Molecular Beam Kinetics: Reactions of K, Rb, and Cs with Br2 and I2

Abstract

Crossed-beam studies have been made of the reactions of K, Rb, and Cs atoms with Br2 and of K and Cs with I2. It is found that for all these systems: (1) The reaction cross sections are remarkably large, ≳150 Å2. (2) Most of the alkali halide product recoils into the forward hemisphere with respect to the incident alkali atom beam, with scattering angle θ≲60° (in the center-of-mass system). However, there also appears to be considerable intensity (∼20% of the forward peak) throughout the backward hemisphere, 90°<θ<180°. (3) The angular distribution (in the c.m. system) of alkali atoms scattered without reaction falls off much more rapidly at wide angles than for collisions between unreactive molecules of comparable size. (4) The shape of the angular distributions of both the reactive and nonreactive scattering is the same for various alkali metals, but differs appreciably for Br2 and I2. (5) Most of the chemical energy released appears as internal excitation of the products. This could include substantial rotational and/or electronic excitation, but other evidence shows that vibrational excitation of the newly formed bond is dominant. All these properties can be accounted for by a model (suggested originally by Polanyi and Magee) which assumes that the attacking alkali atom transfers its valence electron to the halogen at large distances (≳7 Å), so that these are in effect ion-recombination reactions.