Alkali-Metal Harpooning Reactions in Li-FCH3 and Li-FH
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy,
Graduate Department of Chemistry,
University of Toronto.
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Alkali-Metal Harpooning Reactions
in Li-FCH l3 and LieeFH
HanE3in Oh
For the degree of Doctor of Philosophy,
Department of Chemisftry, University of Toronto
2ao 1
The van der Waals complexes, Lib4FCH3and Li..FH, have been forrned for the first time. using a laser-ablation method. The complexes were identified by photoionization tirne-of-fiight mass spectrometry. The excitation of the complexes accessed selected configura'tions of the Transition State (TS) of the electronically-excited reaction, ~i'(2p~~)+ XR -, LiX + R, leading thereafter to depletion of the complexes. Depletion oif complexes was obser~edacross a broad range of excitation wavelengths. 'The experimentally observed depletion spectrum was compared to the results oTf ab initio calculations, perfomed by others. These ab inifio calculations permitted identification of the electronically- excited states reached by photoexcitatiorn of the Li..XR complex and assisted in the interpretation of the depletion spectra. Reaction proceeds by surface- hopping from the TS to the ground state; the TS lifetime was obtained by an optical saturation method, and was founai to be 5 1.3ns for both Li reactions. To Boln, SuMin, and JiHye Acknowledgements
I sincerely thank John Polanyi for his guidance and support during my stay in Toronto. I have gained a lot of confidence while working under his guidance. I have learned from him that the physical insight and the understanding of nature are far more important than any technical details.
I am indebted to Andrew Hudson for teaching me experimental techniques. I have enjoyed working companionship with him for last 4 years. I would like to thank TaeGeol, Javier, Jiaxi, and Sergei for providing a cozy environment in the laboratotyloffice and for their wam welcome-smile every morning. Duncan has always been kind to me whenever I needed his heip.
Maria and Anastasia have patiently listened to my complaints and silly stories.
Fedor Naumkin explained his theoretical results to me in a very understandable language. Especially his calculations for Li..FCH3 were so great than even 1 were able to appreciate it irnrnediately! Prof. Piotr Piecuch was a good mentor to me and showed me the beauty of ab initio calculations. Serguei Raspopov patiently followed al1 the algebra and corrected the wrong equation for TS lifetime.
I think that I could not finish rny Ph.D without Boln's sacrifice and her endless love. Boln spent the whole summer parenting SuMin and JiHye while I was struggling with writing my poor thesis. I sincerely thank my parents, parents- in-law, BoAe, SungHan, and HanJong for being supportive and encouraging me al1 the time. iii
Table of Contents
Chapter 1- Introduction
1-1. Chernical Dynamics of the reaction event
2 History of TSS
Line broadening in non-reactive system
Line broadening in the reactive system
Continuum resonance Raman Scattering
(Bound-free transition)
TSS in van der Waals complexes
Negativeion photoelectron spectroscopy
Femtosecond Tmnsition Spectroscopy (FTS)
TSS performed previously in thiJ labaratory
1-3. Alkali metal reaction with halide molecule
M + X2 reaction
M + CHgreaction
M + HX reaction
4. Thesis preview
Chapter 2. Experiments
24. Apparatus
2-1 Vacuum system
2-1-2. Laser-ablation method Motivation for use of laser-ablation method
Formation of van der Waals complexes from
the laser-ablation source
Pulsed valves
Description of the mechanical part of the ablation
assembly
Procedures for making the Li sample-rod
2-1-3. Laser system
Excitation laser, LI
lonization laser, L2
2-1-4. Timeof-flight mass spectrometer (TOFMS)
TOFMS
Data acquisition & timing circuitry
2-2. Data analysis
Mass spectra
Measurernents of the photodepletion cross-section
2-3. Summary
Chapter 3. Transition State Spectroscopy for the reaction of
Li'(2p 'P) + CHjF -+ LiF + CH3
3 Introduction
Experiments
3-2. Identification of the LLFCH3 vdW complex 3-3. Measurements of the photodepletion cross-section 62
for Li..FCH3
34. Assignments of the depletion peaks 65
Theoretical results and Discussion
3-5. Po tenfial-en ergy-surfaces 69
3-6. Simulation of the photoabsorption spectrum 76
3-7. Measurements of fhe TS lifetlme by optical 80
'saturation method'
3-8. Summary 85
Chapter 4. Transition State Spectroscopy for the reaction of 87
Li'(2p 'P) + HF + LiF + H
4- 1. Introduction
-4-2. Experiments
Results
4-3. Identification of the Li-FH complexes
4-4. Photodepletion spectrum for the LLoFH complex
Discussion
4-5. Assignment of peaks in the photodepletion spectrum 1O4
4-6. Dynamics of harpooning 107
4-7. Summary 910 Chapter 5. Thesis summary and future directions
5-1. Thesis summary
5-2 Future directions
Conclusions
Appendix A
References vii
List of Figures
Figure 2-1 : Side view of the vacuum system
Figure 2-2: Schematic diagrarn of the laser-ablation assembly
Figure 2-3: Schematic diagram of the pulsed valve
Figure 24: Schematic drawing of the laser-ablation source assembly
Figure 25: (a) The procedure for rnolding the Li support-rod
(b) The procedure for molding the Li support-rod
Figure 2-6: Depletion-signal versus delay between two lasers
Figure 2-7: Schematic drawing of the experimental setup
Figure 2-8: Depletion-signal versus excitation-laser fluence
Figure 3-1: TOF mass spectrurn for the Li-FCH3 complexes. The ion
Signal was measured following photoionization by laser La (h2 =248nm).
The dotted line represents ion-signals before photoexcitation and the solid line is the ion-signal after photoexcitation.
Figure 3-2: Power dependence of the ionization signal ((A2 =248nm) for Li-FCH3. The straight line is a linear least-square fit to the date points. The slope of the line is 1.02.
Figure 3-3: Action spectra for (a) LLFCH3 and (b) NammFCH3
Complexes, showing the depletion cross-section in A2 vs. the wavelength hl of Li in nm.
Figure 34: Potential-energy curves for the ground and the electronicallv-excited states of LimoFCH3.alona an a~~roxirnateminirnum- viii energy path for the collinear PES. The observed spectrum is shown, aginst the appropriate energies, inset at the top left.
Figure 3-5: Potential-energy contour polts (a) for the ground 2s and (b) (c) (d) for the electronically-excited states 2px, Zp,, 2pz of
LieeFCH3.
Figure 3-6: Minimumenergy structure for the LieeFCH3complex.
Figure 3-7: Sirnulated photoabsorption spectrum of LieeFCH3for
T=250K. V~brationallevels up to v=10 on the ground and excited
States were included.
Figure 3-8: Laserexcitation and rate processes initiated in the complex.
Figure 3-9: Simulation of the TS lifetime for LiaeFCH3at II =640nm, using Eq. 3-7 (see text). The heave dashed-line gives comparable data for NaeaFCH3at hl = 690nm.
Figure 4-1 : Crossed-beam assembly for formation of the Li-FH
Complexes.
Figure 4-2: TOF spectrurn illustrating the formation of the Li-FH
VdW Complexes.
Figure 43: Action spectra for LFH(above) and NaaeFH(below)
Complexes (the latter being from Ref. 24), showing the depletion
Cross-section in A2 VS. the wavelength of laser Li in nrn. The heavy
Vertical Iine is the atomic D-line (2p t 2s for Li, and 3p t 3s for Na).
Vibrational spacing indicative of anhannonicity is identified for a series
viii of four vibrational levels (see text).
Figure 4-4: Ab initio calculation of the vertical-transition energies for 106 excitation from the minimum-energy geometry of the LLFH and
Na-FH complexes (the latter being from Ref. 40) on the ground-state
PES, q2A',to the lowest-lying excited states. z2A',32A', 12A".
Figure 45: Potential-energy curves for the ground and the
Electronically-excited states of LiFH. along an approximate minimum-
Energy path for the collinear PES. The equilibrium values for the Li-F and H-F bond lengths are indicated on their respective axes by downward pointing arrows at r~=ro (Li-F) = 1.564A and at r2 =ro (H-F) = 0.917A.
Vibration in the ground electronic state LLFH (tentatively identified with the vibrational progression of Fig. 4-3) is indicated for v = 0-3. List of Tables
Table 2-1: Voltages on the plates and the flight tube in the TOFMS
Table 3-1 : Calculated electronic-vibrational transitions in Li-FCH3
(wavelength in nm, integrated absorption coefficient in cnilmole). Chapter 1. Introduction
11 Chernical Dynamics of the reacüon event
Understanding the dynamics of the reaction event is a goal of chemical
dynamics. The reaction event includes processes occurring during the collision.
The reaction event involves breaking of old chernical bonds and sirnultaneous
formation of new chemical bonds. The co~certednessof this event is evident in
that average collision energies for successful reactions are typically less than
10% of the dissociation energies of the old bonds. This concerted event occurs
on an extremely short time scale. Proceeding from reactants to products
involves changes in the internuclear separation totaling -1 0A. If the atoms
moved at thermal speed of 10~-10~crn/s,then the entire event would take 10"~-
10%. In this work the 'Transition State' (TS) is defined as al1 configurations through which the reacting particle evolves en route from reactants to
prod~cts.'~
The direct observation of this concerted reaction event was not
previously feasible, due to the short time scale of the event. Most of Our
information regarding the reaction event came from the experimental
measurement of disappearance of reactants or of appearance of products. This
information was combined with theory to infer the dynamics intemediate
between the reactants and the products. The importance of 'Transition State
Spectroscopy' (TSS) is that it accesses and probes the intermediate species (transition state) that exist between the reactants and products. The accuracy of ab initio calculation in the TS region can today be checked by the direct TSS observation.
2 History of the TSS
Line broadening in the non-reactive system
The history of TSS can be traced back to the observation of the line broadening by Lorentz in the early 20" ~entur~.~*~The broadening of the line spectrum of an atom A was due to the meeting interaction of A with a collision partner B. Each frequency in the Lorentzian wing of the atomic line corresponds to the internuclear separation of AB- The wide range of a collisional internuclear separation leaves a broad wing as a -fingerprint.
The wing can be divided into various regions according to the location from the atomic Iine center. The 'impact region' or %ne~re'~ corresponds to a region which arises in the wing near to the atomic line center, with a relatively sharp emission intensity but several orders-of-magnitude smaller than the line center. This region is characterized by frequencies Av lit,, where T, is the duration of a collision, -10-l2 S. Only limited information can be obtained experimentally around a potential region where an emitter is perturbed slightly by a long-range collision. The 'far wing' and 'extreme wing' regions are obsewed with a broader intensity outside the 'impact region' in the emission line spectrum. This region carries information of short-range collisions where the details of the potential are important. Thus, the potentialenergy curve can be reconstructed from the analysis of the 'impact region', the 'far wing', and the
'extreme wing'. For example, the Rb-Xe adiabatic potentials could be inferred from the observation of the line broadening at different temperatures6
Line broadening in the reactive system
The line broadening could be observed in the reactive collisions as well as in the non-reactive collisions. The first observation of emission from the reactive intermediate species (TS) was made for the reaction of Eq. (1-1 ).7*8
F + Naz + [FN~N~]*+NaF + Na' (1-1)
The 'far wing' emission extending -1 OOnm away from the alkali-metal line center was observed at pressures lower than those at which collisional pressure broadening would nomaliy be observed. The most intense sodium D-line emission at the line center indicated that most of emission occurred long after the products had separated. However, a small fraction of Na* emitted while it was separating from the NaF fragment. The total integrated intensity of the emission wing was -do4 that of the sodium D-line emission. From this, the lifetime of the intermediate species, [~NaNal*,could be estimated to be 1O'X
[the lifetime of Na D-line (-1 0" s)] = -1 0-l2S. The experimentally observed wing of the emission spectrurn could be reproduced from classical trajectory studies that estimated the ernission intensity by accumulating the time the trajectories spent near certain geornetry corresponding to the certain wavelength of the ernission. Likewise. the line broadening in the absorption of the reactive intermediate species US) was also observed for many reactive systems such as K + ~a~i~*'~,K + ~g~r~", K + N~BT'*.
Continuum resonance Raman scattering (Bound-free transition)
The structure such as vibrational or rotational signatures was sparse in the wings of both absorption and emission spectrurn. A rich structure could be obtained if the TS was prepared in well-defined configurations. The 'continuum resonance Raman scattering' experiment, for example. on ~~31'~and 0314 showed a rich vibrational structure of the ground potential-energy-surface. The
TS on the repulsive potential-su rface was prepared in a narrow range of configurations by laser excitation of a bound molecule. While the TS were dissociating, the TS ernitted to the quantum states of the ground electronic state. The emission intensity was deterrnined by the Franck-Condon overlap for successive configurations as the TS dissociated to products. In this kind of experiment, information related to the repulsive electronic state and the highest vibrational levels in the ground electronic state can be obtained. However, the interpretation of this ernission spectrum could be complicated if there would be other intersecting electronic states leading to dissociation. TSS in the van der Waals complexes
In the above experiments, the TS was prepared from a bound state
which had a defined geometry such as internuclear distances and bond angles.
Another recent example of TSS, pioneered by Soep and co-~orkers'~~~and by
Wttig and et 26, was accessing the TS from a reaction precursor. In the
former case. the TS of an excited Ca reactions with the hydrogen-halide
molecules was accessed by excitation of the ground-state van der Waals (vdW)
complexes. Ca-HX (X=F, CI, Br. and 1). which was formed in a supersonic
expansion. The complexes were speculated to be most stable in the linear
geometry.
By changing the excitation-laser wavelength, different configurations on
the potential-energy-surface (PES) of ~a*('P or 'D) -HX could be accessed.
The 'action spectra' were measured by monitoring the cherniluminescence of the product, CaX, as the excitation laser was scanned in the vicinity of the atomic transitions of calcium. A quite extensive vibrational structure (bound-to- bound transition) was observed in the 'action spectra'. In addition, the branching ratio between the product channels was obtained by analysis of the dispersed-fluorescence spectra of the product CaX (A*II or B~E'). The reaction probability and the branching ratio were observed to Vary dramatically from one halogen to the other. Due to many PESs involved in the excited-state reaction, the dynamics of the excited-state reactions were very complicated. This approach has been extended to the classic alkali-metal (Na and Li) atom 'harpooningJreactions in this laborat~r~'~~~.which is the main subject of this thesis and will be described in detail in the following chapters.
A similar approach to TSS, initiating an intracluster reaction, was developed by Wttig and CO-worken.25126 For example, the vdW cornplex,
XHeeOCO,was irradiated with a 193 nm photolysis laser which directed a photofragment, H, towards the OC0 end of the complex at preferred angles defined by the geometry of the vdW complex. In this intracluster reaction. the translational energy of the incident H-atom could be related to the photon energy used for photolysis of HX and to the intracluster collision occurred with a restricted range of impact parameters. Depending on the structure of vdW complex, the reaction probabiiity varied dramatically. For example, the bent
BrH-OC0 complex, with Br close to the carbon atom, was observed in the analysis of OH product yield to be -40 times more reactive than the linear
CIH-OC0 complex.
An interesting rnethod was developed by Lester and CO-workers,who state-selectively excited the pure OH overtone (von=2 t von=O) vibrational transition of O-HpOH complex (T-shaped) to induce a half-collision inelastic scattering between the OH radical and ortho-Hz under restricted initial orientation conditions. 27-30 By probing the interna1 energy of inelastically scattered OH (v=l), the state-to-state energy transfer processes that occur between the vibrationally-excited OH radical and O-H2 could be understood. In addition, the lifetimè of O-H2-OH (vOH=2), which was related to the sum of the vibrational predissociation (inelastic scattering in full collision) and chemical reaction rate, was measured to be long (115+ 26ns) in the pump (IR overtone excitation)-probe (LIF of OH(v=l)) timedelay experiment.
Negative-ion photoelectron spectroscopy
One of the most informative approaches to TSS is to photodetach a stable negative-ion to form an unstable neutral complex in the TS region for reaction. This 'negative-ion photoelectron spectroscopy' has been developed
mainly by Neumark and CO-workers. 3'932 For example, they fomed FHF complex by clustering F' with both parahydrogen and normal hydrogen." The electron was detached from FHf complex by laser light with a fixed wavelength that transfomed the negative-ion to the labile [FHHI*, TS for the reaction F + Hz
-+ HF + H. The translational energy of the detached-electron carried information of the PES of the neutral complex. The photaelectron spectrum of - the negative-ion yielded the resolved vibrational structure characteristic of the unstable [FHHJ* complex. This vibrational structure came mostly from the vibrational mode orthogonal (bending mode) to the reaction coordinate in the neutral complex.
