Spectral and kinetic properties of the 4.4-eV photoluminescence band in alpha-SiO2: Effects of gamma irradiation

Spectral and decay properties of the photoluminescence (PL) emission in the range 3.5-5.0 eV are investigated in various types of silica (a-SiO2), as grown and after gamma irradiation. We report experimental results on the ultraviolet (5.0 eV) absorption, on the emission and excitation spectra of the stationary FL, excited by ultraviolet (5.0 eV) and vacuum ultraviolet (6.0-8.5 eV) light, on the decay times of the transient PL emission, at room temperature, and at 10 K. Our results show that gamma irradiation causes the appearance of a PL band, centered at 4.37 eV, which can be excited at 5.0 eV and at 6.8 eV. This band, which is induced in all the investigated samples, brings a close similarity to the PL band alpha(1) that is peculiar of unirradiated oxygen-deficient natural silica. However, small but appreciable differences between the two bands can be inferred from our experimental data. The results are consistent with an energy level scheme with two singlet-singlet transitions. We tentatively ascribe the small differences between the two PL bands to different dynamic environments surrounding the intrinsic and the gamma-induced centers.