anions. To find the optimal temporal overlap between the massselected anions and the laser beam in the ion-laser interaction region, we monitor the neutrals
generated after photodetachment by deflecting the anions prior to the inline detector. Also, a setof horizontal deflectors provides a small correction to the ion trajectory prior to the ion-laserinteraction region, which optimizes the VMI focusing. The pressure in the detection region, which
is pumped by 230 l/s and 685 l/s turbomolecular pumps in parallel, is about 3x10-9 Torr undernormal operational conditions.

2.4 Velocity-Map Imaging Photoelectron Spectrometer
The photoelectron imaging spectrometer is based on the original concept of Eppink andParker.26 Its high sensitivity is an advantage compared to other methods of photoelectron detection.
The VMI analyzer is located at the spatial focus of the ion beam, where photodetached electrons
are extracted perpendicular to the ion beam. The VMI assembly is composed of three oxygen-freehigh-conductivity copper electrostatic plates (a solid repeller, annular ground, and extractor), a
20- cm long electron flight tube, an annealed µ-metal, and a 40-mm diameter microchannel plateelectron multiplier. The diameter and thickness of the three plates are 3″ and 0.03″, respectively.
The ground and extractor plates have a 1″ central aperture. These plates are spaced by 1″. In orderto repel the photodetached electrons, the repeller plate is pulsed between -10 and -150 V,
depending on the desired electron kinetic energy range. The middle plate is held fixed at earth
ground (0 V), and the typical voltage on the extractor is fixed between +30 and +450 V, a value

18

selected to extract photoelectrons toward the electron detector over the desired electron kinetic
energy range under velocity-map imaging conditions. At optimized velocity-focusing conditions,the ratio of the Vrepeller to Vextractor is -1:2.78. In addition, the extractor is electrically connected to
the flight tube and the front plate of the electron detector to create a field-free flight region. The
whole assembly is surrounded by µ-metal shielding to protect the photodetached electrons fromexternal magnetic fields, which alter the trajectories of electrons.
After the photodetached electrons travel through the field-free flight region, they map onto a
position sensitive detector, which includes a gated pair of 40-mm MCPs (gain ~ 106), a phosphor
screen (Burle, Inc.), and a cooled (-11 OC) charge-coupled device (CCD) camera (LaVision). The
kinetic energy and angular distribution of the photoelectrons determines the shape and distributionof the measured image, which is transferred to the computer by the DaVis image acquisition
software. The imaging-quality MCPs (chevron configuration) are held at + 1350 V to minimize
background signals and then are pulsed to + 2 kV for ~ 2 μs by a home-built pulser to have fullgain of the signal. Additionally, the overlap timing between anion and laser beam is set at thecenter of the pulse of the MCPs to avoid the voltage instability. The voltage on the phosphor screen
is kept +3 kV higher than the voltage on the MCPs, i.e., + 5 kV. To capture the image on thephosphor screen, we use the CCD camera. The camera has a wide angle lens, and therefore, theheight needs to be adjusted to focus on the back of the phosphor screen (the image). Typically, thecamera exposure time is 5 seconds for the sequence summing function and 500 ms for the event
counting mode. The event counting mode utilizes the centroiding procedure, which finds the centerof each electron impact (a few pixels in diameter) and reduces it to a single pixel. In contrast, thesequence summing function without centroiding collects data faster but with slightly worse energy
resolution. In our instrument, the energy resolution, ∆Ek/Ek, with centroiding is about 2% and19without centroiding is 3%. For 2% resolution, if the electron kinetic energy is 0.3 eV, then the ∆Ekwill be 6 meV (i.e., 48 cm-1). For electron kinetic energies below 0.1 eV, the spectral resolution
cannot be less than 3 meV due to the velocity spread within the anion beam.

2.5 Laser System
The laser radiation for these photodetachment experiments include the fundamental (1064nm), second harmonic generation (SHG, 532 nm), and third harmonic generation (THG, 355 nm)of the neodymium doped yttrium aluminum garnet (Nd:YAG, Coherent, Inc., Infinity) laser. To
gain more tunability in the radiation used, we also employ a 532-nm pumped dye laser (PDL-1),
355-nm pumped optical parametric oscillator (OPO), and difference frequency generation (DFG).
The various setups allow us to tune the radiation between the IR and UV and are described in moredetail below.
In this thesis, the nanosecond Nd:YAG laser system operates at 80 Hz and typically outputs 5
to 300 mJ/pulse of 1064-nm light, depending on applications. The 1064-nm light can be doubled
in a temperature-controlled type-I β-barium borate crystal (BBO) to generate 532-nm light. For
THG, the 1064-nm and 532-nm light with the same polarization are combined in a second
temperature-controlled type-I BBO crystal with different phase-matching angles to produce 355nm light. In order to have the fundamental and SHG at the same polarization, a half-waveplate isused in the beam path between the SHG and THG crystals to rotate the 1064-nm polarization by
90° for the sum-frequency generation (SFG) process. By tuning the angle of the SHG and THG
crystals, the power of 532-nm and 355-nm light can be optimized. This Nd:YAG laser also has abuilt-in 355-nm pumped OPO and BBO-based SHG stage for doubling signal or idler output from20

the OPO, which provides a wavelength range of 220−2000 nm. The typical linewidth of the signal
and idler light is 5−10 cm-1; however, light near the degeneracy, 710 nm, has an increased
linewidth, 20−100 cm-1. To obtain a narrower linewidth spectra, the PDL-1 dye laser is used, andthe schematic layout is shown in Fig. 2.3. In the dye laser, there are three cells, an oscillator, a preamplifier, and an amplifier. This dye laser can be side- or end-pumped by 532-nm light withvertical polarization, depending on the requirements of different dyes. The fundamental output ofthe dye laser can be doubled in an additional BBO crystal to generate UV light. This provides anoptional wavelength range of 250−850 nm with energies of 0.1 to 30 mJ/pulse.
For the data obtained in Chapter 4, we needed mid-IR 3236-nm light, which cannot be
produced by the setups discussed above. Therefore, we configured a DFG setup to produce themid-IR light by mixing 801-nm light (from the 532-nm pumped LDS798 dye laser) with theresidual 1064-nm light in a 1x1x3 cm3 magnesium oxide doped lithium niobate (MgO:LiNbO3)
crystal. The schematic is depicted in Fig. 2.3. The MgO:LiNbO3 crystal, purchased from the United
Crystals Inc., has type-I phase-matching angle at 44.1°, an AR-coating at 1064 nm and 750 – 830
nm on the entrance surface, and AR-coating at 2500 – 3800 nm on the exit surface. The 801-nm
and 1064-nm lights have to simultaneously reach the non-linear crystal, i.e., the same traveling
distance. The polarizations of the dye laser output and 1064-nm light are vertical and horizontal,
respectively, and therefore, the resulting polarization of mid-IR light is horizontal. Using a band
filter to absorb 801-nm and 1064-nm lights, the power of the 3236-nm light can be monitored bya power meter and optimized by tuning the phase-matching angle of the MgO:LiNbO3 crystal. The
mid-IR light, energy between 50 and 100 µJ/pulse, is focused into the anion-laser interaction region
using a 1-m CaF2 lens. We measure the dye wavelengths using a wavemeter (Atos, LRL-005)

21

calibrated by a He-Ne laser. Based on the dye wavelength, we then calculate the exact mid-IR
wavelength:(*+,-./0

=

(
*123

−

(
*5678 9+.

22

Figure 2.3 Schematic layout of mid-IR generation utilizing a Coherent Infinity Nd:YAG laser anda QuantaRay PDL-1 dye laser. The dye cells (yellow) in the day laser are pumped by the 532-nm
light (green), which laser polarization is flipped to be vertical using a half-wave plate. The residual
1064-nm light (red) with horizontal laser polarization is aligned through the MgO:LiNbO3 crystal,and is mixed with the dye laser output (orange) to generate a mid-IR light with horizontal laser
polarization. The delay stage is used to simultaneously get both lights to the non-linear crystal,i.e., the same traveling distance. The power of the mid-IR light is optimized by rotating the phasematching angle of the MgO:LiNbO3 crystal.

23

For the reconstruction of the photoelectron image in this experimental setup, the laser
polarization is required to be parallel (horizontal polarization) to the imaging MCPs and at thecorrect height to enter the vacuum chamber. Therefore, vertically polarized light is rotated 90° by
a twisted periscope, in which two mirrors are perpendicularly arranged, and horizontally polarized
light only has the height adjusted by a normal periscope, parallel arrangement of two mirrors.
Before the laser beam is aligned through the anion-laser interaction region, the laser power isreduced to 0.1 – 3 mJ/pulse to avoid “drilling” the exit window on the chamber. To further avoid
damaging the exit window, the light is softly focused through the detection region with a 1-m focal
length lens. The overlap timing of the laser with the anion beam is adjusted by a Stanford Digital
Delay pulse generator (Stanford Research, DG535), set to obtain the strongest detachment signals.
The laser-light wavelengths are measured using a wavemeter (Atos, LRL-005) calibrated by a HeNe laser or using the known spin-orbit splittings of atoms, such as I and S, from the measured
photoelectron image.

2.6 Data Analysis
Following photodetachment of the electrons, the three dimensional (3-D) laboratory frame
photoelectron distribution is projected onto a two dimensional (2-D) imaging MCP detector, which
is digitized by the DaVis camera software, and transferred to a computer for analysis. The raw 2-D
images must first be reconstructed back into the nascent 3-D photoelectron distribution. This isaccomplished by using the basis set expansion (BASEX) Abel inversion software package
developed by Dribinski et al., which ultimately converts the raw 2-D photoelectron images into
nascent 3-D velocity and angular distributions.49 The velocity distribution is the angle-integrated
intensity as a function of radial pixel (typically half pixel) from the center of the ring image. Hence,

24

the center of the photoelectron image is very crucial that a symmetrization procedure is developed.
Take vertical symmetrization of the bottom half of the image as an example; we get rid of the top
half of the image, duplicate the data of the bottom half, flip it vertically, and paste it back onto the
top half. The center is determined by using different (x,y) coordinates to carry out verticalsymmetrization of the image until velocity distributions of symmetrized images are matched.
Likewise, horizontal symmetrization is carried out using left and right half of the image. Both thesignal and background images are individually reconstructed by the BASEX using the same center.
If the quadrant-symmetrized or symmetrized image results in a photoelectron spectrum with better
signal-to-noise ratio or lower uncertainty, we report those data.
To obtain the photoelectron spectrum in terms of the electron kinetic energy, eKE, thevelocity domain photoelectron spectrum is transformed to the energy domain using the appropriate
Jacobian transformation, Eq. 2.1.,
I(E) ∝ I(v)/v(2.1)

where v is velocity. In addition to the electron velocity distribution, the reconstructed images
contain the differential cross section, which provides the photoelectron angular distribution withrespect to the cylindrical symmetry axis, defined by the laser polarization direction. Anisotropy
parameters obtained from the photoelectron angular distributions are useful to assign electronic
states of neutral molecules and were discussed in Chapter 1.
The calibration of the energy scale for the reported data is performed using well-known
peak energies. In particular, EAs and spin-orbit splittings of atomic species are mostly used inthese experiments because of narrower peak widths due to a lack of an internal energy. We reportphotoelectron spectra as a function of electron binding energy (eBE= ħω – eKE), which is
independent of the laser wavelength used for photodetachment. Figure 2.4 demonstrates the 532

25

nm (2.330 eV) photoelectron spectrum of S–, which is used for calibration.50-52 The spectrum isreconstructed from the quadrant-symmetrized photoelectron image with centroiding. The S– anions
are produced by dissociative electron attachment of CS2 with slow secondary electrons. The
observed peaks, labeled a through f, are well-separated, and the corresponding transitions areshown in the inset. The spectral resolution is about 2% and defined as ∆Ek/Ek, in which ∆Ek is themeasured energy width of the peaks and Ek is electron kinetic energy at the center of the peak.

26

Figure 2.4 Photoelectron spectrum of S– at 532 nm. The spectrum is reconstructed from thequadrant-symmetrized photoelectron image. The peaks are labeled a through f, and the peak widths
are also shown. The corresponding known transitions are presented in the inset.

