The 2.7 eV photoluminescence band in high-purity synthetic silica

The 2.7 eV photoluminescence emission ('gamma' band) has been studied in neutron-irradiated high-purity synthetic silica, both under UV and VUV (vacuum ultraviolet) excitation. Some aspects of the 5 eV excitation band, which is already known to exist, have been studied in great detail, while an excitation band centred at about 7.7 eV has been analysed for the first time. The intensity of the gamma emission for 5 eV excitation changes as a function of temperature in a completely different way to that for 7.7 eV excitation. In the latter case the emission intensity is constant up to about 300 K; a non-radiative decay predominates at higher temperatures. For excitation at 5 eV, the 'anomalous' increase of the emission that is already known to occur is observed as the temperature rises up to 500 K. We have developed a phenomenological model relating to the electronic levels of the luminescence centre that fits the experimental data well.