Interferometric calorimeter for the measurement of water-vapor absorption

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Abstract
An interferometric calorimeter has been developed to measure the optical-absorption coefficient of gases at the level of 10−10 cm−1. The system operates by measuring the phase shift on a cw He–Ne laser probe beam, which is caused by the expansion of the gas after it has been heated in the focal volume of a pulsed excitation laser beam. The minimum detectable fringe shift is approximately 2 × 10−5 waves. For a 1-J excitation laser this shift corresponds to an absorption coefficient of 2 × 10−10 cm−1. The continuum absorption of water vapor at 9466 cm−1 has been measured to be 6 × 10−10 cm−1 at a 16.5-Torr partial pressure in a 1-atm (760 Torr) N2 buffer at 30°C.
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