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

Abstract

In a crossed beam experiment, cross sections have been measured for the ion–molecule reactions Ar++H2→ArH++H and Ar++D2→ArD++D. Low collision energies (0.025≤E≤ 1 eV) and high resolution (ΔE∼10 meV, half-width at half-maximum) have been obtained using the method of guiding the ion beam by an octopole field and the technique of supersonic beams for H2 or D2. A structure in the energy dependence of cross sections has been found and attributed to a manifestation of vibronic resonances. Calculations are presented and compared to experimental findings to illustrate this effect, which arises because of the successive population of vibronic levels of the charge transfer complex Ar–H2+ or Ar–D2+, which are the intermediates for these reactions. Empirical potential energy surfaces for the entrance channels have been constructed accounting explicitly for the open shell nature and spin–orbit effects in Ar+(2PJ); symmetry considerations have also been used to establish the sequence of pertinent vibronic surfaces of the charge transfer intermediate 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 surfaces—quantum mechanical tunneling has been found crucial for the proper description 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 supported by the vibronic levels of the intermediate charge transfer complex, appears to be washed out by the finite experimental resolution. It is also shown that finite experimental resolution had been the reason for the failure of detecting vibronic resonances in previous experiments and that the present ones are in general agreement with them when resolution is artificially lowered. Finally, it is pointed out that the present approach, when applied to charge transfer processes, provides a model which appears consistent with existing measurements. It also accounts for the observed selective reactivity of the fine structure components of argon ions.