Diffusion on a face-centred cubic(111) surface in the presence of two non-equivalent adsorption sites

The interrelation between the surface symmetry and diffusion of adsorbed at oms is studied theoretically on a model face-centred cubic (fcc) (111) surf ace within the lattice-gas approximation. Diffusion proceeds through two no n-equivalent sites on the fcc(111) surface. Short-range repulsive interacti on between particles in the nearest-neighbour non-equivalent sites is consi dered. This repulsion leads to a modification of the occupation numbers of both kinds of site and to changes in the energy barrier height for diffusio n. The chemical surface diffusion coefficient, D, has been found to be strongl y dependent on coverage, Theta, the interaction strength between adatoms, a nd the difference between the activation energies, Delta E. In the case of a small Delta E value, the diffusion coefficient D increases with Theta, as was expected for repulsive interaction. In our model the increase of D at Theta=0.5 is reduced for higher values of Delta E and a local maximum of D( Theta) is observed. (C) 2000 Elsevier Science B.V. All rights reserved.