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Rotational Distributions in Vibrational Transfer

journal contribution
posted on 2023-06-07, 19:24 authored by Anthony J McCaffery, Richard J Marsh
Rotational distributions vary widely among the different collisional interactions that initiate chemical and physical change, processes that are often regarded as differing in kind. Here the commonality of mechanism among a variety of collision-induced processes is emphasized. This mechanism is the conversion of linear-to-angular momentum at the hard wall of the intermolecular potential. Its operation is constrained by (i) the existence of quantized molecular eigenstates and (ii) boundary conditions set by energy conservation. The wide variation of these boundary conditions under differing kinematic circumstances gives rise to the wide variety of rotational distributions that is observed experimentally. Three cases of vibrotation transfer (VRT), namely Li2-Ne, NO-NO, and HF-H are considered in detail. It is shown that the natural distribution in VRT is best described as “frustrated exponential-like”, only recognized as such by observing the development of rotational distribution shape as the vibrational momentum “gap” steadily increases, as in the cases considered. The low ?j region of the distribution becomes severely truncated as this gap increases, giving distribution shapes which are superficially Boltzmann in appearance. The analysis here indicates that derivation of rotational “temperatures” based on this apparent similarity is likely to give misleading results. Velocity-angular momentum diagrams are used to give physical insight into the operation of the mechanism, the effect of energy boundary conditions and to predict rotational distribution shapes and peak values. The analysis also suggests that in determining vibrational transfer cross section, inaccurate results will generally result unless initial rotational state ji 0 and the whole manifold of rotational states in vf is summed.


Publication status

  • Published


Journal of Physical Chemistry A




American Chemical Society





Page range

10442 - 10451



Department affiliated with

  • Chemistry Publications

Full text available

  • No

Peer reviewed?

  • Yes

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