Rotational quenching of rotationally-excited H$_2$O in collisions with He

TL;DR

This paper presents detailed theoretical calculations of rotational quenching cross sections and rate coefficients for water molecules colliding with helium, covering a wide range of excited states and temperatures relevant to astrophysics.

Contribution

It provides new state-to-state and total deexcitation rate coefficients for ortho- and para-H$_2$O due to He collisions, including low-temperature data and comparison with previous results.

Findings

Good agreement with previous high-temperature results

Significant discrepancies at low temperatures due to potential energy surface differences

Comprehensive data for a wide range of rotational states and temperatures

Abstract

Theoretical rotational quenching cross sections and rate coefficients of ortho- and para-H$_2$O due to collisions with He atoms are presented. The complete angular momentum close-coupling approach as well as the coupled-states approximation for angular momentum decoupling were applied to solve the scattering problem for a large range of rotationally-excited states of water. Results are obtained for quenching from initial levels 1$_{1,0}$, 2$_{1,2}$, 2$_{2,1}$, 3$_{0,3}$, 3$_{1,2}$, 3$_{2,1}$, 4$_{1,4}$, 3$_{3,0}$, and 4$_{2,3}$ of ortho-H$_2$O and from initial levels 1$_{1,1}$, 2$_{0,2}$, 2$_{1,1}$, 2$_{2,0}$, 3$_{1,3}$, 3$_{2,2}$, 4$_{0,4}$, 4$_{1,3}$, and 3$_{3,1}$ of para-H$_2$O for kinetic energies from 10$^{-5}$ to 10$^4$ cm$^{-1}$. State-to-state and total deexcitation cross sections and rate coefficients for temperatures between 0.1 and 3000 K are reported. The present state-to-state rate coefficients are found to be in good agreement with previous results obtained by Green and coworkers at high temperatures, but significant discrepancies are obtained at lower temperatures likely due to differences in the adopted potential energy surfaces. Astrophysical applications of the current rate coefficients are briefly discussed.