Luminescence activity of surface and interior Ge-oxygen deficient centers in silica

We report a comparative study on the optical activity of surface and interior Ge-oxygen deficient centers in pressed porous and sol-gel Ge-doped silica, respectively. The experimental approach is based on the temperature dependence of the two photoluminescence bands at 4.2 (singlet-singlet emission, S1 ->S0) and 3.1 eV (triplet-singlet emission, T1 ->S0), excited within the absorption band at about 5 eV. Our data show that the phonon assisted intersystem crossing process, linking the two excited electronic states, is more effective for surface than for interior centers in the temperature range 5-300 K. For both centers, a distribution of the activation energies of the process is found. Based on the results of quantum chemical calculations of the electronic structure of (HO)2Ge: molecule it is suggested that the electronic de-excitation pathway involves two excited triplet states (S1 -> T2 -> T1 -> S0) and shows a structural dependence on the O-Ge-O angle.