Phonon coupling of non-bridging oxygen hole center with the silica environment: Temperature dependence of the 1.9Â eV emission spectra

We report an experimental study on the shape of the 1.9 eV emission associated with non-bridging oxygen hole centers in silica and its temperature dependence, from 4 up to 300 K, under visible and ultraviolet excitation. Our analysis points out that these defects are coupled with their environment by phonons whose contribution can be described by the single mode of mean frequency between 300-400 cm−1 and Huang-Rhys factor of ∼3. On increasing the temperature, the luminescence intensity undergoes a thermal quenching caused by non-radiative processes, its deviation from a pure Arrhenius law can be accounted for by an uniform distribution of activation energy, from 0.002 to 0.05 eV. Meanwhile the emission band gets narrower, this contrasts with the broadening with temperature observed in other silica defects and suggests that the vibrational levels associated with the excited state are not thermally populated according to Boltzmann's law.