Reactions of alkali metal atoms with carbon tetrachloride. Rainbow-like coupling of product angle and energy distributions

Abstract

Velocity distributions of alkali metal chlorides reactively scattered from crossed thermal beams of K, Rb or Cs and CCl4 have been measured over the range 100–1000 m s–1 at laboratory angles from 10 to 100° with respect to the parent alkali beam. The differential cross sections for reactive scattering in the centre-of-mass system show strong coupling between the peak position of the product angular distribution [graphic omitted] and the final relative translational energy E′, and vary markedly with the identity of the alkali metal atom. For a given alkali metal, [graphic omitted] shifts to smaller angles as E′ increases, and as K → Rb → Cs the entire pattern shifts toward the forward hemisphere. These properties suggest an analogy to the rainbow effect familiar in elastic scattering. The product distributions can be simulated by a simple dynamical model. The most important features are the reaction probability as a function of initial impact parameter, the repulsive force causing dissociation of the unstable CCl4– intermediate formed by transfer of the alkali metal valence electron, and the T → V, R energy transfer induced by release of this repulsion during formation of the product bond. The product velocities obtained 25 years ago for Cs + CCl4 by Bull and Moon agree with our data within ≈ 10%.