Rotational distributions in vibrational transfer

Rotational distributions vary widely among the different collisional intera ctions that initiate chemical and physical change, processes that are often regarded as differing in kind. Here the commonality of mechanism among a v ariety of collision-induced processes is emphasized. This mechanism is the conversion of linear-to-angular momentum at the hard wall of the intermolec ular potential, its operation is constrained by (i) the existence of quanti zed molecular eigenstates and (ii) boundary conditions set by energy conser vation. The wide variation of these boundary conditions under differing kin ematic circumstances gives rise to the wide variety of rotational distribut ions that is observed experimentally. Three cases of vibrotation transfer ( VRT), namely Li-2-Ne, NO-NO, and HF-H are considered in detail. It is shown that the natural distribution in VRT is best described as "frustrated expo nential-like", only recognized as such by observing the development of rota tional distribution shape as the vibrational momentum ''gap" steadily incre ases, as in the cases considered. The low Deltaj region of the distribution becomes severely truncated as this gap increases, giving distribution shap es which are superficially Boltzmann in appearance. The analysis here indic ates that derivation of rotational "temperatures" based on this apparent si milarity is likely to give misleading results. Velocity-angular momentum di agrams are used to give physical insight into the operation of the mechanis m, the effect of energy boundary conditions and to predict rotational distr ibution shapes and peak values. The analysis also suggests that in determin ing vibrational transfer cross section, inaccurate results will generally r esult unless initial rotational state j(i) similar or equal to 0 and the wh ole manifold of rotational states in nu (f) is summed.