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.