Vacuum ultraviolet excitation of the 1.9-eV emission band related to nonbridging oxygen hole centers in silica

We investigated the red-luminescence band at 1.9 eV in a gamma ray irradiated synthetic wet silica, under ultraviolet and vacuum-ultraviolet excitations in the temperature range 17-300 K. The excitation profile detected at T=17 K evidences a broad band peaked at 6.4 eV, with full width at half maximum of 1.7 eV, besides the well-known component at 4.8 eV. On increasing the temperature up to T=300 K, for both ultraviolet and vacuum-ultraviolet excitations, the red luminescence decreases by a factor 10. These results give experimental support to the assignment of the vacuum-ultraviolet absorption transition to a nonbridging oxygen hole center, proposed in a recent paper [T. Suzuki , Phys. Rev. Lett. 90, 186404 (2003)]. Moreover, the thermal quenching of the red-luminescence evidences the activation of a nonradiative relaxation channel inhibiting the radiative emission from the excited state.