Harry D. Downing

Optical constants of solid ammonia in the infrared

On the basis of transmission measurements of samples at temperatures near the melting point, we have determined the Lambert absorption coefficient α(ν) for solid ammonia in the spectral range 950–5300 cm−1. The imaginary part k of the complex index of refraction Nˆ=n+ik can be obtained from the defining relation k(ν) = λα(ν)/4π. Using the measured values of α(ν), we have employed subtractive Kramers–Kronig analysis to obtain the real part n(ν) of Nˆ in the range 960–5300 cm−1. In this analysis, we have used a computed value n(ν) at 4800 cm−1 based on a Lorentz–Lorenz relation and the value of n(ν) for liquid ammonia that we measured previously. The resulting optical constants k(ν) and n(ν) for solid ammonia are presented graphically and tabularly.

Optical constants of ammonium sulfate in the infrared

We have measured the spectral reflectance at near-normal incidence for 3.2M, 2.4M, and 1.6M solutions of ammonium sulfate, a compound found in the aerosols in the earth’s atmosphere. Values of the optical constants n(v) and k(v) have been determined from the measured spectral reflectance by Kramers-Kronig analysis. We have attempted to obtain a synthetic spectrum of crystalline ammonium sulfate by extrapolation of the k(v) values obtained for the solution to the absorber number densities of the NH4+ and SO4-- ions characteristic of the crystal. By subtractive Kramers-Kronig analysis we then obtain n(v) for the crystal. This method shows some promise of obtaining approximate values of the optical constants of soluble materials that cannot be obtained as large single crystals.

Infrared band intensities in ammonium hydroxide and ammonium salts

We have applied Kramers-Kronig analysis to reflection spectra to determine the optical constants of ammonium hydroxide and of aqueous solutions of ammonium chloride and bromide. From considerations of the absorption indices k(ν) we conclude that ammonium hydroxide consists of a solution of NH3 in water, in which NH3 molecules are hydrogen bonded to neighboring water molecules. The spectrum of ammonium hydroxide differs from the spectra of ammonium salts, in which bands characteristic of NH4+ ions are prominent. The existence of ammonium hydroxide as an aerosol in planetary atmospheres is briefly discussed.

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.

Infrared optical constants of aqueous solutions of sodium sulfate

The optical constants n(ν) and k(ν) for 1.0- and 0.5-M concentrations of sodium sulfate at ambient temperatures were determined by Kramers–Kronig phase-shift analyses of the near-normal reflectances over the spectral region from 2 to 20 μm. For the absorption that is due to the ν3 vibration of the SO42− ion, total band strengths, in reciprocal centimeters/(mol/liter), of 14.2 and 13.6 were measured for the 1.0- and 0.5-M concentrations, respectively. Changes in band position and intensity are discussed for this absorption.

Orthorhombic Sulfur (α-S)

Publisher Summary The cyclo-S8 molecules constitute a large fraction of the melt throughout the liquid phase. Cyclo octasulfur is frequently referred to as λ-sulfur (λ-S). From λ-S, strongly birefringent orthorhombic crystals or weakly birefringent monoclinic crystals can be assimilated. These two crystallographic allotropes are known as “α-S” and “β-S,” respectively. At the room temperature, α-S is the only thermodynamically stable modification of the element. The morphology of both natural and synthetic specimens of α-S range from cryptocrystalline masses or fine-grained, aggregates to large crystals of well-formed basal pinachoidal or dipyramidal habits. Crystalline sulfur is a soft, brittle material, and is difficult to polish. The crystals display poor cleavage and tend to fracture conchoidally. Sulfur is a poor conductor of heat, and thermal expansion from handling can result in fracturing or spallation of a specimen. This chapter discusses the optical properties of α-S and liquid sulfur. The primary crystal data is restricted torefractive-index measurements with polarization components along the a and b-axes, and thus exclude polarization with components along the c-axis of the α-S crystal.