Kopin Liu

Application of time-sliced ion velocity imaging to crossed molecular beam experiments

A three-dimensional (3D) ion velocity imaging method was developed to measure the product velocity distributions in crossed molecular beam experiments. While maintaining conventional two-dimension velocity mapping, the third velocity component was mapped linearly to the ion time of flight. A weak extraction field was used to spread the ion turnaround time to several hundred nanoseconds, which permits good resolution for selection of the longitudinal velocity. A fast gated (⩾5 ns) intensified charge coupled device camera was used to record time-sliced ion images. Calibration of the apparatus was done by measuring O+ images from the multiphoton dissociation/ionization of O2. The resolution in velocity achieved was about 1% (Δv/v) in slicing through the center of a Newton sphere. The overall performance was examined by observing product ion images from the F+CD4→DF+CD3 reaction. To detect CD3+ with kinetic energy release of about 1 eV, 50 ns time slicing provides sufficient velocity resolution, such that res...

Do vibrational excitations of CHD3 preferentially promote reactivity toward the chlorine atom

The influence of vibrational excitation on chemical reaction dynamics is well understood in triatomic reactions, but the multiple modes in larger systems complicate efforts toward the validation of a predictive framework. Although recent experiments support selective vibrational enhancements of reactivities, such studies generally do not properly account for the differing amounts of total energy deposited by the excitation of different modes. By precise tuning of translational energies, we measured the relative efficiencies of vibration and translation in promoting the gas-phase reaction of CHD3 with the Cl atom to form HCl and CD3. Unexpectedly, we observed that C–H stretch excitation is no more effective than an equivalent amount of translational energy in raising the overall reaction efficiency; CD3 bend excitation is only slightly more effective. However, vibrational excitation does have a strong impact on product state and angular distributions, with C–H stretch-excited reactants leading to predominantly forward-scattered, vibrationally excited HCl.

CH Stretching Excitation in the Early Barrier F + CHD3 Reaction Inhibits CH Bond Cleavage

Shaking Prevents Breaking Intuition suggests that vibrational excitation of a molecular bond ought to increase the likelihood of its breaking in an ensuing chemical reaction. W. Zhang et al. (p. 303) observe precisely the opposite outcome in a spectroscopic study of the F + CHD3 reaction. Exciting the C-H stretch leads exclusively to the formation of DF and CHD2 products, in contrast to the more abundant yields of both HF and DF observed in the absence of CH vibration. Though the mechanism underlying this effect remains unclear, the result highlights unanticipated complexity in the reaction dynamics of a relatively simple molecule. A molecular bond vibration unexpectedly inhibits, rather than promotes, cleavage of the carbon–hydrogen bond. Most studies of the impact of vibrational excitation on molecular reactivity have focused on reactions with a late barrier (that is, a transition state resembling the products). For an early barrier reaction, conventional wisdom predicts that a reactant’s vibration should not couple efficiently to the reaction coordinate and thus should have little impact on the outcome. We report here an in-depth experimental study of the reactivity effects exerted by reactant C-H stretching excitation in a prototypical early-barrier reaction, F + CHD3. Rather counterintuitively, we find that the vibration hinders the overall reaction rate, inhibits scission of the excited bond itself (favoring the DF + CHD2 product channel), and influences the coproduct vibrational distribution despite being conserved in the CHD2 product. The results highlight substantial gaps in our predictive framework for state-selective polyatomic reactivity.

Observation of a transition state resonance in the integral cross section of the F+HD reaction

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 channel 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 steplike 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 transition state. Theoretical scattering calculations have been carried out using the Stark–Werner potential energy surface, which accurately ...

Tracking the energy flow along the reaction path

We report a comprehensive study of the quantum-state correlation property of product pairs from reactions of chlorine atoms with both the ground-state and the CH stretch-excited CHD3. In light of available ab initio theoretical results, this set of experimental data provides a conceptual framework to visualize the energy-flow pattern along the reaction path, to classify the activity of different vibrational modes in a reactive encounter, to gain deeper insight into the concept of vibrational adiabaticity, and to elucidate the intermode coupling in the transition-state region. This exploratory approach not only opens up an avenue to understand polyatomic reaction dynamics, even for motions at the molecular level in the fleeting transition-state region, but it also leads to a generalization of Polanyis rules to reactions involving a polyatomic molecule.

Reactive excitation functions for F+p-H2/n-H2/D2 and the vibrational branching for F+HD

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*(2P1/2) atom is found to be ...

Reaction dynamics of O(1D)+H2, D2, and HD: Direct evidence for the elusive abstraction pathway and the estimation of its branching

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.

Crossed-beam scattering of F+CD4→DF+CD3(νNK): The integral cross sections

The title reaction was investigated in a crossed-beam experiment. A (2+1) resonance-enhanced multiphon ionization technique was used to interrogate the internal-state distributions of the CD3 product at three different collision energies. Only the ν2 (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 ν2=0–3. A reaction barrier of 0.5 kcal/mol was deduced.

EXPLORING THE SPIN-ORBIT REACTIVITY IN THE SIMPLEST CHLORINE ATOM REACTION

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*(2P1/2) 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*(2P1/2) atom is surprisingly more reactive to H2 than the ground state Cl(2P3/2) 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.

Steric Control of the Reaction of CH Stretch–Excited CHD3 with Chlorine Atom

Spectroscopy elucidates the complex interplay between orientational and vibrational effects in a simple chemical reaction. Exciting the CH-stretching mode of CHD3 (where D is deuterium) is known to promote the C-H bond’s reactivity toward chlorine (Cl) atom. Conventional wisdom ascribes the vibrational-rate enhancement to a widening of the cone of acceptance (i.e., the collective Cl approach trajectories that lead to reaction). A previous study of this reaction indicated an intriguing alignment effect by infrared laser–excited reagents, which on intuitive grounds is not fully compatible with the above interpretation. We report here an in-depth experimental study of reagent alignment effects in this reaction. Pronounced impacts are evident not only in total reactivity but also in product state and angular distributions. By contrasting the data with previously reported stereodynamics in reactions of unpolarized, excited CHD3 with fluorine (F) and O(3P), we elucidate the decisive role of long-range anisotropic interactions in steric control of this chemical reaction.