Dudley Williams

Optical properties of water in the near infrared

The real n (ν) and imaginary k (ν) parts of the complex refractive index Nˆ = n + i k of water at 27 °C have been determined from measurements of spectral reflectance at near-normal incidence and from measurements of the transmittance of water in carefully constructed absorption cells. Values of n (ν) are reported in graphical and tabular form for the spectral region 3800–27 800 cm−1; values of the Lambert absorption coefficient α(ν) are presented graphically and in tabular form, along with k (ν) for the region 3800–14 500 cm−1. Upper limits of k (ν) are established for the region 14 500–27 800 cm−1. The results are compared with earlier studies.

Optical Constants of Sulfuric Acid; Application to the Clouds of Venus?

With the purpose of obtaining the real and imaginary parts of the complex refractive index N; = n + ik, we have made quantitative measurements of spectral transmission and reflection of sulfuric acid solutions in the visible and near infrared. On the basis of the results, we have obtained values for n throughout the entire region and values of k in the near infrared together with upper limits for k in the visible region. These optical constants can be used to interpret the results of polarization studies of solar radiation that has been scattered by the clouds of Venus. We have Kramers-Kronig phase-shift analysis to obtain values of n and k from reflection measurements in the intermediate infrared region (400-4000 cm(-1)). Our measurements were made at 300 K on sulfuric acid solutions having concentrations by weight of 95.6, 84.5, 75, 50, 38, and 25%. If the particles in the Venus clouds consist of liquid droplets of sulfuric acid at a temperature of 250 K, comparison of existing Venus data with our data suggests that the acid concentration is probably higher than 70%. Various possibilities are discussed.

Absorption Line Broadening in the Infrared

The effects of various gases on the absorption bands of nitrous oxide, carbon monoxide, methane, carbon dioxide, and water vapor have been investigated. Self-broadening effects for each of these gases have been compared with the effects of nitrogen in broadening the rotational lines within various vibration-rotation bands; the results can be expressed in terms of self-broadening coefficients. The effects produced by various foreign gases have also been compared with those of nitrogen; the results are expressed in terms of relative foreign broadening coefficients and relative collision diameters. The foreign gases studied include the nonabsorbing gases helium, oxygen, argon, hydrogen, and nitrogen and also carbon monoxide, carbon dioxide, and methane in spectral regions where there are no overlapping bands.

Lambert Absorption Coefficients of Water in the Infrared

By use of a wedge-shaped cell providing an absorbing layer tapering in thickness from less than one wavelength of visible light at one end to approximately 20 μm at the other end, we have measured the Lambert absorption coefficient for water in the spectral region between 4000 and 288 cm−1. After proper initial alignment of the cell windows had been established by the observation of interference fringes in the visible, we measured film thicknesses at various positions along the wedge by interferometric methods, employing convenient wavelengths in the infrared. We present the results of the study in graphical and tabular form.

Far-Infrared Spectrum of Liquid Water*

The infrared spectra of H2O and D2O in the liquid state at ambient temperature (30°C) have been remapped in the spectral region between 10 and 330 μ. The major features observed were extremely intense absorption bands with maxima at 685 and 505 cm−1 in H2O and D2O, respectively. These major bands are overlapped at the low-frequency ends by much less intense bands producing transmittance minima near 193 and 187 cm−1, respectively. No evidence was obtained for the series of narrow bands recently reported by Stanevich and Yaroslavskii. Extinction coefficients have been determined for the range 170–50 cm−1 and are compared with recent data; present data on linear absorption coefficients for H2O in the range 1500–1100 cm−1 are in fair agreement with the results of previous workers. The influence of temperature variations on the frequencies of infrared bands has been studied for all bands in the region between 4000 and 32 cm−1. Theoretical interpretation of the results is discussed briefly.

Optical Constants of Water in the Infrared

The infrared reflectance of water in the region 5000–300 cm−1 has been measured at near-normal incidence and at an incidence angle of 53°. On the basis of the measured values of spectral reflectance and the existing data on spectral transmittance, we have obtained values for the real and imaginary parts of the refractive index of water. The resulting values, which are presented in both graphical and tabular form, are compared with recent determinations by other investigators.

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.

Collisional Broadening of CO Absorption Lines by Foreign Gases

Foreign-gas broadening coefficients F, which give the ratio of the line-broadening ability of various gases relative to that of nitrogen, have been determined for 26 rotational lines in the fundamental band of carbon monoxide. The gases studied include H2, D2, He, Ne, Ar, Kr, Xe, CO2, NH3, CH4. For monatomic and diatomic gases with molecular masses less than that of N2, the F values for lines in the band wings are greater than for lines near the band center; the reverse is the case for monatomic gases with molecular masses greater than that of N2 and for CO2. The F values for NH3 and CH4 showed little variation for lines in different parts of the band. The results for the monatomic and diatomic gases are in excellent agreement with Chai’s recent independent measurements of the self broadening of carbon monoxide. The results of the present study have been used in combination with those of Chai and those of Hunt, Toth, and Plyler to obtain values of cross sections of foreign gases for collision broadening of the carbon monoxide absorption lines.

Optical constants of ice in the infrared

The normal-incidence spectral reflectance of ice at −7° C has been measured in the range 300–5000 cm−1. A Kramers–Kronig phase-shift analysis of the measured spectral reflectance has been employed to provide values of the real and imaginary parts of the refractive index. The resulting values of these optical constants are suitable for use in Mie-theory computations of scattering by ice particles in planetary atmospheres. The optical constants of ice at −7° C are compared in detail with those of liquid water at several temperatures and with those recently determined for ice at −170° C.

Influence of Temperature on the Spectrum of Water

The normal-incidence spectral reflectance of water at 5, 27, and 70°C has been measured in the spectral region between 5000 and 350 cm−1. From the measured values of spectral reflectance we have determined the optical constants nr and ni by Kramers–Kronig methods. The band strengths SB = ∫ni(ν) dν and bandwidths have been determined for the absorption bands near 3400, 1640, and 600 cm−1 at each temperature. A similar study of deuterium oxide at 27°C has been conducted for purposes of comparison.