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Chemistry & Biochemistry Graduate Theses &Dissertations

Chemistry & Biochemistry

Spring 1-1-2017

Quantum State-Resolved Collision Dynamics ofNitric Oxide at Ionic Liquid and Molten Metal
Surfaces
Amelia Marie Zutz
University of Colorado at Boulder, mia.zutz@gmail.com

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Quantum State-Resolved Collision Dynamics
of Nitric Oxide at Ionic Liquid and Molten Metal SurfacesbyAmelia Marie Zutz
B.A., Carleton College, 2010

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirement for the degree ofDoctor of Philosophy
Department of Chemistry and Biochemistry2017This thesis entitled:
Quantum State-Resolved Collision Dynamics
of Nitric Oxide at Ionic Liquid and Molten Metal Surfaces
written by Amelia M. Zutz
has been approved for the program of Chemical Physics by:_____________________________David J. Nesbitt_____________________________
W. Carl Lineberger

Date _______________

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

iiZutz, Amelia M. (Ph.D., Chemical Physics)
Quantum State-Resolved Collision Dynamics of Nitric Oxide at Ionic Liquid and Molten Metal
Surfaces
Thesis directed by Professor David J. Nesbitt

Detailed molecular scale interactions at the gas–liquid interface are explored withquantum state-to-state resolved scattering of a jet-cooled beam of NO(2Π1/2; N = 0) from ionicliquid and molten metal surfaces. The scattered distributions are probed via laser-induced
fluorescence methods, which yield rotational and spin-orbit state populations that elucidate thedynamics of energy transfer at the gas-liquid interface. These collision dynamics are explored asa function of incident collision energy, surface temperature, scattering angle, and liquid identity,
all of which are found to substantially affect the degree of rotational, electronic and vibrational
excitation of NO via collisions at the liquid surface.
Rotational distributions observed reveal two distinct scattering pathways, (i) moleculesthat trap, thermalize and eventually desorb from the surface (trapping-desorption, TD), and (ii)those that undergo prompt recoil (impulsive scattering, IS) prior to complete equilibration withthe liquid surface. Thermally desorbing NO molecules are found to have rotational temperatures
close to, but slightly cooler than the surface temperature, indicative of rotational dependent
sticking probabilities on liquid surfaces. Nitric oxide is a radical with multiple low-lying
electronic states that serves as an ideal candidate for exploring nonadiabatic state-changing
collision dynamics at the gas-liquid interface, which induce significant excitation from ground(2Π1/2) to excited (2Π3/2) spin–orbit states. Molecular beam scattering of supersonically cooled
NO from hot molten metals (Ga and Au, Ts = 300 – 1400 K) is also explored, which provide
preliminary evidence for vibrational excitation of NO mediated by thermally populated electron-iii

hole pairs in the hot, conducting liquid metals. The results highlight the presence ofelectronically nonadiabatic effects and build toward a more complete characterization of energy
transfer dynamics at gas-liquid interfaces.

iv

To Andyv

ACKNOWLEDGMENTS

I have many people to thank for not only helping me during my Ph.D., but also prior to
my arrival in Boulder. I started my undergraduate degree at Carleton with many possible majors
in mind, but eventually Will Hollingsworth and Marion Cass piqued my interest in Physical
Chemistry. Their passion for science and endless curiosity was infectious, as they were always
excited to explore new ideas and even created a new class for myself and four other students whowanted to learn more about molecular reaction dynamics.
I want to thank David Nesbitt, my research advisor and mentor, who has constantly
pushed me to think more deeply about my data and to understand underlying fundamental
principles of a problem. David has shown me the importance of distilling complex ideas down tosimple physical pictures, as well as the power of clear communication and scientific storytelling.
To my thesis committee—Eric Cornell, Barney Ellison, Heather Lewandowski, and Carl
Lineberger—thank you for taking the time to learn about and provide feedback on my research. I
want to further express my appreciation to Eric, Barney and Carl for tracking my progress overthe last few years and bringing an outside perspective to my research.
Over my time here at JILA, the support and assistance of the entire JILA staff has been
invaluable. The machine shop and electronics shop staffs have helped me consider, discuss andsolve countless problems with their extensive knowledge and constant willingness to help. In
particular, Kyle was incredibly helpful in designing and building new components for myexperiment, as well as Hans for his enthusiasm and skill in brainstorming and tackling everyviconceivable problem. The computing support and supply staff have also proven to be crucial in
helping to keep my experiment up and running.
The Nesbitt group has been an intellectually stimulating and scientifically diverse
environment, which has been incredibly rewarding. Many fellow Nesbitt group members havehelped immensely throughout my Ph.D. Mike Ziemkiewicz built up this experiment and was aninvaluable teacher and mentor. I learned so much working with him, especially from themultitude of angles from which he attacks problems. Thank you also to Nick Dupuis, Grant
Buckingham, Rob Roscioli, Kevin Early, Monika Grutter, Andrej Grubisic, Melanie Reber, Erik
Holmstrom, Tim Livingston-Large, Jake Pettine, and Mikhail Ryazanov for countless scientificand nonscientific discussions.
I am incredibly grateful for the support of all my friends and family and am especiallyappreciative of the community of friends I’ve gathered in Boulder with whom I ski and playsoccer/squash/ultimate frisbee. All of my friends outside of Colorado have continued to provide
support from afar, in particular both my brothers. Mom and Dad, thank you for all theopportunities you’ve made available to me and for your boundless love and support. Finally,
Andy, thank for all your relentless support throughout this process, for being there always andfor bringing so much joy to my life.

vii

CONTENTS

Chapter 1

Introduction ............................................................................................................... 1

References for Chapter 1 ........................................................................................................ 14

Chapter 2

Experimental Apparatus ........................................................................................... 20

2.1 Introduction .................................................................................................................... 20
2.2 LIF Detection of Nitric Oxide ........................................................................................ 21
2.3 Incident Molecular Beam ............................................................................................... 26
2.4 Liquid Surfaces .............................................................................................................. 33
2.5 Molecular Scattering and Detection Geometry .............................................................. 38
2.6 Data Analysis ................................................................................................................. 45
2.7 Signal to Noise Estimate ................................................................................................ 48
2.8 Experimental Changes to Study Hot Molten Metals ..................................................... 50
2.8.1
2.8.2
2.8.3
2.8.4
2.8.5

Resistive Heating Setup ................................................................................. 51
Temperature Measurement ............................................................................. 56
Metal Evaporation ......................................................................................... 59
Surface Cleaning ........................................................................................... 63
Detection of NO(v = 1) ................................................................................. 64

References for Chapter 2 ....................................................................................................... 68

Chapter 3 Nonadiabatic Spin−Orbit Excitation Dynamics in Quantum-State ResolvedNO(2Π1/2) Scattering at the Gas−Room Temperature Ionic Liquid Interface ........... 70
3.1 Introduction ..................................................................................................................... 70
3.2 Experimental .................................................................................................................. 76
3.3 Results and Analysis ...................................................................................................... 79
3.3.1 Low Einc Rotational Distributions (Einc = 2.7(9) kcal/mol) ............................ 79
3.3.2 High Incident Energy Rotational Distributions (Einc = 20 kcal/mol) ............. 86viii

3.4 Discussion ...................................................................................................................... 92
3.4.1 Nonadiabatic Electronic Effects .................................................................... 92
3.4.1 Theoretical Considerations ............................................................................ 94
3.5 Summary and Conclusions ............................................................................................ 99
References for Chapter 3 ....................................................................................................... 102
Chapter 4 Quantum State-resolved Molecular Scattering of NO(2Π1/2) at the Gas[Cnmim][Tf2N] Room Temperature Ionic Liquid Interface: Dependence on
Alkyl Chain Length, Collision Energy, and Temperature ........................................ 106
4.1 Introduction .................................................................................................................... 106
4.2 Experimental .................................................................................................................. 110
4.3 Results ............................................................................................................................ 112
4.4 Discussion ....................................................................................................................... 117
4.4.1 Alkyl Chain Length Dependence .................................................................. 117
4.4.2 Spin-orbit Distribution: Dependence on Cn ................................................... 126
4.4.3 Dependence on Surface Temperature ............................................................ 127
4.5 Conclusions .................................................................................................................... 130
References for Chapter 4 ....................................................................................................... 132

Chapter 5 Angle-resolved Molecular Beam Scattering of NO at the Gas- Liquid Interface .... 136
5.1 Introduction .................................................................................................................... 136
5.2 Experimental .................................................................................................................. 140
5.3 Results ............................................................................................................................ 143
5.3.1 Low Energy Collisions: Cos(s) Angular Distributions andThermal Desorption Dynamics ...................................................................... 144
5.3.2 High Energy Collisions: Angle-Dependent Rotational Distributions ........... 149
5.3.3 Angle-Dependent Spin-Orbit Distributions at Low andHigh Collision Energies ................................................................................ 154
5.3.4 NO Scattering from Squalane and PFPE ....................................................... 156
5.4 Discussion ...................................................................................................................... 158

ix

5.4.1 Rotational State Dependence on θs ................................................................ 158
5.4.2 Spin-orbit Dependence on θs ......................................................................... 162
5.4.3 Strong Correlation Between <s> and Angular Momentum N ..................... 164
5.5 Conclusion ..................................................................................................................... 167
References for Chapter 5 ....................................................................................................... 170
Chapter 6 Incident Collision Energy Dependence of NO Scattering Dynamics at theGas-RTIL Interface .................................................................................................. 174
6.1 Introduction .................................................................................................................... 174
6.2 Experimental .................................................................................................................. 175
6.3 Results and Analysis ...................................................................................................... 175
6.3.1 Isolating Thermal Desorption at Low Collision Energies ............................. 179
6.3.2 Branching into Multiple Scattering Pathways at High Collision Energies ... 181
References for Chapter 6 ....................................................................................................... 185

Chapter 7 State-to-state Resolved Scattering of NO from Molten Gold and Gallium .............. 186
7.1 Introduction ...................................................................................................................... 186
7.2 Experimental .................................................................................................................... 188
7.3 Results .............................................................................................................................. 191
7.3.1 NO Scattered from Au(l) ............................................................................... 192
7.3.2 NO Scattered from Ga(l) ............................................................................... 196
7.4 Discussion ........................................................................................................................ 198
7.4.1 Temperature Dependence of NO scattered from Ga(l) .................................. 198
7.4.2 Energy Transfer at the NO + Ga(l)/Au(l) Interfaces ..................................... 201
7.5 Conclusion ....................................................................................................................... 203
References for Chapter 7 ....................................................................................................... 205

Bibliography ................................................................................................................................. 207

x

FIGURES

Figure 1.1

Molecular scattering of NO from liquid surfaces with LIF detection ...................... 6

Figure 1.2

Impulsive scattering and trapping-desorption scattering pathways ......................... 7

Figure 1.3

Room-temperature Ionic Liquid molecular structures ........................................... 10

Figure 2.1

NO electronic transition probed by LIF detection................................................... 21

Figure 2.2

Sample LIF spectrum of NO(v = 0) ....................................................................... 22

Figure 2.3

Production of pulsed UV light schematic .............................................................. 24

Figure 2.4

LIF signal curve of growth vs. laser energy ........................................................... 27

Figure 2.5

Incident beam pulse shapes .................................................................................... 28

Figure 2.6

Incident beam rotational temperature and LIF spectrum ....................................... 29

Figure 2.7

Growth curve of fractional NO in molecular beam ................................................ 31

Figure 2.8

Incident beam velocity measurements ................................................................... 32

Figure 2.9

Molecular structures of liquids studied .................................................................. 34

Figure 2.10 Birefringence of [C12mim][Br] liquid crystal formation ........................................ 37
Figure 2.11 Schematic of LIF detection .................................................................................... 39
Figure 2.12 LIF fluorescence and UG5 filter transmission spectra ........................................... 41
Figure 2.13 Final angle resolved molecular scattering .............................................................. 42
Figure 2.14 Angular distributions for detection at various scattered angles ............................. 43
Figure 2.15 Model fit to v = 0 LIF spectrum ............................................................................. 47
Figure 2.16 Room temperature NO distributions ..................................................................... 48
Figure 2.17 Molten metal vapor pressure curves ....................................................................... 51
Figure 2.18 Molten metal heating setup .................................................................................... 53
Figure 2.19 Temperature measurements of tungsten crucible and molten metals ..................... 57
Figure 2.20 Calibration of molten metal temperature measurements ........................................ 59xiFigure 2.21 Gold sample in vacuum chamber ........................................................................... 60
Figure 2.22 Imaging lens cone ................................................................................................... 61
Figure 2.23 Blackbody radiation effect on LIF detection .......................................................... 62
Figure 2.24 LIF detection scheme for NO(v = 1) ...................................................................... 65
Figure 2.25 NO A2Σ+(v = 0) ⟵ X2П(v = 1) spectrum .............................................................. 66
Figure 2.26 Transmission spectra of UG5 and XUV0325 filters .............................................. 67
Figure 3.1

Molecular scattering at gas-liquid surface .............................................................. 77

Figure 3.2

Sample LIF spectrum and corresponding NO rotational distributions .................. 80

Figure 3.3

Low collision energy rotational Boltzmann analysis ............................................. 81

Figure 3.4

Low collision energy rotational temperatures vs. surface temperature .................. 82

Figure 3.5

Spin-orbit and lambda-doublet electronic state ratios ............................................ 86

Figure 3.6

High collision energy Boltzmann analysis with two-temperature fit ..................... 89

Figure 3.7

Two-temperature fit parameters, α and TIS, as a function of surface
temperature ............................................................................................................. 90

Figure 3.8

Spin-orbit ratios in the TD and IS channel vs. surface temperature ...................... 93

Figure 4.1

Experimental molecular scattering cartoon with [Cnmim][Tf2N]
molecular structures and sample LIF spectra of incident and scattered NO ........... 108

Figure 4.2

Rotational Boltzmann plots of scattered NO at Einc = 2.7 and 20 kcal/mol ........... 114

Figure 4.3

Rotational temperatures of low collision energy NO scattered from
[Cnmim][Tf2N] for n = 2, 4, 8, 12, 16 .................................................................... 116

Figure 4.4

Two-temperature fits to rotational distributions of scattered NO(Einc = 2 and 20 kcal/mol) for varied hydrocarbon chain lengths .......................... 120

Figure 4.5

Two-temperature fit parameters of scattered NO exhibit shift fromshort (n ≤ 4 ) to longer (n ≥ 8) chain lengths) ........................................................ 121

Figure 4.6

Spin-orbit ratio of scattered NO at low and high collision energies as afunction of hydrocarbon chain length .................................................................... 127

xii

Figure 4.7

Temperature dependence of α and TIS for [C2mim][Tf2N] and[C12mim][Tf2N] ...................................................................................................... 128

Figure 5.1

Angle-resolved LIF detection of molecular scattering ........................................... 141

Figure 5.2

Sample LIF spectra of NO scattered at θs = -30º and 30º ...................................... 143

Figure 5.3

Single temperature rotational Boltzmann plots for scattered NO(2Π1/2) atvarious final scattering angles (Einc = 2.7 kcal/mol) ............................................. 145

Figure 5.4

Angle-resolved rotational temperatures and integrated populations for NO
scattered at Einc = 2.7 kcalmol ................................................................................. 147

Figure 5.5

Rotational population distributions (Einc = 20 kcal/mol) with TD/IS fits asa function of final scattering angle θs ..................................................................... 150

Figure 5.6

Angle-resolved two-temperature fit parameters (α and TIS) of hyperthermal
NO scattered from [bmim][Tf2N] ........................................................................... 153

Figure 5.7

Spin-orbit temperatures vs. final scattering angle for low and highcollision energies ..................................................................................................... 156

Figure 5.8

Scattering parameters (Telec, TIS and α) for NO scattered fromsqualane, PFPE and [bmim][Tf2N] at Einc = 20kcal/mol ......................................... 159

Figure 5.9

Normalized angular distributions for specific J statesat Einc = 2.7 and 20 kcal/mol ................................................................................... 161

Figure 5.10 Spin-orbit ratios of scattered NO vs. total angular momentum(θs = - 60º and 60º) .................................................................................................. 162
Figure 5.11 Average NO scattering angle as a function of total angular momentum ................ 165
Figure 6.1

LIF Spectra of scattered NO at Einc = 2.7 and 20 kcal/mol ..................................... 176

Figure 6.2

Collision energy dependence of rotational population distributions ....................... 177

Figure 6.3

Spin-orbit electronic temperature vs. collision energy............................................ 178

Figure 6.4

Rotational Boltzmann plot for Einc varied from 2.0 to 4.0 kcal/mol ....................... 179

Figure 6.5

Rotational temperatures for scattered NO at varied low Einc .................................. 180

Figure 6.6

Two-temperature rotational fits for Einc = 4.9 – 20 kcal/mol .................................. 181

Figure 6.7

Thermal accommodation coefficient as a function of collision energy .................. 183

xiii

Figure 6.8

IS rotational temperature vs. incident collision energy ........................................... 184

Figure 7.1

Molecular scattering apparatus to study molten metals .......................................... 189

Figure 7.2

LIF spectra of NO(v = 0 and v = 1) scattered from molten gold (Ts = 1400 K)
at high collision energies ......................................................................................... 191

Figure 7.3

Vibrational Boltzmann plot of NO scattered from Au(l). ....................................... 193

Figure 7.4

Rotational Boltzmann plots of NO(v = 0) scattered from Au(l) ............................. 194

Figure 7.5

Rotational Boltzmann plots of NO(v = 1) scattered from Au(l) ............................. 196

Figure 7.6

Rotational Boltzmann plots of NO (v = 0 and v =1) scattered from Ga(l) ............. 197

Figure 7.7

Vibrational excitation of NO from Ga(l) as a function of surface temperature. ..... 199

Figure 7.8

Probability of vibrational excitation vs. inverse TS. ................................................ 200

Figure 7.9

Summary of energy transfer from NO scattered from Au(l) and Ga(l)into the electronic, rotational and vibrational degrees of freedom. ......................... 202

xiv

TABLES

Table 3.1

Ab initio equilibrium geometries for Cl- + NO and well depths for the
A’ and A” potential surfaces ................................................................................. 97

Table 4.1

Summary of least squares fit parameters for NO scattered from
[Cnmim][Tf2N] .......................................................................................................118

Table 5.1

Angle-dependent rotational and spin-orbit temperatures of NO scatteredfrom squalane, PFPE and [bmim][Tf2N] at low collision energies .......................157

xv

Chapter 1

Introduction

The gas-liquid interface is a complex and remarkably important chemical environment,
yet the chemical physics community is still seeking a comprehensive understanding of thechemical dynamics occurring at a molecular level. The interaction of gases with liquid surfaces
is fundamental to many atmospheric, environmental, biological and catalytic processes,1-7
however it is still not possible to predict how individual gas molecules will interact with a liquidsurface. As a gas molecule approaches a surface, there are many properties that govern whether amolecule sticks to or bounces away from the surface. Are there barriers for molecules adsorbing
on the surface? Do these barriers depend on angle of approach, rotational state, vibrational state,molecular orientation, or surface temperature? For molecules that do stick, how long do they taketo thermally equilibrate, or do the molecules escape before fully thermalizing? Does an adsorbed
molecule on the surface remain intact, migrate across the surface and possibly dissolve? If theprobe molecule is chemically reactive, does it have sufficient time to react with the surface
before desorbing? Additionally, if the molecule bounces away from the surface, how is energy
transferred between the liquid surface and gas molecules? What degrees of freedom (i.e.,
rotational, translational, vibrational, electronic) are involved in this process? By isolating
individual gas-liquid systems, we can build a more detailed molecular level picture of energy

1transfer at gas-liquid interfaces and develop a more predictive understanding of collision
dynamics and adsorption/solvation of gases into the liquid phase.
The interfacial layer of a liquid next to a gas looks and behaves fundamentally differently
from the bulk of the liquid. Depending on the gas-liquid combination, the interfacial region
extends several monolayers into the liquid, ranging from several angstroms to a few nanometers,
and has a preferred orientation due to the gas-liquid interfacial asymmetry.8 Many experiments
can now characterize the structure and composition of liquid surfaces,9-14 with experimental
techniques that include: sum frequency generation spectroscopy (SFG),10, 15, 16 surface tension
measurements,17 angle-resolved X-ray photoelectron spectroscopy (ARXPS),18 X-ray
diffraction,19, 20 low energy ion scattering (LEIS),21 metastable atom electron spectroscopy(MAES),22 and neutron reflectivity.23, 24 All of these experimental techniques have varying
levels of surface sensitivity, ranging from the topmost surface layer to a region that extends
several monolayers deep. Additionally, these techniques are all capable of extracting a variety ofgas-liquid interface specific information. For example, SFG yields information on molecularcomposition and orientation in the interfacial region, whereas ARXPS, MAES and LEIS offer
insight into atomic composition at the surface. These experiments, combined with theoreticalefforts,25-32 paint a physical picture of how molecules arrange at liquid surfaces.
In order to explore gases adsorption on and solvation into liquid surfaces, a number ofother experiments have examined solubilities and rates of trace gases dissolving in liquids,33 suchas droplet train flow reactors, wetted wall flow reactors, and bubble train reactors. In theseexperiments, gas molecules are brought into contact with a liquid surface in a controlled way,such that the amount of gas dissolved in the liquid over a known time can be determined. Whilethese experiments yield important information on mass accommodation rates and bulk

2solubilities of gases for liquids, they are unable to address the fundamental processes that govern
the adsorption of gases on liquid surfaces.
Molecular and atomic scattering experiments, however, provide insight on the dynamical
processes occurring at gas-liquid interfaces and help to elucidate the elementary steps involvedin collisions of gases at liquid surfaces. The first gas-liquid scattering experiments were
performed by Hurlbut and Beck in 1959,34 who investigated Ar and N2 interactions with molten
gallium and indium surfaces. The field was expanded further by Fenn,35, 36 Cohen,37-39 andNathanson40, 41 who continued to develop and explore atomic and molecular scattering from low
vapor pressure liquid surfaces. In general, most scattering experiments involve a wellcharacterized incident molecular or atomic beam, which is directed at a clean liquid surface invacuum. Scattered molecules are then detected either via time of flight-mass spectrometry (TOFMS)26, 32, 41-45 or laser based quantum state resolved techniques (e.g., laser induced
fluorescence,46-51 direct IR absorption spectroscopy,52-56 resonant multiphoton ionization +velocity map imaging57-60). These experiments extract information from gas-moleculeinteractions with the topmost surface layer, which allow investigation of energy transfer between
gas molecules and the liquid surface.
In addition to the Nesbitt group, current research by the groups of Nathanson,
McKendrick and Minton have explored gas-liquid interfaces via inelastic and reactive scatteringof molecules and atoms.26, 32, 40-69 Scattering experiments have been performed with a wide rangeof molecular and atomic probes, including inert gases (Ar, Ne, Xe),41, 44, 62, 63 reactive atoms (O,F),32, 49-51, 68, 69 diatomics (NO, N2, HCl, DCl)42, 46, 47, 57, 59, 61 and larger polyatomic molecules(CO2, CH4 NH3, D2O)52-56, 65. The liquids studied have traditionally been low vapor pressureliquids like squalane (C30H62), squalene (C30H50), perfluoropolyether (PFPE), glycerol, room3

temperature ionic liquids (RTILs), and molten metals. Collectively, results from theseexperiments have built a more complete picture of chemical dynamics at the gas-liquid interface
by studying energy transfer into and out of internal molecular degrees of freedom (i.e. rotational,translational, vibrational and electronic). This energy transfer has been explored as function of (i)incident and final scattering angles,26, 54, 55 (ii) incident collision energy,28, 52 (iii) mass/identity ofatomic or molecular probe,42, 44, 65 and (iv) surface temperature41, 46, 53, 62. From theseexperiments, a number of parameters have been found to influence sticking probabilities. For
example, compared to atoms, molecules adsorb more readily on surfaces due to their additionalrotational and vibrational degrees of freedom that facilitate energy transfer between the gas andthe liquid surface. Heavier incident projectiles can more efficiently dissipate translational energy
into the liquid, allowing them to trap more easily on surfaces. Additionally, incident moleculeswith more momentum normal to the surface are more likely to trap and thermalize on thesurface, which include molecules with higher collision energies and projectiles approaching thesurface at less glancing angles. Finally, hotter liquids with larger amplitude fluctuations at thesurface create rougher surfaces that increase the trapping probability for incident molecules.
Reactive scattering experiments probe both the atomic composition as well as thechemical reactivities at the topmost liquid surface layer. For example, hydrogen abstraction
experiments (F + squalane → HF and O + RTILs/squalane/squalene → OH) have been able toprobe relative concentrations of H atoms at surfaces by comparing the yield of OH and HF
between different liquids.32, 49, 51, 68, 70 In more recent work, high vapor pressure liquids arestudied in microjet experiments where gas molecules are investigated after evaporating and/or
scattering from a ~10-20 μm diameter narrow stream of liquid.13, 42, 63, 64 Liquids with vapor
pressures as high as several Torr can be studied using liquid microjets. The simple reason for this4

is that the vapor density decreases with distance from a liquid jet more rapidly than from a planar
liquid surface, thus suppressing collisions of the molecular probe with evaporated gas phasemolecules and enabling detection of collision dynamics occurring at the gas-liquid surface. Thisis a capability not previously possible, which points to a new and exciting direction in the fieldthat will allow species such as NO and HCl evaporated from critically relevant liquids like water
to be studied with quantum state resolution.
In the experiments described in this thesis, nitric oxide is used as the molecular probe ofliquid surfaces and its internal rovibronic states are probed with laser induced fluorescence (LIF).
The experimental setup, described in detail in Chapter 2, involves a supersonically cooled beamof NO (2Π1/2, J= 0.5) directed at a clean liquid surface in vacuum with quantum state resolved
detection of scattered molecules (Figure 1.1). These state-to-state resolved scatteringexperiments investigate energy transfer and collision dynamics occurring at gas-liquid interfaces
by comparing the internal molecular quantum states between incoming vs. outgoing NO
molecules. NO is an ideal molecular probe with a well-characterized LIF spectrum on the A2Σ+
⟵X2П γ band that has strong transition probabilities. Moreover, as a radical with multiple low
lying electronic states, the use of NO facilitates the investigation of collisional energy transfer
into the electronic degrees of freedom.
The 2Π electronic state of NO is split by spin-orbit coupling into the 2Π1/2 ground and2

Π3/2 excited states, which are separated by a spin-orbit splitting of Eso =120 cm-1 at J=1.5.71 Eachspin-orbit state is also further split into two lambda-doublet states of opposite spectroscopic
parity, with energy spacings of ~0.01 cm-1 for lowest rotational state. These small electronicenergy spacings invite the investigation of surface-hopping between electronic states fromcollisions at conductive liquid surfaces, like ionic liquids and molten metals. Each of these5Figure 1.1 Molecular scattering of supersonically cooled NO (2Π1/2, J = 0.5) from a roomtemperature ionic liquid surface ([bmim][Tf2N]). Scattered NO molecules are detected via LIF
with quantum state resolution, where collisions excite both spin-orbit states and rotational states
up to J = 50.5. The LIF spectra are shown for the incident jet-cooled beam and the scattered flux.
The liquid surface is generated from molecular dynamics simulations by Schatz et al.72

electronic states, as well as the rotational distributions within each state, can be probed
independently with LIF detection.
Many molecular scattering experiments have produced rotational and translational
distributions of scattered molecules that provide evidence for two simple physical pathways,40, 41,
51, 53, 57, 73

whereby incident gas molecules interact and transfer energy with a liquid surface andcan (i) trap and thermalize on the surface before desorbing (trapping desorption, TD), or (ii)6scatter directly from the surface (impulsive scattering, IS), as illustrated in Figure 1.2. Molecular
dynamics simulations reveal the presence of IS scattering pathways, whereby incident molecules
collide once or twice with the liquid surface before scattering away at near specular scattering
angles, whereas molecules that thermalize typically do so in multiple collisions with the surface.
By the time such thermalized gas molecules desorb, it is reasonable to assume that they have lost
memory of the incident collision conditions and, in the absence of exit channel barriers, often
leave the surface in near cos(θs) distributions. While it seems reasonable to expect a continuumof scattering pathways in between these two limits, as suggested by trajectory simulations,25, 27, 28,
30, 74

this bimodal picture empirically fits data from our NO molecular beam scatteringexperiments over a dynamical range of more than two orders of magnitude in rotational state
distributions and offers useful quantitative parameters with which to characterize rotational
excitation.

Figure 1.2 Molecular scattering pathways whereby incident molecules can either directly
scatter (impulsively scattering, IS) or trap and thermalize on the surface before desorbing(trapping/thermal-desorption, TD).

7

In the thermal desorption scattering pathway, decades of gas-liquid scatteringexperiments have supported a simple physical picture where molecules thermalize/trap on thesurface and subsequently desorb back into the gas phase with translational and rotational
distributions that are described by the surface temperature. This can be understood with thesubtle, yet remarkably powerful concept of detailed balance,75 which is most easily described byconsidering a system of gas molecules in equilibrium with a liquid surface. In order for thissystem at equilibrium to remain at equilibrium, the flux of gas phase molecules approaching thesurface must be equal to the flux of molecules leaving the surface. Therefore, if we assume unity
sticking coefficients for all molecules striking the liquid, then all molecules leaving the surface
must be thermally desorbing from the surface with rotational and translational temperature
distributions characteristic of the surface temperature. These ideas still hold true for molecular
scattering experiments in vacuum even though the system is not in equilibrium. The basic reason
why is that, once a molecule has thermalized on the surface, it can’t “know” in advance whether
a thermal distribution of gas or vacuum conditions are situated above it, therefore these
thermalized molecules on the surface must behave as they would under equilibrium conditions.
These ideas are discussed in more detail in Chapters 3 and 5 where nonunity sticking
probabilities are also considered, which can arise from the presence of dynamical barriers toadsorption/desorption at the surface.
Room temperature ionic liquids (RTILs) are a novel class of green, designer solventswith various applications spanning environmental, biological, electrochemical, and industrial
processes.76, 77 These liquids are molten salts, in that they consist only of ions, yet unlike thefamiliar table salt, these RTILs are liquid at or near room temperature. The presence ofinterfacial ions makes these liquids an intriguing surface to explore nonadiabatic surface hopping8

effects, which might arise from partial electron transfer with incident NO molecules. RTILs are
remarkably versatile, with low vapor pressures, high solubilities, and chemical/thermal
stability.78-80 These RTILs are also known as “designer solvents”, since by changing the identityof either the anion or cation, these liquids can be tuned to solvate specific species with more than
106 possible combinations.77 RTILs are being developed for solar cells,81, 82 Li-ion batteries,83, 84
drug delivery,85 catalytic reactions,86-88 and many extraction schemes.89-91 For example, thetunability of RTILs is being used to engineer ionic liquids as CO2 scrubbers92-94 and also toseparate H2 from CO in steam gas reformation.95 For these applications, molecular beam
scattering experiments can help us to develop an understanding of what parameters promote
specific gas molecules to trap and dissolve into a liquid, while inhibiting other gases fromsolvating. The liquid surface acts as a barrier to solvation for an approaching gas molecule, thus
the investigation of collision dynamics at the gas-liquid interface represents a crucial first step
toward a predictive understanding of gas solvation processes in RTILs.
Imidazolium-based ionic liquids, in particular, are relevant in many of the applications
mentioned above. The cation of these RTILs has its charge delocalized over an imidazolium ring,with nonpolar alkyl side chains on each of the nitrogen atoms. The specific cations studied here
are referred to as 1-alkyl-3-methylimidazolium, or [Cnmim], and are pictured in Figure 1.3.
Common anion pairings are often inorganic molecules like [BF4]-, [TF2N](bis(trifluoromethylsulfonyl)imide, N(CF3SO2)2-), and Cl-, which are also illustrated in Figure
1.3. A number of gas-RTIL interface studies have investigated the atomic composition at theseliquid surfaces, which have found both anions and cations present at the surface.12, 22, 51, 72, 96-100
The studies presented in Chapters 3-6 of this thesis focus on collision dynamics at the surface of9these imidazolium based ionic liquids and how the atomic composition of these varied RTILs
affects the interfacial dynamics.

Figure 1.3 RTIL molecular structures of a) 1-alkyl-3-methylimidazolium cation ([Cnmim]+)and b) [TF2N]-, [BF4]-, and Cl- anions

Molten metal surfaces represent another intriguing opportunity to explore nonadiabatic
collision dynamics with NO. Metals have a continuum of electronic states that allow electron
hole pair (ehp) formation to be resonant with any vibrational or spin-orbit excitation in an NO
molecule. This means that ehp formation can couple to degrees of freedom in incoming/outgoinggas phase molecules via nonadiabatic mechanisms such as surface-hopping. Manyexperimental101-105 and theoretical studies106, 107 have examined scattering dynamics ofvibrationally excited NO (v = 12 and v = 15) from solid crystalline surfaces like Au(111) andCu(111), which have demonstrated evidence of ehp mediated energy transfer in gas-surface
collisions. Specifically, research by Wodtke and coworkers101, 103, 104 has demonstrated theseeffects lead to multiquanta vibrational excitation and relaxation of NO from Au(111), a process
which is not present in insulting surfaces such as LiF. Molecular and atomic scatteringexperiments have also been performed from liquid metallic surfaces. Nathanson et. al. has
scattered inert gases from Ga, In, and Bi liquid surfaces44, 108 to investigate the effects of surface

10

mass and surface tension on scattering dynamics. Additionally, in previous work in the Nesbitt
group, surface temperature dependent studies were performed on NO scattering from liquid Ga
from Ts = 300 to 600 K, where both efficient rotation and spin-orbit excitation were detected.46
These spin-orbit changing collision dynamics observed necessitate surface-hopping between twoelectronic potential energy surfaces. There are many opportunities to further explore these
nonadiabatic collision dynamics from molten metal surfaces, especially via energy transfer into
the vibrational degree of freedom, which will be presented in the final chapter of this thesis.
The organization of this thesis is as follows. Chapter 2 provides a detailed description of
the experimental apparatus, in addition to data collection and analysis. Recent changes to theexperiment that allow studies of molecular scattering from molten metal surfaces at Ts ≤ 1400 K
are also presented. Experimental results are first introduced in Chapter 3, which examine NO
scattering from [bmim][Tf2N] at varying surface temperatures and collision energies. The simple
physical picture of thermally desorbing molecules with rotational and translational distributions
reflective of the surface temperature has been supported in scattering experiments of CO2 fromperfluoropolyether (PFPE)53 and HCl from a liquid-mimetic self-assembled monolayers(SAMs).57 Nitric oxide, however, exhibits subthermal rotational and spin-orbit temperatures at
low collision energies in these experiments, consistent with rotational/spin-orbit dependent
sticking coefficients of NO adsorption/desorption on liquid surfaces. This chapter also presents
scattering results at higher incident collision energies, where both the TD and IS scattering
pathways are populated, to explore the thermal accommodation coefficient, α, as the surface
temperature is varied.
In Chapter 4, dynamics at the gas-RTIL interface are investigated as a function of thecation alkyl chain length, with a specific focus on [Cnmim][Tf2N] RTILs, where n = 2, 4, 8, 12,11

16 and n represents the number of carbon atoms in the hydrocarbon chain on the cation (Figure1.3). The complexities of the intermolecular interactions (i.e., hydrogen bonding, van der Waals
forces, Coulomb attraction between ions, and π-stacking) govern how the molecules organize inthe bulk and at the liquid surface. The relative strength of these interactions change as thehydrocarbon chains are lengthened. For example, the dispersion forces between alkyl chains start
to dominate for RTILs with longer chains (n ≥ 6), resulting in bilayer formation in the bulk
where alternating layers of nonpolar hydrocarbons form between layers of charged anions andimidazolium rings. Additionally, longer hydrocarbon chains are preferentially located at thesurface and tend to protrude out into the vacuum. As the cationic alkyl chains are lengthened andthe RTIL liquid surface is altered, the experiments in this chapter explore how these changes
affect NO collision dynamics from RTIL surfaces. The results in Chapter 4 show a distinct shift
in scattering dynamics from shorter (n ≤ 4) to longer (n ≥ 8) hydrocarbons, which is the range ofalkyl chain lengths over which the most significant changes in surface composition take place.
Angle resolved scattering of NO is investigated in Chapter 5. These studies are motivatedby the observation of subthermal NO rotational distributions presented in Chapters 3 and 4, inorder to determine if molecules thermally desorbing at all scattering angle have the samerotational temperatures. Additionally, these experiments explore whether or not all molecules arethermalizing on the surface at low collision energies and if there are incident angle or rotational
state dependent sticking coefficients. Scattered molecules are detected at backward and forward
scattering angles of θs = -60º, -30º, 0º, 30º, 45º and 60º with respect to surface normal, while heincident angle is kept constant at θinc = 45º. Collision dynamics are investigated at the liquidsurfaces of squalane, PFPE, and [bmim][Tf2N] to determine whether NO scattering trends persist
at different, more insulating liquid surfaces.

