ROTATIONAL RELAXATION IN PARAHYDROGEN AND ITS MIXTURES WITH HELIUM NEON AND ARGON AT 300 DEGREES K

Ultrasonic-velocity dispersion measurements have been performed in parahydrogen and its mixtures with helium, neon, and argon, all at 300°K. In each case the experimental dispersion curves can be matched successfully to those calculated under the assumption that the 0-2 rotational transition relaxes separately from the 2-4 and higher-order terms. For pure pH2 we find a relaxation time τ20 of 1.30X10-8 sec for the 2→0 transition and a τ42 of 3.90X 10-8 sec for the 4→2 transition. Comparison with the quantum-mechanical theories of Roberts and of Davison for H2-H2 collisions using Morse potentials shows good agreement for τ20 over the temperature range of 75°-300°K. The Morse-potential asymmetry parameter yielding the best fit is β=0.113 for Roberts' calculation and 0.108 for Davison's. It is found that He-pH2 collisions are more effective than pH2-pH2 in producing the J=0 to J=2 transition, but less effective for higherorder transitions. Collisions of neon with pH2 are found to be more effective at room temperature for inducing the 0→ 2→4 transitions than either helium or argon.