The results of our earlier studies of reflection and absorption in various spectral regions are reviewed and then used to provide values of the complex index of refraction l• = n + ik of water at 27øC in the spectral range 5000-10 cm -x, corresponding to wavelengths in the range 2 #m to 1 mm. Values of n, k, and the Lambert absorption coefficient a, which are presented graphically and in tabular form, should prove useful in studies of the scattering of infrared radiation by water droplets in the atmosphere and in studies of radiative heat balance at water surfaces. Although the infrared spectrum of water had been the subject of numerous investigations, Irvine andPollack [1968] made a critical survey of published results that revealed many inconsistencies and a general paucity of quantitative data on which to base values of the real and imaginary parts of the complex index of refraction • = n + ik. In view of the importance of n and k in calculations of the transmission, scattering, and absorption of electromagnetic radiation by water droplets in the earth's atmosphere, our laboratory group has devoted considerable attention to the quantitative determination of the optical properties of water in the infrared. We have based our earlier listings of the optical constants n and k on quantitative measurements of various types in various spectral regions. The purpose of the present paper is to give a critical small k and agreed, within the stated limits of uncertainty, in the centers of absorption bands where k is large; in the spectral range v < 600 cm -• the uncertainties in k became larger because of limitations imposed by the spectrometers employed. The values of k were measured in this lowfrequency region by Robertson et al. [1973], who used a far infrared grating instrument to determine a in the spectral range between 800 and 50 cm-•; these authors also obtained values of n by means of a KK analysis of measured values of a. In the spectroscopy of the remote infrared, interferometers used with Fourier transform techniques have marked advantages over conventional grating instruments. Using interferometric methods, John Chamberlain and his associates at the National Physical Laboratory (NPL) have obtained values of review of our earlier studies with the purpose of providing a set n and k in the range 100-20 cm-X; in the course of this work, of 'best values' for use in atmospheric studies. Davies et al. [1970] employed absorption techniques, and Zafar In our initial study, covering the 5000-to 400-cm -x region, et al. [1973] employed reflection techniques. Existing water Querry et al. [1969] attempted to measure the reflectance of data in the microwave and radiofrequency regions have been polarized radiation at two large angles of incidence and to summarized by Ray [1972]. determine n and k by solution of the generalized Fresnel equations. In the range 5000-330 cm -x, Rusk et al. [1971] PRESENT STUDY employed reflectance measurements at near-normal incidence In preparing the present summary of our work on water we and...

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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.

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