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After 20 years of experiments on double electron capture and molecular fragmentation, I focus my attention on classical calculations to understand inelastic processes in atom-atom, ion-atom, and ion-molecule collisions, at low and high projectile energies. These present works were initiated by an observation : Quantum mechanics is unable to get evidence for oscillations due to interferences caused by a Auger electron scattering on two protons, following 30keV He2+ + H2 collisions.

Evidence for oscillations in Auger electron angular distributions following He2+ + H2 collisions.

A four-body semiclassical model has been developed and applied to collisions between Auger electrons and two protons, in order to describe interference phenomena observed experimentally in the angular distribution of Auger electrons following double electron capture in 30-keV He2+ + H2 collisions. In contrast with calculation based on quantum mechanics, the present model is able to reproduce, at least qualitatively, but also quantitatively, the angular distribution of Auger electrons, as well as energy distribution of Auger electrons at a fixed angle (Figures 1 and 2).

At present, there is no explanation for this surprising result. A more detailed analysis of electron trajectories that lead to maximum and minimum intensity is thus needed. Since only autoionization has been studied, the next step in our future work will be to include the double electron capture process in the calculation. Indeed, if the separation of the primary process (double capture) and the postcollision process (autoionization) is well adapted to calculate, for example, total or partial cross sections, both processes have to be included to study interferences, since interference phenomena are already present before the collision in the H2 molecule itself. The classical treatment of the whole collision is thus a challenge, which first requires that we correctly simulate the H2 molecule, by the inclusion of electron-electron interaction.

Contact : François Frémont

Semiclassical analysis of angular differential cross sections for single electron capture

A three-body semiclassical model has been developed and applied to discuss the role of interferences in 250-eV H+ + H collisions. First, total capture cross section has been determined, and is found to be smaller than the recommended cross section by a factor of ∼3. Then, differential cross section for electron capture has been calculated, including (or not) interferences, and compared with previous experiment and theoretical results based on quantum mechanics. In contrast with impact parameter distribution, angular distribution clearly exhibits oscillations (Figure 3), whose amplitude increases when interferences are taken into account.

Figure 3 : Angular distribution o H atoms following electron capture H+ + H collisions. Full black circles, experiment ; empty circles, quantum theory ; blue curve, present calculation without any interferences ; red curve, present calculation with inclusion of interferences.

Surprisingly, while the absence of interferences in the calculations gives rise to cross sections that are very similar to that found experimentally, the inclusion of interferences fails to reproduce the experiment, but is in qualitative agreement with the quantum theory.
Therefore, the interpretation of oscillations in the angular distribution of H atoms following electron capture is questionable. Classically, the oscillations are related to the number of swaps the electron experiences between the target and the projectile centers during the collision. In quantum mechanics, the oscillations originate from the interference of two different collisional paths. Both interpretations, in view of our calculations, are reasonable.

Contact : François Frémont