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 i n 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 theoretical ly obtained molecular potential curves for the H-F- molecular system in the LiF crystal. We find that molecular potentials which correspond To the pro motion of two 2p electrons of F- along the 3d sigma one-electron orbital en ter the lowest one-electron continuum at an H-F- distance of similar to 2.5 atomic units, so that autoionization at smaller distances becomes possible during the collision. With these potentials, electron spectra an calculate d for certain impact parameters using a semiclassical eikonal approximation . Calculated spectra that can be compared to the experimental ones are obta ined by integrating over an impact parameter distribution which is extracte d from Monte Carlo calculations of trajectories. It is found that satisfact ory agreement of experimental and theoretical spectra can be achieved if it is assumed that electron promotion leads with high probability to excitati on of autoionizing doubly excited F- states embedded in the conduction band of LiF. [S0163-1829(99)11915-5].