The oxidation of a polycrystalline tin foil was examined using Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), ion scattering spectroscopy (ISS), and electron-stimulated desorption (ESD). An oxidation state which does not correspond to SnO or SnO2 has been identified using XPS and retarding mode AES at low oxygen exposures (≤3000 L). It has been suggested in previous studies that oxygen penetrates beneath the outermost layers during the early stages of oxidation of certain metals such as tin. This suggestion has been verified in this study using ISS and ESD. Very small amounts of oxygen are detected with ISS on a sputter-cleaned Sn surface, and this O signal does not increase with oxygen exposure even though XPS and AES show a significant oxygen uptake. Additionally, no O+signal is detected with ESD whereas large ESD O+ signals are obtained from surfaces of bulk tin oxides. The oxygen concentration is uniform across the sample surface at these exposures as determined by SAM implying that grain boundaries do not provide preferential adsorption sites.
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Asbury et al. (1987) studied this question.