Femtosecond Transition-State Spectroscopy (FTS)
The advent of femtochemistry particularly due to Zewail and CO-workers offered an opportunity to observe a molecular system in We continuous process of its evolution from reactants to transition states and then to products. 34.35 At the early stage of the femtosecond transition-state spectroscopy (FTS), the dissociation reactions of alkali-halide molecules were studied as good examples of visualization of the localization and of resonance dynamics along the reaction coordinate. 36-38
For the alkali halide system, the covalent [denoted by M (alkali-metal atom)+ X (halogen)] potential and the ionic (M+ + X-) potentials cross at a large internuclear separation. For the example of Nal, two potentials cross at an internuclear distance of 7A. The internuclear separation of the MX at the crossing point can be roughly estimated from the ionization potential of the alkali-metal and the electron affinity of the halogen atorn (see equation below).
, where 1. P. and E. A. stand for the ionization potential and the electron affinity respsctively, and Rc represents the internuclear distance at the curve-crossing point. The 'pump' femtosecond pulse took the ion-pair M'X- to the covalent MX potential and the wave-packet traveled towards the ionic [M+-X~ potential as its internuclear separation increased. Some of the wave-packet dissociated to products and some of it bounced back from the crossing point of covalent and ionic potential curves, with a finite probability with which a halogen atorn was
'harpooned' again. For the example of Nal, the Nal oscillates about 1O times on the upper PES before completely dissociating to Na + 1. With a series of 'probe' pulses, delayed in time from the 'pump' pulse, the nuclear motion along the dissociation reaction-coordinate towards the final state, M + XI could be followed. The observation of the free alkali-metal M and the [M-XI** showed a quantized and coherent buildup of free alkali-metal and the damped sequential recurrences of the [M~..x]**.
TSS perfonned previously in this laboratory
In recent times, the TSS approach, complexing the reactants in the TS configuration, has been applied to the reactions of excited alkali-metal (sodium) with halide-molecules. l2The TS of the excited-state sodium reactions could be reached by photoexcitation of the ground-state vdW complexes, Na..XR, where X represented a halogen-atom and R denoted hydr~gen~~,methyl 21 -23 , or pheny120radical. The TS corresponded to the configurations intermediate between reactants (Na'(3p 2~)+ XR) and products (NaX + R). Following the excitation of the ground-state vd W complexes, charge-transfer occurred in a narrow range of the TS configurations, around an avoided-crossing region between the ionic (Na+ + XR') and covalent (Na + XR) potentials. Afier the charge-transfer, XR dissociated to yield NaX + R. This process can be symbolized in Eq. 1-3.
Na-XR + hvl -+ [N~*..xR]* + [N~+~*XR~*+ NaX + R
where a double-dagger symbolizes the transition state. ln the above experiments, the disappearance of the vdW complexes after photoexcitation of the ground-state complexes, 'photodepletion', was rneasured as a function of excitation-laser wavelength, hi. In the particular case of Na..FH, high accuracy ab initio PESs were ca~culated.~~Comparison of ab inifio calculation and the experimental 'photodepletion' spectrum provided detailed information related to the PES and reaction dynarnics of the excited- state reaction. 39,40
13 Alkali-metal atom reactions with hallde-molecules
In this thesis, the experimental studies for the reactions of excited-state
Li* (2p 2~)with the halide molecules (CH3F and HF) will be described.
Therefore, it is worthwhile to check the characteristics of the alkali-metal atorn
(M) reactions with various halide rnolecules (X2, CH3X, and HX).
M + X2 reaction
The alkali-metal atom (M) and halogen-molecule (X2) reactions were first studied in the classic M. Polanyi flame e~perirnent.~'From the width of the salt deposit on the wall of the glassware and the time scale provided by inter- diffusion of M and XZl it was demonstrated that these reactions were remarkably fast, with reaction proceeding on more than every gas-kinetic collision. In order to explain high reaction cross-section, 'electron jump' or 'harpooning' mechanism was proposed early by ~a~ee~*and by ~erschbach~~. According to this rnodel, the charge-transfer occurs at a large intemuclear distance between an alkali-metal atom with a low ionization potential (1.P) and a halogen- molecule with a positive electron affinity (EA). TMX; then dissociates to yield the product MX.
In the crossed molecular-beams experirnents, high reaction cross-sections
(in many cases, or > 100 A2) were observed for i'M + X2 reaction. For this reaction, the product, MX, was found to scatter into the fonvard direction. Such anisotropic angular distributions suggested that the reaction process must be over before colliding reactants had time to execute several rotations around one another. Therefore, the reaction time could be estimated to be shorter than a rotational period (typically -10''~s), for the reaction whose angular distributions were anisotropic.
In addition, analysis of the velocity distribution for the scattered K and KBr from collisions of crossed bearns of K + Br2 showed that most of the chemical energy released in these reactions appeared as an interna1 energy of the produ~ts.~~*~~The high reaction cross-section amd the forward scattering of the recoiling product can be explained in ternis of the 'harpooning' mechanism.
Since the charge-transfer can occur at a large internuclear distance for this reaction, collisions with very large impact parameters are still very effective in yielding the products. In the case of the large imtpact parameter collision, the alkali-metal simply picks up the halogen atom and carries it foward with little deflection. This mode1 is called a 'stnpping mechanism'.
The reactivity of excited Na* atoms with halogen-molecules were
studied systematically by Y. T. Lee and his CO-workers. The ground-state
sodium atoms were excited to Na* (32~32,42~5~2) states in the beam-interaction
region, using CW dye lasers. Wah increasing electronic energy, the reaction
cross-section increased and the angular distribution was less forward peaked.
As the electron jump distance moves to larger separation for
successively higher Na electronic states (with successively lower ionization
potential), the reaction cross-section, or = ZR& increases, where R is the
radius at which the covalent and ionic curves cross (see Eq. 1-2). The electron- jump at a larger separation was thought to give longer tirne for halogen-ion to
dissociate before the arriva1 of the alkali-metal ion. Thus, the 'secondary
encounter' between the-alkali-metal and the departing halogen atom through
which the ground-state Na reactions give a fomvard angular distribution could be
reduced, and wnsequently the products of the excited-state reactions were less
forward scattered.%
M + CH& reaction
The alkali-metal (M) reactions with the methyl-halides (CH&) have also
been studied extensively, both experirnental~y~~~~and via theoretical ca~culations~~~.In the studies of the dependence of the total reaction cross- section on the translational energy (E&)),for Rb(or K) + CH,Br(or CH31) reaction, the threshold above which the reaction product was detected was observed. 51.52
After a sharp increase of the reaction cross-section above the threshold, the reaction cross-section reached a maximum followed by a monotonie decline.
An Arrhenius-like positive energy dependence of the reaction cross-section near the threshold was due to an energy barrier originated from the low (or negative) electron affinity of the halide molecule.
It was thought that the X-CH3 had to be stretched significantly, Le. E.A.
(X-CH1) > E.A. (X-CH~)'~,in order to sway the unfavorable electron afFinity of the methyl-halide molecule towards the successful reaction (electron jurnp).
Thus, the charge-transfer could occur only when the internuclear distance of M-
X was quite close to the equilibriurn distance of M-X, and X-CH3 bond was stretched significantly. This is in accord with the experimental observation showing the relatively lower reaction cross-section for the M + CH& system than that for the M + X2 system.
For the example of Rb(or K) + CH3Br (or CH31) reaction, the angular . distribution of the product was found to be anisotmpic in the backward direction relative to the alkali-metal. 51052 If is not surprising, considering that the charge- transfer can occur only at a short internuclear distance of M-X, and thus it requires a low impact parameter collision for reaction to happen ('rebound mechanism'). In ali subsequent experiments of the reactions of alkali-metal atoms with alkyl mono-halide, the 'rebound mechanism' was found to be responsible for al1 the observations.
Lee and CO-workersstudied the reaction of an excited sodium (3P, 4D) with methyl-halide rno~ecules!~Wth increasing electronic energy (thus lowering 1. P. of the alkali-metal), the relative reaction cross-sections increased, in accordance with the 'harpooning' model. The wider experimental center-of- mass angular distributions supported the idea that an opening of the steric angle of acceptance for reaction occun with increasing electronic energy and thus sodium orbital size. However, the excited sodium reaction with methyl- halide still proceeded via the 'rebound mechanism' (backward scattering of the product).
M + HX reaction
The first chemical reaction studied using crossed molecular beams was a K + HBr system, investigated in 1955 by Taylor and ~atz.'~However, the reactions of alkali-metal atoms with hydrogen-halide molecules were not studied in as much detail in molecular beams until more recently, due to unfavorable kinematics in the center-of-mass distributions for these systems. Experimental
60,61 studies have now been performed by many groups, such as Brooks ,
Loesch6267 , ~ee~&~~,and Dü ren's7376 g rou p.
The dependence of the reaction rate on the vibrational and rotational excitation were studied in this laboratory for the ground-state endothennic reactions, HX(v,, or j;v)+ Na -+ H + NaX (X=F, CI) using the
cherniluminescence depletion rnethod."17' The vibrational excitation was found to be very effective in promoting the reaction, which was consistent with a 'late' barrier characteristic of the endothermic reactions. 78-79
On the other hand, Brooks and coworkers looked at the effect of the translational energy on the slightly endothermic K + HCI reaction? They found an increase in reaction probability as the collision energy was increased from 3 to 10 kcalfrnole, and then a fall-off as the collisional energy was further increased to 18 kcalfmole. The Brook's group also performed molecular-bearn studies of the relative contributions made by translational and vibrational energy for the same reactions of K + HCI + KCI + H.~~~~~By using a resonant HCI chemical laser, HCI was excited to the first vibrational state and the reaction cross-section was observed to increased about 2 orders-of-magnitude. The cornplementary addition of translational energy accomplished by the seeding technique was rnuch less efficient than the vibrational excitation in increasing the reaction cross-section.
The above reaction, K + HCI + KCI + H, and the more endothermic reaction of K +HF + KCI + H were studied in greater detail by Loesch and co- workers. 62v63 However, they observed the reaction probability for these reactions rises steadily until reaching a constant value for collision energies from 20 kcallmole up to over 40 kcalfmole. They also studied the effect of reagent vibration excitation on the reaction probability. At low collision energies, they observed similar results for K + HCI (v=0,1) to those cf Brook's, but saw a decrease in the vibrational enhancernent with increasing collision energy. The dropping-off of the vibrational enhancement was due more to the increase in the reaction cross-section at v=O with increasing collision energy, than to a large drop in the cross-section at v=l.
The effect of electronic excitation on the reaction cross-section and dynamics was studied in Lee and Düren's group for Na + HX reactions using the crossed molecular beams method. For the reactions of ground-state alkali- metal atoms with hydrogen halides, the electron-jump, harpooning, does not take place at a long distance since hydrogen halides have negative electron affinities.
Lee and his CO-workersfound that with increasing electronic energy of
Na, the reaction cross-section increased.68-72 For the reactions of Na* (5S, 4D), due to lower ionization potentials, early electron transfer could occur at large
Na-HCI separations. The H atom departed very quickly after the electron- transfer and the early departure of H thus caused the low product recoil energy.
For the Na* (5S, 4D) reactions, a DIPR-DIP model which characterizes the reaction by sequentiai two-body steps: electron transfer from Na to HCI, departure of H from CI-, and, finally association of ~a+and CI- to form NaCI, could explain the reactions well.
For the Na* (3P) reactions, a large fraction of excess energy was observed to go into the translational energy of the products. In this case, the
electron transfer occurs at a smaller distance and thus the dynamics of H atom departure are no longer govemed by the dissociation of HCI_ The repulsive departure between H and NaCl and the stronger coupling among al1 three atorns are thus responsible for a large fraction of the excess energy going into product recoil energy. The product NaCl was backward scattered with respect to the incorning Na atom in the center-of-mass frame of reference, for the reactions of Na (3S, 3P, 4D, and 5s) with HCI. This indicated that the reactions proceeded directly even with the electronic excitation.
Düren et al. have studied the reaction of Na(3S, 3P) + HF using a sensitive hot wire dete~tor.~~'~They could observe the reaction products, NaF, which Lee's group could not detect due to a low sensitivity of their detection method, for the excited-state Na reactions. Analysis of the rainbow structure of the non-reactive channel revealed the well-depths of the ground-state and the excited-state reactions. Due to a high anisotropy of the HF molecule, a double rainbow reflecting the scattering from the two ends of the target could be seen.
Analysis of the rainbow scattering provided well-depths of es,= 17e, cs2= 65f1 meV and 92k3, sp2= 337k5 meV for the ground-state and excited-state reactions respe~tively.~~
They also found that for the excited-state reaction, the double- differential center-of-mass cross-section was in the foward direction with minor contribution in the backward direction. They discussed the reaction mechanism for the excited-state reaction in two difterent ways; first, on an adia'ibatic basis, the system starts to evolve on the electronically excited surfaces where surfaces of 2A' symrnetry are non-adiabatically coupled with the grmund state surface which is of 2~ symmetry. After passing the region connecaing the two surfaces (a surface-hopping model), the system proceeds to evolv= on the ground state potential surface. Secondly, the system could be thouight to evolve on the wrresponding diabatic surface. Therefore, the excited-state reaction could be seen as an exothermic reaction with an early downhill surface, which explained well the observed feature that the energy part of the cross- section showed a wide distribution of the interna1 energy.
The reaction Li + HF is the simplest system arnong the alkali-metal atom plus halide-molecule reaction system. This system, involving rnetal and halide atoms of the lowest atomic nurnber, has been identified as ai prototype of the alkali-atom plus halide-molecule system, whose reaction proceseds through charge-transfer 'harpooning' reaction mechanism. This was clear im a
Herschbach's statement made in 1971.80 He stated that " An ab inmtio calculation of (H, Li, F) would be of interest both to study a new species of hydrogen bond and to elucidate the problem of connecting the H + K+Br- portion of the surface to the regions involved in the 'electron jump' process. ." The simplicity of the system drew a lot of attention, especially from the tlrheoretical community and the detailed literature survey for the experimental ai nd theoretical studies of this prototype reaction will be provided in the achapter 4. 1-4. Thesis preview
Photoinitiated reaction in the van der Waals (vdW) complexes, LiooFCH3 and LiooFH,provides a novel approach to the study of the excited Li* (2p 2~) reactions with halide molecules, Li'(2p 'P) +CH3F(or HF) + LiF + CH3 (or H).
A selected TS configuration between the reactants, Li* + FR, and the products,
LiR + F, can be accessed by photoexcitation of Li-FR. The configuration of the
TS is detemined by the geometry of the ground-state vdW cornplex and the excitation-laser wavelength, hl.
The transition-state of the above reaction was studied by monitorhg the depletion of LiooFR as a function of hi. This provided the so-called photodepletion 'action' spectra. These spectra will be compared with the results of high-level ab inifio calculations performed by others. Theory assisted in the interpretation of the observed spectra and indicated the dynamics of the excited-state Li* reaction with XR. The excited-state Li* reactions have been suggested to proceed vis surface-hopping from the TS to the ground state. The
TS lifetime, related with the surface-hopping probability, was measured by observation of an optical 'saturation'. The concept behind this observation, and the results will be described in this thesis. Chapter 2. Experiments
2 Apparatus
2-1-1. Vacuum system
Experiments were perforrned in 3-stage differentially-pumped vacuum chambers shown in fig. 2-1. These chambers composed a typical Wiley-
McLarlen type of time-of-flig ht mass spectrometer (TO FMS) . Positive-ions fonned by an ionization-laser beam at a region denoted by d of fig. 2-1 in a main chamber were flown to a detector in a third chamber by deflectionlacceleration electric fields. The ions could be separated according to their mass, due to differences in their flight times. The overall length of the machine was -1.5 m.
The main chamber was a stage where van der Waals (vdW) complexes were formed and were electronically excited or ionized by lasers. The ions formed by the ionization laser were extracted into a second chamber by a -100
Vlcrn electric field between the first and the second extraction plates (see a,b in fig. 2-1). The pressure in the main chamber was relatively high because gaseous reactants and inert gases were injected into this chamber during the experiment.
The pressure was rnaintained below 1o-~ torr by a 5000 Ils 10" diffusion-pump
(Varian VHS 10) connected to a two-stage rotary backing pump (Edwards
E2M40). In order to prevent an oil backstream, the diffusion pump was baffled by a liquid-nitrogen trap (Varian 0362 Cryo baffle with NRC882 LN2 controller). The chamber could be separated from the baffled diffusion purnp by a butterfiy valve. Figure 2-1: Side view of the vacuum system The second chamber B housed a series of electrodes (plates) at the early
stage, which enabled ions to accelerate to high kinetic energy. This chamber
was pumped by a 4" difhsion pump (Varian VHS 4) baffled by another liquid
nitrogen trap. This chamber could also be isolated from the baffled diffusion
pump by another butterfiy valve. The ion signal was measured in chamber C that
was purnped &y a 2" diffusion pump (Edwards Diffstak series 63). The 4" and Zn
diffusion pumps were both backed by a single-stage rotary backing pump
(Edwards ElM5). The pressure in each chamber was measured by a Bayard-
Alpert ionization gauge (Kurt J. Lesker model4336, with Granville-Phillips
controller, series 260). The pressure in charnber C should be maintained 51
torr for the detector to have a good signal-to-noise ratio. The power source to the
detector was connected to a safety switch that would turn off the detector in
response to an abrupt pressure rise. The average background pressures under
typical operating condaions were CIO", clOb, and 1 torr respectively for
chamber A, B, and C.
