Growth and decay kinetics of defect centers in high-purity fused silicas irradiated at 77 K with X-Rays or 6.4-eV laser light

Abstract

High-purity synthetic fused silicas (Suprasil 1, 1200 ppm OH; Suprasil Wl. < 5 ppm OH) were subjected to 100-keV X-rays or 6.4-eV laser photons at 77 K and their electron spin resonance (ESR) spectra were recorded as a function of dose (or laser fluence) and. subsequently, as a function of 5-min isochronal anneals to 300 K. For Suprasil 1: E' centers (Si·) grew as the ~ 0.88 power of X-ray dose from ~10 4 to 10 7 rad; oxygen hole centers (OHCs) followed a similar growth law except for an additional component which saturated at a dose of ~ 10 5 rad. Atomic hydrogen grew linearly with X-ray dose, becoming the dominant paramagnetic species above ~3×10 5 rad. The growth of H 0 and OHCs under laser irradiation consisted of a saturating component due to single-photon photolysis of OH groups, as well as a linear component of H 0 growth due to two-photon excitations. For Suprasil Wl: Both E' centers and OHCs grew as the ~ 0.7 power of X-ray dose. The OHC spectra of Suprasil Wl are identified as arising from the superimposed signals of peroxy radicals (Si-O-O·) and self-trapped holes (STHs). For both materials: E' centers are not produced in significant numbers by 6.4-eV irradiation at 77 K. Both the E' centers and the OHCs are believed to he of the trapped hole type: their isochronal anneal curves are interpreted in terms of the detrapping of STHs in a well-defined stage ~ 150–200 K and the more gradual release of trapped electrons from a non-paramagnetic trap which cannot he populated by 6.4-eV light in low-OH silicas. A much more stable electron trap. believed to be an interstitial O 2 molecule, is preferentially photopopulated in Suprasil Wl. It is found that low-OH silica which is also O 2 free is virtually immune to 6.4-eV photon damage at 77 K.