researchdic

Application of Time-Sliced Ion Velocity Imaging to Crossed Molecular Beam Experiments
Jim J. Lin, Jingang Zhou, Weicheng Shiu, and Kopin Liu
Rev. Sci. Instrum. Vol. 74 No. 4 (2003) pp. 2495-2500
A three-dimensional (3D) ion velocity imaging method was developed to measure the product velocity distributions in crossed molecular beam experiments. While maintaining the conventional two-dimension (2D) velocity mapping, the third velocity component was mapped linearly to the ion time-of-flight. Weak extraction field was used to spread the ion turn-around-time to several hundred nanoseconds, which permits good resolution for the longitudinal velocity selection. A fast gated ( ³ 5 ns) intensified CCD camera was used to record the time-sliced ion images. The calibration of the apparatus was done by measuring O⁺ images from multiphoton dissociation/ionization of O₂. The velocity resolution achieved was about 1% ( D v/v) while slicing through the center of the Newton sphere. The overall performance was examined by observing product ion images from the F + CD₄ → DF + CD₃ reaction. For detecting CD₃⁺ with kinetic energy release about 1 eV, a 50 ns time slicing provides sufficient velocity resolution, such that the image resolution is mainly limited by the velocity spreads of the two molecular beams. This ion optics can focus a large volume of ion cloud, which is crucial in a crossed molecular beam experiments. Direct 3D imaging also simplifies the data analysis. This direct 3D ion imaging method provides a powerful tool to study systems without cylindrical symmetry.


Crossed-Beam Scattering of F + CD₄ → DF + CD₃(vNK): the Integral Cross Sections
Jingang Zhou, Jim J. Lin, Weicheng Shiu, Shih-Chieh Pu, and Kopin Liu
J. Chem. Phys. Vol. (2003) pp.
The title reaction was investigated in a crossed-beam experiment. A (2 + 1) REMPI technique was used to interrogate the internal state distributions of the CD₃ product at three different collision energies. Only the v₂(umbrella) mode excitation was observed. Its distribution changes from a monotonically declined distribution at low energy to a slightly inverted one at higher collision energy. Although the rotational excitations of CD₃ were small, a strong preference for K = 0 was found, indicative of the dominance of the tumbling rotation motion of the CD₃ product. The vibration-resolved excitation functions were also measured for v₂ = 0-3. A reaction barrier of 0.5 kcal/mol was deduced.


State-Specific Correlation of Coincident Product Pairs in the F + CD₄ Reaction
Jim J. Lin, Jingang Zhou, Weicheng Shiu, and Kopin Liu
Science Vol.300 No.5621 (2003) pp. 966-969 Abstract Full Text

When a chemical reaction forms two molecular products, even if the state-resolved differential cross section (DCS) for each product is obtained individually, the coincident attributes of the co-products are still lacking. We exploit a method that provides coincidence information by measuring the state-resolved, pair-correlated DCS. Exemplified by the reaction F + CD₄ → DF + CD₃ , a time-sliced ion velocity imaging technique was used to measure the velocity distribution of a state-selected CD₃ product and to reveal the information of the coincident DF in a state-correlated manner. The correlation of different product state pairs shows striking difference, which opens up a new way to unravel the complexity of a polyatomic reaction.


Mode-Correlated Product Pairs in the F + CHD₃ → DF + CHD₂ Reaction
Jingang Zhou, Jim J. Lin, and Kopin Liu
J. Chem. Phys. Vol.119 No.16 (2003) pp. 8289-8296
The title reaction was investigated at three different collision energies in a pulsed, crossed-beam apparatus. The (2 + 1) REMPI spectra of the CHD₂ products revealed, in addition to the anticipated 4ⁿ_m vibronic bands, a hitherto unobserved feature. The new feature was shown and assigned to the 3¹₁ band. A time-sliced ion velocity imaging technique was applied to map out the coincident DF attributes of the two product states 4₂ and 3₁, whose energy levels lie nearly degenerate. Remarkably similar results were found for the two states in every aspect at all three collision energies. A simple model of Fermi-coupled states was proposed to rationalize this, at first sight, surprising finding. Implications to collisional processes which involve mixed molecular basis states in general are outlined. Possible quantum interference phenomenon is suggested.


