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 c- and b-ray irradiation doses. We found that the intensities of these optical bands are linearly correlated in agreement with a model in which they are assigned to a single defect. This finding allows the determination of spectroscopic parameters related to the optical transitions efficiencies. In this case the absorption oscillator strength at 4.8 eV is 200 times higher than that at 2.0 eV; while 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 the literature for the non-bridging oxygen hole center. Moreover, they account for the excitation ! luminescence pathways occurring after UV and visible absorption.