Improved H2–He and H2-H2 collision-induced absorption models and application to outer-planet atmospheres

Using state-of-the-art ab initio interaction-induced dipole and potential-energy surfaces for hydrogen–helium (H₂–He) pairs, we compute the rototranslational collision-induced absorption coefficient at 40-400 K for frequencies covering 0-4000 cm⁻¹. The quantum mechanical scattering calculations account for the full anisotropic interaction potential, replacing the isotropic approximation. The absorption data are expected to be accurate with an uncertainty of 2% or better up to 2500 cm⁻¹. The uncertainty is slightly higher at the highest frequencies where the rototranslational absorption is largely obscured by the rovibrational band. Our improved agreement with measurements at 200-800 cm⁻¹ result from the improvement of the potential energy surface. The previously available rototranslational data set for H₂–H₂ pairs (Fletcher et al., Astrophys. J. Supp. 235, 24 (2018)) is also extended up to 4000 cm⁻¹. In the rovibrational band previous isotropic potential calculations for H₂–He (Gustafsson et al., J. Chem. Phys. 113, 3641 (2000)) and H₂–H₂ (Borysow, Icarus 92, 273 (1992)) have been extended to complement the rototranslational data set. The absorption coefficients are tabulated for ortho-to-para ratios from normal-H₂ to pure para-H₂, as well as equilibrium-H₂, over 40-400 K . The effect of these updates are simulated for the cold atmosphere of Uranus and warmer atmosphere of Jupiter. They are equivalent to a brightness temperature difference of a fraction of a degree in the rototranslational region but up to 4 degrees in the rovibrational region. Our state-of-the-art modifications correct an otherwise +2% error in determining the He/H₂ ratio in Uranus from its spectrum alone.