Rotational excitation of small carbon chains by collisions with He in the ISM

¹ LERMA – UMR 8112, Observatoire de Paris, 5 place Jules Janssen, Meudon, France
² LOMC – UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP. 540, 76058 Le Havre, France
³ Departmento de Fisica y Quimica Teóricas, Instituto de Estructura de la Materia, CSIC, Calle Serrano 121, 28006 Madrid, Spain

Abstract

Modeling of molecular emission from interstellar clouds requires the calculation of rate coefficients for excitation by collision with the most abundant species. Based on new highly correlated potential energy surfaces, rotational excitation of C₂H and C₂H⁻ radicals in collision with He are investigated. C₂H fine and hyperfine structure is taken into account. As already found for other systems, rate coefficients for the C₂H⁻ ion are larger by a factor up to 3 compared to the neutral species.