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 (less tha n or equal to 10 keV) H+, H-2(+), He+, Ne+, Ne2+, Ar+ and Ar2+ ions on atom ically clean poly and monocrystalline aluminum surfaces. In these spectra b road peaks at 11 and 6.6 eV result from one-electron decay of Al bulk and s urface plasmons, respectively. Their relative intensity has been studied fo r different angles of ion incidence and electron emission. "Direct" or "pot ential" excitation of plasmons can take place if the available ion neutrali zation energy E-pot 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 s tronger than of surface plasmons. Potential excitation of bulk plasmons sho ws a quasiresonant dependence on E-pot and is not significantly more import ant for multiply than for singly charged ions. With too small potential ene rgy plasmons can only "indirectly" be excited by fast electrons from concom itant kinetic emission. For proton impact on Al(111), structures in the ele ctron spectra which have earlier been ascribed to plasmon excitation are he re explained by diffraction of electrons from kinetic emission which underg o multiple scattering in the uppermost surface layers before escape. (C) 20 01 Elsevier Science B.V. All rights reserved.