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 excitati on band, which is already known to exist, have been studied in great d etail, while an excitation band centred at about 7.7 eV has been analy sed for the first time. The intensity of the gamma emission for 5 eV e xcitation changes as a function of temperature in a completely differe nt 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 predomi nates 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 phenomenologic al model relating to the electronic levels of the luminescence centre that fits the experimental data well.