Adsorption of water on Al(100)

Abstract

The interaction of water with a clean and oxygen precovered Al(100) surface at about 100 K has been studied by thermal desorption spectroscopy (TDS), nuclear reaction analysis (NRA) and measurements of work function changes (ΔΦ). TDS and NRA show that an amount of water equivalent to 2.2 monolayers (ML; 1 ML = 1.22 × 10 15 H 2O cm −2) dissociates either upon adsorption or during subsequent heating to ~ 200 K. The surface is then saturated and the additional water is adsorbed in ice layers which desorb in the molecular form. TD spectra show the zero order desorption of undissociated water accompanied by the release of hydrogen from the dissociated water. On the oxygen precovered surface much less dissociation occurs and nearly all of the adsorbed water desorbs. Hydrogen desorption spectra are compared to desorption spectra obtained from a surface exposed to oxygen and atomic hydrogen. ΔΦ decreases on water exposure of both clean and O-exposed Al(100) surfaces, but the decrease is more rapid and exhibits a lower (i.e. more negative) saturation value for the O-precovered surface. Desorption of water and hydrogen from the water-exposed clean Al(100) surface, is accompanied by a decrease in ΔΦ to values characteristic of water-exposed oxidized Al(100) surfaces. Further heating causes ΔΦ to increase again, accompanied by desorption of hydrogen. This is discussed in terms of the disproportionation of surface −OH groups.