Matrix effects in the neutralization of He ions at a metal surface containing oxygen

Charge exchange between He ions and a Ni(111) surface containing oxygen was studied by Low-Energy Ion Scattering, using 1.25 keV He+ as primary ions. The energy resolved yield of positive ions was detected after backscattering from Ni or O for different exposures of Ni(111) to molecular oxygen. Pronounced changes in the neutralization efficiency due to the presence of oxygen are observed for both, the adsorbate phase at low oxygen dose, and the NiO phase at high dose. The presence of O in the surface makes resonant charge transfer in a close collision possible. Evidence for a strong matrix effect is found: O in NiO neutralizes much more efficiently than O in the adsorbate phase. Independently, the different interaction stages of Ni–O and the surface structure were monitored by Photoelectron-Emission-Microscopy and Low-Energy Electron Diffraction.