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 occurring along the (111) directions in the plane D110/D110 = D110*/D100* = 2 for all adsorbate-adsorbate interactions; unstarred quantities refer to the chemical, starred to the tracer diffusion coefficients. The value 2 was in agreement with experimental results for O/W(110). For H/W(110) experiments showed D110*/D100* ≅ 1 for virtually all coverages and temperatures. The present simulations indicated that this could be accounted for by allowing a fixed ratio of lateral to diagonal, i.e. (100)/(111) directed jumps. The ratio of chemical diffusion coefficients was still equal to that of the tracers in this case, but D110/D100 was in general a function of coverage. It was possible, however, to find interactions for which D110/D100 remains virtually constant. Theoretical arguments, based on the so-called "conventional approximation" were 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 did not affect D110/D100. Finally, the experimentally observed orientation dependence of mean square fluctuations was confirmed by simulations, using anisotropic adsorbate-adsorbate interactions.

Models of Surface Diffusion. I. Anisotropy in Activated Diffusion. M.Tringides, R.Gomer: Surface Science, 1986, 166[2-3], 419-39