Abstract

Crossed-beam studies have been made of reactions of alkali atoms with about fifteen different halogen compounds. Kinematic analysis of the angular distributions of the alkali halide product indicates that in all these reactions the final relative translational kinetic energy of the products is comparable to the initial kinetic energy of the reactants. This has been confirmed for the K + Br₂ reaction by direct velocity analysis of the KBr. Thus most of the chemical energy released (difference in strength of the new and old bonds) appears as internal excitation of the products. In some cases rotational and/or electronic excitation could be important; however, indirect arguments suggest that vibrational excitation of the newly formed bond is often dominant, at least for the reactions of alkali metals with diatomic halogen molecules. For these reactions, the cross section σ_r varies from $\stackrel{\sim }{<}10{\AA }^2$ to $\stackrel{\sim }{>}100{\AA }^2$. As σ_r increases, the favored direction for recoil of the alkali halide product gradually shifts from backward (rebound mechanism) to forward (stripping mechanism) and the angular distribution of alkali atoms scattered without reaction shows an increasingly pronounced fall-off at wide angles. These correlations and the product excitation are discussed in terms of simple models for the reaction dynamics.

© 1965 Optical Society of America

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