Excitonic and other interband transitions inTlInS2single crystals

A study of the optical properties of the layered compound ${\mathrm{TlInS}}_2$ is presented. Reflection spectra were measured at room temperature, in the energy region 1.5–6.2 eV. Transmission spectra were measured in the temperature range 10–290 K, in the energy region 1.5–3.5 eV. Two structures, at energy positions 2.58 and 2.87 eV (10 K), are attributed to transitions to ground exciton states, from valence states split because of the spin-orbit interaction, at the point Γ of the Brillouin zone. Two other structures at higher energies are attributed to transitions at ${\mathit{M}}_0$ and ${\mathit{M}}_3$ critical points, respectively. The second-derivative spectra ${\mathit{d}}^2$ɛ̃/d${\mathrm{\omega }}^2$ were fitted, accordingly, to analytic two-dimensional critical-point line shapes. Lorentzian line shapes are used to fit the structures of excitonic origin. The temperature dependence of the peaks and their broadening parameters are also presented.