Mechanisms for ion-induced plasmon excitation in metals

We have studied the excitation of plasmons produced by 100 eV He+, Ne+ and Ar+ and by 5-100 keV H+ and He+ projectiles in Al and Mg through the observation of electrons from plasmon decay, ejected from clean and cesiated surfaces. At low velocities, plasmon excitation occurs only for ions of high potential energy and is independent of velocity. The effect of Cs adsorption on this potential plasmon-excitation mechanism on Al surfaces suggests that the excited plasmons are not bulk plasmons, as was assumed previously, but short-wavelength surface plasmons. For ions moving faster than a threshold velocity v(h) similar to 1.3 v(Fermi) predicted by electron gas theories, kinetic plasmon excitation can occur because the valence electrons cannot respond instantaneously to screen the moving charge. We found that, contrary to theoretical expectations, plasmon excitation by H+ and He+ projectiles occurs below v(th). With the aid of a simple model, we suggest that this sub-threshold excitation results from energetic secondary electrons. (C) 1999 Elsevier Science B.V. All rights reserved.