Richard R. Patty

Absorption of Infrared Radiation by CO 2 and H 2 O. Experimental Techniques

The experimental techniques used in an extensive laboratory investigation of the infrared absorption and emission by CO2 and H2O are described. Apparatus discussed include two multiple-pass absorption cells with base lengths of approximately 1 and 29 m, gas-handling systems, a prism spectrometer and a grating spectrometer with their accessories, and a system used to digitize absorption spectra. Also discussed are methods used in gas sampling, checking for possible sources of error, and recording and analyzing the data.

Further Studies of Pressure-Modulated Infrared Absorption

A “pressure modulator” has been constructed for use in studies of modulated infrared emission and absorption spectra. The modulator provides sinusoidal variation of the volume of a gas sample at a frequency of 8 cps with compression ratios of 2:1, 3:1, 4:1, 5:1, and 6:1. The general results obtained in grating studies of the fundamental and first-overtone absorption bands of carbon monoxide can be satisfactorily interpreted on the basis of an adiabatic modulation cycle, although detailed checks of the expected behavior for transitions between high rotational states were not entirely successful. In the adiabatic modulation of a gas for which γ=1.66, temperatures as high as 640°K and as low as 225°K are attained in the present modulator with a compression ratio of 5:1 without raising the cell temperature more than 45 K deg above room temperature.

Infrared Emission from a Pressure-Modulated Gas*

The volume of enclosed gas samples has been subjected to sinusoidal variation at a frequency of 8 cps; this volume modulation cycle is approximately adiabatic and involves temperature variations of several hundred degrees. Several emission bands of nitrous oxide in the spectral region between 3000 cm−1 and 1000 cm−1 have been investigated as a function of compression ratio for samples of pure nitrous oxide, nitrous oxide diluted with argon, and nitrous oxide diluted with various argon-nitrogen mixtures. Emission near the centers of strong absorption bands is that to be expected from a temperature-modulated blackbody provided the absorber concentration is sufficiently great to give complete absorption near the band center. For weaker bands and for strong bands with low values of absorber concentration, the effects produced by absorber concentration modulation and pressure modulation are superimposed on the effects attributable to temperature modulation. All observed effects can be interpreted on a semiquantitative basis in terms of an adiabatic modulation process.

Pressure-Modulation Study of the Absorption of Non-Black Thermal Radiation*

The pressure-modulated near-infrared emission spectrum of a gas consists of a set of vibration-rotation bands. The intensity of the emitted radiation is a rapidly varying function of frequency. The transmission of this radiation through various gas samples has been investigated for absorbing atomspheric gases such as carbon dioxide, carbon monoxide, nitrous oxide, and methane; the effects of pressure-broadening of spectral lines by inert gases such as argon and nitrogen have been considered. The transmittance of radiant power has been shown to depend on the composition of the emitting sample and the compression ratio employed as well as on the composition of the absorbing sample. Possible application of pressure-modulation techniques to atmospheric transmission studies is suggested.