E′center in glassy SiO2: Microwave saturation properties and confirmation of the primarySi29hyperfine structure

Electron-spin-resonance studies of a series of air-annealed samples of glassy Si${\mathrm{O}}_2$ having various degrees of enrichment (or depletion) in the ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ isotope have confirmed that a $\gamma$-ray-induced doublet of 420-G splitting is the ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ hyperfine structure of the well-known $E^{\prime }$ center. This finding validates the widely accepted model of the $E^{\prime }$ center as an unpaired electron spin in a dangling $s{p}^3$ hybrid orbital of a silicon bonded to three oxygens in the glass structure and eliminates a recently proposed alternative model. Continuous-wave microwave saturation measurements at ∼9.2 GHz were carried out in order to establish spin-lattice relaxation behavior and to determine absolute line intensities in the low-power limit. The spin-lattice relaxation process for the ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ $E^{\prime }$ center is shown to be dominated by a hyperfine mechanism. Spin-lattice relaxation times $T_1$ could be extracted from the cw saturation data only by means of a semiempirical formulation differing from the usual approaches found in the literature.