27

Chapter 3
Versatile, Pulsed Anion Source Utilizing PlasmaEntrainment: Characterization and Applications
3.1 Introduction
Negative ion photoelectron spectroscopy has been widely applied to investigate thespectroscopic properties of neutral species2-4 and the dynamics of photodissociation,6,9 cage
recombination,5,7 SN2 reaction,8 isomerization,11 and charge transfer.10,12 In order to study
extensive aspects, a variety of anion sources have been developed for continuous wave (cw) andpulsed instruments. The flowing afterglow ion source,53-56 which has great capabilities for
multistep ion syntheses, was adapted from the conventional continuous anion-laser beam
photoelectron spectrometer.57,15 However, no pulsed anion sources have attempted to carry out
rational ion syntheses in a supersonic expansion. In this chapter, we characterize the design of anovel, pulsed anion source based upon entrainment of ions and/or electrons using a plasma source.
This pulsed anion source permits multistep syntheses of cold anionic species that are very difficult
to produce using conventional pulsed anion sources. Some applications of the development arealso demonstrated.
A pulsed anion beam is typically constructed using a pulsed supersonic jet that coaxiallyor perpendicularly collides with a continuous beam of 1-keV electrons from an electron gun.27 For
the formation of some anions, an electrical discharge source28-31 or a laser vaporization source32-35
is required and results in a high temperature environment. All of these sources use a main pulsed
expansion to create ions. Generally, a supersonic expansion of molecules in a carrier gas leads tosufficient cooling of the internal degrees of freedom for high-resolution spectroscopic applications.
However, a gas expansion that passes through a hot plasma, for example, contains too manyenergetically excited species to completely cool the anions formed. The insufficient internal

28

cooling of anions in the supersonic anion jet broadens peaks and, possibly, generates dense spectra,
both of which raise the difficulty of extracting spectroscopic information.
There are two methods to reduce the internal energy of the anions studied, the Ar-tagging
technique46,58 and cryogenic ion trapping.59,60 For the Ar-tagging technique a “cold” subset of theanions are selected. Since Ar atoms weakly bind to anions, mass selecting for anion-Ar clusters
limits the internal energy of the anion, suppressing hot bands and reducing spectral congestion.6164

The supersonic entrainment technique is often employed to attach Ar solvent atoms to anions

and build large complexes. The Ar atoms are boiled off to cool the anion, thus the anions in thelarge clusters have low internal energy.46 Another method is to use a cryogenic ion trap. In thismethod, an ion beam is stored in a low-temperature environment for milliseconds or seconds,
which is capable of cooling ions via radiative relaxation or collisions with a cold buffer gas. The
resulting cold anions are released from the trap having low internal energies. Using a cold ion
technique can increase the resolution of photoelectron spectroscopy to obtain more accurate
spectroscopic properties of neutral molecules. Although both of these methods have ion-coolingcapability, they do not allow for great control over rational anion syntheses, as in the flowing
afterglow source.
Yet another method, the continuous pick-up method, is capable of making cold anionicclusters. The cluster anion of the O–(Ar)n (n = 0 ~ 34) has been formed in a continuous expansionof about 9-atm Ar gas, maintained at 196 K, that has been intersected with O– (made by theinteraction of electrons, emitted from a filament, with N2O gas).65 One can expect that O– waspicked up by neutral Ar clusters that were formed in the cold, condensed Ar gas tank. Other ion
sources have been employed to investigate large ionic systems relevant to the condensed phase.
Electrospray ionization (ESI) sources are frequently used to transfer ionic species from solution

29

into the gas phase. These ions can then be mass-selected, stored, and cooled in a cryogenic ion trap
via collisions with a cold buffer gas for further different measurements.36-39 Additionally, Rydberg
electron transfer66 and a laser-assisted desorption technique67 have been used to attach electrons to
nucleobases and amino acids, respectively.
In this chapter, a detailed description of the pulsed plasma-entrainment anion source isgiven. Its performance is documented with a variety of applications. This anion source utilizes twopulsed valves: one serves to create the supersonic expansion, the other is placed perpendicular tothis main expansion and utilizes an electrical discharge. Anions are made in a side pulsed dischargethat creates a violent, high-energy environment. The Ar beam picks up the anions in a region ofhigh density, providing collisional cooling to the anions. It is informative to explore the many usesof such a source. This pulsed anion source has successfully generated OH–(Ar)n (n = 0 ~ 32), andwe obtain the photoelectron spectra of n=0–3, 7, 12, and 18. Further, using this anion source, we
are able to perform condensation reaction, and we demonstrate this by producing HOCO– isomers
and CH3OO–. The photoelectron spectra of bare HOCO– are presented and compared to the
published spectrum taken under a cryogenic environment.68 Lastly, the photoelectron spectra of
CH3OO– formed by an association reaction in this pulsed anion source and by a proton abstraction
reaction in a continuous anion source,69 are compared, illustrating the possibilities of making
transient species through rational chemical means.

3.2 Anion Source Details
A thorough description of the apparatus can be found in Chapter 2; the design of theplasma-entrainment anion source is described here in more detail. Figure 3.1 shows a schematicarrangement of this pulsed anion source. The setup employs two pulsed General Valves (Parker-

30

Hannifin, series 9): one functions to generate the supersonic expansion, and the other is oriented
perpendicular to this main expansion and utilizes an electrical discharge. Since the two valves are
independently controlled by a commercial multi-channel Iota-1 driver and a digital delay
generator, temporal overlap of the two gas pulses can be adjusted in real time. A plasma is createdin an electric discharge that is mounted onto the faceplate of the side valve, and is introduced at a
distance of about 3 mm beyond the faceplate of the main valve. The side discharge nozzle operates
under choked-flow conditions (the gas throughput is roughly one order of magnitude less than thatfrom the main valve) with a 250-µs gas pulse, and entrains plasma into the main expansion of Ar.
The Ar acts as a carrier gas and provides additional collisional cooling. The principle of supersonicentrainment was first characterized and extensively studied by Campargue; it was determined thatgas outside the shock-wave structure of the jet was drawn into the free-jet zone of silence far fromthe orifice.45 This study confirms that the low-density gas from the side pulsed valve does not
disturb the main Ar expansion, and that it is possible to entrain electrons, cations, anions, radicals,
and solvent species into the main Ar expansion. In order to measure the time spread of the ion
packet that is entrained into the main expansion, we vary the timing of the extraction plate that
directs anions down the time-of-flight mass spectrometer; we find that the temporal FWHM of theion-packet is about 20 µs, indicating a small spatial coverage of ions.

31

Figure 3.1 Schematic arrangement of the pulsed anion source. The source is capable ofentraining ions from a side expansion.

32

In order to increase the forward intensity of the main expansion, we use a 40° conicalnozzle with a 0.5-mm orifice (DO) and an aspect ratio (L/DO) of 7.70 The nozzle seals in a similar
fashion as a commercial faceplate and, thus, couples well with a General Valve body. Even though
we run the pulsed discharge at a duration time of 140 µs to produce a large quantity of anions,internally cold anions can also be generated in the main expansion after entrainment. Furthermore,sequential addition of neutral reactants to cold anions provides for the possibility of implementing
stepwise ion syntheses in the pulsed expansion. This new source gives multiple pathways to theformation of a wide variety of anions prior to extraction into the TOF MS and VMI detector.
The pulsed discharge source is comprised of two stainless steel electrodes, one is attached
to a commercial General Valve faceplate (0.8-mm orifice) held at ground and the other is pulsed
to negative high voltage; a cylindrical MACOR plate insulates the two place from each other. A
second MACOR ring is added at the end of the whole assembly. The grounded S.S. electrode has
an inner channel with diameter of ø0.04″ and a thickness of 0.075″. The inner channel diameter ofthe MACOR insulator in between the two electrodes is ø0.06″, and its thickness is 0.04″. The S.S.
electrode that is pulsed to high negative-voltage has a 40° conical nozzle with a ø0.04″ orifice
diameter and a thickness of 0.32″. The inner diameter and thickness of the final MACOR ring areø0.39″ and 0.15″, respectively. Each plate has four holes, which hold the whole stack together
using nylon screws. The outer diameter of all the plates is ø1.5″. Appendix A shows a schematic
drawing of the home-made nozzle and all the discharge components. Appendix B gives a blowout figure of the discharging region. We use a pulsed voltage between -800 and -2000 V tomaximize the ion intensities and the stability of the discharge. The negative-voltage pulse isprovided by a pulse generator (Directed Energy, Inc. (DEI), GRX-3.0K-H) with a DC power
supply (Bertan associates, Inc., 205-03R). The pulser (triggered by a digital delay generator)

33

controls the discharge timing and pulse duration, both of which are adjusted to optimize the ion
intensities. Additionally, using two 10kΩ resistors in series with the cathode increases thedischarge stability. Time traces for the pulser output voltage and the voltage measured across theelectrode for a -2 kV, 40-µs discharge are shown in Fig. 3.2, with an inset illustrating the circuit
diagram. When -2 kV (in a 40-µs pulse) is applied through the two 10 kΩ resistors, the voltage
drop across two resistors is -1.5 kV, corresponding to a discharge current of 75 mA. The ballast
resistor depicted allows the -2 kV between the plates to initiate the discharge, but then limits thedischarge current to a maximum of 100 mA, providing current regulation and arc suppression.

34

Figure 3.2 Time trace of a -2kV discharge voltage for OH– production. Through two 10kΩ
resistors, the voltage drops about 1500 V, which corresponds to the peak discharge current of 75
mA. The inset represents the circuit diagram, in which the discharge electrodes are under vacuum
condition in the source chamber.

35

For the preparation of the desired anions, a suitable gas mixture is crucial. For example,
OH– is readily formed in the discharge of 1% O2 and 25% H2 in Ar at a backing pressure of 4.7
atm. We use this mixture to study OH–(Ar)n clusters; Appendix C lists the experimental conditionsof OH–(Ar)n cluster formation. We have also tried various combinations of N2O/CH4/Ar,
N2O/H2/Ar, and O2/CH4/Ar for OH– production; however, in all cases the discharge of these gas
mixtures produce OH– intensities that are less intense than the O2/H2/Ar mixture. Furthermore, forthese mixtures many N2O– or CH4–solvated anionic complexes, e.g., OH–(N2O)n or OH–(CH4)n,also form at a number of mass-to-charge ratios, resulting in an undesirable background. The gas
mixtures used in the formation of various other anions will be pointed out in each section.

3.3 Results and Discussion
In this section, we demonstrate the performance of this pulsed anion source using mass
spectrometry and photoelectron spectroscopy. First, we discuss electron entrainment into the main
expansion from the side valve. Then, we present and discuss photoelectron spectra of CH3–,
OH-(Ar)n, CH3O–(Ar), CH3O–(H2)n, HOCO–, and CH3OO– to confirm our ability to make a widearray of cold anion with this novel anion source.
3.3.1 Electron Entrainment
After initiating a discharge in the side expansion, the gas forms a plasma (a neutral medium
consisting of many charged particles: electrons, cations, and anions) with a Debye shielding length
which is much shorter than the physical dimension of the plasma. These electrons will become
entrained into the main expansion, making the formation of anions by attachment to neutral
molecules in the supersonic jet possible. However, the charged species of a plasma do not moveindependently. The electric field of the quickly moving electrons drags ions along with them,36

creating an ambipolar diffusion that results in the electrons and ions having the same diffusion
rate.71 This ambipolar diffusion makes it possible to form anions through attachment of slow
entrained electrons. We demonstrate this in Fig. 3.3, which shows a comparison between two mass
spectra obtained using neat Ar (black) and 1% O2 in Ar (red) in the main expansion. In both cases,
neat Ar is used in the side discharge source (-2 kV in a 140-µs pulse). The addition of O2 in themain expansion causes the formation of O– and O2– anions, reinforcing the electron-entrainment
application of this pulsed anion source. It is worth noting that the ion intensities obtained are atleast as intense as other common sources, making this method a plausible way to form anions
without too much unwanted “heating” of the anions.

37

Figure 3.3 Comparison between TOF mass spectra from experiments using neat Ar (black) and
1% O2/Ar (red) for the main expansion. Neat Ar gas is used for both experiments in the side
discharge source. Entrainment of electrons is demonstrated from the additional formation of theO– and O2– anions.

