Atomic Hydrogen in Gamma-Irradiated Silicon Dioxide

The hyperfine structure of the components of the doublet EPR spectrum of atomic hydrogen in silicon dioxide (silica gel, quartz, or molybdenum glass) gamma-irradiated at 77 K is due to the overlap of two doublets with alpha = 50.5 and 50.4 mT, a standard singlet and a resolved multiplet, responsible for the accumulation and stabilization of two types of surface and bulk atomic hydrogen. Surface hydrogen atoms formed upon the dissociation of surface OH groups and water molecules are stabilized in dry traps, and they have a standard doublet with alpha = 50.5 mT in the EPR spectra. Bulk hydrogen atoms are formed (upon radiolysis) and stabilized in internal water clusters of SiO2. In the water medium of clusters, bulk hydrogen atoms are affected by the protons of the water environment and, interacting with protons, they exhibit a multiplet doublet with alpha = 50.4 mT in the EPR spectra. The contribution of the multiplet doublet to the total spectrum depends on the type of SiO2 and the radiation dose. The fraction of a multiplet doublet in macroporous glass at a dose of 120 kGy was 40%. When water was completely removed from a SiO2 matrix, bulk hydrogen atoms disappeared, and a standard doublet of atomic hydrogen was recorded in the EPR spectra after an irradiation dose of 40 kGy or higher.