Models of surface diffusion: I. Anisotropy in activated diffusion

Abstract

Monte Carlo simulations of surface diffusion on a bcc(110) plane were carried out to investigate the anisotropy of diffusion. For the unreconstructed plane with jumps occuring along the (111) directions in the plane D 110/D 110 = D 110 ∗/D 100 ∗ = 2 for all adsorbate-adsorbate interactions; unstarred quantities refer to the chemical, starred to the tracer diffusion coefficients. The value 2 is in agreement with experimental results for O/W(110). For H/W(110) experiments showed D 110 ∗/D 100 ∗ ≅ 1 for virtually all coverages and temperatures. The present simulations indicate that this can be accounted for by allowing a fixed ratio of lateral to diagonal, i.e. (100)/(111) directed jumps. The ratio of chemical diffusion coefficients is still equal to that of the tracers in this case, but D 110/ D 100 is in general a function of coverage. It is possible, however, to find interactions for which D 110/ D 100 remains virtually constant. Theoretical arguments, based on the socalled “conventional approximation” are presented to rationalize the equality of the ratio of tracer and chemical diffusion coefficients, and to evaluate the ratio for simple cases. Additional simulations showed that diffusion chains do not affect D 110/ D 100. Finally, the experimentally observed orientation dependence of mean square fluctuations was confirmed by simulations, using anisotropic adsorbate-adsorbate interactions.