F. J. Aoiz

Product rotational polarization in photon‐initiated bimolecular reactions

This paper concerns the semiclassical description, calculation and measurement of angular momentum polarization in the products of elementary gas‐phase bimolecular reactions. A unified, semiclassical treatment of the centre‐of‐mass correlated (k,k′,j′) angular distribution involving the reagent and product relative velocity and the product angular momentum vectors is described, and is related to other methodologies already existing in the literature. Explicit expressions are provided enabling experimentalists to extract rotational polarization information from crossed‐molecular beam and photon‐initiated reaction studies, under a variety of experimental conditions. Furthermore, the strategy developed is well suited to the theoretical calculation of reaction product polarization, in particular, using classical trajectory methods. An illustrative example of such a calculation is presented, and the centre‐of‐mass polarization data provided is used to simulate the laboratory frame rotational moments that can b...

Quasiclassical state to state reaction cross sections for D+H2(v=0, j=0)→HD(v’,j’)+H. Formation and characteristics of short‐lived collision complexes

State resolved total and differential reaction cross sections, as well as reaction probabilities, have been calculated by the quasiclassical trajectory (QCT) method for the D+H2(v=0, j=0)→HD(v’,j’)+H reaction on the Liu–Siegbahn–Truhlar–Horowitz potential energy surface in the collision energy range 0.30–1.25 eV. Thus a detailed comparison with existing fully converged quantum mechanical (QM) calculations has been performed. The general agreement between both sets of results is good with some differences. QCT integral reaction cross sections for the production of HD(v’=0) are lower than the corresponding QM ones by 10%–15% for collision energies higher than 0.6 eV, and the energy dependence of the QCT reaction probability with a total angular momentum J equal to zero shows no structure when summed over all j’ states (contrary to the QM case). The differential cross sections for the lowest j’ values show, when represented as a function of energy, a ‘‘ridge’’ feature similar to the one found in exact QM cal...

Experimental Studies and Theoretical Predictions for the H + D2 → > HD + D Reaction

The H + H2 exchange reaction constitutes an excellent benchmark with which to test dynamical theories against experiments. The H + D2 (vibrational quantum number v = 0, rotational quantum number j = 0) reaction has been studied in crossed molecular beams at a collision energy of 1.28 electron volts, with the use of the technique of Rydberg atom time-of-flight spectroscopy. The experimental resolution achieved permits the determination of fully rovibrational state-resolved differential cross sections. The high-resolution data allow a detailed assessment of the applicability and quality of quasi-classical trajectory (QCT) and quantum mechanical (QM) calculations. The experimental results are in excellent agreement with the QM results and in slightly worse agreement with the QCT results. This theoretical reproduction of the experimental data was achieved without explicit consideration of geometric phase effects.

A unified quantal and classical description of the stereodynamics of elementary chemical reactions: State-resolved k–k′–j′ vector correlation for the H+D2(v=0, j=0) reaction

We present a detailed and quantitative comparison of the quantum mechanical (QM) and quasiclassical (QCT) descriptions of the stereodynamics of elementary chemical reactions. Analogous formulations of the QM and QCT k–k′–j′ vector correlation in atom–diatom reactions have been derived and shown to be equivalent in the correspondence principle limit. The comparison between the results obtained from the application of the QM and QCT methodologies to the H+D2(v=0, j=0)→HD(v′,j′)+D reaction at a collision energy of 1.29 eV renders an almost quantitative agreement.

Classical dynamics for the F + H2 → HF + H reaction on a new ab initio potential energy surface. A direct comparison with experiment

Abstract Quasi-classical trajectory calculations have been carried out for the F + H2 reaction on a new ab initio potential energy surface. There is remarkably good agreement between the calculated and experimental vibrationally resolved differential and integral cross sections for all three collision energies studied experimentally. As in the experiments, the production of HF in the ν′=2 vibrational state dominates, with smaller fractions in ν′= 1 and ν = 3. A forward peak appears selectively for ν′ = 3, but is somewhat smaller than the experimental one.

