Rotational state-to-state transition rate coefficients for H₂O + H₂O collisions at nonequilibrium conditions

¹ Chemistry Department, Wehr Chemistry Building, Marquette University, Milwaukee, Wisconsin 53201-1881, USA
e-mail: dmitri.babikov@marquette.edu
² Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziącka Street 5, 87-100 Toruń, Poland
³ Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Institute for Astrophysics and Computational Sciences, The Catholic University of America, Washington, DC 20064, USA
⁵ University of Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France

Received: 16 May 2024
Accepted: 5 July 2024

Abstract

Aims. The goal is to develop a database of rate coefficients for rotational state-to-state transitions in H₂O + H₂O collisions that is suitable for the modeling of energy transfer in nonequilibrium conditions, in which the distribution of rotational states of H₂O deviates from local thermodynamic equilibrium.

Methods. A two-temperature model was employed that assumed that although there is no equilibrium between all possible degrees of freedom in the system, the translational and rotational degrees of freedom can be expected to achieve their own equilibria independently, and that they can be approximately characterized by Boltzmann distributions at two different temperatures, T_kin and T_rot.

Results. Upon introducing our new parameterization of the collisional rates, taking into account their dependence on both T_kin and T_rot, we find a change of up to 20% in the H₂O rotational level populations for both ortho and para-H₂O for the part of the cometary coma where the nonequilibrium regime occurs.