Molecular Beam Kinetics: Reactions of K, Rb, and Cs with ICl and IBr

Crossed‐beam studies of the reactions of alkali atoms with ICl and IBr show that the main features are very similar to those found for the Cl₂, Br₂, and I₂ reactions: (1) The reaction cross sections are extremely large, >̃150 Ų. (2) Most of the alkali halide product appears in the forward hemisphere with respect to the incident alkali‐atom beam, although there is also considerable intensity (∼15%–25% of the forward peak) throughout the backward hemisphere. (3) The angular distribution of the alkali atoms scattered without reaction falls off much more rapidly at wide angles than for collisions between unreactive molecules of comparable size. (4) Most of the chemical energy released appears as internal excitation of the products. The surface ionization detectors used in these experiments do not reveal whether the product of the M + IX reactions is MI or MX (or both). However, kinematic analysis of the scattering and results of chemiluminescence experiments indicate that (5) MX is the principal product. The electron‐jump model for alkali reactions again offers a plausible chemical basis for the observed reaction dynamics, including Property (5). A qualitative molecular‐orbital argument predicts that the attacking alkali atom transfers its valence electron to an antibonding $\mathrm{\sigma ̄}$ orbital of IX which is made up primarily of the ${\text{5p}}_z$ orbital of the I atom, but the charge shifts to the X atom as the intermediate (IX)⁻ molecule–ion dissociates in the field of the incoming M⁺ ion.