The thermoluminescence of α-phase samples was studied at temperatures ranging from 100 to 480K. The crystals had been subjected to X-ray and ultra-violet irradiation at an average temperature of about 170K. The thermoluminescence signal was characterized by 4 bands which were detected at wavelengths of 300, 418, 696 and 750nm; thus reflecting the emission of F+ centers, F centers, Cr ionic impurities and Ti ionic impurities, respectively. A profile change in the trap levels was proposed in order to explain the difference in thermoluminescence response after the exposure of the sapphire crystals to X-ray and ultra-violet radiation. On the basis of the thermoluminescence results for α-Al2O3 which had been subjected to ultra-violet illumination (4.8eV), it was concluded that the energy level of the ground state of the F center lay in the band-gap at about 4.5eV above the top of the valence band, while the F+ center was associated with a fundamental level that was lower than that of the F center by about 2eV.

M.Ghamnia, C.Jardin: Philosophical Magazine B, 1997, 76[6], 875-85