Electronic structure of molecular arsenic chalcogenides

The valence bands of ${\mathrm{As}}_4$ ${\mathrm{S}}_4$ and ${\mathrm{As}}_4$ ${\mathrm{Se}}_4$, were studied by x-ray-photoemission spectroscopy (XPS) and interpreted utilizing a complete-neglect-of-differential-overlap self-consistent-field molecular-orbital(MO) formalism. The upper valence band originates almost entirely from $p$ electrons, and the lower mostly from $s$ electrons. A mixing of nonbonding and bonding $p$-derived levels occurs, even within the first x-ray-photoemission peak. These results differ from the origins of peaks in XPS spectra proposed for similar materials in the literature. In addition, the self-consistent-field MO computation agrees with the XPS data to a much greater degree than the results of previous MO calculations.