Translational and rotational relaxation

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Abstract
The translational relaxation and the translational nonequilibrium effects on the rotational relaxation are studied by solving the semiclassical Boltzmann equation with Monte Carlo simulation in the HCl–H2 system, where HCl formed in the exothermic reaction H+Cl2→HCl+Cl with nonequilibrium rotational and translational distributions with an excess translational energy is dispersed in the heat bath of H2. A simple hard sphere model with the line-of-centers cross section of upward rotational transition, the exponential model of downward transition probability proposed by Polanyi and Woodall, and the selection rule ‖ΔJ‖=1 is used. The explicit time-dependent translational and rotational distributions from initial nonequilibrium to final equilibrium are obtained. Even when the translational distribution is far from equilibrium at the initial stage of rotational relaxation, the difference in the rotational distribution between translational nonequilibrium and equilibrium is less than 10% at the initial stage. Therefore, the estimation of the initial rotational distribution by Polanyi and Woodall with the master equation, where the translational equilibrium is assumed, is valid.