  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.
