Excitation of volume plasmons in glancing collisions of protons with Al(111) surfaces

We studied electron emission from Al(111) surfaces induced by 4-100 keV protons at glancing incidence. The electron energy distributions show a structure due to the decay of volume plasmons. The intensity of the plasmon structure was measured as a function of angle of incidence to the surface. We found that volume plasmons are excited even under conditions where protons do not enter in the solid. The results are consistent with an indirect excitation mechanism by fast secondary electrons produced in binary ion-electron collisions. This mechanism is more important than direct excitations by ions and excitations produced by Auger electrons.