Sub-threshold plasmon excitation in free-electron metals by helium ions

In this work, we re-examine sub-threshold plasmon excitation by ions measuring the energy distributions of electrons emitted from Al and Mg under the impact of 0.16-4 keV single charged Helium ions. Electron energy distributions for the case of He+ projectiles contain well distinguished structures that confirm the excitation of multipole surface plasmons, due to the potential energy released by the neutralization of incoming ions. It is shown that electron emission from multipole plasmon decay is more intense than from Auger neutralization (AN). Our experiments indicate that scattering and cascade in the solid of electrons excited by plasmon decay and by AN may give an important contribution to electron emission. (C) 2003 Elsevier B.V. All rights reserved.