| This thesis presents a non perturbative theory to describe multi-electronic processes occurring in the course of ion-atom collisions.The treatment is semiclassical in that the relative target-projectile motion is described by classical straight-line constant velocity trajectories,while the electronic dynamics is treated quantum mechanically,by solving non perturbatively the time-dependent Schodinger equation.The treatment has been im-plemented in a new version of two-active-electron computer code.Besides the long and complex development and tests of the code,the last three years have been especially de-voted to understanding of the physics of specific heavy particle scattering events.We have undertaken the study of three collision systems with various features:(i)low charge ion-ion collisions with an extreme importance of electronic correlation(H++H-collisions),(ii)multiply charged projectile-atom collisions(C4++He)and(iii)He++He collisions with the dynamical treatment of the three electrons.Our guideline was always to target systems for which experimental and theoretical results were available(at least in some energy do-main),with still open questions,related,for example,to strong disagreement between the various data.We have tried as much as our computing resources and allowed it to produce results with controlled convergence.These investigations were carried out in a very wide,up to three decades,energy domain with same collision description(i.e.same basis sets),which brought continuity and coherence on the predictions and the interpretations of the results and of the underlying mechanisms giving rise to the processes considered.Firstly,we have investigated the double electron capture(DEC)process in the H++H-collision system.Despite the apparent simplicity of this highly correlated system,all previous calculations fail to reproduce the measured experimental total cross sections.Our results reproduce well the experimental data in both magnitude and shape.Furthermore,we demonstrate that the oscillations stem from coherence effects between double electron capture and other two-electron inelastic channels,namely the transfer-excitation processes.An extended Rosenthal-like model based on a molecular treatment of the collision supports our interpretation.Our results shed new light on this old but challenging problem.Secondly,the electron capture processes in C4++He collisions have been studied in a wide energy domain.The results of our calculations are compared with available theoretical predictions and experimental measurements:very good agreements are found for both total and state-selective single electron capture(SEC)and DEC cross sections.We extend the knowledge on that system to high energies for which only a single series of data exists.The mechanisms responsible of SEC and DEC processes as well as the role of electronic correlations in the collisions have been further studied by additional restricted two-active-electron and single-active-electron calculations.Furthermore,the observed oscillations in the small-angle angular-differential cross sections for both SEC and DEC have also been investigated by the simulated Fraunhofer-type diffraction pattern.Lastly,an extended three active electrons approach has been adopted on the study of Hc+He collisions.Total,state-selective and angular-differential cross sections are presented and compared with available experimental and theoretical results.A prominent oscillatory energy dependence structure in the transfer-target-excitation cross sections is observed and explained by a strong competition between these channels and the projectile-excitation processes.Moreover,the angular-differential cross sections considered in this study exhibit an oscillatory structure which is interpreted within a simple Fraunhofer-type diffraction model.For the two highest considered collision energies,the cross sections show a different pattern for which both Fraunhofer-type diffraction and Thomas mechanism have to be advocated. |
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