  The activated surface diffusion of interacting adsorbates is described in
terms of the so-called interacting single adsorbate approximation, which is
applied to the diffusion of Na atoms on Cu(001) for coverages up to 20% in
quasielastic He atom scattering experiments. This approximation essentially
consists of solving the standard Langevin equation with two noise sources and
frictions: a Gaussian white noise accounting for the friction with the
substrate, and a white shot noise characterized by a collisional friction
simulating the adsorbate-adsorbate collisions. The broadenings undergone by the
quasielastic peak are found to be in very good agreement with the experimental
data reported at two surface temperatures 200 and 300 K.

Statistical Mechanics;Materials Science;Quantum Physics