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. (C) 2003 Elsevier B.V. All rights reserved.