Alkali-Atom—Halogen-Molecule Reactions in Molecular Beams; The Spectator Stripping Model

Abstract

Crossed‐molecular‐beam techniques have been used to study reactive collisions in some alkali‐metal—halogen systems (Cs+Br₂, K+I₂, K+Br₂, Rb+IBr). Both the absence of rainbow scattering in elastic‐scattering distributions and the magnitude of the reactive collision yield suggest large reaction cross sections in the order of 100 Ų for all the systems studied. The position and width of the angular distributions of alkali halide product molecules indicate forward scattering in the center‐of‐mass system with recoil energies close to zero. These observations suggest a stripping mechanism for these reactions and a model, based upon the simplest possible stripping mechanism, has been proposed to explain the observations. This model, called spectator stripping, predicts the gross features of the distributions and leads to a prediction of the maximum intermolecular distance at which reaction can occur. This corresponds to a reaction cross section of approximately 100 Ų. The effect of electron transfer from the alkali atom to the halogen molecule at large distances is shown to be almost equivalent to spectator stripping for these reactions if the vertical electron affinity of the halogen molecule is used. Even larger cross sections would be predicted if the adiabatic electron affinity were used; however, an evaluation of the probability of adiabatic adjustment of the halogen internuclear distances during the collision time appears to preclude this possibility.