Auger and autoionization features of clean and oxygen-exposed iron

Studies have been made of the $M_{2,3}\mathrm{VV}$ Auger transition and the $3p$-core loss feature for the Fe(110) surface, both for the clean surface and after various dosages of oxygen. The peaks within the Auger feature are shown to be directly related to the oxygen-induced changes in the $d$ band observed in ultraviolet photoemission spectroscopy studies. Changes in intensity and position of the anomalous peak above the $M_{2,3}\mathrm{VV}$ Auger threshold and the feature past the $3p$-core loss threshold after oxygen exposure suggest a correlation between the two features. The major core loss feature is interpreted as a resonant excitation of $3p$ electrons to a localized state above the Fermi level, with the decay of this state by direct recombination, resulting in an autoionization peak above the Auger threshold. Although changes in autoionization features are rare as they are quasiatomic and hence less likely to be affected by chemical environment, we interpret our results to be due to changes in the excited state from $3d$ for the clean surface to a $4s$ state, localized by the presence of the $3p$ hole, in the oxide.