Broadening of H$_2$O rotational lines by collision with He atoms at low temperature

TL;DR

This study provides detailed pressure broadening coefficients for water vapor's rotational lines caused by helium collisions at low temperatures, aiding astrophysical observations and future experiments.

Contribution

It offers the first comprehensive set of pressure broadening coefficients for H₂O:He collisions at low temperatures, combining experimental and theoretical data.

Findings

Broadening coefficients vary significantly between lines.

Coefficients increase markedly at lower temperatures.

Elastic contributions dominate the broadening effects.

Abstract

We report pressure broadening coefficients for the 21 electric-dipole transitions between the eight lowest rotational levels of ortho-H$_2$O and para-H$_2$O molecules by collisions with He at temperatures from 20 to 120 K. These coefficients are derived from recently published experimental state-to-state rate coefficients for H$_2$O:He inelastic collisions, plus an elastic contribution from close coupling calculations. The resulting coefficients are compared to the available experimental data. Mostly due to the elastic contribution, the pressure broadening coefficients differ much from line to line, and increase markedly at low temperature. The present results are meant as a guide for future experiments and astrophysical observations.