12

In Chapter 6, unpublished studies are presented of incident energy dependence on NO
scattered from a [bmim][Tf2N] liquid surface. Rotational distributions of scattered NO are
explored as the incident collision energy is tuned. Additionally, a series of low collision energies
are also examined to investigate translational energy dependence on the sticking coefficient in
the thermal desorption scattering channel.
Finally, molecular scattering of NO from molten metal surfaces is presented in Chapter 7.
A new heating setup has been built (Chapter 2) to investigate scattering dynamics at molten Au
and Ga surfaces, with heating capabilities up to Ts = 1400 K. The molten metal surface
temperatures are hotter than have previously been explored in molecular scatteringexperiments44, 46, 108 and represent first molecular scattering experiments performed from molten
gold. Vibrationally excited NO is detected from both molten Ga and Au metal surfaces,suggestive of electron hole pair formation at the NO-molten metal interface. Additionally,
scattering experiments from liquid gold are compared to the extensive molecular scatteringstudies of NO from Au(111), which demonstrate differences in how molecules scatter from solidand liquid surfaces.

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19

Chapter 2

Experimental Apparatus

2.1 Introduction
The experiment described in this section utilizes quantum state resolved scattering of NO
from liquid surfaces to explore energy transfer and collision dynamics at the gas-liquid interface.
2

A supersonically cooled beam of NO( Π1/2; J = 0.5) strikes a low vapor pressure liquid invacuum and the scattered NO molecules are detected via laser induced fluorescence (LIF),
yielding full vibrational, rotational and spin-orbit distributions. Fundamental to the success of
this experiment are the following components: (i) full characterization of the incident molecularbeam, (ii) quantum-state resolved characterization of the scattered flux, (iii) a clean liquidsurface and, (iv) confidence that changes in internal states of the molecules are due to collisions
with the liquid surface and not from interactions with gas phase molecules. The state-to-state
resolved measurements in this experiment enable dynamics and energy transfer at the gas-liquidinterface to be investigated by probing changes between the initial and final state distributions ofNO. This experiment was built up by Mike Ziemkiewicz, whose thesis delves into a number ofdetails not discussed in this chapter.1 The first half of this experimental section broadly describes
the experimental setup and data analysis routines, with an emphasis on experimental methods
and changes made to the apparatus. The latter half of this chapter describes recent experimental

20

changes made to enable systematic, controlled heating of molten metals up to Ts = 1400 K,
which allow molecular beam scattering experiments to be performed from molten gold, among
other liquid metals.

2.2 LIF Detection of Nitric Oxide
NO molecules are detected via LIF, which is a quantum state specific and density
dependent measurement. A UV laser pulse excites NO molecules in the ground X(2П) electronic
state to the first A(2Σ+) excited electronic state, which then fluoresce back down to the groundstate with a ~ 210 ns radiative lifetime2, 3 and nearly unity quantum yields (Figure 2.1). Forradiative lifetimes independent of upper state, the intensity of the fluorescent light will be strictly
proportional to the population in a given quantum state. Thus, by tuning the wavelength of the

Figure 2.1 LIF detection of NO on the A2Σ+ (v = 0) ⟵ X2П(v = 0) γ band. NO molecules are
excited by a ~225 nm pulsed, tunable UV laser. Fluorescence back down to the X2П state is
proportional to the NO population in the X2П rovibronic state being probed.

21

laser, NO rovibronic distributions up to N = 50 in both spin-orbit states and vibrational states v =(0, 1) can be observed and quantitatively measured. The energy required to excite the A2Σ+
⟵X2П γ band electronic transition is ~ 5.5 eV, which is obtained from a pulsed, tunable UV
laser (~225 nm) generated from the tripled output of a Nd:YAG-pumped dye laser. A sample NOA2Σ+ (v = 0) ⟵ X2П(v = 0) spectrum is shown in Figure 2.2 with the P, Q and R branches
2

e

2

f

labeled for all spin-orbit and lambda-doublet electronic states, Π 1/2 (Q21, P11, R11), Π 1/2 (Q11,eP21, R21), Π 3/2 (Q12, P22, R22), and Πf3/2 (Q22, P12, R12). The subscript labels on each of these22branches refer to spin-splitting in the upper A2Σ and lower X2П electronic states, whichcorrespond to the first and second subscripts, respectively.
The ground state of NO has nonzero angular momentum, as the unpaired electron lies inan antibonding π* orbital. The ground 2Π electronic state is therefore split into two spin-orbit
states, ground (2Π1/2) and excited (2Π3/2), which in Hund’s case (a) are separated by a spin-orbit

Figure 2.2 Sample LIF spectrum of scattered NO (v = 0) at hyperthermal collision energies (Einc22= 20 kcal/mol), where quantum states up to N = 50 in both spin-orbit states ( Π1/2 and Π3/2) are
populated. The length of the arrows extend up to N = 40 for each branch.

22splitting of Eso = 123 cm-1 at J = 1.5.4 These spin-orbit states physically correspond to theprojections of the spin and orbital angular momenta either aligned (Ω = 3/2) or anti-aligned (Ω =1/2) with respect to the internuclear axis. NO molecules in low N states, where N represents endover-end tumbling angular momenta, have energy spacings between rotational states (~2BNON,
where BNO =1.72 cm-1) that are small compared to Eso. The energy level structures for thesemolecules are well described by Hund’s case (a), where both the orbital and spin angularmomenta are strongly coupled to the NO internuclear axis. This case holds up to N ~ 35, where
the end-over-end tumbling energy spacing nears the spin-orbit energy splitting and the spin
angular momentum decouples from the NO internuclear axis (Hund’s case (b)). In theexperiments presented, however, nearly all the rotational states of the NO molecules detected fall
into the Hund’s case (a) scenario where the spin-orbit state label (Ω) remains valid. Each spinorbit state is also further split into two lambda-doublet states of opposite spectroscopic parity.
These spacings are much smaller, at ~0.01 cm-1 for the lowest rotational state and physicallycorrespond to the half-filled π* orbital either parallel (e) or perpendicular (f) to the plane ofmolecular rotation.5 Each of these electronic states, as well as the rotational distributions withineach state, can be probed independently with LIF detection.
A schematic of pulsed UV light production is shown in Figure 2.3. The primary source ofthe laser pulse is a Nd:YAG (Continuum PL-8010). The laser medium is a neodymium-doped
yttrium aluminum garnet (Nd:YAG) crystal that is optically pumped with flashlamps, which are
typically run at an applied voltage of 1.2 kV. The lasing cavity is Q-switched with a Pockels cellthat rotates 1064 nm photons a quarter wave of rotation when a voltage is applied, which opens
the cavity for optical amplification and outputs a fast, high intensity laser pulse. A digital delay

23

Figure 2.3 Pulsed UV light produced from the tripled output of a Nd:YAG pumped dye laser,which is used to study molecular scattering of NO from liquid surfaces.

generator (DDG, SRS DG535) first triggers the flashlamps to optically excite the Nd:YAG
crystal and a second trigger delivers a high voltage pulse to the Pockels cell once an optimum
population inversion in the Nd ions is achieved. The 1064 nm light is doubled via second
harmonic generation in a beta barium borate (BBO) crystal, which delivers ~ 4 W of 532 nm
light in 8 ns pulses, or 400 mJ pulses at 10 Hz.
The doubled Nd:YAG laser pumps a tunable dye laser (Continuum ND-6000) operatingwith LDS-698 (Pyridine-1) dye, which outputs wavelengths from 661 to 740 nm. Optimal dye
concentrations in methanol are 5 x 10-5 M and 2 x 10-4 M for the amplifier and oscillator,
respectively, which are circulated through each dye cell window (Continuum DCP 6000 dye
circulator). A high resolution diffraction grating within a ‘Moya oscillator’ allows for laser
frequency tuning and good wavelength selectivity. This grating is operated by a sine bar drive,
whereby a linear displacement in the translation stage is transformed into a linear change in24

wavelength. To ensure smooth wavelength tuning and to avoid mechanical backlash on the sinebar drive, the laser should be scanned from blue to red frequencies. The dye laser is typicallyscanned at a rate of 0.1 cm-1 per second, by a LabView program ‘Definitive Scanner’.1 This dyelaser is comprised of one oscillator and two amplifiers, although only one amplifier is needed toobtain sufficient UV laser pulse energies. With the second amplifier blocked, average dye laser
output powers of ~100 mW are obtained at 680 nm, or 10 mJ/pulse at 10 Hz repetition rates.
The dye laser output then passes through two nonlinear, BBO crystals that triple thewavelength of the dye laser. The first BBO crystal (phase matching angle θ = 34.7º, Inrad)combines two photons in the incoming laser beam via second harmonic generation and outputs
light at twice the frequency of the incident light. The second BBO crystal (θ = 53.2º, Inrad) thenadds the fundamental and doubled photons together via sum frequency generation to produce
frequency tripled light. These BBO crystals reside in autotrackers (Inrad Autotracker III) thatrotate the crystals to maintain optimum frequency mixing production while scanning thefrequency of the dye laser with an active feedback servo design. The pulsed laser light output
from these autotrackers is a mixture of the fundamental, doubled and tripled light. A series offour prisms in a UV harmonic separator (Inrad, Model 752-104) are used to separate thesewavelengths, outputting just the tripled wavelength from the dye laser. This light is verticallypolarized with wavelength ranges between 220-246 nm (40500 to 45400 cm-1) and a 0.4 cm-1
linewidth, as determined by widths of isolated NO peaks in the LIF spectrum.
The pulsed UV laser enters and exits the vacuum chamber through 2 mm calcium
fluoride (CaF2) Brewster angle windows, which allow the p-polarized laser beam to pass throughwhile minimizing stray light from entering into the chamber. Each of these Brewster windows sitat the end of 24” baffle arms, inside of which a series of aluminum irises are situated with25

increasing diameters, ranging from 5 mm to 11 mm, that further help to ensure a zero photon
background for LIF detection. The laser beam is aligned through two irises on either side of thechamber. The laser power pulse-to-pulse fluctuations are substantial, with a standard deviation ofσ ≈ 25%. The energy of each laser pulse is measured after passing through the vacuum chamber
on a pyroelectric photodetector (Molectron J3-09 Pyroelectric Joulemeter) and its signal isintegrated over a 300 ns window with a boxcar integrator (SRS 250). The power measured foreach laser pulse is used to normalize the fluorescence measurements described below.
A typical LIF scan of NO (v = 0) on the (A2Σ+ ← X2Π) γ band spans roughly 800 cm-1,
from 44050 to 44850 cm-1, and probes NO rotational states in the ground (2Π1/2) and excited(2Π3/2) spin-orbit manifolds from J = 0 to J = 50.5. For one of these spectra, it takes ~ 2.5 hours
to scan the dye laser at 0.1 cm-1 per second. Throughout each scan it is crucial that the LIF signal
is linear with laser power. In order to ensure LIF transitions are not saturated, the UV laser
powers are kept at ≤ 3 μJ/pulse levels. This lack of saturation is confirmed by a LIF signal curveof growth vs. laser power in Figure 2.4, which shows these 3 μJ pulse energies are safely within
the linear regime.

2.3 Incident Molecular Beam
The incident molecular beam is supersonically expanded from an Even-Lavie valve. This
expansion yields a narrow distribution of translational velocities and a highly rotationally cold
beam. In order to produce a supersonic expansion, where the molecules and atoms in themolecular beam are adiabatically cooled, two conditions must be fulfilled: (i) gas must flow froma high pressure region to a low pressure region, which must differ by at least a factor of two inpressure and (ii) the mean free path inside the high pressure region must be much larger than the26

Figure 2.4 LIF signal vs. laser energy curve of growth. These data are taken from the P21(J =1.5-3.5) bandhead peak at 44198cm-1. Laser pulse energies are typically kept ≤ 3 μJ/pulse to
avoid saturation of LIF transitions.

diameter of the pinhole (λ >> d). The Even-Lavie valve (Type E.L.-5-C-2005) produces thissupersonic expansion via a solenoid based mechanism coupled with a plunger that presses up
against a kapton gasket with a 100 micron diameter pinhole and a stainless steel conical nozzle.
These Even-Lavie valves can operate at high pressures (100 bar), high temperatures (400 K), andwith short opening times (15 μs), which result in remarkably cold molecular beams.
Rotational cooling in the supersonic molecular beam expansion is very efficient. By way
of example, an LIF spectrum of the incident molecular beam of 1% NO in Ne-70 is shown in
Figure 2.5. More than 99% of NO is in the lowest J = 0.5 state, which translates into a rotationaltemperature < 1 K. These rotational temperatures vary slightly depending on the buffer gas
mixed with NO. For example, H2 as a diatomic buffer gas is a less efficient cooler than27

Figure 2.5 LIF spectrum of the supersonically cooled incident molecular beam comprised of 1%
NO in Ne-70 (70% Ne, 30% He). The incident beam has a rotational temperature of < 1 K
where more than 99% of all NO molecules are in the J = 0.5 state.

monatomic rare gases and yields rotational temperatures just over 2 K. For all expansion
conditions, however, all NO molecules in the incident beam are in the ground 2П1/2 state with thelambda-doublet states equally populated.
Pulse shapes for the incident beam are shown in Figure 2.6 for varying opening times.
The width of these pulses is controlled by the Even-Lavie ‘E.L. Driver’. As seen in Figure 2.6,
rebound pulses or “bounces” of the plunger appear for longer opening times. To ensure the valve
is opening all the way and the molecular beam is fully cooled, pulse widths ≥ 50 μs must be used
(Figure 2.6); However, due to corrosion inside the pulsed valve, opening times ≥ 80 μs are now
required for complete opening of the valve and maximum cooling. The interior of the valve has

28

Figure 2.6 Incident molecular beam time profiles from the Even-Lavie pulsed valve at various
pulse widths. Rebound pulses are observed for longer opening times.

been cleaned extensively on multiple occasions by scrubbing, scraping and sonicating theplunger, spring, ceramic spacers and high-pressure tube. While this cleaning procedure has
improved function, the pulsed valve continues to not open fully at pulse widths < 80 μs.
The gas mixtures are made in 1 L stainless steel cylinders with 1% NO and 99% buffergas, consisting of either H2, He, Ne or some mixture of those three. A gas manifold made ofwelded stainless steel with Swagelok VCO fittings and teflon tubing is used to combine and store
these gas mixtures. The manifold connects the mixing cylinders, gas cylinders and Even-Lavie
pulsed valve, which allows mixing of a variety of gases and can be evacuated with a small
mechanical vacuum pump. Gas mixtures are made by first adding NO and subsequently adding

29

the buffer gas in bursts to facilitate turbulent mixing. The mixtures are made at a total pressureof 5,000 Torr and are left overnight, or at least 8 hours, to achieve a uniform composition. A
10,000 Torr Baratron is used to measure the partial pressures of each gas, which can bequantitatively confirmed with the relative percent gas mixtures measured by a residual gas
analyzer (RGA) in the chamber. A corrosion-resistant, stainless steel Monel regulator stands
between these gas mixture cylinders and the pulsed valve, limiting the backing pressure of thevalve to ~ 3,000 Torr. Additionally, a metal mesh filter is situated before the valve to remove any
particulate matter and keep the interior of the pulsed valve clear of obstructions.
The density of the gas in the supersonic expansion a distance x from the valve opening
can be calculated using the following empirical formula, Pjet = C * P0 * (d/x)2, where P0 is the is
the stagnation pressure in the valve, d is the pinhole diameter, and C is a constant related to theheat capacities, which is ~ 0.15 for monatomic gases.6 The liquid sample sits 13 cm from thepulsed valve, for which the beam conditions translate into a density of 9 x 1012 #/cm3 at thesurface, including both NO and the buffer gas. Under these experimental conditions, an 80 μs gas
pulse of molecules moving at 2 x 105 cm/s results in an incident molecular flux of 1.5 x 1014#/cm2 at the surface. There are roughly 5 x 1015 sites/cm2 (calculated based on the size of anincident NO molecule) and, to ensure isolated interactions with the surface, the incident gas
pulse flux should be < 0.1 monolayers per pulse. The calculations above confirm this condition ismet, with ~ 0.03 monolayers/pulse of incident atoms and molecules striking the surface.
Fractional NO concentrations in the incident beam are chosen to maximize LIF signal
while ensuring negligible cluster formation of NO2 or NO-X (X = diluent gas) in the supersonic
expansion, which can occur due to both larger NO partial pressures and high backing pressuresin the pulsed valve. To monitor cluster formation in the incident beam, curve of growth30

Figure 2.7 LIF signal as a function of fractional NO concentration in the incident molecularbeam. Signals are integrated over the R11(0.5) peak and increase linearly with NO partialpressures in the region studied. Gas mixtures with Ar have consistently 4x lower signals,
possibly to due freezing of the molecular beam.

measurements have been made with varying fractional NO partial pressures for all buffer gases
used. Mixtures of 1% NO in all carrier gases are safely within a linear regime of LIF signal vs.
NO fraction (see Figure 2.7), indicative of negligible cluster formation in the incident molecularbeam. NO mixtures with Ar consistently yielded 4x lower signal relative to other buffer gases
mentioned above (Figure 2.7), even with NO concentrations as low as 0.01%. This could be a
result of either cluster formation of NO-Ar van der Waal’s complexes or freezing of themolecular beam, therefore Ar was not used in these experiments. The mass of the buffer gas
atoms and/or molecules control the speed of NO in the molecular beam. Thus, by changing thebuffer gas, the speed of the incident NO molecules is varied. The terminal velocity of the31supersonic expansion can be calculated using the following equation:7
√

(

)

(2.1)

where m is the average molecular weight of the gas mixture, γ is the heat capacity ratio (CP/CV),and T is the temperature in the stagnation region. This terminal velocity for supersonic
expansions is reached within x/d ~ 5, i.e., five times the pinhole diameter away from the valve
opening.7
These theoretical expectations are compared to experimental measurements withmicrophone time of flight measurements of gas pulses. A hearing aid microphone is placed on ametal rod in vacuum through an Ultra-Torr feedthrough. This microphone is placed in front of

Figure 2.8 Microphone measurements of the incident molecular beam are used to calculate NO
velocities and collision energies in various gas mixtures, which correspond to velocities ranging
from 8 x 104 to 2.5 x 105 m/s. Measurements are made by translating a microphone along theaxis of the pulsed valve expansion through an Ultra-Torr feedthrough, as illustrated at the top of
the figure.

32

the pulsed valve such that it can be translated along the axis of the supersonic expansion from thepinhole of the valve (see top of Figure 2.8). Molecular speeds are calculated by plotting thedisplacement of the microphone (Δx) against the difference in peak signal arrival times (Δt), as
presented in Figure 2.8 for gas mixtures of 1 % NO in Ne-70 (70% Ne, 30% He), He and H2.
For example, NO speeds of 1.76 x 105 cm/s are measured for gas mixtures of 1% NO in He,
which correspond to Einc = 11.1(7) kcal/mol, i.e., within experimental uncertainty, but slightly
faster than theoretical predictions of 10.5 kcal/mol. This range of buffer gases, from H2 to Ne,
allows creation of tunable incident beam energies from 2.0 to 20 kcal/mol, or velocities from 8 x
104 to 2.5 x 105 m/s.

2.4 Liquid Surfaces
In these experiments, NO acts as a molecular probe of the gas-liquid interface; therefore,
it is crucial that the incident NO molecules, after leaving the pulsed valve, collide only with theliquid surface and do not collide with other gas phase molecules before reaching the LIF
detection region. To ensure this condition is met, experiments must take place under vacuum
conditions where the mean free path of gas molecules is much larger than the distance an NO
molecule travels from the exit of the pulsed valve to the detection region (~15 cm). Furthermore,
the vapor pressures of the liquids studied must also correspond to mean free paths (λ) at least anorder of magnitude larger than 15 cm. This condition is satisfied by liquids with vapor pressures≤10-4 Torr, for which λ ≥ 500 cm. In this regime, the state-to-state resolved NO populations
observed are truly representative of collision dynamics occurring solely at the gas-liquidinterface.

33

Two liquids that fit this requirement include squalane and perfluoropolyether (PFPE),
which both have low vapor pressures at room temperature and have been studied in previous gasliquid scattering experiments.8-11 Room temperature ionic liquids (RTILs) are a new class ofgreen solvents that are molten salts with melting temperatures below 100ºC. These ionic liquids
are an attractive alternative to many organic solvents due to their low vapor pressures, which also
makes them suitable for study in vacuum. A variety of imidazolium based RTILs have beenstudied with various anion and cation combinations. Experiments have been performed with 1alkyl-3-methylimidazolium ([Cnmim], where n = 2, 4, 8, 12 and 16) cation and three different
anions: i) chloride, ii) tetrafluoroborate ([BF4]-) and (iii) bis(trifluoromethylsulfonyl)imide([Tf2N]-). In Figure 2.9, the molecular structures of all liquid surfaces studied are illustrated.
Liquid metals have also been studied, the preparation and heating of which will be discussed
further in Section 2.9.

Figure 2.9 Molecular structures of liquids studied. a) RTIL cation, [Cn-mim]+ (1-alkyl-3methylimidazolium), with varying hydrocarbon chain lengths, b) RTIL anions: [Tf2N](N(CF3SO2)2- ), [BF4]- and Cl- c) PFPE (F(CF(CF3)CF2O)14CF2CF3) and squalane (C30H62)34

All RTILs were purchased from Io-li-tec with ≥ 99% purity, while both squalane (C30H62,
99% purity) and perfluoropolyether (PFPE, F(CF(CF3)CF2O)14CF2CF3, Krytox 1506) werebought from Sigma-Aldrich. Prior to placement in vacuum, the liquids are heated and stirred forat least 4 hours while being pumped on by a mechanical pump with a liquid nitrogen trap toremove volatile impurities from the liquid samples. In particular, this degassing procedure
removes dissolved H2O, as many of these ionic liquids are quite hygroscopic. The degassed
liquids are then poured into a stainless steel crucible in the vacuum chamber with a 2.4 mL
reservoir (4 x 2.4 x 0.25 cm3) and maintained at low pressures (Pchamber < 5 x 10-6 Torr) before
and during any experimental runs.
One method used by many researchers to ensure that a liquid surface is clean and free of
impurities is Ar+ sputtering, where the surface is bombarded with high energy (i.e. KE = 2 kV)
Ar ions. For the aforementioned ionic liquids studied in these experiments, however, this process
ablates and vaporizes the liquid through local heating. Unfortunately, these vaporized liquids canin turn coat the imaging lens, resulting in decreased LIF signal. Therefore, to ensure cleanlinessof liquid surfaces beyond the degassing procedure, experiments have been performed where thesurface is refreshed every five minutes. This is done manually by moving a rod with a wireattached through an Ultra-Torr feedthrough that scrapes the topmost layer of the liquid surface.
Scattering results from these refreshed surfaces are in excellent agreement with scattering data
for the same experimental parameters where the surface was not scraped. Furthermore, as theseliquids remain in vacuum and are bombarded with NO molecules and inert gases, the surfaces do
not change over the course of multiple days of running experiments, indicating that the surfaces
remain consistent and clean throughout these experiments.

35

The temperature of the crucible and liquid is measured by a Type K thermocouple that issecured with a set screw in a small hole drilled into the side of the crucible. This stainless steel
crucible is heated by two NiCr cartridge heaters that are sandwiched between the crucible and alower plate. Both of these pieces are machined with grooves to fit the cylindrical heaters and areheld together with threaded stainless steel rods and tightened nuts. A variable transformer (Staco,
3PN1010) delivers power to these heaters and is coupled to a solid state relay hooked up to a PID
controller (Omega, CNi 3224-C24), which is able to maintain crucible temperatures within± 1 K. To confirm thermal equilibration, bare thermocouples have also been submerged in theliquid reservoir, which reveal temperatures in agreement with the crucible temperature. This
heating system is capable of achieving liquid temperatures up to 600 K, however, the vapor
pressure of many of these liquids increases rapidly with surface temperature, thus constraining
the experimental upper temperature limit. Higher vapor pressures of liquids can not onlyinterfere with NO molecules, but the evaporating liquids can also coat the imaging lens anddecrease the fluorescence signal collection efficiency. For most of the RTILs studied, surface
temperatures from 293 – 393 K were investigated, and for both squalane and PFPE only a 293 K
surface temperature was studied.
Many of the imidazolium RTILs investigated also have liquid crystal phases. For
example, [C12mim][Br] forms a smectic liquid crystal phase between 40ºC and 110ºC.12 In thiscrystal formation, the long alkyl chains on the cations align between hydrophilic layers of thecharged imidazolium rings and anions, as illustrated in the Figure 2.10 inset. These well-definedlayers can slide over one another, giving this phase ‘liquid’ like characteristics. An image of thisliquid crystal formation (Figure 2.10) between two cross-polarized glass slides shows strong
birefringence. Each domain corresponds to regions where liquid crystal molecules are well36

Figure 2.10 Liquid crystal phase of [C12mim]+[Br]-, as viewed between two cross-polarized
glass slides, revealing strong birefringence indicative of smectic ordering. The inset shows theordering of the cations and anions for the smectic A liquid crystal structure.

ordered, while black shaded regions in between domains indicate isotropic distributions where
the direction of alignment is changing from one domain to another.
Scattering experiments of NO from [C12mim][Br] have been carried out in both the liquid
crystal and isotropic liquid phases. Differential scanning calorimetry measurements are
performed to confirm phase transition temperatures, the results of which found the solid to liquid
crystal phase change at 40ºC and the liquid crystal to isotropic liquid at 100ºC, in good
agreement with literature values.12 The sample temperature is varied from 80ºC to 130ºC in thehope of observing differences in scattering dynamics between the rough, fluctuating liquidsurfaces and more ordered liquid crystal surfaces. However, no differences were observed inscattering between these two phases, both in terms of overall signal and the extracted
rotational/electronic temperatures at both low (2.0 kcal/mol) and high (20 kcal/mol) collisionenergies. If any differences in scattering from these two different phases exist, they were below

37the sensitivity of these experimental measurements, i.e., rotational temperatures of scatteredmolecules were within 5 K and the integrated signal was within the standard deviation of theaverage signal. These same studies have also been performed with [C12mim][BF4], where again
no differences in scattering dynamics from its liquid crystal and isotropic phases were observed.
This could either indicate that the surface structure presented to incoming gas molecules is notaffected by the long-range order present in liquid crystals, or more likely the fact that themolecular beam spot size averages over many different surface domains and the large range ofscattering angles detected blurred any major differences.

2.5 Molecular Scattering and Detection Geometry
All molecular scattering takes place within a 90 L vacuum chamber with 1” thickaluminum flanges and a stainless steel frame. The chamber is pumped on by a 1450 L/sturbomolecular pump (Pfeiffer TPH 1801 UP) which is backed by a 25 L/s two stage rotary vanepump (Edwards E2M 80). A stainless steel coaxial metal sieve trap (Nor-Cal) sits between thesetwo pumps to prevent backstreaming of oil from the mechanical pump into the turbopump. This
pumping configuration yields base pressures of 2 x 10-8 Torr in the vacuum chamber, as
measured with a Bayard-Alpert style ionization gauge. The largest contributor to backgroundpressure is water that leaks out from pores in the metal walls of the chamber, as measured by aresidual gas analyzer (SRS RGA200). This RGA is mounted into the vacuum chamber with amass selection range of 1-200 amu, which is useful for not only monitoring background gas
concentrations but also observing and characterizing the evaporation from liquids in thechamber. Additionally, the RGA can serve a leak detector for the vacuum chamber, by externallypassing a gas like helium along the flanges and monitoring the m/z = 2 peak.

38

The molecular scattering geometry and spatial configuration of LIF detection are
illustrated in Figure 2.11. The incident molecular beam passes through a 3 mm skimmer (BeamDynamics) 5.3 cm downstream from the pulsed valve. This collimated beam then travels another
8 cm before striking the liquid surface at a 45º angle, with a 1º half angle divergence. These
geometric parameters result in a 1.1 cm x 0.8 cm spot size of incident NO molecules hitting theliquid surface (i.e. well within the 4 cm x 2.4 cm available area). The alignment of the pulsed
valve with respect to the liquid surface can be checked by replacing the pulsed valve with acoaxial laser pointer, to ensure the beam passes through the skimmer and lands at the center ofthe liquid reservoir. The pulsed valve and liquid surface are mounted on an 80/20 structure
within the vacuum chamber,1 with the pulsed valve on a rotatable piece that allows variation of
the incident molecular beam angle with respect to the surface. Additionally, this arm can be

Figure 2.11 Detection of LIF signal, which is measured on a photomultiplier tube andnormalized to the laser energy, as measured by a pyroelectric photodetector. Note that the axis of
the detection lens and PMT is actually oriented normal to the scattering plane.

39rotated so that the pulsed valve is directed toward the LIF detection spot within the chamber,enabling quantum state resolved LIF characterization of the incident beam.
The pulsed UV laser passes 1.6 cm above the surface and in the plane of specular
scattering, exciting all NO molecules in its path. Excited NO molecules fluoresce down to theground state, but only fluorescent photons from NO in a 4 mm section of the laser beam at thecenter of the chamber are detected and imaged onto a photomultiplier tube (PMT). The PMT sits
in an aluminum tube extending into the vacuum chamber, with a viewing axis perpendicular tothe plane of specular scattering. Directly in front of the PMT lies a 4 mm pinhole mask andbeyond that a 1:1 imaging setup with two plano-convex silica lenses (diameter and focal length =5 cm) with one lens forming the vacuum seal. This spatial filtering allows only NO in the
scattered flux at a specified scattering angle to be detected and prevents NO in the incident beam,
which the laser beam also passes through, from contaminating signal of scattered NO. In order toblock stray photons in the visible, as well as incident laser light, there is a 4 mm colored glass
UV bandpass filter, (240 - 395 nm, UG5 filter, ThorLabs) between the pinhole mask and the
PMT. The fluorescent photons are also in the UV and are mostly redder than the initial laser
pulse, as illustrated in Figure 2.12. The transmission spectrum of this filter is graphed in Figure
2.12, in addition to the transition probabilities, or Franck-Condon factors, of the fluorescence
spectrum from A2Σ+ (v’ = 0) to X2П (v’’= 0 – 10).13 The strongest Franck-Condon factor occurs
at A2Σ+(v = 0) → X2П (v = 1), which is due to the shorter equilibrium N-O bond length in theA2Σ+ state relative to the X2П state. Plotting these two quantities together reveals that the filter
blocks ~90% of the incident laser light and, by summing over the transmission of all vibrational
states, transmits ~ 60% of all fluorescent photons.

40

Figure 2.12 Transmission spectrum of the UG5 filter and transition probabilities of fluorescencefrom A2Σ+(v’=0) → X2П(v”). The filter transmits 60% of all fluorescence and blocks 90% of theincident laser light (~225 nm). The width of the fluorescence bars for each v” state isrepresentative of the wavelength range for each rovibronic spectrum.

The PMT is set up to image fluorescence from the center point of the vacuum chamber.
Therefore, in order to measure different scattering angles, the entire scattering apparatus within
the chamber must be moved. The pulsed valve, skimmer and liquid surface within the chamber
can be horizontally translated along the axis of the laser to allow detection of a variety of
scattering angles. The scattering apparatus can also be translated perpendicular to the plane of
specular scattering in order to image off axis scattering, although this degree of freedom hasneither been explored in current nor previous thesis work.

41

Figure 2.13 shows the scattering geometry for studies where the scattered molecules aredetected at θs = -60º, -30º, 0º, 30º, 45º and 60º. There is a finite range of scattering angles
corresponding to each of these experimental conditions due to the detection volume and the spot
size of the molecular beam hitting the surface. The geometric limits for scattering angles
detected around θs = 45º, ranging from a minimum of θs = 36º and a maximum of θs = 56º, isillustrated in Figure 2.14(a) where all angles are measured with respect to the surface normal.
Figure 2.14(b) shows the contribution of all scattering angles for a given experimental
configuration. These angular distributions can be estimated by calculating all straight line
trajectories from the incident liquid surface to the LIF detection focal volume. The uncertainties
in these angles, θs = -60(3)º, -30(9)º, 0(12)º, 30(9)º, 45(6)º and 60(3)º, represent 1 standard
deviations based on statistical sampling of all these straight line trajectories. These calculations
are purely geometric and do not make any assumptions about the angular distribution of thescattering molecules (e.g. cos(θs) or otherwise). Though far from competing with the angular

Figure 2.13 Angle resolved molecular scattering of NO at θinc = 45º and θs = -60º, -30º, 0º, 30º,
45º and 60º. The LIF detection setup images a small volume in the center of the vacuum
chamber, therefore, in order to image different scattering angles the pulsed valve and liquidsurface are translated horizontally along the probe laser axis with respect to the imaging volume.