38

3.3.2 Exotic Species: CH3–
Using conventional pulsed anion sources, it is extremely challenging to produce CH3–.
However, this pulsed ion source provides a straightforward way for making cold CH3–. In 1978,
Ellison et al. successfully measured the photoelectron spectrum of CH3– (formed in a discharge)
and determined the EA of the methyl radical.72 The CH3– spectrum revealed an extensive
progression in the out-of-plane bending (umbrella) mode of the methyl radical in its electronic
ground state. In this experiment, we use an electrical discharge of neat CH4 gas in the side pulsed
valve, and in the main valve we have a neat Ar expansion (3.7 atm). The pulsed electrical discharge
takes place when the CH4 gas passes through a stainless steel cathode, which is pulsed (140 µs) at
a potential of -900 V in series with two 10kΩ resistors (for increased discharge stability), to astainless steel grounded anode. The ions formed are then picked up by the Ar expansion and further
cooled. Interestingly, CH3– anions were not observed using a dilute gas mixture of 3% CH4 in Ar
in the discharge; see Fig. 3.4. Figure 3.5 presents the photoelectron spectrum of CH3– using
1064-nm (1.165 eV) radiation. This spectrum is consistent with the previous spectrum taken in
1978, and further discussions on this system can be found in Chapter 4.

39

Figure 3.4 Time-of-flight mass spectrum from experiments using (a) 3% CH4/Ar and (b) neat CH4
in the side discharge source. Neat Ar gas is used as the main expansion for both experiments. The
chemical identity of the mass-selected anions is confirmed by comparing their photoelectron
spectra with other known anion photoelectron spectra. F– comes from chemical contamination insample line.

40

Figure 3.5 The 1064-nm photoelectron spectrum of CH3– showing an extensive progression in
the out-of-plane bending (umbrella) mode of CH3.

41

3.3.3 Ar-tagging: OH–(Ar)nFigure 3.6 shows composite mass spectra of Ar-tagged OH– ranging from 0 to 32 Ar atoms,
OH–(Ar)n=0-32. Below m/z=50, three predominant peaks occur: O–, OH–, and O2–. These ions areformed in the discharge source and drawn into the main expansion of neat Ar. Once in the main
expansion, some of the OH– anions cluster with Ar atoms generating OH–(Ar)n; the portions of themass spectrum showing 7 ≤ n ≤ 32 are magnified and shown in two insets. The peaks for n = 12
and 18 are prominent, suggesting that 12 and 18 are “magic numbers” that indicate a particularly
stable cluster geometry.73,74 These magic numbers (of 12 and 18) are also observed in O–(Ar)n andcorrespond to the closing of the solvation shell in an icosahedral (12 Ar atoms) and double
icosahedral (18 Ar atoms) structure.65 Similar solvation shells likely account for the peakintensities observed for n = 12 and 18 in the OH–(Ar)n mass spectrum.

42

Figure 3.6 Composite mass spectra of OH–(Ar)n=0-32. The insets show the color-coded 5x (redcurve) and 20x (blue curve) magnifications of the selected mass spectral region to better view thecluster distribution and magic numbers of n = 12 and 18.

43

Figure 3.7 shows a series of representative photoelectron spectra for OH–(Ar)n –clusters
with n =0, 1, 2, 3, 7, 12 and 18 are obtained using three different photodetachment wavelengths:459 (2.703 eV), 532 (2.330 eV), and 602 (2.060 eV) nm. In the photoelectron spectrum of bareOH–, the rotational branches of the OH radical in the 2Π3/2 and 2Π1/2 states are resolved. In the
spectra for all of the complexes, there is a single broad peak that shifts to higher electron binding
energy with an increasing in the number of Ar atoms. The peak shift is a result of the greaterbinding energy of the solvent to the anionic cluster than to the neutral cluster. Solvation by the
first Ar atom increases the electron binding energy by about 0.12 eV. The next two consecutive
Ar additions (n=2 and 3) shift the vertical detachment energy (VDE) by about 0.07 eV. Thus, theVDE of OH–(Ar)3 is 0.07 eV higher than the VDE of OH–(Ar)2, 0.14 eV above the VDE of OH–
Ar, and 0.26 eV above the EA of OH. The solvation energies for the various cluster sizes studied
are shown in the Fig. 3.7, and demonstrate that the average solvation energy per Ar atom decreases
as the cluster size increases. Since Ar atoms in outer solvation shells interact less with the localized
negative charge, the addition of Ar atoms beyond n=12 only shifts the VDE minimally; in fact, thedifference in the VDE of n = 12 and n = 18 is only ~0.06 eV, suggesting an averaged sizedependent binding energy of 0.01 eV per Ar atom. This solvation behavior is consistent withstudies of I2–(Ar)n and O–(Ar)n clusters.65,61
The FWHMs of the photoelectron peak in the spectra of all the cluster sizes are similar(approximately 0.06 eV) and twice as broad as that of the bare OH– peak (about 0.027 eV). Recall
that the peak FWHM reported here is dependent of eKE, since the spectral resolution in VMI
spectrometer degrades with increasing eKE (decreasing eBE). Hence we use three different
wavelengths for the various clusters in an attempt to make a fair comparison of the FWHM. In
order to understand the peak broadening that occurs upon complexation of OH– with Ar, it is useful

44

to begin with the simpler and analogous system, O–(Ar). Garand et al. has assigned the EA of
O(Ar) and deduced75 that the solvation-energy shift from the EA of O is about 700 cm-1. Using
theoretical simulations, they determined an increased O–Ar bond distance of 0.371 Å, changing
from 2.976 to 3.347 Å, upon photodetachment. The slow electron velocity-map imaging (SEVI)
spectrum of O–(Ar) resolved the fundamental vibrational frequencies of the neutral (31 cm-1) andthe anionic (86 cm-1) ground state. In comparison to the O(Ar) complex, the neutral and anionic
OH(Ar) clusters have a low HO–Ar stretching frequency and even lower frequency bending
vibrations, making it much more difficult to resolve these transitions. Furthermore, any rotational
excitation in the OH–(Ar) cluster will also lend to the difficulty. As a result, the observed
broadening is believed to arise from unresolved low-frequency vibrational modes of the neutral
and/or initial excitation of the low-frequency internal degrees of freedom of the anion. However,
it is possible that the broadening is due to the Franck-Condon overlap, causing the anion (in its
ground state geometry) onto the repulsive wall of the neutral because of a large change in theequilibrium geometry between the anion and neutral species. Additional efforts for high-resolution
experiments need to be made to understand the true reason of the peak broadening.

45

Figure 3.7 Photoelectron spectra of OH–(Ar)n=0, 1, 2, 3, 7, 12, and 18 at three different wavelengths: 459(top), 532 (middle), and 602 (bottom) nm.

46

3.3.4 H2-tagging: CH3O–(H2)nUsing this pulsed anion source, we have successfully attached one Ar atom and up to threeH2 molecules on CH3O–, yielding CH3O–(Ar) and CH3O–(H2)1-3 clusters. By comparing thesecomplexes, we gain insight into the different solvation effects of Ar and H2. In particular, H2 hasa smaller polarizability than Ar; thus, we expect a weaker interaction and perturbation from H2than Ar. We also hope to obtain sharper peaks in the photoelectron spectrum and a minimalsolvation-energy shift. The photoelectron spectroscopy of doubly charged anions aggregated withseveral H2 molecules has been investigated previously.76 These spectra reveal a slight blue shift ofthe binding energy, having ~34 meV per H2 on average. This binding energy is lower than the
typical binding energy of an Ar atom (~60 meV).46 In contrast, studies of F–(H2) and OH–(H2) thataccess the transition state region using negative ion photoelectron spectroscopy display a larger
H2 solvation energy of about 250 meV.77-79 Our measurement is consistent with the larger shift inbinding energy, as discussion in detail later.
One application of this technique that has been developed is messenger spectroscopy. First
recorded using predissociation of H2–solvated species as a function of photon energy, it has
facilitated the measurement of infrared spectra of cationic clusters in a continuous beam.80-82
Recently, vibrational predissociation spectroscopy of macromolecular ions, tagged with H2
molecules, has been realized using a ESI source and a cryogenic ion trap.38 By analysis ofvibrational bands in the infrared action spectrum and electronic structure calculations, thestructural characterization of these ions was obtained.
Figure 3.8 shows a comparison between two mass spectra for the production of CH3O–(m/z = 31). The CH3O– which is formed in the side discharge source (–800V, 140 µs) fromexpansion of Ar gas (2.0 atm) bubbling through liquid CH3OH (99.8%, MacronTM Chemicals) kept

47

at room temperature. The plasma is then entrained into a main expansion of either neat Ar (black)or a mixture of 25% H2 in Ar (red) at a backing pressure of 3.7 atm. In the neat Ar expansion,
CH3O–(Ar) is formed; we confirm this by photoelectron spectroscopy. When H2 is added, using agas mixture of 25% H2 seeded in Ar, the intensities at m/z = 33, 35, and 37 increase, denoting theformation of CH3O–(H2)1-3 clusters. When optimizing for the cluster intensities, the intensity of
m/z = 31 is about 20 times stronger than the m/z = 33 intensity.

48

Figure 3.8 Comparison of the CH3O–(H2)n=0-3 TOF mass spectra from experiments using neat Ar(black) and 25% H2/Ar (red) for the main expansion. The CH3O– is made in the side dischargesource.

49

Figure 3.9 shows the 532-nm (2.330 eV) photoelectron spectra of CH3O– and CH3O–(Ar),
together with the 459-nm (2.703 eV) photoelectron spectra of CH3O– solvated with one, two, andthree H2 molecules. The photoelectron spectrum of CH3O– has been studied previously.83-85 The
photoelectron spectrum of CH3O– shows a Jahn-Teller vibronic transition that is assigned to e’degenerate vibrational modes of the ground state (2E) methoxy radical. The SEVI spectrum ofCH3O– has resolved the spin-orbit features.85 Direct comparison of the photoelectron spectra for
CH3O– and CH3O–(Ar), top panel of Fig. 3.9, shows that the CH3O–(Ar) spectrum (purple) exhibits
a significant blue shift of about 95 meV from the CH3O– spectrum (black). Besides the shift, thevibrational progression in the photoelectron spectra are nearly identical, indicating a weakperturbation from the Ar solvent. The story is rather different when CH3O– is tagged with onehydrogen molecule. The photoelectron spectrum of CH3O–(H2) is significantly broadened, and thefeatures are no longer resolvable. This is also shifted to a higher binding energy than the argontagged complex. If the solvation energy of the H2 molecule was much small, creating only a little
shift, it would be expected that by tagging CH3O–(H2) with more H2 solvent molecules theinternally energy of the anions and the spectral congestion would be reduced. However, thephotoelectron spectra of CH3O–(H2)2 and CH3O–(H2)3 are still very broad. Unlike the CH3O– andCH3O–(Ar) spectra, which provide information about the anion-neutral geometry change, theelectron affinity, and vibrational frequencies, the H2–tagged spectra presented herein provide very
limited information.

50

Figure 3.9 Photoelectron spectra of CH3O–, CH3O–(Ar), and CH3O–(H2)n=1-3 using 532- and459-nm light. The color-coded traces correspond to the indicated anion complexes.

51

In comparison to Ar solvation, H2 tagging shows a greater solvation energy and only abroad feature. The blue-shifts of the electron binding energy as determined by the m/z = 71, 33,
35, and 37 spectra are ~ 95, 250, 450 and 590 meV, respectively, from the CH3O origin. Thesefindings contradict the idea of a small perturbation from H2 solvation, as we had expected. If H2solvation leads to internally colder anionic clusters, why are the spectra of the CH3O–(H2)1-3
clusters more congested? There are several likely reasons. First, the stronger interaction in theanionic complexes gives rise to a shorter bond distance between the solute and the solvent than in
the case of the neutral complexes. The broad spectral feature observed for the H2–tagged species
may result from accessing the steep repulsive wall of the neutral potential upon photodetachment.
Second, partial H-bonding in the CH3O–·H−H form an intermediate that may complicate the anion
photoelectron spectrum; photodetachment may access the transition state region of neutral surface,analogous to F–(H2) and OH–(H2). As a consequence, adding H2 does not simplify the CH3O–
spectrum. Additional efforts for high-resolution experiments and theoretical calculations need tobe carried out to explain the peak broadening due to H2 solvation.

3.3.5 HOCO–
We demonstrated large, cold OH–(Ar)n clusters in Section 3.3.3. With a cold OH– source,a well-controlled chemical reaction can facilitate the formation of the anionic forms of OH reaction
intermediates that are of interest in the atmosphere and in combustion. Here we take HOCO– as an
example, and compare our results with data obtained from the work of Johnson et al. who stored
HOCO– in a cryogenic electrostatic ion beam trap.68 To make internally cold HOCO– in the present
experiment, we perform a condensation reaction between a neutral CO molecule and an OH– anion,
after which the newly formed HOCO– cools in the supersonic expansion.