2-1-2. Laser-ablation mefhod
The van der Waals (vdW) complexes, LieeFR(R=CH3, H), were forrned by crossing a Li beam generated by a laser-ablation method with another pulsed bearn of appropriate fluoride (CH3F, HF) in an inert-carrier gas (argon or helium).
An experimental arrangement of the laser-ablation assembly for generating Liom
FH vdW complexes was a little dmerent from that for Li&H3 complexes and it 23 will be described in chapter 4. In this chapter, only the arrangement for generation of LimmFCH3complexes will be described in detail.
Motivation for use of laser ablation.
In this laboratory, detailed studies on the sodium-atom plus halide- molecule complexes, Na-XR (X=F, Ci, Br and R=H, CH3, Ph), have been performed over the last decade. 19-24 In those studies, complexes were forrned by crossing a Na beam with the expansion region of a pulsed-supersonic jet of the halide in an inert carrier. However, in those cases, the Na beam was generated by a stainless-steel oven heated to a temperature of 500°C. The vapor pressure of Na at this temperature is -5 torr, which would require us to heat up the oven to
-830 OC in order to get the same vapor pressure for ~i.~'Although a few laboratories were able to generate the Li beams using oven-based methods6W8 , it is dificuit to produce a stable Li beam in this rnanner.
A method which uses a laser beam to produce a non-volatile metal beam was pioneered by Smalley and his CO-workersin 1981.82-84 Since then, this method has been applied to generation of metal and carbon-family beams by many groups. Recently, ~oep's'"'~and ~rena's~~~groups applied the laser- ablation method to production of alkali/alkaIi-earth metal beams. For Li, Fuke et al. ablated a Li rod to study the LÏsolvated by H20 and NH3 molecules.88-90 Formation of the van der Waals complexes from the laser-ablation source
A schematic diagram of the laser-ablation source is shown in fig . 2-2. This assembly was modified from the Smalley-type assernbly with the aid of suggestions made by Prof. F. Davis of Cornell University. The ablation block consisted of two stainless-steel sub-blocks. The Li beam was generated in an upper block, while the Li beam was mixed with a CH3F (Matheson, 99 wt. % min.)
/argon beam in a lower block.
The processes occurring in the upper block during the laser ablation experiment were as follows; the argon carrier gas (Matheson, Ultra High Purity,
99.999% min.) was flowed over a Li surface of the sample-rod along a 2 mm diameter channel (see fig. 2-2). Argon gas was chosen as a carrier, since the Li bearn was more stable when argon was used than when helium was used. This was consistent with the previous experirnental observations where Na-FCH3 vdW complexes were more stable with an argon carrier gas. "i23 This carrier gas was expanded from a homemade piezoelectrically-actuated pulsed valve (PV), called hereafter PVI. menthe carrier-gas pulse passed over the Li surface, a
532nm beam of Nd:YAG laser (Quantel International, 6608-50) hit the Li surface for -10"s. The Li was vaporized in form of ~i*,Li+, Li-, U2*, and etc. and this Li plume was swept away by the carrier gas. The Li plasma was believed to be quenched and themalized to some degree when mixed with the carrier gas. gas inlet K~J.f
upper block 1 mm diameter / ablation laser hole ablation laser
gas inlet CH3F/Ar 1 lower block
Figure 2-2: Schematic diagram of the laser- ablation assembly The second hannonic of the Nd:YAG laser beam was focused onto the Li
surface through a 1mmdiameter hole (see fig. 2-2). using a IOOcm focal length
lens. The power of the ablation laser was rneasured between 10-1 5mJlpulse
before the main charnber fiange. In aligning the ablation-laser beam throug h the
lmm hole, a He-Ne alignment laser beam was used as a reference beam. The
He-Ne beam was sent through the ablation block in the opposite direction to the
ablation laser. Alignment of the abIation laser was carefully made through the
1mm hole in order to utilize as much of the laser beam as possible for the
ablation process. About 40% of the ablation-laser beam went through the 1mm
hole to hit the Li surface. During the experiment, the ablation-laser beam path
could be changed by adjusting the focusing lens to optimize the Li beam
generation.
The Li sample-rod was translated and rotated by a screw mechanism, so
that a fresh Li surface was exposed to the ablation-laser beam. The stability of
the sample-rod rotary motion was observed to be important in generating a stable
Li atomic beam.
The Li beam generated in the upper block was mixed with a 50%
CH3F/argon beam pulsed from another piezoelectric pulsed valve (called PV2) in the lower block (see fig. 2-2). These two beams crossed each other at the right angle as visualized in fig. 2-2. Since Li and CHjF are ready to react directly (Li +
CHsF + LiF +CH3 , AH < 0, exothemic), the Li beam was mixed with CH3F in the lower block after the Li plasma was thermalized to a certain degree while they were flowing down. The mixed LilCH3F/Arbeam expanded adiabatically out of the lower block. The desired vdW complexes, Lia.FCH3, were formed through many-body collisions during the expansion. Experimentally, the relative amount of CH3F to that of the Li gas was an important parameter in obtaining high concentration of the vdW complexes. The arnount of CH3F pulsed out of valve
PV2 could be adjusted through the control of the driver as will be discussed in detail later.
A typical argon backing pressure behind PV1 was between 200 kPa and
250 kPa. The backing pressure behind PV2 was around 200 kPa and the composition of CH3F in argon was -50%. The flow of the substrates, CH3F and the carrier gas (argon), into the copper gas line was controlled by a precision needle valve. The partial pressure of gases was measured by a Baratron pressure gauge (MUS instruments, Inc. 2228) with digital readout (PDR-D-1).
Once the gas line leading to the PV2 was filled with the appropriate gas mixture. complexes could be generated for a sufficiently long time before the mixture had to be replaced. The argon and CH3F gases were used in the experiment without further purification.
Pulsed valvesg'
As described above, two home-built piezoelectrically-actuated pulsed valves were used in this experiment. These valves are the Proch-and-Trickl types of pulsed valves. A schematic diagram of this valve is shown in fig. 2-3.92
A piezo-dis k (P hysik Instrurnente, PieZoelectrïc Translators (PAS), p286.23) located at the center of the valve (a in fig. 2-3)vibrated in response ta high negative-voltage pulses from a homemade puised-valve driver. The piezo-disk was coated lightly with the Fomblin@ fluorinated grease to be protected from the possible fluorine attack. A plunger (c in fig. 2-3) was attached to the piezo-disk so that it vibrated with the disk. An O-ring Witon, 2-003 size) on the tip of the plunger sealed on and off a Irnm diameter noule hole in the snout of the pulsed valve (see d, e, f in fig.2-3). The length and the inner diarneter of the snout were
4.13 and 0.71 cm respectively.
The amount of gas coming out of the pulsed valve could be adj usted by two means. First, it could be controlled by changing the negative-voltage applied to the pulsed valve. A second method was to change a duration-tirne of the negative pulse, i.e. pulse-width, to the pulsed valve. Both methods could be applied on the pulsed-valve drivers for PV1 and PV2. For this experiment, the pulse-duration was fixed at 400 and 250 psec respectively for the pulsed-valve driver 1 and 2. The voltage of the pulse to PV1 was fixed at 450V and the voltage to PV2 was adjusted between -550 and -700V to maximize the concentration of LioeFCH3.The voltage applied to PV2 was very sensitive in foning high concentration of LiaaFCH3complexes, and consequently it was adjusted frequently during the experiment. 7f, lmm diameter hole
c- d, snout pulsed valve
and nut
h, gas inlet \ i, electtic connecter
" Not scaled
Figure 23: Schernatic diagram of a pulsed valve The amount of gas corning out of the pulsed valve could also be controlled mechanically. Mena pulsed valve was assembled, a plunger could be tightened against a wall of the snout (d in fig. 2-3) in such a way that the O-ring could seal the Imm noule hole (f in fig. 2-3) either tightly or loosely. For the present experiments, the PV2 was assembled more tightly than the PV1, allowing the smaller amount of gas to come out of PV2. Both pulsed valves were triggered almost at the same time. However, PV2 was expected to pulse slightly later iihan PVI, since the more tightly assembled PV2 would have a longer mechanical pulsing delay.
The operation of the assembled pulsed valves was tested externally before being installed to the laser-ablation assembly. For this purpose, a hole (-
5 cm in diameter) was made on the main chamber Range. Using an adapter flange (-8cm in diameter), the pulsed valve could be attached onto the main chamber flange. The operation of the pulsed valve was monitored on an anaiog ion-gauge pressure reader (Granville-Phillips controller, series 260). It was observed that the analog-meter needle moved in response to every gas pulse, especialiy when the pulsed valve was operated at low frequencies, Le. at 1-4 Hz.
The amount of gas from the valve could be changed by adjusting the tightness of the plunger against the noule hole while monitoring the pressure reading. Description of the mechanical part of the ablation assembly
A schematic view of the laser-ablation assembly is illustrated in fig . 24.
The ablation blocks (d in fig. 24) were attached to an end of a -25 cm long alurninum beam (j in fig. 2-4). The aluminum beam was reinforced by a strut (n in fig. 24) to minimize lateral distortion. Due to space limitations, the ablation- laser beam was designed to be turned 90° by a right-angle prism (A=C=6.25,
B=9.0 mm, Edmund Scientmc optics, BK7, a in the figure) before hitting the Li surface (see fig . 2-4).
A support-rod for Li (see c in fig. 24) was made of stainless-steel (0.25" in diameter, 8.0 cm in length). As will be described in fig. 2-5, a 1.5 cm long Li was molded ont0 this stainless-steel template, 1.5 cm away from the end of the support-rod. The procedure for rnaking this Li sarnple-rod will be described in detail in the next section. The support-rod was connected to a brass threaded- rod (h in fig. 2-4). It could be translated and rotated by the screw motion of the threaded rod (h) (0.75" in diameter) through a female thread block (1 in fig. 2-4).
To prevent precession of the support-rod, the thread on the rod (h) was made very finely (0.75"-40) and a Teflon guide-ring (e in the figure) was attached onto the entrance face of the ablation block. In order to minimize the precession of the support-rod, the female threaded block (1 in the figure), the Teflon guide for the threaded rod (m), and the Teflon ring guide for the Li rod (e) had to be aligned carefully. These three elements were repeatedly adjusted until the to mot
a: right angle prism f: pulsed valve, PV2 k: 90 gear boxes b: 532nm ablation laser g: motor direction reversion switches 1: female threaded block c: Li sample rod h: threaded rod (rod h) m: Teflon guide for a threaded- d: ablation block i: connecting rod(rod i) n: strut e: Teflon ring guide for Li rod j: aluminum barn O: pulsed valve, PVI threaded-rod (h) rotated srnoothly and the support-rod rotated without noticeable precession.
The threaded-rod (h) was connected to a 90" gearbox (MDC, M.S 90" gearbox, #680300) through a thin stainless-steel rod (i in fig. 2-4) which had a square-shaped end-tip toward the threaded-rod (h). There was a -5crn deep square hole at the end of the threaded-rod (h) to allow the connecting rod (i)to slide into the threaded-rod (h). When the rod (i) was rotated, this rotational motion was converted to the translational/rotational motion of the threaded-rod through the screw mechanism.
The Li suppokt-rod was rotated by an extemal rnotor (Edrnund Scientific 1 rpm 12 Volt, #41860) at the speed of -1 rpm. The rotational movement of the external motor was transferred to another 90" gearbox through a vacuum-sealed rotary feedthrough (MDC, DDRM-275, #652100). The direction and the speed of the motor were controlled by a homemade motor driver. The rotation speed of the motor was detemined by the DC voltage applied to the motor. The 16 DC
Volt was provided to the motor driver from a DC power-supply (Hewlett Packard,
E3611A DC Power Supply). The direction of the sample-rod rotation was designed to reverse whenever the threaded-rod (h) touched switches (see g in fig. 24) insulated on Teflon plates. The location of these switches was carefully chosen to make sure that the ablation laser hits only the Li surface (1 -5 cm long) on the support-rod. Procedure for making the Li sample-rod3
Lithium is a soft metal, so it can not be rotated intact, nor connected directly to the threaded-rod (h in fig. 24). Instead, the lithium was molded ont0 a stainless-steel support-rod (or template). The suppoit-rod had a -3cm long male
Uiread at the end. The Li was pressed ont0 this thread 1.5 cm long and a fernale-threaded cap was put on the rest of the thread. The procedure for making the Li sample-rod was learned from Prof. Fuke in ~apan.~~The details are as follows; the lithium reacts with oxygen and even with nitrogen in the air, so that the Li sample-rod had to be made inside a glove box (Bel-Art, Scienceware@
H50025-0000, Plexiglas@)purged by argon gas for about 15 min. The procedure for making the Li sample-rod is described pictorially in fig. 2-5 (a) and (W.
In step 1, a 0.5" diarneter Li-ingot (Alfa Aesar, 99.9% purity. Natural abundance 7~i(91%), 6~i(9%))was cut -1 -5 cm long using a razor. In step 2, the center of this disk-shaped ingot was penetrated using the sharp end of the stainless support-rod. Then, a female-threaded cap was put on the threaded part of the support-rod. The cap and the support-rod were locked together by a small
Allen setscrew (it is not shown in fig. 2-5). The Li-ingot had a bigger diameter than the sample-rod; the excess part of the ingot had to be cut off using the razor in step 3. This roughly-shaped Li sample-tod was pressed into the Li rod mold.
As can be seen from the cross-sectional view (step 4 of fig. 2-5 (b)), the mold razor step 1
Cut Li
cap (closeci upper end) step 2 of support-r Insert support-rod Li . '; /
step 3
Shave Li
Figure 2-5: (a) The prodecedure for molding the Li support-rod Press Step4 /
Shape Li
cross-section Step5
Shave again
Repeat step 4 and step 5 several times a a
Step6
Final product
Figure 2-5: (b) The prodecedure for molding the Li support-rod had a sharp edge around the Li sample-rod and some space into which excess Li
could be squeezed. This excess lithium was cut off in step 5. At a further step,
this tnmmed Li rod was once more pressed, but this time the Li rod was rotated
90" so that the Li surface could be molded cylindrically. Steps 4 and 5 were
repeated 4-5 times until the Li rod was smooth and round. The completed Li
sample-rod was transferred in an argon-filled bottle to the main chamber for
installation in the laser-ablation assembly. It usually took 15-20 min to make a
smooth Li rod, excluding -15min. argon purging.
2-1-3. Laser system
Excitation laser, L,
The van der Waals complexes forrned in the laser-ablation source were
carried down by the expanding jet a distance of 15mm into the laser-interaction
region (see fig. 2-1, d). The complexes were electronically excited by a tunable
dye laser (LI; Lambda Physik, LPD3000) pumped by an excimer laser (Lambda
Physik, LPX200). The excimer laser was operated with a XelHCVNe 308nm gas fiIl and had a -400 mJIpulse operating power.
The excitation laser was triggered after a delay of 450-550 ps following the trigger to the PV1. The laser beam entered the main chamber through a series of light baffles in a side am. This arrangement was designed to collimate LI and reduce the amount of scattered light in the laser-interaction region. The pulse energy of the excitation-laser beam was adjusted using a prism attenuator
(Newport Model 935-5). A beam splitter was used to reflect -10% of the laser power of LI ont0 a Joule-meter (Molectron Model J3-02DW), the signal from which was amplified and measured by a sampl'e-and-hold circuit (Molectron, JD
1000 Joulemeter display). The signal from the Joule-rneter had to be calibrated to obtain the actual laser power used for photoexcitation of the vdW complexes.
lonization laser, Lz
The van der Waals complexes were ionized by an excimer laser (L2.
