Auger-induced valence processes in alkali halides following alkali core-hole creation

Abstract

Ab initio calculated values are presented for the interatomic Auger and radiative decay rates for initial (2p${)}^2$ and 2p hole states of the Na ion in NaF. A comparison of the rates shows that these hole states decay overwhelmingly by the nonradiative Auger process. The knowledge of the rates makes possible a clarification of the mechanism of photon-stimulated positive-ion desorption presented by Parks et al. [Phys. Rev. B 28, 4793 (1983)]. Following the creation of a Na(1s) hole in NaF, the ensuing Auger cascade results in the creation of four to six valence holes, or one ${\mathrm{F}}^{+}$ ion and two to four valence holes, on the neighboring negative fluorine ions within an elapsed time of a few times ${10}^{\mathrm{-}14}$ s. The majority of the valence holes are produced as hot holes, and are likely to diffuse several tens to hundreds of angstroms in times of the order of picoseconds before self-trapping to form $V_K$ centers. The fast diffusion of these holes greatly reduces their chance of participating in ion desorption processes. The possibility that a fraction of the holes self-trap immediately and assist in the desorption of positive ions is considered. It is found that positive-ion desorption can occur if enough holes self-trap immediately. However, the required configurations are so special that they may occur with too small a probability to account for the observed positive-ion current. The fast hole diffusion also makes it unlikely that the $V_K$ centers from the cascade will exhibit the clustering effects which would have been expected had the holes self-trapped in place. An increase in the yield of electron-hole pairs of about 6% is predicted to occur at the Na K edge.