Determination of the Visible-Range Optical Absorption Spectrum of Peroxy Radicals in Gamma-Irradiated Fused Silica

Abstract

The most fundamental defect centers induced by high-energy radiations (x rays, γ rays, electrons, protons, neutrons, or intense UV light) in amorphous silicon dioxide are E′ centers (≡Si·), nonbridging-oxygen hole centers (NBOHCs: ≡Si-O·), peroxy radicals (PORs: ≡Si-O-O·), and self-trapped holes (STHs), where the notation "≡"1). The E′ center has a strong optical absorption in the UV at 215 nm and no known absorptions in the visible range. Both the NBOHC and the POR are believed to have absorption bands near 260 nm, and the former exhibits a low-oscillator-strength band (f ≈ 4 – 8×10-4) near 600 nm.1-5 On fundamental grounds, the POR would be expected to have a corresponding transition near the same wavelength. Based on data for drawing-induced defects in optical fibers,4 the likely existence of such a band was previously inferred,1 but proof has been lacking. We report here unambigous spectral resolution of a POR absorption band near 630 nm, based on ESR and differential-optical isochronal-anneal studies of γ-irradiated high-purity silicas in bulk and optical-fiber forms, respectively.