Lumonics. TE 861-T-3). The excimer laser (ionization laser) was operated with a
Kr/F2/He 248nm gas fiIl and had a 60-1 00 rnJ/pulse operating power. The ionization laser, L2,was fired alone and then subsequent to the excitation laser,
Li,after a delay of 100-150 ns to estimate the depletion intensity (it will be explained in detail in the following sections). The laser L2 was focused rnildly by an f=100cm lens to fit its geometric cross-section into an area within the diarneter of the excitation laser. The laser L2 entered the main chamber from the opposite side to Li through another side arm containing the light baffles. The laser beams,
Li and L2, were aligned to counter-propagate. Special -re was taken of in aligning the ionization-laser beam through the main charnber in order to make sure that the ionization-laser bearn did not hit the extraction plates (a, b in fig. 2- The depletion signal, NINo, as a fuimtion of delay time between the excitation and the ionization laser is shown in fig. 2-6,where N and Noare the concentrations, Le. intensity of ion-signals, of the ground-state LiemFCH3vdW complexes with and without the excitation laser. The excitation laser initiated the excited-state reactions to yield the products and thus the concentration of the ground-state complexes decreased after rthe excitation laser was fired. The depletion signal was not obtained when the ionization laser was fired before the excitation laser. This indicated that photminduced fragmentation of [Lio.FCH3]+ did not occur. The depletion signal of neutral complexes was observed to be constant with respect to delay time betw-en 100 and 400ns. The zero for the delay time was broadened by tirne-jitter bcetween the two lasers, and also by the laser duration (20ns). After 400ns. the despletion signai intensity NINOstarted to increase as complexes escaped from the extraction region of the TOFMS. In this experiment, -1 00ns delay was used betwveen the excitation and the ionization laser.
2-7-4. Time-of-flight mass spectrometew (TOFMS)
TOFMS
The vdW complexes ionized by the ionization laser, L2, in the laser interaction region (see d in fig. 2-1) were extracted into the second chamber by a
+100V/cm electric field applied between the first extraction plate (+100V) and the -400 -200 O 200 400 time delay, At= t (L2)-t (L,) / ns
Figure 2-6: Depletion-signal versus delay between two lasers second extraction plate (grounded). The first extraction plate (a in fig. 2-1) consisted of a 6.3 cm x 3.2 cm stainless-steel plate supported by 4 insulated
Teflon legs from the grounded second extraction stainless-steel plate (b in fig -2-
1). A constant -+100V was applied on the first extraction plate from the power- supply (Spellman High Voltage DC Supply). The electric wiring is illustrated in fig. 2-7; schematic drawing of experimental setup. lons generated during the laser-ablation process, not by photoionization, were rernoved by a constant electric field between the first and second extraction plates, above the laser- interaction region (d in fig. 2-1).
lons extracted from the laser-interaction region was focused and accelerated to 2,200eV kinetic energy by ion-optics located at the early stage of the second chamber. The ion-optics was composed of 12 equally-spaced stainless-steel plates (see e(1-12) in fig. 2-1). These plates were isolated frorn one another by Teflon spacers. The voltages shown in table 2-1 was applied to each plate from the high-voltage power supply (Spellman RHSR 5PN50 power supply with a home-built potential divider). The voltages had an overall pattern that increased linearly. The specific voltages on the plates were obtained through a MacSimon simulation program. The simulation showed that a Iinear increase of voltage in the early part of ion-optics helped an ion-beam to be collimated and the high voltage on the later plates focused the ion-bearn around a MCP (MicroChannel Plate, Galileo, 1396-0060 matched pair) detector. Figure 2-7: Schematic drawing of experimental setup plate voltageN 1 plate voltage N
Table 2-1 : Voltages on the plates and the flight-tubes in the TOFMS The same negative voltage on the last plate of the ion-optics was applied to the flight tube (f in fig. 2-1) in Chamber C to maintain the focusing of ion-beam.
This flight-tube ended with a wire mesh that would protect the MCP from an abruptly big ion-signal. The ions accelerated by the ion-optics flew through a
60cm free space and were detected at the chevron-type MCP (4cm in diameter).
When a particle with a high kinetic energy hit the surface of the MCP, an electron was emitted and amplified into large number of electrons through the microchannels. The structure of the MCP is roughly sketched as h in fig. 2-1. A -
1kV voltage was applied on each plate of MCP from the power supply (Bertan power supply, model # 215). The amplified electrons were collected at an electrode. This signal was then once again amplified by a factor of 10x using a
300 MHz preamplifier (Philips Scientific Model6950).
Data acquisition & timing circuitry
A block diagram of the data-acquisition system is illustrated in fig. 2-7.
The electric connections involved in the laser ablation, the photoexcitation, and the ionization processes have already been described above in detail. Thus, only the electric connections for the dataacquisition will be described below.
A preamplified ion-signal from the MCP was connected to a 'sample' port on a Boxcar gated-integrator and signal-averager (Stanford Research SR250).
An ion signal corresponding to the van der Waals complexes was integrated and fed into #1 input port of an NDconverter (Stanford Research SR245). Along with ion-signal collected at the MCP electrode, the electric voltage representing the excitation-laser power was also connected to #3 input port of the ND converter. The signals in the NDconverter was sent to the personal computer
(Pentium il 200 MHz) through a GPlB (General Purpose Interface Bus) interface.
The depletion intensity and the excitation-laser power were stored in the PC hard-drive by a 'depletion master' computer prograrn coded using a 'Labview' graphic language program.
The amplified ion-signal from the MCP was monitored on a 100 MHz oscilloscope (Tektronix 2232) through the Boxcar averager/integrator. The time- of-flight spectra, consisting of 2048 data points taken at interval of 40ns, were recorded at an 'average mode' of the oscilloscope. Averaging of the ion-signal frorn -100 spectra recorded in sequence provided a smooth time-of-flight spectrum. This spectrum was stored in the hard drive of the personal computer using the GPlB interface and a 'read the tof program coded by the Labview program.
All the tirne-sequences involved in this experiment were controlled through a 5-channel digital-delay/pulse-generator (Stanford Research, DG535). The connections for timing-control are illustrated in detail in fig. 2-7. The port To and
A on the DG535 was used for triggering the pulsed-valve drivers for PV1 and
PV2. The delay between two ports was almost zero, however, the PV1 was expected to pulse slightly earlier than the PV2, due to the longer mechanical
delay of the tig htly-assem bled PV2.
The pulse from the port B on the DG535 triggered the ablation laser. This
pulse and the delayed-pulse generated by another homebuilt pulse-delayer
initiated the lamps and the Q-switch of the Nd:YAG laser respedively.
The port C and D on the DG535 were used for triggering the Lambda
Physik Excimer laser, which was the pump laser for excitation of the vdW
complexes, and the ionization laser (L2)- These two lasers had both very small
internai delays, i.e. -500ns. so that they were fired virtually at the same time. For
the photodepletion intensity measurement of the vdW complexes, Li&R (R=H,
CH3), the Lambda Physik Excirner laser had to be fired at every second trigger
(the reason will be explained in the following section). This firing from every
second pulse was achieved using a hornemade flip-flop circuit.
2-2. Data analysis
Mass spectra
The rnass-number of the ions which corresponded to a particular peak in the TOF spectrum was identified according to their flight time. Since the kinetic energy, ~~rn$/2,of each accelerated ion is the same, the ion mass can be related to the flight time, t, by
where is defined by the time at which the ionization laser, LZ,fired and c is a constant to be determined using known mass numbers and their flight tirne. In this experirnent, 6~i,'~iand Li..0H2 (m=25) was used to obtain c and b. The
Li-OH2 peak was observed at the early stage of the experiment, but could be removed by argon gas-line purging and overnight pumping. The plots of fi versus the fiight time could be fitted into the form of eq. 2-1, which in turn converted the TOF spectrum into the rnass spectrum.
Measurements of the photodepletion cross-section
The transition-state of the excited-state Li'(2p 2~)reactions with halide molecules, Li'(2p 'P) + FR + LiF + R, have been studied in this work by measuring the photodepletion cross-section, ode,, of the corresponding vdW complexes, Li-FR, as a function of the excitation-laser wavelength. The radiation initiated the excited-state reaction in its transition-state region. The photodepletion cross-section is defined as 1 -No a,, a,, = -ln(-) Fl N
where No,N represents the concentration of the ground-state complexes before
and after the excited-state reactions are initiated by the photoexcitation. and FI is the excitation-laser fluence at a particular excitation-laser wavelength, hl, in A*.
The depletion cross-section provides a nomalized scale that can be used to compare the extent of depletion for different vdW complexes and also as a function of the excitation wavelength. The origin of the eq. 2.2 will be discussed
in the following paragraphs.
The depletion of the complex Li..FR following the excitation with a wavelength. 4 (hence vl), is given by
Li-FR + hv1 -, [L~**.FR]' + products (2-3)
If it is assumed that al1 the excited complexes react, leading to depletion of
Li-FR, then
where N is the concentration of the vdW complexes (Le. the ion-signal in the TOF spectrum). 11 is the photon-flux of the excitation laser in photons/(cm2s) and t is the time elapsed during the excitation laser pulse (Le, O duration of the laser pulse). After the excitation laser htasfired, the concentration of complexes will be reduced from No to N. An expression for the depletion cross-section in eq 2.2 can be calculated using The applicability of eq. 2-2 to the system studied here [Li-FCH3] is confirmed by the linear part of the plot in fig. 2-8 for a fluence of the excitation laser below -7 m~/crn~for a specific excitation wavelength of 640nm. This photodepletion cross-section equation bears a similarity to the well-known Beer-Lambert law. In deriving the Beer-Lambert law equation, the decrease of the intensity, 1, can be expressed as since the decrease in the intensity of the incident light along a light path is proportional to the intensity of light and the concentration of the light-absorbing molecules, N. Equation 24and 2-8 are equivalent. In equation of 2-8,the 2 4 6 8 excitation-laser fiuence, FI /rn~crn-~ Figure 2-8: Depletion signal versus excitation-laser fluence proportional constant. K, replaced the photodepletion cross-section, G~~~~of equation 24. Equation 2-8 can be made to yield the Beer-Lambert law. The measurements of adep at a particular wavelength were typically averaged over 100 repetitions of the excitation and ionization laser pulses. The photodepletion cross-section of the complexes was found to fluctuate between measurements by t10%. This was due to a shot-to-shot instability in formation of the complexes by the laser-ablation rnethod. In addition. the magnitude of the cross-section in the photodepletion spectrum was found to Vary day-to-day by +20%. This fluctuation was probably due to changes either in the alignment of the overlapping laser beams or the homogeneity of the excitation-laser beam. The measurement of the photodepletion cross-section as a function of excitation- laser wavelength constitutes an 'action spectrum'. The action spectra were used to obtain information on the transition-state of the excited-state reaction, Li* + FR -+ LiF + R. In order to study the excited-state Li' (2p 2~)reactions with halide- molecules starting in transition-state configurations of the excited-state reactions, the corresponding van der Waals (vdW) complexes, Li-XR (X=F, R=H, CH3)' were formed and were photoexcited. The complexes were formed by laser ablation. The laser-ablation apparatus was the Smalley-type ablation source. A lithium metal pulsed beam was generated by laser ablation and was crossed with an appropriate halide pulsed beam inside the ablation block. Two piezo- electn'cally-actuated pulsed valves were used to sweep the lithium plume in an inert gas carrier, and to mix it with pulses of the halide molecules. A detailed description of the laser-ablation apparatus and of the pulsed valves is to be found in this chapter. The mathematical form of the photodepletion cross-section is given and the experimental setups for measurement of the photodepletion cross- section are described. Chapter 3. Transition State Spectroscopy for the reaction of Lie(2p 'P) + CHIF + LiF + CH3 3 Introduction This chapter will present studies of the transition state (TS) spectrum for the reaction of the excited-state Li' with the rnethyl-fluoride molecule, Li* (2p *P) + CH3F + LiF + CH3. The TS of the above reaction couid be accessed by photoexcitation of the corresponding van der Waals (vdW) cornplex, Li-FCH3. This methodology was originally developed by Soep et al. and by Wttig and his CO-workers. Their work was briefly described in chapter 1. The vdW complexes, Li-FCH3, were fomed using the laser-ablation method as described in chapte; 2. Monochromatic excitation of the complexes accessed a potential-energy-surface (PES) traversed by the reaction Li'(2p *P) + FCH3 + LiF + CH3, at a configuration in the TS region. The charge-transfer dissociation following the photoexcitation led to depletion of the complexes. The extent of the depletion, which was expressed in the fom of the 'depletion cross- section', was rneasured as the wavelength of the excitation laser was scanned. This procedure produced a photodepletion 'action' spectrum for the LLFCH3 complexes, the analysis of which provided information regarding the TS for the reaction of the excited-state Li' with the methyl fluoride. In the past, reactions of the excited-state sodium with the halide molecules were studied in this laboratory by measuring the photodepletion cross-section of the Na..XR (X=F, CI, or Br and R= H, CH3, or ph) vd~complexes's24. In the present studies it is hoped that the increased simplicity obtained by using a Li atom rather than a Na atom as a chromophore will lead to a more accurate analysis of the PES using ab initio calculations. In this chapter, the experimental depletion spectrum will be cornpared with a simulated photoabsorption spectrurn obtained by ab initio calculations. These ab inifio calculations were perfomed by Dr- F. Naumkin in this laboratory. Comparison of the experimental and the theoretical spectra will assist in the assignment of the observed peaks in the experimental spectrum. Experiments 3-2. Identification of the Li..FC& vdW complexes The Li-FCH3 vdW complexes were formed using the laser-ablation method. The ablation source was described in detail in chapter 2,and thus only a very brief description of the ablation source and the experimental setup will be given below, along with a detailed explanation for formation of Li..FCH3 complexes. A 532nm beam of the Nd:YAG laser was focused ont0 a Li rod and generated a Li plume. This plume was swept away by argon carrier pulsed from a piezo-electric pulsed valve located above the Li sarnple-rod (see fig. 2-2). The Li plume was quenched and themalized by collisions with the carrier gas while travelling down along a narrow channel (2mm in diameter) in the stainless-steel ablation block. This beam was then mixed with another pulsed beam of -50% CH3F/argon, in the lower ablation block. Then, the mixed beam of LiJCH3F/Ar expanded out of the ablation block, and the desired vdW complexes were fomed through many-body collisions. The exchange reactions such as are responsible for formation of the LiaoFCH3complexes, since in order for two molecules (Li and CH3F) to be trapped in a van der Waals potential-well, there should be a third body to carry away any excess energy. Among the three candidate pathways, the last two are the most plausible since the concentration of Li dimer was found to be low in the photoionization time-of-flight (TOF) spectrum. A typical TOF mass spectrum produced from the mixing of a -50% CH3F/argon (at 250kPa backing pressure) with a Li beam is illustrated in fig. 3-1. Li.. FCH, **--- before excitation - after excitation O 20 40 60 80 ion mass / a.u. Figure 3-1: TOF mass spectrum for the LieeFCH3cornplexeç. The ion signal was rneasured following photoionization by laser L2 (L2=248nm). The dotted line represents ion-signals before photoexcitation and the solid line is the ion-signal after photoexcitation. The ion-signals were obtained using a KrF (248nm) excimer laser, L2. The laser power was usually maintained in the range of 60-1 OOmJ/pulse and was focused loosely by a 100cm focal length lens. A strong signal from Li and a weak signal from Li2 are evident in the TOF spectrurn. Lithium has two major isotopes; 6~i and 7~iwith 9% and 91% natural abundance respectively. In the TOF spectrum, these isotopes are not well resolved. However, in the TOF spectrum obtained using only argon carrier without CH3F,the two isotopes were ciearly resolved. The Li [I.P.(lonization Potential) = 5.4eV] and Li2 [I.P. = 5.leVI must be ionized by a two photon at 248nrn since the ionization laser has E=S.OeV energy?' This suggests that the concentration of Li in the molecular beam is higher than is seen in the TOF specirum, since Li ion signal, Li', was obtained in the less efficient two-photon process. The TOF spectrum obtained without CH3F also indicated that a Li monomer was a dominant species in the molecular beam. A similar observation was made in the experimental studies by Davis et al. who used the same type of the laser-ablation apparatus. 94g95 Although the laser- ablation method is generally known to produce a significant amount of metal clustering, Davis et al. observed rnolybdenum monomer as a dominant species from the laser-ablation of this rnetai. In our TOF spectrum, Li clusters bigger than Li2 are not observed despite of the fact that the I.P. of these ctusters are believed to be below the ionization-laser energy. A broad peak corresponding to mass 41 was obtained in fig. 3-1. This peak was assigned to LLeFCH3,and was shown to be single-photon ionized in fig. 3-2. The dependence of the ion-signal on the ionization-laser fluence was measured for the LiooFCH3complexes, showing a linear dependence with a dope of unity (see fig. 3-2). This indicates a single-photon ionization mechanisrn of the Li-FCH3 complex. This observation provides an upper Iimit to the ionization potential of the LiUFCH3cornpiex, i.e. I.P.