Theoretical studies of surface diffusion and of photo-induced surface processes

Theoretical study of three types of processes associated with adsorbate-substrate interactions will be discussed. The first process considered is the surface diffusion mechanism of large adsorbates. It will be demonstrated that the conventional random walk mechanism, associated with the surface diffusion of small adsorbates, has to be largely modified when description of the motion of large adsorbates along the surface is desired. The results of these calculations shows good qualitative agreement when compared with experimental data. In the second study reported here, the branching ratio between desorption and dissociation for large model adsorbates on a laser heated substrate is examined. It is found that the branching ratio is very sensitive to a number of quantities including the adsorbate size, adsorption geometry and the relative strength between the adsorbate-surface bond and the intramolecular bond which is broken during the dissociation process. The last group of calculations investigate the dynamics of photodissociation products and their relation to the orientation and alignment of the adsorbates on the solid surface. The calculations are carried out for the HBr/LiF(001) system. Quantities such as the photoreaction probability, the angular and energy distributions of the scattered photofragment are shown to depend strongly on the initial alignment of the adsorbate. In addition, the quantum nature of the H photoproduct is examined using a 2-D quantum mechanical model. It is shown that for some systems the angular distribution of the scattered H-atom may exhibit a complicated diffraction pattern which can be related to the structure of the interface between the adsorbate layer and the substrate.