Theoretical study of an unusual reactive collision Cs(7p)+H2→CsH+H. Diabatic approach of the collinear collision potential energy surfaces

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Abstract
The Cs(7p)+(X 1∑+g, v=0) H2→(X 2∑+, v=0) CsH+H reactive collision was recently experimentally observed from a crossed beam experiment by Crépin et al. This reaction is rather unusual since it starts from a highly excited state of the system (11th potential surface) and must reach the ground state surface in the product channel without energy loss. Accurate nonempirical CI calculations of the potential energy surface for the collinear collision are reported, using large basis sets and a nonempirical relativistic pseudopotential for the Cs atom. The adiabatic potential surfaces, which exhibit irregular behavior, are reinterpreted in terms of an ab initio nearly diabatic effective Hamiltonian spanned by five neutral repulsive channels Cs(6s)×H2, Cs(6p)×H2,..., Cs(7p)×H2 and an ionic very flat Cs+H−2 channel which tends to the product wave function (Cs+H−)×H. The intersection of these diabatic potential surfaces are accessible from the entrance energy and this picture supports a harpooning mechanism, the 7p electron jumping on the H2 system at RCsH ≊6 bohr, the reactive systems being those which remain diabatically on this ionic surface through the avoided crossings with the lower Cs(7s)×H2,...,Cs(6s)×H2 neutral surfaces. The electronic couplings between the diabatic states are given as functions of the RCsH and RHH coordinates and their behavior is qualitatively understood. The existence of accidental conical intersections between some neutral diabatic states and the ionic state potential surfaces is demonstrated; these conical intersections should increase the reaction cross section. This diabatic study confirms, therefore, the harpooning mechanism previously proposed to interpret this paradoxical photochemical reaction.