Autoionization and Auger features of clean and oxygen-exposed surfaces of Fe, Co and Ni

Abstract

The $M_2$,3VV Auger and 3p core loss spectra for clean and oxygen-exposed surfaces of the transition metals Fe, Co, and Ni have been studied. All the peaks observed in the Auger spectra are accounted for in terms of both atomic and bandlike $M_2$,3${\mathrm{M}}_4$,5${\mathrm{M}}_4$,5 transitions and an autoionization emission involving the decay of a localized excited state. For the clean surfaces the localized excitation and subsequent deexcitation by autoionization emission is considered, within a quasiatomic model, to be 3$p^6$3$d^n$→3$p^5$3$d^n$ $d^{\mathrm{*}}$→3p ${}_{}{}^6{}_{}{}^{}$3$d^{n\mathrm{-}1}$+ef. However, upon oxidation of these surfaces, the splitting observed in the 3p core loss spectra and the energy of the observed autoionization peaks indicate changes in the nature of the excitation due to the chemical environment. This is contrary to the accepted notion for such quasiatomic processes, which are considered essentially unaffected by the chemical environment. Further, for these oxidized surfaces, the observed autoionization features are shown to be associated with the excitation of 3p electrons to a localized state at the bottom of the conduction band as opposed to the empty d states at the top of the valence band.