(Lie.FCH3) c 5.0 eV. This means that the ionization potential of Li was lowered significantly when Li was complexed with FCH3, from an atomic value of 5.39eV. A similar decrease in the ionization potential was also observed for NaaeFCH3complexes with respect to atomic Na; I.P. (Na)= 5.14eV and I.P. (NaoeFCH3)= 4.3ev." Complexes with more than a single FCH3 molecule attached to the Li atom, Le. Lioo(FCH3),with ns, are not found in the TOF spectrum. This is in contrast to earlier measurements of the TOF mass spectra for the crossed- molecular beams Na + FCH3 using an oven as the source of Na, in which the complexes Naoo(FCH3), with m 1 5 were observed.2223 The depleted ion-signal for the LimoFCH3complex was measured by exciting the complex prior to photoionization. The dotted line in the TOF spectrum (see fig. 3-1) represents the ion-signals obtained by the ionization laser, L2, in the absence of the photoexcitation laser, LI, and the solid line is the ion-signals in the presence of Li prior to L2. For the purposes of this TOF Slope = 1.02 Figure 3-2: Power dependence of the ionization signal (A2 =248nrn) for LLFCH3. The straight line is a linear least-squares fit to the data points. The dope of the line is 1.02. spectrum, a 640nrn excitation-laser wavelength was used. The wavelength was chosen merely to illustrate the depletion of the complex. The laser fiuences for Li and L2was kept constant for al1 measurements. For the purpose of fig. 3-1, the fluence of LI was kept low (0.5-1.5m~/cm~),resulting in depletion of -50%. The Ruence of Li was kept low to ensure that al1 the photodepletion measurements were performed in the region where the Beer-Lambert law applied (see section 2-2). Photodepletion involves formation of the harpooning- reaction products, LiF + CH3 and Li + CH3F- Water impurities in the gas iine leading to both pulsed valves gave rise to formation of the LieeOH2cornplex. This [LioeOH2]+ion-signal with m/z=25 was used for mass-calibration of the TOF spectrum along with li+(m/z=7) and ~i*+ (m/z=14) peaks. This ion-signal was removed from the TOF spectrum through overnight gas-line pumping. The fi uctuation of ion-signals corresponding to Li and Li2 was relatively high (-25%). This is mainly due to a shot-to-shot instability of the laser-ablation process. Although the Li rod was rotated constantly, and also pre-ablated before the experiment to burn-off the oxidized layer on the Li surface, the Li surface exposed to the ablation laser must be irregular on a shot-to-shot basis. This was supported by the obsenration that the stability of ion-peaks corresponding to Li and Li2 varied when the speed of the rod rotation was changed. Despite the above. the shot-to-shot fluctuation of the Li-FCY ion-signal was relatively small (45%). This is due to the fact that CH3F was the lirniting reagent in formation of the complexes. The amount of CH3Ffrom the tightly- assembled valve PV2 was smaller than that of Li gas from PV1. Therefore, the concentration of Li-FCH3 complexes could be maintained relatively constant notwithstanding the fluctuations of the overall Li concentration on a shot-to-shot basis. A discrepancy iii fluctuation of different ion-peaks made it difficult to obtain reproducible TOF spectra even at a short time interval. The logarithm of the depletion signal, In(No/N), was shown as a function of the excitation-laser fluence, FI, in fig. 2-8. A linear increase of In(NdN) up to 7 m~lcrn*(corresponding to a depletion intensity of -95%) was observed at a fixed excitation wavelength of 640nrn, in accordznce with the photodepletion cross- section equation. A similar dependence of the depletion was observed at other excitation wavelengths. At a higher excitation-laser fluence, Le. ~7rn~/cm~, leveling-off of the ion-signal was observed. This leveling-off was due to the complete depletion of the vdW complexes, not to optical 'saturation' resulting from the finite lifetime of the excited-state vdW complexes. The measurement in fig. 2-8 is in contrast to a similar measurement for Na-FCH3 for which the optical 'saturation' of the depletion intensity was found at moderate fractional depletion as FI was increased. The consequence of this latter observation will be discussed Iater in this chapter, in which an estimate of the lifetime for the transition state will be obtained from the observation of 'saturation'. 3-3. Photodepletionspectnrm for the LLFCH3 complex The transition-state (TS) of the reaction, Li*(Pp 'P) + CH3F + LiF + CH3, was studied by measuring the photodepletion cross-section, ad,,, of the LimoFCH3 complexes. The concentration of the complexes before and after irradiation with the excitation laser, LI, was measured as described in chapter 2. The depletion cross-section in A2 was calculated using the equation, ode, = 1IFl[ln(NdN)]. where No and N represent the ion-signal of the LiooFCH3complexes before and after photodepletion, and Fi refers to the excitation-laser fluence in k2.The photodepletion cross-section was measured across a broad range of wavelengths, hl, from 570 to 860nm with a step size of 0.25nm. The depletion spectrum was obtained from an average of 4 to 6 scans of the wavelength region, Al, covered by each dye laser. Each scan was obtained by averaging the depletion intensity, In(NdN), from 25 repetitions of the laser pulses at each wavelength of Al, Le. with the excitation laser, LI, and the ionization laser. L2, fired together followed by L2 fired alone. The 'action' spectrum for the depletion of the LieoFCH3complexes consisted of two broad regions in which a significant depletion was observed, as illustrated in fig. 3-3. A first region with two sharp depletion maxima was observed between 700 and 850nm, and the second region between 570 and 680nm was a broad symmetrical peak with weakly resolved structure. For cornparison, the 'action' spectrum for depletion of Nao.FCH3 is also shown below excitation energy, El/eV excitation wavelength, h ,hm Figure 3-3: Action spectra for (a) LiaqFCH3and (b) NaeaFCH3complexes, showing the depletion cross-section in A2 VS. the wavelength hl of laser L1 in nm. the action spectrum for Li-FCY. This spectrum of Na-FCH3 was previously pub~ished.~~It exhibits hivo broad regions of depletion as in the case of LiooFCH3. The maximum photodepletion cross-section in the NaooFCH3action spectrum was -0.7A2 at 590nm, which is to be compared with the larger maximum cross- section of -7A2 obsewed here for Li..FCH3. A thick Iine located at 671 and 589 nm in each spectrum indicates the Li(2p 2~ t 2s 2~)and the Na(3p 2~ t 3s 2~) atomic transition respectively- The most notable difference between the LCFCH3 and Na..FCH3 spectra is in the appearance of the depletion peak observed furthest to the red. For the earlier example of Nao.FCH3, this peak was of comparable size to that observed toward the blue. However, for LLFCH3, the depletion cross-section at the red side was larger than that to the blue, and more structured. In the NaOmFCH3spectrum, the depletion peak located further to the blue was obsewed in the middle of the Na atornic transition. In contrast to that, for Li.. FCH3,the broad envelope was observed to the blue of the Li atomic transition (2p *P t 2s 's) between 570 and 680nm. This depletion peak extended approximately 0.33eV to the blue of the Li atomic transition. This blue shift iç believed to be related to a deep potential-well of the ground state Li..FCH3 complex and a high energy potential-well above the Li (2p 2~)+ CH3F asymptote (see below). 3-4. Assignments of the depletion peaks An assignment of the peaks observed in the photodepletion spectrum for Na-FCH3 can be made based on different electronically-excited states of the complex, N~*~.FcH~.~~The peaks observed in the LLFCH3 spectrum can be assigned in a similar rnanner. The assignment is illustrated in fig. 3-3, and is described in the following paragraphs. In discussion section, a detailed description of the action spectrurn for LieeFCH3will be given based on ab initio calcu lations. Electron-density contour plots, obtained by ab initio calculations in ref. 23, for the highest-occupied molecular orbital in the ground and electronically-excited states of NaaeFCH3(see fig. 9 in ref. 23) indicated that the electron-distribution around the Na atom resembled distinct atomic orbitals. An assignment of the electronic states of either NaeaFCH3or LiemFCH3can, therefore, be based on the atomic states of the alkali-metal atom which are seen as being perturbed by the FCH3 molecule. The appropriate atomic transition which correlates with excitation to the electronically-excited states of the complex accessed in the wavelength range illustrated in fig. 3-3 are np (2~)t ns (*s), where n=2 for Li and n=3 for Na. The atomic transitions (D-line) are indicated by bold lines in fig. 3-3. The orientation of the p orbitals in the excited alkali-metal atom can give rise to either a Z-type interaction from the pz orbital, or a II-type interaction from the px and p, orbitals, with the closed-shell FCH3 molecule in the complex. The vdW interaction between Li and FCH3 induces a splitting between the 2~ states arising from the atomic orbitals 2p, and 2p,,. The interaction between the paired electrons in the orbitals of the closed-shell halide, FCH3, and the p, orbital of Li' (i.e. gives rise to stabilization of the complex. The interaction becomes repulsive in Li*(2p, 2~)due to unfavorable overlap between the occupied 2p, orbital and the closed-shell halide rnolecule. However, attractive dispersion interactions in Li*-FCH3 are sufficient to stabilize the excited-state complex. The peak observed furthest to the red in the photodepletion spectra is assigned to the superposition of the depletion cross-section in which the complexes are excited to the npx2~ and np, 2~ states. The peak observed furthest to the blue corresponds to excitation to the npz2~ state (the subscript in the atomic orbital occupation represents the orientation of the np orbital relative to the axis of the van der Waals bond). Structure was observed in the depletion peaks of the spectra for both Li.. FCH3and Na..FCH3 complexes. The origin of this structure has been identified earlier, for the example of NaooFCH3.as arising from vibrational modes in both the ground state and the TS. However, in the LL.FCH3 spectrum it was not possible to identify progressions of vibrational lines in the depletion peaks (fig. 3- 3). The depletion peak measured for LioeFCH3from hl = 570 to 680nm appeared to contain numerous superimposed progressions of vibrational peaks. In contrast, it appeared that only a few vibration modes were excited in the depletion region between 700 and 850nm. The origin of the intense vibrational maxima in this depletion region will be described later in this chapter. The integrated intensity of the depletion illustrated in fig. 3-3 is proportional to the oscillator strength, fnp+ns, for the electronic transition within the complex. Depletion of the complex occurs following absorption of a photon. Therefore, the value of the measured depletion should reflect the cross-section for absorption by the complex (this is further justified in ref. 40). In ref. 96, the cross-section for absorption integrated across the appropriate energy interval for each electronicaily-excited state, i, was shown to be equal to (ah2/2me)fi,where a is the unitless fine-structure constant (a = 11137). h is Planck's constant. me is the mass of an electron and fi is the oscillator strength for the electronically- excited state, i. In order to determine the oscillator strength for the np t ns transition, the depletion cross-sections, ~d~~ (in A'), for Li..FCH3 and NaabFCH3 have been integrated across the energy interval (in eV; where El=hc/h~)for the wavelength region observed in fig. 3-3. The values obtained were hp- -0-6for Li..FCH3 and f3,,% -0.2 for Na-FCH3. The oscillator strength, f,,,,,, for both Lib.FCH3 (n = 2) and NaoeFCH3(n = 3) complexes are much reduced relative to f = 1 for the corresponding metal atom. This is in accord with measurements made by Brockhaus et al. for Naob (NH~)~.'~In their work, f = 0.36 was obtained for NabaNH3,and f = 0.024 for Naab (Ni-& The surn of the oscillator strength for al1 possible transitions must, however, remain unity. Excitaüons in the complex other than 2p 2~ t 2s 2~ for Li&CH3 and 3p 2~ t 3s 'S for Na-FCH3 rnust therefore make up the difference, as in the case of the Na..(NH& complexes. This is indicative of a substantial difference in transition probabilities between the free and the complexed alkali- metal atom. Photodepletion of the Lie.FCH3 complex was probed across an additional range of excitation wavelengths, between 450 and 570nm. The intention was to measure depietion occurnng through excitation to sorne of the higher electronically-excited states, e.g. 3s 2~.It was expected that there should be signifiant stabilization of the 3s 2S state of LiooFCH3.This would be in accord with measurernents of photodepletion for NaooFCH3reported in ref. 23 in which depletion was observed from the 4s 2~ excited state of the complex at a wavelength much longer than that needed to excite the atomic transition, 4s 'S c 3s 2~,in Na. Depletion via higher electronically-excited states of LIoFCH3was not observed, perhaps because the region in which the atomic transition 3s 2~ c 2s 2~ occurs in Li (368nm)was not approached. Theoretical Results and Discussion 3-5. Pot en fial-energy-surfaces Ab initio Li(2s, 2p, 3s) + FCH3 potential-energy surfaces (PES) were calculated using the multireference configuration interaction (MRCI) method by Dr. Fedor Naumkin of this group. The calculated interaction potentials for the 2s and 2p,,, states of LLFCH:, are shown in fig. 3-4 for Li approaching the F-end of the FCH3 molecule along the (F-C) z-axis (Le. O (Li-F-C) = 180Q),with the molecule fked in its equilibrium geometry. Both the ground and 2px,, states are bound more strongly.for the electronically excited states, as was the case for the O 1 2 3 4 5 6 irtemuclear distance, r(Li-F) Figure 34: Potential-energy curves for the ground and the electronically- excited states of LI-FCH3, along an approximate minimum-energ y path for the coliinear PES. The observed spectrum is shown, against the appropriate energies, inset at the top left. analogous sodium cornp~ex.*~The 2pr state. however, exhibits a quasi-bound character, with a potential bamer ahead of the well. The higher energy for the 2pz state is connected with the stronger repulsion between the electron densities of FCH3 and of Li Pp-orbital extended along the z-axis. For a preliminary identification. the observed depletion spectrum is plotted in fig. 3-4 along the ordinate at the appropriate energy. The energy gaps between the ground (2s) and excited (2p,,,) states, obtained ab initio, can be seen to be in broad agreement with the observed excitation energies in photodepletion. A more detailed cornparison between theory and experiment will be given later in this section. The potential energy for the 3s state of Li..FCH3 was also calculated. Since the vertical-excitation energy (-2.8 eV) required to access this state from the equilibrium configuration of the ground state lies outside the region of wavelengths it is understandable that this transition was not present in this photodepletion spectra. Potential-energy contour maps for the ground and excited states of Li- FCH3 are illustrated in fig. 3-5. The energy is shown as function of the position of the Li atom relative to the F atom (located at the origin), with the central axis of Li(2s) PES Li(2p,) PES contour step 50 meV contour step 50 meV Li(2pJ PES Li(2pz) PES contour step 50 meV -contour step SO meV Figure 3-5: Potential-energy contour plots (a) for the ground state 2s and (b) (c) (d) for the electronically-excited states 2px, Zp,, 2p, of Li-FCH3. the CH3 umbrella onented along the z-axis. For both the ground and excited states of the cornplex, potential energies changed only weakly as the bond angle, @(Li-F-C),decreased from 180" to -120°, but increased substantially when it approached 90". The minimum-energy structure for the ground-state complex, LiomFCH3, was found to have a bent configuration with @.(Li-F-C) 440" (see fig. 3-6), larger by about 6" than for Na4.FCH3 and by about 30" than for LimmFHfit will be described in the next chapter). The van der Waals bond length was determined to be r&-F) =1.91A, which is shorter than re(Na-F) = 2.36 A in the sodium complex due to the smaller size of Li. The r,(Li-F) value is close to that for Li-FH (1.89R see chapter 4) and can be cornpared with the bond length for the product LiF rnolecule, re4-56 A. The binding energy for the ground-state complex was detemined to be Dea0.2leV. similar to that for Na-FCH3 (D. ~0.21 and Li- FH (De=0.24eV, see chapter 4), al1 of which are more strongly bound than Na- FH (De =0.07e~~~). The 2px,states are nearly degenerate and more strongly bound than the ground state of the complex, with the binding energies more than twice as large (see fig. 3-6). The equilibrium geometries of these excited states are close to that for the ground-state cornplex. By contrast, the potential-energy minimum for the Zp, state is quite different from that for the ground-state complex. The equilibrium geometry is collinear and the vdW bond length r,(Li-F) is 0.1 5A Figure 3-6: Minimum-energy structures for the Li..FCH3 complex shorter than for the ground state. For decreasing r(Li-F) this excited state is reached across a potential barrier of -0.3eV. These calculations are consistent with the appearance of the photodepletion spectrum for LieeFCH3.Stronger binding in the 2px,, states of the complex relative to the ground state is reflected in the location of a photodepletion peak in fig. 3-3 substantially to the red of the atomic transition for Li. Photoexcitation of LLFCH3in this region of wavelengths can be expected to be dominated by the transitions between the vl,2 = O levels of the vdW stretching (vl) and Li-F-C bending (v2) modes, since r, for the ground state, 2s, and the excited states, 2px,,, are ver-similar. This is reflected in fig. 3-3 by the appearance of two sharp peaks in the depletion cross section at approx. 800 and 830nm, presumably arising from excitation to tle2= O levels of the 2px and 2p, states respectively as judged from their relative energies at equilibrium. Excitations to vals2t 1 levels of the 2p,,, states may give rise to the less intense features observed in this region of the spectrurn. These preliminary assignments are supported by the spectral simulations given later. The appearance of the broad depletion peak for Li-FCH3 observed in the region 570-680nm of fig. 3-3 is consistent with the calculated Iinear geometry of the 2p, state. The significant differences in geometry between the ground and the 2p, excited state results in excitation to a number of different vibrational states of 2pr. This is evident in the simulations in the next section. 