Velocity correlations and memory functions in surface diffusion

We introduce a generalized velocity correlation function and the correspond ing memory function for the study of tracer diffusion of interacting partic les on surfaces. These functions can be obtained by using discrete particle displacement variables either from experimental or simulation data. We stu dy the behavior of these functions for diverse systems and find that in mos t cases, for both functions, there is an intermediate power-law decay propo rtional to t(-x) which spans about two orders of magnitude in time. The exp onent x depends on the range and the strength of interactions. For the Lang muir gas with on-site exclusions only, x approximate to 2. For other intera cting systems we find that, when strong attractive interactions are present , x tends to decrease from two to unity, while strong repulsion may cause x to be larger than two. These results demonstrate that the velocity correla tion function can be used to gage the importance of interaction effects for surface diffusion. (C) 2000 Published by Elsevier Science B.V. All rights reserved.