42

Figure 2.14 a) Range of scattered angles detected at θs = 45º, b) Calculated final angle
scattering distributions with 1 standard deviations of θs = -60(3)º, -30(9)º, 0(12)º, 30(9)º, 45(6)ºand 60(3)º (see text for details).

resolution achievable with flux detection in a rotating molecular beam chamber, this simple
spatial filtering scheme nevertheless provides quite useful quantitative angular information on
the scattering dynamics (see Chapter 5).
The PMT (Electron Tubes, model 9813QB) has high sensitivity (capable of detecting
single photon events), low noise and a fast response (3 ns), making it ideal for fluorescence
detection. When a photon strikes the photocathode of the PMT, following the photoelectriceffect, it is converted into an electron with 30% efficiency. The bialkali photocathode has a 2”
active area diameter and a broad spectral response from 160 to 630 nm. The photoelectrons

43generated at the photocathode are accelerated by an electric field toward a series of 14 dynodes
set at continuously increasing voltages. At each dynode, a single electron is converted into
multiple electrons via secondary electron emission. This avalanche of electrons for a single
photon event eventually reaches the anode, where a sharp voltage pulse is produced andmeasured over a 50 Ω resistor. The electron multiplication from all the dynodes results in a gainof 2.2(0.9) x107 for a single electron, when the photocathode set is at -2000 V relative to theanode.
The voltage is supplied to the PMT by a high voltage power supply (SRS PS300), withthe voltage on the anode that is typically applied for these experiments varying between 1600
and 2000 V relative to the photocathode. Larger voltages yield higher electron gains and thus
higher sensitivity, whereas lower voltages are desirable in experiments with strong fluorescencesignals to avoid saturating the PMT. Different PMT voltages are often used over the course of asingle data scan to maximize the dynamic range of quantum state populations detected. The
following conversion factors have been measured and are used to ensure linearity in signal overthe course of a scan: 1.0 (VPMT = 2000 V), 3.07 (VPMT = 1800 V), and 12.01 (VPMT = 1600 V).
The PMT voltage is then integrated over a 200 ns gate with a boxcar integrator (SRS 250). This
module utilizes a gate generator and fast gated integrator to output a voltage that is proportional
to the input signal and normalized to the gate width. After the PMT and before the boxcar, bothsignal and noise are amplified with a 20 dB preamplifier (Mini-Circuits, ZFL-1000LN) toeliminate loss of S/N due to input noise on the boxcar.
All timing of the system is controlled by two digital delay generators, which first trigger
the flashlamps and Q-switch of the Nd:YAG laser. The PMT is gated with a boxcar integrator ~60 ns after the initial laser pulse to avoid detection of photons from the incident laser. After

44

passing through the vacuum chamber, the energy of each laser pulse is also gated with anotherboxcar integrator. An analog to digital converter then delivers these digitized signals to a
LabView program (‘Definitive Scanner’), with the values saved along with the wavelength foreach laser pulse. In addition to scanning the dye laser, this program can also perform scans (at afixed laser frequency) in the time domain, which is useful for looking at the pulse shape of theincident beam (Figure 2.6) as well as the scattered flux. This function is utilized to choose theoptimal time delay setting for the pulse valve, which is selected in order to image fluorescencefrom the peak signal of scattered NO.

2.6 Data Analysis
The data collection and analysis programs were written by Mike Ziemkiewicz, whose
thesis contains more in depth information,1 the following is a brief overview of these programs
and small changes made to them. The ‘Definitive Scanner’ LabView program outputs data withcorresponding laser frequencies, LIF signals (V) and laser energies (J) for each spectral scan. To
analyze these data, the LIF background for each spectrum is estimated as a linear correction,
based on background measurements from the beginning and end of each scan. The LIF signal isthen normalized to the laser power for each data point/laser pulse. If the PMT voltage is varied
throughout a scan, the correct conversion factors for each PMT setting must be applied before
stitching together the spectrum, which is then smoothed with a Savitzky-Golay smoothing
function.14
As the dye laser is scanned over the course of an NO spectrum, the frequency reported by
the laser can unfortunately vary from the true frequency by up to 0.2 cm-1. Therefore, in order toextract populations from a convoluted NO spectrum with many overlapping peaks, the45

frequencies need to be calibrated. This is done with a LabView program ‘Frequency Calibration’that uses the known frequencies of NO transitions to perform a linear interpolation/calibration of
the laser frequency.1 This spectrum is then fit to a model with STARPAC nonlinear least squares
fitting by a program entitled ‘LIFfits’. This program uses the known line positions andabsorption coefficients of the NO LIF transitions (from LIFBASE15) to fit spectra to a leastsquares model and extract populations for all rovibronic quantum states populated. This program
can be used for analyzing any NO LIF spectrum, notably both A2Σ+ (v = 0) ⟵ X2П(v = 0, v =1); the only changes required are the wavelength range as well as the input file containing line
positions and Einstein B coefficients for all transitions in the region of interest. The ‘LIFfits’
program outputs populations, up to J = 70.5 (or for however many rotational states are entered
2

e

2

f

2

e

2

f

into the input file) for all electronic states, Π 1/2, Π 1/2, Π 3/2, and Π 3/2, within the vibrational
state detected, as well as the model fit to the spectrum. An example of the NO A2Σ+ (v = 0) ⟵X2П(v = 0) spectrum and model fit to the smoothed data is show in Figure 2.15.
The rovibronic populations can be used in two other programs for quick analysis of therotational and electronic distributions. The program ‘Averages’ calculates an average rotationaltemperature for each electronic state and an electronic temperature (where [2П3/2]/[2П1/2] = exp(Eso/kTelec) and Eso = 123 cm-1). The program ‘Double_Exp’ fits rotational distributions to adouble exponential, or two-temperature Boltzmann fit, which physically represents the TD and
IS scattering pathways:(2.2)
Here, α = PTD/(PTD +PIS) is the branching ratio into the TD channel, PTD/IS represents therespective fractions of the TD and IS component for a given rotational state, and Qrot(TD/IS) is

46

2

Figure 2.15 Sample LIF spectrum of scattered NO( Π1/2, v = 0) and the least squares model fit.

the canonical partition function defined by the rotational temperature. This program allows allparameters (TTD, TIS and α) to be fixed or floated, and the most recent version ‘DoubleExp2013’,
allows different TTD values to be fixed at separate temperatures for the two spin-orbit states,
while the lambda-doublet states within each spin-orbit state are kept the same.
In order to evaluate the capability and accuracy of this analysis and LIF detection, a room
temperature scan of NO thermalized in the vacuum chamber has been measured. This is done bydetecting fluorescence of NO just before the pulsed valve fires, so that instead of imaging jetcooled NO molecules freshly scattered from the liquid surface, NO molecules that have bounced
around the vacuum chamber and completely thermalized to room temperature are detected. The
Boltzmann rotational distributions of thermalized NO are shown in Figure 2.16, with a global fit

47

to all four electronic states that yields Trot = 295(4) K. This rotational temperature and the spinorbit temperature, Telec = 298(10) K, are consistent with NO molecules thermalized with a 293 K
chamber and corroborate the validity of the analysis routine.

Figure 2.16 Room temperature rotational distributions of NO thermalized with a 293(1) K
vacuum chamber. Both the electronic and rotational temperatures are within uncertainties for thechamber temperature, confirming lack of saturation in LIF detection and reliability of the data
analysis routine.

2.7 Signal to Noise Estimate
By way of preparation for these experiments, signal to noise calculations are presented
for LIF detection of NO scattered from liquid surfaces. In general, there are four numbers needed
to calculate the signal of this experiment: (i) number of scattered NO molecules in the detection48

volume, (ii) probability of NO being excited by a photon, (iii) fraction of the fluorescence
imaged onto the photomultiplier tube, and (iv) the signal of a single photon detected on the PMT.
As discussed in Section 2.3, the molecular beam density (NO and buffer gas molecules) at thesurface, 13 cm away from the pulsed valve, is 9 x 1012 #/cm3. Assuming roughly half themolecules scatter specularly, there is a density of 3 x 1012 #/cm3 15 cm away from the pulsed
valve at the detection spot. The detection volume is 0.013 cm3, which is limited by the width ofthe laser beam (2 mm) and the 4 mm mask on the PMT. Gas mixtures consist of 1% NO in abuffer gas and approximately 1% of the NO molecules are in a given quantum state, which yields
an upper limit of ~ 4 x 106 NO molecules in the detection volume for the rovibronic state being
probed.
The probability that an NO molecule absorbs a photon can be calculated with Pabs = 1 –exp(-σ*Nphotons/Abeam). The cross section, σ, is 4.5 x 10-16 cm2 for a strong line in the spectrum(P21(1.5 – 3.5) bandhead at 44198 cm-1) with an Einstein B coefficient of B = 6.0 x 108 m2/Js anda laser linewidth of 0.4 cm-1. The laser beam operates at ~225 nm and 3 μJ/pulse, with 3.4 x 1012
photons/pulse and an area of ~0.03 cm2. This yields a ~ 5 % probability that a NO molecule inthe detection region absorbs a photon. With fluorescent quantum yields of ~ 1, this means thatfor 4 x 106 NO molecules, 2 x 105 of these NO molecules in a given quantum state will be
excited.
The fluorescence of NO is emitted in 4π steradians, however the position of the imaging
lens with respect to the detection volume results in only ~7% of the fluorescence imaged onto the
PMT. There are ~5% reflective losses from each surface of the convex lenses, through which
81% of photons are transmitted while only 60% of the fluorescence is transmitted through the
UG5 filter (Figure 2.12). Additionally, the 200 ns boxcar gating of the LIF signal, which begins

49

60 ns after the laser pulse, collects ~47% of the fluorescence based on the 200 ns lifetime of NO.
Finally, the photocathode of the PMT has a conversion efficiency of 30%. Totaling all theselosses, ≈ 0.5 % of the fluorescent photons are detected by the PMT. For 2 x 105 NO molecules
excited, we therefore expect ~1 x 103 photons on the PMT for one of the highest signal peaks.
A single photon on the PMT results in a ~10 mV signal, which is calculated from VPMT =geR/Δt, where g is the gain of the PMT, e is the electronic charge, Δt is the 2 ns response timeand R is the 50 Ω readout resistor. The signal from the PMT has a width of 3 ns and is detected
within a 200 ns gate by the boxcar. Thus, for 1 x103 photons detected by the PMT, a 150 mV
signal will be measured. While the experiment is running, off peak noise on the PMT is ~300 μV
when the anode is set at 1600 V relative to the photocathode. This results in S/N ~ 500 for one of
the stronger peaks in the LIF spectrum. On peak noise due to large fluctuations in the gain of the
PMT (~40%) is the largest source of noise in these experiments. Rovibronic populations
extracted from three data scans are typically averaged together in order to reduce uncertainties
for quantum state populations.

2.8 Experimental Changes to Study Hot Metals
In order to study specific metals of interest at hot surface temperatures (Ts > 1000 K), a
new heating setup within the chamber was built that is capable of heating the crucible and metalof interest up to 1500 K (and even hotter for metals with low vapor pressures). Additionally,
since the vapor pressures of many metals becomes significant near 1000 K, line of sightprotection has been installed for the imaging lens and turbopump to prevent evaporating metal
atoms from depositing on these surfaces. The melting temperatures and vapor pressures for anumber of metals considered for scattering experiments are presented in Figure 2.17.16, 17 Gold50

Figure 2.17 Vapor pressures of various molten metals as a function of temperature,16, 17 where
each vapor pressure curve starts at the melting point for a given metal. The experimental vapor
pressure limit is set a 10-4 Torr to ensure a single collision environment for molecular scattering.
Therefore, metals like Ag and Ni are not suitable for molecular scattering experiments in theirmolten state.

represents one of the more difficult metals to study experimentally, with a melting temperatureof 1337 K and a corresponding vapor pressure of 1.5x10-5 Torr. Vapor pressures ≤ 10-4 Torr, arelow enough to allow single-collision conditions for molecular scattering experiments frommolten metal surfaces. The following sections discuss the specific experimental changes made to
the apparatus to study these hot molten metals.

2.8.1 Resistive Heating Setup
For study of molten metals, the choice of reservoir material is crucially important since
many liquid metals like Au and Ga readily amalgamate with other metals at high temperatures. If

51

a liquid metal were to gradually dissolve atoms of the crucible, this would clearly decrease thepurity of the bulk metal and especially the surface of the liquid metal. Tungsten is extremely
nonreactive with many metals, including Au and Ga. The Au-Ga equilibrium phase diagram
reveals less than 0.128% mixing of tungsten with Au at Tmelt = 1337 K and the solubility oftungsten in Ga is negligible.17, 18 In order to prevent metals like gold from wetting, or creeping up
and over the crucible walls, an alumina coating is put around the edge of the crucible. Only thesides of the crucible are coated with alumina, which creates a liquid pool with a macroscopicallyflat liquid surface forming in the reservoir and avoids the formation of metallic droplets/spheresthat can form when the bottom of the crucible is also coated. The crucibles used in thisexperiment are 4” long, ¾” wide, 0.010” thick tungsten pieces with a 1.5” x ½” reservoir that is
1/8” deep and has a 0.005” alumina coating on the edge of the reservoir (S21-AO-W, R.D.
Mathis). This ensures a large enough liquid metal surface area with respect to the 1.1 x 0.8 cm2
spot size of the incident beam hitting the molten metal surface.
The new heating setup heats metal samples via resistive heating, whereby a heating
element, in this case a tungsten boat, is heated by passing an electric current through it. The
power delivered to the heating element is P = I2*Rboat, therefore all other cables and connectionsin this circuit are designed to have minimum resistances to ensure maximum power is delivered
to the crucible. The tungsten crucible has a 7.2 mΩ resistance, and requires roughly 300 W ofpower to heat up to 1473 K (as quoted by R.D. Mathis). The power supplied to this circuit comesfrom a 4 kW, high current, low voltage power supply (LV400, R.D. Mathis), which can output
up to 0-5 VAC at 800 A. The power supply is connected by 7.7 awg, 500 A rated cables to two
1” solid Cu feedthroughs (RD Mathis, RDM-FT-400). These feedthroughs are each attached to
two 1” x 1/8” Cu bars (oxygen free Copper, alloy 101) which rise 8” above the feedthroughs and52sandwich the tungsten boat, overlapping 1” of the boat on each side, as shown in Figure 2.18.
This particular setup is situated to detect specularly scattered molecules at θs = 45º. However, inorder to vary the scattered angle observed by the PMT, new copper bars would simply need to bemachined to translate the liquid surface with respect to the detection volume (as was done withthe previous setup and discussed in Section 2.5).

Figure 2.18 Molten metal resistive heating setup. A 4kW, high current, low voltage power
supply is connected through the Cu feedthroughs and bars to the boat holding the metal sample(powdered gold pictured). The crucible is made of tungsten with an alumina coating on the edgesof the reservoir.

53

This heating circuit can be modeled as resistors in series, with the resistance of the power
supply, feedthroughs, copper bars, evaporation boat and the connections between all of theseelements. These resistances total to Rtot ~8.2 mΩ, where the power supply impedance isestimated at 1 mΩ, Rfeedthru ≈ 3 μΩ, RCu bars ≈ 13 μΩ, Rboat = 7.2 mΩ at Tboat =1473 K. In order toheat an empty evaporation boat to 1473 K, the company that manufactures these crucibles (R.D.
Mathis) states that 313 W need to be delivered to the boat with a voltage drop of 1.7 V across theboat at 212 A. For these numbers, 0.12 W will be delivered to each feedthrough and 0.60 W will
be delivered to each set of Cu bars. Based on the mass of these Cu elements, their heat capacity
and the amount of power delivered, the Cu feedthroughs (uncooled) of will heat up ~ 2.5 K/hour
and the Cu bars will heat up ~ 50 K/hour. However, due to residual gas conductance from thecopper bars, black body radiation cooling, and cooling to the vacuum flange, this heating rate is
an upper limit.
The resistances of these materials also change with temperature. For example, theresistance of the tungsten evaporation boat at 1473 K is 7.2 mΩ. The resistivity of tungsten (5.6
x 10-8 Ωm) and its temperature coefficient (α = 0.0045/ºC) can be used to calculate the resistanceof the evaporation boat at other temperatures: RW boat(1333 K) = 4.96 mΩ, RW boat(1300 K) =4.05 mΩ and RW boat(300 K) = 1.15 mΩ. Therefore, as the tungsten boat gets hotter, its resistanceincreases and a larger fraction of power is delivered to the evaporation boat relative to the othercomponents. The resistivity of the Cu elements will also grow slightly as they warm, but thesechanges will be negligible with respect to the total resistance of the circuit.
At high temperatures, there will also be significant conductive and radiative heat loss
from the system. Conductive heat loss is calculated by, qc = -kAΔT/Δx, where k is the thermal
conductivity, A is the cross sectional area, and ΔT is the change in temperature along the54

direction of heat flow of length Δx. The conductive heat loss from the tungsten boat will occur onboth sides through each set of Cu bars, which have a cross sectional area of 1.6 cm2 and are 25
cm long. For a temperature gradient from 1337 K to 300 K, ~60 W will be dissipated on eitherside of the Cu bars. Additionally, the power dissipated by the alumel and chromel 4 awg wires of
the Type K thermocouple is inconsequential at less than 1 W at 1337 K. Thermal radiation
represents another source of heat loss and is calculated by, qrad = σεA(T4), where σ is theBoltzmann constant, ε is the emissivity of the surface, A is the surface area, and T is thetemperature. For a 25 cm2 surface area tungsten boat with 0.04 emissivity at 1337 K, 18 W ofpower is radiated. For Cu heated to ~600 K, with 260 cm2 surface area and 0.03 emissivity, 5 W
of power is radiated. While these radiative heat losses seem small in comparison to conductive
power losses, radiative heat loss can rapidly dominate with increasing surface temperature as qrad
grows with T4.
There are wires secured to each Cu feedthrough and on both sides of the evaporation boatthat are connected to voltmeters outside the chamber. This enables the measurement of thevoltage drop from each Cu feedthrough across the Cu bars to the evaporation boat, whereminimal voltage drop is expected and required in order to deliver maximum power to theevaporation boat. Additionally, the voltage drop across the tungsten boat can be measured andmonitored. As the potentiometer on the power supply is turned up, the voltage drop across the
feedthroughs and Cu bars remains low and relatively constant, between 20 to 30 mV, while thevoltage drop across the boat increases significantly. When the tungsten boat is filled with gold
and heated to 1400 K, a voltage drop across the boat of Vboat = 490 mV is measured with acurrent of 136 A (67 W). The amount of power required to heat this system to such temperatures
is significantly less than what is quoted by the company who manufactures both the evaporation

55

boats and the power supply, even though their numbers are quoted for a slightly hotter
temperature at 1473 K and for an empty boat. This could be due to better base pressures in our
vacuum chamber (Pchamber ~ 2 x 10-8 Torr), where less power is lost via gas conductance.
A final consideration is the thermal expansion of Cu during heating. If the copper bars areheated to 600 K, their thermal expansion in height would be < 1 mm. While this will lead to aslight change the scattered angles of NO detected, these differences are small in comparison to
the range of angles detected at θs = 45(6)o.

2.8.2 Temperature Measurement
A number of possible methods have been considered for measuring the temperature ofmolten metals with temperature ranges from TS = 293 to 1400 K. Bare type K thermocouples,
which have an upper temperature limit of 1523 K, have been submerged in the puddle of molten
gallium. Temperature measurements were made up to ~ 1100 K, at which temperature thegallium had fully corroded the thermocouple (Figure 2.19(a)). A Type K thermocouple was also
placed in a ceramic thermowell, where the thermocouple was positioned in a close-fitting hole
drilled in an alumina block. Alumina does not deteriorate when put in contact with these metals
as it is used in the evaporation boats to prevent wetting; however, since alumina repels molten
metals like gallium and gold, that also leads to poor, irregular thermal contact between the metal
and thermowell (Figure 2.19(b)). These thermowell measurements were calibrated against a bareType K thermocouple submerged in the metal, which showed the thermowell temperatures to be~100-200 K colder than the metal itself. Optical pyrometers were also considered, but again
presented a few problems, the first being that there are no windows on the vacuum chamber.
Only the imaging lens in front of the PMT can be used as a viewport into the chamber when the56

Figure 2.19 Various temperature measurement methods of the evaporation boat: a) Bare type K
thermocouple immersed in and corroded by liquid gallium, b) Type K thermocouple in alumina
thermowell, which is repelled by the liquid gallium, c) optical pyrometer calibration through theimaging tube and lens, and d) Type K thermocouple pressed against the bottom of evaporation
boat.

experiment is not running. Therefore, in order for an optical pyrometer to have visual access to
the liquid surface, it would need to be inside the chamber, possibly with a fiber optics bundle.
This bundle would need to be directed toward the surface and near the surface, which would
mean it would not only get hot and but it would be coated in evaporating metals, thus rendering
the instrument unable to measure radiative photons. Furthermore, optical pyrometers only workat temperatures above 1040 K, which would severely limit the ability to study scattering
dynamics at lower temperature ranges.
Ultimately, the temperature measurement solution is that a Type K thermocouple has
been secured to the bottom of the crucible (Figure 2.19(d)), which can be calibrated against these57other temperature measurements and serves a secondary standard. This thermocouple is in goodthermal contact with the boat on one side but exposed to vacuum on its other side, leading tocooler temperatures than the crucible and its molten metal contents. Attempts were made to spot
weld a strip of metal over the thermocouple onto the boat to maximize thermal contact, however,tungsten is difficult to spot weld and these attempts were not successful. This thermocouple on
the evaporation boat has been calibrated against (i) optical pyrometer measurements of an empty
boat through the imaging lens (Figure 2.19(c)) and (ii) a bare Type K thermocouple submergedin the metal. The observed temperatures plotted in Figure 2.20 represent measurements frommultiple samples over the course of a week. The thermocouple on the boat has a reliable linear
relationship with respect to the true temperature of the hot metal and evaporation boat, whichallows a consistent measurement of the liquid metal without interference or contamination. Overthe course of a 2 hour scan for a full spectrum, the temperature on the boat thermocouple willvary within ± 5 K. Propagation of uncertainty for these temperature fluctuations combined withstandard errors in the linear fit in Figure 2.20 lead to uncertainties of 35 K for a 1083 K surface
temperature.
Thermocouples are also placed throughout the chamber to monitor heating of other elements tomake sure O-rings and other important experimental components are not overheating. Type K
thermocouples have been placed (i) in the imaging tube next to the PMT, (ii) on the copper bars
sandwiching the tungsten boat, and (iii) on the copper feedthroughs in vacuum. Additionally, a
handheld infrared thermometer has been used to monitor the temperature of the vacuum flanges
and the exterior portion of the copper feedthroughs to verify that the O-rings (Viton is rated up to~ 200ºC) and insulating teflon spacers (Tmelt = 327ºC) on the copper feedthroughs remain
thermally unstressed.

58

Figure 2.20 Correlation of temperature measurements for molten metals in a tungsten boat.
Tmetal represents the true temperature of the metal and evaporation boat, which is measured eitherwith a bare thermocouple in the molten metal (for T < 1000 K) or with an optical pyrometer for(T > 1100). This is plotted against temperatures from a thermocouple on the bottom of thetungsten boat, called TTC boat, which serves as a secondary standard to determine the true metal
temperature while running experiments.

2.8.3 Metal Evaporation
The vapor pressure of liquids increase with temperature following the expression
exp(-ΔHvap/kT) and hotter molten Au has a vapor pressure of 1.5 x 10-5 Torr at its melting
temperature. This vapor pressure is low enough that the mean free paths are still sufficiently
large (λ ≈ 300 m) with respect to NO distances traveled to detection, but it also means that many
metal atoms will be evaporating and coating surfaces throughout the vacuum chamber, as

59

depicted in Figure 2.21. The one way flux of Au atoms leaving the surface for this vapor
pressure is 4.5 x 1015 molecules/cm2/s, which translates to the evaporation of ~30 monolayer/s
for liquid gold densities of 17.31 g/cm3. The gold sample is several millimeters deep and withthese evaporation rates it would take ~34 hours for 1 mm of gold to evaporate at 1337 K. If weexamine one point on the surface and integrate the solid angle of evaporating atoms (assuming acosine distribution), approximately 7% of the evaporating atoms are directed toward the imaging
lens. If all these Au atoms were to stick, then the imaging lens would be coated with gold atoms
at a rate of ~ 125 monolayers/minute. This would make imaging fluorescence through this lensfor spectral scans of 2 hours impossible. Additionally, even if this rate were slower and allowed
for transmission of light, it would result in a decrease in signal over time and yield systematic
errors in relative intensities over the course of a scan.

Figure 2.21 Vacuum chamber after molecular scattering experiments from molten gold, whichstill resides in the tungsten/alumina crucible. High vapor pressures of gold at its melting point
result in gold deposition on all line-of-sight surfaces in the vacuum chamber.

60

As discussed in Section 2.5 and illustrated in Figure 2.11, the fluorescing photons that
arrive at the photocathode of the PMT originate from a single spot along the laser beam. A
stainless steel cone has been constructed that sits directly in front of the imaging lens (Figure2.22) and allows all photons of interest, which originate from the detection volume along thelaser beam path, to be collected by the PMT. This cone blocks a direct line of sight from theliquid surface to the imaging lens, thus keeping the lens clear for LIF detection when performing
scattering experiments from hot metal temperatures with higher vapor pressures.

Figure 2.22 A stainless steel cone is placed on the imaging tube to block evaporating metals
from coating the imaging lens. As the picture shows, gold has been deposited on the cone,
keeping the imaging lens clear for fluorescent detection measurements.

Blackbody radiation from the hot crucible and molten metal grows as T4, resulting inmany stray photons inside the vacuum chamber at high temperatures. Therefore, the cone on theimaging tube not only obstructs metal atom deposition on the lens, but also blocks many of theseblackbody photons emitted by the liquid surface and evaporation boat from reaching the PMT.
Figure 2.23 displays the transmission of the UG5 filter as well as the spectral distribution of61

blackbody radiation at 1400 K. While the peak of the blackbody curve is in the IR (at ~ 2000nm), there is still a significant amount of radiation in the UV. Figure 2.23 also notes the spectral
range of the PMT (from 160 nm to 630 nm), which prevents detection of blackbody photons in
the IR, where there is significant transmission in the UG5 Filter, from being converted into
signal at the photocathode of the PMT. To estimate the background noise of these photons
detected by the PMT that also pass through the transmission filter, we can integrate theblackbody radiation from 240 to 395 nm (where there is ≥ 50% transmission on the UG5 filter)

Figure 2.23 A log scale plot of the blackbody radiation spectrum for a 1400 K object (red line)
in relation to the transmission spectrum of UG5 filter (black line) and the spectral response rangeof the PMT (grey shaded region).

62

for a 200 ns gate, at 1400 K for a 8 cm2 surface (top surface area of the tungsten boat not covered
radiation divided by the power of a single 300 nm photon yields ~ 1.6x104 photons that would by
Cu bars), where only ~7% of photons are directed toward the imaging lens. This blackbody reachthe photocathode of the PMT for a single laser pulse. These calculations are for a perfect
blackbody, whereas emissivities of gold range from 0.05 to 0.47 depending on how polished thesurface is. Whether this background is 100 or 10,000 photons, this blackbody radiation
represents a significant background for LIF detection that the cone on the imaging lens blocks.
While there are still scattered photons present in the chamber, this cone significantly limits thosethat make it to the PMT.
Within the vacuum chamber, the liquid surface sits directly beneath the turbopump. A
metal shield has been placed in the chamber to protect the turbopump blades from getting coatedby evaporating metals, which could lead to an imbalance and malfunction of the pump. A
circular sheet of aluminum, with the same diameter as the opening of the turbopump (8”), sits ~5” beneath the turbopump opening to block a direct line of sight from the liquid surface.

2.8.4 Surface Cleaning
The surface of these metals is cleaned via Ar+ sputtering (IG2 Model 32-165 RBD
Instruments). In this process, the vacuum chamber is filled with 99.995% purity Ar to 4 x 10-5
Torr. The Ar ions are generated within a dual filament ionization chamber via electron impact.
They are then focused toward the liquid surface target with energies ranging from 0.5 to 2 kV.
The Ar+ beam is produced 5 cm away from the liquid surface and is directed at a 45º angle withrespect to surface normal. The Ar ions bombard the surface and impart their momentum tosurface atoms, which then have enough energy to escape the surface. This is particularly useful63for cleaning the oxide layer from metal surfaces, notably Ga2O3 oxide layers. The metal surfaces
are cleaned by operating the Ar+ sputterer for at least 30 minutes prior to every scan. A decreasein scattered NO signal is observed after the first cleaning for a new metal sample put in vacuum.
After this first cleaning NO scattering signals remain constant over time, however, Ar+ sputtering
is continued to ensure NO isn’t reacting on the surface by creating an oxide laser over the courseof multiple experiments.

2.8.5 Detection of NO(v = 1)
For a thermalized distribution of NO molecules at 1000 K, the following ratios ofvibrational states are populated, [v = 1]/[v = 0] = 0.064 and [v = 2]/[v = 0] = 4 x 10-3([Ev=1 – Ev=0] = ΔEvib = 1879 cm-1). In order to probe NO(v = 1) with LIF, NO can be excited toeither v = 0 or v = 1 of the A electronic state (Figure 2.24). The A2Σ+(v = 1) ⟵ X2П(v = 1)
spectrum starts at ~44550 cm-1, which is toward the end of the NO A2Σ+(v = 0) ⟵ X2П(v = 0)
spectrum shown in Figure 2.2. Another option for vibrationally excited NO detection is A2Σ+(v =0) ⟵ X2П(v = 1), which requires an excitation energy of ~ 42200 cm-1. This latter detection
scheme is favored since the absorption and emission coefficients are on average 2.7 and 2.3
times larger, respectively, than the former detection scheme. Moreover, not only does thisdetection of NO(v = 1) have the same emission step as NO(v = 0) detection, making comparisons
between vibrational state populations more straightforward, but the A2Σ+(v = 0) ⟵X2П(v = 1)
spectrum also does not overlap with the A2Σ+(v = 0) ⟵ X2П(v = 0) spectrum, which further
simplifies the analysis procedure.
A sample NO(v = 1) spectrum, exciting to the A2Σ+(v = 0) state, is show in Figure 2.25.
This spectrum is again fit to a model with the ‘LIFfits’ program that extracts rovibronic

64

populations, which then undergo the same analysis as NO(v = 0). The signal to noise on thehighest peaks in this spectrum is ~ 30. The NO v = 2 spectrum for A2Σ+(v = 0) ⟵X2П(v = 2), at~ 40400 cm-1, has similar transition probabilities as the A2Σ+(v = 0) ⟵X2П(v = 1) band;
however, the v = 2 state is ~ 20 times less populated than the v = 1 state at 1000 K, which puts
the largest peaks right at the S/N limit. LIF detection of NO(v = 2) was attempted on the X2П(v”= 2) ⟵ A2Σ+(v’ = 0) transition, however, no peaks were detected in this region. The signal was
integrated over wavelength range of expected signal to determine an upper limit for the [v = 2]/[v= 0] population of 1.5 x 10-3.

Figure 2.24 LIF detection schemes for NO(v = 1) where a laser beam with energies between
44450 and 45000 cm-1 excites A2Σ+(v = 1) ⟵ X2П(v = 1) or energies between 42150 and 42700
cm-1 excite A2Σ+(v = 0) ⟵X2П(v = 1). This latter detection scheme has been used to detectNO(v = 1) since it has stronger transition probabilities as well as an isolated spectrum.

65Figure 2.25 NO(v = 1) spectrum with the inset showing representative signal to noise.

A new filter is required in front of the PMT to block incident laser light, which is shiftedto lower frequencies for the v = 1 and v = 2 detection. For example, with the current UG5 filter,
there is ~40% transmission for A2Σ+(v = 0) ⟵X2П(v = 1) excitation at 236 nm and ~70%transmission for A2Σ+(v = 0) ⟵X2П(v = 2) excitation at 247 nm (Figure 2.12). For both of thesedetection schemes a shortpass filter from Asahi Spectra (XUV0325) is employed. As seen in
Figure 2.26, this filter has low transmission for the incident laser light while still collecting 33%
of the fluorescence signal.

66

Figure 2.26 Transmission spectrum of the XUV0325 and UG5 filters in addition to transition
probabilities of fluorescence from A2Σ+(v’= 0) ⟵ X2П(v”). The XUV0325 is employed for thedetection of NO(v = 1) and NO(v = 2), which require incident laser pulses of 236 and 247 nm,
respectively, which is blocked by the XUV0325 filter (see text for details). The XUV0325 filtertransmits 33% of all fluorescence, roughly half the signal collected from the UG5 filter (the filter
used for NO(v = 0) detection).

67

References for Chapter 2

1.

M. P. Ziemkiewicz, Molecular and Electronic Dynamics in Van der Waals Cluster
Spectroscopy, Hydrogen Abstraction Reactions, and Inelastic Collisions at Liquid
Surfaces (Ph.D.). University of Colorado at Boulder, 2012.

2.

H. Zacharias, J. B. Halpern and K. H. Welge, Chem. Phys. Lett., 1976, 43, 41-44.

3.

J. Brzozowski, N. Elander and P. Erman, Phys. Scripta, 1974, 9, 99-103.

4.

G. Herzberg, Spectra and Molecular-Structure I. Spectra of Diatomic-Molecules,
Princeton Univ. Press, Princeton, NJ, 1968.

5.

M. H. Alexander, P. Andresen, R. Bacis, R. Bersohn, F. J. Comes, P. J. Dagdigian, R. N.
Dixon, R. W. Field, G. W. Flynn, K. H. Gericke, E. R. Grant, B. J. Howard, J. R. Huber,
D. S. King, J. L. Kinsey, K. Kleinermanns, K. Kuchitsu, A. C. Luntz, A. J. Mccaffery, B.
Pouilly, H. Reisler, S. Rosenwaks, E. W. Rothe, M. Shapiro, J. P. Simons, R. Vasudev, J.
R. Wiesenfeld, C. Wittig and R. N. Zare, J. Chem. Phys., 1988, 89, 1749-1753.

6.

F. B. Dunning, R. G. Hulet and M. D. Morse, Atomic, Molecular, and Optical Physics:
Atoms and Molecules, Academic Press, San Diego, 1996.

7.

G. Scoles, Atomic and Molecular Beam Methods, Oxford University Press, New York,
1988.

8.

B. G. Perkins and D. J. Nesbitt, J. Phys. Chem. B, 2006, 110, 17126-17137.

9.

A. M. Zolot, P. J. Dagdigian and D. J. Nesbitt, J. Chem. Phys., 2008, 129, 194705.

10.

J. J. Nogueira, S. A. Vazquez, O. A. Mazyar, W. L. Hase, B. G. Perkins, D. J. Nesbitt andE. Martinez-Nunez, J. Phys. Chem. A, 2009, 113, 3850-3865.

11.

M. E. King, G. M. Nathanson, M. A. Hanninglee and T. K. Minton, Phys. Rev. Lett.,
1993, 70, 1026-1029.

12.

M. Yang, B. Mallick and A. V. Mudring, Cryst. Growth Des., 2013, 13, 3068-3077.

13.

H. Scheingraber and C. R. Vidal, J. Opt. Soc. Am. B, 1985, 2, 343-354.

14.

A. Savitzky and M. J. E. Golay, Anal. Chem., 1964, 36, 1627-&.

15.

J. Luque and D. R. Crosley, LIFBASE: Database and Spectral Simulation (version 1.5),
Report SRI International Report MP 99-009, 1999.

16.

C. B. Alcock, V. P. Itkin and M. K. Horrigan, Can. Metall. Quart., 1984, 23, 309-313.

17.

W. E. Forsythe, Smithsonian Physical Tables, Knovel, Norwich, N.Y., 9th rev. edn.,
2003.
68

18.

