Temperature and excitation energy dependence of decay processes of luminescence in Ge-doped silica

We report experimental results on the time decay of photoluminescence at 4.2 eV in Ge-doped silica. This optical emission is assigned to a singlet-singlet transition between electronic states localized on an oxygen deficiency nearby a Ge atom and its radiative decay rate is in competition with an intersystem crossing mechanism that populates an excited triplet state. We investigate the dependence of the lifetime of this photoluminescence on the temperature, in the 6-295 K range, and on the excitation energy, in the ultraviolet and vacuum ultraviolet region. The mean value of the decay time decreases on increasing the temperature, in agreement with the phonon-assisted nature of the intersystem crossing process whose activation energy is estimated similar to90 meV. In particular, at temperatures higher than 145 K, the time decay of the 4.2 eV emission deviates from a single exponential law and depends on the excitation energy, suggesting the presence of a distribution of intersystem crossing rates. These features are ascribed to the structural heterogeneity of vitreous matrix embedding the optical active point defects.