Slow-ion induced electron emission from clean metal surfaces: "Subthreshold kinetic emission" and "potential excitation of plasmons"

We discuss some recent advances in the understanding of potential (PE) and kinetic electron emission (KE) for impact of slow ions on clean metal surfa ces. Below the "classical threshold" for KE by direct momentum transfer to quasi-free metal electrons, electron emission can already occur by three ot her mechanisms, i.e., electron promotion into the continuum in binary ion-t arget collisions, quasi-localization of electrons at the surface and, being only relevant at very low impact velocity, a partially localized electroni c excitation in the impact zone. Furthermore, we regard potential emission of electrons (PE) from the one-electron decay of plasmons which are excited by impact of slow ions on quasifree electron metal surfaces (e.g., poly- a nd mono-crystalline aluminum). Electron spectra for impact of 3-10 keV sing ly and doubly charged ions show that such plasmons result either from the i on potential energy ("potential excitation of plasmons - PEP") or, in a sec ondary process, from sufficiently fast electrons from KE. Contributions fro m these processes to the respective total electron yield are typically in t he percent range. A considerably more pronounced structure in the electron spectra resulting from impact of H+ on Al(111), which has formerly been asc ribed also to plasmon decay, is probably caused by diffraction of slow KE e lectrons undergoing multiple scattering in the uppermost target layers. (C) 2001 Elsevier Science B.V. All rights reserved.