Electron promotion in collisions of protons with a LiF surface

Electron spectra arising from grazing incidence collisions in the keV range of protons with a LiF surface grown on a tungsten substrate are analyzed in terms of the mechanism of electron promotion followed by autoionization. The analysis consists in a comparison of the experimental electron spectra with calculated electron spectra. The calculations are based on theoretically obtained molecular potential curves for the H-F^- molecular system in the LiF crystal. We find that molecular potentials which correspond to the promotion of two $2p$ electrons of F^- along the 3dσ one-electron orbital enter the lowest one-electron continuum at an H-F^- distance of {~}2.5 atomic units, so that autoionization at smaller distances becomes possible during the collision. With these potentials, electron spectra are calculated for certain impact parameters using a semiclassical eikonal approximation. Calculated spectra that can be compared to the experimental ones are obtained by integrating over an impact parameter distribution which is extracted from Monte Carlo calculations of trajectories. It is found that satisfactory agreement of experimental and theoretical spectra can be achieved if it is assumed that electron promotion leads with high probability to excitation of autoionizing doubly excited F^- states embedded in the conduction band of LiF.