Rotational Relaxation in Dilute Gas Mixtures

The relaxation of rigid rotator molecules dilutely dispersed in a bath of inert gas molecules is investigated. The systems studied correspond to hydrogen and hydrogen deuteride molecules in an excess of helium and neon, and hydrogen fluoride in an excess of helium. Fairly accurate quantum mechanical estimates of the potential energy governing the translation to internal energy transfer process have been used in this investigation. In those cases where the interaction energy was not available from previous work (${\mathrm{H}}_2–\mathrm{Ne}$ and $\mathrm{HF}–\mathrm{He}$ systems) the appropriate calculations were carried out to determine this quantity. Approximate solutions to the Boltzmann equation which describes the relaxation process were obtained by expanding the distribution function in orthogonal polynomials and evaluating the necessary integrals by a Monte Carlo method. The properties of the rotational energy autocorrelation function and relaxation time are examined.