Rotational Relaxation in Parahydrogen and Its Mixtures with Helium, Neon, and Argon at 300°K

Abstract

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 ${\tau }_{20}$ of 1.30 × 10⁻⁸ sec for the 2 → 0 transition and a ${\tau }_{42}$ of 3.90 × 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 ${\tau }_{20}$ over the temperature range of 75°–300°K. The Morse‐potential asymmetry parameter yielding the best fit is $\text{β\hspace{0.17em}=\hspace{0.17em}0.113}$ 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 $\text{J\hspace{0.17em}=\hspace{0.17em}0}$ to $\text{J\hspace{0.17em}=\hspace{0.17em}2}$ transition, but less effective for higher‐order transitions. Collisions of neon with pH₂ are found to be more effective at room temperature for inducing the 0 → 2 and 2 → 4 transitions than either helium or argon.