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

Abstract

We discuss some recent advances in the understanding of potential (PE) and kinetic electron emission (KE) for impact of slow ions on clean metal surfaces. Below the “classical threshold” for KE by direct momentum transfer to quasi-free metal electrons, electron emission can already occur by three other mechanisms, i.e., electron promotion into the continuum in binary ion-target collisions, quasi-localization of electrons at the surface and, being only relevant at very low impact velocity, a partially localized electronic 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 quasi-free electron metal surfaces (e.g., poly- and mono-crystalline aluminum). Electron spectra for impact of 3–10 keV singly and doubly charged ions show that such plasmons result either from the ion potential energy (“potential excitation of plasmons – PEP”) or, in a secondary process, from sufficiently fast electrons from KE. Contributions from these processes to the respective total electron yield are typically in the percent range. A considerably more pronounced structure in the electron spectra resulting from impact of H + on Al(1 1 1), which has formerly been ascribed also to plasmon decay, is probably caused by diffraction of slow KE electrons undergoing multiple scattering in the uppermost target layers.