Conformational disorder and optical properties of point defects in vitreous silica.

AbstractDisordered systems are characterized by the presence of local conformationalheterogeneity, which reflects the complex landscape of the potential energy ofthe vitreous state. Optical properties of defects embedded in a vitreous matrixare also determined by the interaction with the surrounding environment; sothe conformational disorder of the system induces spectral inhomogeneity. Asa consequence, detailed experimental investigation of absorption and photoluminescencebands can give information on configurational substates aroundthe chromophore. We focused our attention on B-type optical activity in silicaglasses, characterized by a singlet emission and a triplet emission, connected via aphonon-assisted intersystem crossing (ISC) process. In particular, the ISC shows astrong dependence on excitation energy and a non-Arrhenius behaviour on varyingthe temperature. We take into account the observed thermal behaviour over awide temperature range, assuming a distribution of the activation energies of theISC process. This approach outlines the correlation between optical propertiesand conformational heterogeneity and indicates that the ISC process is driven bythe entropic contribution, revealing the fundamental role of the dynamics of thesurrounding matrix.