Plasma excitation in solids by sub-threshold velocity ions

Electron energy distributions arising from 4 keV protons scattering off Al (111) and Cu(110) are presented. Although the protons have an energy that is an order of magnitude smaller than the generally accepted threshold for plasmon excitation, the spectra clearly show that excitation and subsequent decay of bulk plasmons takes place. The results are interpreted in terms of an excitation process in which not only energy and momentum are transferred to the plasmon but also, mainly momentum, to the surface. The results suggest that excited plasmons have a finite momentum that depends on the proton energy