He+ scattering on clean and oxygen covered Al(1Â 1Â 1)

We present results of a study of He+ neutralisation on clean and oxygen covered Al(1 1 1). For clean Al(1 1 1) we find a small ion fraction which rapidly increases with increasing ion energy attaining about (0.4 ± 0.1)% at 4 keV. The overall behaviour is quite different from that observed on previously studied Ag surfaces and is attributed to reionisation processes, a conclusion supported by measurements using He atoms. When O2 is adsorbed onto Al(1 1 1) at room temperature, we observe a rapid increase in the ion fraction, which attains a broad plateau for exposures higher than about 50 L. Experiments with incident He atoms lead to an ion fraction that is smaller, indicating that in this case a substantial fraction of ions is due to survival of incident ions. We attribute these changes to a decrease of the Auger neutralisation rates on the oxidic surface.