Irradiation induced germanium lone pair centers in Ge-doped sol–gel SiO2: Luminescence lifetime and temperature dependence

We studied the temperature dependence of the emission profile and of the lifetime, measured at 4.3 eV, related to the germanium lone pair centers (GLPC) induced by gamma ray at 5 MGy in a Ge-doped silica sample and in an analogous sample irradiated at 10 MGy, in which by a successive thermal treatment up to 415 °C the induced GLPC has been modified (named residual GLPC in the following). The measurements were recorded in the temperature range 10–300 K using an excitation of ∼5.2 eV. The data show that the energy level scheme of the induced and the residual GLPC is very similar to that of the native defects generated during the synthesis, and the intersystem crossing process (ISC) of the induced GLPC is similar to that of the native centers, whereas it changes for the residual GLPC. Moreover, we have found that the efficiency of the singlet–singlet transition is similar for the three GLPC types. We suggest that the observed differences are due to the pre-exponential factor of the Arrhenius law describing this type of phonon assisted process and it can be attributed to changes in the entropic or the structural contributions to the ISC.