Type of optical transitions at the edge of fundamental absorption of TlGaSe$_{2}$ and TInS$_{2}$ crystals subjected to $\gamma$ irradiation

The effect of $\gamma$ irradiation on the optical properties of layered TlGaSe$_{2}$ and TInS$_{2}$ crystals in the wavelength range of 400–1100 nm at 300 K is studied. From the analysis of optical absorption spectra, the energies of direct and indirect optical interband transitions before and after $\gamma$ irradiation are determined. It is shown that as the $\gamma$-irradiation dose is accumulated in the range of 0–25 Mrad by TlGaSe$_{2}$ and TInS$_{2}$ single crystals, the energies of direct and indirect allowed optical transitions increase from $E_{\operatorname{gd}}$ = 2.06 eV and $E_{\operatorname{gi}}$ = 1.90 eV at $D$ = 0 Mrad to $E_{\operatorname{gd}}$ = 2.11 eV and $E_{\operatorname{gi}}$ = 1.98 eV at $D$ = 25 Mrad for TlGaSe$_{2}$ and $E_{\operatorname{gd}}$ = 2.32 eV crystals and $E_{\operatorname{gi}}$ = 2.27 eV at $D$ = 0 Mrad to $E_{\operatorname{gd}}$ = 2.35 and $E_{\operatorname{gi}}$ = 2.32 eV at $D$ = 25 Mrad for TInS$_{2}$ crystals. A decrease in the transmittance at doses from 0 to 5 Mrad and a further increase in the transmittance at the radiation dose $D$ = 25 Mrad are observed.