F. Aguillon

Wave-packet study of H2 formation on a graphite surface through the Langmuir-Hinshelwood mechanism.

We have studied the formation of the H2 molecule on a graphite surface, when both H atoms are initially physisorbed. The graphite surface is assumed to be planar. The interaction potential is modeled to reproduce the experimental properties of H physisorption on graphite. Extending our previous work [S. Morisset, F. Aguillon, M. Sizun, and V. Sidis, J. Chem. Phys. 121, 6493 (2004)], full-dimensionality quantum calculations are presented for collision energies ranging from 4 to 50 meV. It is shown that the reaction occurs with a large cross section and produces the H2 molecule with a considerable amount of vibrational energy. The mechanism is either direct or involves the formation of an intermediate complex.

Theory for the nonadiabatic multichannel fragmentation of the Na3+ cluster ion following collision with a He atom

A general theoretical procedure is developed that treats the fragmentation of a polyatomic cluster ion following excitation by a fast rare gas atom. The process involves multiple electronic states of the cluster ion that are described by the diatomics-in-molecule (DIM) procedure. The interaction of the cluster ion with the rare gas atom is obtained by extending the DIM model and by including three-center interactions. The atom-cluster collision is treated using the semiclassical path procedure and the sudden approximation for the cluster. Finally, the fragmentation is studied using the trajectory surface hopping procedure. The method is applied to the Na3+–He system, which can fragment into three product channels. For each channel doubly differential cross sections are computed and compared with the available experimental data. The calculations give much insight into the fragmentation process of Na3+.

Coupled wave packets study of the dynamics of dissociative ion–molecule charge exchange

A coupled wave packet method is presented which allows us to treat exactly the vibrational and dissociative motions in nonadiabatic atom–molecule collisions, whose relative motion is described by a classical trajectory. It consists of a time propagation of the coupled vibrational wave functions defined over a grid. It is applied to the dissociative charge exchange (DCE) process in atom–molecule collisions. Model cases are investigated that put forward the basic characteristics of the DCE process. Analysis of the time evolution of the wave packets leads to a direct view of the collision dynamics and to an interpretation of the dissociation mechanisms. Striking phenomena are revealed, that were not predictable by the previously existing approximate methods.

Fragmentation of Na3+ clusters by He impact: effect of initial cluster temperature on non-adiabatic phenomena

Abstract A theoretical study has been carried out of the fragmentation of Na 3 + ions following a collision with He. The collision leaves Na 3 + in any of three electronic states, and each of these can fragment into three product channels (Na 2 + +Na, Na 2 +Na + and Na+Na+Na + ). The Na 3 + system has a conical intersection and an avoided crossing; these permit transitions between the different states. Cross-sections for each of the nine fragmentation pathways are presented for three different values of the initial vibrational excitation of Na 3 + . The cross-sections are very sensitive to this vibrational energy.

Dalitz plot analysis of three-body fragmentation of Na3+ excited by He impact

Three-body fragmentation of Na3+ ions to Na++Na(3s)+Na(3s) following excitation by He is studied experimentally and theoretically. The three reduced kinetic energies of the products in the center-of-mass are determined for each fragmentation event, and the results are displayed in a Dalitz plot. The fragmentation involves three adiabatic 1A′ electronic states of Na3+ that become degenerate at the detector. It is possible to determine the final electronic state for each event, and here we show that each of the three product states appears in a particular sector of the Dalitz plot. Theoretical and experimental Dalitz plots for the three-body fragmentation of Na3+ are presented, and the results are related to various mechanisms for three-body fragmentation of this system.

Fragmentation of Na3+ clusters following He impact: Theoretical analysis of fragmentation mechanisms

The process of Na3+ cluster ion fragmentation following excitation by a fast He atom is studied using a theoretical procedure developed earlier. The collision with He leaves Na3+ in any of three electronic states, and each of these can fragment into three product channels (Na2++Na, Na2+Na+ and Na+Na+Na+). Attention is focused on understanding both the dynamics of the Na3+–He interaction and the post-collisional fragmentation of the excited cluster. Four simple fragmentation mechanisms are proposed to describe the major features of the process. Contributions of these mechanisms to different fragmentation pathways are determined and their dependence on the initial internal energy of the cluster is studied. Fragmentation intensity maps are calculated and good agreement with experiment is obtained.

Coupled wavepackets study of the dynamics of a model ion-molecule charge exchange

The dynamics of the ion-molecule charge exchange process A + BC + → A + + BC is investigated in a semiclassical framework. The treatment makes use of a semiclassical approximation whereby the collisional motion evolves along a classical trajectory, whereas the vibrational wavefunctions associated with each electronic state are treated quantally. The wavefunctions are defined over a grid and their time evolution is handled numerically. Besides its ability to solve the collision equations, the wavepacket method is shown to provide a useful physical picture of the collision. The goal of this paper is to put forward the basic characteristics of charge exchange processes in term of coupled wave-packets dynamics. In order to do that, a very simple model case has been investigated in the frame of the impact parameter approximation. This model provides deep physical insight into the dynamics of the process, especially in the intermediate energy domain where the non-adiabatic transitions are neither vertical, nor ...

Coupled-wavepacket study of model dissociative charge exchange in collisions between a diatomic ion and a diatomic molecule

Abstract The dynamics of the dissociative charge exchange process AB+CD + →A + +B+CD is investigated, extending previous coupled-wavepacket studies of dissociative and non-dissociative charge exchange in ion-molecule collisions to the case where both collision partners are diatomic species. Here too, the treatment makes use of a semiclassical approximation whereby the relative motion between the two molecules evolves along a classical trajectory, whereas the vibrational motions of the two molecules are treated quantally. The associated wavefunctions are defined over a two-dimensional grid and their time evolution is handled numerically. In order to put forward the basic characteristics of charge exchange processes, a very simple model case has been investigated in the frame of the impact parameter approximation. The model provides physical insight into the dynamics of the process, especially in the intermediate energy domain where the non-adiabatic transitions are neither vertical nor horizontal. At high and low collision energies, the results can be directly compared to the atom-diatom case; at intermediate energies, specific effects are found which reveal an influence of the vibrational movement of CD on the probability of dissociation of the AB molecule.

Distribution of final electronic states following three-body fragmentation of Na3+ excited by He impact

Abstract Three-body fragmentation of Na 3 + ions to Na + +Na(1s)+Na(1s) is studied experimentally and theoretically. A procedure is developed for distinguishing the three adiabatic electronic 1A ′ states in the product region, even though these states are degenerate at the detector. The procedure requires knowledge only of the three atom–atom and ion–atom relative velocities, and this information is obtained in our coincidence experiments. This paper reports the first experimental determination of the populations of the three-product adiabatic states in a three-body fragmentation process. The results, which are different for each state (despite of degeneracy), are compared with theoretical calculations.