Study of the Reaction of K with HBr in Crossed Molecular Beams

Abstract

The reaction of K+HBr→KBr+H is studied by measuring elastic and reactive scattering in crossed molecular beams. The K beam is well collimated and made nearly monoenergetic by a mechanical velocity selector. The HBr effuses from a low-temperature source as a diverging beam. Angular distributions of KBr at fixed K energy and the variation of the total chemical reaction cross section with initial relative energy indicate a reaction threshold energy below 0.4 kcal/mole and a nearly constant reaction cross section at higher energies up to about 4.5 kcal/mole. The difference between the measured scattered K and the K intensity calculated classically for hypothetical nonreacting molecules scattered elastically leads to a determination of the variation of the probability of reaction with impact parameter or with the potential energy at the distance of closest approach in the collision. The influence of the requirement of conservation of angular momentum on the probability of reaction is evident. At impact parameters below about 3.5 Å, 90% of the collisions lead to reaction. Above threshold the reaction cross section is found to be about 34 Å2.