Spectroscopy at extreme infra-red wavelengths III. Astrophysical and atmospheric measurements

Abstract
Observations of solar, lunar and atmospheric radiation between 1 and 4 mm wavelength are described. The optical arrangement employed parabolic reflecting telescopes with Golay cells as detectors and gauzes or objective gratings for wavelength selection. The atmospheric results include measurements of transmission through various types of cloud and also the detection of an absorption line at about 1·6 mm which is almost certainly the 3-2-22 pure rotation line of water predicted by Van Vleck. More generally the atmospheric measurements confirm earlier experimental results showing that the absorption due to the tails of submillimetre absorption lines is greater than that predicted by Van Vleck. Some direct laboratory measurements have been made of pressure broadening of the water vapour line at 0.538 mm. Combined solar and lunar measurements give an effective solar temperature of 5800 °K at 1·3 mm and 6700 °K at 2·5 mm wavelength and a value of δ = 0.36 for the ratio of thermal to electromagnetic attenuation coefficients for lunar surface material.

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