A well-defined type of O deficiency was introduced into the silica network by introducing Si-Si groups through molecular doping based upon sol-gel synthesis. In this material, the visible and UV photoluminescence (up to 7eV) arising from localized states excited in the energy range of 3 to 12eV, by synchrotron radiation, was investigated. The photoluminescence excitation spectrum was dominated by a single main sub-bandgap excitation band, peaked at 7.1eV with a full-width of half-maximum of 0.8eV. No photoluminescence arose at 5eV; that is, no photoluminescence band at 4.4 or 2.7eV was observed thus ruling out the formation of 2-fold coordinated -Si- sites. These data confirmed all previous theoretical and experimental attributions of transitions, at about 7 to 8eV, to localized excitations of neutral O vacancy sites. Exciting in the vacuum-UV above 7eV, the investigated material did not exhibit the typical 4.4 and 2.7eV photoluminescence observed in fused silica. The peculiar neutral O-vacancy configuration appeared to inhibit the photoconversion process responsible, in fused silica, for vacuum-UV excitation of the 2-fold coordinated Si emission. The main emission peaked at about 3.7eV, with a photoluminescence lifetime of about 1µs. A minor component with a much slower lifetime (800µs) was detected at 2.9eV, with a further minor excitation channel at about 6eV. All emissions excited at 7.1eV exhibited an intensity decrease with temperature, but a negligible thermal change in the lifetime. The results gave an indication of the possible energy level structure of the neutral O vacancy and indicated an efficient non-radiative decay mechanism of the excited state; caused by a strong electron-phonon interaction during vacuum-UV excitation of the defect. Analysis of the data suggested a large Si-Si bond relaxation of about 0.1nm; giving definite experimental confirmation of previous theoretical calculations.

Radiative Decay of Vacuum-Ultraviolet Excitation of Silica Synthesized by Molecular Precursors of Si-Si Sites - an Indicator of Intracenter Relaxation of Neutral Oxygen Vacancies. A.Paleari, N.Chiodini, D.Di Martino, F.Meinardi: Physical Review B, 2005, 71[7], 075101 (7pp)