P. P. Sethna

Optical constants of water in the infrared: Influence of temperature*

We have compared the near-normal-incidence spectral reflectance of water at 1, 16, 39, and 50 °C with the corresponding spectral reflectance of water at 27°C, at which temperature the optical constants n(ν) and k(ν) have been previously determined. By applying Kramers-Kronig analyses to the resulting values of spectral reflectance we have obtained the optical constants of water at each of the above temperatures. We present the results in graphical form for the spectral region 400–5000 cm−1 and in tabular form in the vicinity of major absorption and dispersion features. The bearing of our results on the intermolecular structure of water is discussed.

Optical constants of D 2 O in the infrared

The near-normal incidence spectral reflectance of deuterium oxide at 27 °C has been measured in the spectral range 6800–350 cm−1. Kramers-Kronig phase shift analysis has been applied to measured reflectance to obtain values of the real and imaginary parts of the complex index of refraction Nˆ(ν) = n(ν) + ik(ν). The results are compared in detail with the corresponding optical constants of ordinary water at 27 °C. In general, the D2O absorption bands are narrower and less intense than the corresponding bands in ordinary water.

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.

Optical constants of liquid methane in the infrared

The near-normal incidence spectral reflectance R(ν) of liquid methane at 98 K has been measured in the infrared in the spectral range 6700–350 cm−1. The resulting values of R(ν) have been subjected to Kramers-Kronig phase-shift analysis to provide values of the real n(ν) and imaginary k(ν) parts of the complex index of refraction N(ν) = n(ν) + ik(ν) in the range 4000–400 cm−1. The results of the present study are presented in graphical form and in tabular form over this range. The strengths S = ∫k(ν) dν of the absorption bands are compared with the corresponding bands in gaseous methane. Techniques for using the present results to obtain approximate values of the corresponding optical constants of solid methane are discussed.

Optical constants of cupric sulfate in the infrared

We have measured the spectral reflectance of a cupric sulfate single crystal for unpolarized radiation at near-normal incidence. Values of the refractive index n(ν) and the absorption index k(ν) have been obtained by subtractive Kramers-Kronig phase-shift analysis. The values of ∫ k(ν) dν for the characteristic SO4−− bands in the spectrum of the crystal are compared with values of ∫ k(ν) dν for these bands obtained by extrapolation of values of k(ν) determined for cupric sulfate solutions.

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.