Rovibrational excitation of HCl in collisions with H₂ and He

In this work we report rotational and vibrational excitation rates for collisions between HCl and H₂ or He. Accurate excitation rates for collisions involving H₂ or He are needed to interpret microwave and infrared observations of the interstellar gas for non-LTE line formation. HCl has been detected in the atmospheres of some planets, as well as in interstellar clouds. It is an important tracer of chlorine and can be used to constrain the chlorine elemental abundance and isotope ratios. We present scattering calculations of HCl with H₂ using a new full-dimensional potential energy surface. State-to-state rate coefficients for rovibrational transitions were calculated using a quantum close-coupling method for vibrational quenching in HCl( v₁=1, j₁ )+H₂( v₂=0, j₂ ) → HCl( v₁'=0, j₁' )+H₂( v₂'=0, j₂') collisions, with j₁=0-5. Rate coefficients ranging from 5 to 1000 K are presented for para-H₂ ( j₂=0 ) and ortho-H₂ ( j₂=1 ) collision partners. These results are compared with those obtained using the 5D coupled-states approximation; this approximation was used to compute rate coefficients for rovibrational transitions between HCl states with v₁ = 0-2 for temperatures between 10 and 3000 K. We also present results involving collisions of HCl and He. These results are an extension of our previous calculation of state-to-state rotational quenching rate coefficients for HCl with initial rotational levels up to j=30 for temperatures between 0.1 and 3000 K using the SAPT potential. The close-coupling method and the coupled-states approximation are applied in calculations of rotational state-to-state cross sections.

Work at UGA is supported by NASA grant No. NNX16AF09G, at UNLV by NSF Grant No. PHY-1806334, and at Penn State by NSF Grant No. PHY-1806180.