T. B. Massalski and H. Okamoto, Binary Alloy Phase Diagrams, ASM International,
Materials Park, Ohio, 2nd edn., 1990.

69

Chapter 3

Non-Adiabatic Spin-Orbit Excitation Dynamics in Quantum State-Resolved
NO (21/2) Scattering at the Gas-Room Temperature Ionic Liquid Interface*

3.1 Introduction
The surface of a liquid represents an unusually complex and yet intensely relevant
nanoscale environment for chemistry, the further elucidation of which is currently one of the
major frontier challenges for the chemical physics community. Detailed molecular energy
transfer and molecular dynamics at gas-liquid interfaces are crucial to understanding chemical
pathways such as i) gas adsorption, accommodation and dissolution into liquids, ii) reactive
processing of atmospheric aerosols and iii) heterogeneous gas-liquid catalytic processes.1-4
Indeed, energy transfer from one molecule to another via inelastic scattering represents afundamental collisional process that, when coupled with molecular scattering experiments, canyield important insights into both the structure and dynamics of liquid interfaces.5 The energy
transfer between translational, rotational, vibrational and electronic degrees of freedom of a gas
projectile and the liquid surface has been probed with the combination of time-of-flight mass
spectrometry and universal detection methods. 6-10 Additional information has been obtained at
the quantum state level arising from more recent laser-based techniques such as direct IRabsorption, multiphoton ionization/velocity map imaging and laser induced fluorescence11-18. In*Published in J Phys. Chem. C, 119, 8596, (2015)70

close conjunction with molecular dynamics simulations6, 19-22, these experiments have begun toelucidate probabilities for gas molecules sticking, dissociating, desorbing and directly scattering
at liquid surfaces. In particular, the synergism between experimental and theoretical efforts has
provided substantial evidence for a simple physical picture of microscopic branching, whereby
incident gas molecules transfer some of their translational energy and can either i) transiently
trap on the surface, thermalize and desorb (trapping-desorption, TD), or ii) scatter more or lessdirectly from the surface without complete loss of initial collision conditions (impulsive
scattering, IS).5, 9, 14, 15, 20 Knowledge of such interactions is crucial to developing a better
predictive understanding of dynamical processes at gas-liquid interfaces, most notably the
kinetics and thermodynamics of adsorption/solvation of gases into the liquid phase.1, 23, 24
To both explore and exploit such fundamental understanding, it is particularly useful tohave “tunable” solvent systems for study, such as afforded by room temperature ionic liquids(RTILs). As the name implies, RTILs represent a novel class of designer solvents that are liquidat room-temperature and yet exhibit extremely low vapor pressures due to strong cation/anion
Coulomb interactions. These molten ionic salts are versatile, green solvents due to their highsolubility, low vapor pressures, and chemical and thermal stability. Most importantly, bychanging either the cation or anion structure, or even altering a functional group within an ion,the properties of ionic liquids can be tuned to selectively dissolve specific molecules, with anenormous range of potential applications in electrochemistry, biological, analytical, andengineering fields.25, 26 More specifically, RTILs are being developed for use in fuel cells27,
solar panels28, electrolytes in batteries29, drug delivery30, and a variety of extraction schemes31, 32.
By way of examples, commercial separation of H2 from CO in steam gas reformation, or CO2
sequestration from power plants based on differential solubility in RTIL solvents are actively

71being considered,32-35 for which a detailed understanding of the gas-RTIL interface would
obviously be a crucial first step. One parallel direction of keen interest is the investigation ofopen shell, radical systems, which now introduce the role of non-Born Oppenheimer surface
hopping between multiple electronic states as well as differential chemical reactivity and evenpotential prospects for stereochemical dynamics.36-39
In order to explore such issues, we have developed novel capabilities for probing internalrovibronic energy distributions of open shell NO molecules scattered at the gas-ionic liquidinterface. Nitric oxide is an open shell radical with nonzero total angular momentum andmultiple low-lying electronic states, which allow the electronic degree of freedom in a gas-liquid
molecular scattering experiment to be explored. The scattered molecules are probed with laserinduced fluorescence (LIF) in the ground X (2) electronic state, which is composed of spinorbit ground (21/2) and excited (23/2) state manifolds separated by a spin-orbit splitting of ESO
123 cm-1.40 For low end-over-end tumbling angular momenta (N), energy differences between

NO rotational states ( 2BNO N) are small with respect to ESO. Thus, both electron orbital andspin angular momenta are strongly coupled to the NO internuclear bond (i.e., Hund’s case (a)),
with the total angular momentum projection along the internuclear axis (Ω = 1/2, 3/2) as a goodquantum number. Within each spin-orbit state sub-manifold, there are also two weakly split Λdoublet states of opposite overall parity, which semiclassically correspond to the half-filled
orbital either parallel (e) or perpendicular (f) to the plane of rotation.41 Most importantly, all fourelectronic states for each rovibrational NO(v, J) can be independently detected by UV laser
induced fluorescence on the A(2)  X (2) band. This permits exploration of surface hopping
between these low lying electronic states arising from collisions at the gas-liquid interface. As
will be seen in more detail, this provides an especially interesting molecular probe of electron72

dynamics during a collisional event, which can be of particular relevance to liquid systems withhigh charge densities such as surface anions in ionic conductors like RTILs as well as freeelectron carrier motion in molten metals.36, 37
Recently, a number of experimental and theoretical studies have explored the gas-RTIL
interface, with imidazolium-based ionic liquids being of specific interest due to their wide range
of applications.25, 26, 33, 42 One particular focus of such research efforts has been on microscopic
surface composition, specifically the relative abundance of anions and cations at the interface, as
well as the placement and orientation of functional groups.16, 17, 22, 43-46 For example, surface
tension measurements43 as a function of bulk composition provided early insights into thesynergism between van der Waals, hydrogen bonding and electrostatic interactions at theinterface. At a more refined molecular level, the surfaces of these liquids have been probed withnonlinear (2) methods such as sum frequency generation spectroscopy (SFG),43, 47, 48 which by
virtue of phase matching constraints only probes molecules within the first few monolayers of
the gas-liquid interface, while rejecting contributions from deeper into the bulk. Similarly, angleresolved X-ray photoelectron spectroscopy (ARXPS) methods have also been used to explore thecomposition at the gas-RTIL interface, exploiting angle and kinetic energy resolved escape ofelectrons into the vacuum from molecules within one electron scattering length from the surface.
44-46, 49

Inelastic16, 18 and reactive6, 12 molecular scattering experiments offer particular advantages
in probing only the topmost molecular layer, and which therefore have provided valuable data
for molecular interactions at the gas-RTIL interface, as well as help characterize the compositionand orientation of molecules present at the surface. For example, hydrogen abstraction scattering
methods (e.g., O(3P) + liquid surface → OH(2) or F(2P) + liquid surface → HF)6, 12, 50 have73

been especially useful in characterizing the presence of alkyl hydrocarbon species at RTIL
surfaces, 2, 4, 38 while inelastic scattering experiments can explore interfacial properties via
composition dependent energy transfer into internal degrees of freedom of the projectile.16, 18 In
conjunction with theoretical calculations6, 22, such experiments have shown that alkyl chains on
imidazolium cations near the surface act quite hydrophobically and preferentially point into thevacuum. Furthermore, theory predicts a significant thermodynamic propensity for bulky anions
to preferentially occupy the topmost layer, which therefore compete with larger alkyl chains forsurface sites.22 This naturally leads to a balance in thermodynamic driving forces between anion
size and alkyl chain length for the top surface layer, which in turn can control the degree ofhydrophobicity in the gas-liquid interfacial region. Indeed, the relative abundances of anions vs.
cations, as well as different functional groups within each of these ions, clearly must influence
the energy transfer and collision dynamics for incident gas molecules striking the RTIL surface.
Of particular interest is the potential role of partial electron transfer between interfacialions and colliding projectiles at the gas-RTIL interface, which could be especially relevant when
considering nonadiabatic surface hopping effects in open shell NO + RTIL collision dynamics.36
RTILs comprise a complex liquid mixture of nevertheless strongly interacting ions, and as aresult exhibit interesting surface electronic properties that can be explored using XPS, ultraviolet
photoelectron spectroscopy (UPS) and soft X-ray emission spectroscopy (SXES).51 Because offinite electron scattering lengths, SXES and UPS are both interfacial region-specific techniquesthat can probe the valence band of liquids with low vapor pressures, which Kanai et al. have
exploited to suggest that the HOMO (highest occupied molecular orbital) state in [bmim]+[Tf2N]is delocalized over both cation and anion species.52 Interestingly, this conclusion was found to beanion-dependent over a series of imidazolium-based RTILs. For example, the HOMO and74

LUMO (lowest unoccupied molecular orbital) states are thought to be localized primarily on the
cation for [bmim]+[BF4]- and [bmim]+[PF6]- RTILs, but which contrasts with what has beenreported for [bmim]+Cl- and [bmim]+Br- RTILs, where the HOMO is attributed primarily to theanion. Indeed, this would also be consistent with typical inorganic ionic liquids like molten
NaCl, where the top valence band is localized on the Cl- anion.53 However, this also invites
comparison with bulky anions such as [TF2N]-, for which molecular orbital calculations on[bmim]+[Tf2N]- suggest that the HOMO is energetically higher than the valence band. This canbe rationalized on the basis of the Madelung potential for each ion , e.g., energy level shifts due
to electrostatic destabilization of the [bmim]+ cation molecular orbitals by the surrounding RTIL
environment, whereas the molecular orbitals of the anion become preferentially stabilized.52, 54
This is particularly relevant with open shell radical projectiles like NO, for which nonadiabatic
effects can play a large role in gas-liquid collision dynamics, with the further intriguing
possibility of transient electron transfer amplitude in the interfacial region.
The present work represents a systematic study of quantum state resolved scattering ofNO from the gas-[bmim]+[Tf2N]- RTIL interface. The organization of this paper is as follows. In
Section 3.2, we provide a brief overview of relevant aspects of the experiment, followed in
Section 3.3 by results and analysis on quantum state resolved final rovibronic distributions as afunction of (i) incident collision energy and (ii) RTIL surface temperature. As a key observation,
we see evidence for strongly nonequilibrium as well nonadiabatic dynamics in the scattered flux,
both at low (2.7(9) kcal/mol) and hyperthermal (20(6) kcal/mol) collision energies. This isfollowed in Section 3.4 by a discussion of possible origins for such nonthermal, surface hopping
phenomena. This is aided by preliminary MOLPRO CCSD(T) and multireference calculations of75

ab initio equilibrium geometries and well depths for the simple model NO + Cl- collision system,with summary and conclusions presented in Section 3.5.

3.2 Experimental
A brief description of the experimental setup is described herein, with more thoroughdescriptions found in previous studies on NO scattering from liquid gallium and imidazoliumbased RTILs.17, 18 A collimated molecular beam of NO is directed at a liquid reservoir and thescattered molecules are detected with rovibrational, electronic spin-orbit and Λ-doublet state
resolution. An illustration of the experimental setup is shown in Figure 3.1, where the[bmim]+[Tf2N]- liquid surface displayed is based on quantum mechanics/molecular mechanics(QM/MM) molecular dynamics simulations by Schatz et. al.22 The incident molecular beam issupersonically expanded from an Even-Lavie pulsed valve (100 μm diameter pinhole, 3000 Torrbacking pressure, 80 μs pulse width) and is rotationally and electronically cold; all NO molecules
are in the ground spin-orbit state (2Π1/2), with equal Λ-doublet populations, and a rotationaltemperature  1 K (as shown in previous work17), corresponding to ~95% of the NO moleculesin the 2Π1/2, N=0 state. Within the limit of our LIF detection sensitivity (1 part in , we detect
no vibrationally excited NO molecules in the incident beam. The molecular beam is composed of
1% NO/ 99% buffer gas, using H2 or Ne-70 (70% Ne, 30% He) in order to vary Einc from 2.7(9)up to 20(6) kcal/mol, respectively, with NO in the buffer gas optimized to eliminate dimer
formation. The molecular beam passes through a 3 mm skimmer 5.3 cm downstream from thevalve orifice and travels another 8.6 cm to the surface, where the collimated beam strikes thesurface at a 45° angle on a 0.8 cm x 1.1 cm area spot size. The gas-liquid molecular scattering

76Figure 3.1 Dynamics at the gas-RTIL interface are probed by directing a molecular beam ofsupersonically cooled NO at RTIL liquid surfaces and detecting scattered molecules with fullquantum state resolution. All incident NO molecules are in the ground spin-orbit state (2Π1/2) andare rotationally cooled (Trot ≈, while rotational levels up to J = 45.5 for both ground andexcited spin-orbit states are detected in the scattered molecules after collisions with the surface.
The liquid studied is 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e.,[bmim]+[Tf2N]-), where [bmim]+ and [Tf2N]- are shown in black and gray, respectively, in thesimulated RTIL interface calculated by Schatz et al.22

occurs in a 96 L stainless steel vacuum chamber with a 1500 L/s turbopump, which establishes abase pressure of 2 x 10-8 Torr.
Scattered molecules are state-selectively detected using laser induced fluorescence (LIF)by electronically exciting NO molecules via the γ-band (A(2Σ+)  X(2Π )) with a UV pulsed
laser and imaging the fluorescence back down to the 2Π ground state on a photomultiplier tube
(PMT). A pulsed UV light source is generated by tripling the output of a YAG-pumped-dye
laser, operating at 10 Hz with LDS-698 dye. The resulting light is ~ 225 nm with 0.4 cm-1 line
width, with pulse energies 5 μJ/pulse to avoid saturation of LIF transitions. The range of laser
intensities is confirmed against studies of NO room temperature distributions and further

77checked for clustering effects by curve of growth measurements. The dye laser is scanned overan 800 cm-1 range in order to detect all electronic, spin-orbit, Λ-doublet and rovibrational states(J  45.5) significantly populated. The fluorescence signal on a PMT is electronically gated intime and normalized to the laser energy for each pulse. The laser beam runs parallel to thesurface (1.6 cm above the surface) and in the plane of specular scattering, and is delayed
approximately 200 μs (varies with Einc) in time with respect to the gas pulse, which is chosen sothat molecules are detected at the peak of the scattered NO signal. A 1:1 confocal lens imaging
setup with a 4 mm iris collects the fluorescence for a well-defined volume of scattered molecules(~ 15 mm3), the size of which is determined by the diameter of the iris and the laser beamwidth/height. The spot size of the incident gas molecules on the surface, along with the LIF
detection volume, results in the range of scattered angles detected at θs = 45° ± 10°, with somepreliminary scattering angle studies obtained at normal incidence (θs = 0° ± 12°). It is worth
noting that any such fluorescence detection measures the density rather than the flux of scatteredmolecules, which therefore proportionally underrepresents flux for species with larger velocity
components perpendicular to the laser probe direction.
The liquid surface studied herein is [bmim]+[Tf2N]- purchased from Iolitec with 99%
purity. The RTIL material was degassed with moderate stirring and heating at ~60ºC for aminimum of 6 hours while being pumped on with a liquid nitrogen cold trap to remove trace
amounts of gaseous species. The degassed liquid is placed in a stainless steel crucible with a 4.4
x 2.4 x 0.5 cm3 reservoir. Resistive heaters can heat the crucible to TS  900 K, though for[bmim]+[Tf2N]-, the accessible range is limited to TS 373 K due to increase in vapor pressure
with temperature.55 Although the bulk of the experiments have been performed for a stationary78

liquid surface, the results are consistent with data obtained under conditions where the interface
is periodically skimmed in vacuo every 5 minutes.

3.3 Results and Analysis
A sample spectrum with typical S/N for NO scattered from [bmim]+[Tf2N]- at Einc =20(6) kcal/mol is shown in Figure 3.2(a). Each spectrum is modeled with a least-squares fit toGaussian line shapes and peak populations are determined by integrating over each peak for
rotational states up to J = 45.5 for both Λ-doublet states (e, f) within each of the two spin-orbit
states (21/2, 23/2). Since all incident molecules are rotationally cooled into the ground spinorbit electronic state, the large number of peaks in the spectrum clearly indicates considerable
amount of energy transfer taking place with the surface. Sample relative populations as afunction of J for each of the electronic states is shown in Figure 3.2(b). Significant differences
are observed in the scattered molecules between the two spin-orbit states due to nonadiabatic
collision dynamics. By way of contrast, no differences are observed between the two Λ-doublet
states within each spin-orbit state, for which we report simple averages unless otherwise stated.
Because of possible quantum state dependence in the adsorption behavior, detailed balance
considerations yields no a priori prediction of the final scattered quantum states. Nevertheless, it
is useful for our purposes to construct Boltzmann plots for populations vs. i) rotational state andii) incident beam energy.

3.3.1 Low Einc Rotational Distributions (Einc= 2.7(9) kcal/mol)
The data at low incident energies (Einc = 2.7(9) kcal/mol) can be quite well described bylinear, single temperature Boltzmann plots, consistent with most NO molecules undergoing79

Figure 3.2 (a) Sample LIF spectrum of scattered NO from [bmim]+[Tf2N]-. Each spectrum is fit
with a least-squares fit (shown in red in the magnified inset), from which quantum-state resolved
populations are extracted. This spectrum is taken at high incident collision energy (Einc = 20
kcal/mol), with Ts =333 K. As illustrated in the upper left hand corner, the signal is only
measured for a small volume of scattered molecules with θscatter = 45° ± 10°. (b) Population
distributions corresponding to the above sample spectrum, where the four electronic states(2Πe1/2, 2Πf1/2, 2Πe3/2, 2Πf3/2) are plotted as a function of rotational J state.

80

Figure 3.3 Boltzmann plot of scattered rotational populations at low incident translationalenergy (Einc 2.7(9) kcal/mol). These distributions display linear fits to a single temperature
distribution, with the Λ-doublet states are experimentally equally populated and therefore
averaged for each spin-orbit state. Note that Ts = 292 K, and thus that Trot(2Π1/2) and Trot(2Π3/2)
are close to but slightly cooler and hotter than the liquid interface, respectively.

trapping-desorption events with complete thermal accommodation (i.e.,   1). These sample
data in Figure 3.3 have been taken at a surface temperature of Ts = 292 K, with the plots
representing each spin-orbit state, 2Π1/2 and 2Π3/2, averaged over nearly equally populated Λdoublet states. Interestingly, despite low enough collision energies for complete thermal
accommodation at the RTIL surface, the scattered molecules observed in the 2Π1/2 and 2Π3/2
manifolds are in fact not in equilibrium with each other and instead exhibit lower (Trot(2Π1/2) =271(8) K) and higher (Trot(2Π3/2) = 308(8) K) rotational temperatures than Ts. These effects canbe explored in more detail by varying the surface temperature, as shown in Figure 3.4. The

81

measured rotational temperatures do increase linearly with surface temperature with a slope of
0.5 and thus distinctly different from unity. More quantitatively, the slopes for the two spin-

orbit states are equal but maintain a constant vertical difference (ΔT ≈ 30 K) as the surface
temperature is varied from 292 to 373 K. By detailed balance considerations, this necessarily
implies deviations from unity sticking behavior at the gas-liquid interface, and suggests a
significant angular and/or spin-orbit dependence to the potential surface foradsorption/desorption.56

Figure 3.4 Rotational temperature dependence on Ts for low collision energy (Einc = 2.7(9)kcal/mol). The dotted purple line represents a line of “complete accommodation,” or what therotational distributions would look like if fully equilibrated with the surface temperature (Ts) andthe sticking coefficient were unity and independent of rotational state. Note that each spin-orbit
state manifold exhibits deviations from but still a sensitivity to Ts. Error bars are based on
multiple measurements at each surface temperature.

82

These results are surprising and differ qualitatively from previous studies with CO2
scattering from PFPE at similarly low incident energies (Einc = 1.6 kcal/mol).14 Specifically, theCO2 rotational and even translational Doppler line width temperatures in these prior studies were
found to be in close agreement with Ts, consistent with the expectation that low energy incident
molecules stick, thermalize and therefore desorb with quantum state distributions in equilibriumwith the surface.14 However, for open shell molecules like NO and OH, there is now growing
evidence that gas-surface scattering dynamics clearly differ. 57,58 For example, inelastic
scattering experiments with NO scattered at low Einc off molten metals and single crystal
surfaces yield rotational temperatures with small differences from Ts near room temperature, butwhich increase significantly as the surface temperature increases, e.g. as much as ΔT ≈ 300 K at
Ts = 800 K.57 From an equivalent perspective, temperature programmed desorption (TPD)
experiments have also been conducted on Ru(001) crystals dosed with NO, which frommicroscopic reversibility samples the quantum state resolved sticking probabilities in the reverse
direction.58 By way of specific example, thermally desorbed NO from Ru(001) is well fit by asingle rotational temperature of 235 K, i.e., again significantly cooler than the surface
temperature of 455 K. Such nonequilibrium dynamics in rotational temperatures between spinorbit states have also been observed in other 2Π projectile scattering experiments. For example,
reactive scattering of O(3P) at squalane liquid surfaces revealed small but statistically significant
differences (~20K) between rotational temperatures for OH product spin-orbit states, with theexcited spin-orbit state appearing slightly hotter.11 Of particular relevance to the present work,
studies by Zare et. al. on NO scatting from metallic Ag(111) at multiple low collision energies
found the excited 2Π3/2 rotational state manifold to be consistently hotter than that of the ground2

Π1/2 state, yet with both not in equilibrium with the surface temperature.57, 59

83

Simple time reversal symmetry and detailed balance ideas56 can be exploited to provide
some insight into the origin of such non-equilibrium dynamics. For a molecular projectile
approaching a gas-condensed phase interface in a specific quantum state, incident energy andangle, there will be a probability to trap long enough to thermalize with the surface andeventually desorb. If this sticking probability is unity for all quantum states, energies and angles,
then, by detailed balance, all molecules that have thermalized on the surface must desorb into aflux weighted equilibrium distribution representative of the surface temperature. This is simply aconsequence of the rigorous kinetic constraint, for conditions of a gas in equilibrium with asurface, that the desorbing flux must maintain the system in its equilibrium status. Conversely,
the nonequilibrium distribution of NO observed in the current low energy scattering experiments
necessarily implies that sticking probabilities for the incoming NO molecular beam depend on
internal quantum state and/or incident energy.
An even stronger prediction is that detailed balance considerations rigorously require theinternal quantum state distributions arising from the surface accommodated fraction () for an
equilibrium flux of incident molecules at Ts to exactly complement those from the non-trapping
fraction (1-), and thereby form a composite distribution in perfect equilibrium with the surface
temperature. It is important to note that even these low incident energy NO beams are cooled
predominantly into the ground rotational and spin orbit state (N = 0, 2П1/2) and therefore far fromequilibrium with Ts. However, detailed balance requires that the fraction of NO molecules ()that trap/accommodate with the surface long enough to “forget” this initial condition must
recapitulate the same distributions (equilibrium or non-equilibrium) as would have been obtained
under fully equilibrium incident beam conditions.

84Based on the observed NO distributions, these simple but powerful detailed balance ideas
make several predictions that stimulate further theoretical exploration. First of all, rotational NO
temperatures lower than Ts imply that the sticking coefficient (α) must i) depend on J and ii)
indeed decrease with increasing rotational excitation. There are many possible ways to achieve
this behavior, e.g., a strong angular dependence on the gas-liquid surface interaction potential.
As a result of such angular anisotropy, incoming jet cooled NO can quantum mechanically mix
in excited rotations to form states oriented with respect to the surface and thereby achieve a
deeper or shallower adsorption well and thus a higher or lower barrier to surface physisorption.
A similar argument applies to the spin-orbit state distributions as well, but now with the hotter
than Ts rotational temperatures (at Ts = 292 K) for the excited spin-orbit manifold suggesting asignificant shift in the angular anisotropy for the 2Π3/2 vs. 2Π1/2 NO –RTIL intermolecularpotential. Indeed, we have made preliminary steps toward such an analysis with high level ab
initio calculations, as will be discussed in more detail later in Section 3.4.
Finally, we note that the formation of excited spin-orbit states from a purely groundelectronic state incident beam requires the presence of nonadiabatic surface hopping events
between the 2Π3/2 and 2Π1/2 manifolds. For the low energy collisional studies above, this could in
principle arise simply from a high probability for trapping-desorption events and therefore
sufficient time to equilibrate the spin-orbit degree of freedom with the surface temperature. For
example, summed over all rotational states, the relative population ratio between ground andexcited spin-orbit states at 292 K is [2Π3/2]/ [2Π1/2] = 0.45(2), which is close to but lower than the
fully equilibrated Ts predictions of 0.54 (Figure 3.5). At the highest RTIL temperatures sampled(T  373 K), this spin-orbit population ratio rises to [2Π3/2]/ [2Π1/2] = 0.50(3), which is again
close to, though still systematically below, the equilibrium value of 0.62. Indeed, much larger

85

Figure 3.5 Comparison of electronic state populations relative to each other as the surface
temperature is varied for Einc = 2.7(9) kcal/mol. (a) Spin-orbit state ratios, where each spin-orbit
state population is summed over all J states and both Λ-doublet states. The spin-orbit ratio[2Π3/2]/[ 2Π1/2] ~ 0.45-0.52 and varies little with Ts. The energy difference between the two spinorbit state manifolds is 123 cm-1. (b) Λ-doublet electronic ratios are shown on the right, where
each Λ-doublet state is summed over all J states and both spin-orbit states. The Λ-doublet states( 0.01 cm-1are nearly equally populated independent of surface temperature.

deviations from near equilibrium behavior in the spin-orbit distributions are observed at highincident collision energies, as discussed below.

3.3.2 High Incident Energy Rotational Distributions (Einc = 20 kcal/mol)
With increasing Einc, deviations from this pure trapping desorption behavior due toimpulsive scattering events become apparent. In particular, at superthermal collision energies,
Einc = 20(6) kcal/mol, there is no longer a linear Boltzmann plot characterized by a single

86temperature, but instead strong curvature that unambiguously signals the presence of additional
nonequilibuium rotational dynamics in the scattered flux. As shown in previous work,8, 15, 19, 23
such distributions can be well fit to a two-temperature model described by Eq. (3.1):(3.1)

where α = PTD/(PTD +PIS) is the branching ratio into the TD channel (with α  1 signaling all
molecules undergoing TD scattering) and PTD and PIS represent the respective fractions in the TD
and IS component for a given rotational state. Although there must clearly be a continuum ofsuch IS scattering channels, the resulting rotational quantum state distributions are surprisingly
well characterized by a “temperature” over 2 orders of magnitude in dynamic range. Such dual
temperature behavior has now been verified in many quantum state resolved studies and formany different gas-liquid molecular scatterers, as well as confirmed theoretically with detailed
trajectory studies.5, 8, 12, 14, 57
The fundamental source of this rather remarkable simplicity is not yet theoretically wellunderstood; however, we offer the following observations. First of all, there is no a priori reason
for the nonthermally accommodating IS dynamics to be described by a superthermal Boltzmann
distribution; indeed, one would expect deviations from such simple behavior to become evident
with sufficient experimental sensitivity. Second, we have recently initiated studies as a functionof scattering angle, exploring quantum state resolved NO distributions at both specular (s = 45º)and normal incidence (s = 0º). Even for microscopically rough gas-RTIL interfaces, thepreliminary results for normal vs. specular scattering angles are consistent with the angulartrends anticipated for TD vs. IS pathways, for example, (i) significant increase (20-30%) in87fractional trapping-desorption probability , (ii) significant growth in TD signals with littlechange in rotational distributions at low Einc, and (iii) significantly colder spin-orbit temperatures(Telec). Lastly, such dual temperature Boltzmann behavior necessarily predicts overlapping TD
and IS contributions at low final state energies, as clearly verified in the extensive molecular
dynamics simulations of Hase and coworkers. 19, 20 In any event, such a dual temperaturedescription in Eq. (3.1) empirically provides a physically motivated, few parametercharacterization of scattered populations over a wide experimental dynamic range (Figure 3.6).
Rotational populations at high collision energies for each spin-orbit state have been fit to
Eq. (1) with α and TIS floated. Consistent with detailed balance considerations, TTD has been
fixed at Trot values obtained from the low Einc scattering results for the corresponding surface
temperature and spin-orbit state. Figure 3.6 shows a sample Boltzmann plot for the ground spinorbit (2Π1/2) manifold at Einc = 20(6) kcal/mol; the fitted α and TIS values plotted in Figure 3.7 asa function of i) surface temperature and ii) electronic state, which reveal two significant trends.
First of all, the IS temperatures are nearly 3-4-fold hotter than Ts, which indicates the presence ofhighly rotationally inelastic collisions at superthermal incident energies. Second, the IS
temperature component is relatively insensitive to increasing Ts, with the 2Π3/2 distributions
consistently hotter than the 2Π1/2 state, a pattern also noted above at low Einc. Bimodal rotational
distributions have also been observed in orientation dependent scattering of NO from Ag(111),
which have found hotter rotational distributions for O-end scattering than for N-end scattering.60
These effects are most pronounced at more glancing scattering angles (i.e., θs = 70º) and moremodest at specular scattering angles. More recent work has been done by Wodtke et. al. looking
at these orientation effects in scattering vibrationally excited NO from Au (111) and with similar
qualitative results.38, 39

88

Figure 3.6 Sample high incident energy (Einc = 20(6) kcal/mol) rotational distributions for the2
1/2 Boltzmann plot. Unlike the linear trends exhibited at low Einc (Figure 3.3), at higher
collision energies, distinct curvature is observed in the rotational Boltzmann populations. This
dynamic range (over 150-fold) is well fit by a dual-temperature distribution, where the lower
temperature (TTD) is fixed at values from the low Einc fits, with the hotter temperature (TIS) andbranching ratio () between to the TD/IS scattering pathways floated.

It is also worth noting that the branching ratios between the TD and IS channels (α =0.15-0.30) are surprisingly small at high collision energy, much smaller, for example, than thecorresponding values (α =0.65-0.72) observed for hyperthermal scattering of CO2 from RTILs.16
This implies that distributions at these higher incident energies are dominated by nonequilibrium gas-liquid scattering dynamics and indeed, by detailed balance considerations, areconsistent with significant deviations from unity sticking behavior also noted in the thermallyaccommodated flux. There is again a clear spin-orbit state dependence to the accommodation

89

Figure 3.7 Dependence of TIS and α as a function of surface temperature at Einc = 20(6)kcal/mol. (a) Branching ratios (α) increase systematically with surface temperature, consistent
with greater thermal roughening of the RTIL interface. (b) Conversely, TIS is relatively
insensitive to surface temperature, with hotter temperatures for 2Π3/2 vs. 2Π1/2 state manifolds (c)Sample fits to a two temperature distribution for 2Π1/2 and 2Π3/2, with total populations
normalized to unity. The rotationally cold TD component (blue) represents a significantly larger
fraction for 2Π1/2 vs. 2Π3/2 states, consistent with strong Ts dependent coupling between rotationaland spin-orbit degrees of freedom.
90

dynamics, with a consistently higher fraction of the spin-orbit conserving collisions (2Π1/2 →2Π1/2) proceeding through the TD channel than for the spin-orbit changing events (2Π1/2 → 2Π3/2).

These differences can also be seen directly in the rotational energy distributions in Figure 3.7(c),which reveal a visibly larger fraction of TD collision dynamics in fits for the 2Π1/2 vs. 2Π3/2
states. The ground electronic state has a higher trapping-desorption component, by roughly 10%,than the excited spin-orbit state. Since all of the incident NO molecules are in the ground spinorbit state, spin-orbit conserving collisions (2Π1/2 → 2Π1/2) at 292 K result in thermal
accommodation ~ 25% of the time, whereas significantly fewer (~15%) molecules undergoingspin-orbit changing collisions (2Π1/2 → 2Π3/2) thermalize with the surface. Alternatively stated,the more highly rotationally inelastic impulsive scattering (IS) channel appears to beaccompanied by a greater propensity for electronically inelastic spin-orbit changing collisions.
This suggests a strong coupling between rotational and electronic degrees of freedom in the gasRTIL collision event and offers interesting new challenges to dynamical theory.
It is also worth noting that the fraction of collisions proceeding via the TD pathway
increases monotonically with surface temperature. This has been noted previously in other gasliquid studies7, 8, 14, and is consistent with increased inelastic energy loss contributions due tothermally induced surface roughening at higher Ts, thus resulting in a larger fraction ofmolecules accommodating on the surface. Interestingly, thermal roughening with increased
surface temperature does not appear to exert any collateral influence on energy transferred into
rotation via the IS channel. Furthermore, despite such strong temperature dependent effects on
the branching ratio α, the corresponding spin-orbit differences are clearly maintained. All ofthese observations reflect nonunity sticking coefficients and the presence of angular/electronic91

features in the NO-RTIL potential surface, which translate into a strong sticking dependence on
both the incident rotational and spin-orbit state.

3.4. Discussion
3.4.1 Nonadiabatic Electronic Effects
Scattering an open shell radical species such as NO provides the ability to probe theprobability of electronically nonadiabatic events at the gas-liquid interface. In order to explore
such surface hopping dynamics in more detail, the rotational populations are summed over all
rotational states and are plotted in Figure 3.5 (Einc = 2.7 kcal/mol) and Figure 3.8 (Einc = 20
kcal/mol) as a function of (i) spin-orbit state, (ii) Λ-doublet state (only at low Einc), and (iii)
RTIL surface temperature. One message from these plots is unambiguous; since the initial jetcooled beam has a spin-orbit ratio indistinguishable from zero [2Π3/2]/[ 2Π1/2] ≈ 0.0 , theobservation of strong spin-orbit excited populations under both low and high incident collisionenergy conditions implies the presence of surface hopping effects in the gas-surface collision
dynamics. More specifically, Figure 3.5(a) reveals that low collision energies achieve spin-orbit
population ratios of [2Π3/2]/[ 2Π1/2] ≈ 0.4 - 0.5, with a relatively weak dependence on surface
temperature. As noted previously, these values are close to but consistently lower than
predictions based on complete equilibration with the surface in the desorbing flux, withequivalent electronic spin-orbit temperatures varying between Telec = 215 and 252 K over Ts =292 – 373 K. By way of contrast, relative populations for NO molecules scattering into e and fparity Λ-doublet states are compared in Figure 3.5(b). As also demonstrated in Figure 3.2, there
are no significant differences in the scattered molecules’ rotational distributions between the Λ-

92

Figure 3.8 Spin-orbit ratios for the TD and IS scattering channels summed over all rotationaland -doublet states for Einc = 20(6) kcal/mol. The ratio of spin-orbit populations within the TD
component (blue) is [2Π3/2]: [2Π1/2]  0.40, corresponding to an electronic temperature of ~ 200 K
and relatively insensitive to surface temperature. By way of comparison, the equivalent ratios forthe impulsively scattered fraction (red) are [2Π3/2]: [2Π1/2] 0.70-0.85 corresponding to a ~ 500 –1000 K range of electronic temperatures and a gradual increase with Ts.

doublet states within each spin-orbit state and the ratio of these Λ-doublet states is largely
insensitive to changes in the surface temperature.
The non-adiabatic dynamics in the scattered flux are further explored further at highcollision energies in Figure 3.8, where the TD and IS contributions are now treated separately,with the spin-orbit ratio within each scattering pathway considered by itself. The TD fraction
exhibits a spin-orbit ratio of [2Π3/2]/[ 2Π1/2] ~ 0.35 - 0.40, with no appreciable dependence on Ts.
As expected, these results are quite similar to those previously discussed at low Einc, despite93nearly 8-fold higher collision energy. This is consistent with what our fits identify as the TD
component, correctly reflecting the fraction of fully thermally accommodated NO trajectories,
which from detailed balance considerations must yield the same spin-orbit and Λ-doublet ratios(and indeed rotational distributions) independent of the incident collision energy. By way ofcontrast, however, this spin-orbit fraction jumps precipitously for the impulsively scatteredcomponent, for which there are nearly as many excited as ground spin-orbit state molecules([2Π3/2]/[ 2Π1/2] ~ 0.71 - 0.85). Expressed in terms of an equivalent electronic “temperature”, thiswould correspond to a range of Telec from 500 to1000 K. It is worth noting that this is moreconsistent with the window of temperatures experimentally observed in the impulsively scatteredrotational distributions, which might suggest a simple dynamical picture of coupling/partial
equilibration between electronic and rotational degrees of freedom on the time scale of an IS
collisional event. Furthermore, there is a clear surface temperature dependence to thenonadiabatic excitation effects in these collisions, with the fraction of excited 2Π3/2 molecules in
the IS channel increasing with increasing Ts and ostensibly correlated with microscopicroughening of the RTIL surface. Finally, this trend again highlights the connection between i)rotationally inelastic and ii) spin-orbit changing (2Π1/2 → 2Π3/2) collision dynamics in theimpulsive scattering pathway, i.e., that rotational and electronic degrees of freedom must be
strongly correlated.

