In this paper Mille's S-matrix variational approach has been extended to study the reaction dynamics for ion-pair formation processes. M＋X~2→M^+＋X~2^- reaction system involves two potential energy surfaces, i.e., the covalence state (M＋X~2) and the ionic state (M^+＋X~2^-) and their crossing effect. The working equations for calculating ion-pair formation cross section have been derivated based on the above two-state model. The translational wavefunctions have been expanded by distributed Gaussian basis sets in calculation of matrix elements. Satisfactory results of numberical calculations using S-matrix variational approach for Cs＋O~2→Cs^+＋O~2^- ion-pair formation system have been obtained.

Key words: ION PAIRS, REACTION KINETICS, VARIATIONAL METHOD, MATRICES, CROSS PROFILE