A multi-spectroscopic study of luminescence sensitivity changes in natural quartz induced by high-temperature annealing

The luminescence sensitivity of natural quartz is significantly enhanced if heated beyond the first and second crystal phase transitions, in the temperature range 500-1100 degreesC. This work employs a number of complementary spectroscopic methods to obtain a fuller understanding of the mechanisms of this sensitization process. Spectrally resolved radioluminescence is used to study the role played by hole trapping recombination sites. Linearly-modulated optically stimulated luminescence is used to probe a number of different optically active electron donor defects. Spectrally resolved thermoluminescence simultaneously monitors the processes of electron eviction from donor centres, and recombination at acceptor sites. Finally, high-frequency electron paramagnetic resonance is used to monitor changes in population and structure of defects present within the lattice, some of which partake (either directly or indirectly) in the emission processes of the samples. The work leads to the conclusion that luminescence is generally sensitized in quartz by the removal of oxygen vacancy E' centres (which act either as non-radiative recombination centres, or as luminescence centres emitting in the deep UV, beyond the range of our instrumentation). In some samples, further luminescence enhancement is accompanied by heat induced creation of donor defects. The spectral results show that, not only is there competition between the accepters for recombining electrons, but also that the emission undergoes significant thermal quenching at elevated temperatures. Comparison of the radioluminescence and thermoluminescence measurements enables the quenching energies of two emission bands (360, 470 nm) to be obtained directly.