Insights Into Dynamics of the F + CD₄ Reaction Via Product Pair Correlation
Jingang Zhou, Jim J. Lin, Weicheng Shiu, and Kopin Liu
J. Chem. Phys. Vol. 119 No. 10 (2003) pp. 4997-5000
To unravel the "extra-atom" complexity of the title reaction, we exploit an experimental approach which, by taking advantage of the correlated information of coincident product pairs, allows us to peel off judiciously the intrinsic complications of a six-atom reaction, extracting the underlying backbone of three-atom dynamics. Examining the collisional energy dependencies of the pair-correlated attributes for a given state(s) of CD₃ products from the title reaction, several of major observations can qualitatively be understood, whereas others await further theoretical investigations. An intriguing possibility for the existence of reactive resonances in this six-atom reaction is surmised.


Imaging the Pair-Correlated Excitation Function: the F + CH₄ → HF(v') + CH₃(v = 0) Reaction
Weicheng Shiu, Jim J. Lin, and Kopin Liu
J. Chem. Phys. Vol. 120 No. 1 (2004) pp. 117-122
The velocity map ion imaging technique was applied to measure the reaction excitation function for the first time. It was found that the "raw" excitation function was significantly distorted by the density-to-flux transformation of the title reaction. Through a systematic investigation, possible reasons for such a dramatic effect are outlined. In addition, the state-resolved, pair-correlated excitation functions and branching ratios are presented. Effects of imperfect time slicing in the time-sliced velocity imaging technique in general are also discussed.


Observation of a Transition State Resonance in the Integral Cross Section of the F + HD Reaction
Rex T. Skodje, Dimitris Skouteris, David E. Manolopoulos, Shih-Huang Lee, Feng Dong, and Kopin Liu
J. Chem. Phys. Vol. 112 No. 10 8 March 2000 pp. 4536-4552
We have studied the reaction F-HD at low collision energies using a combination of experimental and theoretical methods. Clear evidence for a reactive resonance is found in the integral cross section for the reactive channer F-HD → HF-D. Using a crossed molecular beam apparatus, the total reactive cross sections for the HF + D and DF + H channels were obtained in the collision energy range of 0.2-5 kcal/mol. In addition, Doppler profiles were obtained over this range of energies, which provide information about the angularly resolved distribution of final vibrational states. The cross section shows a distinctive step-like feature near 0.5 kcal/mol. Furthermore, the Doppler profiles reveal a dramatic change in the angular distribution of products over a narrow energy range centered at 0.5 kcal/mol. This feature is shown to arise from a reactive resonance localized near the trasition state. Theoretical scattering calculations have been carried out using the Stark-Werner potential energy surface, which accurately reproduce the shape of the resonance feature. A detailed analysis of quantum dynamics using the spectral quantization method reveals that a quantum resonance exists near 0.52 kcal/mol, which is localized about the collinear FHD geometry. At collision energies below 1 kcal/mol, the reaction was found to proceed almost exclusively through resonant tunneling with very little contribution from direct, over the barrier, reaction. The properties of the quantum resonance, such as the position, lifetime, and partial widths were found to correlate well with the experimental results and the quantum scattering calculations.


Resonance-Mediated Chemical Reaction: F + HD → HF + D Rex T. Skodje, Dimitris Skouteris, David E. Manolopoulos, Shih-Huang Lee, Feng Dong, and Kopin Liu Phys. Rev. Lett. Vol. 85 (2000) pp. 1206-1209 Conclusive evidence is presented for the existence of a reactive resonance in the F + HD reaction. In a molecular beam experiment, the resonance appears in the integral cross section as a distinct steplike feature, while in the differential cross section it is manifested as sharply varying forward-backward peaks in the product distribution. A detailed analysis of the quantum dynamics establishes that a reactive resonance localized in the transition-state region is responsible for these remarkable observations. At collision energies below 1 kcal/mol, the reaction proceeds almost exclusively through resonant tunneling with very little contribution from the more conventional direct mechanism.


