Fine and hyperfine excitation of C 2H by collisions with He at low temperature

Abstract

Modelling of molecular emission from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. From a new, highly correlated, two-dimensional potential energy surface, rotational excitation of the C 2H(X 2Σ +) molecule by collision with He is investigated. State-to-state collisional excitation cross-sections between the 25 first fine structure levels of C 2H are calculated for energies up to 800cm -1 which yields after thermal averaging rate coefficients up to T= 100K. The exact spin splitting of the energy levels is taken into account. The recoupling technique introduced by Alexander & Dagdigian allows us to deduce the corresponding temperature-dependent hyperfine state-to-state rate coefficients. Propensity rules are discussed. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.