Plasmon excitation in the interaction of protons and electrons with clean and potassium-covered Al surfaces

We present a study of the low-energy electron emission resulting from the interaction of 5–100 keV protons colliding with clean and K-covered Al(111) surfaces at grazing incidence. In the whole energy range the spectra show structures that are associated with decay of both bulk and surface plasmons. We discuss the dependence of these structures with K coverage and compare them with those seen in the loss spectrum induced by 500 eV electron bombardment. We observe that the surface mode presents a stronger dependence with K coverage. Finally, we discuss the role of the ion velocity on the thresholds for surface and bulk plasmon production by direct excitation, by secondary electrons, and by electron capture processes.