Rotational excitation of SiS molecules by collisions with He atoms

¹ LERMA and UMR 8112 of CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France e-mail: francois.lique@obspm.fr
² DAMIR, Instituto de Estructura de la Materia, CSIC, Calle Serrano 121, 28006 Madrid, Spain
³ Department of Chemistry and Biochemistry, University of Maryland, College park, Maryland 20742-2021, USA

Received: 14 May 2007
Accepted: 21 June 2007

Abstract

Context.Observations of molecular emission at millimeter and infrared wavelengths, supplemented by careful and detailed modeling, are powerful tools for investigating the physical and chemical conditions of astrophysical objects. Modeling of molecular emission requires excitation calculations using radiative as well as collisional rates with the most abundant species.

Aims.The present paper focuses on the calculation of rate coefficients among the first rotational levels of the SiS molecule in its ground vibrational state in collision with He.

Methods.A new accurate two dimensional potential energy surface system was calculated for the SiS-He system using highly correlated ab initio calculations. Dynamical calculations of pure rotational (de)excitation of SiS by collisions with He were performed for the first rotational levels using the near exact close-coupling approach.

Results.Collisional cross sections among the 26 first rotational levels of SiS were calculated for kinetic energies up to 1500 cm^-1. Rate coefficients between the rotational levels were calculated for temperatures ranging from 5 to 200 K. A propensity rule that favors even transitions is found and is explained by the small asymmetry of the SiS-He potential energy surface.