Medium-range structural order and fractal annealing kinetics of radiolytic atomic hydrogen in high-purity silica

Thermal annealing of radiation-induced atomic hydrogen in OH-containing high-purity silicas was studied by electron-spin resonance (ESR) in the temperature range 75–185 K. Both 100-keV x rays and 6.4-eV excimer-laser light were used to generate the ${\mathrm{H}}^0$. In general, the isothermal anneal curves consisted of two components, one approaching a $t^{\mathrm{-}1}$ law at long times, and the other taking on a $t^{\mathrm{-}2/3}$ dependence at still longer times. All data are interpreted in terms of bimolecular kinetics, assuming dimerization of the radiolytic ${\mathrm{H}}^0$ and/or its reaction with other paramagnetic species in the glass. The $t^{\mathrm{-}1}$ behavior follows directly from classical second-order kinetics. Fractal kinetics is used to explain the long-time fractional–power-law annealing behaviors. Diffusion in medium-range structural channels in silica is suggested as the origin of the observed fractal kinetics. Data consistent with this proposal are presented.