3-6. Simulation of the phofoabsotptionspectrum To interpret the relative intensities in the spectra, transition dipole moments p were calculated between the electronic states involved. The 2s-+2pXsytransitions in Li are somewhat amplified when Li approaches the F-end of FCH3, and the 2s+2pZ are reduced. This accounts partly for the higher intensities of the 2p,, peaks as compared with 2pz. When the wmplex bends, the transition moments are almost unaltered, indicating a constant perturbation of Li by FCH3 at a given Li-F distance. The simulated photoabsorption spectrum using the calculated potential energy and transition moment functions is shown in fig. 3-7. The first ten vibrational levels in each of the ground and excited states (without rotation) were included, giving rise to a hundred lines for each transition. The vibrational temperature of the complex was assumed to be -250 K, identified in ref. 40 to be the approximate temperature of the Na-FH cornplex (Though laser-ablation was used in the present work, presumably yielding higher temperature Li, this was subsequently cooled by entrainment in Ar followed by supersonic expansion). The spectra for the 2s-+2px,, transitions are similar, forming two dense groups of lines around 800nm which account for the two sharp peaks in the experimental spectrum in this wavelength region. Each group of lines in the 800nm region is dominated by a single strongest line originating from the 600 650 700 750 800 850 Excitation wavelength hm Figure 3-7: Simulated photoabsorption spectrum of Li-FCH3 for T=250K. Vibrational levels up to v=10 on the ground and excited states were included. transition between the ground vibrational states. The 2s+2pZ spectrum, by contrast, has a relatively wide envelope at about 600nm, composed of several lines of comparable intensity; this accounts for the other component of the observed spectrum. In the 600nrn region the most intense lines correspond to transitions between the v=O state of the lower electronic state and v>O states of the upper state (see Table 3-1). This leads to the smaller wavefunction overlaps, and helps explain the smaller spectral intensity observed, as compared to the 2s+2px,, transitions. The differences between the calculated and experimental spectra lie mainly in the positions of the peaks, particularly for the 2s+2pz transition. These discrepancies are thought to be due to the Iimited basis set and/or the restricted number of reference states. The use of a larger basis set for the linear geometry showed a slight decrease in energy for the 2p, state while leaving the ground state energy unchanged, thus leading to a red shift of the 2s+2pZ spectrum, in qualitative accord with observation. The predicted spectrum will become more continuous with inclusion of rotational excitation, more closely approaching the observed spectrum. More refined calculations can therefore be expected to improve the agreement with experiment. In work presently under way, the range of wavelengths in the depletion spectrum is being extended beyond the present limit of 850nm (fig. 3-3) into the region corresponding to vibrational excitation of C-F to examine the effect of C-F The designation 0,Oetc. refer to vl,v2, where vl,vp refer to the stretching and bending modes. Table 3-1 : CâÇculated electronic-vibrational transitions in Li-FCH3 (wavelength in nm, integrated absorption coefficient in crnlmol) stretch on the dynamics. 3-7. Measurements of the TS lifetime by an op tical 'saturation'. The lifetime of the transition-state, [L~S~FCH~I*,can be measured by analysis of the observed saturation of dep letion, In(NdN) , versus the excitation- laser fiuence. Figure 3-8 shows the laser-excitations and the rate processes initiated in the complex. The kinetic mode1 can be written as follows (see fig. 3- 8) 1 Lie.FCH3 + hv, + [L~*~.FCH~]* rate = BpvN (34) [L~S~FCH~I*+ hv7 +P Li-FCH3 + 2hv7 rate = BpN* (3-5) [L~*-FCH~]*+ LiF + CH3 or Li + FCH3(vr) rate = (1/rS) N* (3-6) , where N and N* denote the concentration of the ground-state (Li-FCH3) and transition-state US) ([L~*~~FcHJ]*)complexes respectively. The factor B is the Einstein coefficient for excitation and stimulated-emission, p~ is the spectral density, and r* is the lifetime of the TS. The Bpv is equai to ~~~~l~,where adep is the photodepletion cross-section measured experirnentally in the Beer-Lambert regime (see chapter 2) and II is the flux of the excitation laser in Equation 3-4 represents photoexcitation of the ground-state cornplex, whereas equation 3- 5 describes the laser-induced stimulated-emission to re-form the ground-state complex. Equation 36 is the depletion process, with a kinetic rate constant of II rS. The rate equations can be solved to yield an expression for the depletion Reaction rate = 1/T* * m [Li*-FCH3]* : Stimulated-emission rate = BpN* Excitation rate i = BpN 9 Lim.FCH3 Figure 3-8: Laser-excitation and rate processes initiated in the complex. intensity, where FI is equal to Ilt, and T =tk*[t is the duration of the excitation-laser pulse, -2Onsl. It has been assumed that the flux of the excitation-laser remains constant during the excitation-laser pulse. Equation 3-7 and 3-8 have been newly derived. The equation to be found in ref 112. (used in A. J. Hudson, PhD Thesis) are incorrect due to a mistake in algebra. The derivation of eqs. 3-7 and 3-8 is to be found in Appendix A. In the low excitation-laser fluence limit, equation 3-7 is reduced to the modified Beer-Lambert equation, I~(N~N)=G~,,F~,which gives a linear relation between adep and In(NdN). In the depletion measurements, a low value of excitation-laser fluence was used to ensure that the depletion of the complexes occurred in the linear region of the Beer-Lambert law. However, at high excitation-laser fluence, it was found that the depletion intensity, In(NdN), of the Li-FCH3 complexes was saturated. This satu ration is due to stimulated-emission from the TS to the ground electronic state. The stimulated-emission becomes important at high excitation-laser fluence since the rate for the process is related with the population (N') of the TS state. Since the stimulated-emission results in re-formation of the stable ground-state cornplex, it sets an upper limit on the percent depletion that can be detemined at high fluence. The dependence of the depletion-intensity on the excitation-laser fluence, F1,at hi = 640nrn, has been measured. The results are shown in fig. 3-9. The percentage depletion is represented on the right vertical axis in fig. 3-9, while the actual value for depletion-intensity is shown on the left vertical axis. The depletion intensity increased linearly up to F1 = 7m~/cm~,and leveled-off at In(NdN) r 3.0 corresponding to 95% depletion at FI >7rn~/cm~.At other hl where depletion was observed, leveling-off was observed at a similar depletion- intensity, In(No/N) 2 3.0. Using equation 3-7, the depletion intensity versus the excitation-laser fluence can be simulated to yield a TS lifetime of r* s 1.3ns at hl = 640nm. Due to the limited detection-sensitivity at depletion intensity higher than 3.0, only an upper lirnit of the TS Iifetime can be measured in this work. To determine r* in the range c 1.3ns requires a precise measurement of depletion in excess of 95% (Le. In(NdN) > 3.0). The upper limit of the TS lifetime rneasured here is one order-of-magnitude less than the T* = 12ns TS lifetime at 690nm that rneasured in earlier optical-saturation work with the sodium complex, for NaooFCH3.Further work, now in progress using the Na-FR complexes, will give the TS lifetime information as a function of the excitation-laser wavelength, and hence rS as a function of the TS configuration. This should provide a guide for future ab initio and dynamical calculations. 5 4 3 2 1 ------ O ,m... 1""'"" 1""'"" 1-"-"--- '"--l O 2 4 6 8 IO Excitation laser fluence in rn~/crn~ Figure 3-9: Simulation of the TS lifetime for LLFCH3at 640nm, using Eq. 3-7 (see text). Expenmentally obtained data points are indicated with red circles with error bars. The experimental data was best fitted with sZ =1.3ns and it was represented with a blue curve. The heavy dashed-line gives comparable data for Na-FCH3 at 690nm.ref 22 3-8. Summary The excited-state Li* reaction with FCHj was studied by measurïng the photodepletion cross-section of the Li-FCH3 vdW complexes across a broad range of excitation-laser wavelengths. The different wavelengths of the excitation-laser enabled the ground-state complex to reach different configurations of the transition-state for the this reaction. The measurement of the photodepletion cross-section as a function of excitation-laser wavelength gave a photodepletion 'action' spectrum. The experimental photodepletion spectrum consisted of two broad regions in which a significant depletion following excitation of the complex was observed. The first region, which was characterized with two high intensity maxima, was observed between 700 and 850nrn and the second region, which had a broad symmetncal feature, was observed between 570 and 680nm. These regions could be assigned based on ab initio calculations. In the wavelength region probed in this experiment, excitation of the complex was correlated to the atomic Li 2p 2~ siate perturbed by FCH3. The 2p 2~ state of atomic Li has 3 degenerated orbitals and these orbitals were split into triplets of 2px, Zp,, 2p, states through the interaction with the FCH3 molecule. Ab initio calculations showed that the minimum-energy structure for the ground-state 86 complex was stabilized by 09leV with respect to the Li + FCH3 asymptote and the 2px,, states were stabilized by 0.51 and 0.49 eV respectively. The ground state and the 2px,, states were calculated to have a similar geometry. The favorable overlap of the wavefunctions due to their similar geometry gave rise to WO sharp high-intensity maxima between 700 and 850nm. In contrast to the 2p,, states, the minimum-energy structure for the 2pz state had a different geometry from that for the ground state. The unfavorable overlap of the wavefunctions led to a very broad symmetrical peak between 570 and 680 nrn. These findings were further confimed by the photoabsorption simulation calculations- The Iifetime of the TS was measured by analysis of the observed saturation of depletion, In(NdN), versus the excitation-laser fluence. The saturation was due to the stirnulated-emission from the TS to the ground electronic state. Using a kinetic model with inclusion of the stimulated-emission process, the saturated could be reproduced to yield the TS Iifetime of T* 5 1.3ns at hl = 640nm. Chapter 4. Transition State Specboscopy for the reaction of Li*(Zp 'P) + HF + LiF + H -The alkali-metal (M) plus halide-molecule (XR) system has played an important role in the development of chernical reaction dynamics since the fint crossed molecular beam experiment in 1955 by Taylor and ~atz.~'Arnong this type of reaction, the Li + HF reaction is the simplest which involves only 13 electrons in total. The M + HX reaction has been known to proceed through a charge-transfer mechanism but with a rather small reaction cross-section due to the negative electron-affinity of HX (see chapter 1). The Li + HF reaction, involving metal and halogen atoms of the lowest atomic number, can be regarded as the prototype of charge-transfer 'harpooning' reactions. This prototype reaction has been studied experimentally in crossed rnolecular beams and the results will be summarizad in the following paragraphs. This system was first studied by Helbing and Rothe in a crossed molecular beam apparat~s.~~They measured the velocity dependence of the non-reactive total cross-section for this system, wh ich showed characteristics of glory undulation. This undulation was expiained in tens of the orientation of HF with respect to Li. For example, in a case of a high velocity collision, a rotational tirne of the molecule was long compared to a characteristic translational time, i.e. smt/rtranç>> 1. and thus the system expenenœd the orientation effect more effciently. Assuming a spherically symmetric interaction with a Lennard-Jones (12-6) potential, the parameter of E, rmwas obtained as E= 0.70 x 10"~erg (-0.0044 eV) and rm=4.ï9A respectively, where E and r, are the well-depth and the position of the potential minimum respectively. About 10 years after the above experirnent, the reaction Li + HF was studied in the crossed-molecular beams by Lee and CO-workersin a more thorough fa~hion.~~They measured angular distributions of product rnolecules at 4 different collision energies ranging from 2 to 9 kcalhole. The product, LiF, was detected at al1 collision energies between 2 and 9 kcalfmole. At low collision energy (3 kcal/mole). the product angular distribution showed the evidence of cornplex-formation with near forward-backward symmetry. However. the angular distribution at high collision energy (8.7 kcal/mole) was quite fonivard-sideways peaked. The product translational energy distribution P(ET1)at both collision energies was observed to give an average ET'of -55% of the total available energy. This appeared consistent with a 'late exit barrief for reaction. From the product translational energy analysis, the reaction exothermicity was also estimated to be AH= -1 -1+ 0.2 kcalfmole for the ground state reaction. From the non-reactive scattering of Li, the rainbow structure was observed at low collision energy, and analysis of the rainbow structure deterrnined a potential well-depth and a minimum-energy position. The resulting value of the well-depth and the minimum-energy position was c= 0.46 kcallmole (-0.020 eV) and rm=4.34A. Recently, Loesch et a/. extended angular distribution measurements into larger center-of-mass (CM) angles in his crossed molecular beam experimentB6 In this study, the backward glory was observed at Etr 1296rneV at large CM angles (-120°) as well as the rainbow structure at small CM angles. A simplified scattering calculation, based on an empirical Lennard-Jones (8,6)PES function, rationalized the rainbow and the backward glory as well as the Elr dependence of these features. The observed angular distribution as a function of Eu was reproduced with a PES with two energy minima. The shallow well region. related with a configuration of Li-H-F, had a maximum well-depth of -33meV. and it was essentially determined by the rainbow scattering. The deep well region featuring a maximum depth of -300 meV was associated with the backward glory. This deep well corresponded to the Li-F-H configuration. Experirnental studies on the steric effect of the orientated HF molecules in the state-specific reaction Li + HF(v=l j=l , m=O) + LiF + H were also performed using a technique of infrared radiation pumping in a Stark field.67 They observed a marked influence of steric effects on the angular distributions, the partition of available energy, and the integral reaction cross-sections. This reaction system has also been a subject of ab initio caiculations. A partial PES for Li + HF was first computed by ab initio means by Lester and Krauss.g9a100 Only the entrance channel was calculated using a Hartree-Fock approximation with HF fixed at the equilibriurn internuclear distance. For the approach of the Li atom toward the F atom, a potential minimum (welldepth = 0.1 57eV) was found for a Li to HF center-of-mass distance of 2.12A.deepening slightly (0.163 eV) for a more bent (1 35O) configuration. A rather strongly bound Li-F-H system was found in an ab initio study by Trenary et al.10' In their studies. the dissociation energy of Li&H was estimated to be 4.2kcal/mole (-0.18 eV). The PESs for Li + HF -+ LiF + H reaction were also obtained through Valence-Bond calculations by Balint-Kurti and yardley.'02 In particular, several surfaces for excited electronic states were also calculated. The grou nd-state PES gave a value of +2.5 kcalhole (-0.1 1 eV) for the reaction endothermicity (without zero-vibration-energy correction). A well-depth in the entrance channel of the PES was predicted to be +2.2 kcaVmole (-0.095 eV) for a Iinear geometry, Le. O(LCF-H)=1 80°. This value increased marginally by -0.25 kcal/mole as bending was introduced to the system. Zeiri and Shapiro also generated the PES for this reaction, using a Valence-Bond semiempirical rneth~d.'~~*'~They showed that an electron can jurnp from Li to the F atom on the ground state PES only when Li was reasonably close to HF molecule. This 'non-harpooning' behavior was explained in ternis of the strong repulsion felt by HF' at a close range. For example, to allow for the electron-transfer to occur, the Li atom had to approach the F atom to alrnaist the LTF' equilibrium distance, and simultaneously the HF bond had to stretch considerably. They also showed that the barrier height reduced and the saddle point moved from the exit channel to the entrance channel as the system moved away from collinearity. This reduction of barrier height was thought to arise frorn the smaller the (HF)- repulsion due to the possibility of the KF t, HF- resonamce, since another ionic configuration, that of L~+H-F,emerged as the systern bent. A small potential-well was observed to be formed around @(Li-F-H)=70° in their calculation. Chen and Schaefer's self-consistent field and configuration interaction (SCF-CI) calculations confirrned the existence of a potential well.lo5 They obtained a depth of 4.5 kcal/mole (-0.195 eV) at a bent geometry of @(Li-F-H) =114*. The ground-state reaction was found to be endothermic by +2.9 kcaVmole (-0.126 eV) without zero-vibrational-energy correction and exothenic by -1.7 kcal/mole (-0.074 eV) with zero-vibrational-energy correction. A minimum-energy barrier of -4 kcalfmole was predicted to be located in the=exit channel at a bent geometry of 74'. More recently, MRDCI (Multi-Reference Configuration-Interaction with single and Double excitation) calculations by Aguado et al. predicted a well depth of 28 kllmole (-0.29 eV) in the entrance-channel at a bent geometry of 106' and also located a minimum-energy barrier of 24 kJ/mole (-0.25eV) above the reactants in the exit channel.lo6 The possibility of perfoming a laser-catalyzed reaction to surmount the energy barrier on the ground state PES led to ab initio calculations of the excited states. Balint-Kurti and Yardley determined the PES for the ground and Iowest electronicallyexcited states of L~FH.'