Rotational Distributions in Vibrational Transfer

Abstract

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 linearto-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 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 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 j i = 0 and the whole manifold of rotational states in V f is summed. +)Li 2 -Ne collisons involving V i ) 2-24, j i ) 30. Second are the rate constants reported by Islam † Part of the special issue "C. Bradley Moore Festschrift".

The role of angular momentum in collision-induced vibration–rotation relaxation in polyatomics

“…4,5 In diatomics the J value at which A and E plots intersect is generally a good guide to the peak of final rotational distribution both in VRT 11 and in the vibrational predissociation of van der Waals molecules. 13 This intersection represents the lowest J f ͑or K f ) for which all b n up to the maximum available may be used without violating energy conservation and constitutes the most probable exit channel.…”

“…The vibrationally close-couple-infinte order sudden ͑VCC-IOS͒ technique introduced by Clary 10 and often used to pre-dict vibration-rotation transfer ͑VRT͒ probabilities in large molecules effectively freezes rotations throughout the collision. In diatomic molecules the efficient disposal of angular momentum is a critical factor in determining cross-sections for pure RT, VRT, 11 electronic energy transfer, 12 vibrational predissociation of van der Waals molecules, 13 and atomdiatomic molecule exchange reactions. 14 Direct calculation of the probability of linear-to-angular momentum conversion is fast and yields accurate results.…”

Rotational distributions following van der Waals molecule dissociation: Comparison between experiment and theory for benzene–Ar

“…33 Our focus is on the subsequent dissociation step and the rotational excitation that ensues. In this section we describe an extension of the "internal collision" model of VP in vdW complexes of diatomic molecules 8 to the polyatomic case. The method utilizes the equivalent rotor model introduced as a means of calculating rotational distributions following inelastic collisions in polyatomic species.…”

“…The model used here for VP in benzene-Ar is a direct analog of that used successfully in diatomics in which dissociation is induced by an internal collision between the molecule and weakly bound species as each executes quasiindependent vibrational motion. 8 In this process the Ar atom is ejected and vibrational excitation is converted to rotational excitation of the 0 0 level. The model is sensitive only to the energy available for transfer to rotation and translation of the products and is independent of how this is distributed within the complex prior to dissociation, i.e., the calculations are not affected by whether dissociation occurs from 6 1 or isoenergetic levels accessed by IVR.…”

“…In diatomic molecules it has been found that the efficient disposal of angular momentum is a critical factor in determining the product state rotational distributions in vibrational predissociation of van der Waals molecules. 8 It has also been shown that angular momentum constraints determine cross sections for rotational transfer ͑RT͒ as well as vibrational-rotational transfer ͑VRT͒, 9,10 electronic energy transfer, 11 and atom-diatomic molecule exchange reactions. [12][13][14] Simple calculations based on a "torque arm" for the process are very successful in reproducing ex-perimental rotational distributions.…”