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Photofragmentation of molecular adsorbats : Theoretical study of photoionization, photodesorption and related processes.

Abstract : This thesis studies photofragmentation of molecules absorbed on solid surfaces namely the photoionisation, photodesorption and the influence of adsorbate coverage. In the first part, the hindered rotation of molecular adsorbate and photoelectron backscattering are presented. The hindered rotation changes the orientation of the molecular axis and consequently the ejection direction of photoelectrons, while backscattering give rise to interference effects between direct and reflected electron wave. In the models of the literature these effects were up to now neglected and our approach includes the influence of the surface in a realistic way. I calculated the angular distributions of electrons excited from the 4 sigma orbital of physisorbed (CO/Ar) and chemisorbed CO (CO/Ni(100)) systems including the hindered rotation. For CO/Ar, the results are strongly influenced by this rotation. For CO/Ni(100) the hindered rotation plays a minor role appearing as a broadening in the structures of the angular distribution of photoelectrons. Moreover I present a recipe that enables the estimation of a mean tilt angle of the adsorbate about the surface normal taking into account the hindered rotation. For CO/Ni, the photoionization cross section including backscattering of electrons was calculated for 5 sigma and 4 sigma electron excitations. It is found to be important for the first but neglectable for the latter. In the second part, the influence of the surface coverage by the adsorbate was studied by classical and quantum mechanical methods. This coverage manifest itself as a rigidification of the angular motion of the adsorbate, particularly for the precession. For a dense coverage, the first rotational states are dominated by four geometries for the adsorbate orientation, which are nearly degenerated in energy. This does not imply a free precession of the adsorbate but four nearly equiprobable positions of the adsorbate on the surface. The third part includes photodesorption in the non-thermal regime. For a photon excitation energy between 2 and 7 eV, the adsorbate is first electronically excited. Then, the deexcitation transfers a part of energy to the nuclear modes of the adsorbate, especially to the one associated to desorption. With the excited electron, another part of energy departs into the solid. A multidimensional model for photodesorption, that couples the nuclear motions, was derived and applied to CO adsorbed on top of Cu(111). The hindered rotation is strongly coupled to the desorption mode but the internal vibration and hindered translation are decoupled from these two modes. Experimentally, a bimodal fragment velocity distribution was found and attributed to a thermal and nonthermal desorption mechanisms. The results of the present model show similar bimodal distributions only for a nonthermal regime.
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Matthias Büchner. Photofragmentation of molecular adsorbats : Theoretical study of photoionization, photodesorption and related processes.. Atomic Physics [physics.atom-ph]. Université Paris Sud - Paris XI, 1995. English. ⟨tel-00586500v2⟩

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