The nature of excess oxygen in as-manufactured and γ-irradiated high-purity synthetic silicas is investigated. Electron-spin-resonance measurements suggest that peroxy radicals (3/4 SiOO·) could be produced either by the cleavage of peroxy linkages (3/4 SiOOSi 3/4) or by the reaction of E'centers (3/4 Si·) with oxygen molecules. The excess oxygen is found to exist in the glass in two forms: as peroxy linkages and as interstitial molecular oxygen. The peroxy linkage is shown to be the cause of optical absorption at 3.8 eV. Heat treatment at 900-1000 °C results in the growth of the 3.8-eV band, that is, the peroxy linkages, through the reaction of oxygen vacancies and interstitial dioxygen molecules. These results indicate that the 5.0- and 3.8-eV bands (which are characteristic of "oxygen-deficient" and "oxygen-surplus" silica, respectively) can coexist in a glass, depending on the synthesis conditions.
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