Valence and core-level binding energy shifts in realgar (As 4S 4) and pararealgar (As 4S 4) arsenic sulfides

Abstract

The core level and valence band X-ray photoelectron spectra (XP spectra) of two arsenic sulfides, As 4S 4 (α-realgar) and As 4S 4 (pararealgar), are reported. Pararealgar is a polymorph of realgar created by irradiation of realgar with visible light. The core level spectra of the materials can be distinguished by small differences in core level binding energies (BEs). For example, the As 3d 5/2 and S 2p 3/2 peaks are at 43.1 and 162.8 eV BE, respectively, in realgar and 43.2 and 162.7 eV BE, respectively, in pararealgar. Density functional theory (DFT) calculations at the B3LYP/cc-pVDZ level were performed on single As 4S 4 clusters to determine the ground state orbital energies for realgar and pararealgar and electrostatic potential-derived atomic charges. The shifts in the core level BEs are qualitatively reproduced in the orbital energies and can be rationalized based on an increased average charge in pararealgar (S=−0.21, As=+0.21) versus realgar (S=−0.19, As=+0.19). A large variation in calculated charge between inequivalent As sites in pararealgar (from +0.09 to +0.31) broadens the observed arsenic XP spectra features. In addition, the four main features in the valence bands of these materials have been identified based on the DFT calculations.