Luminescence properties of point defects in silica
  The optical properties of point defects in as-grown natural silica are reviewed. Two emissions peaked at 4.2 eV ($\alpha_E$ band) and at 3.1 eV ($\beta$ band), related to an absorption band at 5.1 eV ($B_{2\beta}$), have been experimentally investigated on the basis of their temperature dependence and their kinetic decay. Our results allow to characterize the excitation pathway of these luminescence bands and to make clear the competition between the radiative relaxation rates and the phonon assisted intersystem crossing process linking the singlet and the triplet excited states from which $\alpha_E$ and $\beta$, respectively, originate. Finally, we discuss the role played by the disordered vitreous matrix in influencing the optical features of defects. 
Marco Cannas