3.4.1 Theoretical Considerations
Though a complete treatment is well beyond the scope of the present work, it is worth
briefly exploring the fundamental source of such open shell nonadiabatic dynamics in NO +RTIL scattering at the gas-liquid interface. To make things simple, we replace the surface by a94

single scattering center, NO + M, which allows us to borrow directly from the pioneering workof Alexander and co-workers.61, 62 From such studies for NO + rare gas dynamics, this yields A’
and A” potential surfaces in Cs symmetry, on which the energy transfer collisions occur. As
elegantly shown by Alexander and coworkers, spin-orbit conserving interactions are governed by
the sum potential, Vsum = (A’ + A’’)/2, while spin-orbit changing interactions depend upon the
difference potential, Vdiff = (A’’- A’)/2. Vdiff provides a description of the electronic anisotropy of
the NO molecule as a 2D function of Jacobi coordinates i) M-N-O bond angle () and ii) centerof mass NO –M separation (R) with respect to the scattering center.62, 63Far from the scattering
center at collinear Cinv (M-N-O or M-O-N) geometries, these two adiabatic surfaces are exactly
degenerate. However, this degeneracy is broken during approach, the magnitude and geometricaldifferences of which control the propensity for spin-orbit state changing dynamics.
It is worth stressing that comparably high levels of spin-orbit excitation have been
observed in other gas-surface scattering systems, in particular NO + molten gallium17 and NO +Ag (111).57 By way of contrast, spin-orbit excitation/deexcitation for crossed molecular beamstudies64 of NO + Ar is diminished by roughly an order of magnitude. This clearly speaks to asignificant increase in the Vdiff potential surface for conducting liquid/single crystal metals and
RTILs. As first suggested in vibrational relaxation studies for NO(v) + Au (111) by Tully37,
Wodtke and coworkers65, and later proposed by Alexander et. al. for NO + Ag(111)66, one
intriguing reason for such large Vdiff potential surfaces for conducting liquids and solids could be
due to partial charge transfer to/from the NO molecule during the collisional process. As
discussed previously for imidazolium-based RTILs52, 54, this could in principle come from either
the anion or cation species at the gas-liquid interface, although charge donation from the anion

95

would tend to be more energetically favorable, the simplest of which would be atomic halide
anions.
In order to explore the magnitude of such nonadiabatic effects, we have therefore
performed preliminary ab initio geometry optimizations for the simple model system, NO + Cl-,
using both high level single reference (CCSD(T)) and multireference (CASSCF+MRCI)
methods on a MOLPRO software platform. The CCSD(T) calculations have been performed
with explicitly correlated F12 electron methods and specially optimized correlation consistent
f12 basis sets of Peterson et al. (denoted VnZ-f12, n = 2,3) 67 and extrapolated to the complete
basis set limit (CBS). Each of the equilibrium geometries for A’, A” symmetry Cl-–NOstructures at the CCSD(T) level are summarized in the left hand column of Table 3.1. Note that
the A’ surface exhibits two such weakly bound minima, one for collinear approach with the O
end pointing inward toward the Cl- anion ( = 180.0o, R = 4.098 Å, rNO = 1.1500 Å, De = 1.12
kcal/mol), and a second for a more bent structure ( = 101.1o, R = 3.620 Å, rNO = 1.1502 Å, De =1.18 kcal/mol), with a very shallow trench and transition state connecting the two equilibriumgeometries. Of particular importance, however, there is also a much more strongly boundequilibrium geometry of A” symmetry, which is bent with the N end pointing preferentiallytoward the Cl- anion and far more deeply bound ( = 67.4o, R = 3.157 Å, rNO = 1.1565 Å, De =3.97 kcal/mol) than either of the two A’ symmetry equilibrium states. This implies a surprisingly
large magnitude of the difference potential (i.e., Vdiff) both in well depth and geometry, whichfrom the scattering calculations of Alexander et. al. begins to rationalize the high propensity forspin-orbit state changing dynamics observed experimentally.
With open shell radical molecular systems such as NO, it is appropriate to be concernedthat single reference ab initio quantum calculations, even at the CCSD(T)-f12 level, might prove96

insufficient for predicting accurate equilibrium geometries and well depths. We therefore haverepeated these A’, A” Cl- – NO geometry optimizations with multireference MOLPRO methods.
Specifically, we start from state-averaged CASSCF (MULTI) calculations with explicitly
correlated VnZ-f12 basis sets (n=2,3), balanced average over the lowest two A’, A” states (2A’,2A”), with (15,4) and (9,1) occupied and closed orbitals, respectively. A subsequent

Table 3.1 Ab initio equilibrium geometries for X- + NO, with X- = Cl- as a simple atomic RTIL
anion. Left side: MOLPRO calculations at the CCSD(T)/VnZ-f12 level for n = 2, 3 andextrapolated to the complete basis set (CBS) limit. Right side: multireference MOLPRO
calculations (MULTI/MRCI(+Q)) for an explicitly correlated VnZ-f12 basis set (n = 2, 3, andCBS limit). RX-NO and rNO in angstroms, X-NO in degrees, De in kcal/mol. Note the dramaticdifference in equilibrium geometries and well depths for the A’ and A” potential surfaces, which
provides a physical mechanism for the non-adiabatic surface hopping dynamics observed
experimentally. See text for details.

multireference calculation with the Davidson correction (MRCI-f12 + Q) is then performed foreach of the lowest A’ and A” symmetry states, with additional reference symmetries specified toensure balanced configuration spaces for both calculations. When extrapolated to the CBS limit
and optimized with respect to all three Jacobi coordinates, the multireference results again reveal
the presence of two weakly bound A’ equilibrium structures, one collinear ( = 180.0o, R = 4.124
Å, rNO = 1.1512 Å, De = 1.22 kcal/mol) and one bent structure ( = 104.2o, R = 3.691 Å, rNO =97

1.1512 Å, De = 1.19 kcal/mol), with a relatively flat trench connecting the two local minima.
Likewise, the mutireference calculations yield a much more strongly bound A” geometry ( =73.8o, R = 3.302 Å, rNO = 1.1542 Å, De = 3.49 kcal/mol), in nearly quantitative agreement with
CCSD(T) predictions (see Table 3.1). Once again, it is worth stressing the large shifts inequilibrium geometry and well depth between the A’ and A” surfaces, which yield both a large
difference potential (Vdiff) and thus provide a clear quantum mechanical mechanism for thestrong spin-orbit changing collision dynamics observed experimentally.
As a final comment, it is worth pointing out that the NO bond lengths for both the
CCSD(T) and multireference A” equilibrium geometries are systematically larger (+ 0.003-0.005
Å) than for either of the corresponding A’ minima. This would suggest a simple physical pictureof partial charge donation from the Cl- anion into the antibonding * NO molecular orbitals, as
can be confirmed by direct inspection of the wave function composition. Such partial chargetransfer mechanisms have been previously invoked to interpret vibrational relaxation andexcitation for NO scattering from metals vs. insulating systems.36, 68, 69 For example, when
vibrationally excited NO (v =12) is scattered from metallic surfaces with low work functions, theNO molecules readily lose significant amounts of vibrational energy. Electron emission(“chemicurrents”) has also been detected from metallic surfaces following collisions with highly
vibrationally excited molecules.69 However, when these same highly vibrationally excitedmolecules were scattered from insulating surfaces, there was only quite modest vibrational
relaxation.68 Such nonadiabatic electronic effects clearly indicate a breakdown of the BornOppenheimer approximation, which assumes the electrons can adjust instantaneously to nuclear
motion while staying on the same electronic adiabatic potential energy surface. As one simple
model of the dynamics, electron-hole pair excitation in metallic surfaces could result in partial98electron donation to the incident molecule to form a transient NO- species, which then flows
back into the metallic Fermi sea as the NO scatters away from the surface.
It is therefore of interest whether such a charge transfer picture could also offer a novel
mechanism for nonadiabatic NO collision dynamics at the gas-RTIL interface. In particular,
charge donation from anions at the gas-liquid interface both into and out of the two *
antibonding molecular orbitals of NO could facilitate particularly efficient spin-orbit excitation,
simply by interchanging the  =  1 projections of the orbital angular momentum and thusachieving the desired change in spin-orbit state ( =  + ) without any requisite flip of theelectron spin projection (). However, additional experimental and theoretical work will beneeded to confirm or refute the importance of such a partial charge transfer mechanism incontrolling nonadiabatic collision dynamics at the gas-RTIL interface, toward which we hope thepresent study has provided some stimulation and useful first insights.

3.5 Summary and Conclusions
The gas-RTIL interface has been explored via quantum state resolved scattering ofsupersonically cooled NO molecules from [bmim]+[Tf2N]-, where collision dynamics aremonitored by observing rovibronic scattered populations as a function of collision energy andsurface temperature. By exploiting open shell NO molecules cooled into the lowest spin-orbit
state, “surface hopping” between its multiple low-lying electronic states has been probed, withthe presence of strong nonadiabatic collisional processes confirmed via spin-orbit excitedmolecules in the scattered flux.
At low collision energies (Einc 2.7(9) kcal/mol), the distribution of scattered NO is welldescribed by a single rotational temperature, however, one that is not in equilibrium with the99surface temperature. Furthermore, there is evidence for nonequilibrium electronic effects as well;for spin-orbit changing collisions (2Π1/2 → 2Π3/2), Trot is roughly 30K hotter than for spin-orbit
conserving collisions with the surface (2Π1/2 → 2Π1/2). While these collision energies are low
enough to facilitate complete accommodation of molecules at the surface, these results clearly
indicate i) a nonunity sticking coefficient as well as ii) rotational and electronic state dependent
sticking probabilities to adsorption/desorption. At higher collision energies (Einc  20(6)kcal/mol), highly nonequilibrium rotational distributions are observed that can be well fit to a
two-temperature distribution which can be associated with fully accommodated (TD) and all
non-accommodated (IS) scattering pathways. The branching ratio into the TD channel, α,
increases with surface temperature, consistent with roughening of the gas-liquid interface.
Differences in these branching ratios are also observed between the two spin-orbit state
manifolds, which reveal that spin-orbit conserving interactions are correlated more strongly withtrapping desorption behavior than the spin-orbit changing collisions. The consistency between
such data at both high and low incident collision energies further reinforces the notion that there
exist dynamical barriers to adsorption/desorption that are dependent both on spin-orbit electronic
state and angular orientation for incident NO molecules striking the RTIL surface.
The propensity for electronic excitation, or nonadiabatic surface hopping, is quantified bysumming over all rotational states within each of the electronic states. Negligible differences ofthe scattered NO molecules are observed between Λ-doublet states, while large differences areseen between the two spin-orbit manifolds. Specifically, at lower collision energies, where thefull accommodation pathway is likely to predominate, [2Π3/2]: [2Π1/2] ~ 0.4 and is close to thermal
equilibrium with the surface temperature. At higher collision energies, conversely, this ratio
becomes closer to [2Π3/2]: [2Π1/2] ~ 0.8, i.e., substantially higher than predicted from a full100

equilibration picture. If we consider the TD and IS channels separately, the relative population of
the excited spin-orbit state is found to be significantly higher for the IS component, with thatpathway therefore being more favorable for electronically inelastic, spin-orbit changing
collisions.
To explore these nonadiabatic effects further, preliminary ab initio calculations for Cl- +NO, where Cl- represents a simple RTIL anion, have been performed on a MOLPRO software
platform using both single reference CCSD(T) and multireference (CASSCF+MRCI) methods.
The results predict large differences, both in well depths and equilibrium geometries, between
the A’ and A’’ potential energy surfaces, which from the work of Alexander and co-workers
begins to explain the high propensity for spin-orbit excitation observed experimentally.
Additionally the equilibrium bond lengths for the A’’ state are found to be systematically longer
than the A’ state, consistent with transient partial charge donation from the Cl- anion into the NO
π* orbital, and which raises the possibility of a novel electron exchange mechanism that changes
in the spin-orbit state. Though there is clearly more work needed to confirm the role of suchnonadiabatic pathways, we hope such calculations will serve as motivation for further theoreticalefforts exploring the detailed mechanism for spin-orbit excitation at the gas-RTIL interface.

101

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105

Chapter 4

Quantum state-resolved molecular scattering of NO (2Π1/2)
at the gas-[Cnmim][Tf2N] room temperature ionic liquid interface:
Dependence on alkyl chain length, collision energy, and temperature*

4.1 Introduction
Room temperature ionic liquids (RTILs) are molten salts with melting temperatures <100 ºC which represent a novel class of green solvents with high electrical/thermal conductivity,1
low vapor pressures,2 and high thermal stability.3 These novel liquids consist purely of ions andhave many valuable applications in catalytic reactions,4-6 Li-ion batteries,7, 8 fuel cells,9, 10 CO2
capture,11-14 and solar cells.15, 16 Of particular practical importance is that interfacial and bulk
properties of RTILs can be targeted for specific applications by either changing the anion/cation
combination,17, 18 or even simply varying functional groups within the ions. This results inenormous flexibility in the “design” of such RTILs, with well over 106 liquids feasible bycombinatorial substitution.19 These effects are important not only in controlling bulk equilibrium
properties (e.g., Henry’s Law solubilities) as well as non-equilibrium dynamical processes, suchas molecular solvation kinetics at the gas-liquid interface. An outstanding challenge to thechemical physics community has been to provide a more fundamental and predictive
understanding of how cation/anion composition (e.g., cation hydrophobicity) might be used to“tune” the interfacial properties of these novel liquids.

*Published in AIP Advances, 6, 105207 (2016)106

Due to utility in several of the above mentioned applications, many RTIL studies to datehave focused on imidazolium based cations with an aliphatic Cn side chain andbis(trifluoromethylsulfonyl)imide anions (denoted [Cnmim][Tf2N]). For such [Cnmim][Tf2N]
RTILs, the charge of the cation is delocalized over the imidazolium ring with a nonpolar alkyl
side chain, while the [Tf2N] anion has its charge centered on the central N(SO2)2 group with twoouter nonpolar CF3 groups (Figure 4.1). The Cn hydrocarbon chain length plays a crucial role inmoderating intermolecular interactions, including hydrogen bonding, dispersion forces, stacking, and Coulomb attraction between the cation and anion. For example, dispersion forces
between alkyl chains increase for longer hydrocarbons, affecting how the ions arrange and thus
overall liquid properties. In particular, theoretical and experimental studies have reported on theformation of bilayers in the bulk for n > 4, where the long alkyl chains align, separating charged
layers of anions and imidazolium cation rings.3, 20-22 Specifically for [Cnmim][Tf2N] RTILs, anincrease in n is accompanied by an increase in bulk viscosity23, 24 and yet a decrease in bothdensity23-25 and ionic conductivity.23 It is important to stress that bulk composition can inprinciple differ fundamentally from the gas-liquid interface. For example, the extreme“hydrophobicity” of the vacuum likely plays a dominant role in structure and chemicalcomposition of the interfacial region, which clearly motivates further study of the top mostinterfacial layer itself.
The gas-liquid interface is particularly crucial for applications such as CO2 sequestrationand supported ionic liquid phase (SILP) catalysis4, where the adsorption of gas molecules on theliquid surface is a critical first step for dissolving in the liquid. Many experimental techniques
have been exploited to investigate the RTIL liquid surface, including surface tension
measurements,24-26 sum frequency generation (SFG),27-31 neutral impact collision ion scattering

107Figure 4.1 Experimental cartoon for studies of supersonically cooled NO scattered at θinc = 45ºfrom [Cnmim][Tf2N] RTILs and detected via laser induced fluorescence (LIF) at near specularangles (θs = 45º). Sample LIF spectra are shown in the top panels, with only the 2Π1/2 state
populated and Trot ~ 1 K in the incident beam (top left), whereas the scattered NO flux (top right)
shows significant electronic and rotational excitation with both spin-orbit states and rotational
states up to J = 45.5 observed. The [C4mim][Tf2N] surface snapshot was constructed bymolecular dynamics simulations by Schatz et. al.,32 where the Tf2N anion is shown in red, thecation imidazolium ring is dark blue and the cation alkyl chain is light blue. The relevant
chemical structures of the ions are shown at the bottom of the figure. In the present work, weinvestigate a series of RTIL cations where the alkyl chain, Cn, is varied over n = [2, 4, 8, 12, 16].

108

spectroscopy (NICISS),21, 33 angle-resolved X-ray photoelectron spectroscopy (ARXPS),33-38 Xray diffraction,31, 39 neutron reflectivity,40 low-energy ion scattering (LEIS),41 metastable atomelectron spectroscopy (MAES),42 and molecular beam scattering,43-48 as well as molecular
dynamics simulations.32, 44, 49 Each of these studies has varying levels of explicit surface vs.
subsurface sensitivity and are therefore capable of extracting different information relevant to theinterface, such as density and atomic composition (ARXPS, NICISS, LEIS) or molecularorientation (SFG, MAES). The combination of experimental and theoretical efforts have begun
to build up a more complete picture of these liquid surfaces, and specifically have identified for[Cnmim][Tf2N] RTILs the presence of both anions and cations at the interface.32, 33, 37, 38, 41, 42, 44,50

In particular, the studies suggest a preference for nonpolar alkyl chains at the surface pointing

into the vacuum with the charged imidazolium rings directly below in the liquid.33, 42, 49
Similarly, the bulky nonpolar CF3 moieties of the Tf2N anions point out from the surface,32, 37, 38,
42, 49resulting in aggregation of both ionophobic (alkyl chains and CF3 groups) and ionophilic(anion centers and cation imidazolium rings) groups.
While experiment and theory agree that both anions and cations are present at the gasliquid interface, the fractional population of anions is found to decrease as n increases.25, 41-44
LEIS atomic composition studies of the liquid surface report that, as the alkyl chain is lengthened
from n = 2 to 12, the anion signals diminish by 4x but do not disappear entirely.41 Elegant
reactive scattering experiments of hyperthermal O(3P) with RTILs producing OH radical
products have also demonstrated an increased propensity for alkyl chains at the surface for larger
n,44, 45 as well as sensitivity to fractional methylenic (CH2) vs. methyl (CH3) hydrocarbon
presence at the interface. Of special interest, a change in the orientation of molecules at the gasliquid interface has been observed as alkyl chains are lengthened in [Cnmim][Tf2N] RTILs. For

109

short alkyl chain lengths (n  4), both experiment29, 31 and simulation32, 44, 49 suggest that the alkyl
chains point parallel to the surface normal, while for longer hydrocarbons (n  5), these appear
to “flop over” and lie more or less parallel to the surface plane.33, 44
The present study significantly extends our previous efforts47, 48 to elucidate
equilibrium/non-equilibrium collision dynamics of radicals at the gas-RTIL interface, and how
these dynamics are affected by conformation changes at the surface. This is done specifically via
quantum state-resolved molecular beam scattering as a function of (i) incident collision energy(Einc), (ii) surface temperature (TS) and (iii) alkyl chain length of the imidazolium cation (n). Fullquantum state-to-state resolution of the scattering dynamics is made possible with asupersonically cooled (1K) beam of NO, with the scattered molecules sensitively detected withlaser induced fluorescence (LIF). The presence of multiple low-lying electronic states in NO
allows nonadiabatic surface hopping dynamics to be observed, in addition to inelastic
rovibrational excitation within the four spin-orbit and Λ-doublet electronic manifolds. In
particular, the results provide additional confirmation of fractional population and orientation ofalkyl chains at the RTIL surface, as well as insight into how such conformational changes affectstate-resolved collision dynamics at the gas-liquid interface.

4.2 Experimental
The apparatus has been described in detail previously; we present only a brief overview
of conditions relevant to the present study.47, 48 Quantum state-resolved molecular beam
experiments at the gas-liquid interface have been carried out for jet cooled NO(21/2 (N=0))
projectiles48, 51 scattering from 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide,or [Cnmim][Tf2N], as a function of i) alkyl chain length (n = 2, 4, 8, 12, and 16), ii) collision110

energy (Einc = 2.7 and 20 kcal/mol) and iii) surface temperature (TS = 293 – 373 K). The center
panel in Figure 4.1 displays a sample theoretical snap shot at the [C4mim][Tf2N] gas-liquidinterface, obtained from quantum mechanics/ molecular mechanics (QM/MM) simulations by
Schatz et. al.32 The different molecular components are colored (anions in red, cation
imidazolium rings in dark blue and cation alkyl chains in light blue), which highlight theabundance of different functional groups populating the liquid surface. All ionic liquids arepurchased from Iolitec with ≥ 98% purity, with samples stirred, heated and degassed for aminimum of 6 hours while pumped on with a liquid nitrogen trap, before being placed in a 4.4 ×
2.4 × 0.5 cm3 crucible in a stainless steel vacuum chamber with base pressures of 2 × 10-8 Torr.
For each scattering study, the liquid temperatures are varied from 293 K (or 315 K for thehighest melting point RTIL [C16mim][Tf2N]20) to a maximum of 373 K, where the upper limit toour operating temperature range reflects the increase in vapor pressure with temperature.52
Scattering experiments are performed on a stationary liquid surface, with indistinguishable
results obtained for sample data skimming of the liquid surface every five minutes. The
molecular beam is produced from an Even-Lavie pulsed valve with a 100 μm diameter pinhole
and 3000 Torr backing pressure, yielding rotationally cold (Trot ≈ 1 K) NO molecules in the
ground spin-orbit state (2Π1/2). The molecular beam is comprised of 1% NO doped into either H2or Ne-70 (70% Ne and 30% He), which yields beam-surface collision energies of Einc = 20(2)and 2.7(9) kcal/mol, respectively. A 3 mm diameter skimmer results in a collimated beam
striking the liquid surface at θinc = 45º with a 1.5º half angle divergence.
Molecular scattering in the specular direction (θs  45º) is detected with quantum state
resolution via LIF, yielding density measurements of rovibrational and electronic state
populations. A pulsed UV light source (225 nm) is generated by tripling the output of a YAG-111

pumped dye laser operating with LDS-698 dye at 10 Hz. Final state distributions of NO 1.6 cm
above the liquid surface are probed on the (A2Σ+ ⟵X2П) γ band, with the laser scanned over 800
cm-1 to access J  45.5 for each electronic manifold, spin-orbit and Λ-doublet state (2Πe1/2, 2Πf1/2,
2

Πe3/2, 2Πf3/2). The fluorescence signals (normalized to laser pulse energy and < 5 μJ/pulse toensure operation in the linear regime) are imaged on a photo-multiplier tube (PMT) with spatial
filtering to limit the probe volume to 15 mm3. Data are primarily taken near θs = 45(6)º, withadditional tests of angle resolved scattering at θs = - 60(3)º, 0(12)º, and 60(3)º. The PMT iselectronically gated at the peak of the arrival time distribution, which, for an 80 μs gas pulse
duration, reflects both thermally desorbing (TD) and hyperthermally scattering (IS) pathways.
LIF spectral scans are analyzed with homemade software and STARPAC least squares
subroutines51, 53 to fit overlapping peaks and extract populations for each quantum state. The
quantitative accuracy of the procedure is confirmed by Boltzmann analysis of room temperature
NO thermally equilibrated (298 K) with the vacuum chamber.

4.3 Results
Sample LIF spectra (Einc = 20(2) kcal/mol) for both i) incident and ii) scattered NO
molecules are found in the top panel of Figure 4.1, which highlight the extreme differences
between incident (1K) and outgoing (collisionally excited) quantum state distributions. Scattered
NO (v=0) molecules are detected with up to J = 45.5 for both e/f parities within each spin-orbit
manifold, 2Π1/2(e,f) and 2Π3/2(e,f). Since all incident NO molecules have been cooled into theground 2Π1/2 state, electronic spin-orbit excitation at the gas-liquid interface represents asignificant pathway, with clear differences between spin-orbit conserving collisions than spinorbit changing collisions. By way of contrast, no statistically significant differences are observed112between the two Λ-doublet states within each spin-orbit manifold, for which we report only
populations averaged over e/f parity channels for the remainder of this paper.
The rotational populations within each spin-orbit manifold can be further analyzed inlogarithmic Boltzmann plots of population scaled by rotational degeneracy vs. total internalenergy, as displayed in Figure 4.2. At low incident collision energies (Einc = 2.7(9) kcal/mol),where all molecules accommodate with the liquid surface, the data are well fit to a single straightline reflecting a single temperature Boltzmann distribution. Note that this rotational temperature
is close to but distinctly cooler ( 30 K) than the surface temperature (TS = 293 K). This nonequilibrium behavior is consistent with our previous NO gas-liquid studies,48 but in clear contrast
to closed shell gas molecules such as CO2 and HCl, which at similarly low Einc desorb withrotational/translational distributions in equilibrium with the surface temperature.54, 55 Fromdetailed balance considerations, the CO2 and HCl data imply unity sticking coefficients in thereverse (i.e., incident) direction, independent of quantum state, translational energy, and angle
impinging on the surface. The dynamics for NO colliding with RTILs at low incident energies isclearly more complicated, as indeed confirmed by observation of similarly sub-thermal
distributions in scattering experiments on other single crystal, liquid and molten metal
surfaces.48, 51, 56-58 At the very least, detailed balance arguments imply a rotational state
dependence to the sticking coefficients for NO in the reverse direction, with the higher NO
rotational states being less likely to trap, accommodate and eventually desorb.
It is worth considering whether all NO molecules at these low collision energies trap on
the surface, or whether there might still be some residual energy dependence to the sticking
coefficient. Towards this end, we have initiated angle-resolved studies and obtained preliminary
data on final angular dependent scattering at these low collision energies, probing at θs = - 60°,113

Figure 4.2 Boltzmann plots of scattered NO rotational populations for Einc = 2.7(9) and 20(2)kcal/mol. At low Einc, rotational populations fit well to a line, describing molecules thermallydesorbing from the surface that are characterized by a single rotational Boltzmann temperature.
At higher Einc, significant curvature is observed, which can be well fit to a simple twotemperature model physically representing the TD and IS scattering pathways. The colder
component is fixed at Trot from the data at low Einc, while the hotter component, TIS, and thebranching ratio into the TD channel, α, are floated.

0°, 45° and 60°. The results consistently support that rotational temperatures for the desorbing
NO flux are i) independent of the final scattering angle and yet ii) weakly sub-thermal ( 30 K)
with respect to the RTIL temperature. Thus, although a sub-thermal nature of the desorbing
population signals a clear rotational state dependence to the adsorption event, the observed
independence with respect to outgoing scattering angle implies an efficiency of trappingdesorption events independent of incident angle (as discussed in greater detail in previous
work48). Specifically, if accommodation of NO on RTILs has no dependence on incident beam114

angle, then, via detailed balance considerations, there can be no difference in the quantum state
distributions probed as a function of scattering angle, as observed experimentally.
At high Einc, distinct curvature is observed in these Boltzmann plots (Figure 4.2), withsignificant contributions from molecules impulsively scattering (IS) from the surface andyielding excess population in high J states. These distributions can be fit to a two-temperaturemodel47, 48 where the total distribution reflects a sum of hot and cold components representing
the IS and TD scattering pathways, and α reflects the microscopic branching between the TD andIS channel, summed over all final J states. To be consistent with this model, the (cooler) TD
temperatures are fixed from the corresponding quantity observed at low Einc, while the (hotter)component, TIS, and branching fraction, α, are determined by least squares fit to the data. As
seen in Figure 4.3, the rotational temperatures obtained under such low Einc = 2.7(9) kcal/mol
scattering conditions appear to be independent of RTIL chain length, within 1 error bars basedon repeated measurements. However, the rotational temperatures for both spin-orbit states areagain systematically below the actual liquid temperature (solid line) for all RTILs probed, with aweak but clear additional sensitivity to the outgoing spin-orbit state. By detailed balance, theseobservations imply the presence of dynamical barriers to adsorption that depend on bothrotational and spin-orbit state, but insensitive to length of the alkyl chain.
It is worth stressing that a simple description of the quantum state resolved scattering
populations microscopically bifurcating into thermal (TD) and hyperthermal (IS) components
must fail at sufficient level of scrutiny. Indeed, a quite plausible anticipation would be to observe
a continuum of possible IS scattering pathways, as suggested in trajectory simulations59-63 withdifferent molecular interaction potentials. In spite of such expectations, it is therefore remarkablethat such a simple, dual-temperature model empirically fits the data over two orders of115

Figure 4.3 Rotational temperatures for NO scattered from [Cnmim][Tf2N] at low Einc = 2.7(9)kcal/mol. These temperatures have been extracted from linear Boltzmann fits (e.g., see Figure4.3) for each spin-orbit state. Note that Trot is systematically lower than TS, while Trot(2Π3/2) is
consistently hotter (by ~30 K) than Trot(2Π1/2). This signals the influence of non-equilibrium
dynamics in the desorbing channel and, by detailed balance considerations, a rotational and spinorbit dependence to the sticking coefficients in the adsorption event. Furthermore, increasing thecation chain length (n) for the RTIL does not appear to affect rotational temperatures at low Einc.

magnitude in dynamic range and, in particular, up to internal rotational energies more than 10fold higher than kTS (see Figure 4.2). Although it is reasonable to exploit such behavior for a
simplest empirical characterization of the data, our intention is for such quantum state resolved
data to provide the requisite stimulus for development and testing of more sophisticatedtheoretical models that go beyond the canonical assumption of a microscopic bifurcation into
TD/IS components.

116With open shell species such as NO, we also probe collisional energy transfer at the gasRTIL interface into low lying electronic degrees of freedom, with information on nonadiabatic
scattering dynamics obtained from relative populations of the final spin-orbit states. As theincident NO molecules are cooled into the ground 2Π1/2 state, the presence of NO(2Π3/2) in the
scattered flux necessarily represents nonadiabatic, surface hopping collisional events.64 As therange of J states sampled is quite large (J  45.5) one should confirm that NO is adequately
described as a Hund’s case (a) molecule, i.e., with spin and orbital angular momenta strongly
coupled to the internuclear axis. This requires differences in NO end-over-end tumbling energies(ΔE ≈ 2BNON) to be comparable to or less than the spin-orbit energy splitting (ESO), a condition
satisfied up to all but the highest few NO rotational states detected. The spin-orbit distributions
obtained as a function of surface temperature and alkyl chain length are summarized in Table 4.1
and will be discussed below in Sec IV.

4.4 Discussion
4.4.1 Alkyl Chain Length Dependence
We first begin by considering the effects of alkyl chain length on the low Einc scattering
dynamics. Sample results are shown in Figure 4.3 for a single liquid temperature (TS = 353 K),
with all error bars representing standard deviations of the mean obtained from multiple
measurements. Although at these energies essentially all NO molecules trap and thermallydesorb from the surface, this does not necessarily mean that the desorbing flux remains inequilibrium with the liquid temperature. Indeed, both ground and spin-orbit excited states are
distinctly cooler rotationally than the surface temperature for all RTILs probed, which signals thepresence of angular anisotropy and/or dynamical barriers for the desorption event as well as,

117

2

Π3/2

2

Π1/2

α at Einc = 20(2) kcal/mol
Ts

n=2

4

8

12

16

293

0.34(3)0.22(2)0.38(2)0.32(2)313

0.33(2)0.26(3)0.39(3)0.29(3)333

0.32(3)0.29(3)0.42(3)0.37(3)0.42(2)353

0.37(3)0.31(3)0.40(3)0.40(3)0.42(3)373

0.39(3)0.33(3)0.43(3)0.41(3)0.41(2)293

0.22(3)0.16(1)0.28(2)0.28(2)313

0.23(2)0.21(3)0.32(3)0.28(3)333

0.26(3)0.25(3)0.34(3)0.33(3)0.38(2)353

0.29(3)0.26(3)0.36(3)0.37(3)0.44(3)373

0.29(3)0.25(3)0.36(3)0.40(3)0.37(3)

TIS (K) at Einc = 20(2) kcal/mol

2

Π3/2

2

Π1/2

Ts

n=2

4

8

12

16

293

1060(70) 1030(40)920(40)980(50)313

1030(40) 1070(90)930(70)940(80)3331070(70) 1160(90)970(70)990(80)880(30)3531120(70) 1210(90)930(70) 1030(80)900(30)373

1140(70) 1070(90) 1050(70) 1180(80)890(50)2931160(70) 1190(40) 1010(40) 1090(50)313

1190(40) 1260(90) 1050(70) 1110(80)3331190(70) 1210(90) 1020(70) 1070(80) 1050(30)3531140(70) 1280(90) 1050(70) 1080(80) 1100(30)3731140(70) 1180(90) 1130(70) 1140(80) 1000(50)Table 4.1 Summary of least squares analysis fit parameters for NO scattered from
[Cnmim][Tf2N] at Einc = 20(2) kcal/mol as a function of TS and Cn, for both ground (2Π1/2) andexcited (2Π3/2) spin-orbit states.
from detailed balance considerations, rotational quantum state dependence in sticking
coefficients for the incoming flux48. Furthermore, the spin-orbit excited (2Π3/2) state NO118

rotational distributions are consistently ~30 K hotter than the ground (2Π1/2) state, indicating
sensitivity in these dynamical barriers and sticking probabilities at the gas-liquid interface tospin-orbit electronic excitation. As mentioned earlier, this behavior is reminiscent of NO
dynamics scattering/desorption from single crystal metals (e.g. Ag(111) and Ru(001)), for which
a similar propensity toward colder rotational distributions was reported.56, 58
Although the results in Figure 4.3 reflect a single surface temperature (TS = 353 K), thesame qualitative trends are observed over a wide range of TS, with desorbing NO rotationaltemperatures from low incidence energy scattering systematically lower than the liquid reservoir.
Interestingly, Figure 4.3 indicates both i) Trot and ii) sub-TS cooling trend in Trot to be relatively
insensitive to alkyl chain length. Based on a nearly unity trapping probability for jet cooled low J
states of NO, such an independence on chain length is not surprising. The persistent cooling
trend in Trot implies that as RTILs are varied by lengthening the hydrocarbon chain, any
dynamical barriers to adsorption/desorption do not change significantly.
While sticking probabilities governing the trapping-desorption channel at low energies
appear to be independent of n, significant differences as a function of chain length are indeed
observed in scattering dynamics at higher collision energies. At Einc = 20(2) kcal/mol, the TD/IS
two-temperature fits to rotational populations are summarized in Figure 4.4, with normalized
populations for the 2Π1/2 state plotted vs. rotational energy and alkyl chain length (n). For eachrotational distribution, the IS component (red) represents the majority species, while the TD
component (blue) comprises the remainder of the scattered flux. As the alkyl chain is lengthened,
the relative intensity of the TD component systematically increases from low n = [2, 4] to high n

119Figure 4.4 Two-temperature fits to rotational populations for NO scattered at Einc = 20(2)kcal/mol from [Cnmim][Tf2N]. As the alkyl chain is lengthened, the relative magnitude of the TD
component increases, consistent with a larger fraction of the incident NO molecules thermallytrapping on the liquid surface. The above data represent surface temperatures of TS = 333 K andonly the ground 2Π1/2 spin-orbit state, though similar trends are observed for the 2Π3/2 spin-orbit
excited state and over multiple liquid temperatures (see text for details).