52

OH– + CO + Ar → trans- and cis-HOCO– (internally excited) + Ar

(3.1)

trans- and cis-HOCO– (internally excited) + nAr → trans- and cis-HOCO– (cooled) (3.2)

The neutral CO reactant is present in the main expansion (1% CO seeded in 3.7 atm of Ar), while
the OH– is formed in the side discharge. The position of the side valve is near the vicinity of thenozzle of the main pulsed valve, described in Section 3.2, so the OH– is taken up very early in themain expansion. The anion and neutral find each other in the high density of the early expansion
and the association reaction take place, forming the HOCO– product, and subsequent collisions of
the anion with Ar result in an internally cold HOCO–
Figure 3.10(a) presents photoelectron spectra of HOCO– taken at three different
wavelengths: 532 (2.330 eV), 602 (2.059 eV), and 764 (1.622 eV) nm. Clearly, there is a strong
cross-section suppression of photodetached electrons near threshold. This behavior is described by
the threshold law (see Section 1.1.4).17,86,18,87,88 (Note that simulating a near threshold spectrum
can alter the Franck-Condon simulation and yield incorrect information about the anion-neutral
geometry change; however, the spectral assignments in the spectrum should not be affected by thethreshold law.) Previously, Johnson et al. adapted the cryogenic ion trap to the photoelectronphotofragment coincidence spectrometer for storing and cooling of HOCO– anions in a ~20 K
environment.68,59 They determine the adiabatic EAs of cis– and trans–HOCO to be 1.51 and 1.38
eV, respectively. Their near-threshold spectrum (775 nm) is reproduced as the blue trace in Fig.
3.10(b).68 The black trace in Fig. 3.10(b) is an enlargement of our 764-nm spectrum seen in Fig.
3.10(a). Comparison between the black and blue traces in Fig. 3.10(b) finds that the peak positions
in both spectra are consistent, illustrating the success of the stepwise HOCO– synthesis. This novel
synthesis also produced the desired anion with low levels of internal excitation. Despite the53similarity in the peak positions, there are still two dissimilarities between the two spectra in Fig.
3.10(b): the peak width and the ratio of cis– and trans–HOCO– isomers. First, the narrower peakwidth in our data could arises from lower initial excitation of anion’s internal degrees of freedom
or from a higher instrument resolution. In either event, the narrower peaks reduce spectralcongestion and provide information with a smaller uncertainty. For example, near an eBE of
1.52 eV, the black spectrum shows two distinct peaks, but these peaks are overlapped in the blue
spectrum. Second, the black spectrum in Fig. 3.10(b) has a larger contribution from the trans–
HOCO– isomer than the blue spectrum. The different proportions of the two isomers are due to thedifferent way in which the anions are formed. In the sample used to obtain the blue spectrum, theHOCO– anions are stored in the ion trap for several hundred milliseconds and most of them arequenched to the global minimum, cis–HOCO–. On the other hand, in the entrained pulsed jet, thetwo isomers are suddenly cooled to their individual minima and shortly thereafter are in acollisionless region, “trapping” more trans isomers. This comparison proves that without using acryogenic ion trap, this low-expense, pulsed anion source has the capability to make cold anions,even when the anions form from exothermic association reactions.

54

Figure 3.10 Photoelectron spectra of HOCO– (a) at three different wavelengths: 532 (red), 602(purple) and 764 (black) nm, and (b) comparison between the 764 nm spectrum (this work) andthe 775 nm spectrum (blue) from Johnson et al.68 The diamond and star symbols denote the EA ofcis– and trans–HOCO–, respectively, from the assignments in the work of Johnson et al.68

55

3.3.6 CH3OO–The CH3OO– anion has been investigated and formed using a proton abstraction reaction;
this reaction was performed by adding CH3OOH downstream of an OH– in a flowing afterglow
ion source at JILA.69 We also generate CH3OO– using the pulsed anion source described in thischapter; here we carry out an association reaction between O2 and CH3–.
CH3– + O2 + Ar → CH3OO– (internally excited) + Ar

(3.3)

CH3OO– (internally excited) + nAr → CH3OO– (cooled)(3.4)

The CH3– reactant, produced in the side discharge pulse (-900V) from 2.4 atm of neat CH4, isadded in the vicinity of the nozzle of main pulsed valve from a perpendicular geometry. It is in themain expansion that the association reaction takes place. The main expansion is created from a gas
mixture that is 1% O2 seeded in Ar at a total backing pressure of 3.7 atm. The CH3OO– adduct isgenerated and efficiently cooled in the Ar supersonic jet. When optimizing for the CH3OO– anion,
the intensity ratio of CH3– to CH3OO– is 3:1.
In order to confirm production of the CH3OO– adduct from the pulsed anion source, wetake the photoelectron spectrum at 532 nm (2.330 eV) and compare it to the spectrum previously
obtained in the flowing afterglow photoelectron spectrometer using 363.8-nm radiation; these two
spectra are shown in Fig. 3.11. The CH3OO– spectrum provides the electron affinity, the term
energy, and the CH3O–O stretching and H3C–O–O bending frequencies. The eBEs of the spectralfeatures in both spectra are consistent, suggesting that both anions are the indeed the same species.
Again, it indicates the capabilities of this novel pulsed anion source. Hence, without using any
explosive compounds, this pulsed anion source performs rational ion synthesis to make transient
species that are internally cold.

56

Figure 3.11 Comparison of the CH3OO– spectra obtained using the wavelengths of 363.8 (blue)and 532 (black) nm. For the blue spectrum from Blanksby et al.,69 CH3OO– is formed via a proton
abstraction reaction following addition of CH3OOH downstream of OH– in a flowing afterglow
ion source at JILA. For the black spectrum (this work), we perform the association reaction of O2
with CH3– in an Ar pulsed jet to generate CH3OO–.

57

3.4 Conclusion
In this chapter, we present a versatile, pulsed anion source that utilizes a plasmaentrainment method suitable for production of cold anions (that can easily be solvated). This
simple and inexpensive setup is also capable of tagging anions with Ar atoms to ensure that theinternal energy of the ions is low. Further, this ion source is able to produce cold anions through awell-controlled ion-molecule reaction, such as the production of HOCO– and CH3OO– via
association reactions. All of these very exothermic reactions have been successfully performed byentraining the charged species into a main expansion containing ~1% of the neutral reactant. Thisversatile anion source allows for the investigation of many exotic or transient species. In addition
to the applications addressed in this chapter, cooling anions and rational ion synthesis, there is also
the possibility of initiating chemistry from size-selected clusters (bigger than binary complexes)
to explore significant portions of reactive potential energy surfaces that are not accessed in thebinary complexes.

58

Chapter 4
Photoelectron Spectroscopy of the Methyl Anion, CH3– andCD3–
4.1 Introduction
CH3− is the simplest carbanion and has a trivalent carbon atom with a lone pair of electrons
and a negative charge.89 Carbanions frequently occur as reactive intermediates in organometallic
chemical reactions and are involved in the formation of Grignard and organolithium reagents.
While in these organometallic reagents, the anions are not free, but instead are stabilized bycounterions, experimental studies in the gas phase can provide a deeper understanding of the
intrinsic properties of isolated carbanions. CH3− can be obtained by removing a proton frommethane. Methane, however, is an extraordinarily weak acid and has an extremely high gas-phase
acidity. Since the difficulty of removing a proton from methane reflects the difficulty of forming
CH3−, the properties of this anion are not well known, despite its fundamental importance. If thesimplest alkyl anion, CH3−, can be characterized, further insight may be gained about larger alkyl
anions.
The inversion splitting between the 0+ and 0− levels of the umbrella-bend statecorresponding XH3 molecules has been explored in the NH3 (ND3)90,91 and H3O+ (D3O+)92-94
systems. Since the CH3− anion is isoelectronic with NH3 and H3O+, it is expected that CH3− will
also have a pyramidal structure with C3v symmetry and a symmetric double-well potential, and,thus, an inversion splitting associated with the umbrella bending vibration. In NH3, this tunnelingmotion has been well-studied due to its use in the first maser, and this splitting is 23.7 GHz(0.79 cm-1). In H3O+, a lower inversion barrier leads to a larger splitting of 55.35 cm-1. Until thepresent work, the inversion splitting of the CH3− anion has not been experimentally observed. In

59addition, this work also utilizes study of the fully deuterated species, CD3−, to understand the massdependence of the tunneling effect. Herein, we report an experimental determination of theinversion splitting of CH3− and CD3−.
The previous experimental studies of CH3− are scant and mostly focus on the electron
affinity (EA) of CH3 and the gas-phase acidity of CH4.95,96,72,97-99 The photoelectron spectrum of
the CH3− anion has been studied previously.72 Ellison et al. reported an early (55-meV resolution)
determination of EA(CH3), 0.08 ± 0.03 eV. The CH3− spectrum exhibited a long progression in thev2 umbrella mode of ground-state CH3, indicating transitions from a pyramidal anion to a planar
2A″ ground state of CH3. The 2E′ valence excited state of the CH3 radical was not observed,

indicating that it lies more than 2.2 eV above the ground state. The low resolution of the energy
analyzer prohibited any determination of the inversion splitting in the anion.
Graul and Squires employed collision-induced dissociation of various carboxylate or
alkoxide anions to generate CH3− and investigated reactions of the anion with neutral molecules.97
Using thermochemical cycles, they determined the gas-phase acidity of methane to be
417 ± 4 kcal/mol. This strong basic property of CH3− allowed for the observation of proton transfer
even from ethylene and NH3 to CH3−. Electron transfer from CH3− to O2, OCS, CS2, and C6F6 wasalso observed, indicating a low electron binding energy for CH3. This low EA is consistent withboth the results of Ellison et al.72 and an autodetachment study of CH3− following IR excitation.99
Finally, Graul and Squires observed both nucleophilic addition and elimination reactions of CH3−
with both protic and aprotic reagents, showing its high nucleophilicity.
There has been considerable theoretical attention directed toward the CH3− anion.100-115 The
stability and structural properties are highly emphasized. Calculated structures are also consistent60

with a pyramidal geometry. However, the EA has been calculated to be both positive and negative(bound and unbound, respectively), depending on the level of theory and basis set used. In
particular, the anharmonicity of the umbrella mode adds difficulty and complexity to thecalculation of the zero point energy, which strongly affects the calculated electron binding energyof this weakly bound anion. For reliable results, a potential energy surface calculation of highquality is required.
In this chapter, we report the photoelectron spectra of cooled CH3− and CD3−. We utilize
the plasma-entrainment anion source described in Chapter 2 to produce reasonable quantities ofcold CH3−, allowing us to investigate its properties with much higher resolution than waspreviously possible. Using CD4 as a precursor, CD3− can also be formed and studied. We measure
the electron affinities of CH3 and CD3, and we significantly improve the measurement uncertainty
using mid-IR light. We observe extended vibrational progressions involving the umbrella bend in
the electronic ground state of both CH3 and CD3. The observation of the well-studied v2 bands of
CH3 and CD3, coupled with the dramatically improved electron energy resolution, enable us todirectly measure the inversion splitting between the 0+ and 0− energy levels in both CH3− and CD3−.
In addition, by employing a thermochemical cycle, we can improve upon the previous
measurements of the gas-phase acidity of methane and report the experimental gas-phase acidity
for d4-methane. Finally, we compare our experimental results with calculated values and discusspossibilities for investigating yet-to-be-detected CH3 2E′ valence excited state.

61

4.2 Experimental Details
In this experiment, we utilize the negative ion photoelectron velocity-map imaging (VMI)
spectrometer, described in more detail in Chapter 2, to obtain anion photoelectron spectra. This
experimental apparatus uses a pulsed anion beam that is mass-separated by a Wiley-McLaren timeof-flight mass spectrometer (TOF MS). The mass-selected anions of interest are photodetachedusing a tunable light source, and then a standard VMI photoelectron detector is utilized to obtain
the photoelectron spectrum.
The CH3− anion is prepared in a novel plasma-entrainment pulsed anion source describedin Chapter 2. This source utilizes two pulsed General Valves (Parker-Hannifin, series 9): one
serves to create the supersonic expansion, the other is placed perpendicular to this main expansion
and utilizes an electric discharge. The electrical discharge takes place when the gas passes througha stainless steel cathode, which is pulsed (140 µs) at a potential of -900 V in series with two 10kΩ
resistors (for increased discharge stability), to a stainless steel grounded anode. This side discharge
nozzle, which operates under choked-flow conditions (2.4-atm neat CH4), is pulsed forapproximately 250 µs, introducing ions into an Ar expansion (backing pressure, 3.7 atm). The Ar
beam picks up the anions moving them in the direction of the main expansion and providing
additional collisional cooling. The cold anions are then extracted into the TOF MS. Figure 4.1(a)shows the mass spectrum under these conditions. We confirm the chemical identity of the massselected anions by comparing their photoelectron spectra with other known anion photoelectron
spectra. Using neat CD4 as a precursor, CD3− can be generated in an analogous manner, and themass spectrum is shown in Fig. 4.1(b).

