Rotational energy relaxation in molecular hydrogen

Theoretical studies of rotational relaxation in para-hydrogen are presented. By using a set of theoretically deduced state-to-state rotational rate constants the master kinetic equations and the relevant fluid mechanical equations are solved numerically. Calculations are performed over the range 100 ≤ T ≤ 1100°K to simulate conditions for free jet expansion, shock tube, and sound absorption experiments. The over-all rotational energy relaxation rates extracted from these calculations are compared with available experimental data and are shown to be in good qualitative agreement. The magnitudes and the temperature dependence of these rates depend critically on the degree and direction of the initial departure from equilibrium as well as the multilevel nature of the relaxation process. The apparent discrepancies in the measured rates from the different experiments are shown to be qualitatively self-consistent in light of the present calculations.