Mechanism of Intrinsic SiE′-Center Photogeneration in High-Purity Silica

Permanent Si $E^{\prime }$ centers with concentrations as high as ${10}^{17}$ spins/g induced in high-purity Si${\mathrm{O}}_2$ by sub-band-gap (6.4 eV) light were studied by electron-spin resonance. Although an electron-trap nitrogen center has been reported as the dominant defect induced by 7.9-eV photons in Suprasil 1 silica, its absence in the present experiment is evidence that free carriers are not produced by 6.4-eV photons. The $E^{\prime }$-center concentration increased as the square of 6.4-eV laser power, confirming the efficacy of two-photon processes for producing point defects in silica with sub-band-gap light. Our evidence supports the view that $E^{\prime }$ centers are generated by nonradiative decay of neutral excitons.