= [8, 12, 16], representing a comparable increase in the fraction of NO molecules trapping at thesurface. Two-temperature fit parameters to these data, in addition to those for the 2Π3/2 state, as afunction of alkyl chain length are plotted in Figure 4.5 at TS = 333 K, with the complete data set
at all liquid temperatures summarized in Table 4.1.
As immediately evident in Figure 4.4 and 4.5(a), the TD branching fraction α for these
RTILs is strikingly low ( = 0.25 - 0.40) for all chain lengths, with nearly 60 - 75% of themolecules experiencing impulsive scattering (IS) events at the gas-RTIL interface. This
contrasts with the nearly 200% larger values (α ≈ 0.70) observed in previous scattering studies

120

Figure 4.5 Two-temperature Boltzmann fit parameters for Einc = 20(2) kcal/mol as a function ofalkyl chain length. As n increases, α, the branching ratio into the TD channel, also increases andthen plateaus at longer alkyl chain lengths. Spin-orbit changing collisions (2Π1/2 → 2Π3/2) have ahigher propensity to impulsively scatter (i.e., lower ) than spin-orbit conserving collisions.
Additionally, the degree of rotational excitation into the IS channel decreases with alkyl chain
length. As indicated by the dashed lines, both accommodation and IS rotational temperature plotssuggest a discontinuous shift between short (n  4) and long (n 8) chain lengths, which would
be consistent with molecular dynamics predictions of surface reorganization of the alkyl groups
over a similar range in n (see text for details).

121

of CO2 from [Cnmim][Tf2N] at Einc = 15 kcal/mol.46 This significant difference inaccommodation is not explained solely by slight discrepancies in the incident energies and, in thelimit of thermal accommodation as the rate limiting step, implies proportionately slower kinetics
for NO vs CO2 solvation into the bulk RTIL liquid. The fundamental reasons must reflect
differences in chemical/physical interactions at the interface. As a simple physical picture, this
could be rationalized to arise from the 4-5 fold larger NO vs. CO2 rotational constants, and thus
the 4-5 fold larger gap between final rotational levels impeding energy transfer from incident
translation into rotation at the gas-liquid interface. This would be interesting to explore
systematically by probing gas-liquid collisional energy transfer into rotation for different
isotopomeric molecular rotors with larger rotational constants such as DCl (B ≈ 5.39 cm-1) andHCl (B ≈ 10.4 cm-1).
Secondly, Figure 4.5(a) indicates a systematic spin-orbit state dependence for thetrapping desorption fraction, i.e., α(2Π1/2) > α(2Π3/2), which based on the incident beam
population in 2Π1/2 implies that electronically adiabatic spin-orbit conserving collisions (2Π1/2 →2Π1/2) undergo trapping-desorption more readily than non-adiabatic spin-orbit changing

collisions (2Π1/2 → 2Π3/2). This has been noted in earlier studies with NO scattering from RTILs,
and provides additional evidence for the presence of non-adiabatic surface hopping dynamics
taking place at the gas-liquid interface.47, 48 An intriguing parallel observation is the fact thatfully accommodated TD collisions desorb from the surface with colder ground (2Π1/2) vs. excited(2Π3/2) spin-orbit state rotational temperatures, which by detailed balance considerations would
suggest i) decreased trapping efficiency for higher J states and ii) lower trapping efficiency forground vs. excited spin-orbit states for a given high J state. This latter prediction would at firstseem to contradict the larger sticking coefficients for the lower spin-orbit state noted above.

122

There is no paradox, however, as these scattering experiments sample very low J states in theincident beam, whereas the detailed balance arguments are based on depletion of high J state
populations in the outgoing NO flux. In any event, the data clearly indicate an unusually richcoupling between i) rotational and ii) spin-orbit state collision dynamics of NO at the gas-RTIL
interface, which offer many challenges for non-adiabatic theoretical study.
Thirdly, the TD branching fraction α shows a clear sensitivity to alkyl chain length (n),with systematically more trapping/desorption with increasing hydrocarbon tail. These trends arein agreement with molecular beam experiments of both CO2 and O(3P) inelastically scatteringfrom [Cnmim][Tf2N] RTILs, which have also shown an increase in the TD fraction for longer
alkyl chains.44, 46 Such behavior is certainly consistent with a propensity for buildup of the softand longer hydrophobic hydrocarbon chains at the interface, as also supported by theoreticalmolecular dynamics simulations.44, 49 Of particular interest, however, is that Figure 4.5(a) and4.5(b) suggest that these shifts do not appear to occur uniformly, but rather with a more nearly
discontinuous upward shift between short (n = 2 - 4) and long (n = 8 - 16) alkyl chains. We will
return to this below, but simply note again that this is the same region of hydrocarbon length
over which a discontinuous shift in gas-RTIL interfacial alkyl chain orientation (extended
outward vs. folded over) is also predicted from MD simulations.33, 44
Figure 4.5(b) displays the measured IS rotational temperatures as a function of n andspin-orbit state. It is worth noting that these rotational temperatures range from 900 to 1200 K,
i.e., considerably hotter than the 333 K surface and indicating significant energy transfer fromhyperthermal translational energy of the beam into the rotational degree of freedom. Secondly,
these IS rotational temperatures cool slightly as n increases and again suggest a break from short(n = 2 - 4) to long (n = 8 - 16) alkyl chains, as similarly noted in Figure 4.5(a) for the sticking

123coefficient . On closer inspection, the excited spin-orbit state is rotationally hotter than the
ground electronic state, which was also previously observed at low Einc. These results are in goodqualitative agreement with both experimental and theoretical work, which suggest that theinterfacial landscape changes significantly for alkyl chain lengths between n = 4 and 8.
Specifically, for larger n, theoretical studies suggest i) alkyl chains preferentially populating thesurface more at the expense of anion representation, ii) alkyl chains reorienting and lying flat on
the surface, as opposed to pointing perpendicular to the surface (as seen for small n) and iii) a
more loosely packed, less dense interfacial region with lower surface tension.
Such compositional changes at the surface have been confirmed with LEIS41 and XPS25,
33

experiments, which are able to extract atomic densities at the gas-liquid interface. Thesestudies have found more hydrocarbons and fewer anions present at the surface for larger n, witha distinct change in the orientation of alkyl chains at the surface observed around n = 4. Based on
combinations of various photoelectron spectroscopy and atom scattering techniques, thehydrocarbon tails for n ≤ 4 [Cnmim][Tf2N] RTILs are found to stick outward into the vacuum,
while the longer hydrocarbons (n ≥ 8) tend to flop over on top of the surface.33, 42 Moreover, as afunction of hydrocarbon length, Kolbeck and coworkers25 found the surface tension to decrease
from n = 1 to 4, with the surface tension then remaining constant out to larger n. These results
support significant topological changes in hydrocarbon/anion surface structure up to n 4,beyond which the alkyl chains have established a dominant compositional presence at the[Cnmim][Tf2N] surface.
Elegant molecular beam scattering studies from RTILs have been performed by Minton,
McKendrick and coworkers, who scattered hyperthermal O(3P) from [C2mim][Tf2N] and[C12mim][Tf2N] and detected inelastically scattered O atoms as well as reactively scattered OH

124

and H2O from hydrogen abstraction at the liquid surface, which provide a probe of fractionalhydrocarbon presence at the surface.44 Given the stoichiometric difference in the number of CH2
groups in each RTILs, (where H-abstraction reactions are expected to occur), they found a higher
than stoichiometric reactivity for n = 12 compared to n = 2, thus inferring an increase in alkyl
chain presence at the surface for [C12mim][Tf2N] RTIL.44 Further efforts by McKendrick et al.
reactively scattered O(3P) from [Cnmim][Tf2N] RTILs, with n = 2, 4, 5, 8, and 12, and detected
OH, the yield of which was found to increase with n.43, 45 These results again support increasing
alkyl coverage at the surface with hydrocarbon length, in good agreement with the current
results, but where the OH yield increases continuously and does not saturate, thus suggestingalkyl chain coverage may still be evolving at n = 12. This may suggest a differential sensitivity to
RTIL surface features between O(3P) and NO(2), for which lengthening of alkyl chains
between n = 8 to 16 does not appear to affect the scattering dynamics. However, given the quite
different reactive vs. inelastic scattering methods of probing the surface, the qualitative trends
between these molecular beam studies are clearly in excellent agreement.
A greater propensity for alkyl groups present at the surface coupled with a more flexible
interface with increasing n is also in very good agreement with inelastic scattering resultspresented in Figures 4.4 and 4.5. As a simple physical picture, smaller chain lengths result in arougher surface, where incident translational energy is more efficiently transferred into therotational degree of freedom. For small n, this would predict hotter IS rotational temperatures,with slightly cooler temperatures for larger n where the alkyl chains are lying flat on the surface.
The branching ratio, α, increases even more dramatically (40%) over this same range of chain
lengths, consistent with a softer, more flexible surface at higher n that can more readily absorb
molecular incident energy. Simply stated, larger n leads to a looser arrangement of alkyl chains125with a lower effective surface mass, whereby molecules can more readily rearrange and becometrapped transiently on the surface. Our data reveal that such a longer, looser arrangement of alkyl
chains on the surface leads to increased trapping of NO gas molecules on the surface, which is anecessary prerequisite for solvation into the bulk.

4.4.2 Spin-Orbit Distribution: Dependence on Cn
We next explore the nonadiabatic interactions occurring at the surface by considering theelectronic spin-orbit ratios of the scattered NO, [2Π3/2]/[2Π1/2], as a function of alkyl chain length(n) and collision energy (see Figure 4.6). At low Einc = 2.7(9) kcal/mol and TS = 353 K, spinorbit ratios of [2Π3/2]/[ 2Π1/2] ≈ 0.5 are observed (Figure 4.6), which implies only roughly onethird of the incident NO molecules are electronically excited (2Π1/2  2Π3/2) in the collisional
process. A simple estimate of an “electronic temperature” can be extracted from the Boltzmann
expression [2Π3/2]/[ 2Π1/2] = exp(-ESO/kT), with the energy difference between the two spin-orbit
states (Eso) equal to 123 cm-1.65 At low Einc , these spin-orbit ratios correspond to ~ 250 K, i.e.,significantly hotter than the ~ 1 K rotational/electronic temperatures in the incident beam and yet
still significantly colder than the 353 K liquid interface itself. Interestingly, the magnitude of thisspin-orbit excitation appears to be insensitive to the alkyl chain length, recapitulating the trends
observed for Trot. This is not surprising, as both rotational and electronic populations resulting
from fully accommodated species could be additionally sensitive to dynamical barriers along thedesorption pathway. At high collision energies (Einc = 20 kcal/mol) which facilitate both TD and
IS scattering pathways, a greater spin-orbit ratio of [2Π3/2]/[ 2Π1/2] ≈ 0.66 is observed (see Figure4.6), corresponding to electronic temperatures now in excess of TS. Since spin-orbit excitationout of the incident 2Π1/2 ground state reflects a non-adiabatic event, a surface hopping sensitivity

126

Figure 4.6 Spin-orbit ratio of scattered NO at both low/high Einc and as a function of alkyl
chain length. The horizontal green line represents the “spin-orbit temperature” corresponding to
the surface temperature where [2Π3/2]/[ 2Π1/2] = e-ESO/kT and ESO = 123 cm-1. Error bars represent
2σ standard deviations.

to incident translational energy would be at least qualitatively consistent with simple 1D LandauZener models for curve-crossing dynamics.66, 67

4.4.3 Dependence on Surface Temperature
Scattering dynamics of NO from this series of [Cnmim][Tf2N] RTILs have also been
investigated as a function of surface temperature, with the resulting two-temperature fit
parameters at high Einc plotted vs. TS in Figure 4.7. For visual clarity, only data for NO scattering

127

Figure 4.7 Two-temperature fit parameters to rotational distributions at Einc = 20(2) kcal/mol as
the surface temperature is varied for two alkyl chain length RTILs, n = 2 and 12. As the liquid
temperature increases and the surface roughens, the branching into the TD channel increases
linearly whereas the IS rotational temperatures remain essentially flat.

128

from [C2mim][Tf2N] and [C12mim][Tf2N] into the excited spin-orbit states are plotted,
with the complete set of data for all RTILs and both spin-orbit states summarized in Table 4.1 asa function of surface temperature. Trends in alkyl chain length discussed earlier are apparent
here as well, with longer alkyl chains resulting in a larger fraction of molecules trapping on thesurface (i.e., higher ) and lower rotational excitation in the impulsively scattered channel (i.e.,colder TIS). As seen in Figure 4.7(a), the trapping-desorption fraction increases very nearlylinearly with TS, as observed in other gas-liquid scattering experiments54, 68, 69 and can be
ascribed to an increase in microscopic surface roughness with surface temperature. The slope isapproximately 0.0015(19)/K for [C12mim][Tf2N], i.e., 15(2)% more NO trap and desorb from thesurface per 100 K increase in TS, and 0.0010(12)/K for [C2mim][Tf2N]. As evident from Table
4.1, similar slopes (e.g., 6(4) – 16(3)% per 100 K) are observed for all RTILs studied and forboth spin-orbit states. By way of comparison, CO2 scattering (at Einc = 15 kcal/mol) fromperfluoropolyether (PFPE) and fluorinated self-assembled monolayer surfaces (F-SAMs) has
also been studied over TS = 232 – 323 K.54 An equivalent analysis of the temperaturedependence of α yields slopes of 17(3)% and 11(6)% per 100 K for the PFPE and F-SAM
surfaces, respectively, i.e., reflecting a comparable range of NO + RTILs values as reported
herein.
The impulsively scattered rotational temperatures are plotted in Figure 4.7(b), again
focusing on the 2Π3/2 state of n = 2 and 12 [Cnmim][Tf2N] RTILs. As noted earlier, the TIS(n = 2)
values are slightly hotter than the TIS(n = 12) values. Furthermore, each of these TIS rotationaltemperatures is significantly hotter than TS, though with remarkably little sensitivity to the actual
surface temperature. Thus, while the branching ratio  into the TD channel is clearly strongly
influenced by TS, the temperature of the liquid surface appears to have little influence over the129

impulsively scattered IS temperatures observed experimentally. One simple physical picture forsuch behavior would be that interaction times with the surface for IS events are too short to besignificantly influenced by liquid temperature, though the fraction of trajectories that proceed via
IS vs. TD channels remains sensitive to thermally induced surface roughness.68

4.5 Conclusions
We report molecular beam scattering results of supersonically cooled NO from
[Cnmim][Tf2N] RTILs with state-to-state resolution. The energy transfer between the surface andincident NO molecules is explored as a function of the incident collision energy, surface
temperature and the cation alkyl chain length, which is varied over n = [2, 4, 8, 12, 16]. At low
collision energies, all molecules accommodate and thermally desorb from the surface in a single
Boltzmann temperature. However, even in this simplest of dynamical scenarios, we observe
rotational and spin-orbit temperatures that are systematically cooler than the surface temperature.
Furthermore, the rotational and spin-orbit temperatures are unaffected by the alkyl chain length.
Thus, while physiochemical changes are occurring on the surface as n is varied in these[Cnmim][Tf2N] RTILs, the sticking probabilities that govern the TD channel for both therotational and electronic degree of freedom are not influenced.
At high incident collision energies (Einc = 20 kcal/mol), rotational populations of
scattered molecules are well described by a two temperature TD/IS model. As these alkyl chainsare lengthened, the landscape of the RTIL surface is changed such that more incident gas
molecules trap on the surface and those that do impulsively scatter leave the surface with lessrotational energy. These results are in agreement with a simple physical picture building on othertheoretical and experimental work,33, 44-48 whereby longer hydrocarbon RTILs have an enhanced130presence of alkyl chains at the surface that result in a more flexible, loosely packed surface moreable to promote energy transfer into the liquid. Additionally, the surface temperaturedependence of these RTILs on scattering dynamics has been explored, revealing for all RTILs
studied that, as TS increases, more molecules trap on the surface and yet the rotational
component of the IS channel remains unaffected. In summary, these studies provide quantitative
data into the rich quantum-state resolved dynamics for both inelastic and non-adiabatic scatteringof open-shell species with the surfaces of room temperature ionic liquids, and how qualitative
conformational changes occuring at the gas-liquid interface affect these dynamics. Detailed
theoretical modeling will clearly be invaluable in interpreting the TS and alkyl chain length
dependence of these interfacial collision dynamics with more molecular level of detail, andtoward which we hope these studies help provide the requisite stimulus.

131

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G. Herzberg, Spectra and Molecular-Structure I. Spectra of Diatomic-Molecules,
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Nitrogen Oxides (NOx), Why and How they are Controlled, Report US EPA Technical
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135

Chapter 5

Angle-Resolved Molecular Beam Scattering of NO at the Gas- Liquid Interface*

5.1 Introduction
Nitric oxide and nitrogen oxides (NOx = NO + NO2) are major pollutants in theatmosphere and play a crucial role in the formation of acid rain and photochemical smog.1-4
While there are natural sources of atmospheric NOx such as lightening, soil emissions and forest
fires, its considerable presence in the atmosphere is largely anthropogenic, with leading emission
sources from power plants and fossil fuel emissions.5, 6 One way to curb the introduction ofexcess NOx into the atmosphere is by the use of wet scrubbers, which when placed insmokestacks can remove NOx from flue gas emissions.4, 7, 8 As a result, NO collisions at gasliquid interfaces have important implications in pollution chemistry. NO also plays an importantrole in the chemistry of atmospheric aerosols,5, 9-12 where thermal adsorption/desorption at thegas-liquid interface significantly influences the subsequent chemical oxidation processes. For
these and many other atmospheric pollution chemistry applications in environmental chemistry, afirst principles understanding of how small molecules such as NO transfer energy to and/or
equilibrate with a gas-liquid interface becomes crucially relevant.
As one of several questions of fundamental interest, we ask what are the energy transfer
dynamics of NO colliding with complex liquid surfaces? In particular, what properties of the*To be submitted to J. Chem. Phys.
136

interface govern whether a gas molecule sticks, thermally accommodates, and dissolves into theliquid, or simply interacts with the liquid surface transiently to achieve full thermal
accommodation with respect to internal rovibronic quantum states? Molecular scattering studies
have provided critically important insights into the nature of such gas-liquid dynamics and how anumber of liquid properties might be engineered to “tune” these interfacial interactions
systematically. Specifically, multiple experiments have investigated the behavior of gas
molecules striking liquid surfaces, probing how surface temperature,13-15 presence (or absence)of solvated ions,16, 17 and different functional groups18, 19 at the interface can influence theresulting collision dynamics. Time of flight mass spectrometry17, 19-22 and quantum state
resolved15, 19, 23-26 detection techniques have also proven to be extremely powerful andcomplementary tools for examining elastic, inelastic, and reactive molecular scattering at liquidsurfaces. In these studies, a diverse range of molecules and atoms (e.g., CO2, HCl, OH, O, noble
gases, etc.), have been scattered from a variety of low vapor pressure liquids, the results of whichhave helped formulate a more predictive framework for energy transfer dynamics of atomic,
diatomic and polyatomic colliders at the gas-liquid interface.
In the limit where molecules have become fully thermally accommodated with andeventually desorb from a liquid interface, the outgoing distributions are often presumed to bewell described by rotational, vibrational, translational and electronic temperatures characteristicof the surface temperature (TS). From microscopic reversibility/detailed balance
considerations,27, 28 this behavior would imply unity sticking coefficients for the corresponding
reverse collisional event, independent of internal quantum state, incident angle, and range of
speeds appreciably sampled in a gas at TS. Indeed, such simple, intuitive behavior of complete
thermal equilibration in the desorbing flux has been verified experimentally for scattering of CO2

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from perfluoropolyether (PFPE)13 and HCl from alkanethiol self-assembled monolayers(SAMs)29 over a wide range of surface temperatures.
In the case of NO, however, the rotational and electronic temperatures of the desorbing
molecules are often not in equilibrium with the liquid interface. Instead, the molecules are bettercharacterized by distinctly subthermal temperatures (i.e., Trot < Ts) for NO desorbing fromcondensed phase interfaces.14, 30-35 A variety of low incident energy molecular scatteringexperiments have revealed these results from (i) crystalline, (ii) organic self-assembled
monolayer, (iii) low vapor pressure liquid and even (iv) molten metal surfaces, all with varying
degrees of rotational cooling observed.14, 18, 34-36 Indeed, arguably the most dramatic example ofthis can be found in temperature programmed desorption (TPD) studies, where absorbed NO
molecules have residence times of minutes to hours and thus certainly have achieved complete
thermal equilibrium with the surface. Nevertheless, cold rotational temperatures on the order of235(35) K have been noted for NO desorbing from Ru(001)30 at Ts = 455 K, withcorrespondingly sub-thermal values (Trot = 600(4) K) for NO desorption from even hotter (TS =1100 K) Pd(111) surfaces.31
Previous work exploiting quantum state-resolved laser induced fluorescence (LIF)
detection of jet-cooled NO molecular beams scattered from the gas-liquid interface has provided
modest but clearly supporting evidence for such non-equilibrium desorption behavior. Rotationaltemperatures have been observed some 30 K cooler than Ts for NO scattering from both
imidazolium-based room temperature ionic liquids (RTILs) and molten gallium. Due togeometrical limitations in the LIF collection optics, however, these studies were only able todetect NO molecules emerging into a single, near-specular scattering angle. Apparatus
modifications implemented in the present work have successfully eliminated such optical138

constraints and now permit quantum state-resolved detection of the scattered NO flux as afunction of solid angle in the 2 upper half plane, most importantly even backscattering into
solid angles more glancing than the incident molecular beam. This new capability makes
possible i) angle- ii) collision energy, and ii) quantum state-resolved investigation of NO
scattering dynamics at a series of gas-liquid interfaces, with which we can help further probe andelucidate the observed non-equilibrium behavior.
Specific questions of interest at low Einc are as follows. Do NO molecules in all quantum
states fully accommodate on the liquid surface, which, for example, would imply scattering withforward/backward symmetry? Does the thermally accommodated NO flux reemerge into acos(s) distribution for all internal quantum states, which would be consistent with unity sticking
coefficients in the reverse direction for all incident angles? What are the NO rotational/electronic
distributions as a function of final scattering angle and what does this suggest with respect to“surface-hopping” dynamics between 2П1/2 and 2П3/2 electronic surfaces? Could orientation
and/or quantum state dependent adsorption dynamics for NO at a liquid interface be responsible
for the non-equilibrium behavior evidenced in subthermal rotational and spin-orbit temperatures?
Finally, how are these experimental trends all consistent with detailed balance and microscopic
reversibility considerations?
It is worth clarifying at the outset that our ability to independently probe angular andinternal quantum state distributions samples two different aspects of the scattering dynamics. For
example, detailed balance considerations and observation of a cos(s) distribution for any one
NO quantum state permit rigorous prediction of the angular independence of sticking coefficientsfor that specific quantum state. However, the existence of such cos(s) angular distributions does
not impose any corresponding constraint on the actual rotational and/or spin-orbit internal139

quantum state distributions of the desorbing NO. Thus, as we shall see, it is entirely possible tohave cos(s) angular distributions mimicking perfect angle independent thermal accommodationof the incident flux, but with non-equilibrium internal state distributions in the outgoing flux.
Indeed, by detailed balance considerations, such observations directly imply quantum state
dependent sticking coefficients and signal the presence of dynamical barriers to adsorption anddesorption processes.
In this work, we explore angle- and quantum-state resolved collision dynamics at the gasliquid interface via molecular beam scattering of NO from low vapor pressure [bmim][Tf2N]
RTIL, squalane and perfluoropolyether (PFPE) liquids. Specifically, we first exploit tunable
beam collision energies at low Einc to investigate the angular dependence of thermallyaccommodated NO desorption dynamics in isolation. Next, angle-resolved IS scattering
dynamics are explored at high Einc to probe the scattering angle dependence of microscopic
branching between TD vs. IS pathways. Finally, we take advantage of our quantum stateresolved LIF detection capabilities at high Einc to examine the explicit angular dependence forrotational and spin-orbit distributions in the scattered NO.

5.2 Experimental
For the present studies, supersonically cooled NO has been scattered at θinc = 45(1)º from
1-butyl-3-methyl-trifluorosulfonylimide ([bmim][Tf2N]), perfluoropolyether (PFPE), andsqualane gas-liquid interfaces, followed by with quantum state and angle-resolved (θs = -60º to60º) LIF detection of the subsequently scattered NO molecules. The scattering takes place in a 90
L vacuum chamber, evacuated with a 1500 L/s turbomolecular pump that achieves basepressures of 2 x10-8 Torr. The jet-cooled molecular beam of NO is produced from an Even-Lavie

140

pulsed valve with an 80 μs pulse width, resulting in rotationally and electronically cold NO with
Trot ~ 1 K and only the ground spin-orbit state significantly populated (2П1/2; J=0.5). Stagnation
gas mixtures consist of 1% NO in either Ne-70 (70:30 mixture of Ne:He) or H2 diluent, which
permits tuning the incident energies from Einc = 2.7(9) to 20(2) kcal/mol, respectively. After
passing through a 3 mm skimmer 5.4 cm downstream of the nozzle, the collimated beam strikes
the liquid surface at inc = 45º in a 4.4 cm x 2.4 cm x 0.5 cm stainless steel reservoir, resulting in
a 1.1 cm x 0.8 cm locus for NO scattering on the gas-liquid interface.
The [bmim][Tf2N] species (IoLiTec, 99% purity) is degassed by stirring/heating for atleast six hours prior to placement in the chamber, with similar degassing protocol for liquid
squalane (C30H62, Sigma-Aldrich, 99% purity) and PFPE (F(CF(CF3)CF2O)14CF2CF3, Krytox
1506, Sigma-Aldrich). All liquid temperatures are held constant at Ts = 293(1) K, with scattered

Figure 5.1 An incident beam of supersonically cooled NO (2П1/2(N=0), Trot ~ 1 K) strikes theliquid surface at 45(1)º and Einc = 2.7(9) or 20(2) kcal/mol. Scattered NO molecules are detectedwith quantum-state resolution via LIF in a 13 mm3 probe volume, which is scanned along the
225 nm excitation laser path to achieve angular detection windows at θs = -60(3)º, -30(9)º,0(12)º, 30(9)º, 45(6)º and 60(3)º.

141

NO molecules detected using laser induced fluorescence (LIF) on the γ band (A2Σ+ ⟵X2П). The
requisite UV pulses (~ 225 nm) are generated via the tripled output of a YAG-pumped dye laser,which enters the chamber through baffle arms and passes 1.6 cm above the gas-liquid interface
and in parallel to the scattering plane (see Fig. 5.1). Fluorescence from the scattered NO isfocused onto a photomultiplier tube (PMT) through a 4 mm diameter pinhole mask in a 1:1
confocal imaging geometry. This spatial filtering permits fluorescence detection from only a
restricted focal region (13 mm3), which, by translating the PMT with respect to the scattering
plane, allows systematic variation of the detection angle into which the NO has been scattered.
Specifically, the measured scattering angle windows are θs = -60(3)º, -30(9)º, 0(12)º, 30(9)º,45(6)º and 60(3)º, where the uncertainties represent 1 standard deviations based on statisticalsampling of all collision free (i.e., straight line) trajectories from the liquid surface to the LIF
detection focal volume.
The pulsed valve duration ( 80 s) is much longer than time scales for the gas-liquid
collision event ( 1-10 ps) and thus the data reflect scattering under steady state conditions. A
boxcar integrator captures the fluorescence signal at the peak of the time domain signal,
therefore NO molecules are probed that arise from both i) full thermal accommodation/desorption (TD) as well as ii) direct, impulsive scattering (IS) from the gas-liquid interface. The LIF
data is least squares fit using STARPAC software to a LIFBASE model of the NO spectrum,35, 37,
38

from which quantum state populations and uncertainties are extracted. The reliability of such

fitting routines is systematically tested on  10-6 Torr samples of NO introduced into the vacuum
chamber, the quantitative spectral analysis of which correctly recapitulates room temperature
rotational and electronic distributions within experimental uncertainty. At least 3 spectral scans
are obtained and least squares fit for each i) collision energy, ii) liquid, and iii) scattering angle,

142

with the results averaged to yield improved standard deviations of the mean as well as a more
realistic assessment of statistical errors.

5.3 Results
By way of initial overview, sample LIF spectra for forward [θs = 30(9)º] and backward[θs = -30(9)º] scattered NO are displayed in Figure 5.2, under hyperthermal incident beam
conditions of Einc = 20(2) kcal/mol and an incident angle of θinc = 45º. In contrast to the incident
NO beam, which has nearly all population supersonically cooled into (2П1/2; J = 0.5), thescattered spectra reveal extensive rotational population up to J  45.5 in both ground (2П1/2) and

Figure 5.2 Sample LIF spectra for θs = -30(9)º and 30(9)º scattering of jet-cooled NO (21/2(N=0)) at Einc = 20(2) kcal/mol from [bmim][Tf2N], which reveal significant inelastic energy
transfer into highly excited rotational (up to J  45.5) and spin-orbit electronic (2П1/2 and 2П3/2)
states. The 3-fold more intense signals observed in the forward vs. backward direction indicate astrong propensity for forward scattering at high collision energies.

143

excited (2П1/2) spin-orbit manifolds. This indicates substantial inelastic rotational energy transfer
as well as significant nonadiabatic spin-orbit “surface-hopping” dynamics at the gas-liquidinterface, for both backward- and forward-scattered NO species. As physically expected under
hyperthermal conditions and inc = 45o, there is a marked predominance for forward scattering in
the near-specular direction, with roughly 3x more signal at θs = 30º than at θs = -30º. Equally
noteworthy is the still quite significant fraction of NO recoiling in the backward direction, evenat θinc = 45º and high incident collision energies. Since collisions from a nominally “flat” liquidsurface potential would conserve parallel momentum and thereby predict specular scattering in
the forward direction, this speaks to the high degree of surface corrugation on the nanometerscale that such gas-liquid interfaces must present to the incoming NO projectile.

5.3.1 Low Energy Collisions: Cos(s) Angular Distributions and Thermal Desorption
Dynamics
To simplify our scattering conditions, we first consider data at low collision energies (Einc= 2.7(9) kcal/mol), where from previous gas-liquid scattering studies, one expects all incident
molecules to stick, have sufficient opportunity to fully thermally accommodate, and eventuallydesorb from the liquid surface. By way of sample data, NO rotational distributions for the ground(2П1/2) spin-orbit state are plotted in Figure 5.3 as a function of final scattering angle (θs), wherein anticipation of a “temperature-like” Maxwell-Boltzmann behavior, the data have beenreported as semilogarithmic plots of degeneracy scaled rotational populations vs. internalrotational energy. The rotational distributions at all θs are indeed well described by a common
temperature, Trot ~ 257(10) K, i.e., considerably cooler than the 293 K surface temperature and,most notably, independent of the final scattering angle. Data for all four spin-orbit/lambdadoublet states are extracted from at least 3 spectral scans at each final angle, with temperature

144

Figure 5.3 Sample rotational Boltzmann plots for ground spin-orbit scattered NO(2П1/2) at Einc= 2.7(9) kcal/mol as a function of scattering angle θs. Note that Trot is i) independent of θs andyet ii) systematically cooler (T  -35 K) than the liquid surface temperature. By detailedbalance considerations, these observations are consistent with i) angle-independent and yet ii)
internal quantum-state dependent sticking coefficients for incoming NO at low incident collisionenergies.

uncertainties estimated from the standard deviation of values obtained for the individual
Boltzmann least squares fits. Since statistically significant differences in the Λ-doublet data are
not evident, the reported populations are therefore simply summed over the final (e/f) parity
states. Similar cooling and angle independent trends are also confirmed for Boltzmann plots forthe excited spin-orbit (2П3/2) state, though with a much smaller magnitude cooler from TS.
Specifically, the Maxwell-Boltzmann plots are again quite linear with no dependence of thescattered rotational temperatures on final scattering angle θs. Thus, evidence for “rotationalcooling” below the surface temperature is observed for both spin-orbit-conserving and spin-orbitchanging trajectories, with Trot(2П1/2) 257(10) K< Trot(2П3/2) 254(10) K (where numbers in145parentheses represent one standard devotion of the mean from multiple measurements) and valueless than the actual surface temperature of Ts  293(1) K.
Rotational temperatures for both ground and spin-orbit excited states of NO averaged
over multiple scans are displayed vs. θs in a polar plot (Fig. 5.4(a)). The data clearly illustrate
two important points: Trot is i) independent of scattered angle and yet ii) statistically different for
the two spin-orbit states, with excited and ground spin-orbit Trot values cooler by ~ 10 K and ~35 K than the surface temperature. We note that such behavior (i.e., Trot < Ts for scattering at lowEinc) is distinctly different from molecules such as HCl and CO2,13, 29 which have been found tothermally desorb from a variety of liquid surfaces with Ttrans  Trot  Ts. As previously
hypothesized14 and as detailed balance arguments insist,27 this necessitates the presence ofquantum state-dependent sticking dynamics that favor an excess of rotationally colder species inthe thermally desorbing NO flux. Specifically, this means that higher energy rotational statescolliding with the interface under equilibrium conditions (Trot  TS) must have smaller sticking
probabilities, and vice versa. Since the reverse desorption rates of thermally accommodated NO
must reflect this behavior precisely by detailed balance, the outgoing NO flux is thereby
systematically depleted in higher J states and thus “colder” than TS, as observed experimentally.
One simple physical mechanism for achieving such behavior would be the presence of NO
orientation dependent potential wells for molecular adsorption at the liquid surface, which could
dynamically allow more slowly rotating J states in the incoming flux to better align and thereby
stick with higher efficiency than more rapidly rotating molecules in higher J states. By way ofcontrast, we see do not see corresponding evidence for any dependence in the desorbing NO
distributions on scattering angle. Again by detailed balance considerations, such an
independence (at low Einc) of quantum state rotational distributions (Trot) with θs is consistent146

Figure 5.4 Polar plots for NO scattering at Einc = 2.7(9) kcal/mol. a) NO rotational temperatures
for each spin-orbit state as a function of final scattering angle s. Note the rotational cooling
effects, with Trot(2П1/2) and Trot(2П3/2) some T  -35 K and T  -10 K cooler than Ts,
respectively. (b) NO populations for each spin-orbit state as a function of s, corrected for
distance to angular detection region (see text). Note that the fits to cos(θs) distributions (dashedcircles) are consistent with complete thermal accommodation at the surface and unity sticking
coefficients for all angles (s) at low collision energies. Error bars represent standard deviationsof the mean (mean) from multiple ( 3) frequency scans.

