The photo-absorption of neutral O vacancy defects in silicate and germanosilicate glasses was studied by using first-principles quantum-chemical techniques. The lowest singlet-singlet excitations in these defects occurred at about 7eV and involved the promotion of an electron, from a bonding orbital between adjacent Si (or Ge) atoms, to a diffuse Rydberg-type orbital. Such excitations were too high in energy to contribute significantly to the 5eV absorption band of silicate and germanosilicate glasses.

B.B.Stefanov, K.Raghavachari: Physical Review B, 1997, 56[9], 5035-8