An acoustical method for determining the thermal and momentum accommodation of gases on solids

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Abstract
The velocity and absorption of sound in He, Ne, and Ar have been measured at room temperature and pressures ranging from 6.0−0.3 torr. The gases were contained in a tungsten tube 0.953 cm i.d. Flashing this tube at approximately 1900°C in a high vacuum before the measurements produced a marked increase in the sound velocity and a marked decrease in the sound absorption. The effect has been attributed to a decrease in the thermal and momentum accommodation coefficients produced by removing the adsorbed gases from the tungsten surface. In the presence of mischmetal getter, the accommodation coefficients for the surface after flashing remained constant for a period of two weeks. When oxygen was admitted to the system momentarily to a pressure of a few torr and then pumped out, the sound velocity and absorption in the noble gases returned to values corresponding to higher accommodation coefficients. The effect was repeatable. Comparison of the measured absorption and velocity with theoretical curves places reasonable limits on the permissible values for the energy and tangential−momentum accommodation coefficients.

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