Electron trapping and hydrogen atoms in oxide glasses

Trapped hydrogen atoms generated in 3 MeV beta-radiolysis of B2O3:OH glass below 140 K were studied using electron paramagnetic resonance (EPR). Two t ypes of trapped H atoms were present in this glass; one was an interstitial atom located in a void between several BOB fragments, another was the atom trapped in a cage between two B3O6 (boroxol) rings connected by hydrogen b onds. The geometry of the trapping site was determined using electron spin echo envelope modulation (ESEEM) spectroscopy. Time-resolved pulsed EPR was used to observe mobile H atoms at 300-500 K. The lifetimes (10-100 mu s) o f the H atoms were controlled by similar to 10(18) cm(-3) of metastable spi n centers. The H atoms migrated with diffusion constant of 1.5x10(7) cm(2)/ s (activation energy of 0.13-0.16 eV), mean residence time at the site of 4 -5 ns, and mean jump length of 0.56 nm (at 300 K). This site-to-site migrat ion causes rapid spin relaxation due to modulation of magnetic interactions , such as dipole-dipole interaction of the unpaired electron of the H atom with B-10 and B-11 nuclei. Though there was no observed H/D kinetic isotope effect on the decay/diffusion of the hydrogen atoms, there was a significa nt isotope effect on their radiolytic yield (alpha approximate to 1.5-1.6). This effect is comparable to the one observed in SiO2:OH and aqueous acid glasses. This similarity suggests that in the room-temperature "wet" SiO2 a nd B2O3 glasses, mobile H atoms are generated via electron trapping at the proton(s) associated with threefold coordinated oxygen (-OH2+ and/or > OHcenters). Semiempirical MNDO simulations were used to estimate energetics o f such electron trapping reactions. [S0021-9606(99)52335-0].