We apply a many-body classical model and a semiclassical coupled-channel approach to study the electronic processes occurring in the course of fast collisions between atomic and molecular species. The methods are applied in a comparative study of electron transfer in He 2+ /Ar 2+ −H 2 + collisions at keV u −1 impact energies.The classical calculations are in agreement with recent experimental data [H. Bräuning et al., J. Phys. B 34, L321 (2001)] for projectile velocities larger than the initial electron velocity. The semiclassical model is based on the sudden approximation where the electronic wave function is expressed by linear combinations of traveling atomic capture states and target molecular states obtained at fixed nuclei. The related charge transfer cross sections are slightly underestimated (overestimated) for He 2+ ͑Ar 2+ ͒ when considering H 2 + in its initial vibrational ground state. These cross sections do, however, depend strongly on the internuclear distance of the hydrogen molecular ion: when involving an initial vibrational excitation of the target, the averaged cross sections obtained from the semiclassical approach become in fair agreement with experiments.

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A (1+1)-dimensional model of H+2 is employed to study the validity of the fixed and the classically moving nuclei approximations commonly applied in theoretical description of ionization of diatomic molecules in short, intense laser fields. Up to a certain threshold intensity the ionization probability is found to be in very good agreement between all models, even though there are some differences in the final electronic probability density. Beyond the threshold intensity the classical and fixed nuclei approximations are seen to be completely inadequate to describe the ionization process. A simple scaling relation for the threshold intensity is derived and shown to be in excellent agreement with the numerical results.

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Abstract:When rare gas atoms are submitted to a short pulse of radiation delivered by an infrared laser with peak intensity beyond I ≈ 10 18 W/cm 2 , highly charged ionic species and fast electrons are produced. In this context, we discuss several issues raised by the results reported in very recent experiments, in which ion yields have been measured. We shall address the questions related to the population dynamics of the successive ionization stages, in order to account for the observed ion production yields in finite duration pulses.2004 Optical Society of America [9-point type]