Rate coefficients for rotational state-to-state transitions in H₂O + H₂ collisions as predicted by mixed quantum–classical theory

Chemistry Department, Wehr Chemistry Building, Marquette University, Milwaukee, Wisconsin 53201-1881, USA

^★ Corresponding author; dmitri.babikov@mu.edu

Received: 23 August 2024
Accepted: 13 November 2024

Abstract

Aims. A new dataset of collisional rate coefficients for transitions between the rotational states of H₂O collided with H₂ background gas is developed. The goal is to expand over the other existing datasets in terms of the rotational states of water (200 states are included here) and the rotational states of hydrogen (10 states). All four symmetries of ortho- and para-water combined with ortho- and para-hydrogen are considered.

Methods. The mixed quantum–classical theory of inelastic scattering implemented in the code MQCT was employed. A detailed comparison with previous datasets was conducted to ensure that this approximate method was sufficiently accurate. Integration over collision energies, summation over the final states of H₂, and averaging over the initial states of H₂ was carried out to provide state-to-state, effective, and thermal rate coefficients in a broad range of temperatures.

Results. The rate coefficients for collisions with highly excited H₂ molecules are presented for the first time. It is found that rate coefficients for rotational transitions in H₂O molecules grow with the rotational excitation of H₂ projectiles and exceed those of the ground state H₂, roughly by a factor of two. These data enable a more accurate description of water molecules in high-temperature environments, where the hydrogen molecules of background gas are rotationally excited, and the H₂O + H₂ collision energy is high. The rate coefficients presented here are expected to be accurate up to the temperature of ~2000 K.