The H+H2 reactive system. Progress in the study of the dynamics of the simplest reaction

Progress in the understanding of the H + H2 reaction and its isotopic variants is reviewed with special attention to the achievements of the last decade. The detailed agreement between theory and experiment attained during this period is emphasized and major experimental and theoretical advances are highlighted. The excellent description of most experimental findings, from state-resolved cross sections to thermal rate constants, provided by the available quantum mechanical treatments, as well as the good overall behaviour of classical mechanics are underlined. Debated issues on short-lived complexes and delayed scattering, resonances and interferences, or geometric phase effects are extensively discussed. Finally, the state-of-the-art is summarized and prospects for future research on this prototypic system are presented.

The F+HD→DF(HF)+H(D) reaction revisited: Quasiclassical trajectory study on an ab initio potential energy surface and comparison with molecular beam experiments

The dynamics of the F+HD reaction has been studied by means of quasiclassical trajectory calculations on an ab initio potential energy surface (PES) at several collision energies. At the collision energy of 85.9 meV and for the DF+H isotopic channel of the reaction, there is a remarkable agreement between calculated and experimental results, in both the center of mass (c.m.) differential cross sections (DCS) and in the simulation of the laboratory (LAB) time of flight (TOF) and angular distributions (AD). The good agreement also extends to the lower collision energy of 58.6 meV for this channel of the reaction. In contrast, the simulation of the LAB angular distributions for the HF+D channel shows strong discrepancies between theory and experiment at both collision energies, which can be traced back to the absence of a forward peak in the calculated c.m. DCS for HF(v’=3). Simulations made from QCT calculations on other PES with important HF(v’=3) forward scattering contributions also fail to reproduce the...

Recent results from quasiclassical trajectory computations of elementary chemical reactions

Recent quasi-classical trajectory (QCT) calculations of the dynamics of some prototypic elementary reactions, from state-resolved differential cross-sections (DCS) to thermal rate constants, are reviewed. The reactions studied are H + H2, F + H2, Cl + H2 and O(1D) + H2, for which reliable potential-energy surfaces (PES) are available. The QCT results are analysed in the light of the most recent quantum mechanical (QM) calculations and experimental findings. In general, QCT integral, differential reaction cross-sections and rate constants are found to be in good agreement with their QM and experimental counterparts, indicating that, for the systems considered, the motion of the nuclei during reactive encounters is largely classical and that quantum effects, such as tunnelling, play a relatively minor role in the overall dynamics. The importance of the zero-point energy of the transition state is highlighted as one of the most important deficiencies of the QC treatment. The need for precise QC and quantal simulations of the actual laboratory measurements, in order to identify experimental quantum effects clearly, is emphasized. Finally, the importance of the calculation and measurement of vector correlations in chemical reactions is stressed and some examples are presented.

Spin–orbit effects in quantum mechanical rate constant calculations for the F+H2→HF+H reaction

Exact and approximate quantum mechanical calculations of reaction probabilities and cumulative reaction probabilities have been carried out for the F+H2 reaction on the ab initio adiabatic potential energy surfaces by Stark and Werner (SW) and by Hartke, Stark, and Werner (HSW), the latter including spin–orbit corrections in the entrance channel. These data have been employed to obtain thermal rate constants for the title reaction in the temperature range 200–700 K. The exact and approximate results have been compared with experimental determinations and previous theoretical predictions. In particular, the reaction probabilities obtained on the HSW surface are found to be in very good agreement with recent calculations by Alexander et al. [J. Chem. Phys. 109, 5710 (1998)] based on the exact treatment of spin–orbit and Coriolis coupling for this system. However, the rate constants calculated on the HSW PES are systematically lower than the experimental values, which indicates that the height of the adiabat...

Dynamics of the Simplest Chlorine Atom Reaction: An Experimental and Theoretical Study

Angular distributions and time-of-flight spectra for the reaction Cl + H2 → HCl + H obtained from a high-resolution, crossed-molecular beam experiment were compared to differential cross sections calculated by both converged quantum mechanical scattering and quasi-classical trajectory methods. Good agreement was found between the experimental results and each theoretical prediction. The results demonstrate that excellent agreement can be obtained between state-of-the-art simulations and experiments for the detailed dynamical properties of this prototype chlorine atom reaction.