COADSORPTION-INDUCED EFFECTS AT SURFACES - COMPRESSION OF A CHEMISORBED OXYGEN LAYER BY GOLD ON RU(001)

An experimental study of the Au/O/Ru(001) system has shown that coadsorption of gold causes dramatic changes in the parameters of oxygen desorption and, consequently, in the selectivity of surface processes. E(A), the activation energy for desorption of oxygen, is lowered, it is coverage independent, and desorption of oxygen is governed by approximately first-order kinetics. Lower E(A) prevents oxygen bulk diffusion, which otherwise competes with desorption from a pure oxygen layer, thus leading to a 2-fold increase in the amount of desorbing oxygen. A complete analysis of desorption data yields E(A) = 339 +/- 17 kj/mol, prefactor-nu = (4 +/- 2) X 10(14) s-1, and reaction order n = 0.94 +/- 0.03. Observed effects are not due to chemical interactions between Au and O, as inferred from TDS, AES, and XPS data. We propose that changes in E(A), nu, and n arise as gold 3-D islands compress oxygen 2-D islands, keeping the local oxygen coverage higher than in a saturated pure oxygen overlayer. As oxygen leaves the Ru surface (before any significant Au desorption), gold adatoms spread over the liberated Ru surface, keeping oxygen compressed and local oxygen coverage constant.