Optical Gap in GeS ₂ Glass: Molecular Orbital Analyses and Raman Scattering Spectroscopy

Ge‐chalcogenide glasses have been attracting considerable interest because of excellent infrared performance with non‐poisonous compositions, while their electronic properties remain vague. To understand the origin of optical absorption edges in Ge–S glasses, we analyze related small clusters containing Ge–S, Ge–Ge, and S–S bonds that could be constituents of the glass, using the ab initio molecular‐orbital software GAMESS. We also scrutinize calculated electronic properties in light of experimental results, including Raman‐scattering spectra. These studies suggest that the absorption edge in GeS ₂ is governed by photo‐electronic transitions in structures containing Ge–Ge homopolar bonds and corner‐sharing GeS _4/2 units, while edge‐sharing units hardly influence it because of symmetry‐forbidden transitions. The Ge–Ge clusters also possess highly uniaxial dipole moments, which could give rise to the so‐called negative photoinduced anisotropy. Since the concentration of the wrong bond critically varies with preparation details, reproducibility of absorption‐edge spectra in GeS ₂ glass becomes unavoidably poor. It is known that in Ge‐ and S‐rich Ge–S glasses the optical gaps become narrower, which could be related with small excitation energies of segments containing Ge–Ge and S–S bonds. Some comments are also given for As ₂ S ₃ and GeSe ₂ glasses.