Observation of collective inner-shell effects for protons backscattered from the Al(110) surface

The surface peak of keV protons backscattered from a clean Al (110) surface has been studied in detail for three different scattering geometries. The corresponding energy-loss distributions are asymmetric due to the single and double ionization of the Al inner-shell electrons (L shell). Effects beyond the independent-electron model (inner-shell collective effects) have been observed by using Monte Carlo simulations for the ion ballistic and coupled-channel calculations for the inelastic energy loss. These effects decrease the width by up to 10% for the full width at half maximum. Correspondingly, the low-energy tail of the surface peak is reduced in intensity by more than 30% in the case of a large number of near central collisions as in shadowing and blocking geometries.