Semiempirical potential surfaces for the alkali–halogen molecule reactions

Abstract

Potential surfaces for the alkali (Li,Na,K,Rb), halogen–molecule (F₂,Cl₂,Br₂,I₂) reactions are presented. The surfaces are constructed by a semiempirical method based on valence bond formalism developed previously by the authors. The method, contrary to diatomics in molecules, uses as input only the ground diatomic potentials. A simplified one‐covalent–two‐ionic configuration model is used to produce the three lowest adiabatic surfaces. Alternatively, the separate covalent ionic and off‐diagonal terms, which are useful to charge exchange colisions, can be given. Comparison with ab initio results for the Li+F₂ are very encouraging. All surfaces are found to have wells in the T‐shape configuration and except for the F₂ case, to have a barrier of a few kcal/mol in the entrance channel. The barrier is minimal in the collinear geometries. The effect of a collinear transition state and a T‐shape well is shown to lead to a systematic change in the differential cross sections of the K+X₂ and Rb+X₂ reactions, as the halogen molecule becomes heavier.