Ultraviolet emission lifetime in Si and Ge oxygen deficient centers in silica

We have measured the temperature dependence, in the range 10–295 K, of the lifetimes of the ultraviolet emissions associated with twofold coordinated Si and Ge, in pure and Ge-doped silica under excitation by synchrotron radiation. The Si-related fluorescence, centered at 4.4 eV, has a single exponential decay and its lifetime decreases from 4.0 ± 0.2 to 3.5 ± 0.2 ns on increasing the temperature. The luminescence at 4.2 eV, associated with the Ge defect, decays with a single exponential dependence below 150 K and with a more complex dependence at higher temperatures, its lifetime reducing from 7.8 ± 0.2 to 3.4 ± 0.2 ns. These features give evidence of the effectiveness of the thermally activated intersystem crossing in the relaxation of the excited singlet states, which is larger in the Ge-related center.