Improved H2-He and H2-H2 Collision-Induced Absorption Models and Application to Outer-Planet Atmospheres

TL;DR

This paper presents advanced collision-induced absorption models for H2-He and H2-H2 pairs, improving accuracy for planetary atmosphere analysis and extending data sets up to 4000 cm-1 with applications to Uranus and Jupiter.

Contribution

The study introduces improved ab initio interaction potentials and quantum calculations for collision-induced absorption, extending existing data sets and enhancing modeling accuracy for planetary atmospheres.

Findings

Achieved 2% or better accuracy in absorption coefficients up to 2500 cm-1.

Extended H2-H2 absorption data set up to 4000 cm-1.

Corrected +2% error in Uranus He/H2 ratio determination.

Abstract

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 results 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. Physics. 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.