Anisotropy of the fundamental absorption edge ofTlSbS2

We have investigated, at pumped-liquid-helium temperature, the near-band-edge optical properties of the ternary layer compound ${\mathrm{TlSbS}}_2$. We resolve first a direct-band-gap exciton. A quantitative analysis of the data allows an accurate determination of the direct-band-gap energy $E_0$=1.907 eV at 2 K. An anisotropic behavior is found, which depends on the direction of polarization of the incident light with respect to the triclinic axes which lie in the plane of the layer. Depending on the polarization, both $E_0$ and the next threshold $E_1$ have strongly different oscillator strengths. These results are discussed in the light of previous results for the monochalcogenide SnS.