Reactive Excitation Functions for F + p -H₂/n -H₂/D₂ and the Vibrational Branching for F + HD
Feng Dong, Shih-Huang Lee, and Kopin Liu
J. Chem. Phys. Vol. 113 No. 9 (2000) pp. 3633-3640
Complementary to our recent report on the F + HD reaction, the reactive excitation functions for the other isotopomers are presented. Through analysis of the differential cross section data, the collisional energy dependencies of product vibrational branchings for F + HD are also reported here. Several important conclusions can be drawn from this work. First, the transition-state properties, in particular the barrier height, of this reaction are well-characterized by the SW PES, despite its neglect of spin-orbit couplings. Second, contrary to the theoretical conclusion in recent literatures, an experimental observation is presented which seems to suggest that a resonance may indeed exist for the F + H₂ reaction in support of the original interpretation proposed by Lee and co-workers. Third, the vibrational branching for the F + HD → HF + D reaction elucidates another facet of resonance effects in the integral cross sections. Finally, the nonadiabatic reactivity of the spin-orbit excited F^*(²P_1/2) atom is found to be small, which is in line with the conclusion inferred from a most recent, full quantum mechanical multisurface calculation.


State-specific Excitation Function for Cl(²P) + H₂ (v=0, j): Effects of Spin-orbit and Rotational States
Shih-Huang Lee, Lih-Huey Lai, and Kopin Liu
J. Chem. Phys. Vol. 110 No. 17 1 May 1999 pp. 8229-8232
By exploiting different sources for both reagents, the state-specific excitation function was deduced for the title reaction for the first time. It was found that for reactions with the ground state Cl(²P_3/2) atom, the rotation of the H₂ reagent has little effect on reactivity at low collision energies, but plays a beneficial role in promoting the reaction at higher E_c's; in contrast to a recent QCT prediction on the G3 surface. Evidence was presented for non-negligible reactivity from the spin-orbit excited Cl(²P_1/2) atom. The ration of the cross sections for a rotating H₂ to a nonrotating one appears to be larger for reaction with the ground state Cl than with the excited Cl,**


Exploring the Spin-orbit Reactivity in the Simplest Chlorine Atom Reaction
Shih-Huang Lee and Kopin Liu
J. Chem. Phys. Vol. 111 No. 14 8 October 1999 pp. 6253-6259
The reaction of Cl(²P) + H₂ → HCl + H serves as a benchmark for understanding the abstraction mechanism and has been the subject of numerous experimental and theoretical studies. Despite such intensive investigations, the reactivity of the low-lying spin-orbit excited Cl(²P_1/2) is unknown and has always been assumed to be negligibly small. By exploiting two different sources for generating the Cl(²P) beam, it is now found that the excited Cl(²P_1/2) atom is surprisingly more reactive to H₂ than the ground state Cl(²P_1/2) reagent, overturning conventional wisdom. By using a technique called Doppler-selected time of flight to directly map out the doubly differential cross sections (angle and speed), the detailed dynamical attributes for both spin-orbit states are also elucidated and contrasted for the first time for any bimolecular reaction.


van der Waals Interactions in the Cl + HD Reaction
Dimitris Skouteris, David E. Manolopoulos, Wensheng Bian, Hans-Joachim Werner, Lih-Huey Lai, Kopin Liu
SCIENCE Vol. 286 No. 26 (1999) pp. 1713-1716
The van der Waals forces in the entrance valley of the Cl + HD reaction are shown here to play a decisive role in the reaction's dynamics. Exact quantum mechanical calculations of reactive scattering on a potential energy surface without Cl-HD van der Waals forces predict that the HCl and DCl products will be produced almost equally, whereas the same calculations on a new ab initio potential energy surface with van der Waals forces show a strong preference for the production of DCl. This preference is also seen in crossed molecular beam experiments on the reacion. The study of chemical reaction dynamics has now advanced to the stage where even comparatively weak van der Waals interactions can no longer be neglected in calculations of the potential energy surfaces of chemical reactions.


