Rotational excitation of H3O+ cations by para-H2: improved collisional data at low temperatures

Abstract

ABSTRACT The hydronium cation plays a crucial role in interstellar oxygen and water chemistry. While its spectroscopy was extensively investigated earlier, the collisional excitation of H3O+ is not well studied yet. In this work, we present state-to-state collisional data for the rotational de-excitation of both ortho- and para-H3O+ due to para-H2 impact. The cross sections are calculated within the close-coupling formalism using our recent, highly accurate, rigid-rotor potential energy surface for this collision system. The corresponding thermal rate coefficients are computed up to 100 K. For para-H3O+, the lowest 20 rotation-inversion states were considered in the calculations, while for ortho-H3O+, the lowest 11 states are involved (up to j ≤ 5), so all levels with rotational energy below 420 K (292 cm−1) are studied. In order to analyse the impact of the new collisional rate coefficients on the excitation of H3O+ in astrophysical environments, radiative transfer calculations are also provided. The most relevant emission lines from an astrophysical point of view are studied, taking into account the transitions at 307, 365, 389, and 396 GHz. We show that our new collisional data have a non-negligible impact (from a few per cents up to about a factor of 3) on the brightness and excitation temperatures of H3O+, justifying the revision of the physical conditions in the appropriate astrophysical observations. The calculated rate coefficients allow one to recalculate the column density of hydronium in interstellar clouds, which can lead to a better understanding of interstellar water and oxygen chemistry.