Darrell E. Burch

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.

Infrared Transmission of Synthetic Atmospheres.* I. Instrumentation

A technique for investigation of the infrared absorption of water vapor and carbon dioxide under simulated atmospheric conditions has been developed. The “total absorption” or area under the curve giving fractional absorption as a function of frequency can be determined for each spectral region in which characteristic absorption occurs. The apparatus includes a 22-m multiple-traversal absorption cell which permits controlled variation of the following parameters: (1) geometrical path length, (2) pressure of absorbing gas, (3) pressure of the nonabsorbing gases nitrogen and oxygen, and (4) the temperature of the gaseous mixture. A prism spectrometer is used to measure fractional absorption as a function of frequency under various experimental conditions. Although the observed shape of a given absorption band depends upon the effective slit widths of the spectrometer, the total absorption of a band depends, within wide limits, only on the foregoing listed parameters. For the range of temperatures encountered in the lower atmosphere, the influence of temperature variation on total absorption is so small that it can be neglected. On the basis of results obtained by the techniques described, it is possible to make accurate predictions of absorption of infrared radiation in the earth’s atmosphere.

Optical and Infrared Properties of Al 2 O 3 at Elevated Temperatures

The absorption coefficient of single-crystal Al2O3 (sapphire) has been found to increase at most wavelengths from 0.56 to 6 μ as the temperature is increased up to 2020°C. As the temperature is increased further until the material melts, the absorption coefficient appears to increase discontinuously by a factor of about 30 or 40. The emissivity of some ceramic material which consists of more than 99% polycrystalline Al2O3 is greater than that of sapphire at the same temperature, but shows the same general dependence on temperature as it is heated up through the melting point.The index of refraction of sapphire at wavelengths less than 4 μ was found to increase 0.05 (+0.01, −0.03) as the temperature increases from ambient to 1700°C.

Total Absorptance of Carbon Dioxide in the Infrared

Total absorptance ∫ A(ν)dν has been determined as a function of absorber concentration w and equivalent pressure Pe for the major infrared absorption bands of carbon dioxide with centers at 3716, 3609, 2350, 1064, and 961 cm−1. The results in the 875–495 cm−1 region are expressed in terms of mean spectral absorptance A¯(ν1−ν2)=∫ν2ν1A(ν)dν/(ν1−ν2) for five separate subregions. The effects of temperature variations on absorption in some regions are discussed. Estimates of band intensity ∫ k(ν)dν are given for each band and are compared with the results of others.

Infrared Transmission of Synthetic Atmospheres.* II. Absorption by Carbon Dioxide

The total absorption ∫Aνdν has been determined for the CO2 bands at 15, 5.2, 4.8, 4.3, 2.7, 2.0, 1.6, and 1.4μ under stimulated atmospheric conditions. The absorber concentrations w ranged from 1 to 1000 atmos cm of CO2 for the strong bands and from 100 to 8600 atmos cm for the weak bands. Nitrogen was added to give total pressures ranging up to atmospheric; the pressure effects of oxygen were found to be similar to those of nitrogen. The observed data can be satisfactorily represented by two types of empirical relations.(1)For small values of total absorption,∫Aνdν=cw12(P+p)k;(2)For large values of total absorption,∫Aνdν=C+D Logw+K Log(P+p),where w is the CO2 absorber concentration, p is the CO2 partial pressure, and P is the total pressure. Values of the constants c, k, C, D, and K are given for each region of characteristic absorption. The present results are compared with those obtained in earlier studies. The use of the empirical relations in calculating atmospheric absorption is discussed.

Total Absorptance of Carbon Monoxide and Methane in the Infrared

The total absorptance ∫A(ν)dν of the major infrared bands of carbon monoxide and methane has been measured as functions of absorber concentration w and equivalent pressure Pe over wide ranges of these variables. The experimental results are presented graphically, and empirical equations relating ∫A(ν)dν, w, and Pe are presented. By employing small values of w and large values of Pe, it has been possible to determine the band strengths or intensities ∫k(ν)dν for the fundamental band of carbon monoxide and for ν2, ν3, and ν4 fundamentals of methane; the values obtained are compared with results of other investigators.

