Optical properties of antimony tri-iodide

Abstract

Single crystals of pure antimony tri-iodide in which the optic axis is normal to the crystal plate have been prepared. The ordinary refractive index of antimony tri-iodide has been determined at wavelengths in the absorption edge and out to a wavelength of 2.5 pm. Crystals of the com pound have been shown to be uniaxial negative and the dispersion of the birefringence has been measured. The ordinary and extraordinary refractive indices calculated from the ionic refractivity of iodine agree with the experimental values. Antimony tri-iodide has a long-wavelength absorption edge at about 5000 Å, the variation of absorption coefficient with photon energy being in agreement with Urbach’s equation. Measurements of the light transmitted through an evaporated film 0.1 pm thick at 125 °K have shown that the absorption spectrum consists of a series of absorption bands having absorption coefficients of about 105 cm -1 . These absorption bands have been attributed to two hydrogenlike series of absorption levels, the first members of each series (separation 0.46 eV) being the halogen doublet due to (3/2, 1/2) splitting. The oscillator strength of the first member of the long-wavelength series is 0.65 x 10 -4 per atom at 125 °K. Reflexion measurements from a single crystal at 173 °K have shown that two reflectivity maxima occur in positions corresponding to the absorption bands of the halogen doublet. Photoconductivity has been observed in single crystals at room temperature. The temperature and light intensity dependence of the photocurrent suggests that the photocurrent arises from the thermal dissociation of the exciton states of the halogen doublet.