Alkali-promoted oxidation of Al(111): [formula omitted] and [formula omitted] coadsorption and the role of surface structure

Abstract

Synchrotron radiation core-level photoemission from the Al 2p, K 3p and Rb 4p states has been used to characterise the role of preadsorbed Rb and K on the interaction of oxygen with an Al(111) surface. Specific precoverages have been used corresponding to two different (√3 × √3)R30° surface structures, a metastable low temperature phase involving alkali atoms in atop sites, and a stable higher temperature phase with substitutional alkali atoms. In all cases a significant promotion of both dissociation and oxidation is seen relative to the activity of the clean Al(111) surface. In comparison with earlier results for Na O coadsorption Rb is found to promote oxidation most strongly and Na least strongly with K being intermediate; the Rb room temperature substitutional phase, in particular, shows oxidation at the lowest oxygen exposures and no indication for the RbO chemisorption precursor comparable with the NaO one identified on the Na-covered surface. By contrast the Rb-atop and K-atop geometry surfaces do show evidence of some discrete chemisorption states in the Al 2p spectra of the type seen on alkali-free Al(111), but only in the presence of other spectral structure assigned to mixed-coordination geometries. At low temperatures the effect of both Rb and K on oxidation, but not on initial oxygen adsorption, is generally suppressed, an effect ascribed to the role of bulk diffusion. Measurements of normal incidence X-ray standing wavefield absorption for the Rb O coadsorption structures at very low oxygen exposure also indicate that no simple single sites are occupied, particularly in the case of the more reactive Rb-substitutional phase at room temperature, although there appears to be a relatively well-defined OAl layer spacing attributed to small but laterally incommensurate oxide islands. Measured work function changes at low oxygen exposure in the Rb O and Na O systems can be reconciled with oxygen penetration of the alkali layer except for the Na-substitutional phase, for which the data are qualitatively consistent with the previously reported atop geometry.