Oxidation of Mg and electron transfer processes

Results of a study of the modification in electron capture processes during the oxidation of metal surfaces are presented by the example of O- formation in oxygen ion/atom scattering. Measurements of scattered-ion fractions were made for a clean Mg surface, which was then exposed to O-2 and also MgO(100). It was found that a strong increase in capture occurs with increasing oxygen coverages from the submonolayer range to very high coverages corresponding to oxide formation. At low coverages these changes are interpreted as resulting from competing local effects at oxygen chemisorption sites and non-local effects corresponding to work function changes. The results for high exposures are similar to what was observed for the MgO(100) crystal. Here scattering on an ionic solid is discussed and non-resonant electron capture on the Odelta- Site has to be considered after inclusion of level shifts in the Madelung held. Here a sequence of collisions, or sufficiently close approaches to Odelta- sites, resulting in non-resonant charge transfer, can lead to a build up the O- population even if the capture probability at a given site is low. (C) 1997 Elsevier Science B.V.