Unraveling the effect of reagent vibrational excitation on the scattering mechanism of the benchmark H + H → H + H hydrogen exchange reaction in the coupled 1E' ground electronic manifold.

The hydrogen exchange reaction, H + H → H + H, along with its isotopic variants, has been the cornerstone for the development of new and novel dynamical mechanisms of gas-phase bimolecular reactions since the 1930s. The dynamics of this reaction are theoretically investigated in this work to elucidate the effect of reagent vibrational excitation on differential cross sections (DCSs) in a nonadiabatic situation. The dynamical calculations are carried out using a time-dependent quantum mechanical method, both on the lower adiabatic potential energy surface and employing a two-state coupled diabatic theoretical model to explicitly include all the nonadiabatic couplings present in the 1E' ground electronic manifold of the H system. Towards this effort, the Boothroyd-Keogh-Martin-Peterson (BKMP2) surface of the lower adiabatic component is coupled with the double many-body expansion (DMBE) surface of the upper one. The smooth variation of energy along the seam of the conical intersections is explicitly confirmed. The coupled two-state calculations are performed only for H ( = 3-4, = 0), as the minimum of the conical intersections becomes energetically accessible for these vibrational levels in the considered energy range. Initial state-selected total and state-to-state DCSs are calculated to elucidate various mechanistic aspects of reagent vibrational excitation. The latter enhances the forward scattering and makes the backward scattering less prominent. Important roles of collision energy in the vibrational energy disposal of both forward- and backward-scattered products are examined. Analysis of the state-to-state DCSs of the vibrationally excited reagents reveals an important correlation among scattering angle, and the product rotational angular momentum and its helicity state. Such an analysis establishes a novel mechanism for the forward scattering of the reaction.