62Figure 4.1 Mass spectrum for the production of CH3– and CD3– by introducing ions formed from(a) a neat methane or (b) a neat methane-d4 pulsed discharge, respectively, into a neat Ar
expansion. F– comes from chemical contamination in sample line.

63

Anions of m/z = 15 or 18 are photodetached by spatially and temporally overlapping themwith laser light. The velocity and angular distribution of the photodetached electrons is thenacquired by the VMI photoelectron spectrometer. The raw data are converted into a velocity
distribution using the BASEX implementations in the required Abel inversion.49 The spectral
resolution in the VMI spectrometer improves as the electron kinetic energy (eKE) decreases. Since
we have access to a tunable light source, we use three different photon energies to obtain
photoelectron spectra of CH3− and CD3−. The 763-nm (1.625-eV) output of a 532-nm pumped dyelaser is used to obtain a survey photoelectron spectrum, and the Nd:YAG fundamental light (1064nm or 1.165 eV) is used to obtain the overview (full eKE range) photoelectron spectrum of theCH3 ground state. A higher energy-resolution spectrum, focusing on the lowest energy transitions,is obtained using 3236-nm (0.383-eV) light. We generate this mid-IR light using difference
frequency generation (DFG) of 801-nm (1.548-eV) light (from the 532-nm pumped LDS798 dyelaser) and the residual 1064-nm (1.165-eV) light (Nd:YAG fundamental) in a 1x1x3 cm3magnesium oxide doped lithium niobate (MgO:LiNbO3) crystal. The mid-IR light, which generally
has energies of 50–100 µJ/pulse, is focused into the laser-anion interaction region using a 1-m
CaF2 lens. To determine the exact photon energy, we directly measure the dye wavelength using awavemeter (Atos, LRL-005) calibrated by a He-Ne laser, and then calculate the resultant mid-IR
wavelength.
In this thesis, we report photoelectron spectra as a function of electron binding energy(eBE= ħω – eKE), which is independent of the laser wavelength. The calibration of the energy
scale for the reported data is performed using the well-measured v2 out-of-plane bending
frequencies in neutral CH3 and CD3 from high-resolution IR absorption studies.116-118 In order to64confirm the validity of this procedure, we also calibrate the energy scale for the images using the
known photoelectron spectrum of S− with 532-nm light to assure agreement.50-52
Since the band origin and the observed peak maximum are not necessarily the same, theuncertainty in the determined eBE is given by combining the uncertainty in the Gaussian-fit peakcenter location with the deviation of the peak center from the simulated and fitted band origin. We
reproduce and fit the rotational envelope of the observed peaks in the photoelectron spectra using
the PGOPHER rotational simulation program.119 The anions and neutrals are treated as rigid
symmetric tops, and the rotational constants for the anions and neutrals are obtained from Kraemer
et al.,110 Yamada et al.,116 and Sears et al..118 The rotational temperature and Gaussian linewidthsof the rotational transitions are varied until the rotational band contour best fits the observedlineshapes. Consequently, each difference between the peak maximum and the simulated band
origin can be accurately obtained.

4.3 Results
Upon photodetachment, the out-of-plane bending angle from the pyramidal CH3− anion to
the planar CH3 radical changes significantly, which leads to an extended Franck-Condon
progression in the umbrella v2 mode. Figure 4.2 shows schematic potential energy curves as afunction of the out-of-plane bending angle for both CH3− and CH3. The curves are plotted using
the parameters found in from Kraemer et al.110 and shifted to fit our experimental findings. The
out-of-plane bending angle is defined as the angle between the C−H bond and the plane of thethree H atoms. For example, a 0 degree angle is formed by the C atom in the H3 plane, whichcorresponds to the equilibrium geometry of the methyl radical. The plotted wavefunctions are
obtained using the Numerov method.120 As shown in Fig. 4.2, the pyramidal CH3− anion exhibits

65

a symmetric double-well potential, which gives rise to an inversion splitting, analogous to NH3and H3O+. The inversion splitting depicted in Fig. 4.2 arises from the wavefunction tunnelingthrough the barrier. The coupling of the two equivalent vibrational states for the two configurationsof CH3− lifts the degeneracy, splitting the energy levels, and creates two discrete parity states.
These ground-state inversion levels of CH3−, denoted here as 0+ and 0−, are linear combinations of
the degenerate states with even and odd symmetries, respectively. Excitation will follow theselection rule ;< = 0+ → ; = 0, 2, 4… and ;< = 0− → ; = 1, 3, 5… to preserve the parity andensure a non-zero Franck-Condon overlap. If both 0+ and 0− states of CH3− are populated in theanion beam, we anticipate the progression in the non-totally symmetric v2 mode to be irregularly
spaced. In other words, the above-stated selection rule requires alternate vibrational levels of themethyl radical to be populated by excitation from either the 0+ or 0− inversion level of the anion.
These Franck-Condon allowed transitions are indicated by the red (;< = 0+) and blue (;< = 0–)
double arrows in Fig. 4.2.

66

Figure 4.2 Schematic potential energy surfaces for the electronic ground states of CH3 (green) andCH3– (black) as a function of the out-of-plane bending angle. The red and blue arrows indicate a
non-zero Franck-Condon overlap for v″=0+ (even wavefunction, red dashed line) and 0– (oddwavefunction, blue dashed line) in the anionic ground state to even and odd quanta of the neutral
state, respectively. The 0+−0– inversion splitting of the CH3– is labeled ∆.

67

The three well-resolved photoelectron spectra of CH3−, shown in Fig. 4.3, agree with our
expectations. These spectra, taken with 763 (1.625 eV, black), 1064 (1.165 eV, red), and 3236
(0.383 eV, blue) nm photodetachment wavelengths, show similar features to the Ellison et al.
photoelectron spectrum, exhibiting an extensive progression of energy levels in the out-of-planebending mode of CH3. The observed peaks (labeled A through G) in Fig. 4.3 are listed and assigned
in Table 4.1. The peak center, in eBE, of each transition is the weighted average of the threeexperimental spectra. Their uncertainties are given by the deviation of the center of the peak fromthe simulated and fitted band origin. As expected, the differences between adjacent peak positions
are irregularly spaced, suggesting transitions from both anion inversion levels.

68

Figure 4.3 Photoelectron spectra of CH3– at three different photodetachment wavelengths: 763(black), 1064 (red), and 3236 (blue) nm.

69

Table 4.1. Experimental peak positions and assignments in the photoelectron spectra of CH3− andCD3−. The values in parentheses are the uncertainties described in the text.
CH3 + e– ← CH3–
Assignment
Peaka Position (cm-1); ← ;<A
763(31)0 ← 0+
B
1349(27)
1 ← 0−
C
2047(22)2 ← 0+
D
2761(17)3 ← 0−
E
3544(45)
4 ← 0+
F
4308(40)5 ← 0−
G
5165(35)
6 ← 0+aPeak labels shown in Fig. 4.3 and 4.4.

Peaka
a′
a
b
c
d
e

CD3 + e– ← CD3−
Assignment
Position (cm-1); ← ;<332(52)691(28)0 ← 0+
1138(22)1 ← 0−
1650(19)2 ← 0+
2190(17)3 ← 0−
2767(14)4 ← 0+

70

Since the spectral resolution in the VMI spectrometer degrades with increasing eKE, we
use the spectrum obtained using 3236-nm light to obtain the highest accuracy measurement of the
EA and inversion splitting. The near-threshold spectrum of CH3− is expanded in Fig. 4.4(a). We
assign peak A, which is at the lowest eBE, to the transition from the lower inversion level, 0+, of
CH3− to the ground vibrational state of CH3. This origin transition corresponds to the adiabatic
electron affinity (EA) of the methyl radical. The most intense transition, peak C, which is assigned
to the vertical transition from the 0+ state of the anion to the ; = 2 state of the neutral, has the best
Franck-Condon overlap and, thus, the smallest geometry change from the anion to the neutral.
Transitions to odd ; quanta of the neutral have to come from the upper inversion level, 0−, of CH3−
in order to have a non-vanishing Franck-Condon overlap. The variation in peak intensity of thefour peaks seen in Fig. 4.4(a) primarily reflects the relative populations of the 0+ and 0− inversion
levels in the ion beam, further reinforcing our assignments. The peak intensities are also subject tothe threshold effect, and, thus, the intensities of peak C and D in the near-threshold spectrum drop
notably compared to other spectra taken with higher photon energies; see Fig. 4.3.

71

Figure 4.4 Mid-IR photoelectron spectra of (a) CH3– and (b) CD3–. The CD3– spectrum is rebinned
to 2.5 pixels to improve the signal-to-noise ratio without changing the spectral appearance.

72

Figure 4.4(b) shows the near-threshold CD3− spectrum taken with 3236-nm laser radiation.
The spectrum is rebinned to 2.5 pixels to improve the signal-to-noise ratio without altering thespectrum. The observed peaks (labeled a′, and a through e) in Fig. 4.4(b) are listed and assigned inTable 4.1. The uncertainties are obtained with the same method described above. Although peaka′ has the lowest eBE in the spectrum, the eBE of peak a′ is less than half of the eBE of peak A(the EA of CH3). Because there is only a small change in the zero-point energy upon deuteration,
it seems very unlikely that this low eBE denotes the EA of CD3. Instead, we assign peak a to theorigin transition. With an expected difference between the CH3 and CD3 EA of less than 10 meV(ca. 81 cm-1), the 9 meV difference between Peaks A and a suggest that this assignment ispreferred.121 Based on this assignment, peak a′ is a hot band that likely arises from initial
population in one or both of the 1+, 1− states of CD3−. As in CH3−, the peaks are irregularly spaced,indicating transitions from the 0+ and 0− states of the anion to even and odd quanta of vibrational
levels of the neutral, respectively.

4.4 Discussion
Photoelectron spectroscopy of negative ions yields information on the structure andvibrational frequency of the corresponding neutral species. However, the fundamental v2
frequency, its overtones, and the planar equilibrium structure of CH3 and CD3 have all been
rigorously determined from absorption and emission studies of the neutral species.116,122,117,118 The
experimental frequencies are summarized in Table 4.2; from this table it becomes apparent thatthe v2 frequencies of CH3 and CD3 show substantial negative anharmonicity (i.e., greater
vibrational frequencies with increasing vibrational levels). Yamada et al. suggest that the negative
anharmonicity may be attributed to a vibronic interaction between the ground and excited

73

electronic states of the neutral molecule.116 In the present experiment, we take advantage of the
known v2 frequencies of CH3 and CD3 umbrella modes to obtain the inversion splitting, ∆EInv., in
CH3− and CD3−. For the ∆EInv. of CH3−, we use the equation shown below:∆EInv.(CH3−, 0+ ↔ 0−) = 3v2(CH3, v2=3-0) + Peak A(0←0+) – Peak D(3←0−).
This equation includes Yamada et al.’s measurement of 3v2 (2019.1657 cm-1, CH3, v2=3←0).116
For the eBE of Peaks A and D, multiple measurements are obtained using mid-IR light. We thentake an average of the computed values over several independent measurements to determine theinversion splitting, and assign an uncertainty that is one standard deviation of the mean. This
procedure yields an inversion splitting of 24(2) cm-1 between the v2 = 0+ and 0− energy levels of
CH3−. Similarly, we apply the same approach to CD3−, giving a 0+−0− inversion splitting of7(2) cm-1. For comparison, we correct the measured v2 frequencies for the inversion splitting of
the anion; these values are listed in Table 4.2. In addition, we use the eBE of Peak D (which has alower uncertainty than Peak A) to improve our initial determination of the electron affinity of CH3.
By using the position of Peak D and the frequency of 3ν2, we determine the EA to be 0.095(2) eV,
which is consistent with the previously reported EA of 0.08(3) eV. Likewise, the electron affinityof CD3 is determined to be 0.086(2) eV using the eBE of Peak e. Table 4.3 summarizes our
experimental determinations of the EAs and inversion splittings.

