The collisional excitation of CS by H₂O for cometary applications

Potential energy surface, rate coefficients, and non-local thermodynamic equilibrium modeling

¹ Univ. Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
² Centro de Astrobiología (CAB, CSIC-INTA), Ctra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
³ Joint Quantum Institute, Department of Physics, University of Maryland, College Park, MD 20742, USA
⁴ Nicolaus Copernicus University in Toruń, Faculty of Chemistry, Gagarina 7, 87-100 Toruń, Poland

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 October 2025
Accepted: 5 December 2025

Abstract

Aims. Collisional rate coefficients with H₂O as the collisional partner for cometary applications are usually missing from the literature due to the high computational cost of such calculations. One notable example is CS, which is among the dominant sulfur-bearing species observed in comets, whose formation path remains to be elucidated.

Methods. To allow for the accurate determination of the physical conditions of comets where CS is detected, we conducted a study on the collisional excitation of CS induced by H₂O.

Results. The first CS–H₂O potential energy surface (PES) was computed using the symmetry adapted perturbation theory along with the density functional theory method (SAPT-DFT). The scattering calculations were performed using the statistical adiabatic channel model (SACM) approach. The rate coefficients were employed in a non-local thermodynamic equilibrium (LTE) excitation model to discuss the impact of the new data on the modeling of CS in cometary comae where H₂O is dominating.

Conclusions. The first CS–H₂O rate coefficients were provided for the 5–100 K temperature range, including all CS levels up to j_CS = 18. It was found that using CS–H₂ rate coefficients or the global cross-section approach to supplement for missing CS–H₂O data are not expected to perform well. Among all approximations, using CO–H₂O rate coefficients – CO being chemically similar to CS – is expected to be the most accurate approximation. Computing collisional rate coefficients with H₂O projectiles to address the lack of cometary data is a highly demanding task. In this work we used the SACM approach to compute the CS–H₂O data, opening the way for further calculations for cometary applications. Further testing of the impact of the approximations used to compensate for this absence in the analysis of cometary observations is required to draw strong conclusions about the inaccuracies they may introduce.