Direct Determination of the Spin-orbit Reactivity in the Cl(²P_3/2, ²P_1/2) + H₂/D₂/HD Reactions
Feng Dong, Shih-Huang Lee, and Kopin Liu
J. Chem. Phys. Vol. 115 No. 3 (2001) pp. 1197-1204
By exploiting two different Cl-beam sources and concurrently monitoring the concentrations of the two reagents (Cl(²P_3/2) and Cl(²P_1/2)) and the H- or D-atom product, the spin-orbit specific excitation functions of the title reactions were determined. The exceptionally large nonadiabatic reactivity for Cl(²P_1/2) + n-H₂, inferred in our previous differential cross section investigation, is now confirmed and quantified. The isotope effects for both the spin-orbit ground and excited reagents are also elucidated.


Isotope Effects and Excitation Functions for the Reactions of S(¹D) + H₂, D₂ and HD
Shih-Huang Lee and Kopin Liu
Chemical Physics Letters Vol.190 (1998) pp. 323-328
Excitation functions for the title reactions were determined from 0.6 to 6 kcal/mol. Contrary to the analogous reaction of O(¹D), it appears that the reaction of S(¹D) proceeds solely through insertion over this energy range. Compared to other reactions, an intriguing H/D isotope effect was revealed. The propensity of the intramolecular H/D branching found under thermal conditions for A + HD reactions appears to be reverse for a supersonically cooled HD reagent. This finding implies that the reagent rotation could have profound influences on radical reactivity not only for an activated abstraction reaction, but for a barrierless inserted one.


Exploring Insertion Reaction Dynamics: A Case Study of S(¹D) + D₂ → SD + D
Shih-Huang Lee and Kopin Liu
J. Phys. Chem. A Vol. 102 No. 45 (1998) pp. 8637-8640
The Doppler-selected time-of-flight method was applied to map out the differential cross section of the title reaction at a collision energy of 5.3 kcal/mol. The angular distribution is highly forward/backward peaking with very slight asymmetry in favor of the forward direction. Vibrational structures of the SD product are resolved in the angle-specific translational energy distribution. A strong coupling between the product angular and speed distributions is found. Phase-space theory gives a fair description about the product translational energy distribution, though significant discrepancies are noted for angular and angle-specific speed distributions. Possible reasons are given, which consequently suggest ways for further theoretical investigation.


Collisional Energy Dependence of Insertion Dynamics: State-Resolved Angular Distributions for S(¹D) + D₂ → SD + D
Shih-Huang Lee and Kopin Liu
Springer book "Advances in Molecular Beam Research and Applications" (Edited by R. Campargue 2000)
The doubly differential cross sectionfor the reaction S(¹D) + D₂ at E_c = 9.6 kJ/mol was mapped out by a newly-developed technique called Doppler-selected TOF method. This new technique is simple and robust, yet its resolution is sufficiently high to observe the vibrational structures directly and to infer the shape of the rotational energy distribution. Detailed analysis procedure was presented. Compared to the previous results at E_c = 22.2 kJ/mol, the dependence of the detailed insertion dynamics on the initial collison energy is then revealed. As shown in Figures 3-5 and summarized in Figures 6 and 7, clearly there is rich dynamical information underneath the statistical treatment to be uncovered for an indirect complex-forming reaction. Theoretical efforts, both the ab initio characterization of the interaction potential and the dynamics calculation, are currently underway. We hope that the work presented here will provide the stimulus for more detailed investigations and for better understanding of insertion reactions in general.


Direct Mapping of Insertion Reaction Dynamics: S(¹D) + H₂ → SH + H
S.-H. Lee, K. Liu
Appl. Phys. B 71 (2000) pp. 627-633
The Doppler-selected time-of-flight method was applied to map out the differential cross sections of the title reaction at two collision energies in a crossed-beam experiment. Roughly symmetric and highly forward-backward peaking angular distributions were observed at both energies. Vibrational structures of the SH product were resolved from the velocity measurements of the counter-product H-atom. Most of the angle-integrated observables can readily be understood on statistical grounds, which suggests that statistics plays the dominant role in determining the outcomes of this prototypical insertion reaction. In terms of more detailed angle-specific reaction attributes, significant discrepancies from statistical considerations were revealed, indicative of some hidden dynamics being buried under the statistical factor.


