Abstract
In order to establish the influence of forbidden transitions, rotational relaxation numbers Z rot of the gases 14N 2, 14N 15N, 15N 2, 12C 16O and 12C 18O were determined from ultrasonic absorption measurements. The absorption coefficient was measured in a single-crystal interferometer at a frequency of 3.007 MHz and a temperature of 307.46 K in the pressure range from 75–200 torr. The ratio of the experimental and the classical absorption coefficients determines Z rot. The following results were obtained: Z rot( 14N 2) = 3.9, Z rot( 14N 15N) = 4.1 Z rot( 15N 2) = 4.5, Z rot( 12C 16O) = 2.8 and Z rot( 12C 18O) = 2.7. No influence on the Z value due to the symmetry of the homonuclear N 2 molecules is observed. A short discussion is given.
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