SIMBAD references

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 C₂H(X²Σ⁺) molecule by collision with He is investigated. State-to-state collisional excitation cross-sections between the 25 first fine structure levels of C₂ H are calculated for energies up to 800/cm which yields after thermal averaging rate coefficients up to T= 100 K. 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.