Elastic and Reactive Scattering of K by HCl and HI in Crossed Molecular Beams

Abstract

The elastic and reactive scattering of K by HCl and HI has been measured at several relative energies in experiments with crossed molecular beams. The interpretation follows that presented previously for K+HBr, with some improvements. For both systems the measurements on K show rainbow scattering, and from it parameters have been calculated for the exp-six potential. From the elastic scattering at large angles and a two-body model for it, the probability of reaction is deduced as a function of the potential energy at the distance of closest approach of the reactants. For K+HCl the potential parameters are ε=0.45±0.02 kcal/mole and rm=3.2±0.3 Å for α=12 (assumed). The activation energy is 0.55±0.1 kcal/mole, as determined from the threshold of the probability of reaction, and is probably just the change in zero-point energy for this endothermic reaction. There are indications that the probability of reaction increases when the potential energy is sufficient for the product KCl to be formed in excited vibrational states. The application of the two-body model at the thresholds of formation of KCl allows another determination of rm which is in good agreement with the value obtained from the rainbow scattering. The total reaction cross section as obtained from the probability of reaction is 3.9 Å2 at Ē=2 kcal/mole. No measurements could be made on KCl. For K+HI the potential parameters are ε=0.67±0.02 kcal/mole and rm=4.2±0.2 Å for α=12 (assumed). The activation energy of this exothermic reaction is 0.2±0.1 kcal/mole as determined from the threshold of the probability of reaction. The total reaction cross section as a function of relative energy has been obtained from measurements on KI and from the probability of reaction deduced from the elastic scattering. These two measures of the amount of KI produced agree within a factor of 2, which is the reproducibility of the measurements on KI. At a relative energy of 2 kcal/mole the total reaction cross section is 28 Å2.