Type of Optical Transitions at the Fundamental Absorption Edge in TlGaSe2 and TlInS2 Crystals Subjected to γ-Radiation

The effect of gamma-radiation on the optical properties of layered TlGaSe2 and TlInS2 crystals has been studied within a wavelength range of 400-1100 nm at 300 K. By means of analysis of optical absorption spectra, the energies of direct and indirect optical interbend transitions before and after gamma-irradiation have been determined. It has been shown that the energies of direct and indirect nonforbidden optical transitions grow with accumulation of gamma-radiation dose within 0-25 \Mrad in TlGaSe2 and TlInS2 single crystals from E-gd = 2.06 eV and E-gi = 1.90 eV at D = 0 \Mrad to E-gd = 2.11 eV and E-gi = 1.98 eV at D = 25 \Mrad for TlGaSe2 crystals and from E-gd = 2.32 eV and E-gi = 2.27 eV at D = 0 \Mrad to E-gd = 2.35 eV and E-gi = 2.32 eV at D = 25 \Mrad for TlInS2 crystals. A decrease in the transmission coefficient at doses from 0 to 5 Mrad with a further increase in the transmission coefficient at a radiation dose of D = 25 \Mrad is observed.