Spectroscopic parameters related to non bridging oxygen hole centers in   amorphous-SiO2
  The relationship between the luminescence at 1.9 eV and the absorption bands at 2.0 eV and at 4.8 eV were investigated in a wide variety of synthetic silica samples exposed to different gamma- and beta-ray irradiation doses. We found that the intensities of these optical bands are linearly correlated in agreement with the model in which they are assigned to a single defect. This finding allows to determine spectroscopic parameters related to optical transitions efficiency: the oscillator strength of the 4.8 eV results ~200 times higher than that of the 2.0 eV; the 1.9 eV luminescence quantum yield under 4.8 eV excitation is lower (by a factor ~3) than that under 2.0 eV excitation. These results are consistent with the energetic level scheme, proposed in literature for non bridging oxygen hole center, and account for the excitation/luminescence pathways occurring after UV and visible absorption 
L. Vaccaro, M. Cannas, B. Boizot