Optical properties of vitreous arsenic trisulphide

Abstract

Absorption coefficients for vitreous arsenic trisulphide, at 77 and 300 K, were determined over the wavenumber range 400 to 50000 cm^-1 from transmission measurements on evaporated films and polished discs. The short wavelength absorption edges are discussed in terms of transitions across an indirect energy gap of 1.072 eV at 300 K and a direct energy gap of 3.308 eV at 300 K. The temperature dependence of the absorption bands in the region 400 to 1500 cm^-1, which are attributed to the vitreous network, shows some anomalous results; cooling from 300 K to 77 K increases the strength of some of these bands by more than a factor of ten. The reflection spectrum has a maximum associated with the short wavelength absorption and the reflectivity values are in good agreement with those calculated from measurements of refractive index and absorption. Measurements were made of the temperature coefficient of refractive index; the dispersion of the refractive index was compared with the Drude dispersion theory.