Calculated spectral properties of self-trapped holes in pure and Ge-doped SiO2

We report first-principles cluster model calculations on the electronic structure and spectral properties of self-trapped holes (STH's) in pure and Ge-doped silica. Two different kinds of STH's are considered: STH1, consisting of a hole trapped at the 2p nonbonding orbital of an O atom bridging two Si atoms, and STH2, a metastable defect where the hole is rapidly tunneling between two adjacent O atoms that we modeled by a hole delocalized over two bridging O atoms. The computed hyperfine coupling constants for the two variants of STH centers fully support the experimental assignments based on the analysis of the electron spin resonance spectra. We also considered the lowest optical transitions of these defect centers, and we compared the results with the experimental bands observed in irradiated quartz and with the position of the energy levels deduced from the analysis of the g matrices. [S0163-1829(99)02838-6].