Correlation of preexisting diamagnetic defect centers with induced paramagnetic defect centers by ultraviolet or vacuum-ultraviolet photons in high-purity silica glasses

Electron-spin-resonance (ESR) and vacuum-ultraviolet (vuv) absorption measurements were performed on a series of high-purity silica glasses exposed to 6.4-eV photons, 7.9-eV photons from excimer lasers, and to rays. The concentration of defect centers varies from 1014 to 1016 cm-3 depending on the method of material fabrication and photon energy of the irradiating lasers. Variation of the defect species with both the incident-photon energy and manufacturing condition is observed by ESR measurements. E centers (°Si) are observed in all types of silicas. Nonbridging-oxygen hole centers (NBOHCs°Si-O) in high-OH silica ([OH]1000 ppm) and peroxy radicals (PRs,°Si-O-O) in oxygen-surplus silica ([OH]<1 ppm) are observed, only when the samples are exposed to 7.9 eV photons. The defect centers observed in -irradiated silica are qualitatively in agreement with those in 7.9 eV laser-irradiated silica. The variation of defect species with manufacturing methods indicates that the observed paramagnetic centers are created from preexisting defects. Concentration of the defects induced by either 6.4 or 7.9 eV laser photons is proportional to the square of the pulse energy, indicating that two-photon absorption process dominates in the defect formation. The defect formation process can be understood in terms of the creation of an electron-hole (e-h) pair by two-photon excitation and the subsequent hole trapping or decay of an e-h pair at the site of preexisting defects. Dependence of the induced-defect species on incident-photon energy can be explained by the variation in the energy level of preexisting defects or in the defect formation energy. Results of vuv-absorption measurements reveal the conversion of diamagnetic precursor defects (e.g.,°Si-Si°,°Si-O-O-Si°, and°Si-OH) introduced during the manufacturing process, into paramagnetic defect centers, the E centers, NBOHCs, and PRs.