Collisional excitation of interstellar HCl by He




Modelling of molecular emission from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. This paper deals with the determination of rate coefficients for the rotational and hyperfine excitation of the HCl molecule in its ground vibrational state due to collisions with He. Calculations of pure rotational (de-)excitation cross-sections of HCl by He were performed using the essentially exact close-coupling method. The calculations are based on a new potential energy surface obtained from highly correlated ab initio calculations. Cross-sections for transitions among the 11 first rotational levels of HCl were calculated for total energies up to 3000 cm−1. The hyperfine cross-sections are then obtained using a recoupling technique. The rotational and hyperfine cross-sections are used to determine collisional rate coefficients for temperatures ranging from 5 to 300 K. A clear propensity rule in favour of odd Δj rotational transitions is observed. The usual ΔjF propensity rule is observed for the hyperfine transitions. The new rate coefficients are compared with the previous results obtained for the HCl molecule. Significant differences are found, mainly due to the use of a new potential energy surface. The new rate coefficients will significantly help in interpreting HCl emission lines observed with current and future telescopes.