Hydrogen-Related Conversion Processes of Ge-Related Point Defects in   Silica Triggered by UV Laser Irradiation
  The conversion processes of Ge-related point defects triggered in amorphous SiO2 by 4.7eV laser exposure were investigated. Our study has focused on the interplay between the (=Ge&#8226;-H) H(II) center and the twofold coordinated Ge defect (=Ge&#8226;&#8226;). The former is generated in the post-irradiation stage, while the latter decays both during and after exposure. The post-irradiation decay kinetics of =Ge&#8226;&#8226; is isolated and found to be anti-correlated to the growth of H(II), at least at short times. From this finding it is suggested that both processes are due to trapping of radiolytic H0 at the diamagnetic defect site. Furthermore, the anti-correlated behavior is preserved also under repeated irradiation: light at 4.7eV destroys the already formed H(II) centers and restore their precursors =Ge&#8226;&#8226;. This process leads to repeatability of the post-irradiation kinetics of the two species after multiple laser exposures. A comprehensive scheme of chemical reactions explaining the observed post-irradiation processes is proposed and tested against experimental data. 
F. Messina, M. Cannas