Total Absorptance by Nitrous Oxide Bands in the Infrared

Instrumentation and experimental techniques employed for the determination of the total absorptance ∫A(ν)dν of the bands of various atmospheric gases are described. The total absorptances of the 2563, 2461, 2224, 1285, 1167, 692, and 589 cm−1 bands of pure N2O and N2O mixed with N2 have been determined as a function of absorber concentration w and equivalent pressure Pe which involves the partial pressures of the two gases. The results are given in graphical form. In general, it is found that in situations in which existing theory predicts absorptance proportional to the square roots of pressure and absorber concentration, the total absorptance is indeed nearly proportional to the square root of absorber concentration but not to the square root of the pressure; for the 2224 cm−1 band, ∫A(ν)dν ∝ Pe0.37. In addition to graphical presentation of results, it is possible to express ∫A(ν)dν in terms of w and Pe by means of empirical equations applicable to certain definite ranges of the variables; the validity and the limitations of such empirical equations are discussed. For samples for which the product of the absorption coefficient k(ν) and the absorber concentration is much less than unity for all frequencies in an absorption band, it is possible to measure the band intensity or band strength ∫k(ν)dν. Values of band intensity for the 2563, 2461, 2224, 1167, and 589 cm−1 N2O bands are listed and compared with values previously reported by others.

Absorption of 3.39-Micron Helium–Neon Laser Emission by Methane in the Atmosphere

The paper describes an experiment on the absorption of the 3s2–3p4 helium–neon laser emission at 2947.903 cm−1 (3.39 μ) by methane. The emission frequency coincides closely to one of the components of the P(F+) branch of the ν3 band of methane. Methane and nitrogen in different mixing ratios were introduced into an absorption cell and the transmittance as a function of pressure was determined. By relating the measured absorption coefficient with the known interaction of collision and Doppler effects on the broadening of the absorption line, the separation of the emission line and the nearest absorption line was deduced to be 0.003±0.002 cm−1.The collision broadened full-width at half-maximum of the absorption line was determined to be 0.13±0.04 cm−1 at atmospheric pressure. At 1 atm in the earth’s atmosphere, the transmittance can be calculated to be T=exp(−1.1 L) by using the published value of the concentration of methane where L is the path length in kilometers. The effects of the laser emission in several possible cavity modes and of the several absorption lines in the methane group which overlap each other at high pressures are discussed.

Infrared Transmission of Synthetic Atmospheres.* III. Absorption by Water Vapor

The total absorption ∫Aνdν has been measured for the water vapor bands at 6.3, 3.2, 2.7, 1.87, 1.38, 1.1, and 0.94μ under simulated atmospheric conditions. The water vapor absorber concentrations studied ranged from 0.004 to 3.8 cm of precipitable water. Nitrogen was added to give total pressures up to atmospheric; the effects of nitrogen and oxygen on total absorption were found to be similar. The experimental data can be satisfactorily represented by two types of empirical relations:(1)For small values of total absorption,∫Aνdν=cw12(P+p)k;(2)For large values of total absorption,∫Aνdν=C+D Logw+K Log(P+p),where w is the H2O absorber concentration, p is the H2O partial pressure, and P is the total pressure. Values of the empirically determined constants c, k, C, D, and K are given for each of the spectral regions of characteristic absorption. The results are compared with those of other experimentalists.

Total Absorptance of Water Vapor in the Near Infrared

The total absorptance ∫ A(ν)dν of water vapor in the vicinity of its vibration-rotation bands near 5332 3700, and 1595 cm−1 has been determined as a function of absorber concentration w and equivalent pressure Pe for pure water vapor samples and samples of water vapor mixed with nitrogen. The present results, together with previously published results of Howard, Burch, and Williams, are presented in graphical form; logarithmic plots give ∫ A(ν)dν for various values of Pe as a function of absorber concentration w. These plots may be used in estimating total absorptance of water vapor in any sample for which w and Pe are known, and they may be applied in atmospheric studies.