THE REACTION OF ARGON IONS WITH HYDROGEN AND DEUTERIUM MOLECULES BY CROSSED BEAMS - LOW-ENERGY RESONANCES AND ROLE OF VIBRONIC LEVELS OF THE INTERMEDIATE COMPLEX

In a crossed beam experiment, cross sections have been measured for th e ion-molecule reactions Ar++H-2 --> ArH++H and Ar++D2 --> ArD++D. Low collision energies (0.025 less-than-or-equal-to E less-than-or-equal- to 1 eV) and high resolution (DELTAE approximately 10 meV, half-width at half-maximum) have been obtained using the method of guiding the io n beam by an octopole field and the technique of supersonic beams for H-2 or D2. A structure in the energy dependence of cross sections has been found and attributed to a manifestation of vibronic resonances. C alculations are presented and compared to experimental findings to ill ustrate this effect, which arises because of the successive population of vibronic levels of the charge transfer complex Ar-H-2+ or Ar-D2+, which are the intermediates for these reactions. Empirical potential e nergy surfaces for the entrance channels have been constructed account ing explicitly for the open shell nature and spin-orbit effects in Ar(2P(J)); symmetry considerations have also been used to establish the sequence of pertinent vibronic surfaces of the charge transfer interme diate complex - the role of configuration interaction in the latter is also discussed. The reaction dynamics has been treated as a sequence of nonadiabatic transitions at crossings of potential energy surface - quantum mechanical tunneling has been found crucial for the proper de scription of the observed energy dependence of the cross sections and the vibronic resonance structure. A higher frequency structure, borne out by the calculations and due to a manifold of metastable states sup ported by the vibronic levels of the intermediate charge transfer comp lex, appears to be washed out by the finite experimental resolution. I t is also shown that finite experimental resolution had been the reaso n for the failure of detecting vibronic resonances in previous experim ents and that the present ones are in general agreement with them when resolution is artificially lowered. Finally, it is pointed out that t he present approach, when applied to charge transfer processes, provid es a model which appears consistent with existing measurements. It als o accounts for the observed selective reactivity of the fine structure components of argon ions.