Origin of fine structure in the Auger spectrum of sulphur on a nickel surface

Additional peaks in the derivative spectrum of sulphur contaminant on a polycrystalline nickel surface have been resolved more readily with a cylindrical mirror analyser than with the retarding field analyser. The peaks appear on the high-energy side of the main sulphur peak at 150 eV. Consecutive Auger spectroscopic and X. p. s. measurements made on the nickel surface without removing it to the atmosphere have established that the fine structure is not due to any chemical shift of the sulphur L2,3 level. Reaction with oxygen by exposure to the residual gas, and by deliberate oxidation showed that the main sulphur Auger peak decreased progressively in amplitude, although the smaller additional peaks appeared to remain constant. Graphical integration of the region around the sulphur peak, followed by the fitting of curves of Gaussian form, revealed the presence of three peaks on the sulphur-covered surface, at 148.5, 153.8, and 157.3 eV. On calculation of quantities proportional to the areas under the two larger peaks, it was found that the current in the principal peak remained almost constant, contrary to the interpretation based on the derivative spectrum amplitudes. The i. n. s. results of Becker & Hagstrum on sulphur- and oxygen-covered low-index faces of nickel are used to identify the peaks in the sulphur Auger spectrum as originating in transitions involving electrons in surface molecular orbitals formed by chemisorption. The effect of oxygen adsorption on the sulphur Auger peaks can be interpreted in the same way.