Rotational Relaxation Processes Parker Formula

Rotational-rotational (RR) and rotational-translational (RT) energy transfer processes are usually non-adiabatic and fast, because rotational quanta and, therefore, the Massey parameter are small. As a result, the collision of a rotator with an atom or another rotator can be considered a classical collision accompanied by essential energy transfer. The Parker formula for calculation of number of collisions, Zrot, required for RT relaxation was proposed by Parker (1959) and Bray and Jonkman (1970)  

Factors Zrot are low at room temperature values of about 3-10 for most molecules except hydrogen and deuterium, where Zrot is about 200-500 due to high rotational quanta and Massey parameters. RT and RR relaxations are fast processes requiring only several collisions, and their kinetic rates are close to those of thermalization. Rotational degrees of freedom, therefore, are often considered in quasi-equilibrium with translational degrees of freedom and characterized by the same temperature To even in strongly non-equilibrium systems. 

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