Weak collision behaviour

We have already summarised, in Chapter 1, the remainder of the experimental measurements of the variation of activation energy with pressure, including some weak collision examples a proper treatment of the latter must await a better understanding of the nature of the bottleneck phenomena (as described in Chapter 8) which underlie the weak collision behaviour. 

Notice that the sufficient condition for the occurrence of strict Lindemann behaviour, given a strong collision relaxation matrix, is that the d, are constant alternatively, if there is only one grain having a non-zero value of d, the rate is also strict Lindemann, regardless of the form of the assumed relaxation matrix [81.VI]. In the past, it had often been assumed that strict Lindemann behaviour was a strong collision property only, but we now know that near-Lindemann behaviour can often occur in weak collision systems at high temperature, see Chapter 8. 

Markovian perturbation theory as well as impact theory describe solely the exponential asymptotic behaviour of rotational relaxation. However, it makes no difference to this theory whether the interaction with a medium is a sequence of pair collisions or a weak collective perturbation. Being binary, the impact theory holds when collisions are well separated (tc < to) while the perturbation theory is broader. If it is valid, a new collision may start before the preceding one has been completed when To < Tc TJ = t0/(1 - y). 

Fig. 6.3. Quasi-static behaviour of relaxation times tgj (upper curves) and r ,i in the case of strong (1,2) and weak (3,4) collisions. The straight lines are the asymptotics of the curves after Q-branch collapse.

As can be seen, the difference in behaviour of orientational relaxation times Te,2 in models of weak and strong collisions is manifested more strongly than in the case of isotropic scattering. Relation (6.26) is... 

The behaviour of te,2 (tj) is qualitatively different. In the dense media this dependence also satisfies the Hubbard relation (6.64), and in logarithmic coordinates of Fig. 6.6 it is rectilinear. As t increases, it passes through the minimum and becomes linear again when results (6.25) and (6.34) hold, correspondingly, for weak and strong collisions ... 

In contrast to reactive atom transfer collisions, relatively little attention has so far been paid to rotational alignment in the products of inelastic, excitation transfer. The l2 (D ->A ) fluorescence excited through collision of I2 in the superthermal atomic beam of Xe( P2) is found to be weakly polarised (see Fig. 2), reflecting a slight preference for its rotation to become aligned perpendicular to the incident relative velocity. Similar behaviour is found in the system [38]... 

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