Optical properties of bismuth tri-iodide

Abstract
The optical absorption spectrum of bismuth tri-iodide has been determined from measurements on single crystals ranging in thickness from 0.35 to 58 µm. At 77 °K the absorption spectrum consists of an absorption doublet A1, B1 (separation 0.38 eV) at 16 900 and 20 000 cm-1 with subsidiary absorption peaks A2 and B2 at 18 400 and 21 400 cm-1. Further absorption bands C and D occur to the short wavelength side of B2 at ca. 23 500 and 27 600 cm-1. The absorption coefficients of the peaks have been determined and the oscillator strength of A1 at room temperature found to be 0.02 per atom (Drude theory). The long wavelength tail of A1 at room temperature is described by Urbach’s equation. The ordinary reflexion spectrum of single crystals at 77 °K shows reflexion maxima associated with the absorption bands. Measurements have been made of the ordinary refractive index and crystal birefringence. Absorption bands A1A2 and B1B2 are attributed to exciton states associated with (3/2, 1/2) splitting of the valence band. The spectral distribution of photocurrent shows peaks associated with the absorption doublet A1B1 and a sharp increase at short wavelengths due to interband transitions.

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