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

Abstract

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 < E_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⁺ for the process H⁺ + H₂ (v = 0, j = 0–5) → H⁺ + H₂ (v' = 0, j'). Cross-sections are then computed from the converged reaction probabilities as a function of total angular momentum (J) 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 (j = 0 → j' = 1).