Conformational disorder and optical properties of point defects in vitreous silica

Disordered systems are characterized by the presence of local conformational heterogeneity, which reflects the complex landscape of the potential energy of the vitreous state. Optical properties of defects embedded in a vitreous matrix are also determined by the interaction with the surrounding environment; so the conformational disorder of the system induces spectral inhomogeneity. As a consequence, detailed experimental investigation of absorption and photoluminescence bands can give information on configurational substates around the chromophore. We focused our attention on B-type optical activity in silica glasses, characterized by a singlet emission and a triplet emission, connected via a phonon-assisted intersystem crossing (ISC) process. In particular, the ISC shows a strong dependence on excitation energy and a non-Arrhenius behaviour on varying the temperature. We take into account the observed thermal behaviour over a wide temperature range, assuming a distribution of the activation energies of the ISC process. This approach outlines the correlation between optical properties and conformational heterogeneity and indicates that the ISC process is driven by the entropic contribution, revealing the fundamental role of the dynamics of the surrounding matrix.