Semiempirical potential surfaces for the alkali-halogen molecule reactions

Potential surfaces for the alkali (Li,Na,K,Rb), halogen-molecule (F ₂,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.