Orientational opacity function for CH3I+Rb reactive backscattering: The steric cone of nonreaction

An improved orientational opacity function is obtained for the reaction of methyl iodide with Rb. This opacity function (i.e., the reaction probability as a function of the ‘‘angle of attack’’ γ) results from an improved deconvolution (for the precessional effect) of the reactive backscattering data of Parker et al. (1981) at a collision energy of 13 kJ mol−1. From this opacity function, the cut-off angle of the steric cone blocked out by the methyl group in the course of a reactive encounter is found to be 53±2°. This is larger than the best previous estimate, and quite comparable with that predicted from a simple steric model. The shape of the orientational opacity function is well represented by the nonspherical rigid-body approximation formula of Evans et al. (1985), with a value of the colliding-pair anisotropy parameter of λ=0.41±0.01 and a cut-off angle of 50±2°.