~'They observed a region of avoided- crossing between the lowest excited state and the ground state PES through which the excited-state reaction CO uld occur via a 'surface-hopping' mechanism. An energy gap between the ground state and the lowest-excited state was srnaller for a linear than for a perpendicular geometry. Thus surface-hopping or non-adiabatic transitions between the two states would take place preferentially in the linear geometry. Recently , Ag uado et al. studied the excited-states reaction in detail .'O6 They found that a potential well in the first excited-state located very close to the region of avoided-crossing with the ground state PES. The energy gap between the ground and the first excited surfaces was estirnated to be 1.04eV for a linear geometry of the system. In order to understand the interaction potentials and the reaction dynamics of Li* + HF + LiF + H reaction, the photodepletion spectrum for LiooFHwas obtained experimentally. The spectrum will be shown in this chapter and a discussion of the features of this spectrum will be assisted by high-level ab initio calculations performed in Prof. Piecuch's group at the Michigan State university.lo7 4-2. Experiments The experimental apparatus and the methodology for measurement of the photodepletion cross-section were the same with those described in chapter 2 and 3. However, the arrangement of the laser-ablation source for formation of the Li-FH van der Waals (vdW) complexes has been changed from that of the Li. .FCki3 complexes. In this section, only modifications made on the laser-ablation source assernbly will, therefore, be described in detail below. The most important difference was that the Li-FH complexes were generated by crossing a lithium-metal bearn with a second rnolecular bearn of HF in the vacuum, not inside the ablation block. The differences are éilustrated in the schematic diagram of the ablation-source assembly of fig. 4-1. The justification for use of this arrangement will be made later. A Li beam was generated using the laser-ablation method. In brief, Li plume was generated by radiation of the second hamonic Nd:YAG laser gas inlet Piezu- actuated valve stainless -steel ablation laser laser- 532nm ablation i block gas inlet --FH/He - valve ionization laser, L 2 : 248nm +v Li--FH .. TOFMS excitation laser. Li : 570-970nm Figure 4-1 : Crossed-beam assembly for formation of the Li-FH complexes (Quantel International; 532nm) beam ont0 the Li surface. A pulse energy of 1- 1OmJ at 532nm was focused to -1 mm diameter spot on the lithium rod using a 100cm focal length lens. The Li plume was then swept away by a pulsed helium (Matheson, UHP grade) from a homemade piezo-actuated valve. The Li beam expanded out of the ablation block without further interactions with HF molecules, in contrast to the experiment for formation of the LioaFCH3complexes. The expanding Li beam was then crossed with a second molecular bearn of 2.5 % HF diluted in helium at 2 atm. from a solenoid-driven valve (General Valve Corp.). This tirne, the Li and the HF molecular beam crossed each other in the vacuum, not inside the ablation block, as illustrated in fig. 4-1. The solenoid pulsed-vaive had a 0.8mm diarneter noule hole and was driven by a commercial driver (IOTA ONE driver, General Valve Corp.); The amount of gas coming out of this valve could be controlled by adjusting the duration-time of the high-voltage pulses applied to this valve and the tightness of a noule-head against a valve- body. The experimental conditions for forming high concentration of the LLFH complexes were so delicate that the LLFH vdW complexes were formed in a very small range of experimental conditions. The mechanisms for formation of the Li-FH complexes are Iikely to be exchange reactions such as The energetics of these reactions were calculated using a binding energy for (HF)2 [ 0.1 2ev'08], for FHIHe [-0.001 eV (Do=7.14cm -1 ) 1091, and a calculated value for the binding in LieeFH(0.24eV, see below). In order to fonthe vdW complexes of any kind, there should be a third body to carry away any excess energy at the moment of collision. The third body can be a carrier gas or other molecule that wnstitutes the van der Waals cornplex. However, a Li monomer can be ruled out as a third body since the concentration of Lb was very low in the TOF spectrum. In contrast, HF is known to easily form dimers through the hydrogen bonding. It is more likely that the cornplex is formed by the former exothemic reaction (eq. 4-1) since the complex FH-He is hard to be formed in our source, due to its low binding energy.'Og A couple of different arrangements were tried to generate the Li-FH complexes before this arrangement was employed, but none of thern was successful in fonning the complexes. For example, the laser-ablation source arrangement used for formation of the Li..FCH3 complexes was attempted, but the LLFH complex was not detected in the TOF spectrum. The CO-expansionof HF with He carrier from the piezo-valve was also attempted, but in vain. This failure is believed to be due to direct reactions between Li and HF since the ground state reaction of Li + HF is exothennic (considering the zero-point vibrational energy). When HF molecular beam is pulsed independently, there are more chances that higher concentration of (HF)2 are produced from the second valve, and thus more Li-FH vdW complexes are fomed [see eq. (4-l)]. 4-3. Identification of the LLFH vdW complexes A time-of-flight spectrum illustrating the formation of Li-FH complexes is shown in fig. 4-2. The dotted line indicates the ion-signal when the ionization laser. L2, was fired alone and the solid line represents the extent of depletion of the Li-FH complexes following excitation by laser, Li, which was set at a wavelength of hl = 650nm and fired prior to LZ. The choice of the excitation-laser wavelength was merely illustrative to show the depletion of the vdW complexes. The wavelength of the ionization laser, L2, was 248nm, corresponding to a photon energy of 5.0eV. Ion-signals corresponding to Li and Li2 are also observed, but the relative natural abundance of 6~ito 7~i ( -1 :9) is not clearly resolved in the TOF spectrurn. In contrast to the TOF spectrum for LimoFCH3,ion-signal corresponding to Li monorner is very small in this TOF spectrum. A broad ion- signal is observed near the LLFH cornplex at mass number -32. This signal with mass number 32 is tentatively assigned as the Li2.*OH2complexes. ....---* before excitation - after excitation Li,. .OH,(?) 10 20 30 mass number (mlz) Figure 4-2: TOF spectrum illustrating the formation of the Li-FH vdW comp!exes However, the ion-signal corresponding to the Li monomer cornplex with H20, Liem OH2, is not observed in this TOF spectrum. The LiemOH2ion-signal appeared at the early stage of the experiments, but was removed through the gas-line purging and overnight pumping . Depletion of ion-signal atter excitation was observed only in the mass number m/z=27 corresponding to the LiemFHcomplex. The Li and Li2 signal were unaffected by the excitation laser, LI - It indicates that Liz does not fragment at the excitation laser wavelength of XI = 650nm. The ion-signal corresponding to the LLFH complex fluctuated by ~15%on a shot-to-shot basis, but the ion- signals corresponding to Li and LipmOH2(at m/z=32) fluctuated noticeably more, i.e. by 20-25%. The low fluctuation of the LimeFHion-signal is believed to be due to the fact that (HF)* or FHemHe,which is responsible for formation of the vdW complexes, was the limiting reagents in the reaction (4-1) or (4-2). Since the solenoid valve is known to operate in a reliable way, the fluctuation in the amount of (HF)2 or FH-He produced from this valve should be low. The concentration of Li or Li2 produced from the ablation source rnust Vary more on a shot-to-shot basis. lonization of Li and Lin arose from multiphoton processes; I.P.(Li) = 5.4eV, I.P.(Liz) = 5.1eV and E(L2)=5.0eV . However, ionization of LimmFHoccurred by a single-photon process at the wavslength of L2, i.e. I.P.(LiemFH)I 5.0eV. Measurement of the ion-intensity as a function of the ionization-laser fluence gave a linear plot The linear dependence indicated the single photon mechanism in ionizing the Li-FH complex. The ionization potential of Li was lowered significantly from an atomic value of 5.39eV when Li was complexed with HF molecule. A similar trend has also been observed for LieeFCH3and Na- XR. 4-4. Photodepletion specfrum for the LLFH complex The Li-FH complex was excited by laser Li to the PES which would be traversed by the reaction Li*(2p 2~)+ FH -t [L~*-FH]*+ [L~*-FH~*+ LiF+ H. The excitation foms the transition state (TS) in a selected configuration. The photodepletion cross-section, cd.,, of LLFH was measured as a function of the excitation wavelength from h=570 to 970nm. The action spectrurn of ad,, against hl is shown in fig. 4-3. The range of wavelengths used for depletion included values far to either side of the atornic transition in Li, 2p 'P t 2s *S (indicated by the solid line at 671 nm in fig. 4-3). The photodepletion action spectrum of Li-FH (Fig. 2) consisted of two broad regions. The first region, characterized by a peak with a large maximum in cross-section, was observed (at the right in fig. 4-3) between 800 and 960nm. The second broader region. with a peak having a lower maximum cross-section, lay between 580 and 790nm at the left of Fig. 4-3. These two broad peaks each showed reproducible structure which was ascribed, as in earlier work on Na-FH excitation energy, El /eV 600 700 800 900 excitation wavelength, A,,nm Figure 4-3: Action spectra for LLFH (above) and Na-FH (below) complexes the latter being from Ref. 24), showing the depletion cross- section in A1 vs. the wavelength of laser LI in nm. The heavy vertical line is the atomic D-line (2p t 2s for Li, and 3p t 3s for Na). Vibrational spacing indicative of an harrnonicity is identified for a series of four vibrational levels (see text) . supported by detailed ab initio cornputation, to vibrational progressions in the transition state. In the case of Li-FH the depth of the ground-state potential-well made it likely that we were observing vibrational structure in the ground state, as well. The action spectrurn for Na..FH (reproduced from ref. 24) is also illustrated in fig. 4-3, for comparison. The most notable difference from the spectrurn for LLFH is the relative position of the depletion peaks. For the Na-FH cornplex, the two peaks were located largely to the red of the atomic transition (D-line). However, in the Li-FH spectrurn the depletion peak with a maximum at 670nm extends approximately 0.25eV to the blue side of the atomic transition 2s 2~ + 2p 'P. This blue-shift is believed to be related to the deep potential-well for the ground state of the Li-FH complex (see below). The vibrational structure was well resolved in the region of the action spectrum for LLFH to the blue of the atomic transition, i.e. between 590-630nm. The spacing between the vibrational lines decreased from the blue to the red (!eft to right, in fig. 4-3) in the depletion spectrum. The spacing between the vibrational lines in the progression has a value of A04=437, Aa2=376, Am3=277 cm" respectively (see fig. 4-3). This change of spacing would arise when excitation occurred frorn different vibrational levels in the ground state of the cornp~ex'~~to a single vibrational level in the TS. However, this would be possible only if the vibrational temperature of the ground-state complexes were abnormally high. It remains an open question at the present time whether the vibrational spacing can be accounted for by excitation from v=O of the grond electronic state to a succession of high vibrataional levels in the excited electronic state. Although vibrational structure was observed in other regions of the action spectrum, it was not possible to identify the individual progressions. The experimental oscillator strength for excitation of the complex, LimeFH, can be obtained using the value of the integrated depletion cross-section (in A' eV) from fig. 4-3. For this purpose we shall assume that excitation always gives rise to depletion of the complex, i.e. that the value of ad., reflects the absorption cross-section for Li-FH. This has been shown to be the case for the Na-FH complex in ref. 40. In general, the absoption cross-section integrated across the energy interval for excitation to an electronically-excited state, i, is equal to (ah2/2m,)fi, where a is the unitless fine-structure constant (a = 1/137), h is Planck's constant, me is the mass of an electron and fi is the oscillator-strength. The oscillator strength for excitation of the Li-FH complex was calculated by this means, from fig. 4-3, to be f - 0.3. This result is surprising when compared with the oscillator strength off - 1 for the equivalent atomic transition in Li, 2p 'P t 2s 's,which is close to the maximum allowed value. Since the sum of the oscillator strength for al1 possible electronic transitions must be unity, we conclude that the oscillator strengths are significant for other transitions in the LLFH complex that do not correlate with 2p 2~ t 2s 2~.The oscillator strength for excitation of the Na..FH complex was also found to be significantly less than unity (see fig. 4-3). A similar observation was made in experiments perfonned by Brockhaus et al. for N~..(NH~),." Discussion 4-5. Assignment of the depletion peaks The photodepletion spectrurn for Li-FH is discussed in the following paragraphs on the basis of accurate ab initio calculations of the ground and excited-state PES for LiFH. The potential energy for a large number of configurations of LiFH in the ground and electronically-excited state have been calculated at the MRDCl (Multi-Reference Configuration-Interaction with single and Double excitations) level, in this work. The minimum-energy configuration for the ground-state complex, LimmFH. was found to have a van der Waals bond length, r(Li..F), of 1.891 A (compared with 1.564 A in LiF(g)) and a bond angle of @(Li-F-H)= -1 10.4'. The binding energy in the gound-state was calculated to be 0.239eV. This value for the well- depth corresponds to the extent to which depletion extends to the blue of the atomic-transition line in fig. 4-3. By contrast, the Na..FH complex is much more weakly bound; Do (Na..FH) = 0.07e~.~~~~~The Li-F and Na..F separation in the respective complexes are both extended by about 20% relative to the isolated alkali halide molecule. The equilibrium geornetry of the complex on the ground- state PES is similarly bent for the sodium case as for the Li case, @(Na-F-H) = -1 18' (see ref. 24). Photoexcitation in the region of wavelengths of the depletion spectra is shown in Fig. 44as arising from excitation to electronic states correlating with the 2p 2~ state of Li and the 3p 2~ state of Na in the comparison spectrum below. In the complex the 2~ state is split into a triplet of states; 2'~'. 12~",and 32A' (using the notation for the Cs point group). The peaks in the depletion spectrum for LLFH have been assigned according to the calculated sequence of excited states. The energies for vertical transitions to each excited state from the minimum-energy configuration on the LLFH ground state have been deterrnined at the MRDCl level of computation. The computed transitions for the Naa.FH complex (taken from ref. 40) are also shown, for comparison. It is evident from fig. 4-4 that the peak in the experimental spectrum of Li-FH observed between 800 and 960 nm can be assigned to depletion arising from excitation to the 22A' state, whereas the broad peak observed between 580 and 790 nm can be assigned to depletion by way of the 32A' state. According to fig. 4-4, excitation to the 12~state appears to contribute to the peak observed between 800 and 960nm. However. the position of this vertical-excitation energy was found to depend on the level of the MRDCl treatment. In a subsequent treatment the location of this energy was found in the region of the peak observed between 580 and 790nm. Therefore, it is Iikely that excitation to the excitation energy, El /eV 2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 excitation wavelength, h / nm Figure 44: Ab initio calculation of the vertical-transition energies for excitation from the minimum-energy geometry of the Li-FH and Na-FH complexes (the latter being from ref. 40) on the ground-state PES. 12A'. to the lowest-lying excited states. 22~'132A' and 12A". 12~state contributes to both depletion peaks observed in the action spectrum in fig. 4-3. For NaeeFH,it has been shown in ref. 40 that the 22~'state contributes to the peak located to the red in the experimental spectrum, and the 1*~and 3'A' states contribute to the peak furthest to the blue. 4-6. Dynarnics of harpooning Figure 4-5 gives approxirnate potential-energy curves for the ground and lowest excited states of LiFH along the minimum-energy path for the collinear reaction Li + FH + LiF + H. The dynamics of the excited-state reaction, Li*..FH, can be understood in ternis of this diagram. The photodepletion spectrum for Li-FH has been reproduced on the vertical axis. It is evident that the separation between the ground and excited states accords with the experimental excitation energy for photodepletion. The ground state (1%') and the first excited state (22~')give rise to an avoided crossing in a region in which the H-F bond is significantly stretched from its equilibrium distance ro = 0.917A. A possible pathway exists from the 22~excited state to the ground-state via a surface hopping rnechanisrn. The ground state is most likely to be accessed ahead of the barrier crest. For reaction it is necessary. therefore, that the systern possesses sufficient momentum to surmount the barrier, giving LiF + H. Othewise the downward hop will forrn Li +FH, following a strong and probably inelastic encounter. Observed spectmm reaction coordinate / A Figure 45: Potential-energy curves for the ground and the electronically-excited states of LiFH. along an approximate minimum-energy path for the collinear PES. The equilibrium values for the Li-F and H-F bond lengths are indicated on their respective axes by downward pointing arrows at r~ = ro(Li-F) = 1.564A and r2 = ro(H-F) = 0.91 7A. Vibration in the ground electronic state LLFH (tentatively identified with the vibrational progression of Fig. 4-3) is indicated for v = O - 3. The probability of surface hopping from the lowest-excited state (22~)to the ground state ('12~')obtained, for example, from the Landau-Zener expression110.111 , decreases exponentially with the magnitude of the energy gap between the upper and lower PES. The smallest energy-gap between the ground state and the lowest excited state is ca. -0.7 eV for the collinear geometry (see fig. 4-5). A rnuch smaller gap is likely to exist in a non-collinear geometry, since, using an optical 'saturation' method for the 12A'+ 22~ transition. we have obtained an experimental upper limit to the lifetime of the [~i*-FH]*transition-state of r* s 1.3ns. This result comes from work in progress. This measured r* is one order-of-magnitude shorter than for [N~*..FH]* for which r* was 16ns. It is unlikely that surface-hopping could occur between the higher excited states (12~" and 32A') and the ground-state PES. The 12~" state does not have the appropriate symmetry for non-adiabatic coupling with the ground state. Ab initio calculation indicates that the 32A' state, which has the appropriate symmetry, does not approach the ground state sufficiently closely for significant coupling to occur. The rnost Iikely pathway from the TS to the products appears to be via the 22~'state. Future experimental work will involve a study of the lifetime of the TS as a function of the initially selected configuration as well as the effect of vibrational excitation of the H-F bond under attack. for this simplest of 'harpooning' reactions. The first observation of Li..FH complexes was made and the experimental setup for this observation was given in detail. These vdW complexes were formed by crossing a Li beam across an expansion region of the supersonic jet of HF. In order to study the TS of the excited-state reaction, Li(2p 2~ ) + HF -+ LiF + Hl the photodepletion cross-section was recorded while the excitation laser was scanned across a broad region of wavelengths corresponding to the 2p 2~ t 2s *S atomic Li transition. The peaks in the photodepletion spectrum could be assigned based on preliminary results of ab initio calculations perfomed in Piecuch's group. The depletion peak observed between 790 and 970nm was calculated to correspond to the excitation to 22~'state and the peak between 580 and 790 nm could be assigned to the excitation to 32A'state. It seemed that the I2A" state contributes to both depletion peaks observed in the action spectrum. The dynamics of the overall process, LLFH + hv, + [L~*..FH]* + [L~+-FH~* -, Li'F' + H or Li + HF, was discussed in terms of fig. 4-5. Once transferred to its excited state, [L~*..FH]* can hop back to the ground state through inter-state coupling. The electronically excited states with A' symmetry can couple with the ground state which also has A' symmetry. The harpooning event corresponded to a non-adiabatic hop from the excited states of A' symmetry to the ground state. The outcome can be the ionic product L~+F+ H or, if the energy and momentum along the reaction wordinate are insuficient, Li + HF. Chapter 5. Thesis surnmary and future directions This thesis describes results of an experimental study on excitation to the transition-state of the Li*(2p 'P) f XR (X=F and R= CH3, H) + LiX + R reactions. Photoexcitation of the ground-state Li-XR van der Waals (vdW) complexes results in their depletion by way of this low-lying excited state. The excited-state reactions are best understood as having occurred through excitation of the alkali- metal atom chromophore followed by charge-transfer dissociation. e. g. Li-XR + hvl + [~i*-XR]*+ [L~*.xR-]* + LiX + R (where the double-dagger, $. indicates the transition state). The experimentally measured spectra are interpreted in terms of ab initio calculations performed here and elsewhere by others. 