A synchrotron radiation photoemission study of the oxidation of tin

The room temperature oxidation of a tin metal foil up to O2 exposures of 8 x 10(11) L (1 L = 10(-6) Torr.s) has been studied by synchrotron radiation photoemission spectroscopy. Valence band (VB) and Sn4d core level energy distribution curves (EDC) have been measured at photon energies of 50 and 90 eV. It resulted that the oxide film formed on tin after oxygen chemisorbtion contained Sn2+ and Sn4+ with relative concentrations determined by the stage of the oxidation. The VB, that at 500 L of O2 had the characteristic profile of the SnO VB, showed at higher exposures the appearance of spectral features due to SnO2. The analysis of the Sn4d core levels allowed us to identify the presence of a chemical shift of 0.73 +/- 0.05 eV between the Sn4+ and Sn2+ peaks. In fact the curves obtained by subtracting the metallic contribution due to the substrate from the Sn 4d peaks measured at increasing O2 exposure, appeared progressively shifted towards higher binding energies, because of the change in the composition of the oxide layer. Best fit curves of the previous peaks, deconvoluted using doublets of Gaussians for the Sn2+ and Sn4+ 4d3/2 and 4d5/2 spin orbit components, were used to evaluate the concentration of the two phases. Only the Sn2+ component was found by the fitting program in the peak observed at 500 L, whereas increasing concentrations of Sn4+ (up to 71% at 8 x 10(11) L) showed up at heavier oxidation. Photoemission spectra, taken at different depths in the oxide layer, etched by low energy (200 eV) Ar+ sputtering, showed the presence of a composition gradient; SnO2 being an overlayer lying above a film containing mostly SnO. The thickness of the oxide film present on the tin foil dosed with 8 x 10(11) L of oxygen was evaluated to be about 17 angstrom