Excitation of plasmons by impact of slow ions on clean mono- and polycrystalline aluminum

Low-energy electron spectra have been measured for impact of slow (⩽10 keV) H+, H2+, He+, Ne+, Ne2+, Ar+ and Ar2+ ions on atomically clean poly and monocrystalline aluminum surfaces. In these spectra broad peaks at 11 and 6.6 eV result from one-electron decay of Al bulk and surface plasmons, respectively. Their relative intensity has been studied for different angles of ion incidence and electron emission. “Direct” or “potential” excitation of plasmons can take place if the available ion neutralization energy Epot exceeds the sum of plasmon energy (15.3 and 10.9 eV for bulk and surface plasmons, respectively) and Al work function (4.3 eV). At our experimental conditions excitation of bulk plasmons was considerably stronger than of surface plasmons. Potential excitation of bulk plasmons shows a quasi-resonant dependence on Epot and is not significantly more important for multiply than for singly charged ions. With too small potential energy plasmons can only “indirectly” be excited by fast electrons from concomitant kinetic emission. For proton impact on Al(1 1 1), structures in the electron spectra which have earlier been ascribed to plasmon excitation are here explained by diffraction of electrons from kinetic emission which undergo multiple scattering in the uppermost surface layers before escape.