Charge transfer and memory loss in keV oxygen-ion scattering from Cu(001)
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Oxygen ion and neutral atom yields were measured for O+ and O- ions incident on a clean Cu(001) surface for ion energies from 0.4 to 5 keV. Scattering geometries were chosen that simplify the selection of single collisions between the incident ions and surface atoms and consequently also simplify the identification of the charge transfer mechanisms occurring in the O-Cu system. It was found that the ratio of O+ to O- ions is independent of the incident ion charge state, demonstrating that memory loss occurs over the entire range of energies and scattering geometries investigated. From general arguments based on the energies of the electronic states of the O-Cu system, we attribute the formation of scattered O- ions and neutral O atoms to a resonant charge transfer process. Evidence is presented that the formation of O+ ions is due to a hard collision between the incident ions and top layer surface atoms. Similar measurements were made for low energy (less than or equal to 4 keV) C+ ions incident on Cu(001), over the same range of scattering geometries; The behavior of the single scattering peaks in the C- spectra is also consistent with a resonant charge transfer process. No C+ ions were observed in the scattered flux for all energies and scattering geometries investigated.