Mechanism of Rotational Relaxation

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Abstract
It has been known for some time from infrared chemiluminescence experiments that a nonthermal rotational distribution of hydrogen halide peaked initially at high rotational quantum number, J, relaxes to a thermal distribution without generating a peak at intermediate J [Discussions Faraday Soc. 44, 183 (1967)]. It is shown in the present study that this characteristic pattern of relaxation is well described by a model according to which ΔJ is unrestricted, except for the relation PJ−ΔJJ=N exp (— CΔE), where PJ−ΔJJ is the probability of a collision−induced transfer from J to J — ΔJ, ΔE is the energy difference between these two rotational states, and N and C are constants. This expression for PJ−ΔJJ ascribes a very much lower probability of rotational deactivation to the higher J levels. Three other, contrasting, models were tested; they were rejected since they failed to describe the observed pattern of relaxation adequately. Upper limits were ascribed to PJ−ΔJJ for ΔJ=1–5 in HCl–H2 collisions. This study provides a further method for correcting infrared chemiluminescence data for modest rotational relaxation and at the same time shows that the simple truncation correction procedure used until now is remarkably good.