Charge transfer in atom-surface collisions: effect of the presence of adsorbates on the surface

A review is presented on the charge transfer process in atom-metal surface collisions, focusing on the modifications of this process induced by the presence of adsorbates on the surface. It concerns primarily the case of low coverages where the adsorbates can be viewed as impurities on a substrate and not the case of very large coverages where the substrate properties disappear. Adsorbed impurities are found to strongly perturb the atom-surface charge transfer process. Two different effects invoked in the interpretation of these perturbations are discussed: the non-local effect due to the change of the surface work-function induced by the adsorbates and the local effect due to the local modifications in the surface electronic structure induced by the adsorbate around itself. Recent experimental and theoretical results on this problem are reviewed, stressing the relative importance of the two effects, i.e. the relative weight of delocalized and localized aspects.