Some effects of 5 eV photons on defects in SiO 2

Abstract

The effects of 5 eV photons on the optical absorption bands and paramagnetic centers produced by a prior γ-ray irradiation of type III and IV silicas have been measured. Silica samples obtained from two commercial sources were irradiated with 137Cs γ-rays to a dose of ∼10 8 R at an ambient temperature of ∼35°C. The samples were then exposed to 5 eV photons supplied by a pulsed KrF excimer laser. The samples were exposed to pulses from 100 to 11 500 with 150 mJ per pulse per cm −2. Sample temperatures during exposure were <40°C. Optical absorption spectra were measured with a double beam spectrometer in the 1–6 eV range at a temperature of 22°C. Electron paramagnetic resonance (EPR) spectra were measured with a homodyne spectrometer at 9.7 GHz, room temperature and 110 K and at various power levels. After 100 pulses, a band with a peak at 5.5 eV was bleached in the spectra of type III samples. No additional bleaching was observed after 1500 pulses. After 11 500 pulses, a decrease in absorption at 5.8 eV was observed. In the EPR spectra, the E′ center concentration increased by a factor of 1.8 and the concentration of the non-bridging oxygen centers (NBOHCs) did not change after 100 pulses. No additional changes were observed after 1500 pulses. After 11 500 pulses, a decrease of the order of 69% in the E′ center concentration and a decrease of the order of 26% in the NBOHCs was observed. In the spectra of the type IV samples, a band with a peak at ∼4.85 eV was bleached by the first 100 pulses and no additional changes were observed up to 1500 pulses. A band with a peak at ∼5.8 eV decreased with exposure to 11 500 pulses. The E′ center concentration increased by a factor of 1.5 after 100 pulses, did not change after 1500 pulses and decreased by 84% after 11 500 pulses. The peroxy radical molecule ion concentration decreased to below detectability after 800 pulses and a new paramagnetic state grew with increasing number of pulses up to 1500 pulses, after which no additional increase was observed. No oxygen-related center was detected for any exposure in the type III samples. It is concluded that there are at least three bands absorbing at ∼5.8 eV, the E′ center, the E″ center and another band. The 4.8 absorption bleached by 5 eV photons is attributed to the peroxy radical. The absorption at ∼5.5 eV bleached by 5 eV photons is not attributed to any of the paramagnetic states.