VACUUM ULTRAVIOLET LUMINESCENCE EXCITATION-SPECTRA OF ALPHA-AL2O3 SINGLE-CRYSTALS

The luminesence excitation spectra of corundum single crystals irradiated with 50 MeV electrons were studied. In the region of fundamental absorption there appeared excitation bands (blue and ultraviolet) which correlated well with the optical absorption bands of the F+-centre in the absorption (reflection) spectra. The excitonic peaks observed in the excitation spectra of red luminescence were explained by the charge energy transfer by Cr2+ and Cr4+ ions at excitonic excitation. At h-nu > 9.5 eV sharp drops appeared which were interpreted as nonirradiative near-surface recombinations, the probability of which sharply increased at high absorption coefficients. The enhancement of the luminescence efficiency in the high-energy region was connected with the decay effect of electron excitations as well as with the influence of volumetric excitations leading to the enhancement of recombination glow.