The identification of surface sulfide and polysulfide species based on the curve fitting of S2p photoelectron spectra and, for the first time, of X-ray excited S KLL Auger spectra has been performed. The different sulfur chemical states present on the surface (sulfide S²⁻, central S and terminal S in polysulfide chains) could be unambiguously assigned in the chemical state plot. Sulfur atoms in the central or terminal position, respectively, are found on a line with slope ca. −3 irrespective of the cation indicating similar initial state effects. On the other hand, for a given polysulfide, e.g. K₂S_n, sulfur atoms both in central or terminal positions are found on the same line with slope −1 indicating similar final state effects. This behavior can be rationalized with the fact that the negative charge in polysulfide chains is located mainly on sulfur atoms in the terminal position; indeed, sulfur present as central S shows a binding energy shift of −0.6 eV with respect to elemental sulfur (S₈), and sulfur in terminal S a shift of −2.4 eV. An application of this approach tested on commercial alkali polysulfides is provided for the curve fitting of SKLL signals and sulfur speciation of three different sulfide minerals enargite (Cu₃AsS₄), chalcopyrite (CuFeS₂) and arsenopyrite (FeAsS). Also for the surface of mineral sulfides, terminal S atoms and central S atoms in the polysulfide chains can successfully be identified.

XPS is exploited to study the composition, the chemical state and the cation distribution of metals in CoFe ₂ O ₄ .