Multiple Intersystem Crossing Processes in Ge-Doped Silica Glass: Emission Mechanism of 2‑Fold Coordinated Ge atoms

It has been well documented that ∼5 eV optical excitation of a 2-fold coordinated Ge atom in Ge-doped silica glass results in a singlet–singlet emission at ∼4.2 eV and a triplet–singlet emission at ∼3.1 eV, which are connected by thermally activated intersystem crossing (ISC). However, the true mechanism of the ISC, whose rate shows an apparent non-Arrhenius temperature dependence, has not been well understood and appreciated. In this work, we perform detailed photoluminescence measurements on highly luminescent Ge-doped silica glass with an internal quantum yield of ∼40% in a wide temperature range from 3 to 500 K. It has been found that there exists at least three triplet excited states of the 2-fold coordinated Ge atom, contributing respectively to three different ISC channels. One is a temperature-independent ISC process, and the others are temperature-dependent processes with activation energies of ∼40 and ∼170 meV. We have also found that nonradiative transitions, which have often been neglected in previous studies, need to be considered for a full description of the entire set of emission characteristics. Our conclusion is that the non-Arrhenius characteristics of the ISC rate are not due to a wide and continuous distribution of activation energies of the ISC process, as is often proposed, but instead results from multiple ISC pathways consisting of higher order triplet states.