ENHANCED PHOTOGENERATION OF E' CENTERS FROM NEUTRAL OXYGEN VACANCIES IN THE PRESENCE OF HYDROGEN IN HIGH-PURITY SILICA GLASS

Electron-spin-resonance (ESR) and ultraviolet-vacuum-ultraviolet (uv-vuv) spectra were measured on two types of oxygen-deficient silicas, i.e., low-OH ([OH] < 1 ppm) and high-OH ([OH] almost-equal-to 20 ppm) silicas, and a high-OH ([OH] almost-equal-to 1000 ppm) stoichiometric silica. The results of ESR show that the E' concentration induced in an OH-containing oxygen-deficient silica is one or two orders of magnitude higher than in low-OH oxygen-deficient and high-OH stoichiometric silicas when exposed to 6.4-eV or 7.9-eV photons. It is shown from the vuv spectra that the conversion of an oxygen vacancy into an E' center is enhanced in the OH-containing oxygen-deficient silica compared with the low-OH oxygen-deficient silica. The conversion into an E' center is considered to be mediated by diffusion of atomic hydrogen released from the Si-H bond. A slight peak shift of the 5.8-eV absorption band due to the E' center suggests that the formation of an E(beta)'-like center is involved in the enhanced E' creation. A possible correlation of the enhanced E' creation with the appearance of a 7.4-mT doublet in the ESR spectrum is also discussed.