74

Table 4.2. Experimentally measured out-of-plane bending frequencies of CH3 and CD3 (cm-1) in
the gas phase.
CH3ν2 band Yamada et al.a Hermann et al.b This workc
1←0606.4531
610(41)2←1681.6369
674(35)3←2729(4)
731.0757
738(28)4←3772(4)
759(48)5←4811(4)
788(60)6←5833(53)aReference 116.
b
Reference 122.
c
Values corrected using the inversion splittings.
dReference 118.

CD3
Sears et al.dThis workc
457.8136
454(36)507.9297
505(29)542.2841
547(26)568.6322
570(22)

75

Table 4.3. Experimentally determined values: adiabatic electron affinity (EA) and inversion
splitting (∆EInv.).
EA(CH3)∆EInv. (CH3−, 0+ ↔ 0−)

0.095(2) eV
24(2) cm-1

EA(CD3)∆EInv. (CD3−, 0+ ↔ 0−)

0.086(2) eV
7(2) cm-1

76

Our experimentally determined EAs and inversion splittings, are in good agreement withthe calculated values. Kraemer et al. investigated the CH3− and CD3− inversion potentials at the
SCF/SRCI and CASSCF/MRCI levels of theory,110 and calculated the electron affinities of CH3
and CD3 to be 0.09 and 0.08 eV, respectively. The inversion splitting was also calculated to be
19 cm-1 for CH3− and 4 cm-1 for CD3−. In addition, Dixon et al. calculated the CH3− ground-state
inversion splitting of 27 cm-1 and the CH3 EA of 0.071±0.013 eV using the CCSD(T) level oftheory.103,113
Depuy et al. have used the gas-phase acidity, or the deprotonation enthalpy (∆acidH), of
methane as a reference in determining alkanes acidities.123 For this molecule to serve as a reliablereference acid, its gas-phase acidity should be known as accurately as possible. From our precise
measurement of the EA(CH3), we are able to refine the gas-phase acidity of methane at 0 K using
a thermochemical cycle involving the H3C−H bond strength, the ionization energy of H, and theelectron affinity of CH3: ∆acidH0(CH4) = D0(H3C−H) + IE(H) – EA(CH3). The bond dissociationenergy of methane, D0(H3C−H), was reported by Ruscic et al. to be 103.42(3) kcal/mol.124 The
ionization energy of hydrogen, IE(H) = 313.59 kcal/mol, is well known. In this work, our measured
value for the EA(CH3) of 0.095(2) eV (2.19±0.05 kcal/mol) yields the revised ∆acidH0(CH4) =414.82(6) kcal/mol. The uncertainty of the resulting gas-phase acidity is one order of magnitude
smaller than the previous measurement. Moreover, the present measurement for the EA(CD3) of
0.086(2) eV (1.98±0.05 kcal/mol), together with the D0(D3C−D) = 105.80(3) kcal/mol125 and thewell-known IE(D) = 313.69 kcal/mol, gives the first determination of the gas-phase acidity of
d4-methane, ∆acidH0(CD4) = 417.51(6) kcal/mol.
Negative ion photoelectron spectroscopy can also be used to investigate the electronicenergy levels, or term energies, of neutrals. The methyl radical has a 2A2″ ground state and a

77

valence excited state, 2E′, that has been predicted theoretically but not observed experimentally.
The 2E′←2A2″ transition is symmetry forbidden in the neutral molecule; however, the 2E′ state isaccessible by photodetachment of the CH3− anion. Various electronic structure calculations show
the vertical excitation energy of the 2E′ level, ranging from 6.8 to 7.6 eV, for the planar equilibriumgeometry of the CH3 ground state.126-132 In particular, the MRCI131 and EOM-CCSD132 results are
7.13 and 7.01 eV, respectively. Using the valence state equilibrium geometry to optimize the CH3
ground state, the excitation energy of the valence state was calculated to be 4.5 or 4.2 eV.129,131
Therefore, the adiabatic excitation energy is believed to be between 4.2 and 7.6 eV. The 1978
photoelectron spectrum of Ellison et al. confirms that the 2E′ level is more than 2.2 eV above theCH3 ground state, and, with our tunable light source, it is tempting to pursue a photoelectron
spectrum of CH3− that would expose the 2E′ state. Unfortunately, the UV light required to accessthe 2E′ state of the CH3 radical from the CH3− anion creates severe background photoelectrons in
our current apparatus, giving an extremely bad signal-to-noise ratio. This experiment may befeasible in an oil free chamber where UV-induced background photoelectrons would be minimal.

4.5 Conclusion
We report the photoelectron spectra of CH3− and CD3−. These spectra show extensive
progressions in the out-of-plane bending vibration of the electronic ground states of the neutral
CH3 and CD3 radicals; this long Franck-Condon progression arises from a substantial change in
the geometry upon photodetachment. The electron affinities of CH3 and CD3 are measured to be
0.095±0.002 and 0.086±0.002 eV, respectively. Using the well-studied v2 bands of CH3 and CD3,
we determine the inversion splitting of the CH3−and CD3− anions to be 24±2 cm-1 and 7±2 cm-1,
respectively. By use of a thermochemical cycle, a precise gas-phase acidity is determined for CH4

78

and CD4 at 0 K: 414.82(6) and 417.51(6) kcal/mol, respectively. While the 2E′ valence excitedstate of CH3 is accessible by removing an electron from CH3−, we were unable to determine theadiabatic excitation energy due a large background – spectroscopic examination of this state awaits
a clean photodetachment experiment using approximately 6-eV photons.

79

Chapter 5
Photoelectron Spectroscopy of Propadienylidene
Anion, H2CCC–
5.1 Introduction
Isomers of C3H2 are important in organic, astrophysical, and combustion chemistry. Singlet
cyclopropenylidene (c-C3H2) is the global minimum structure (defied as E = 0 kcal/mol), which
has two low-lying isomers are triplet propargylene (also called propynylidene, HCCCH, ∆E =10 kcal/mol) and singlet propadienylidene (H2C=C=C: or abbreviated to H2CCC, ∆E =13 kcal/mol).133 The first experiment on H2CCC was performed using IR spectroscopy in a matrix,
and they confirmed the structure of H2CCC using the comparison between the experimental andcalculated IR spectrum.134 Then additional laboratory characterization of this species was
accomplished with microwave spectroscopy in the gas phase, where they determined the rotationaland centrifugal distortion constants of H2CCC.135 Subsequently, H2CCC was detected by radio
astronomy in two dense clouds by identifying four rotational lines.136 A combination of microwave
studies and theoretical calculations determined the equilibrium geometry of H2CCC to be a nearprolate top in a C2v symmetry.137-139 The electronic absorption spectrum of H2CCC in Ar and Ne
matrices was also measured and resulted in identifying the three lowest-lying singlet excited statesand vibrational structures in each state.140,141 Recently, using cavity ring-down spectroscopy,
Maier et al. measured the electronic spectrum of H2CCC at 4881 Å and 5450 Å belonging toB̃ 1B1 ← X̃ 1A1 transition in the gas phase, coincident with two unidentified diffuse interstellar
bands (DIBs).142 They proposed that the broad DIBs at these two wavelengths are produced by aground-state electronic transition of H2CCC. However, this assignment has been disputed andcriticized.143-145 More discussion can be found in the discussion section.

80

The photoelectron spectrum of H2CCC– anion has been previously measured by Oakes etal.146 and Robinson et al.121 using 488-nm and 351-nm lights, respectively. In the 488-nm
spectrum, the electron affinity (EA) of H2CCC has been measured to be 1.794(25) eV, but Oakes
et al. did not observe any excited electronic states. Using 351-nm light, Robinson et al. observed
the ground electronic state (X̃ 1A1) and the first excited electronic state (ã 3B1) of H2CCC withphotoelectron spectroscopy. They reported a more precise EA measurement of 1.794(8) eV andthe singlet−triplet splitting of 29.7(2) kcal/mol. In comparison to conventional absorption
spectroscopy, both singlet and triplet (spin-forbidden from neutral H2CCC) electronic excitedstates can be investigated using negative-ion photoelectron spectroscopy.
In this chapter, we report the photoelectron spectrum of H2CCC– anion using radiation
between 245 and 330 nm. We utilize a neutral−entrainment anion source to produce H2CCC–,
allowing us to investigate its electronic excited states (b̃ 3A2, Ã 1A2, B̃ 1B1 and c̃ 3A1). Vibrationaltransitions in the electronic states are also observed and assigned, allowing us to determine theterm energies of H2CCC. Finally, we compare our experimental results with calculated values andthe experimental data previously collected.

5.2 Experimental Details
The VMI apparatus has been described in detail in Chapter 2. In general, the apparatus
consists of a pulsed anion source, a Wiley-McLaren time-of-flight mass spectrometer (TOF MS),24
a velocity-map imaging (VMI)25,26 photoelectron spectrometer, and a ns-pulsed, tunable-laser
system. Only experimental details that are specifically relevant to obtaining the H2CCC spectrum
will be discussed further.

81

Propadienylidene anion (H2CCC–) is formed via H2+ abstraction from allene by O–146,121,147
in a pulsed supersonic entrainment reactor.46 The entrainment reactor comprises a central and twoside pulsed solenoid valves (General Valve Series 9, 0.8-mm orifice) operating at 80 Hz withindividual timings and pulse widths. The middle pulsed valve provides the main expansion using
1% allene (H2CCCH2; 97%, Sigma-Aldrich or 96%, Pfaltz & Bauer) seeded in Ar at a backing
pressure of 4.4 atm. One of the adjacent pulsed valves entrains neat N2O (99%, Scott Specialty
Gases) at a backing pressure of 1.1 atm into the allene/Ar gas expansion. A guided, collinear beamof 1-keV electrons, emitted from a continuous electron gun with thoriated tungsten filament,
collides with the gas expansion to produce slow secondary electrons. The interaction of slow
secondary electrons with N2O yields the O– reactant by dissociative electron attachment. The O–
anion subsequently reacts with allene to produce H2CCC– anion. If the N2O entrainment valve isturned off, the H2CCC– anion signal disappears.
The anions are perpendicularly extracted and mass-separated via the Wiley-McLaren TOF
MS. The H2CCC– anions are photodetached by temporally overlapping the ion beam with a linearlypolarized laser pulse tunable from 245 to 330 nm. The tunable light is produced by frequency
doubling the signal output of a 355-nm pumped OPO in a BBO crystal. Following
photodetachment of the H2CCC– anions, the VMI photoelectron spectrometer acquires the kinetic
energy distribution of the photodetached electrons. In this chapter, we report the photoelectron
spectra as a function of electron binding energy (eBE= ħω – eKE), which is independent of thelaser wavelength used for photodetachment. The calibration of the energy scale for the reported
data is performed using the known transitions148,149-151 of O–and I–. These spectra typically extend
over a 0 – 8,000 cm-1 eKE range, with a corresponding 80 – 400 cm-1 energy resolution. When82

peaks in a H2CCC– spectrum do not overlap, the uncertainty in the absolute eBE is approximately
the HWHM (half width at half maximum) of the peak.

5.3 Results
Photodetachment of H2CCC– anions result in various electronic states of the H2CCC radical
by selectively removing an electron from different orbitals within the ground state anion.
Figure 5.1 shows the ground state electronic configuration (X̃ 2B1) of the H2CCC– anion:[core](1b1)2(7a1)2(2b2)2(2b1)1. Removing the unpaired electron yields the X̃ 1A1 ground electronic
state of the corresponding neutral molecule. Detachment of an α electron from the 2b2, 7a1, or 1b1
molecular orbitals gives the Ã 1A2, B̃ 1B1, or C̃ 1A1 singlet electronic states, respectively, while a β
electron photodetachment from the three orbitals results in the b̃ 3A2, ã 3B1, or c̃ 3A1 triplet states,
respectively. According to Hund’s rule, these triplet states are lower in energy than the singlet
states with the same symmetry (e.g., 3A2 < 1A2). Transitions to both the ground state and the lowest
triplet state of the H2CCC radical have been reported using 351.1-nm (3.531-eV) light.121
Interestingly, the lowest triplet state is 3B1 instead of 3A2; however, this is counterintuitive to what
one would hypothesize from the orbital energy ordering of H2CCC– (X̃ 2B1). The actual energy
ordering (3B1 < 3A2) can be rationalized by considering that two unpaired electrons between singlet
and triplet states with B1 symmetry have stronger coupling than the two A2 states; therefore, thesinglet-triplet splitting is significantly larger for the B1 pair of states than for the A2 states. This
results in the following measured energy ordering: 3B1 < 3A2 < 1A2 < 1B1. To investigate thesehigher lying electronic excited states, laser radiation with wavelengths shorter than 351.1 nm are
employed.

