L. Bañares

Quantum mechanical and quasi-classical trajectory study of the C(1D)+H2 reaction dynamics

First accurate quantum mechanical (QM) calculations of integral and differential cross sections for the C(1D)+H2(v=0,j=0,1) insertion reaction have been performed on a newly developed ab initio potential energy surface [B. Bussery-Honvault et al., J. Chem. Phys. 115, 10701 (2001)]. These results have been compared with those obtained with a quasi-classical trajectory (QCT) method. A Gaussian-weighted binning procedure to assign product quantum states in the QCT calculations yields vibrational branching ratios and rotational distributions in better agreement with the QM calculations than those obtained when the usual histogramatic binning method is employed. This is the first time that the Gaussian-weighted binning procedure is used for an insertion reaction.

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.

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.

Quantum mechanical and quasiclassical simulations of molecular beam experiments for the F+H2→HF+H reaction on two ab initio potential energy surfaces

Laboratory (LAB) angular distributions (AD) measured in molecular beam experiments by Lee and co-workers in 1985 and very recently by Keil and co-workers for the prototypic F+H2 reaction have been simulated using new quantum mechanical (QM) and quasiclassical trajectory (QCT) state-resolved differential cross sections (DCS) calculated on the ab initio potential energy surfaces (PES) by Stark and Werner (SW) and by Hartke, Stark and Werner (HSW); the latter PES includes spin-orbit coupling corrections added to the entrance channel of the former. The simulations of the 1985 LAB ADs performed using the new QM calculations on the SW PES show a very good agreement with the experimental results for all final vibrational states of the HF product. The inclusion of spin-orbit coupling corrections in the ab initio HSW PES does not seem to improve the agreement between theoretical and experimental results. As for the simulation of the recent experiments of Keil and co-workers, the LAB ADs are very well reproduced by...

A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH3I

The real time photodissociation dynamics of CH(3)I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH(3) fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH(3)(nu=0) and I((2)P(32)) and I(*)((2)P(12)) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I(*) channels when the CH(3) fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.

The dynamics of the hydrogen exchange reaction at 2.20 eV collision energy: Comparison of experimental and theoretical differential cross sections

The H+D2(v=0,j=0)→HD(v′,j′)+D isotopic variant of the hydrogen atom exchange reaction has been studied in a crossed molecular beam experiment at a collision energy of 2.20 eV. Kinetic energy spectra of the nascent D atoms were obtained by using the Rydberg atom time-of-flight technique. The extensive set of spectra collected has permitted the derivation of rovibrationally state-resolved differential cross sections in the center-of-mass frame for most of the internal states of the HD product molecules, allowing a direct comparison with theoretical predictions. Accurate 3D quantum mechanical calculations have been carried out on the refined version of the latest Boothroyd–Keogh–Martin–Peterson potential energy surface, yielding an excellent agreement with the experimentally determined differential cross sections. The comparison of the results from quasi-classical trajectory calculations on the same potential surface reveals some discrepancies with the measured data, but shows a good global accordance. The t...