Thermo stimulated luminescence and electron spin resonance in X-ray- and photon-irradiated oxygen-deficient silica

Influences of oxygen-deficiency on radiation properties of high-purity, low-OH fused silica were studied. It is found that thermostimulated luminescence (TSL) peaks are different for photo (7.7 eV) and X-ray excitation at 77 K. X-ray excitation produces TSL peaks at 125 and 170 K corresponding to the anneal temperatures of two types of self trapped holes centers STH(2) and STH(1), respectively, detected by electron spin resonance (ESR). Oxygen-deficiency apparently increases the number of electron traps, stabilizing a larger number of STHs in the continuous defect-free silica network than is observed in similarly X-irradiated stoichiometric silica glasses. Photoexcitation of oxygen-deficient glass at 7.7 eV produces TSL peaks at 105 and 200 K, presently attributed to STHs at perturbed sites in the immediate vicinity of oxygen-deficiency centers (ODCs). High temperature TSL peaks at 240 and 400 K are produced by both types of irradiations and are followed by ESR detection of E' centers only. All observed TSL and ESR signals were proportional to the level of oxygen-deficiency. The main spectral band in TSL near 2.7 eV is a triplet-singlet transition, ascribable to the twofold-coordinated silicon center (ODC)(II)) modified by its nearest structure. It is proposed that this recombination process results when a thermally detrapped STH encounters an electron trapped at the site of ODC(l), and is transmuted into a modified ODC(II):e(-). In principle, any such ODC:e(-)defect should be paramagnetic. However, no trapped-electron centers were detected by ESR in the present experiments. Based on the recent work of others, the E' centers that were observed are believed to be trapped hole centers. Thus, for reasons unknown, the postulated ODC(II):e- centers, which may be the primary electron traps in oxygen-deficient silicas, appear to be ESR-silent. (C) 2007 Elsevier B.V. All rights reserved.