We have re-examined critical experiments on collision induced rotational transfer (RT) and conclude that the probability of RT is controlled by the factors that control the probability of angular momentum (AM) change. The probability of energy change seems less important in this respect. In the light of this we suggest a model for RT in which the probability of AM change is calculated directly and present a formalism for this purpose. We demonstrate that such a calculation leads to an exponential-like fall of RT probabilities with transferred AM, a consequence of the radial dependence of the repulsive part of the intermolecular potential. Thus in this AM model, the exponential gap law has a simple physical origin. The AM model we describe may be used as the basis of an inversion routine through which it is possible to convert RT data into a probability density of the repulsive anisotropy. Through this model therefore it is possible to relate experimental RT data directly to the forces that are responsible for rotational transfer. The hard ellipse model is used in this work to relate calculated anisotropies to a form that includes an isotropic component. The result is a representation of the intermolecular potential through which new insights into the RT process are gained.

We have formulated a law for state-to-state rotational transfer (RT) in diatomic molecules based on the angular momentum (AM) theory proposed by McCaffery et al. [J. Chem. Phys. 98, 4586 (1993)]. In this, the probability of angular momentum change in the rotor is calculated by assuming the dominant process to be the conversion of linear to angular momentum at the repulsive wall of the intermolecular potential. The result is a very simple expression containing three variable parameters, each of which has physical significance in the context of the model. Fits to known RT data are very good and suggest strongly that linear to angular momentum change is indeed the controlling process in RT. The parameters of the fit are sufficiently available to give the model predictive power. Using this formulation, RT probabilities may be calculated for an unknown system with little more than the atomic masses, bond length, and velocity distribution. We feel that this represents an important step in the development of a simple physical picture of the RT process.

Note that this treatment concerns only the 'normal' Faraday effect and excludes the quadratic Faraday effect where the angle of rotation is proportional to the square of the field strength and which may occur only in optically active molecules. We also exclude the optical Faraday effect in which circular birefringence is induced by an intense beam of circularly polarised light. These two phenomena have recently been discussed in detail by Atkins and Miller.''# l8 l5

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".

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Articles you may be interested inVibrational and rotational energy transfers involving the CH B Σ − 2 v = 1 vibrational level in collisions with Ar, CO, and N 2 O We describe a quantitative angular momentum ͑AM͒ model for predicting rotational transfer ͑RT͒ and vibrotational transfer ͑VRT͒ in collisions between CO 2 and hot H atoms. This molecule is important in several contexts, not least as a bridge between the relative simplicity of diatomic molecules and the complexities of polyatomic RT and VRT. We show that for pure RT, an AM constraint dominates but that this changes to a dominant energetic constraint in the case of VRT. The requirement that the ͑001͒ vibrational channel be opened simultaneously with the generation of AM imposes special restrictions which effectively limit the trajectories that lead to VRT. The origin of this is a constraint-induced restriction on the effective impact parameter (b n max ) for individual ⌬ j channels and the effect is manifest as reduced probability for populating low ⌬ j channels. In CO 2 -H* this leads to a shift in the peak of ͑VRT͒ ⌬j probabilities away from zero as found experimentally for the ͑001͒ vibrational mode. We report a Monte Carlo trajectory calculation similar to that of Kreutz and Flynn ͓J. Chem. Phys. 93, 452 ͑1990͔͒ but predict an exponential-like dependence of pure RT on ⌬ j. For VRT to ͑001͒ the constraint-induced restrictions on b n max are incorporated quantitatively and the vibrational channel-opening velocity is treated as a vector quantity. The results of these calculations are in good agreement with experiment. The underlying mechanism, likely to be general in VRT, is clearly revealed in plots of relative velocity versus rotational AM change.

An unanswered question in collision-induced rotational transfer (RT) centers on the similarities that characterize the distributions of ∆j states despite very large differences in mass and chemical composition of collision partners (