147with angle-independent sticking/adsorption probabilities for incident NO striking the gas-liquidinterface in a specific rovibronic state.
We can rigorously test these detailed balance predictions by examining actual quantum
state-resolved density distributions of the scattered NO. For a probe laser beam parallel to thegas-liquid interface, the NO densities must first be corrected for variation in distance from thesurface (see Figure 5.1), which we can model by averaging over the 1/r2 density drop off fromthe liquid surface to all points in the detection volume. This procedure yields corrected quantum
state-resolved NO densities corresponding along some arc at fixed radial distance from the gasliquid collision center. These NO densities are plotted as a function of scattered angle in Figure
5.4(b), where for simplicity we have further summed over all rotational states within each spinorbit state manifold. The data fit very well to a cos(s) angular distribution, which is again
consistent with the simple physical picture of NO molecules at all incident angles sticking to andthermally accommodating with the surface. By detailed balance arguments, such data, along withthe experimentally-observed angle-independence of the rotational temperatures, strongly
supports a simple physical picture of NO scattering from the liquid at low Einc arising from all
trajectories fully thermally accommodating and eventually desorbing from the gas-liquidinterface.
Two final comments in this sub-section are worth noting. First of all, the presence of such
uniform cos(s) angular distributions does not require sticking coefficients to be independent ofinternal quantum state. Indeed, this is already clearly demonstrated in the rotational Boltzmann
distributions for the desorbing NO, which are distinctly out of equilibrium with the surface
temperature TS. Furthermore, significant deviations from equilibrium behavior are also observedin electronic degrees of freedom. For example, as is clear in Fig. 5.4(b), NO signals out of the148excited spin-orbit manifold are only 2 times smaller than that of the ground state manifold.
This would correspond to a spin-orbit temperature of Telec  220 K, i.e., again very much out of
equilibrium with and substantially colder than TS.
Secondly, we note that these LIF-based measurements yield densities rather than fluxesof the probed NO species. If the desorbing speed distributions of NO are the same at all
scattering angles, then the density-to-flux conversion factor would be the same for each smeasurement and a cosine density distribution would also be a cosine flux distribution. By
detailed balance arguments,27 this would stipulate the sticking coefficients for NO collisions at
low Einc to be independent of incident inc and speed (that is, for a range of thermal speeds
sampled by thermally desorbing molecules, which breaks down at hyperthermal energies). In
turn, this would require all outgoing distributions (speed, J, spin-orbit, vibration, etc.) fromthermally accommodated NO to be independent of s. It is worth stressing that sticking
coefficients being independent of angle and speed is not equivalent to sticking coefficients being
unity for all NO internal quantum states. Indeed, by detailed balance, a perfectly unity sticking
coefficient would result in the much stronger requirement that Trot  Telec  TS, which is in fact
not observed experimentally. Our LIF detection of NO in this experiment does not directly
measure velocities of the scattered molecules. However, the results presented above are certainly
consistent with the simplest Occam’s razor physical picture of both angle and speed independent
sticking coefficients, at least for the range of incident energies commensurate with TS.

5.3.2 High Energy Collisions: Angle-Dependent Rotational Distributions
We next consider how such angular independence in the scattering data may be
influenced by magnitude of the incident collision energy. At higher incident speeds, the149Boltzmann plots become significantly non-linear, which has been interpreted in numerous gasliquid scattering systems in terms of a linear combination of thermal desorption (TD) and
impulsive scattering (IS) pathways. With the present experimental capability for final
angle/quantum-state resolved detection, we can now actually isolate and probe the dynamics ofthese two separate scattering pathways as a function of θs. In Figure 5.5, the TD and IS rotational
populations obtained from LIF spectral fits at Einc = 20(2) kcal/mol and θinc = 45º are shown for
the ground 2П1/2 spin-orbit state in a waterfall plot vs. scattering angle and rotational energy, withthe distributions at each angle normalized to unity. Significant rotational excitation up to 4000
cm-1 (J = 45.5) is observed and, quite differently than behavior described previously at low Einc,the rotational distributions now clearly evolve toward hotter temperatures with increasing

Figure 5.5 Rotational distributions for hyperthermal NO scattering (Einc = 20(2) kcal/mol) as afunction of final angle s. Least squares fits into trapping desorption (TD) and impulsive
scattering (IS) components are shown (see text for details). Note the strong dependence of IS vs.
TD signals with final s, which are dominated by IS and TD components for forward andbackward scattering, respectively.

150

scattering angle. Stated alternatively, the lower J state populations are preferentially enhanced at
backward scattering angles (θs < 0º), whereas higher J rotational energy states are preferentiallyscattered in the forward direction (θs > 0º).
More quantitatively, the data can be fit to a two-temperature Boltzmann model consistingof cold and hot scattering components that correspond to TD and IS channels, respectively.
Specifically, Pop(J) = [αPTD(J) + (1- α)PIS(J)] where PTD(J) and PIS(J) can be used to represent
the normalized probabilities for scattering into a specific rotational J state via either the TD or IS
scattering channel. The corresponding Boltzmann probabilities are represented by PTD/IS(J) =(2J+1)exp(-Erot/(kTrot(TD/IS))/Qrot(TD/IS)). It is worth noting that such a “microscopic
branching” description of these populations is not an obvious choice; indeed, one might
physically expect more of a continuum of IS scattering pathways to be taking place.39-43
Nevertheless, there is an impressive amount of experimental support for such a two- temperaturemodel, with, for example, recent REMPI studies of HCl scattered from self-assembled
monolayers with both quantum state and 3D velocity map imaging resolution.29 The current
experimental results on NO scattering from [bmim][Tf2N] provide yet another independentconfirmation for the presence of a Boltzmann-like IS component in the rotational distributions.29
From a purely empirical perspective, such a two-temperature model fits the data remarkably wellover two orders of magnitude in dynamic range and therefore permits one to extract usefulparameters with which to more quantitatively characterize and interpret angle and/or collisionenergy dependent changes in the rotational/spin-orbit state distributions.
In such two temperature analyses (see Fig. 5.5), the thermally accommodated rotationaltemperatures (TTD) are held fixed at values determined from low collision energy scattering at
Einc = 2.7(9) kcal/mol, with the trapping-desorption fraction (α) and IS temperature (TIS) floated151

in a least squares process. The use of the low Einc rotational temperatures for the TD component
is supported by the angular independence of the desorbing flux presented above, which confirm
that these low Einc results arise from fully thermally accommodated TD scattering events. The fit
parameters to the two-temperature model at high Einc are graphed in Figure 5.6 as a function of
scattering angle and spin-orbit state (Einc = 20(2) kcal/mol and θinc = 45º). With more molecules
scattering impulsively at higher incident speeds, a simple physical intuition leads to an
expectation for a relatively larger fraction of molecules undergoing IS scattering at near specular
scattered angles (θs ≈ 45º) and correspondingly fewer at back scattered angles. This is indeed
observed (see Fig. 5.6a) where α, the branching ratio into the TD channel decreases from 0.52 to
0.28 with the scattering angle increasing from θs = 60º to -60º. Thus, the fraction of TD
molecules detected nearly doubles from forward to backward scattering angles, in agreementwith expectations for a larger fractional IS contribution in the forward scattering direction.
It is remarkable, however, that even at strongly backscattered angles (θs = -60º), nearly
half of the NO molecules still arise from the rotationally hot IS channel. In previous experimentsof CO2 scattered from PFPE,44 similar results were also observed, with α = 0.6 at backward
scattering angles for θinc = 45º and Einc = 10.6(8) kcal/mol. While there are obviously differencesin the molecular systems studied, these results and the magnitude of the changes are inagreement what those in Figure 5.6(a). This is in contrast to observations of NO scattering fromsingle crystal surfaces, where a significantly smaller fraction of molecules are found for NO
backscattered angles from graphite32 and Pt(111)32. This again speaks to the substantial
differences in the microtopography for gas-liquid vs. gas-crystalline interfaces. In contrast to
atomically flat crystalline surfaces, liquid surfaces are microscopically rough with thermally
excited capillary wave activity. At the atomic level, functional groups extending out from the152Figure 5.6 Polar plots of two-temperature (IS/TD) Boltzmann fits at hyperthermal collisionenergies (Einc = 20 kcal/mol) to rotational/spin-orbit distributions as a function of final scattering
angle s. (a) Branching ratios () into the thermal accommodation TD channel, which decrease
dramatically in the forward direction. (b) IS rotational temperatures, which systematicallyincrease with scattering angle (TIS  800-1200 K) and are 3-4-fold hotter than the liquid (Ts =293(3) K). Note that spin-orbit conserving (21/2 →21/2) vs. spin orbit changing (21/2 → 23/2)
collisions are both i) systematically hotter and ii) less prone to thermal accommodation (i.e.
result in smaller α values).

153

gas-liquid interface into the vacuum can promote significant backward scattering into the IS
channel as well as facilitate significant hyperthermal rotational energy transfer.
NO rotational distributions in the IS scattering channel are probed further in Figure
5.6(b), where the fitted rotational temperature (TIS) is explored as a function of θs for both
ground and excited spin-orbit states. Substantially hyperthermal temperatures from TIS = 800 1200 K are detected, indicative of significant impulsive translational-to-rotational energy transfer
occurring at the gas-liquid interface. Interestingly, the hotter temperatures are now clearlyobserved in the forward scattered direction, which implies the degree of rotational excitationmust increase with scattering angle. These results are consistent with a simple physical picture of
forward scattering, hyperthermal collisions from a microscopically rough liquid surface. When adiatomic molecule strikes a liquid surface, the torques tend to arise from contact with functionalgroups protruding out into the vacuum, resulting in strong rotational excitation of the projectile
coupled with recoil from the surface in a predominantly forward scattered direction. Indeed,molecular dynamics simulations have confirmed this expectation theoretically.45, 46 Further
confirmation of these trends has been provided by infrared quantum state resolved studies of gasliquid scattering experiments, which reveal the IS component to be preferentially peaked in theforward direction and well fit to a lobular cosn(θs-θ’) distribution with n greater than unity.47-50

5.3.3 Angle-Dependent Spin-Orbit Distributions at Low and High Collision Energies
NO molecules in the incident beam are strongly supersonically cooled in the ground[2П1/2] spin-orbit state; however, collision dynamics at the gas-liquid interface clearly lead tosignificant fractional population of spin-orbit excited NO molecules in the scattered flux. It is
worth noting that such spin-orbit conversion dynamics require “surface hopping” between two154

electronic potential energy surfaces.51 Thus, by detailed balance considerations, angle-dependent
NO electronic state distributions in the scattered flux offer first glimpses into spin-orbit state
dependent sticking coefficients in an intrinsically non-adiabatic process. For simplicity oflanguage, we simply express this spin-orbit ratio as an electronic “temperature,” where Telec is
calculated from with [2П3/2]/[2П1/2] = exp(Eso/kTelec) and Eso = 123 cm-1.52
This electronic temperature Telec is plotted in Figure 5.7 as a function of scattering angle,
θs at both low (2.7(9) kcal/mol) and hyperthermal (20(2) kcal/mol) collision energies. First of all,the spin-orbit temperatures) at low Einc (Telec  227(15) K, [2П3/2]/[2П1/2] ~ 0.45) are robustly
independent of final scattering angle. This is completely consistent with the observed angularinsensitivity in the NO rotational distributions noted above and is again consistent with angleindependent NO sticking coefficients in the corresponding gas-to-liquid interfacial collision
events. Secondly, and perhaps more surprisingly, these electronic temperatures are now
significantly colder than the surface liquid temperature (ΔT ≈ 66 K), which predicts there to be a
substantial sensitivity in sticking probabilities with spin-orbit quantum state. As there are only
vanishingly small concentrations of spin-orbit excited NO in the incident beam, the observationof [2П3/2] NO population in the scattered flux unambiguously signals the presence of
nonadiabatic “surface-hopping” dynamics in NO collisions at the gas-liquid interface.
With increased collision energies (Einc = 20(2) kcal/mol) and summing over all rotational
states and scattering channels, we observe increased opportunities for non-adiabatic excitation
into the spin-orbit excited state (Telec  325 - 425 K). Of particular dynamic interest, Telec
increases by as much as T  100 K between backward and forward scattering direction,equivalent to spin-orbit ratios of [2П3/2]/[2П1/2] ~ 0.58-0.66. Simply stated, ~ 15% more scattering
trajectories arise from spin-orbit changing interactions into forward scattering relative to155

Figure 5.7 Spin-orbit temperatures (estimated via [2П3/2]/[2П1/2]  exp(-Eso/kTelec), Eso = 123cm-1) as a function of scattering angle s at Einc = 2.7 and 20 kcal/mol. Note that at low collisionenergies, the spin-orbit temperature (Telec  227(15) K) is i) independent of θs and ii)
considerably colder than the gas-liquid interface (Ts = 293(1) K). At hyperthermal collisionenergies, the spin-orbit temperatures increase significantly for forward vs. backward scattering,consistent with greater inelastic spin-orbit excitation in the predominantly forward scattered IS
channel.

backward scattering angles. As noted previously,14 there is a higher propensity for spin-orbit
excitation in the impulsive scattering (IS) channel rather than the thermal desorption (TD)scattering pathway, which is of course consistent with an increase in spin-orbit excitation for fluxin the forward scattering half plane.

5.3.4 NO Scattering from Squalane and PFPE
The data thus far have all focused on NO scattering from a single room-temperature ionicliquid, [bmim][Tf2N]. In order to establish a broader context for these results, we now explore

156

similar angle-resolved trends in gas-liquid scattering dynamics with squalane (C30H62) andperfluoropolyether (PFPE) fluids, which in conjunction with the RTIL studies span a dramatic
range of surface compositions. The rotational and spin-orbit temperatures obtained at low Einc asa function of liquid and θs are presented in Table 5.1. All data have been obtained at Ts = 293 K
and θinc = 45º, with scattered molecules probed in the backward (θs = -60o), normal (θs = 0o), andforward (θs = 45o) scattered directions. Despite the wide range of liquid surface properties, theobserved Trot and Telec values for NO + squalane and NO + PFPE at low Einc are in surprisingagreement with NO + [bmim][Tf2N], indeed, essentially indistinguishable at all scattering angles
within experimental uncertainty. Of particular dynamical interest, the 21/2 rotational
distributions from all three liquids (see Table 5.1) are each distinctly colder (Trot = 259(9)K) than

Table 5.1 Angle-dependent rotational (21/2) and spin-orbit electronic temperatures from jetcooled NO (Einc = 2.7(9) kcal/mol) desorbing from three different gas-liquid interfaces. Note themuch colder rotational (Trot ≈ 259(9) K) and electronic (Telec ≈ 217(10) K) vs. liquid surface (TS≈ 293(1) K) temperatures, which unambiguously signal the presence of non-equilibrium, exit
channel dynamics in the desorption process. Note also the relative insensitivity of these results to
s, which, by detailed balance considerations, is consistent with a constant sticking coefficient as
a function of incident angle. Data for the excited spin-orbit state (23/2) reveal a similarinsensitivity to scattering angle, though with warmer values (Trot ≈ 284(9) K) approaching the Ts= 293(1) K surface temperature.

157

the actual liquid temperature (TS = 293(1)K), with an even cooler temperature for the spin-orbit
degree of freedom (Telec = 217(10)K)). This also is in agreement with previous studies18 of low
collision energy NO scattering from [Cnmim][Tf2N] RTILs, for which Trot at low Einc was foundto be surprisingly independent of the Cn alkyl chain length (n = 2, 4, 8, 12, and 16). As therotational and electronic distributions are consistent within experimental error over the threeliquids and range of scattering angles studied, the data constitute exceptionally strong support fora physical picture at low Einc of i) complete thermal accommodation and ii) subsequentdesorption from an equilibrium distribution of states at the gas-liquid interface.
As one obvious corollary, this implies that the out-of-equilibrium rotational andelectronic distributions in the asymptotic NO flux must arise from exit channel dynamics
occurring in the desorption process, which, by detailed balance considerations, predict a
rotational and spin-orbit dependence to sticking coefficients for the reverse adsorption event. In
the interest of completeness, we have also extended scattering studies on these liquids to highincident collision energies (Einc = 20(2)), with spin-orbit Telec, impulsive scattering TIS andsticking coefficient  summarized in Figure 5.8 as a function of scattering angle (s = -60o, 0o,45o) for squalane, PFPE and [bmim][Tf2N]. Once again, each of the three liquids exhibit verymuch the same trends, specifically with Telec and TIS gently increasing and  strongly decreasing
as a function of increasing scattering angle.

5.4 Discussion
5.4.1 Rotational State Dependence on θs
A key feature of the present experiment is the ability to obtain complete rovibronic
distributions at the gas-liquid interface as a function of θs and Einc. We exploit this further by

158

Figure 5.8 Comparison of (a) spin-orbit temperature (Telec), (b) impulsive scattering rotationaltemperature (TIS) and c) thermal accommodation coefficient () for hyperthermal NO scattering(Einc = 20(2) kcal/mol) from a range of low vapor pressure liquids: [bmim][Tf2N],
perfluoropolyether (PFPE) and squalane (C30H62). Note the overall agreement in trends for all
three liquids, despite substantial differences in chemical composition. There is also a systematic
decrease in α with θs for all three liquids, with 2-fold differences between forward (θs = 60o)and backward (θs = -60o) scattering of the NO.

159

probing the dependence of individual J state populations with scattering angle in Figure 5.9.
Here, the relative population of specific J states is extracted from normalized rotational
distributions at each θs. Under low Einc conditions (Figure 5.9(a)), which isolate the full thermal
accommodation (TD) channel, the rotational distributions are essentially flat with respect to final
scattering angle. This is not surprising and indeed reflects a similar message equivalent to thesequence of parallel Boltzmann plots shown in Figure 5.3. At higher Einc (Figure 5.9(b)),
however, the net rotational distributions do vary significantly with scattering angle. This echoes
the data presented in Fig. 5.6(b), but requires no additional simplifying assumptions of a twotemperature Boltzmann model. For hyperthermal scattering into low J states, (J =1.5 – 19.5), therelative rotational state populations are again very nearly flat with respect to θs. This is consistent
with NO signals from these lower J states being dominated by trapping-desorption (TD) events,
which should reflect a fully thermally accommodated rotational distribution. As final NO
rotation increases (J  24.5), however, the populations become increasingly peaked in theforward direction. The data specifically confirm that maximally efficient translational torotational (T-R) energy transfer events at the gas-liquid interface result in forward scatteringcollisions, in agreement with physical intuition. The effects are quite significant: high J states are
as much as ~2-3 times more likely to be detected under forward (θs = 60º) vs. backscattering (θs= -60º) conditions. Fig. 5.9(b) only contains data for the manifold of spin-orbit changing
collisions ([2П1/2] → [2П3/2]), though equivalent trends are observed for the spin-orbit conserving([2П1/2] → [2П1/2]) collisions as well.

160

Figure 5.9 Polar plots of NO(23/2) rotational distributions as a function of final scattering
angle s. (a) At low Einc conditions with NO thermally desorbing from the surface, the NO
rotational distributions are relatively insensitive to θs. (b) At high Einc, however, the rotational
distributions evolve considerably with increasing θs, with a greater propensity for high J state
populations scattered in the forward direction.

161

5.4.2 Spin-orbit Dependence on θs
The spin-orbit ratio as a function of both scattered angle and total angular momentum, N,
are considered in Figure 5.10. Previously (see Figure 5.7), the spin-orbit temperature has beenplotted vs. θs at high Einc, summing over all rotational states. Here we focus on how the spinorbit state populations evolve explicitly with N, which represents the angular momentum
quantum number exclusive of electronic spin (N = (J+1/2) for 2П1/2 and N = (J-1/2) for 2П3/2). In
such a plot, the spin-orbit ratio, [2П3/2(N)]/[2П1/2(N)] has been appropriately multiplied by
N/(N+1) to correctly account for the respective 2J+1 degeneracies associated with each spin-

Figure 5.10 Spin-orbit ratios as a function of NO rotational quantum number and final
scattering angle under hyperthermal incident conditions. Note the strong dependence on
rotational quantum state, with a 2-fold increase in spin-orbit changing (21/2 → 23/2) vs. spinorbit conserving (21/2 →21/2) collisions over the N  0-5 range. A smaller but still quite
significant increase (20%) in spin-orbit ratio for forward vs. backscattered NO species is also
observed.

162

orbit state for the same N value. At high Einc and θs = 60º, the spin-orbit ratio increases withangular momentum (N) by more than two-fold , with [2П3/2]/[2П1/2] 0.30 at N 1 and[2П3/2]/[2П1/2]  0.70 by N  25. By way of comparison, equilibration with the liquid (293 K)
would correspond to a spin orbit ratio of [2П3/2]/[2П1/2] = 0.54. Thus, the lowest angularmomentum states (N < 10) leave the surface with sub-thermal electronic temperatures, while thehigher rotational states (N > 10) scatter with hyperthermal spin-orbit temperatures. Again, theselower rotational states are dominated by the TD channel, notably N ≌ 1 - 2, while the higher
rotational states predominately reflect the IS events. The ground spin-orbit state population getsto be low enough at N > 25, that the error on the ratio measurement makes comparisons difficult.
However, the N < 25 data show a clear increase in spin-orbit excitation with N. At backscattering
angles, θs = - 60º, these same trends are observed but with consistently smaller spin-orbit ratios.
These data also confirm spin-orbit excitation is more favored for forward vs. backward
scattering, which is to say there is an increased degree of electronic nonadiabaticity in theforward direction.
Even in these high N states, the two spin-orbit state populations do not fully equilibrate.
Studies by the McKendrick group found similar trends for OH formed by reactively scattered
O(3P) from hydrocarbon liquids,53 whereby the ground spin-orbit state was also found to bepreferred in the scattered flux (2П3/2 state is the ground state for OH). Furthermore, the ratio of
the excited spin-orbit state grew with N, but the two spin-orbit populations did not equilibrate atlarge N, as we also observe in in Figure 5.10. This is in contrast to experiments of NO scatteringfrom Ag(111) at Einc = 19.8 kcal/mol,54 where an increase in the spin-orbit ratio with N was
observed, however the spin-orbit ratio centered around unity at high N states (N > 20). These
differences in the spin-orbit ratio plateau at high J are likely a result of the silver crystalline163

surface facilitating spin-orbit excitation of NO via electron transfer mechanisms.51 Experimental
and theoretical work by Wodtke, Tully and coworkers on the NO + Au(111) system haveprovided strong evidence for nonadiabatic surface hopping dynamics via electron transfer at the
NO-metal interface.51, 55-58 The relative stability of NO- near a metallic vs. ionic liquid surface
leads to the differences in these spin-orbit excitation comparisons, though more theory is clearlyneeded on this subject to help elucidate the nonadiabatic dynamics occurring in these systems.

5.4.3 Strong Correlation Between <s> and Angular Momentum N
As a final topic, we take a more quantitative look at angular distributions in the scatteredNO flux under hyperthermal scattering conditions (Einc = 20 kcal/mol). As these angular
distributions are relatively broad, it offers more physical insight to express the data in terms of an
average scattering angle, <s>, as an explicit function of N (see Fig. 5.11). Note that such aparsing of the angular data by N is rigorous and does not require any model dependentassumptions concerning branching between TD vs. IS pathways. Furthermore, since all incident
projectiles are initially in the ground (N ≈ 0) rotational state, the final angular momentum N also
serves as a metric of the net torque delivered to the projectile by collision with the gas-liquidinterface. At low N values, the average scattering angle in Figure 5.11 approaches zero within
experimental uncertainty. Though no such assumptions are necessary for this analysis, thevanishing of <θs> at low N would be completely consistent with full thermal accommodationand subsequent desorption of NO, yielding a perfectly forward/backward symmetric, indeed,cos(θs) distribution centered around θs ~ 0º. Most intriguingly, however, the data in Figure 5.11
reveal a striking correlation between i) average scattering angle and ii) net torque experienced by
the NO, with equally good agreement for both spin-orbit conserving and spin-orbit changing

164

Figure 5.11 Average NO scattering angle <s> as a function of angular momentum quantum
number excluding electron spin (N) for both ground (2П1/2, N = J+1/2) and excited (2П3/2, N = J1/2) spin-orbit states. Scattering into low N states is dominated by thermal accommodation (TD),for which <θs> ≈ 0º is in excellent agreement with a cos(θs) distribution. For scattering into highN states, the IS channel is favored and the distribution tends toward specular scattering. Note theremarkably strong linear correlation between i) average angle <s> and ii) amount of torque (NN) delivered to the NO molecule, with a slope of m 0.5 degree per N. See text for a simple
physical interpretation of the dynamics.

manifolds. More quantitatively, <θs> increases remarkably linearly with N up to N  20, with anapproximate slope of 0.5 degree per ħ transferred in the gas-liquid collision. Furthermore, there
appear to be some indications of saturation (<θs>max  15º - 20º) in this average angle above N 
25.
Though a detailed analysis of such a correlation is beyond the scope of the present work,
we note that this represents an ideal target for further exploration with gas-liquid molecular165

dynamics simulations, toward which we defer more quantitative attention in a later venue.
However, we can offer the following preliminary observations. To start, all qualitative trends
observed in the data are completely consistent with the microscopic branching, two-temperaturemodel presented above. For example, the NO flux desorbing into low angular momentum states
will tend to be dominated (though not completely) by TD scattering dynamics,48 for which wewould expect perfectly forward/backward symmetric scattering with an average value of <s> 
0, as observed experimentally. Furthermore, for this two-temperature model, the NO flux into
intermediate N values and rotational energies will accumulate progressively more from the IS
scattering channel, which will tend to tilt the average scattering angle in the forward scattering
direction and therefore predict a systematic increase in <s>. Indeed, in the limit of high N androtational energy, the two-temperature model contribution from IS scattering must rigorously
become predominant. Since IS behavior is anticipated to be strongly forward scattered into thenear specular direction, this would even predict the onset of saturation effects in average angle as
a function of N, toward which the present data already offer strikingly good qualitative
agreement with experiment.
As a parting comment, we take this one step further with a very simplified physicalmodel for collisional torques acting at the gas-liquid interface. Specifically, the microscopicallyrough liquid surface provides a source of friction for exerting unbalanced torques on theincoming NO projectile over some small duration of collisional time tcoll. The net angularmomentum accrued by the NO molecule is directly reflected in the final N value, which in asemiclassical picture would translate into an asymptotic rotational frequency of   2BNON. As azeroth-order picture, one might crudely estimate the net forward rotation () of themolecule-fixed NO frame during the gas-liquid collision event by   tcoll  2BNON tcoll,

166where the collision duration (tcoll) is the net time for application of any surface torques. Thoughmore theoretical consideration will be necessary to explore this connection further, it is worth
noting that even such a simple heuristic model does correctly predict a linear scaling of theaverage scattering angle with the final angular momentum quantum number N, as indeed
experimentally observed.

5.5 Conclusion
The dynamics of NO colliding at liquid surfaces has been investigated as a function of
final scattering angle and with quantum state-to-state and final scattering angle resolution.
Specifically, rovibronic populations of scattered NO have been explored as a function of i)scattering angle (θs = -60(3)º, -30(9)º, 0(12)º, 30(9)º, 45(6)º and 60(3)º), ii) incident collisionenergy (Einc = 2.7(9) and 20(2) kcal/mol), and iii) liquid composition ([bmim][Tf2N], squalane,
and PFPE). The extensive data set firmly supports the conclusion that at low collision energies(Einc = 2.7(9) kcal/mol), all incident NO molecules trap, thermalize and desorb from the surface
for all three liquids. Furthermore, the angular studies indicate that the NO molecules desorb fromthe surface in a cos(θs) distribution and with rotational temperatures independent of θs. These
results are in agreement with detailed balance expectations based on uniform (but not necessarily
unity) sticking coefficients for a given internal NO quantum state as a function of incident angle
and speed. However, this simple dynamical behavior in the angular coordinate is clearly not thecase for internal rotational and electronic quantum states, which reveal rotational distributions in
the asymptotic NO flux systematically colder (Trot (21/2)  257(10), Trot (23/2)  284(10)) thanthe liquid surface temperature (Ts = 293 K) and colder still (Telec  227(15)) in the corresponding
spin-orbit electronic temperatures. These trends have been quantitatively confirmed for NO

167

scattering from a variety of liquid systems ranging from completely ionic liquids (RTIL) toliquids with strongly polar bonds (PFPE) to fully non-polar saturated hydrocarbons (squalane),
which yield the same rotational and electronic temperatures for the three liquids within
experimental uncertainty. By detailed balance, this novel behavior, i.e., the desorbing NO
angular distributions are purely cos(s) whereas the internal quantum state distributions are
significantly out of equilibrium with the surface temperature, rigorously implies clearly non-unity
sticking coefficients for NO and thus the presence of dynamical barriers to adsorption as afunction of rotational and/or spin-orbit quantum state. As changes in the internal spin-orbit state
reflect non-adiabatic surface hopping events between two electronic surfaces, theadsorption/desorption dynamical barriers of such NO collisional events at the gas-liquid interface
are richly complex and deserve further detailed theoretical attention. These results provide
insight into the fundamental interactions between NO and liquid surfaces, for example, slowly
vs. rapidly rotating NO molecules are more likely to trap on liquid surfaces, which helps build
toward a more predictive kinetic understanding of collision and solvation dynamics at gas-liquidinterfaces relevant for environmental applications.
By way of comparison, we have also explored angle dependent rotational/spin-orbit
scattering dynamics as a function of incident beam energy. Whereas the rovibronic quantum state
distributions are clearly quite insensitive to θs at low Einc, at higher collision energies, themolecules are far more likely to scatter in the forward direction at more nearly specular angles
and into correspondingly higher rotational states. A two-temperature trapping desorption(TD)/impulsive scattering (IS) Boltzmann fit to the rotational distributions at high Einc is quiteconsistent with this picture, which accounts for more molecules impulsively scattering into
forward angles and with  3-4 fold hotter rotational temperatures in the IS vs. TD channels.

168

Conversely, the fractional branching coefficient ( between trapping desorption and impulsive
scattering pathways clearly depends quite significantly on the choice of scattering angle, varying
by nearly two-fold between forward and backward scattering directions. The spin-orbit ratios of
the hyperthermal scattered NO are also found to depend sensitively on final rotational state (twofold increase between N = 0-25) with a clear propensity for higher spin-orbit excitation in the IS
channel and for forward (θs = 60o) vs. backward (θs = -60o) scattering. As such data highlight theeffects of hyperthermal collision energy on electronic surface hopping events, this provides
particular incentive for further theoretical exploration of such non-adiabatic, state-to-statecollision dynamics at the gas-liquid interface.

169

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Y. Huang, A. M. Wodtke, H. Hou, C. T. Rettner and D. J. Auerbach, Phys. Rev. Lett.,
2000, 84, 2985-2988.

172

57.

Y. H. Huang, C. T. Rettner, D. J. Auerbach and A. M. Wodtke, Science, 2000, 290, 111114.

58.

A. M. Wodtke, Y. H. Huang and D. J. Auerbach, J. Chem. Phys., 2003, 118, 8033-8041.

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Chapter 6

Incident Collision Energy Dependence of NO
Scattering Dynamics at the Gas-RTIL Interface

6.1 Introduction
In this chapter unpublished results are presented for incident collision energy dependence
on NO scattered from the gas-[bmim][Tf2N] interface. Previous molecular and atomic scatteringexperiments from a variety of low vapor pressure liquids,1-4 in addition to nitric oxide scatteredfrom crystalline surfaces,5-7 have explored atomic/molecular collisional energy dependence.
These experimental results have provided evidence for thermal desorption (TD) and impulsive
scattering (IS) pathways; at low collision energies all incident molecules trap on the surface, incontrast to increasing collisional speeds where more molecules inelastically scatter. Stated more
broadly, a decrease in sticking coefficients is observed at higher incident collision energies.
The molecular scattering experiments presented in this chapter, which examine thecollision energy dependence of NO at liquid surfaces, are largely motivated by the prediction ofrotational and spin-orbit dependent sticking probabilities, as discussed in Chapters 3.3.1 and5.3.1. These results suggest the possibility of translational energy dependent NO sticking
coefficients on liquid surfaces. In order to investigate this theory, experiments have been
performed with a series of molecular beams at low incident collision energies (Einc = 2.0 – 4.0
kcal/mol) where the thermal desorption channel is isolated. Scattering experiments are also

174

performed for a series of increasingly higher collision energies (Einc = 4.9 – 20 kcal/mol) to
explore how the microscopic branching ratio between the TD and IS scattering pathways are
impacted, as well as the degree of excitation into molecular rotational and spin-orbit degrees of
freedom.

6.2 Experimental
A complete description of the experimental apparatus can be found in Chapter 2. The
speed of the incident molecular beam is varied by changing the identity/mass of the buffer gas inwhich NO is seeded. Gas mixtures of 1% NO are seeded in Ne, He, and H2, which generate
collision energies of 2.0(2), 11.1(7) and 20(2) kcal/mol, respectively. These kinetic energies
correspond to molecular speeds ranging from 7.5 x 104 to 2.4 x 105 m/s. Buffer gases are also
mixed in specific concentrations to create intermediate incident beam energies. In principle,
heavier buffer gases, such as Ar and Kr, could be used to achieve even slower molecular beam
speeds, however, these mixtures result in low LIF signal levels, likely due to cluster formation in
the molecular beam. All molecular scattering in this chapter is from a 293 K [bmim][Tf2N] RTIL
surface. Incident supersonically cooled NO(2П1/2, J = 0.5) molecules approach at θinc = 45º andscattered molecules are detected via LIF at specular scattering angles, θs = 45(6)º.

6.3 Results and Analysis
LIF spectra of scattered NO (v=0) molecules are shown in Figure 6.1 for incident
collision energies Einc = 2.7 and 20 kcal/mol. Significant spin-orbit and rotational excitation are
observed at both collision energies, although Figure 6.1 reveals that the LIF spectra arising fromscattering at faster incident speeds extend to higher wavenumbers (~44800 cm-1), which175

correspond to larger populations in higher rotational states (up to J = 50.5). This understandably
indicates that more energy is transferred from the translational to rotational degree of freedom athigher collision energies when there is more translational energy available to the system. The
LIF signal at lower frequencies (44050 to 44300 cm-1), and correspondingly lower J states, isalso stronger for faster incident speeds (ranging from 10 - 50% more for Einc = 20 vs. 2.7
kcal/mol) due to the specular scattering detection geometry of the experiment. Specifically, atlower collision energies all molecules are expected to thermalize on the surface and desorb innear cos(θs) distributions; consequently a small fraction of these molecules are directed toward

Figure 6.1 LIF spectra of NO scattered from a Ts = 293 K [bmim][Tf2N] surface at incident
collision energies Einc = 2.7 and 20 kcal/mol. Decreased rotational excitation coupled with asmaller LIF signal is observed at lower collision energies, where all molecules are thermallydesorbing from the surface in a cos(θs) distribution with colder rotational temperatures. At higher
incident speeds, however, more molecules impulsively scatter into near specular angles (i.e.,
toward the detection region) with hyperthermal rotational excitation.

