Relaxation in polyatomic gases

The experimental results for the relaxation of internal energy in polyatomic gases have been well summarised in various places, see e.g. [61.C 77.L1]. The basic observations are as follows rotational relaxation rates appear to be commensurate with the collision rate itself and are in general rather poorly characterised, partly because of their great speed, but also because of the intrinsic ambiguity in their definition as I have just shown vibrational relaxation rates, on the other hand, are quite well defined and reasonably well understood [59.H 77.L1], and take place on time scales of from a few tens to a few thousands of collisions. 

Let us concentrate for the time being on those molecules with fast vibrational relaxation rates these are generally the so-called strong colliders. If a collection of such molecules is subjected to some perturbation, then their rotational and vibrational populations will return to 

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