Luminescence and absorption spectroscopy of Sn-related impurity centers in silica

We report an experimental study on the absorption and luminescence spectra of oxygen deficient point defects in Sn-doped silica. The absorption band at 4.9 eV (B_2β band) and the two related photoluminescence bands at ∼4.2 eV (singlet–singlet emission, S₁ → S₀) and at ∼3.2 eV (triplet–singlet emission, T₁ → S₀), linked by a thermally activated T₁ → S₁ inter-system crossing process (ISC), are studied as a function of temperature from 300 to 20 K. This approach allows us to investigate the dynamics properties of the matrix in the surroundings of the point defects and the effects of local disorder on the two relaxation processes from S₁: the radiative channel to S₀ and the ISC process to T₁. We observe that the S₁ → S₀ decay kinetics at higher temperatures do not follow a single-exponential law and the ISC rate shows a temperature dependence that cannot be rationalized by a single activation process, suggesting the presence of a complex landscape of configurational energies. The comparison with analogous data for Ge-doped silica reveals that the local dynamics of the matrix, the defect–matrix electron–phonon coupling, and the ISC rate dispersion are not substantially modified by the isoelectronic and isostructural substitution Sn–Ge. On the contrary, the Sn-related ISC process is ∼5 times more efficient than the Ge-related one. Since we observed that the coupling with local phonons increases the ISC efficiency by four order of magnitudes in the investigated temperature range, the reported data strongly suggest that, even if the presence of the spin–orbit coupling is needed for ISC processes, it has not play a primary role in the ISC processes in silica, where it acts as a homogenous and temperature-independent scale factor.