Reactions of chlorine and bromine with hydrogen astatide

Abstract

Angle-differential cross sections are reported for the Reactions (2), Cl+Hat→HCl+At and (3), Br+Hat→HBr+At. Total reaction cross sections at 6.5 kcal/mole initial translational energy are 25.6±4.2 Å2 for (2) and 1.48±0.34 Å2 for (3). Mean energies in product translation are 7.2±0.6 kcal/mole for (2) and 1.0±0.2 kcal/mole for (3). The center-of-mass angular distribution of (2) is forward–backward peaked and almost symmetric about 90°. For (3), about as much product scatters into the forward hemisphere as into the backward. The measured cross sections for both (2) and (3) are close to the limiting value, which is calculated by correcting an assumed total cross section for inelastic plus reactive processes of 50 Å2 for kinematic restrictions independent of spatial configuration and for the effects of electronic adiabaticity. This means that the smaller cross section for (3) is consistent with its smaller mean product translational energy. It also means that for the direction and position of either attack or retreat (or both) no limitations can be claimed if electronic adiabaticity is assumed, and only loose restrictions if such adiabaticity is relaxed. The data thus do not require a model that specifies attack and retreat at small impact parameters exclusively. Reactions of Hat with Cl and Br are broadly similar to the corresponding reactions of HI, although there are obvious differences of detail. The information obtained in this experiment demonstrates the feasibility of using reagents labeled with short-lived nuclides for the study of reactive scattering in crossed molecular beams.