Identification of the Mechanism for Kinetic Emission in keV Proton Cu(110) Collisions

Absolute electron spectra arising from grazing collisions of keV protons with a single-crystal Cu(110) surface are reported. From the experimental data, and from a comparison with known ionization cross sections in gas phase collisions, it is concluded that the mechanism responsible for kinetic emission is the perturbation of localized electrons during several ``distant collisions'' of the neutralized H atom in its ground state. For the first time an approximate theoretical description for kinetic emission formulated for keV collision energies is found to be consistent with the experimental data.