83

Figure 5.1 Ground-state electronic configuration of propadienylidene anion, H2CCC–, andcorresponding singlet or triplet states formed by removal of an α or a β electron.

84

Figure 5.2 shows the experimental photoelectron spectra taken with three different photon
energies: 3.809 (black), 4.164 (red), and 4.704 (blue) eV. (Note the ground-state radical
photoelectron spectrum is shown and discussed later in the chapter.) The peaks at the two lowest
binding energies (eBE = 3.074(6) and 3.207(6) eV) in the black trace are assigned to the origin
transition and the v4 fundamental (H2C−CC stretching), respectively, of the first excited electronic
state (ã 3B1). These values are both in good agreement with the previous observations by Robinson
et al.121 The v4 frequency in the ã 3B1 state is measured to be 1073(68) cm-1. To identify the higher
lying features, we use previous measurements on neutral H2CCC in addition to the measured EA,
which is determined to have a value of 1.7957 eV and discussed in more detail below. In theelectronic absorption spectrum of the H2CCC radicals in a matrix, the two lowest-lying singlet
excited states, Ã 1A2 and B̃ 1B1, were observed 1.73 and 2.00 eV, respectively, higher in energy
than the neutral radical in its ground electronic state.140,141 The addition of the measured EA value
to these previously measured term energies gives the expected detachment energy of the Ã 1A2
state from the X̃ 2B1 anion to be ~3.5 eV. Therefore, the feature appearing at eBE = 3.492(6) eV inboth the black and red traces of Fig. 5.2 is assigned to the origin transition of the Ã 1A2 state. The
peak at eBE = 3.685(10) eV is assigned as the v2 level in the Ã 1A2 state, yielding a v2 frequency(H2CC−C stretching) of 1557(90) cm-1. Similarly, the origin transition of the B̃ 1B1 state is expected
to be ~3.8 eV. However, there is only one broad peak appearing at eBE = 4.065(6) eV, which is
0.265 eV (2137 cm-1) higher in energy than the estimated origin of this state. We assign this peakas the v2 transition in the B̃ 1B1 state, and the origin is unobservable, which is probably due tostrong vibronic coupling between the Ã 1A2 and B̃ 1B1 states.

85

Figure 5.2 Photoelectron spectra of H2CCC– anion taken with photon energies of 3.809 (black),
4.164 (red), and 4.704 (blue) eV. The labels denote all origin transitions except the B̃ 1B1 state.

86

There are some features around 3.3 eV, which were not observed in the work of Robinson
et al.121 probably due to the threshold effect. Since the term energy of the b̃ 3A2 state lies between
the Ã 1A2 and ã 3B1 states, the peaks at eBE = 3.269(7) and 3.307(7) eV are assigned to the origin
transition and the first overtone of the v9 level (CCC in-plane bending), respectively, of the b̃ 3A2
state. The v9 mode (b2 symmetry) in the b̃ 3A2 state becomes active due to a vibronic coupling withthe ã 3B1 state, lending intensity to this transition.152 However, the peak at eBE = 3.348(7) eV in
the black trace shown in Fig. 5.2 cannot be assigned to any level of what state.
Unlike the complicated features below 4 eV, the structure of the c̃ 3A1 state in the blue traceof Fig. 5.2 is straightforward. The origin of this state appears at 4.391(4) eV, and the position of
the adjacent peak is 4.510(5) eV. The spacing is 0.119(7) eV or 960(56) cm-1, and this transitionis assigned as the v4 fundamental (H2C−CC stretching). The position of next peak, 4.629(5) eV,
may be the 2v4 level. However, there is a Fermi resonance between the 2v4 and v2 modes, so things
become more complicated and uncertain.152

5.4 Discussion
This work was combined with Negative-Ion Photoelectron Spectroscopy (NIPES) at JILAand Slow Electron Velocity-map Imaging (SEVI) at Berkeley to obtain a moderately highresolution photoelectron spectrum over the range of eBE from 1.49 to 4.71 eV, shown in Fig. 5.3.
Before discussing details, one can notice that data are all consistent, and particularly, SEVI offers
the highest resolution measurement of the three techniques. However, some SEVI signals candisappear due to threshold effects. In addition, NIPES has constant energy resolution over theentire spectrum, but the range of eBE is limited by the fixed-laser frequency, prohibiting
investigation of higher-lying excited states of the H2CCC radical.

87

Figure 5.3 Composite photoelectron spectrum of H2CCC– anion using NIPES, VMI, and SEVI
techniques. The three techniques are color-coded in black, blue, and cyan, respectively. The labels
indicate the origins of different electronic states of H2CCC radical except the B̃ 1B1 state.

88

From SEVI measurements, the more precise EA of the H2CCC radical is determined to be
1.7957(10) eV, in excellent agreement with the previously reported EA of 1.794(8) eV by
Robinson et al.121 The dominant vibrational progression includes v2 (H2CC−C stretching) and v4(H2C−CC stretching) modes. Minor contributions from the 2v6 (CCC out-of-plane bending) and2v5 (HCH out-of-plane wagging) levels are also revealed. The peak positions of the ã 3B1, b̃ 3A2,
Ã 1A2 and c̃ 3A1 origins are 3.0794(10), 3.2773(25), 3.4914(10) and 4.3923(10) eV, respectively.
The corresponding term energies are 1.2837(14), 1.4816(27), 1.6957(14), and 2.5966(14) eV,
respectively. These experimental values are summarized in Table 5.1. Additionally, there is a 2v9(CCC in-plane bending) level in the ã 3B1 state resolved in the NIPES and SEVI spectra because
the vibronic coupling between ã 3B1 and b̃ 3A2 states lends intensity to the v9 mode. Some
vibrational levels missing in the SEVI spectra are captured with our VMI measurement anddescribed in the results section. With the aid of calculations and simulations, we gain further
insight into the observations and assignments, such as the complex singlet−triplet splittings of theexcited states of B1 and A2 symmetries. Table 5.1 summarizes the experimental and calculated
values for comparison.

89

Table 5.1. Comparison between experimental and calculated detachment energies and term
energies.
Detach. Energy (eV)Experiment
Adiabatic

Term Energy (eV)ExperimentCalculation
EOM-CCSDTQSpeciesState

H2CCC–

X̃ 2B10

H2CCC

X̃ 1A11.7957 ± 0.0010

00ã 3 B13.0794 ± 0.0010

1.2837 ± 0.0014

1.3480b ̃ 3 A2

3.2773 ± 0.0025

1.4816 ± 0.0027

1.5637Ã 1A2

3.4914 ± 0.0010

1.6957 ± 0.0014

1.7302B̃ 1B1c̃ 3A12.0281
4.3923 ± 0.0010

2.5966 ± 0.0014

2.5967

90

Maier et al. recently proposed142 that broad DIBs at 4881 Å and 5450 Å are attributed to
the B̃ 1B1 ← X̃ 1A1 transition of H2CCC. Using our experimental results, the position of the v2 bandin the B̃ 1B1 state, eBE = 4.065(6) eV, lies 2.269(6) eV above the H2CCC ground state (EA =1.7957(10) eV), which corresponds to a wavelength range between 5449 and 5479 Å. This result
is consistent with the electronic absorption of H2CCC at 5450 Å in the work of Maier et al.
However, the astronomical assignment is controversial and has been criticized.143-145 Araki andcoworkers searched for the rotational transition of H2CCC toward HD 183143 using a radio
telescope.145 They concluded that the major DIBs carrier at 4881 and 5450 Å should not be
attributed to H2CCC due to low column density. Similarly, Liszt et al. directly measured thecolumn density of H2CCC with radio telescopes in four diffuse clouds and TMC-1 (Taurus
molecular cloud 1).144 They found that the observed column density of H2CCC is three orders ofmagnitude lower than the required column density to explain the DIBs, suggesting that H2CCC is
not a possible carrier of DIBs due to apparently low abundance. Thus, the hypothesis by Maier etal.142 is very unlikely to be correct.

5.5 Conclusion
In this work, we measured the photoelectron spectra of H2CCC– anion with three different
photon energies. The five lowest-lying excited states are observed and assigned as well as theactive vibrational levels. The B̃ 1B1 origin is not observed and not assigned since strong vibronic
coupling between the Ã 1A2 and B̃ 1B1 states makes the origin transition unclear. Finally, combining
experiment and theory gains further insight into the spectral observations and assignments.

91

Chapter 6
Photoelectron Spectroscopy of Propargylene Anion,
HCCCH–
6.1 Introduction
Isomers of C3H2 are important in organic, astrophysical, and combustion chemistry. Singlet
cyclopropenylidene (c-C3H2) is the global minimum structure, which has two low-lying isomers:
triplet propargylene (HCCCH, ∆E = 10 kcal/mol) and singlet propadienylidene (H2C=C=C:, ∆E =13 kcal/mol).133 Cyclopropenylidene is one of the most abundant organic molecules in interstellar
clouds,153,144 and propadienylidene has been suggested as a carrier of the diffuse interstellar bands(DIBs), which is still being debated and described in Chapter 5. Propargylene, also called
propynylidene, has not yet been detected in space due to its small dipole moment. HCCCH has a3

B electronic ground state with C2 symmetry, according to matrix EPR and IR spectroscopy

studies, as well as theoretical calculations.154 The electronic absorption spectrum of HCCCH in an
Ar matrix has been investigated, exhibiting broad absorption bands and vibrational structures.133,154
The lowest broad band, spanning from 350 to 275 nm, can be assigned to a combination of the Ã
3

B and B̃ 3A states. However, several excited electronic states, including singlet and triplet states,

have been computed to lie within this range.155,156 Since no conclusive experimental assignments
have been reported thus far, further characterization and assignment of the excited electronic statesis necessary.
In this chapter, we report the photoelectron spectrum of HCCCH– anion. We utilize aneutral−entrainment anion source to produce HCCCH–, allowing us to investigate the neutralelectronic excited states following electron detachment. We measure the term energies and92vibrational levels for multiple excited electronic states of HCCCH. Finally, we compare theexperimental results, collected in two laboratories, with calculated values.

6.2 Experimental Details
The VMI photoelectron spectrometer has been described in detail in Chapter 2. This
experimental apparatus uses a pulsed anion beam, mass separation by a Wiley-McLaren time-offlight mass spectrometer (TOF MS),24 a tunable laser source, and a VMI detector.
For anion preparation, we employ the O− + propyne reaction first observed by Dawson etal.157 and form HCCCH− in a pulsed supersonic entrainment reactor.46 Dawson and coworkers
found that the O− + propyne (H3CC≡CH) reaction produces both H2CCC− and HCCCH− products
whereas the reaction of O− with propyne’s isomer, allene (H2C=C=CH2), produces C3H2− in nearlypure H2CCC− isomeric form. The photoelectron spectra of the m/z = 38 anion arising from bothreactions (O− + propyne and O− + allene) are taken under the same experimental conditions in thepresent experiment. The entrainment reactor consists of a central and two side pulsed General
Valves operating at a repetition rate of 80 Hz with separate timings and pulse widths. The middle
valve provides the main expansion using 1% propyne or 1% allene seeded in Ar at a stagnationpressure of 4.1 atm. One of the side valves entrains neat N2O into the main expansion. Collisionsof the gas expansion with a guided 1-keV electron beam yield slow secondary electrons that form
the O− reactant through dissociative electron attachment of N2O. Subsequently, the O− reacts withpropyne or allene to form the m/z = 38 anion. If the N2O entrainment valve is turned off, the m/z= 38 signal disappears.

93

The anions are perpendicularly extracted and mass-separated using a Wiley-McLaren TOF
MS. Anions of m/z = 38 are photodetached by spatially and temporally overlapping them with thefourth harmonic generation (FHG, 266 nm or 4.660 eV) of an Nd:YAG laser or a tunable-lightpulse (5.158, 4.704 or 2.352 eV) from the frequency-doubled or fundamental signal output of a355-nm pumped optical parametric oscillator. Then, the kinetic energy distribution of the ejected
electrons is acquired by the VMI photoelectron spectrometer. In this chapter, we reportphotoelectron spectra as a function of electron binding energy (eBE= ħω – eKE), which is
independent of the laser wavelength used for photodetachment. The energy scale for the reported
images is calibrated using the well-known photoelectron spectrum of I−.149-151 The spectra reported
here cover an eKE range from 0 to 1.2 eV, with a declining energy resolution of 10 to 60 meV
across this range.

