The initial stage of adsorption and beryllium oxidation by water (clearly a nonadiabatic process) was studied for a wide temperature range, using AES, XPS, DRS, and CPD measurements. The mechanism of room temperature (RT) oxidation by water vapor was found to be by nucleation and growth of 3 monolayer oxide islands, laterally spreading until coalescence takes place. When a full oxide layer is achieved, a further slow oxidation takes place, virtually stopping at ∼6 monolayer depth. Exposure of the surface to water vapor at 150 K yielded dissociation to H and OH, chemisorbed on the surface, as detected by an XPS chemical shift. The lack of such a shift at RT indicates a full dissociation of the water molecule on the surface. A giant effect of Be electron-stimulated oxidation (ESO) by water vapor, as opposed to Be mild ESO by O2, was observed, reaching the maximal possible oxidation rate for the ratio of ≥150 impinging electrons per water molecule. It is suggested that the mechanism is a Mott-Cabrera-like one, enabled by a combination of an electric field applied by negative OH and/or oxygen ions formed at the surface, probably by secondary electron attachment, and a very fast diffusion of Be2+ ions enabled by the presence of hydrogen in the oxide bulk. The water vapor ESO exhibits an inverse dependence on the substrate temperature, presumably due to the decrease with temperature of hydroxyl surface concentration, leading to the weakening of the electric field formed across the oxide.
ASJC Scopus subject areas
- Chemistry (all)