Nucleation and growth of p(2 x 2) and c(2 x 2) adsorption structures during oxygen adsorption on the (100) surface of metals at low temperatures

The nucleation and growth of p(2 x 2) and c(2 x 2) adsorption structures du ring dissociative chemisorption of oxygen on a metal surface with a square lattice of adsorption sites has been studied by Monte Carlo simulation at l ow temperature (300 K) when the surface diffusion of chemisorbed atoms is s low (i.e., under non-equilibrium conditions). An adsorption model taking in to account both direct and indirect adsorption pathways, lateral interactio ns between chemisorbed particles and their immobility, was used. The p(2 x 2) islands nucleate due to direct dissociative chemisorption of O-2 molecul es on 3NN (third-nearest neighbor) adsorption sites occurring via occupatio n of nearest neighbor (NN) sites by the oxygen atoms followed by hopping of one of the atoms to a 3NN site. The p(2 x 2) islands grow as a consequence of indirect adsorption occurring via trapping of O-2 molecules into precur sor states followed by migration above the adsorption island and chemisorpt ion on similar sites at the edges of the adsorption islands. The c(2 x 2) i slands are mostly formed by the merger of antiphase p(2 x 2) islands that a re spatially not coherent with respect to each other in both directions, an d grow also as a result of indirect adsorption. Antiphase boundaries are fo rmed by the merger of antiphase p(2 x 2) islands that are spatially not coh erent with respect to each other in one direction. (C) 1999 Elsevier Scienc e B.V. All rights reserved.