Generation and excitation of point defects in silica by synchrotron radiation above the absorption edge

We report photoluminescence measurements carried out on amorphous SiO2 upon excitation by synchrotron light. Exposure of the as-grown material to above-edge light at low temperature induces the formation of nonbridging oxygen hole centers (NBOHC) localized in a thin layer below the surface limited by the penetration depth (tens of nm) of impinging light. After concluding the exposure to 11 eV light, stable defects are revealed by observing their characteristic 1.9 eV photoemission band excited at 4.8eV. The local concentration of induced defects, supposedly formed by nonradiative decay of excitons, is very high (close to 10^21 cm−3) and independent of the previous history of the material. On the other side, we also observe the 1.9 eV emission upon excitation between 9 and 14 eV, accompanied by the 2.5 eV luminescence ascribed to self-trapped excitons. This 1.9 eV band exhibits a temperature dependence different from that measured by excitation within the absorption bands at 4.8 and 6.4eV of NBOHC, and is proposed to arise from the fact that NBOHC are generated by above-edge light in their excited electronic state.