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In the present note the nature of colour (CC) and luminescence (LC) centres occurring in yttrium-aluminium garnet (YaG) crystals is discussed on the basis of optical absorption (OA) and photoluminescence (PL) spectra. YAG crystals, grown by Czochralski and horizontal directed crystallization methods, were examined. The samples have been irradiated by neutrons of energy E > 0.2 MeV, creating structure damages in crystals. 1619 2 ~~~

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Self-trapped hole (STH) and self-trapped exciton (STE) states which are formed in Y3AI5Ol2, Y,SiO,, and TR,Si,O, crystals under y-irradiation in the temperature interval of 77 to 200 K are investigated. The STH is shown to be molecular 0;-, absorbing a t 1.6 eV (for Y,A1501z) and a t 1.65 eV (for Y,SiO,) and causing the eleven-component EPR signal ; the corresponding S!I' E emits a t 3.9 and 3.6 eV. In metal-doped crystals the exciton and hole delocalization lead to a decreasing of the emission energy, to a short-wave shift of the absorption band, and to the growth of the anneal temperature. At the self-localization temperature excitons are proved to migrate by jump diffusion and to transfer energy to the impurity ions exciting their emission.