Spin-polarized H/D atoms and radiation chemistry in amorphous silica

Time-resolved pulsed electron paramagnetic resonance (EPR) was used to study spin-polarized mobile H and D atoms in radiolysis of silica glass containing ∼1200 ppm of OH or OD groups. The EPR spectra of the H/D atoms indicate the occurrence of CIDEP in reactions of H/D atoms with radiolytically induced metastable spin centers. The EPR kinetics observed at 180–530 K exhibit fast growth and slow decay. The slow component (Ea=15–18 kJ/mol) is due to diffusion-controlled scavenging of H and D atoms by defects. For D atoms, this scavenging is 1.1–1.5 times slower than for H atoms; the formation rate of spin polarization is the same for H and D atoms. Below Tc=320±20 K, the decay kinetics can be fit by [H]t=[H]0 exp[−(kst)β], with β≈T/Tc; above Tc, β≈1. This behavior is indicative of dispersive diffusive transport of the H/D atoms through the glass network. The fast component with Ea=6–8 kJ/mol is due to rapid decay of a mobile spin partner of the H/D atoms, which we assume to be a polaron, in reactions with OH groups. It is argued that the large difference in the EPR signals from H and D atoms, relative to the concentration of the OH/OD defects (approximately 4–8 times), must be due to the substantial isotope effect on radiolytic yield of the H/D atoms (α∼1.5±0.2). The most likely reaction that can yield such an isotope effect is recombination of metastable protons with electrons. Our result seems to indicate that the latter reaction is the main route to the H atoms in room-temperature silica. At low temperatures, the H atoms are formed mainly due to dissociation of the OH defects following their reaction with triplet excitons, and α≈1.