Reaction Dynamics of O(¹D) + H₂, D₂, and HD: Direct Evidence for the Elusive Abstraction Pathway and the Estimation of Its Branching
Yen-Tsung Hsu, Jeng-Han Wang, and Kopin Liu
J. Chem. Phys. Vol. 107 Issue 7 15 August 1997 pp. 2351-2356
The dependencies of the integral cross sections for the title reactions on collision energies were determined from 0.6 kcal/mol to nearly 6 kcal/mol. The result provides, for the first time, direct and unambiguous experimental evidence for the existence of an abstraction pathway, in addition to the widely accepted inserted one. A reaction barrier of about 1.8 kcal/mol was found for this elusive abstraction channel. The branching of these two microscopic pathways was estimated. An intriguing H/D isotope effect was revealed, which called for further studies.


Effect of Reagent Rotation in O(¹D) + H₂ (v = 0, j): A Sensitive Probe of the Accuracy of the ab initio Excited Surfaces?
Shih-Huang Lee and Kopin Liu
J. Chem. Phys. Vol. 111 No. 9 1 September 1999 pp. 4351-4352


Reaction Dynamics of O(¹D) + HD. I. The Insertion Pathway
Yen-Tsung Hsu and Kopin Liu
J. Chem. Phys. Vol. 111 No. 17 1 September 1999 pp. 7921-7930
The doubly differential cross sections (angle and speed) for the reactions O(¹D) + HD → OH(OD) + D(H) at E_c = 2.05 kcal/mol were directly mapped out using the Doppler-selected time-of-flight technique in a crossed-beam experiment. Except for the previously noted discrepancy in the H/D isotope branching ratio, good agreement with QCT (quasiclassical trajectory) results on a high quality ab initio surface are found for all dynamical features, including the product angular and traslational energy distributions and the very detailed angle-specific translational energy release.


Reaction Dyanmics of O(¹D) + HD. II. Effects of Excited Surfaces
Yen-Tsung Hsu and Kopin Liu
J. Chem. Phys. Vol. 111 No. 17 1 September 1999 pp. 7931-7944
To explore the effects of excited surfaces on the title reaction, an in-depth crossed-beam investigation was carried out at two collision energies, 3.7 and 4.6 kcal/mol. In combination with the results obtained at the lower energy, 2.05 kcal/mol (the preceding paper), the extent of abstraction contribution to total reactivity was deduced from the doubly differential cross-section measurements and its dominant dynamical features were revealed for the first time. The former was found to be in reasonable agreement with our previous estimate based on the excitation function data. We also present quasiclassical trajectory results based on accurate potential energy surfaces for the 1¹A'and 1¹A" surfaces. The trajectory results give approximately the same breakdown into insertion and abstraction components as in the experiments, and some aspects of the product distributions, such as the angular distributions, are in rough agreement. However, other aspects of the product distributions, such as the abstraction component of the product translational distributions, are in serious disagreement. Possible deficiencies in the potential surfaces and dynamics that could be responsible for these results are considered. 　 A Crossed-Beam Study of the Reaction O(¹D) + D₂: Collisional Energy Dependence of Differential Cross-Section P. Hermine, Y.-T. Hsu and K. Liu Phys. Chem. Chem. Phys. Vol. 2 (2000) pp. 581-587
The dynamics of the title reaction were investigated in a crossed-beam investigation at four different collision energies, ranging from 1.79 to 4.87 kcal mol^-1.At the level of detail of the doubly differential cross section (in angle and speed), a dramatic and systematic variation with the increase in collision energy was noted. We ascribed the variation to the contribution from an additional abstraction pathway to the reaction. Comparing the results at higher collision energies with that at 1.79 kcal mol^-1, which was assumed to be insertion only, the extent of abstraction contribution to total reactivity was deduced and its dominant dynamical features revealed. The former was found to be in reasonable agreement with our previous estimate based on the excitation function data. Major aspects of the latter were found in corroboration of those reported recently for the analogous O(¹D) + HD reaction.


