Hyperfine excitation of linear molecules by para- and ortho-H2: application to the HCl-H2 system.

The determination of hyperfine structure resolved excitation cross sections and rate coefficients due to H2 collisions is required to interpret astronomical spectra. In this paper, we present several theoretical approaches to compute these data. An almost exact recoupling approach and approximate sudden methods are presented. We apply these different approaches to the HCl-H2 collisional system in order to evaluate their respective accuracy. HCl-H2 hyperfine structure resolved cross sections and rate coefficients are then computed using recoupling and approximate sudden methods. As expected, the approximate sudden approaches are more accurate when the collision energy increases and the results suggest that these approaches work better for para-H2 than for ortho-H2 colliding partner. For the first time, we present HCl-H2 hyperfine structure resolved rate coefficients, computed here for temperatures ranging from 5 to 300 K. The usual Δj1 = ΔF1 propensity rules are observed for the hyperfine transitions. The new rate coefficients will significantly help the interpretation of interstellar HCl emission lines observed with current and future telescopes. We expect that these new data will allow a better determination of the HCl abundance in the interstellar medium, that is crucial to understand the interstellar chlorine chemistry.