THE INFLUENCE OF WORK FUNCTION CHANGES ON THE CHARGE-EXCHANGE IN LOW-ENERGY ION-SCATTERING

The influence of adsorbate-induced work function changes on the neutralization of noble gas and Li+ ions scattered from a Ni(100) surface was studied. The resonance neutralization of Li+ ions is unaffected by a work function increase due to CO adsorption, however neutralization becomes more effective as the work function decreases with Cs adsorption. He+ and Ne+ ions are neutralized by Auger type processes which also holds in the case of CO adsorption. Lowering the work function opens an additional neutralization channel by resonant charge transfer: while the scattering from Ni surface atoms shows the usual increase of the ion yield with increasing energy, scattering from Cs at the same surface is dramatically different, showing a decrease of the ion yield with increasing energy. This different neutralization behaviour shows the strong localisation of the charge exchange processes involved. In all cases where resonant charge exhange is dominant, the data can be consistently described by a model in which the distance of closest approach during the collision is explicitly taken into account, thus demonstrating that the resonant charge exchange itself is a local effect.