LOW-TEMPERATURE PHOTOLUMINESCENCE SPECTROSCOPY RELATIONSHIP BETWEEN 3.1 AND 4.2 EV BANDS IN VITREOUS SILICA

We report experimental results on the thermal behavior, in the tempera ture range 350 to 10 K, of the two correlated photoluminescence bands centered at 4.2 and 3.1 eV in a-SiO2. The spectral features and their temperature dependence are compatible with a scheme of optical transit ions from two excited electronic states, a singlet and a tripler, pert aining to a single point defect and strictly connected via intersystem crossing. The analysis of the interconversion process between the sin glet and triplet stare, as a function of temperature, suggests the pre sence of local environments (conformational substrates) surrounding th e chromophore that are frozen at low temperature and reach thermal equ ilibrium above T similar or equal to 120 K. (C) 1997 Elsevier Science B.V.