Role of vitreous matrix on the optical activity of Ge-doped silica

We report an experimental study on the relationship between the optical activity of Ge-oxygen deficient centers and dynamic properties and conformational heterogeneity of vitreous matrix in silica. We focus our attention on the absorption band at similar to5.2 eV (B-2beta) and on the two related emissions at similar to4.2 eV (alpha(E)) and at similar to3.1 eV (beta). From the temperature dependence of B-2beta band we estimate a mean energy value of 26 meV for local vibrational modes coupled to the electronic transition, suggesting that the chromophore and its surrounding have access to low frequency dynamics. From the thermal behavior of the two emissions we distinguish the two competitive relaxation processes from the first singlet excited state S-1: the radiative one, giving rise the alpha(E) band, and the thermally activated intersystem-crossing process between S-1 and the triplet state T-1, originating the beta band. The intersystem-crossing rate increases on increasing the temperature, determining an opposite thermal behavior of the intensity of the two emissions. However, this temperature dependence cannot be rationalized by a simple Arrhenius law and the alpha(E) decay kinetics at high temperatures do not follow a single exponential law, suggesting a complex landscape of configurational energies of the process. (C) 2003 Elsevier Ltd. All rights reserved.