Positive- and negative-ion formation in low-energy O+-Cu(001) scattering

For incident 0.4-7-keV O+ beams scattered from Cu(001), we have measured energy and angular distributions of O+ and O- ions in the scattered flux, and performed accurate quantitative measurements of the O+/O- ratios. The O+/O- ratios depend on the scattering geometry and, for a given geometry, decrease with decreasing scattered perpendicular velocity. The qualitative differences between the energy and angular spectra for the scattered positive and negative ions indicate that this toad is due to the rapid decrease of the O+ fraction. These data and general arguments based on the energies of the states of the O-Cu system an used to identify key mechanisms responsible for the presence of O+ and O- in the scattered flux. We conclude that the resonant and Auger charge transfer mechanisms are both important. Furthermore, the data indicate that the O+ ions are generated in the hard collision between the incident atom and a surface atom. These collisions explain the geometry dependence of the measured O+/O- ratios. [S0163-1829(98)00640-7].