Ortho–Para Conversion for H⁺ + H₂ Collision in Low Temperature: A Fully Close-Coupled Time-Dependent Wave Packet Study

The collision-induced rate coefficients of ortho-para conversion for the H⁺ + H₂ reaction provide accurate information to probe the lifetime of cold environments in interstellar media. Rotationally resolved reaction probabilities are calculated at the low collision energy regime (0 < <i>E</i>_col ≤ 0.3 eV) by employing the coupled three-dimensional (3D) time-dependent wave packet (TDWP) formalism in hyperspherical coordinates on a recently constructed ab initio ground adiabatic potential energy surface of H₃⁺ [<i>J. Chem. Phys.</i> <b>2014</b>, <i>141</i>, 204306] for the process H⁺ + H₂ (<i>v</i> = 0, <i>j</i> = 0-5) → H⁺ + H₂ (<i>v</i>' = 0, <i>j</i>'). Cross-sections are then computed from the converged reaction probabilities as a function of total angular momentum (<i>J</i>) over the same energy regime and subsequently employed to obtain the rate constants for the ortho-to-para (O-P) and para-to-ortho (P-O) conversions and their ratio. The ratio of ortho-para conversion shows (a) appropriate convergence with the inclusion of a higher number of initial rotational states as well as a reasonable agreement with the results from a quantum statistical method and (b) a peak at lower temperature that could be due to the available collision energy for transitions involving lower rotational states (<i>j</i> = 0 → <i>j</i>' = 1).