Optical Spectra of Orthorhombic SnBr2 Crystals

Polarized reflection spectra of orthorhombic SnBr 2 crystals have been measured up to the VUV region at 12 K. Correspondence between the spectral features of SnBr 2 and PbBr 2 is found, which indicates that the electronic structures of both materials are similar to each other. The first exciton band at 3.4 eV shows pronounced polarization dependence. The exciton transition is explained in terms of the 5s → 5p cationic excitation in the crystal field of Cs symmetry. A small structure ascribed to the n = 2 exciton state is found on the high-energy side of the exciton peak and the exciton-binding energy is estimated to be 32 meV. The fundamental absorption tail of SnBr 2 has been also examined at temperatures from 6 to 300 K. The observed absorption spectra suggest that the minimum band gap is of direct type. From the analysis of the absorption spectra at various temperatures, the high-temperature steepness parameter σ 0 is estimated to be 0.70 for E ∥ a and 0.74 for E ∥ b polarization, which suggests that the exciton-phonon coupling is of strong type.