Collision energy dependence of the reactive scattering of Br with HAt

Abstract

Laboratory angle-differential cross sections were measured for the reaction Br+HAt→HBr+At at collision energies from 4.2 to 11.7 kcal/mol. Transformation of these data into the center-of-mass system revealed that the mean energy in product translation is extraordinarily small, rising from 1.5±0.2 to 3.3±0.6 kcal/mol. The ratio of products scattered forward to products scattered backward is about 1.0 to 1.5 at all collision energies studied, but more detailed angular information could not be obtained. The total reaction cross section is in the range of 1.5 to 2.0 Å2 from 4.2 to 8.3 kcal/mol of collision energy, and rises to 2.9±0.34 Å2 at 11.7 kcal/mol. (The uncertainties cited for the cross section values represent random errors only, systematic errors contribute an additional ±15%.) These results confirm the observations made earlier at a collision energy of 5.8 kcal/mol. Phase space calculations show that for collision energies greater than about 7 kcal/mol the total reaction cross section is approaching the maximum value that the small average energy in product translation permits it to have, which means that energy barriers and geometric restrictions are of secondary importance. For collision energies less than 5 kcal/mol the total reaction cross section becomes significantly smaller than the permissible maximum, and some constraint(s) must therefore be effective; for example, the data are consistent with an entrance-channel barrier of 2 to 3.5 kcal/mol. The mean energy in product translation is always far less than the collision energy, and this means that neither direct reactions nor completely statistical reactions are important, a conclusion supported by the forward/backward ratio near unity in the angular distributions. A model intermediate between direct and statistical is proposed in which the rapidly-moving hydrogen atom can possess too much energy to permit separation of HBr and At, and at the same time too little energy to permit spearation of Br and HAt, so that the bromine and astatine atoms are temporarily trapped in one another’s vicinity. This behavior is associated with potential hypersurfaces characterized by a pronounced widening in the neighborhood of the transition state. Such surfaces give a high probability of snarled trajectories, broad angular distributions of products, and small energies in product translation. The results of preliminary calculations are presented to support the model.