CORRELATION-FUNCTIONS IN SURFACE-DIFFUSION - THE MULTIPLE-JUMP REGIME

Although the theories (transition state theory and jump diffusion) usually employed to describe surface diffusion cannot give information about the motion of adsorbed particles inside the potential wells, interesting results were obtained recently by MD simulations showing enhanced oscillations in the mean-square displacement before the linear behaviour in time is finally reached; at the same time evidence of long correlated jumps was found. In this paper the single-particle diffusion on surfaces is studied in the framework of the continuous Brownian model (Klein-Kramers equation). The Klein-Kramers dynamics is first analyzed by qualitative considerations about the dissipation integral, obtaining necessary and sufficient conditions on typical time scales in order to get different migration mechanisms. In particular, at high barriers, conditions are found for multiple jumps to be inhibited or to take place with small and considerable probability respectively. Then starting from the dynamic structure factor, the relevant correlation functions (velocity self-correlation spectrum and mean-square displacement) are evaluated, together with the jump probabilities, at the same potential barrier and friction of the MD calculations. At these values of the parameters diffusion proceeds, as expected, by a considerable fraction of multiple correlated jumps and many oscillations are found in the mean-square displacement in good agreement with MD results.