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 H 2 –He pairs, we compute the rototranslational collision-induced absorption coefficient at 40-400 K for frequencies covering 0-4000 cm −1 and are expected to be accurate to 2% or better up to 2500 cm −1 . • Previously available rototranslational absorption coefficients for H 2 –H 2 pairs are also extended up to 4000 cm −1 . • Previous isotropic potential calculations for H 2 –He and H 2 –H 2 in the rovibrational band have been extended to complement the rototranslational data. • The absorption coefficients are tabulated for ortho -to- para ratios from normal-H 2 to pure para -H 2 , as well as equilibrium-H 2 , over 40-400 K, with the updates changing brightness temperature difference of a fraction of a degree in the rototranslational region as much as 4 degrees in the rovibrational region. • Our state-of-the-art modifications correct an otherwise +2% error in determining the He/H 2 ratio in Uranus from its spectrum alone. Using state-of-the-art ab initio interaction-induced dipole and potential-energy surfaces for hydrogen–helium (H 2 –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 H 2 –H 2 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 H 2 –He (Gustafsson et al., J. Chem. Phys . 113 , 3641 (2000)) and H 2 –H 2 (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-H 2 to pure para -H 2 , as well as equilibrium-H 2 , 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/H 2 ratio in Uranus from its spectrum alone.