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

Abstract

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 $\ensuremath{\gamma}$-ray-induced doublet of 420-G splitting is the $^{29}\mathrm{Si}$ hyperfine structure of the well-known ${E}^{\ensuremath{'}}$ center. This finding validates the widely accepted model of the ${E}^{\ensuremath{'}}$ 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 \ensuremath{\sim}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 $^{29}\mathrm{Si}$ ${E}^{\ensuremath{'}}$ 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.