Understanding the destruction of CH⁺ with atomic hydrogen at low temperatures: a non-adiabatic dynamical study

Carbon hydrides play a crucial role in the formation of complex organic molecules in highly UV illuminated regions of the interstellar medium (ISM). The formation of CH⁺ is the first step in the reactions leading to the formation of various carbon hydrides. CH⁺ formation is relatively well understood with strong agreement between theoretical and experimental results. However, its destruction by collision with the H atom, at low temperatures of interest in the ISM, is in contrast still not well understood and there is a large discrepancy between theoretical and experimental data [R. Plasil <i>et al.</i>, <i>AstroPhys. J.</i>, 2011, <b>737</b>, 1], which are almost an order of magnitude smaller than various classical and quantum mechanical calculations. In this work we have computed and fitted a new set of non-adiabatic potential energy surfaces (PES) for the title system, including the three lower adiabatic states. We then investigate three possible sources of disagreement with the experimental results: non-adiabatic effects from regions near the conical intersections, and rotational and vibrational excitation of the CH⁺ molecule. We conclude that vibrational excitation of the CH⁺ plays a major role in reducing the reactivity at low temperatures, and we raise the question of whether vibrational thermalization of the CH⁺ is not fully achieved in the experiment. Such non-thermalized conditions could explain the decrease of the measured reaction rate constant.