On the interpretation of ion neutralisation spectra at metal surfaces

Abstract

We present a theoretical analysis of the energy distribution of the Auger electrons emitted from a Ni(111) surface during the neutralisation of He+ ions incident on the surface with very low energy, i.e. ∼0.05 eV. We calculate the energy distribution of the emitted electrons in two limiting cases. In case A, it is assumed that the matrix element of the Auger transition is determined by the tails of the electronic wavefunctions on the vacuum side of the metal-vacuum interface. In case B, it is assumed that the ejected electron originates within the metal, in which case the energy distribution of the emitted electrons is, to a first approximation, proportional to an integral fold of the density of states in the bulk of the metal and the density of states at the position of the ion on the vacuum side of the interface ( ∼2.5–3.5 A from the last plane of atoms). The calculations take into account the broadening of the energy distribution due to the position-dependence of the valence level of the ion. The results obtained from case B, unlike those of case A, are in reasonable agreement with the experimental spectrum. A discrepancy between theory and experiment remains at the lower energy part of the spectrum, but this is, probably, to a large extent, due to inaccurate subtraction of secondary electron emission from the experimental data.