Diffusion of an adsorbed particle: Dependence on the adatom-substrate interaction

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Abstract
Based on a general microscopic theory given in an earlier paper we present here a study of the diffusive motion of an atom adsorbed on a solid surface. This is done through an investigation of the quasielastic peak in the dynamic structure factor. Its width goes to zero at each reciprocal-lattice point and the q dependence is found to contain valuable information on the mechanism for the diffusion process. The theory incorporates the dynamics of the adatom-substrate interaction in a more proper way than by using the Brownian-motion theory. All calculations are performed at a temperature corresponding to thermally activated diffusion. If the adatom is identical to the substrate atoms the diffusion process is well described by a jump diffusion model. For a more weakly bound adatom the situation is more complicated. Because of the small friction in this case, adatoms escape only slowly whenever their initial energy is less than the barrier energy for diffusion. On the other hand, adatoms with higher energies move rather freely and give rise to a large contribution to the diffusion constant.