H(II) Centers in natural silica under repeated UV laser irradiations

We investigated the kinetics of H(II) centers (Ge-H) in natural silica (fused from alpha quartz powder) under repeated UV irradiations at 266 nm wavelength performed by a Nd:YAG pulsed laser. These photons temporarily destroy the paramagnetic defects produced by the irradiations, their reduction being complete within 250 pulses. After re-irradiation, H(II) centers grow again, and the observed recovery kinetics depends on the irradiation dose; multiple 2000 pulses re-irradiations induce the same post-irradiation kinetics of H(II) centers after each exposure cycle. It is suggested that this repeatability effect is achieved by the photo-decomposition of previously formed defects and by the ability of each exposure to reset the concentrations of the species involved in the post-irradiation processes to equilibrium values independent from the previous history of the sample.