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 A 1 , B 1 (separation 0.38 eV) at 16 900 and 20 000 cm -1 with subsidiary absorption peaks A 2 and B 2 at 18 400 and 21 400 cm -1 . Further absorption bands C and D occur to the short wavelength side of B 2 at ca . 23 500 and 27 600 cm -1 . The absorption coefficients of the peaks have been determined and the oscillator strength of A 1 at room temperature found to be 0.02 per atom (Drude theory). The long wavelength tail of A 1 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 A 1 A 2 and B 1 B 2 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 A 1 B 1 and a sharp increase at short wavelengths due to interband transitions.