A well-defined type of O deficiency was introduced into the silica network by producing Si-Si groups by molecular doping based upon sol-gel synthesis. An investigation was made of the visible and ultra-violet photoluminescence, up to 7eV, which arose from localized states that were excited within the energy range of 3 to 12eV by synchrotron radiation. The photoluminescence excitation spectrum was dominated by one main sub-bandgap excitation band, which peaked at 7.1 with a full-width at half-maximum of 0.8eV. No photoluminescence arose upon exciting at 5eV. No photoluminescence bands at 4.4 and 2.7eV were observed, thus ruling out the formation of twofold coordinated -Si- sites. The data confirmed all previous theoretical and experimental assignments, of transitions at 7 to 8eV, to localized excitations of neutral O-vacancy sites. When excited in the vacuum ultra-violet above 7eV, the 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 which was responsible, in fused silica, for the vacuum ultra-violet excitation of the twofold coordinated Si emission. The main emission peaked at about 3.7eV, with a photoluminescence lifetime of about 1µs. A minor component with a much shorter 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 lifetime. The results gave an indication of the possible energy level structure of neutral O-vacancy and provided evidence of an efficient non-radiative decay mechanism of the excited state; caused by a strong electron-phonon interaction during vacuum ultra-violet excitation of the defect. Analysis of the data suggested a Si-Si bond relaxation of about 0.1nm; providing 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)