Electron spectra from an extensive series of sulfur compounds have been studied. A correlation has been established between the observed position of inner electron lines of sulfur and structure. The influence of structure on the electron binding energies is discussed in terms of a calculated atomic charge, based on the concepts of electronegativity and partial ionic character of bonds. The results are useful for the study of bonding and structure in sulfur chemistry, and are applied particularly to the discussion of the sulfur-oxygen bond (S=O).

Carbon 1s energies are measured by ESCA for a series of aliphatic saturated compounds, carbonyl compounds, and some aromatic compounds. For convenient use in chemical structure analysis the binding energy shifts are correlated with a charge parameter obtained from electronegativity considerations. The shifts are also analyzed in terms of group shifts from which group electronegativities are derived. A comparison is made between the shifts in solid and gaseous samples and it is shown that solid state effects are small for non-ionic compounds. The observed shifts are then compared with results of semi-empirical and ab initio molecular orbital calculations on free molecules. The theoretical calculations are simplified by use of an electrostatic potential model.

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The formation of self-assembled chemisorbed layers of cystamine, cysteamine, and 4-aminothiophenol on gold has been studied by XPS and voltammetry. These compounds, often used in the preparation of biosensors and modified electrodes, are shown to yield surface coverages of approximately 80% of that of a octadecanethiol monolayer within 5 min in millimolar aqueous and ethanolic solutions. The results of the XPS experiments reveal that a shoulder on the S 2p3/2 peak (situated at 162.1 eV) develops at 161.3 eV upon increasing the adsorption time from minutes to 1 week and that the initial rate of formation of the shoulder is higher for cystamine than for cysteamine. This shoulder is believed to be due to the presence of a sulfur species with a higher coordination number with respect to gold. Increased adsorption times also give rise to increased amounts of oxidized carbon and sulfur in the films. The oxidation of the sulfur in the thiols results in a detachment of the molecules from the gold surface, as indicated by XPS experiments with different takeoff angles. The main N 1s peak for cystamine is shifted toward higher binding energies for increasing adsorption times while two prominent nitrogen peaks are generally seen for cysteamine. For cysteamine, increasing adsorption times result in an increase of the main nitrogen component at the higher binding energy, yielding an apparent shift in the nitrogen peak with time similar to that seen for cystamine. Possible explanations for these experimental findings are discussed. Cystamine, cysteamine, and 4-aminothiophenol films on gold are shown to be irreversibly oxidized in the gold oxide formation region. On the basis of evaluation of the oxidation charge, surface coverages of approximately 1 × 10-9 mol/cm2 were obtained for adsorption times between 5 min and 1 week.

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