Molecular beam kinetics: Optical model for reactive scattering of alkali atoms and alkyl iodides

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Abstract
Angular distributions of reactively scattered alkali iodides have been measured for reactions of thermal beams of Rb and Cs atoms with beams of alkyl iodides RI, where R=CH3, C2H5, n‐ and i‐C3H7, n‐C4H9, and n‐C5H11. When shifts expected from kinematic differences are taken into account, the qualitative features are found to confirm the results previously obtained for the analogous K atom reactions. The reaction cross sections increase with the size of the alkali atom, from ∼35 Å2 for K+CH3I to ∼45 Å2 for Rb and ∼75 Å2 for Cs. The fall off in intensity of elastically scattered alkali atoms at wide angles also increases for K→Rb→Cs. The anisotropic angular distribution of products, the magnitude of the reaction cross section, and the fall off of the wide‐angle elastic scattering for the K, Rb, and Cs systems are correlated in terms of an extended optical model which simply assumes hard sphere interactions for both the entrance and exit trajectories. The reaction probability derived from the data by use of the optical model is large only for collisions which reach the repulsive wall region but even there it is well below unity for thermal energy collisions.