Experimental evidence of different contributions to the photoluminescence at 4.4 eV in synthetic silica

Photoluminescence activity in high-purity synthetic silica samples, both as grown and gamma-irradiated, was investigated by exciting in the vacuum-ultraviolet region. An emission band centred at 4.4 eV, excited within the absorption band at 7.6 eV and exhibiting a strong temperature dependence, was unequivocally evident. All of these features, together with its very fast decay time (2.3 ns at T = 10 K) make this emission distinguishable from the well known isoenergetic bands detected in oxygen-deficient or in gamma-irradiated silica. Our results are discussed in the light of structural models reported in the literature and seem to be consistent with the occurrence of a change in the structure of a point defect, from the single oxygen vacancy, responsible for the absorption at 7.6 eV, to the twofold-coordinated silicon, giving rise to the photoluminescence at 4.4 eV.