The photo-absorption and photoluminescence of divalent defects in silicate and germanosilicate glasses were studied by using first-principles quantum-chemical techniques. It was noted that divalent Si and divalent Ge defects had very similar excitations. They exhibited singlet-to-singlet excitations at 5.2eV (Si) and 5.1eV (Ge), and singlet-to-triplet excitations at 3.1eV (Si) and 3.4eV (Ge). The excited-state geometries were relaxed so as to obtain the corresponding photoluminescence energies. Singlet-to-singlet luminescence transitions occurred at 4.5eV (Si) and 4.1eV (Ge), and triplet-to-singlet transitions occurred at 2.5eV (Si) and 2.7eV (Ge). An excellent agreement with experimental data suggested that divalent Si and Ge defects contributed to the 5eV absorption band, and to subsequent photo-emissions from silicate and germanosilicate glasses.

B.L.Zhang, K.Raghavachari: Physical Review B, 1997, 55[24], R15993-6