Bleaching of optical activity induced by UV laser exposure in natural silica

We report experimental data on two types of natural silica, differing in their OH content, irradiated with UV photons (4.66 eV) from a pulsed Nd:YAG laser. Irradiation induces a reduction of the absorption band at 5.12 eV and of the associated emissions at 3.14 eV and 4.28 eV, ascribed to twofold coordinated Ge(@Ge) centers pre-existing in our samples. The bleaching is mainly due to the post-irradiation conversion of @Ge into the paramagnetic H(II) center via trapping of a H atom. Comparison with literature data points out the peculiarities of silica with a low Ge concentration as regards UV-induced transformations.