Generation mechanism of photoinduced paramagnetic centers from preexisting precursors in high-purity silicas

High-purity silicas synthesized by various manufacturing methods were studied by electron-spin resonance after being irradiated by ArF excimer laser (6.4 eV) at room temperature. E’ centers (?Si⋅) are induced in all samples, while nonbridging oxygen hole centers (?Si-O⋅) appear only in oxygen-surplus silicas and in a sample which has an absorption band at 5.1 eV. The concentration of E’ centers varies from sample to sample, ranging between ${10}^{14}$ and ${10}^{16}$ spins/${\mathrm{cm}}^3$ for the exposure at the average power density of 28 mJ/${\mathrm{cm}}^2$ per pulse at 15 Hz for 1 h. The sample dependence regarding the species and concentrations of photoinduced defects is well explained in terms of transformation of preexisting precursors to paramagnetic defects through a two-photon-absorption process.