On the viability of single atom abstraction in the dissociative chemisorption of O-2 on the Al(111) surface

The dissociative chemisorption of O-2 on the Al(111) surface is investigate d by means of a Monte Carlo simulation that incorporates two mechanisms tha t have been proposed for this reaction in the literature: single atom abstr action and two-atom adsorption that generates translationally hot atoms on the surface. A comparison is made to the much-debated STM results of Brune [J. Chem. Phys. 99, 2128 (1993)], in which the oxygen island density (numbe r of islands per binding site) was determined as a function of coverage. Si nce the two-atom channel has been discussed heavily in the literature, we f ocus primarily on the abstraction mechanism. We show that atom abstraction in its basic form is incompatible with the STM results; however, we propose two simple modifications that enable atom abstraction to reproduce the STM results. In the first modification, the probability of dissociation is hig her at sites next to preexisting O adatoms. In essence, we are proposing th at the increased Al-O bond strength at sites next to preexisting O adatoms [Jacobsen , Phys. Rev. B 52, 14954 (1995)] stabilizes the transition state for dissociation. If atom abstraction is assumed to be the only operative m echanism, and if its probability increases by a factor of similar to 10 nex t to a site that is occupied versus unoccupied, the STM island density data can be approximately reproduced. In the second modification, the abstracte d atom is permitted to make a single hop in the direction of a preexisting, nearby O adatom. The allowance of merely a single, directed hop has a dram atic effect on the coverage dependence of the island density. (C) 2000 Amer ican Institute of Physics. [S0021-9606(00)71315-8].