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%.