The Disulfide Bond in S L _2,3 X-ray Emission Spectroscopy of Transition Metal Sulfides

S L2,3 X-ray emission spectroscopy (XES) and spectra calculations based on density functional theory are used to study the electronic valence structure of Mn, Fe, Co, and Cu transition metal sulfides, without (MnS, FeS, CoS, and Cu2S) and with (MnS2, FeS2, CoS2, and CuS) sulfur dimers in the crystal structure. We find the upper valence band region in the experimental spectra to be very sensitive to the transition metal, while a splitting in the transitions from the S 3s derived bands indicates the presence of sulfur dimers. This makes S L2,3 XES particularly well suited to the chemical speciation of these compounds. The spectral changes are reproduced by our spectra calculations, which allows for a detailed understanding of the electronic structure based on the band structure and the projected density of states. The splitting of the S 3s derived bands in the dimer compounds is found to increase with decreasing sulfur–sulfur distance, indicating an increase of covalency. The splitting in our experimental XES spectra is found to be smaller than in photoemission data and our spectra calculations, which we attribute to nuclear dynamics on the time scale of the X-ray emission process. In particular, the high sensitivity of S L2,3 XES to the presence of sulfur dimers, together with its suitability for operando studies, makes it a key technique for future studies on a wide range of relevant material systems.