Abstract
The reflection spectra of BiI s single crystals were measured between 1.5 and 5.5 eV at 6, 77 and 286 K. A Kramers-Kronig dispersion analysis of the data was made, and the intrinsic absorption coefficient and the real and imaginary parts of the dielectric constants were calculated. The large peak at 2.0 eV is ascribed to direct excitonic transition near the fundamental absorption edge. The main features observed in the absorption spectra are discussed in terms of two proposed models for anionic and cationic excitons in heavy metal halides. Measurements have also been made on the ordinary refractive index near the absorption edge. The data are well described by a two-oscillator model.
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