Nonadiabatic effects in auger-induced ionic desorption

Abstract

The Knotek-Feibelman mechanism of desorption is analysed in the context of a microscopic dynamical theory that has been proposed recently for closed-band systems. To understand the physical contents of the general equations, the probabilityP of ionic desorption is derived analytically in several selected model situations as a function of the characteristic desorption timet o. Although typicallyt o is quite long compared to the inverse level widthW B −1, the results show very significant deviations from the limit of small velocities. This is due to the fact thatP is not primarly determined by the level width, but by the structures, like peaks and singularities, that are contained in the two-hole continuum. Such features, that are completely missed in the traditional approach that represents the continuum by a Lorentzian, are naturally enbodied in the present theory. They may originate from the position of the adatom energy level ɛ relative to the substrate band and/or by hole-hole correlation. When split-off localized states arise,P may become comparable with unity even in the limit of small velocities, giving raise to the Knoteck-Feibelman reneutralization bottleneck; however, this is not the case when the localized states are readsorbed by the continuum in the course of desorption. An explicit example of this behaviour is given. On the other hand, when ions desorb from ionic crystals with a large Madelung energy, the bond-breaking process may approach the sudden limit. A strongly nonadiabatic behaviour is then predicted, including a correlation-induced enhancement ofP by several orders of magnitude, even in bandlike situations. This rapid variation leads to suggestions for a new experiment that should enable us to see the effects of a fine tuning of the electronic parameters.