A post-irradiation increase in the amplitude of the electron spin resonance doublet split by 11.8mT, associated with the H(II) centers (=Ge.---H), was measured in dry and wet natural silica that was irradiated at room temperature with ultra-violet photons (266nm) from a Nd:YAG laser. The concentration of these paramagnetic defects increased upon increasing the delay time after ultra-violet exposure; with a time-scale of few hours. Its final magnitude depended upon the number of laser shots and the OH content. The generation of H(II) centers was related to bleaching of the 5.1eV absorption band which was attributed to twofold-coordinated Ge (=Ge..); thus supporting the occurrence of a conversion process between these defects, whose kinetics were governed by the diffusion of molecular H in the silica matrix.

Growth of H(II) Centers in Natural Silica after UV Laser Exposure. M.Cannas, S.Agnello, R.Boscaino, S.Costa, F.M.Gelardi, F.Messina: Journal of Non-Crystalline Solids, 2003, 322[1-3], 90-4