Growth of H(II) centers in natural silica after UV laser exposure

The post-irradiation increase in the amplitude of the electron spin resonance doublet split by 11.8 mT, associated with the H(II) centers (GeH), was measured in dry and wet natural silica irradiated at room temperature with UV photons at 266 nm from a Nd:YAG laser. The concentration of these paramagnetic defects increases on increasing the delay time after the UV exposure, in a time scale of few hours and its final magnitude depends on the number of laser shots and on the OH content. The generation of H(II) centers is correlated with the bleaching of the 5.1 eV absorption band ascribed to the twofold coordinated Ge (Ge) so supporting the occurrence of a conversion process between these defects whose kinetics is governed by the diffusion of molecular hydrogen in the silica matrix.