Translational, Rotational and Vibrational Relaxation

Proceed now to discussing various types of relaxation processes which may be considered, to a certain extent, independent of each other. 

Assume now that molecules A represent a small admixture in the gas consisting of molecules B in equilibrium (relaxation in the thermal bath). This process is described either by Eq. (8.34) or its simplified version (8.36) provided condition (8.37) is fulfilled. For example, let the A molecule mass be much lower than that of B. Then (see Section IV. 12) 

Substituting this relation into Eq. (8.41) and taking into account that here C = 3k/2 we obtain the following estimate of the translational relaxation time 

If is small, the translational relaxation time may be much larger than the average 

It follows from these considerations that in a mixture of two gases with substantially different molecular masses, for example with relaxation process occurs via 

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