Xavier Giménez

Semiclassical description of quantum coherence effects and their quenching: A forward-backward initial value representation study

The forward–backward (FB) version of the semiclassical (SC) initial value representation (IVR) is used to study quantum coherence effects in the time-dependent probability distribution of an anharmonic vibrational coordinate and its quenching when coupled to a thermal bath. It is shown that the FB-IVR accurately reproduces the detailed quantum coherent structure in the weak coupling regime, and also describes how this coherence is quenched with an increase of the system–bath coupling and/or the bath temperature. Comparisons are made with other approximations and the physical implications are discussed.

The He + H2+ reaction: a dynamical test on potential energy surfaces for a system exhibiting a pronounced resonance pattern

Quantum mechanical calculations on three potential energy surfaces for the prototype ion-molecule reaction He + H2+ → HeH+ + H have been performed in order to test the influence of their accuracies on reaction probabilities and cross sections. The ab initio points of McLaughlin and Thompson (1979) fitted by two different functional forms, and a fit of a new set of ab initio points have been used. Dynamical results, in particular the rich resonance pattern, illustrate the dependence both on the nature of the potential energy surface, and on the type of functional form used to fit the same ab initio data.

Hyperquantization algorithm. II. Implementation for the F+H2 reaction dynamics including open-shell and spin-orbit interactions

This work focuses on numerical aspects and performances of the hyperquantization algorithm, presented in the preceding paper, for a prototypical atom–diatom reaction. Here we provide also the extensions which allow the treatment of excited electronic surfaces. Test calculations have been carried out on the reaction F+H2 at a total nuclear angular momentum equal to zero, the fine structure of the fluorine atom being also explicitly taken into account. The technique presented is shown to be simple and effective for applications to reactive scattering problems, and the results are competitive with those obtained applying other current methods, especially in the strong triatomic interaction region.

Semiclassical description of diffraction and its quenching by the forward–backward version of the initial value representation

It is shown that the forward–backward (FB) version of the semiclassical (SC) initial value representation (IVR) is able to describe quantum interference/coherence (i.e., diffraction) of particles transmitted by a two-slit potential. (In contrast, the linearized approximation to the SC-IVR, which leads to the classical Wigner model, is unable to do so.) FB-IVR calculations are also used to describe the (partial) quenching of this interference structure (i.e., “de-coherence”) when the two-slit potential is coupled to a bath of harmonic oscillators.

Ab initio dynamics of the He + H+ 2 → HeH+ +H reaction: a new potential energy surface and quantum mechanical cross-sections

The reaction He + H+ 2(v,j = 0) → HeH+(v′ = 0, j′) for v = 0, 1,2 and 3 and for scattering energies near the threshold (0.95–1.15 eV) has been studied by calculating ab initio points at MRCI level and ‘exact’ integral quantum reactive cross-sections. More than 1400 nuclear geometries have been chosen to cover the most important regions for the dynamics, an extended set of points being taken directly on a hyperspherical coordinate grid. A many-body expansion with a large number of terms permits an accurate analytical representation of the potential energy surface with a root-mean-square deviation <12meV. The hyperquantization algorithm has been extended to obtain quantum mechanical integral cross-sections which are compared with previous calculations and with experimental results.

A comparison between experimental, quantum and quasiclassical properties for the N(4S) + O2 (X3Σg) → NO (X2Π) + O(3P) reaction

Abstract A detailed reactive infinite order sudden approximation (R-IOSA) study of the reactivity of the N ( 4 S ) + O 2 system has been carried out in the 0.53–1.5 eV translational energy range. Results have been compared with experimental and quasiclassical trajectory (QCT) data. The general dynamical features already obtained within the QCT framework have been reproduced. In addition, the R-IOSA product vibrational distribution matches almost quantitatively the experimental one. The relative importance of the different ν′ contributions, both to the classical and quantum more averaged quantities has been interpreted with the help of the state-to-state opacity function. Finally, the alternating vibrational distribution and its evolution with translational energy are discussed within the context of a Franck-Condon type model.

Reactivity enhanced by under-barrier tunneling and resonances: the F+H2→HF+H reaction

Accurate quantum mechanical rate constants (all contributing partial waves, fine energy grid Boltzmann averaging) for the title reaction are obtained by the hyperquantization algorithm, covering the range from above room temperature down to the cold regime (few K). The good agreement with available experiments down to � 200 K, obtained by blending ab initio description of the transition state and molecular beam scattering experimental characterization of the entrance channel, establishes the reliability of the approach to describe deviations from Arrhenius behavior at those low temperatures where quantum mechanics can induce specific selectivity in chemical reactivity 2003 Elsevier Science B.V. All rights reserved.

Accurate 3 dimensional quantum dynamical study of the Ne+H2+→NeH++H reaction

In this work a comprehensive, fully converged coupled states (CS) quantum mechanical (QM) study of the endothermic Ne+H2+ ion-molecule reaction is presented. The computed dynamical properties are compared with quasi-classical trajectory (QCT) and with the available experimental data. To this end, the analytical potential energy surface of Pendergast, Heck, Hayes, and Jacquet was employed. The two main features of the dynamical behavior for this system are: (1) the rich structure present in the state-selected integral cross section energy-dependent curves, which may be attributed to resonances surviving the partial wave summation; and (2) the large differences between the quantum and the QCT cross sections which are caused by the inability of classical mechanics to conserve the zero point energy. Also noteworthy are the strong enhancement of the reactivity due to higher vibrational states and the effect of the activated complex, formed during the reaction process, on the angular and the rotational distribu...

Dynamics of the N(4S)+NO(X 2Π)→N2(X 1Σ+g)+O(3P) atmospheric reaction on the 3A″ ground potential energy surface. III. Quantum dynamical study and comparison with quasiclassical and experimental results

A detailed reactive–infinite‐order sudden approximation (R‐IOSA) study of the reactivity of the N+NO→N2+O system has been carried out in the 0.0038 to 1.388 eV translational energy range and the results have been compared with the existing quasiclassical trajectory (QCT) and experimental data available. The general features already observed in the previous QCT studies are reproduced qualitatively in the quantum study, even though some differences arise in the product vibrational distributions and state‐to‐state opacity functions in the low energy range. The observed differences have been justified in terms of the anisotropy of the potential energy surface and the vibrational barriers to reaction at fixed angles. A strong vibrational adiabaticity is observed quantally in the low translational energy range, disappearing at moderately high collision energies (around 0.3 eV), where a simple Franck–Condon type model is capable of describing the evolution of the vibrational distribution with translational energ...

THE APPLICATION OF COMPLEX ABSORBING POTENTIALS TO AN INVARIANT EMBEDDING SCATTERING METHOD: I. THEORY AND COMPUTATIONAL DETAILS

In this article, an extension of quantum scattering methods based on propagative R-matrix techniques to deal with negative imaginary potentials is presented. Reactive probabilities can be then obtained, considering only the reactants arrangement channel and Jacobi coordinates. It has been necessary to generalize the R-matrix propagation method, in order to consider the complex-valued nature of the interaction matrix. The new formulation has been particularized, in the present case, to the Infinite-Order Sudden Approximation, for which several results, focusing on the reliability and numerical performances of the method, will be shown.