F AND F-EMISSION THERMALLY ACTIVATED IN SAPPHIRE (ALPHA-AL2O3)( LIGHT)

Thermoluminescence (TL) of alpha-Al2O3 samples has been studied in the temperature range 100-480 K. The crystals were previously submitted t o X-ray and UV irradiation at an average temperature T-a approximate t o 170 K. The TL signal is characterized by four bands detected at wave lengths 300, 418, 696 and 750 nm revealing the respective emission of F+-centres, F-centres, chromium ionic impurities and titanium ionic im purities. A profile change of trap levels is proposed to explain the d ifference of TL response after exposure of the sapphire crystal to X a nd UV radiation. From the TL results of alpha-Al2O3 submitted to UV il lumination (4.8 eV), we conclude that the energy level of the ground s tate of the F-centre lies in the bandgap at around 4.5 eV above the to p of the valence band, while the F+-centre presents a fundamental leve l lower than that of the F-centre by roughly 2 eV.