  We report experimental data on two types of natural silica, differing for
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 5.12eV and of
the associated emissions at 3.14eV and 4.28eV, 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.
