THEORETICAL-STUDIES OF SURFACE-DIFFUSION AND OF PHOTOINDUCED SURFACE PROCESSES

Theoretical study of three types of processes associated with adsorbat e-substrate interactions will be discussed. The first process consider ed is the surface diffusion mechanism of large adsorbates. It will be demonstrated that the conventional random walk mechanism, associated w ith the surface diffusion of small adsorbates, has to be largely modif ied when description of the motion of large adsorbates along the surfa ce is desired. The results of these calculations shows good qualitativ e 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. I t is found that the branching ratio is very sensitive to a number of q uantities including the adsorbate size, adsorption geometry and the re lative strength between the adsorbate-surface bond and the intramolecu lar bond which is broken during the dissociation process. The last gro up of calculations investigate the dynamics of photodissociation produ cts and their relation to the orientation and alignment of the adsorba tes on the solid surface. The calculations are carried out for the HBr /LiF(001) system. Quantities such as the photoreaction probability, th e 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 inte rface between the adsorbate layer and the substrate.