Infrared optical properties of orthorhombic sulfur.

Because of recent developments in planetary astronomy, there has been a resurgence of interest in the optical and thermodynamic properties of elemental sulfur. An encounter between the space probe Galileo and the Jovian moons, particularly Io, is expected to contribute further to this interest. A thorough investigation of the optical properties of orthorhombic sulfur from 2 to 56 microm (5000-180 cm(-1)) is presented. Since less care was taken in many past studies of this element than was warranted, a critical review of some of the relevant literature is included. The near-normal specular reflectance of the (111) face of an orthorhombic sulfur crystal has been measured in both polarized and unpolarized radiation at room temperature. The reflectance of a cryptocrystalline melt freeze has also been obtained. Associated optical constants are determined from a Kramers-Kronig phase shift analysis of the reflectance data. The average reflectance and absolute refractive index n were found to vary with polarization from 0.100 to 0.125 and from 1.92 to 2.09, respectively. Between eight and eleven mostly weak absorption bands of the cyclo-S(8) molecule were discernible, but the attenuation index k remains small throughout most of the region studied. The crystal spectra were found to be quite sensitive to polarization in the neighborhood of the v(4) fundamental. Extrapolation of n to other temperatures and to the liquid phase through the use of the Lorentz-Lorenz relation is discussed.