Electronic excitation and transient defects in As₂S₃ glass

We have carried out ab initio quantum-chemical calculations on clusters of atoms modeling the structure of As₂S₃ glass in its electronically excited states. In the spin-singlet excited state, one of the As-S bonds in the model cluster is preferentially elongated, but the resultant elongated bond still has a substantial bonding character. When the excited state undergoes a spin flip to give a triplet state, the elongated As-S bond is almost completely broken, resulting in a self-trapped triplet exciton. The photoinduced structural changes observed in As₂S₃ glass can be interpreted in terms of this singlet-to-triplet exciton conversion and the concomitant electron-hole recombination, allowing further atomic rearrangements in the glass network.