Approaches of weak and strong collisions

Pressure induced broadening and narrowing of a whole spectrum are described by the quasi-static approximation and the perturbation theory, correspondingly. Comparing inequalities (6.13) and (2.53) one can see that the border between the stages is determined by the criterion coij 1, 

In fact, such a method was proposed by Sack in the classical work [99], which was far ahead of its time. This method provides the general solution of Eq. (6.4) in the form of a continuous fraction, which is, however, rather difficult to analyse. In the case of weak collisions, there is no good alternative to this method, but for strong collisions, the solution can be found analytically. Let us first consider this case. 

We need only one simplification neglect of the Q-branch rotational structure. It is conditioned by Eq. (6.29). This assumption restricts the buffer gas densities, which should not be too low  

In other words, the following description of the orientational relaxation spectrum is valid at such pressures when the Q-branch has already collapsed and starts to broaden. 

Thus defined, the problem can be solved by the standard method, which was used already in Section 3.4 and in [27]. However, there is a more 

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