176

the LIF detection volume at θs = 45(6)º. At faster incident speeds, however, increasingly more
molecules impulsively scatter into near specular angles, i.e., toward the LIF detection region,
which results in larger LIF signals at θs = 45(6)º relative to the low Einc case where all molecules
are thermally desorbing from the surface in a cos(θs) distribution.
The corresponding rotational distributions derived from the LIF spectra in Figure 6.1 aredisplayed in Figure 6.2 for the ground 2П1/2 spin-orbit state, as well as those for two intermediate
incident energies (Einc = 11 and 15 kcal/mol). The population distributions at each collisionenergy are normalized to unity so that the relative distribution of J states populated can becompared between different incident molecular beam conditions. As also predicted from the LIF

Figure 6.2 Scattered NO rotational distributions for varied Einc, where each population
distribution is normalized to unity and from the ground (2П1/2) spin-orbit state. At faster incident
speeds molecules scatter into relatively higher J states, revealing significantly more translationalto-rotational energy transfer than at lower Einc.

177spectra in Figure 6.1, these data reveal that lower collision energies result in fractionally more
molecules scattered into low rotational states, whereas higher collision speeds result inpreferential population of high rotational states. The spin-orbit excitation of NO as a function ofcollision energy is considered in Figure 6.3. All incident NO molecules are prepared in the
ground 2П1/2 spin-orbit state, thus scattered NO molecules detected in the 2П3/2 state represent
spin-orbit changing collisions. As seen below in Figure 6.3, spin-orbit excitation of NO scatteredfrom a RTIL liquid surface grows with increasing collision energy. As discussed in Chapter 5,
spin-orbit excitation is more favored in the IS channel relative to the TD scattering pathway.
Therefore, as more molecules impulsively scatter as Einc increases, spin-orbit excitation in the
scattered flux also increases.

Figure 6.3 Spin-orbit excitation of NO as a function of incident collision energy. Note theincrease in spin-orbit changing collisions (2П1/2 → 2П3/2) as Einc increases, with electronic
temperatures colder than the Ts = 293 K liquid at low Einc and above Ts at high Einc.

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6.3.1 Isolating Thermal Desorption at Low Collision Energies
Rotational distributions from low incident energy scattering are displayed in a Boltzmann
plot in Figure 6.4, where the populations of each rotational state are normalized to theirrespective 2J+1 degeneracy and plotted on a log scale. At low collision energies (Einc = 2.0 – 4.0
kcal/mol) there is no clear evidence for dual temperature scattering behavior (i.e., microscopic
branching) and thus the rotational distributions are fit to single temperatures, which arecharacteristic of 100% sticking coefficients. These rotational temperatures, particularly for the2

П1/2 state, are considerably cooler than the liquid surface (Ts = 293 K) and largely independentof Einc.

Figure 6.4 Boltzmann rotational distributions for scattered NO at Einc = 2.0 – 4.0 kcal/mol, withcorresponding single temperature fits representative of complete thermal accommodation at thesurface. All scattering takes place from a 293 K [bmim][Tf2N] surface at θinc = 45º and θs =45(6)º.

179

In Figure 6.5, the single temperature rotational Boltzmann fits are plotted for both spinorbit states at Einc = 2.0, 2.7, 3.3, and 4.0 kcal/mol. The 2П1/2 rotational temperatures are
consistently ~30-40 K cooler than the 293 K surface temperature, while Trot(2П3/2) is closer toand often within experimental uncertainty of the surface temperature. The rotationaltemperatures for each spin-orbit state are largely insensitive to collision energy in this range,
consistent with there being no translational energy dependence to the sticking probability at thesurface for the range of low collision energies sampled. These results combined with the angleresolved data presented in Chapter 5.3.1 indicate that (i) at low incident energies (Einc = 2.0 –Figure 6.5 Single temperature Boltzmann fits for NO scattered at low collision energies (Einc =2.0 – 4.0 kcal/mol), where all molecules thermalized and subsequently desorbed from the liquidsurface. The rotational temperatures are largely unchanged as Einc is varied, consistent with notranslational energy dependence to the sticking coefficient at low Einc.

180

4.0 kcal/mol), all scattered NO molecules follow a thermal desorption scattering pathway,(ii) there exist rotational and spin-orbit state dependent sticking coefficients that yieldrotational/electronic cooling of the desorbing flux, and (iii) the surface sticking probabilities do
not depend on incident translational energy at low Einc. These conclusions are consistent withadsorption/desorption dynamical barriers that are dependent on angular orientation and give riseto quantum state dependent sticking coefficients.

Figure 6.6 Boltzmann rotational distributions for scattered NO at hyperthermal collisionenergies from Einc = 4.9 to 20 kcal/mol. Two-temperature fits to the data are also plotted, which
represent rotational excitation into the TD and IS scattering pathways as well as the branching
between those two channels. Here the colder component (TTD) is fixed at Trot(2П1/2) from low Eincscattering results where the TD channel is isolated.

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6.3.2 Branching into Multiple Scattering Pathways at High Collision Energies
The rotational distribution of scattered NO at higher collision energies (Einc = 4.9 - 20
kcal/mol) are analyzed in a Boltzmann plot in Figure 6.6. Unlike at low Einc, these rotational
distributions exhibit significant curvature, which can be well fit to a double exponential. This
two-temperature model physically corresponds to the trapping-desorption (TD) and impulsive
scattering (IS) pathways, as discussed in more detail in Chapter 3.3, 4.3 and 5.3. These rotational
Boltzmann fits displayed in Figure 6.6 consist of three parameters: (i) the colder temperature,
TTD, corresponds to the thermally desorption scattering pathway, (ii) the hotter temperaturecomponent, TIS, describes the impulsive scattering pathway, and (iii) α represents the branchingratio into the TD channel. Both TIS and α are floated, while TTD is fixed at the value of Trot foreach spin-orbit state from low collision energy scattering (Trot(2П1/2) ≈ 260 K and Trot(2П3/2) ≈285 K , Figure 6.4) where the TD component is isolated.
The branching ratio (α) from these two-temperature fits is plotted as a function ofincident collision energy in Figure 6.7. At each of the low collision energies (2.0 to 4.0
kcal/mol), α is fixed at unity (corresponding to single temperature fits in Figure 6.4). However,as collision speeds are increased beyond 4 kcal/mol, α decreases monotonically. This decrease in
α with Einc is consistent with NO preferentially impulsively scattering from the surface at higher
incident speeds, as observed in previous studies.1-4, 6, 8 Furthermore, there appears to be someminimum translational energy required (i.e. ≈ 4.0 kcal/mol) to scatter inelastically instead of
100% trapping-desorption on the surface. For NO to trap on the liquid surface, we can imagine ascenario where an approaching NO molecule must transfer some fraction of incident translationalenergy to the liquid. Thus for larger Einc, NO colliding with the surface has increasingly moreenergy to dispose and with which to escape the liquid surface before thermalizing. The results

182

Figure 6.7 Thermal accommodation coefficient, α, as a function of collision energy. At thelowest collision energies (Einc = 2.0 - 4.0 kcal/mol) α is fixed at unity, whereas all otherbranching ratios are from two-temperature Boltzmann fits to rotational distributions of scattered
NO. These data reveal a decrease in α with Einc, i.e. more molecules impulsively scatter at higher
incident speeds.

presented in this chapter suggest that this threshold would be just above Einc = 4 kcal/mol for the
NO + RTIL system studied.
The impulsively scattered rotational temperatures are summarized in Figure 6.8 andexhibit a nearly linear increase with collision energy. These rotational temperatures are displayedfor both spin-orbit states of NO scattered from [bmim][Tf2N], in addition to previous resultsfrom the Nesbitt group of CO2 scattered from perfluoropolyether (PFPE).4 Despite thedifferences in these molecular systems, the two data sets reveal similar efficiencies oftranslational-to-rotational (T-R) energy transfer at each gas-liquid interface studied.

183

Hyperthermal rotational temperatures over a range of ~ 500 – 1200 K are observed in the IS
channel, which correspond to ~1.0 – 2.5 kcal/mol (where Erot(IS) = 2/2 kTIS). At Einc = 4.9
kcal/mol, just under 25% of the incident collision energy is converted into rotational excitation in
the IS channel. This fraction decreases with increasing incident speeds down to ~11% oftranslation-to-IS rotation energy conversion occurring at the largest incident energy studied (Einc= 20 kcal/mol). These results are consistent with numerous other molecular scatteringexperiments from liquid, crystalline and self-assembled-monolayer surfaces, which also reveal
an increase in T-R energy transfer (and a decrease in factional energy transfer) with increasing
collision energy.4-10

Figure 6.8 Incident collision energy dependence on IS rotational temperature. Data are shownfor both spin-orbit states of NO scattered from [bmim][Tf2N], as well as CO2 scattered from
PFPE,4 which reveal similar efficiencies and trends of translational to rotational energy transfer.

184

References for Chapter 6

1.

M. E. Saecker, S. T. Govoni, D. V. Kowalski, M. E. King and G. M. Nathanson, Science,
1991, 252, 1421-1424.

2.

M. E. Saecker and G. M. Nathanson, J. Chem. Phys., 1993, 99, 7056-7075.

3.

M. E. King, M. E. Saecker and G. M. Nathanson, J. Chem. Phys., 1994, 101, 2539-2547.

4.

B. G. Perkins and D. J. Nesbitt, J. Phys. Chem. B, 2006, 110, 17126-17137.

5.

A. W. Kleyn, A. C. Luntz and D. J. Auerbach, Phys. Rev. Lett., 1981, 47, 1169-1172.

6.

G. D. Kubiak, J. E. Hurst, H. G. Rennagel, G. M. McClelland and R. N. Zare, J. Chem.
Phys., 1983, 79, 5163-5178.

7.

J. Kimman, C. T. Rettner, D. J. Auerbach, J. A. Barker and J. C. Tully, Phys. Rev. Lett.,
1986, 57, 2053-2056.

8.

C. H. Hoffman and D. J. Nesbitt, J. Phys. Chem. C, 2016, 120, 16687–16698.

9.

M. P. Ziemkiewicz, J. R. Roscioli and D. J. Nesbitt, J. Chem. Phys., 2011, 134, 234703.

10.

K. R. Lykke and B. D. Kay, J. Chem. Phys., 1990, 92, 2614-2623.

185

Chapter 7

State-to-State Resolved Scattering of NO from Molten Gold and Gallium

7.1 Introduction
Electronically nonadiabatic interactions have been investigated extensively in molecularcollisions with solid metallic surfaces,1-10 yet there has been little study of these molecular detailsof energy transfer between gas phase molecules at molten metal surfaces. The conduction bandof a metal consists of a continuum of infinitesimally spaced electronic states, from which an
electron hole pair can be formed when an electron is excited above the Fermi level, then creatinga hole in the conduction band. Because of the continuum of electronic states present, theseelectron hole pairs can be resonant with any rovibronic excitation of an incident molecule on ametallic surface, thereby presenting a potential avenue for efficient energy transfer between themetal and gas molecule. This electron hole pair mediated rovibronic excitation of incident gas
molecules is an intrinsically nonadiabatic electronic process, whereby electronic degrees of
freedom in the metal couple to vibrational degrees of freedom in the adsorbate.
A number of experiments have studied how nitric oxide transfers energy from a variety ofsolid crystalline surfaces, which have provided evidence for electron hole pair mediatedvibrational and electronic energy transfer between NO and metal surfaces. For example,
vibrationally excited NO(v = 15) has been scattered from Au(111) with relaxation of ~ 8

186vibrational quanta observed in the scattered flux.9 Conversely, when NO(v = 12) was scatteredfrom an insulting LiF surface,7 vibrational relaxation was almost entirely absent, possibly due tothe lack of resonant electron hole pair states with the incoming NO molecule. These results
provide strong evidence that the metal surface is responsible for multiquanta vibrational
relaxation. Additional experiments investigated electron emission from surfaces of Au(111)covered in Cs, which lowered the work function of the surface down to 1.6 eV.10 NO molecules
were directed at the surface with tunable incident vibrational states, which yielded detection ofelectron emission only when the incident NO vibrational energy was greater than the work
function of surface. These results provide strong evidence for coupling of molecular vibrationalenergy to surface electrons in the metal, which represents a fundamental breakdown of the BornOppenheimer approximation.
Numerous experiments dating back to the mid-1980’s have observed vibrational
excitation of NO scattered from solid crystalline surfaces.1, 11-16 These experiments havedemonstrated Arrhenius behavior of vibrational excitation probabilities with surface
temperature,1, 11, 12, 16 where the activation energy is equal to the vibrational energy spacing ofNO. This behavior has been shown to be characteristic of strong coupling between NO and themetallic surface where electron hole pair formation in the metal couples to vibrational excitationof scattered NO molecules. This behavior has become well observed and established forscattering from solid crystalline surfaces, however, in all previous scattering experiments frommolten metal surfaces,17-21 vibrational excitation collision dynamics have not been reported.
The experiments in this chapter not only represent first quantum state resolved scatteringfrom liquid gold, but also first reported vibrational excitation of scattered molecules from amolten metal surface. We investigate NO scattering dynamics from liquid metals as a function of187

(i) metal (Ga and Au), (ii) collision energy (Einc = 2 and 20 kcal/mol) and (iii) surface
temperature (Ga, Ts = 600 – 1000 K). These studies further explore electronically nonadiabatic
dynamics at the gas-metal interface with surface temperatures up to Ts = 1400 K for Au and Ts =1000 K for Ga, which is hotter than has previously been studied for these systems.17, 18, 20 Theseexperiments show first results of vibrationally excited NO from collisions with molten Ga andAu surfaces, as well as evidence suggestive of energy transfer between electron hole pairs in theliquid metal with the NO vibrational degree of freedom. This electronically nonadiabatic
behavior has been well studied for NO + crystalline surfaces, but has not previously been
observed from liquid metal surfaces.

7.2 Experimental
The gas-liquid scattering apparatus has been described in detail previously.20, 22 Here we
provide a brief description of the setup while highlighting experimental changes made to theheating system that enable study of molten metals up to temperatures Ts = 1400 K. In summary,supersonically cooled NO(2П1/2, N = 0) is scattered from molten Ga and Au metal surfaces withquantum state resolved detection. A molecular beam of rotationally and electronically cooled NO
is produced from an Even-Lavie valve, with all molecules cooled to the ground 2П1/2 spin-orbit
state and Trot ~ 1 K. The NO is seeded in a buffer gas of either Ne or H2 (1:99 ratio), which yield
incident collision energies of 2 and 20 kcal/mol (0.09 and 0.87 eV), respectively. The molecularbeam passes through a 3 mm skimmer, creating a collimated beam that strikes the liquid surface
at an angle of 45º with 1º half angle divergence. The molten metal and scattering apparatus
reside in a 90 L stainless steel vacuum chamber that is pumped on by a 1500 L/s turbomolecularpump (base pressure ~ 2 x 10-8 Torr).

188

The liquid metal sits in a tungsten crucible with an alumina coating along the edge toprevent wetting (S21-AO-W, R.D. Mathis). This boat is resistively heated by a 4kW power
supply (LV400, R.D. Mathis), which is connected by a series of cables, feedthroughs and copperbars with low resistivities to minimize heat loss and maximize power delivered to the tungsten
boat (~ 300 W required to heat a 7 mΩ crucible to 1400 K). More specifically, the power supply
is connected to two 1” solid Cu feedthroughs (R.D. Mathis, RDM-FT-400) that are each attached
to two 1” x 1/8” Cu bars (oxygen free Copper, alloy 101) that rise 8” above the feedthroughs andsandwich the tungsten boat, overlapping 1” of the boat on each side.
The scattering apparatus and heating system are pictured in Figure 7.1. The reservoir is
3.8 cm x 1.3 cm, i.e. sufficiently larger than the 0.8 cm x 1.1 cm spot size of the incident beam

Figure 7.1 Molecular scattering apparatus. Supersonically cooled NO (2П1/2, N=0) is scatteredfrom molten Au (pictured) and Ga, which sit in a resistively heated tungsten crucible and can beheated up to Ts = 1500 K. Scattered NO molecules are detected with quantum state resolution via
LIF.

189

hitting the surface. Both Ga (99.9999%, Acros Organics) and Au (99.95%, Goodfellow) arestudied in the present work. Metal surfaces are cleaned by 30 minutes of Ar+ sputtering at 2kV
while heating the surface prior to each data scan. The temperature is measured by a Type K
thermocouple on the bottom of the tungsten boat, which has been calibrated against
measurements from (i) an optical pyrometer and (ii) a thermocouple submerged in the liquid
metal. The experimental upper temperature limit of these metals is constrained by their vapor
pressures at high temperatures. Gallium (Tmelt = 295 K) is studied from Ts = 600 –1000 K whilegold (Tmelt = 1337 K) is studied at a single liquid temperature 1400(40) K, i.e. above
experimental uncertainty such that the sample is fully melted, but cool enough to maintain low
vapor pressures above the liquid surface to ensure a single collision environment for NO
scattering.
Scattered NO molecules are detected with quantum state resolution via laser induced
fluorescence (LIF) on the γ band, A2Σ+(v = 0) ⟵X2П(v = 0, 1, 2). A pulsed UV light source(223-246 nm) is produced from the tripled output of a Nd:YAG pumped dye laser laser operatingwith LDS698 dye. The laser beam passes 1.6 cm above the liquid surface and in the plane of
specular scattering with the laser polarization parallel to surface normal. The fluorescence of
scattered molecules is imaged onto a photomultiplier tube (PMT), but only for a 13 mm3 volume
along the laser path due a 1:1 confocal lens imaging setup with a 4 mm pinhole mask in front of
the PMT. This results in detection of specularly scattered molecules at θs = 45(6)o. For theseexperiments, a cone is placed in front of the imaging lens to block direct line of sight from theliquid surface to the imaging lens, thus keeping the lens clear of metal deposition for LIF
detection; moreover, this cone still allows all photons of interest that originate from the detection
volume along the laser beam path to be collected by the PMT. A UV bandpass filter sits in front190

of the PMT to block incident laser light while maximizing fluorescence gathered. Eachvibrational band is detected at a different wavelength range, therefore different UV filters werenecessary to block the wavelength of each incident laser beam: UG5, Thorlabs for NO(v = 0);
XUV0325, Asahi Spectra for NO(v = 1); UG11, Thorlabs for NO(v = 2). All comparisons
between vibrational states correct for the varying fluorescence transmission spectra of thesefilters. The PMT signal is integrated for 200 ns at the peak of scattered NO signal in time andnormalized to the laser power for each pulse.

7.3 Results
Sample LIF spectra for scattered NO(v = 0) and NO(v = 1) are displayed in Figure 7.2,
where fluorescence is gathered from the X2П(v” = 0, 1) ⟵ A2Σ+(v’ = 0) transition. The spectra
are fit to a least squares model from which rovibronic populations are collected.20, 23 This data
analysis has been confirmed with room temperature rotational and spin-orbit distributions of

Figure 7.2 LIF spectra of scattered NO(v” = 0, 1) scattered from Au(l) (Ts = 1400 K) withfluorescence from the X2П(v”) ⟵ A2Σ+(v’ = 0) transition.

191

thermalized NO in the vacuum chamber. The data in Figure 7.2 represent scattering of NO (Einc= 20 kcal/mol) from a Ts = 1400(40) K liquid gold surface. Only specularly scattered molecules
are detected, where θinc = θs = 45º. All incident NO molecules directed at the molten metal
surface are cooled into the ground vibrational state, thus all NO(v = 1) detected result fromvibrationally exciting collisions at the gas-metal interface. Significant rotational and spin-orbit
excitation of NO by the metal surface is also observed by the large number of peaks within each
vibrational state. The substantial energy transfer into spin-orbit and vibrational degrees of
freedom indicate nonadiabatic, surface hopping collision dynamics occurring at the NO-Au(l)interface.

7.3.1 NO scattered from Au(l)A vibrational temperature of NO scattered from Au(l) can be extracted by integratingover all rotational and spin-orbit populations within each vibrational state via [v = 1]/[v = 0] =exp[-ΔEvib/(kTvib)], where the vibrational energy spacing between NO(v = 0) and NO(v = 1) is
ΔEvib = 1876 cm-1. All populations are appropriately corrected for relevant differences in FranckCondon factors and fluorescence transmission of the UV filter. A two-point Boltzmann plot in
Figure 7.3 illustrates the vibrational temperature calculation with a linear fit corresponding to
Tvib = 708(35) K, i.e. significantly cooler than the 1400(40) K liquid gold surface. Thistemperature is equivalent to a vibrational excitation probability for (NO(v = 0 → 1) of ~ 0.019.
LIF detection of NO(v = 2) was attempted on the X2П(v” = 2) ⟵ A2Σ+(v’ = 0)transition, however, no peaks were detected in this region. The signal was integrated over therange of expected signal to determine an upper limit of [v = 2]/[v = 0] ≤ 1.5 x 10-3. This LIF
detection limit is indicated on Figure 7.3 by the gray dashed line. As illustrated in Figure 7.3, an

192

Figure 7.3 Vibrational Boltzmann plot of NO scattered from Au(l). The natural log ofNO(v = 0) and NO(v = 1) populations are plotted against vibrational energy. The slope of thelinear fit corresponds to a vibrational temperature of Tvib = 708 (35) K, from the equation[v = 1]/[v = 0] = exp[-ΔEvib/(kTvib)]. No population in the v = 2 state was detected, although an
extrapolation of the fit shows the expected population below the LIF detection limit of thisexperiment, i.e. [v = 2]/[v = 0] ≤ 1.5 x 10-3.

extrapolation of the linear fit from the [v = 0] and [v = 1] populations reveals the expected[v = 2] population to lie below the detection sensitivity of this experiment.
We now consider the rovibronic distributions of NO scattered from Au(l). In Figure 7.4,
rotational Boltzmann distributions of NO(v = 0) are displayed for scattering at low (Einc = 2
kcal/mol) and high (Einc = 20 kcal/mol) collision energies, where the ln[Population(J)/(2J+1)] is
plotted vs. rotational energy. These rotational distributions also exhibit nonlinear curvature andare fit to double exponential or two-temperature fits. At higher collision energies (Einc = 20

193

kcal/mol), the rotational distribution fit yields temperatures of Tcold = 360(30) K and Thot =1420(80) K, i.e., within uncertainty of the 1400 K surface temperature. At Einc = 2 kcal/mol,slightly colder temperatures of Tcold = 170(40) K and Thot = 830(20) K are observed. These data
are from the excited 2П3/2 spin-orbit states, which exhibit the same trends/quantities observed
from the ground 2П1/2 spin-orbit state; moreover, since the incident beam is populated only by
the 2П1/2 spin-orbit state, the colder component of these 2П3/2 rotational distributions is clearly
not due to incident beam contamination.

Figure 7.4 Rotational Boltzmann plot of scattered NO(v = 0) from Au (TS =1400 K) at Einc = 2
and 20 kcal/mol. Two-temperature fits to both rotational distributions are also displayed.

In previous molecular scattering experiments from liquid surfaces, a two-temperature fit
has been used to describe a colder, thermal desorption (TD) scattering pathway, with rotationaltemperatures near the surface temperature, and a rotationally hotter, impulsive scattering (IS)194

channel. At lower surface temperatures, this physically motivated model has been useful incharacterizing rotational distributions. However, at higher surface temperatures, i.e. Ts = 1400 K,
while the rotational distributions are well fit a two-temperature model, the rotationaltemperatures may not necessarily lend themselves to a simple TD/IS scattering picture. The
reason for this is due to a combination of high temperatures and weak gas-liquid surface
interaction strengths. Thus, the colder rotational temperatures derived from these fits could
represent weaker interactions of NO molecules with the molten metals surfaces, whereby NO
scatters away with little rotational excitation. Meanwhile, in the high Einc case, the rotationallyhotter NO molecules may either thermally accommodate with the Au surface in the rotational
degree of freedom or the excess of incident translational energy may be excited into rotation in
the collisional process.
In order to investigate potential rotational accommodation with the liquid surface further,
we now consider the rotational distributions of vibrationally excited NO(v = 1) in Figure 7.5,
again for incident collision energies at both Einc = 2 and 20 kcal/mol. There is considerably more
scatter in these rotational populations relative to NO(v = 0) as the NO(v = 1) state is ~50 times
less populated, i.e., [v = 1]/[v = 0] = 0.019. These rotational distributions are fit to a single
temperature, yielding equivalent rotational temperatures at both collision energies: Trot = 980(70)K at high Einc and Trot = 970(70) K at low Einc. Especially striking is the observation ofvibrationally excited NO at Einc = 2 kcal/mol, where the collision energy is less than thevibrational energy spacing, i.e. Einc = 700 cm-1 < ΔEvib = 1876 cm-1, and therefore insufficient toexcite NO vibrations. This suggests an excitation process dominated by thermal electron holepair formation in the metals that is insensitive to incident collision energy. While phonon modesin the metal have also been considered to vibrationally excite NO, they are an unlikely source as

195

Figure 7.5 Rotational distributions of vibrationally excited NO(v = 0 → 1) from collisions with
an Au(l), 1400 K surface for Einc = 2 and 20 kcal/mol. Single-temperature fits reveal similarrotational temperatures at both collision energies that are ~ 400 K cooler than the liquid gold
surface.

a multiphonon process between NO + metal would require interactions times on the order of 50
ps to excite molecular vibrations. Vibrational excitation via a nonadiabatic, electron hole pairmediated excitation process will be explored further in section 7.4.1.

7.3.2 NO scattered from Ga(l)Molecular scattering of NO from liquid gallium at Ts = 873(30) K has also been
investigated at both low and high collision energies (Einc = 2 and 20 kcal/mol) where
vibrationally excited NO(v = 1) is also detected. Rotational Boltzmann distributions are
displayed in Figure 7.5 for both NO(v = 0) and NO(v = 1) at collision energies Einc = 2 and 20
kcal/mol. Broadly summarized, the shapes of these rotational distributions are qualitatively very

196

Figure 7.6 Boltzmann rotational distribution of NO scattered from Ga (Ts = 873(30) K) at both
low and high collision energies. a) Two-temperature fits to NO(v = 0) rotational distributions andb) single-temperature fits to vibrationally excited NO(v = 1), with Trot at both low and highcollision energies within uncertainty of Ts.

197

similar to those observed for NO + Au(l) (Figures 7.4 and 7.5); two-temperature fits to NO(v =0) show clear differences between the two collision energies, while NO(v = 1) rotational
distributions are fit to single-temperature linear fits that are remarkably similar for both Einc.
The double exponential fits to the rotational distributions of NO(v = 0) at low collisions
yield Tcold = 270(40) K and Thot = 730(30) K while at higher collision energies Tcold = 380(20) K
and Thot = 1230(40) K. The single temperature fits of vibrationally excited NO are 810(70) K and900(40) K for the respective Einc = 2 and 20 kcal/mol scattering conditions. Both of theserotational temperatures are within uncertainty of the 873 K liquid gallium surface, suggestive ofrotational thermal accommodation with the surface in the fractional scattering pathways leading
to vibrationally excited NO. This thermal accommodation of the rotational degree of freedom isunlike scattering from the significantly hotter 1400 K gold surface, which yielded rotationaltemperature some 400 K cooler than the surface temperature for NO(v = 1).

7.4 Discussion
7.4.1 Temperature Dependence of NO scattered from Ga(l)As a way to test the nature of collisional NO vibrational excitation at molten metal
surfaces, we now investigate scattering dynamics as a function of the liquid gallium surface
temperature. LIF spectra containing all rotational and spin-orbit states populated within NO(v =0) and NO(v = 1) were taken at Ts = 873 K, from which the Boltzmann rotational populations
shown above were derived. In order to extract a vibrational temperature as a function of Ts,
however, a smaller piece of the spectrum with lines from a wide range of rotational states issufficent. Here, the P12 branch was scanned over for both vibrational states over the bandheadcontaining peaks from J = 1.5 - 19.5, which yields representative vibrational state populations.

198

This band is isolated at the lower energy end of each spectrum and is integrated over for NO
scattered from gallium, at surface temperatures ranging from Ts = 600 – 1000 K. The extracted
vibrational temperatures vs. surface temperature are displayed in Figure 7.7. These data reveal
increasing vibrational excitation with surface temperature, yet with subthermal temperatures, inaccordance with molecular scattering results of NO scattered from solid crystalline surfaces.15
In previous scattering experiments of NO from crystalline surfaces, an Arrhenius surface
temperature dependence of vibrational excitation has been interpreted as characteristic evidence
for nonadiabatic, electron hole pair mediated energy transfer between NO and metal surfaces.1, 11,
12, 16, 24

In Figure 7.8, the vibrational excitation probability, Pv = [v = 1]/[v = 0], is considered as a

Figure 7.7 Vibrational excitation of NO(v = 0 → 1) scattered from Au(l) (Ts = 1400 K). The
vibrational temperature is calculated from [v=1]/[v=0] = exp[-ΔEvib/(kTvib)] where ΔEvib ~ 1876
cm-1.

199

Figure 7.8 Natural log of the vibrational excitation probability of NO(v = 0) + Au(l) → NO(v =1) vs. inverse surface temperature. The blue line represents the linear fit with Ea = 1850(200) cm1, i.e. within uncertainty of the vibrational energy spacing of 1876 cm-1.

function of the inverse surface temperature following the equation: Pv = A*exp[-Ea/kTs]. The
data reveal linear behavior with varied surface temperature and an activation energy of Ea =1850(200) cm-1, i.e., clearly in excellent agreement with the v = 0 → 1 vibrational energy spacing
ΔEvib = 1876 cm-1. This suggests strong electronic coupling between NO + liquid metal surface
and these results represent first evidence of electron hole pair mediated vibrational excitation of
NO at liquid metal surfaces.
Wodtke, Tully and coworkers have contributed extensively toward understanding these
nonadiabatic, electron hole pair mediated collision dynamics occurring at NO + Au(111)
surfaces.5, 9, 12, 13, 25-28 Evidence has been presented that suggests vibrational excitation anddeexcitation of NO molecules at a metal surface are mediated by the formation of electron hole200

pairs in the metal. In this mechanism, as a NO molecule nears the surface, an electron from thelowest unoccupied molecular orbital (LUMO) of the metal can ‘hop’ onto the NO molecule
forming a transient anion that is stabilized by a Coulomb image charge in the metal. As the NO
molecule leaves the surface, the electron hops back to the metal and during this process
vibrational modes in the NO molecule can be excited (or relaxed).

7.4.2 Energy Transfer at the NO + Ga(l)/Au(l) Interfaces
The data in Figure 7.9 summarize the final energy distributions of NO scattered fromboth Au(l) (Ts = 1400 K) and Ga(l) (Ts = 873 K) at low (Einc = 2 kcal/mol) and high (Einc = 20
kcal/mol) collision energies. The electronic, rotational and vibrational temperatures plotted are
calculated as follows: (i) The spin-orbit/electronic temperatures are computed via [2П3/2]/[2П1/2] =exp(-Eso/kTelec) where Eso = 123 cm-1, (ii) average rotational temperatures are presented for each
vibrational state, and (iii) vibrational temperatures are calculated by integrating over all
rotational and spin-orbit states populated with each vibrational state via [v=1]/[v=0] = exp[ΔEvib/(kTvib)], as done in Figure 7.3. Most visible in the Au(l) scattering data in Figure 7.9(a) isthe lack of thermal accommodation for the NO molecular degrees of freedom (i.e. rotational,
electronic, vibrational) with the molten gold surface at both collision energies studied. Similar
trends are observed for scattered NO from a cooler 873 K gallium surface, with the exception ofsome rotational thermal accommodation. Broadly speaking, these scattering data from Au(l) andGa(l) reveal more energy transfer into NO internal rotation and electronic degrees of freedom for
faster incident speeds, whereas, in contrast, there is no effect of collision energy on theefficiency of vibrational excitation. The higher incident collision energies introduce more201

Figure 7.9 Summary of rovibronic temperatures for NO scattered at Einc = 2 and 20 kcal/mol
from a) Au(l) Ts =1400 K and b) Ga(l) Ts = 873 K.

202

available energy into the system, resulting in increased translational to rotational/spin-orbit
energy transfer.
As noted earlier, the detection of NO(v = 1) scattered at Einc = 2 kcal/mol signifies thatthe bulk of the energy must be coming from the metal as Einc < Evib, i.e. incident translationalenergy is insufficient to vibrationally excite NO(v = 1). Also noteworthy is that the vibrationaltemperatures are nearly identical for both collision energies. This is in contrast to thetemperatures observed in the spin-orbit and rotational degree of freedom, which are highly
dependent on collision energy, especially for NO(v = 0 → 0) collisions. A closer look at therotational and spin-orbit temperatures for NO scattering from Au(l) reveals an increase of ~200
K from Telec(v = 0) at low Einc relative to high Einc, however, there is only < 100 K difference inthe spin-orbit temperatures of the two collision energies for the vibrationally excited NO
distributions. This same trend is observed in the rotational energies, revealing ~ 500 K differencein Trot between the two collision energies for NO(v = 0), while only ~ 100 K for NO(v = 1).
These data trends, which are reiterated in the Ga(l) results, indicate that for vibration-conserving
collisions, energy transfer from the translational to rotational/spin-orbit degree of freedom ismore prominent. However, for collisions where NO is vibrationally excited, NO(v = 0 → 1) the
rotational and spin-orbit distributions are less sensitive to incoming incident speeds, where theenergy for excitation is provided by the liquid metal surface.

7.5 Conclusion
Quantum state resolved molecular beam scattering techniques are utilized to investigate
collision dynamics of NO at molten gold and gallium surfaces. The results presented in thischapter represent the first detection of NO vibrationally excited from collisions at liquid metal203surfaces. Additionally, the surface temperature dependence of NO (v = 0 → 1) vibrational
excitation probabilities exhibit activation energies equivalent to the vibrational energy spacing,suggestive of electron hole pair mediated vibrational excitation. Rovibronic distributions of NO
reveal strong collision energy dependence for vibrationally unexcited NO(v = 0 → 0) wheremore rotational and spin-orbit excitation is found for increasing Einc. Higher incident
translational energies bring more energy into the system that can be redistributed into internalrotational and electronic degrees of freedom. The fraction of vibrationally excited NO, however,is relatively insensitive to collision energy for both liquid gold and gallium. Moreover, therotational and spin-orbit distributions are remarkably similar for NO(v = 1) scattered from low
and high collision energies for both Ga(l) and Au(l), indicating that these internal NO modes are
excited by thermal electron hole pairs in the metal surface. These results are consistent withstrong NO-metal coupling and electron hole pair mediated excitation of NO internal degrees of
freedom.

204

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