Crossed-Beam Studies of Neutral Reactions: State-Specific Differential Cross Sections
Kopin Liu
Annu. Rev. Phys. Chem. Vol. 52 (2001) pp. 139-164
Crossed-molecular-beam and laser techniques have enabled experimentalists to measure the state-resolved differential cross sections of elementary chemical reactions. This article reviews recent progress in this area. Particular emphasis is placed on some intriguing physical phenomena associated with a few benchmark reactions, and how these measurements help in answering fundamental questions about reaction dynamics. We examine specifically the geometric phase effects in the reaction H + D₂, the dynamical resonance phenomenon in F + HD, the unusually large spin-orbit reactivity in Cl(²P) + H₂, the insertion reaction O(¹D) + H₂, and the mode-specific reactivity in Cl + CH₄(v). The give-and-take between experiment and theory in unraveling the physical picture of the dynamics is illustrated throughout this review.


Excitation Functions of Elementary Chemical Reactions: A Direct Link from Crossed-beam Dynamics to Thermal Kinetics?
Kopin Liu
Intern. Rev. Phys. Chem. Vol. 20 (2001) pp. 189-217
Excitation function refers to the translational energy dependency of the integral cross section of a bimolecular collision process. Exemplified by a number of elementary chemical reactions, the information content of excitation functions is critically surveyed. Particular emphasis is placed on the close comparison with the available thermal kinetics data. The reactivity for an activated reaction was found to depend sensitively on the rotational state of the reagent, indicative of stereodynamical effects. The intramolecular isotope branching ratio, for the reaction A + HD, exhibits a strong dependency on collision energies. And its isotopic propensity reverses for a non-rotating than for a rotating reagent. By way of contrast, the reactive behavior of a barrierless reaction shows little dependency on the initial rotational state, and the intramolecular isotope branching ratio instead becomes nearly independent of collision energies. In this case, the excitation function obtained from crossed-beam experiments then provides a direct and reliable route to extrapolate or to compare with available thermal rate constants.


Resonances in Bimolecular Reactions
Kopin Liu, Rex T. Skodje and David E. Manolopoulos
Phys. Chem. Comm. Vol. 5 (2002) pp. 27-33
In this Perspective we briefly review our recent studies which prove unequivocally the existence of a quantum dynamical resonance in the F + HD → HF + D reaction. The signatures of the resonance in the integral and differential cross sections of this reaction are elucidated. The interplay between experiment and theory is crucial in establishing the existence of a resonance in a bimolecular reaction and in revealing its physical characteristics.


Photodissociation NO₂ at 355nm: Pair Correlation
Chi-Hsun Hsieh, Yi-Shyuan Lee, Asuka Fujii, Shih-Huang Lee, Kopin Liu
Chemical Physics Letters Vol. 277 (1997) pp. 33-38
The core-sampling technique, combined with the state-selective REMPI detection, is applied for the study of the scalar pair-correlation between the internal state distributions of the NO and O(³P_2,1,0) photofragments. The rovibrational distributions of the NO cofragment revealed by the time-of-flight (translational energy) measurement of the O(³P_j) fragment show clear j-dependence. The angular distribution of each individual fine-structure state of the O-atom also exhibits distinct correlation with the internal states of the counter NO fragments.


Correlated Photofragmentations
C-H. Hsieh, S-H. Lee, A. Fujii, and K. Liu
SPIE Vol. 3271 (1998) pp. 7-14
A simple, cost-effective method has been developed to investigate the quantum-state correlation of the two fragments and the vector properties of a photofragmentation process. The method combines the commonly used state-selective REMPI detection technique with the core-sampling concept in high resolution ion velocity measurements. To demonstrate the merits of this approach and to explore the new dynamical information obtained from these correlation measurements, two examples involving O(³P_j) fragments are presented for illustrations.


