Experimental evidence of the composite nature of the 3.1 eV luminescence in natural silica

The 3.1 eV photoluminescence of silica has been investigated on several samples of commercial origin, by measuring both time-resolved and stationary spectra. This structure, known as the beta band, displayed the well known peculiarities: 3.1 eV energy position, 0.4 eV full width at half maximum, excitation peak at 5.1 eV and 100 mu s decay time. However, an accurate analysis of the experimental data has shown the double nature of the beta band in all natural silica samples here studied. The characteristic values of the two components beta(1) and beta(2) are: peak energy 3.04 and 3.20 eV, full width at half maximum of 0.44 and 0.35 eV and decay time of 115 and 97 mu s respectively. In addition the intensity ratio between the luminescent emissions at 3.1 and 4.2 eV was found to be constant in the whole set of samples. The occurrence of two beta components with so close spectral and time peculiarities might be ascribed to fluorescence processes involving the same oxygen deficient centers surrounded by two slightly different environments.