Models of surface diffusion: II. Tunneling

Abstract

Monte Carlo simulations were carried out to see if the diffusion behavior of 1H, 2H and 3H at 27 K could be reproduced. The basic assumptions were that there is strong mass renormalization, which can depend on local environment and that local barrier heights are affected by adsorbate-adsorbate interactions. In order to account for the lack of diffusion anisotropy jumps resulting in displacements along (100) only, as well as “normal” jumps along (111) and concerted 2 atom tunneling events were also allowed. It was found that the maximum in the D versus θ curves at θ = 0.3 can be reproduced very well by the competition between repulsive nearest neighbor and attractive linear triplet interactions, and that such interactions are also in accord with what is known about the H/W (110) phase diagram. Several sets of interactions were used. All do a good job of reproducing the 1H and 3H curves and also lead to the upturn in D seen for 2H at θ > 0.6, if the effect of quantum statistivs on 2 atom and (100) directed jumps is taken into account. Only one of the models reproduces the 2H curve semiquantitatively, but the interactions used in this model do not lead to the correct phase diagram. Various deficiencies of the models are also discussed.