PHOTOABSORPTION AND PHOTOLUMINESCENCE OF DIVALENT DEFECTS IN SILICATEAND GERMANOSILICATE GLASSES - FIRST-PRINCIPLES CALCULATIONS

The photoabsorption and photoluminescence of divalent defects in silic ate and germanosilicate glasses have been studied by first-principles quantum-chemical techniques. Divalent Si and divalent Ge defects have very similar excitations. They have singlet-to-singlet excitations at 5.2 eV (Si) and 5.1 eV (Ge), and singlet-to-triplet excitations at 3.1 eV (Si) and 3.4 eV (Ge). The excited-state geometries have been relax ed to obtain the corresponding photoluminescence energies. Singlet-to- singlet luminescence transitions occur at 4.5 eV (Si) and 4.1 eV (Ge), and triplet-to-singlet transitions occur at 2.5 eV (Si) and 2.7 eV (G e). Excellent agreement with the corresponding experimental values sug gests that divalent Si and Ge defects contribute to the 5-eV absorptio n band and subsequent photoemissions in silicate and germanosilicate g lasses.