6.3 Results
Figure 6.1 shows the photoelectron spectra of the product anions from O− + allene and O−+ propyne with a photon energy of 2.352 eV. O− can abstract a proton and a hydrogen atom froma compound at the same time (i.e., H2+ abstraction) and produce a new radical anion along withwater. For the reaction of O− with C3H4, the isomer distribution of the C3H2− product anions
depends on the isomeric form of the C3H4 reactant.157 The O− + allene reaction results in thephotoelectron spectrum of H2CCC−, plotted in black, which has been described in Chapter 5. In
contrast, when O− reacts with propyne, the red trace in Fig. 6.1 shows many new features in
addition to those seen in the black trace. This comparison provides direct evidence that the reactionof O− with propyne produces at least one additional isomer of C3H2−. In a separate experiment,

94

Osborn and coworkers have taken the photoelectron spectra of the product anions from O− +propyne reaction using the Negative Ion Photoelectron Spectroscopy (NIPES) instrument at
JILA.158 They confirmed that the O− + propyne reaction produces both H2CCC− and HCCCH−
products. Although the neutral−entrainment anion source in the VMI instrument is different fromthe flowing afterglow ion source in the NIPES instrument, the features from 1.4 to 2.1 eV binding
energy between two instruments show the same isomer-specific chemistry. Therefore, for all
photon energies in this chapter, the black spectra (O− + allene) arise from photodetachment of
H2CCC−, whereas features in the red spectra (O− + propyne) that are not present in the black spectra
can be assigned to transitions from the propargylene anion, HCCCH−. For this Chapter, the focus
will be on the assignment of those peaks attributed to detachment from the HCCCH− isomer.

95

Figure 6.1 Photoelectron spectra of the product anions from O− + allene (black), and O− + propyne(red) acquired with the VMI photoelectron spectrometer. Spectra were taken with ħω = 2.352 eV.

96

Using higher photon energy of 4.660 and 4.704 eV, we observe the photoelectron spectra
shown in Fig. 6.2, which could not be acquired on the NIPES instrument. From this comparison,
the features in the red spectrum that are not present in the black spectrum exhibit a well-resolved,
short vibrational progression. We attribute the peaks at eBE = 4.454(5) and 4.590(5) eV in the red
spectrum to an electronic excited state of HCCCH, assigned in the Discussion section. In order toaccess still higher electronic states of HCCCH, we use a greater photon energy of 5.158 eV. The
photoelectron spectra, presented in Fig. 6.3, show two additional features at eBE = 4.738(5) and
4.869(5) eV.

97

Figure 6.2 Photoelectron spectra of the product anions from O− + allene (black) and O− + propyne(red) reactions. Spectra in red were acquired with ħω = 4.660 eV, and in black with ħω = 4.704
eV.

98

Figure 6.3 Photoelectron spectra of the product anions from O− + allene (black) and O− + propyne(red) reactions. Both spectra were taken with ħω = 5.158 eV.

99

6.4 Discussion
This work is combined with NIPES at JILA to cover the eBE range of 0.9 to 5.1 eV. We
compare these results with high-level electronic structure calculations to gain further insight into
the electronic structure of HCCCH. Before going into detail about the spectra obtained in the VMI
instrument, a brief overview of the findings from the NIPES experiment is presented.158 Throughcomparison of the HCCCH− and DCCCD− photoelectron spectra, the EA of HCCCH is determined3
1
to be 1.156 ± 00..010
095 eV, with a singlet-triplet splitting between the X̃ B and the ã A states of ∆EST
1
0.22
= 0.500 ±0.10
0.01 eV. The experimental term energy of the higher excited state, b̃ B, is 0.94 ± 0.20 eV.

Transitions to both X̃ 3B and b̃ 1B states exhibit long vibrational progressions, a portion of which
can be seen in Fig. 6.1, and are assigned to the v4 mode (in-plane symmetric CCH bend) of theX̃ 3B state, indicating a large geometry change from the anion to the neutral.
Regarding the electronic structure calculation, Osborn and coworkers employ CCSD(T)for the lowest electronic state of each symmetry and multi-reference configuration interaction(MRCI) level of theory for all electronic states in order to calculate the geometries and transitionenergies.158 Note that there is a low-lying excited state, Ã 2A, in the anion, which could be
populated in the anion beam. However, since there is no experimentally observed splitting arisingfrom the two predicted anion states, we consider only one anion state in our interpretation oftransitions to all neutral states. The experimental and calculated values are summarized in Table
6.1. The calculated large CCH bond angle change from anion (126°) to neutral (X̃ 3B, 160°; b̃ 1B,
180°) is consistent with the long vibrational progression of the CCH bend seen in the NIPES
experiment. The other calculated geometries presented in Table 6.1 will be used below to aid in
the assignment of higher excited electronic states seen in the VMI data.

100

The peak assignments in the VMI data are rather complicated. Background signals inherent
in these experiments prohibit quantifying the angular distributions from the VMI apparatus, whichcontains crucial information for the assignment of electronic states. Based on the peak intensities,
the 4.454 eV peak (Fig. 6.2-6.3) is likely the origin of an electronic state. Above the 4.454 eV
peak, the peak at 4.590 eV is tentatively assigned as the CCC stretch mode (v2), indicating a v2
frequency of 0.136 eV (1,097 cm-1). However, a third peak is located at 4.738 eV, lying 0.284 eV(2,291 cm-1) above the 4.454 eV peak. This spacing cannot be due to a fundamental vibrationalmode or, based on intensities, a combination band. Therefore, we assign the peak at 4.738 eV as
the origin of an additional excited electronic state of HCCCH with a similar geometry to the anion.
According to the MRCI results in Table 6.1, the c̃ 1A and Ã 3B states both have large geometry
changes compared to the anion, which would lead to extensive vibrational progressions or makethem difficult to observe. Thus, this 4.738 eV feature is not due to the c̃ 1A or Ã 3B states. In
contrast, the d̃ 1A and B̃ 3A states have small geometry changes compared to the Ã 2A and X̃ 2B
anionic states, respectively, which is consistent with the trends seen in the photoelectron spectrum.
Based on calculated energetics, the best match with the experimental origin peak at 4.454 eV isthe d̃ 1A2 state, and the peak at 4.738 eV is assigned as the B̃ 3A state. The feature at 4.869 eV lies
0.131 eV (1057 cm-1) above the origin of the B̃ 3A state. We assign this peak as excitation of thev2 mode in the B̃ 3A state. Thus, the term energies of the d̃ 1A and B̃ 3A states are 3.298 ±0.10
0.01 and3
3.582 ±0.10
0.01 eV, respectively. The term energy of the B̃ A state is also in a good agreement with

the UV/Vis absorption spectrum of the triplet ground-state HCCCH, which exhibited a broad
absorption from 350 to 275 nm and can be assigned to an absorption to a combination of the Ã 3B
and B̃ 3A states.133,154

101

Table 6.1. Comparison between experimental and calculated detachment energies and term
energies.
SpeciesState–∠ CCC (°) ∠ HCC (°)

Adiabatic Detachment Energy (eV)
CCSD(T)/CBS MRCI/CBS
Experiment

HCCCH

2
X̃ B

179

126

0

0

2

163

123

0.026

0.014

HCCCH

Ã A
3
X̃ B

173

160

1.146

1.03

1

Term Energy (eV)Experiment01.156 ± 0 .095

0 . 010

0
0.10
0.01

ã A
1
b̃ B

180

129

1.48

1.165 ± 0.005

0.500 ±

180

180

1.82

2.100 ± 0.2

10.940 ± 00..22
20

c̃ A
3
Ã B

180

180

2.48

108

131

4.26

1

160

135

4.29

4.454 ± 0.005

3.298 ± 0.10
0.01

180

128

4.36

4.738 ± 0.005

3.582 ± 0.01

d̃ A
3
B̃ A

0.10

102

6.5 Conclusion
In this work, we recorded the photoelectron spectra of the HCCCH– anion using threedifferent photon energies. The three lowest-lying electronic states (X̃ 3B, ã 1A, and b̃ 1B) were
measured in the NIPES instrument to obtain the EA and singlet-triplet splitting of HCCCH. From
the VMI experiments, origin transitions of the d̃ 1A2 and B̃ 3A2 states are assigned and characterizedbased on a small geometry change from the anion. The HCCCH c̃ 1A and Ã 3B states were notobserved.

103

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116

Appendix A: Schematic Drawings of the Parts of the Discharge
SourceFigure A.1 Schematic arrangement of the pulsed plasma-entrainment anion source employing
two solenoid valves. Part 1 is a custom-made conical nozzle coupling with a General Valve
body, part 2 is a cylindrical MACOR plate, part 3 is an electrode (stainless steel or aluminum)
pulsed to negative high voltage, part 4 is a thin MACOR insulator between two electrodes, andpart 5 is a thin electrode (stainless steel) attached to a commercial General Valve faceplate heldat earth ground. These components are expanded below.

117

Figure A.2 Picture of the setup showing how both pulsed valves are mounted in the sourcechamber. This picture also displays how the stack of insulators and electrodes is held together. Allcomponents of the discharge device have four bolt holes and are fastened onto the faceplate of theside solenoid valve using two nylon screws.

118Part 1: 40°-cone, 3.5-mm thick, 500-µm conical nozzle
In order to increase the forward intensity of the main expansion, we use a 40° conicalnozzle with a 0.5-mm orifice (DO) and an aspect ratio (L/DO) of 7.70 The nozzle seals in a similar
fashion as a commercial faceplate and, thus, couples well with a General Valve body.

Part 2: The ended MACOR plate:This MACOR plate separates the main pulsed valve and the electrode (part 3) that is pulsed
to negative high voltage.

119Part 3: The electrode (S.S. or aluminum) pulsed to negative high voltage:
A pulsed voltage between -800 and -2000 V is used to maximize the ion intensities and thestability of the discharge. Additionally, we use a 40° conical nozzle with a 1-mm orifice tocollimate generated plasma. In order to have stable discharge, the edge of the orifice is required to
be clean and smooth. We clean the surface using #400 sand papers and make the edge smoothusing the electropolishing machine at JILA machine shop.

Part 4: The thin MACOR insulator between two electrodes:
The central hole of this MACOR insulator is very fast to get dirty since the discharge occurs
at this region. We use sand “wires” or small driller to get rid of thin layers from the surface of the
middle hole.

120Part 5: The thin electrode (S.S.) held at earth ground:We use this plate to avoid heating up or discharging to the faceplate of the solenoid valve.
This plate is also fast to get dirty and damaged, so the surface of the electrode needs to be cleanand smooth. We clean the surface using #400 sand papers and make the surface smooth using theelectropolishing machine at JILA machine shop.

121

Appendix B: Expanded Sectional View of the Discharging Region
In this design, the gas density at the discharge region between two electrodes is high enoughto produce plasma. The diameters of both central holes in two electrodes are 1 mm, while thediameter of the middle hole in the MACOR insulator is slightly larger, 1.5 mm.

122

Appendix C: Source Conditions for OH−(Ar)n Cluster Formation

Main expansion: 3.7-atm neat Ar
Side expansion: 4.7-atm 1% O2 / 25% H2 / Ar
(Although the backing pressures of both gas mixtures are similar, the side pulsed valve opens
shortly, resulting in that the ion-gauge pressure reading for the side valve is roughly one order ofmagnitude less than that for the main valve.)
Pulsed discharge voltage: -2000 V(The electrode is in series with two 10 kΩ resistors for increasing discharge stability.)
Pulsed discharge duration: 40 ~ 140 µs(Once ions are found with a long pulsed discharge duration (e.g., 140 µs), decreasing the duration
with small steps (10 µs) and optimizing the discharge timing until reaching a duration of 40 µs arehelpful to achieve cold anion beam.)
Discharge current: 75 mA(The voltage drop across two 10 kΩ resistors is -1.5 kV, corresponding to a discharge current of
75 mA.)
Temporal FWHM of the ion packet: 20 µs(We vary the timing of the extraction plate that directs anions down the TOF MS to measure thetime spread of the ion packet in the main expansion.)