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No description available.

This thesis is concerned with the use of L-squared or square integrable functions in electron atom scattering at intermediate energies, and tests the success of various L-squared approximations in model problems of electron hydrogen atom scattering. The representation of part or all of the wave and Green's functions by a set of L-squared pseudostates, and the associated occurrence of unphysical pseudoresonances at the pseudostate thresholds is discussed. The original work of this thesis is in two parts. In the first, a model coupled channel problem is considered in which an L-squared optical potential is used to represent the effect of additional (Q space) channels on the first (P space) channel. A method of Bransden and Stelbovics used successfully for a two channel problem is extended to the case of several channels. Numerical results are presented for the cases of two and three channels and the success of the procedure is assessed. The rest of the research presented here concerns the use of the Schwinger variational method in a restricted model of electron hydrogen atom scattering in which all states are assumed to be spherically symmetric. The method is used successfully to solve coupled channel problems using L-squared pseudostates to represent the s-wave continuum. The origins of the pseudoresonances that occur in these problems are investigated and a method of removing pseudoresonances before T matrix elements are calculated is considered. The limitations and instabilities of the Schwinger method when applied to the full model problem with different representations of hydrogen states in the trial and Green’s functions are investigated, and various modifications are considered in attempts to stabilise results where necessary in these more general cases.

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AUCUN

An extension of the nonadiabatic quantum molecular dynamics approach is presented to account for electron-nuclear correlations in the dynamics of atomic many-body systems. The method combines electron dynamics described within time-dependent density-functional or Hartree-Fock theory with trajectory-surface-hopping dynamics for the nuclei, allowing us to take into account explicitly a possible external laser field. As a case study, a model system of H++H collisions is considered where full quantum-mechanical calculations are available for comparison. For this benchmark system the extended surface-hopping scheme exactly reproduces the full quantum results. Future applications are briefly outlined.

De façon générale, la physique des collisions concerne l'étude des phénomènes induits par l'interaction de particules en mouvement. En physicochimie moléculaire et en physique atomique, cadres dans lesquels s'inscrit cette thèse, les interactions mises en jeu sont coulombiennes et les partenaires de la collision sont des espèces atomiques ou moléculaires, neutres ou chargées. Celles-ci sont susceptibles de subir au cours de la collision des modifications importantes de leur cortège électronique, à la source même de processus secondaires variés présentant un grand intérêt, par exemple, dans la modélisation de systèmes complexes tels que les plasmas, les milieux astrophysiques ou biologiques. Notre étude s'appuie sur une description théorique semi-classique non-perturbative des processus multi-électroniques au cours de collisions atomiques et moléculaires, à des énergies telles que la vitesse relative des partenaires est comparable à celle de leur électrons de valence. Dans deux systèmes " benchmark " (H+ - Li , He - H2+), nous avons mis en évidence respectivement l'existence de couplages complexes entre voies de réaction impliquant les électrons internes et de valence du lithium et des phénomènes d'interférences et de diffraction d'ondes de matière. Nous avons également étudié des systèmes de collision plus complexes impliquant le carbone, en analysant tout particulièrement des phénomènes multi-électroniques (collisions C(+) - He) - hors approximation des électrons indépendants - et multi-centriques (collisions proton-graphène). Pour ce dernier système, les résultats obtenus ont permis de mettre en évidence les principes d'une nouvelle technique d'imagerie de matériaux 2D. / In general, the Physics of collisions concerns the study of phenomena induced by the relative motion of interacting particles. In chemical physics and atomic physics, which are the area covered by this PhD, the interactions are Coulombic and the colliding partners are atoms or molecules which can be neutral or charged. During the collision, they are likely to undergo important modifications of their electronic environment, which can be the source of various secondary processes that are of great interest, for example, in the modelling of complex systems such as plasmas or astrophysical and biological media. Our study is based on a close-coupling semi-classical description of the multielectronic processes occurring in the course of atomic and molecular collision at impact energy such as the relative velocity of the partners are of the same order of magnitude than the classical velocity of their valence electrons. We have studied two ?benchmark? systems (H+ - Li , He ? H2+), for which we have respectively highlighted the existence of couplings between channels involving inner and outer-shell electrons of lithium, and, wave matter interferences and diffraction phenomena. We have also studied more complex colliding systems involving the carbon nucleus in analyzing multielectronic (C(+) ? He collisions) and multicentric (proton-graphene collisions) phenomena. For the latter system, the results obtained have allowed us to bring out the principles of a new two-dimensional material imaging technique.

An extension of the nonadiabatic quantum molecular dynamics approach is presented to account for electron-nuclear correlations in the dynamics of atomic many-body systems. The method combines electron dynamics described within time-dependent density-functional or Hartree-Fock theory with trajectory-surface-hopping dynamics for the nuclei, allowing us to take into account explicitly a possible external laser field. As a case study, a model system of H++H collisions is considered where full quantum-mechanical calculations are available for comparison. For this benchmark system the extended surface-hopping scheme exactly reproduces the full quantum results. Future applications are briefly outlined.