Translational excitation of the molecular beam reaction K+HCl→KCl+H

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Abstract
The translational energy dependence of the reaction K+HCl→KCl+H has been studied in crossed beams in the c.m. energy range 2.1–12.1 kcal/mole (0.09–0.53 eV). Beams of HCl were generated at different translational energies by hydrodynamic expansion of various H2/HCl mixtures and the laboratory energy of the HCl was measured by time‐of‐flight techniques. Angular distributions of the product were measured and integrated for each energy and the total reactive cross section was found to increase monotonically with energy. The functional form of the energy dependence is well approximated by the traditional line‐of‐centers relation. Although the cross section increases with energy, translational excitation is not as effective as vibrational excitation and modest extrapolation of the data suggests that still larger amounts of translational energy will be similarly ineffective. The cross section might be expected to depend upon the number of states which are accessible to the products. But this number grows rapidly as the reagent energy is increased and the influence upon the cross section of this large statistical effect can obscure any subtle dynamic effects due to the nature of the potential surface. In order to remove this statistical bias, it is convenient to express the results in terms of the average state‐to‐state transition rate ?, the experimental rate constant divided by the density of possible product states. Even though the detailed product state distribution is not known, the density of possible product states can be calculated from the conservation laws and molecular properties of the asymptotic products. The state‐to‐state transition rate so obtained is found to decay exponentially with the square root of the energy in excess of the thermodynmic threshold: ?=A exp[−β (E′)1/2]. An extrapolation to energies higher than those studied predicts an eventual decrease in the reaction cross section.