Optical Properties of Orthorhombic Thallous Iodide

Abstract

Reflection spectra of a single crystal of orthorhombic TlI have been measured for the polarization E⁄⁄c and E⁄⁄a over a range of photon energy from 2.6 to 4.1 eV at 2 K. The spectra show pronounced anisotropy between these polarizations. The lowest exciton transition at 2.87 eV is confirmed to occur in a cationic sublattice (Tl⁺ 6s→6p). However, the admixture of the cation s orbital in the top valence band is found to be not so large as in cubic thallous halides. The binding energy of the first exciton is estimated to be 17.1 meV.