Photodissociation Dynamics of C₂H₂, C₂D₂, and C₂HD at 121.6 nm
Jeng-Han Wang, Yen-Tsung Hsu, and Kopin Liu
J. Phys. Chem. A Vol. 101 No. 36 (1997) pp. 6593-6602
The product translational energy distribution P(E_t) and the recoil anisotropy parameterfor acetylene (and isotopic variants) photodissociation at 121.6 nm have been obtained by detecting the H(D) atom fragment using the Doppler-selected time-of-flight (TOF) technique. A strong propensity toward the formation of the fragment was revealed. The resolution of this technique is sufficiently high to resolve the vibrational structures of a small polyatomic molecule. The vibrational spectroscopic constants of the C-H and C-C stretching modes for the state were obtained for the first time, and the mode-specific vibrational state distributions were deduced. It is conjectured that two distinct dissociation pathways, arising from the Rydberg-valence surface intersections, are involved in the title processes. The branching of these two pathways is governed by the initial absorption step.


VUV Photochemistry of CH₄ and Isotopomers. I. Dynamics and Dissociation Pathway of the H/D-atom Elimination Channel
Jen-Han Wang and Kopin Liu
J. Chem. Phys. Vol. 109 Issue 17 (1998) pp. 7105-7112
The Doppler-selected time-of-flight technique was used to study the formation of H and D in the photolysis of CH₄ and its isotopomers. The combination of measurements for the photofragment kinetic energy release P(E_t) and the anisotropy parameterdistributions allows us to differentiate, for the first time, three distinct pathways which are involved in C-H (C-D) bond fission. In conjunction with a recent ab initio theoretical investigation, the mechanisms for this complicated multichannel dissociation process are proposed. In particular, two distinct dissociation pathways are elucidated for the two-fragments channel + H. One pathway invokes a perpendicular-type transition in absorption, which subsequently undergoes intersystem crossing to the triplet surface and then dissociates. The fragmentation via this route yields fast CH₃ + H with a negative parameter. Alternatively, a parallel-type excitation is involved, followed by internal conversion to the ground-state surface on which dissociation occurs. This pathway results in less kinetic energy release and yields a positive parameter. An intriguing isotope effect is revealed, which calls for further theoretical investigations.


On the H-atom Formation After Lyman-αExcitation of CHF₂Cl
Lih-Huey Lai, Yen-Tsung Hsu, Kopin Liu
Chemical Physics Letters Vol. 307 (1999) pp. 385-390
Contrary to what has previously been suggested from an (often)observed bimodal Doppler profile, three distinct features are actually revealed in the H-fragment speed distribution for the title process via the Doppler-selected time-of-flight measurement. In combination with the results from an additional VUV photoionization measurement, the underlying dissociation pathways are elucidated. An alternative reacion mechanism is proposed, which is subject to future experimental verifications.


Vacuum Ultraviolet Photochemistry of CH₄ and Isotopomers. II. Product Channel Fields and Absorption Spectra
Jen-Han Wang and Kopin Liu, Zhiyuan Min, Hongmei Su, and Richard Bersohn, Jack Preses and John Z. Larese
J. Chem. Phys. Vol. 113 No. 10 (2000) pp.4146-4152
In part I of this work the relative velocities and anisotropies of the atomic H and D fragments from methane photolysis at 10.2 eV were measured. In this paper the relative abundance of the methyl and methylene fragments are reported. A complete set of quantum yields for the different photodissociation channels of each isotopomer is obtained by combining the two sets of data. Previously it was found that H atoms are almost four times more likely than D atoms to be ejected; now it is found that hydrogen molecule photofragments are much richer in H atoms than in D. Overall, the heavier D atoms are more likely than the H atoms to remain attached to the carbon atom. An implication for astrophysics is discussed. The VUV absorption spectra of CH₄ and CH₃D are almost identical both at room temperature and 75 K. There is, as expected, no variation in the absorption spectrum with temperature. Evidence is given that all or almost all of the methylene is produced in the a¹A₁ and not in the ground ³B₁ state.