5 Thesis summary Efforts to shed light on the transition state region have resulted in the emergence of the field of 'transition-state spectroscopy' (TSS). This field, originating from the observation of the emission and absorption spectra of reacting systems, cames to maturity in recent years. This thesis uses one approach to TSS, accessing the reactive TS by photoexcitation of a weakly bound vdW cornplex. The complexes Li-XR, which have not previously been reported, were formed using a laser-ablation method, in which a Li-metal bearn crossed a halide-rnolecule beam inside an ablation block for Li&CH3, and as expanding beams in the case of Li-FH. The Li-metal beam was generated by laser-ablation of Li, and the halide-molecule beam came from a second pulsed-valve. The depletion of the vdW complexes was measured by photoionization (laser. L2) time-of-flight mass-spectrometry applied before and after illumination by the tunable laser, LI. Excitation of the ground-state complex accessed the potential-energy- surface (PES) that would be traversed by the reaction of the excited-state lithium- metal atom with halide molecule (XR). The excited-state could be formed in different selected configurations. through which reacting atoms and molecules would evolve on their way from reagents to products, by varying the excitation- laser wavelength, XI. For Lio.FCH3, the depletion cross-section, udep, of the vdW complexes was measured across a broad region of visible excitation-wavelengths, hl. The photodepletion action spectrum exhibited two broad regions of depletion on either side of the Li atomic 2p 2~ t 2s 2~ transition (671nm). The first region, observed between 570 and 680nm. had a broad symmetric appearance, and the second, between 700 and 850nm. consisted of a few of high-intensity maxima, in contrast to earlier work performed on NadR (X=F, CI, Br and R=H, CH3, Ph). The depletion spectrum for LioefCH3could be understood in terms of the excitation of the Li chromophore (Li 2p 2~ t 2s 2~ atomic transition) perturbed by FCH3. The electronic energies and geometry of the ground and excited-states of the LLFCH3cornplex were obtained by ab initio wlculations. The Li 2p 2~ atomic state split into 3 states (2px2~, 2py 2~,and 2pz 2~),depending on the orientation of Li toward F-CH3 molecule. The favorable overlap of rovibrational- wavefunctions of the 2s 2~ ground state and the 2px,, 2~ excited states due to their similar geornetry, Ied to the high intensity maxima observed between 700 and 850nm in the experimental spectrum. The second system studied, Li + HF, is a prototype reaction of the alkali- metal-atom plus halide molecule systern, whose reaction proceeds throug h 'harpooning' mechanism, i.e. charge-transfer from the metal to the halide. In order to study the transition-state of Li'(2p 2~)+ HF -, LiF + H reaction, the photodepletion cross-section of the LLFH cornplex was measured across a broad range of excitation-wavelength, Le. between 570 and 970nrn. The depletion action spectrum consisted of two broad regions, one on either side of the atornic Li 2p 'P c 2s 2~ transition. The first depletion region with a lower maximum cross-section was observed between 570 and 790nm, and exhibited a clearly-resolved vibrational structure between 590 and 630nm. The second region, characterized by a peak with a large maximum in cross-section, was observed between 800 and 960nrn. The depletion spectrum for LFHwas discussed on the basis of preiiminary ab initio calculations of the ground and low-lying excited states PES, performed at MRDCl level. The peaks in the depletion spectrum could be understood as arising from excitation to electronic states correlating with the 2p 2~ state of Li. The 2p 2~ state is split into a triplet of states; 22~,12~", and 3'~' (using the notation for the Cspoint group). The excitation to the 22A'state gave rise to the depletion peak observed between 800 and 960nm and the excitation to the 32A'state gave the broad peak observed between 580 and 790nm. Excitation to the l*~"state contributed to both depletion peaks. The PESs obtained by ab initio calculations showed an avoided crossing between the ground state (12fY) and the first excited state (22~)in a region in which the H-F bond was significantly stretched from its equilibrium distance. A possible pathway for reaction existed from the 22~'excited state to the ground state via a surface-hopping mechanism. The smallest energy gap between the lowest-excited state to the ground state, which is important in determining the probability of surface-hopping, for example, according to the Landau-Zener expression, was found to be -0.7eV for collinear geometry. The TS state initially prepared in the higher excited states, namely 12~and 32~, seemed likely to proceed to products via the 22A'state. A complete PES including al1 bent geornetries is not yet available. The lifetime of the TS, which is related to the surface-hopping probability was measured by analysis of the observed saturation of depletion, In(NdN), versus the excitation-laser fiuence. The saturation was due to the stimulated emission from the TS to the ground electronic state, which became important at high excitation-laser fluence. Since this stimulated emission results in re- formation of the stable ground-state LLFR, it sets an upper lima on the percent depletion that can be measured at high fluence. Using a kinetic model with inclusion of the stirnulated-emission process, the saturated CU rve could be reproduced to yield the TS lifetime at a specific excitation wavelength. For Li complexes, Li..FCH3 and Li-FH, the upper limit of the TS lifetime was found to be s 1.3ns for both complexes, at al1 the wavelengths where depletion was observed. The lifetimes of the TS measured here contrast with those previously obtained for the sodium complexes, Na..FCH3 and Na-FH, for which rS was found to be 13ns and 16ns respective~y."~Further investigation using the 'optical saturation method' is under way, to provide the TS lifetime information as a function of excitation-wavelength. This should provide a guide for future ab inifio and dynamics calculations. 5-2. Future directions Future experiments are planned to study the effect of a state-selective infrared excitation of the X-R stretching vibrational mode of the M-XR complex. The M + XR -+ MX + R ground-state reactions are known to have a late barrier; the reaction cross-section will therefore be greatly enhanced by initial vibrational excitation of the molecular reactant. '13*' l4 Theoretical wlculation for the infrared absorption line-strengths of the Na..FH showed that excitation of the HF stretching mode in Na..FH is enhanced by a factor of 2.2~~21x, and 53x in the fundamental, the first, and the second overtones, compared to the isolated HF rno~ecule."~The excitation to the second overtone vibration of HF (Le. v=3 t O) will provide sufficient energy to overcome the energy barrier for the ground-state reaction. For the Li + FH reaction on the ground-state PES, theoretical studies by Paniagua et a/. showed that the probability of forming LiF products was very high (>go%) when the HF vibrational mode was initially excited, as compared to the lower reaction probabilities for Li + HF collisions with the sarne total energy vested in relative motion of Li and F."~"* The enhanced reactivity can be understood in terms of increased electron affinity of stretched H-F at the vibrationally excited state, giving rise to a lower energy-barrier for the ground- state 'harpooning' reaction which involves charge-transfer from alkali metal to HF. Another application of the initial vibrational excitation of HF, more clearly related to this thesis, is to excite the HF vibrational mode within the complex prior to formation of electronically-excited [M*..FH]* in order to probe regions on an excited-state PES norrnally inaccessible from the ground state. In particular, the NaFH adiabatic PESs (not yet available for Li) indicated that there existed a second region of avoided-crossing between the lowest excited state and the ground state at a configuration of NaFH in which both the Na-F and H-F bond lengths were extended. The region of this second avoided crossing could be reached only if the HF were vibrationally excited before excitation to the TS. The lifetime of N~-(FH)' with respect to dissociation to Na + FH on the ground PES is thought to be long enough to permit this type of double-resonance (IR-Vis) experimental study. We are also planning to probe the reaction products in order to explore the relative reactivity following excitation to different initial configurations of the transition-state. The detection and characterization of the products, (alkali-halide rnolecule, MX, and the radical, H or CH3) by resonant-enhanced multiphoton ionization (REMPI) is going to be attempted. Such experiments will provide information on the branching ratio between 'successful' reactions leading to MX + R and 'failed' reactions re-forming the reactants with altered interna1 energy. The CH3 radicals can be ionized using (3+1)REMPI ~cheme"~,and the translational energy distribution of recoiling H atoms can be measured by Rydberg-atom tirne- of-flight spectros~opy.~~~Both detection schemes have been previously applied in this laboratory. A detailed analysis of the energy distribution of the products will be of value in comparing with the results of classical trajectory-surface-hopping (TSH) calculations and of quantum scattering calculations. The system Li*-FH reported here has a substantial energy gap (>-0.7eV in the collinear geometry) between the lowest excited state and the ground state, offering a good opportunity to test the TSH calculations; TSH calculations for a system with an energy gap more than 0.5eV have yet to be developed. The study of vdW clusters could at a future date serve to bridge the gap between gas-phase and gas-surface photoreactions. This would be achieved by systematically increasing the size of the metallic component in the cluster. In this thesis I have described a new laser-ablation source with which it is hoped that the study of higher clusters. Le. MnaeXR(n >1), will be possible, at a later date. Conclusions The van der Waals (vdW) complexes LioXR(X=F and R= CH3, H) were formed for the first tirne, üsing a laser-ablation method. The Li vapor were produced by directing the 2"* harmonic of Nd:YAG laser ont0 the surface of a Li- metal rod. For LieoFCHt,pulsed argon carrier gas containing the Li vapor was mixed with another pulsed beam of -50% CH3F in argon, inside the ablation chamber. For LLFH the complexes were formed in crossed beams in which the Li metal bearn was crossed with the expansion region of a pulsed-molecular- beam containing 2.5% HF diluted in He. In these experiments. the potential-energy surface for the excited-state reaction, Li'(2p 'P) + XR + LiX + R, was accessed at a configuration in the transition state (TS) region by monochromatic excitation of the cornplex. Charge-transfer ('harpooning' of the halide) following photoexcitation, led to depletion of the complex. For Li..FCH3, photoinduced depletion of the complex was measured across a broad range of values of the excitation wavelength, hl, from 570 to 850nm. The photodepletion spectrum consisted of two broad regions located to either side of the atomic transition line of Li (2p 'P t 2s 2~,671 nrn). The first region. between 700 and 850nm. was dominated by two sharp maxima in the depletion intensity. A broad peak with weakly-resolved structure characterized the second region between 570 and 680nm. These findings were interpreted by means of high-level ab inifio calculations (performed by others) of potential- energy surfaces and associated transition moments. The first region in the depletion spectrum could be assigned to the electronic transition to the 2px,, 'P states of the complex, whereas the second region was assigned to the transition to the 2pz 2~ states. The shapes and intensities of the observed peaks in the depletion spectrum could in large part be explained in ternis of a simulated absorption spectrum. For the reaction Li + HF - which because of its sirnplicity is the prototype of charge-transfer 'harpooning' reactions- the TS of the excited-state reaction was probed by varying the excitation wavelength over the range of 570-970nm. while recording the photodepletion of the complex (the so-called 'action spectrum'). This photodepletion spectrum consisted of two broad regions: the first region, between 790 and 970nrn, was characterized by a peak with a large maximum in cross-section, and the second broader region, between 570 and 790nm, exhibited a lower maximum cross-section. Prelirninary ab initio studies showed that the peak observed between 790 and 970nm could be assigned to depletion arising from excitation to the 2*~'state (using the notation for the Cs point group), while the second broader region could be assigned to depletion by way of the 32~state. Additionally, excitation to the I~A"state contributes to both regions. A possible pathway exists from the 22~excited state to the ground-state, via a surface hopping mechanism. An experimental upper Iirnit to the lifetime of the [L~S~XRJ*transition-state was measured using an optical 'saturation' method. For Li-FCH3, the depletion intensity, In(NdN), increased linearly up to 7mJ/cm2 excitation-laser fluence, FI, at hl= 640nm and leveled-off with In(NdN) 2 3.0 corresponding to 95% depletion at FI 2 7mJ/cm2. Using a kinetic model that includes stimulated-emission, the depletion-intensity versus excitation-laser fluence could be simulated in shape to yield r* s -1 -3ns for the lifetime of the transition-state, [L~*~~FCH&For LieeFH, the depletion intensity increased linearly up to FI -1 0m~lcrn~and thereafter leveled-off (as for the previous complex) at In(NdN) 2 3.0,for Al= 900nm. The depletion curve at high F1, i.e. > 10m~/cm~,was also analyzed to give a lifetime for [L~*-FH]? T* s -1.3ns. The TS lifetirnes for both of these complexes are one order-of-rnagnitude less than those rneasured in earlier work, by optical saturation, for Na-FH. We conclude that in the case of the two Li-atorn harpooning TS, studied for the first time here, surface-hopping from the TS to the ground-state, to form reaction products, is far facile. Theory is expected to show a closer-approach of the TS of the excited-state reaction to the ground-state potentialenergy surface (PES) for the Li reaction. Exploration of the relevant 30 PES is under way presently, in other laboratories.* A private communication from Prof. Donald Truhlar (University of Minnesota) and Prof. Piotr Piecuch (Michigan State University) gives the result that the value of rs calculated ab initio, combined with trajectory surface- hopping, is several orders-of-magnitude shorter for Li-FH than for Na-FH, as found experimentally. lAug 28 2000. Appendix A. Derivation of the Equation Governing Depletion. The following derivation of eqs. 3-7 and 3-8 was kindly provided by Dr. S. Raspopov of this laboratory. TS, N* reaction rate = irnulatad emission ("'*))/ excitationTst rate 1 1 rate = &N* ground state , N Figure A-1. A simple kinetic model representing laser-induced processes in M-FR complexes. A two level kinetic model representing laser-induced processes is shown in fig. A-1 . In this rnodel, the excitation, stimulated emission, and reaction were considered in order to derive the depletion equation. A spontaneous emission process was ignored in the derivation since the spontaneous ernission process is much slower than the other processes. The time derivatives of the population in the ground-state complex, N, and in the TS, N*, are given by following equations. where CJ represents the absorption cross-section and II is the excitation-laser intensity. The eq. (A-1) can be rearranged to give N* in terms of N. (A- 1a) The eq. (A-la) can be differentiated with time to give eq. (A-16). The eq. (A-1 a) and (A-1b) can be applied to both sides of eq. (A-2) to yield and this eq. (A-2a) is rearranged to give the following second-order differential equation. This equation can be solved using a trial function (A-4). kt N=e (A-4) The eq. (A-3) can be rephrased as eq. (A-3a) by piugging the eq. (A-4) into the eq. (A-3). This quadratic equation can be solved to give two specific solutions. Therefore, the general solution of eq. 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