I n Mg-and Ca-doped cc-Al,O, crystals irradiated by X-rays a t 80 K new paramagnetic colour centres are observed having an absorption band a t about 255 nm and an emission band a t 310 nm. The hyperfine structure of the EPR spectrum indicates that the unpaired electron spin interacts mainly with two AlZ7 nuclei. The optical and EPR spectra are consistent with the model of a centre involving one electron trapped a t an anion vacancy adjoining the substitutional divalent ion ( F~Q or Fc,). The luminescence band a t 290 nm observed in the TSL peak a t 225 K is suggested to result from the recombination of the holes with the FM~(Fc,) centres.
The introduction of Gas+ ions into cc-Al,O, crystals results in a new complex luminescence band at about 5 eV. The main optjcal excitation maximum is observed a t 8.5 eV indicating the bound exciton nature of the excited state. I n the cathodoluminescence spectra a t 80 K three sub-bands can be detected with maxima at 4.61, 5.15, and 6.58 eV. The luminescence decay in all the three bands has two exponential components -a short one with T = (15 & 1) ns and a long one with T about 1 4 s . The Ga-related luminescence can be observed under the excitation of X-rays up to 600 K; a drop in intensity takes place at about 210 K corresponding to the Ga-related TSL peak. The activation energy of the release of electrons a t this peak as determined by the fractional-
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