Electron spectroscopic studies of clean and oxidized iron

Abstract

X-ray photoelectron spectroscopy (XPS) and soft X-ray appearance potential spectroscopy (APS) together with Auger electron spectroscopy (AES) were used to study the electronic properties of clean and oxidized (Fe 3O 4) iron surfaces. The features arising from excitations of electrons from Fe 2p core levels are discussed consistently within the common one-electron picture (i.e. neglecting final state effects). For pure Fe the shape of the APS L 3 peak is evaluated taking into consideration the theoretical density of states above the Fermi level and is found to agree well with that observed. As a consequence it is shown that in this case the appearance potential is about 1 eV larger than the threshold energy for the excitation of a core electron to the Fermi level. Thus for 2p 3 2 electrons this quantity results to be 704.8 eV from both XPS and APS techniques. Successive oxidation at 500°C leads to an increase of the appearance potentials of the Fe 2p levels by only 0.5 eV, whereas the positions of the corresponding XPS peaks are shifted by as much as 3.5 eV. However this apparent disagreement can be eliminated by taking into account the above mentioned effect concerning the appearance potentials from pure Fe and the fact that the threshold energies (which determine the appearance potentials) of the XPS signals are shifted only by 1.7 eV. This example demonstrates that considerable care has to be taken in discussing “binding energies” or “chemical shifts” as derived from different electron spectroscopic techniques. The observed splitting of the MVV Auger transition of Fe at 47 eV upon oxidation is interpreted in terms of the qualitative features of the valence band structure of Fe 3O 4 and ascribed to the participation of a cross-transition between O 2p and Fe 3p states.