It was noted that disordered systems were characterized by the presence of local conformational heterogeneity, which reflected the complex landscape of the potential energy of the vitreous state. Optical properties of defects embedded in a vitreous matrix were also determined by the interaction with the surrounding environment, so the conformational disorder of the system introduced spectral inhomogeneity. As a result, detailed experimental investigation of absorption and photoluminescence bands could provide information on configurational substrates around the chromophore. Attention was focussed on B-type optical activity in silica glasses; characterized by a singlet emission and a triplet emission, connected via a phonon-assisted intersystem crossing process. In particular, the intersystem crossing exhibited a strong dependence upon excitation energy and a non-Arrhenius behaviour upon varying the temperature. The observed thermal behaviour over a wide temperature range was taken into account; assuming a distribution of the activation energies of the intersystem crossing process. This approach revealed a correlation between optical properties and conformational heterogeneity, and indicated that the intersystem crossing process was driven by the entropic contribution.

Conformational Disorder and Optical Properties of Point Defects in Vitreous Silica. A.Cannizzo, M.Leone: Philosophical Magazine, 2004, 84[13-16], 1651-7