Abstract
Rotational relaxation numbers Z rot of the gases N 2 , O 2 , CO, CO 2 and CH 4 were determined at a mean temperature of 500°K by means of thermal transpiration measurements. Special attention was given to possible differences in Z rot -values of isotopic molecules: CH 4 and CD 4 , 14 N 15 N and 14 N 2 , 12 C 16 O and 12 C 18 O. The following results were obtained: Z rot (N 2 ) = 3.8, Z rot ( CO 2 ) = 3.9, Z rot ( CO ) = 1.9, Z rot ( O 2 ) = = 4.2 and Z rot ( CH 4 ) = 14.8. No differences in Z rot -values were found between the different isotopic molecules in the case of nitrogen and carbonmonoxide, whereas Z rot (CD 4 ) was about half the value obtained for CH 4 , in agreement with acoustical results.
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