  Despite being one of the weakest dimers in nature, low-spectral-resolution
Voyager/IRIS observations revealed the presence of (H$_2$)$_2$ dimers on
Jupiter and Saturn in the 1980s. However, the collision-induced H$_2$-H$_2$
opacity databases widely used in planetary science (Borysow et al., 1985; Orton
et al., 2007; Richard et al., 2012) have thus far only included free-to-free
transitions and have neglected the contributions of dimers. Dimer spectra have
both fine-scale structure near the S$(0)$ and S$(1)$ quadrupole lines (354 and
587 cm$^{-1}$, respectively), and broad continuum absorption contributions up
to $\pm50$ cm$^{-1}$ from the line centres. We develop a new ab initio model
for the free-to-bound, bound-to-free and bound-to-bound transitions of the
hydrogen dimer for a range of temperatures (40-400 K) and para-hydrogen
fractions (0.25-1.0). The model is validated against low-temperature laboratory
experiments, and used to simulate the spectra of the giant planets. The new
collision-induced opacity database permits high-resolution (0.5-1.0 cm$^{-1}$)
spectral modelling of dimer spectra near S$(0)$ and S$(1)$ in both Cassini
Composite Infrared Spectrometer (CIRS) observations of Jupiter and Saturn, and
in Spitzer Infrared Spectrometer (IRS) observations of Uranus and Neptune for
the first time. Furthermore, the model reproduces the dimer signatures observed
in Voyager/IRIS data near S$(0)$ (McKellar et al., 1984) on Jupiter and Saturn,
and generally lowers the amount of para-H$_2$ (and the extent of
disequilibrium) required to reproduce IRIS observations.

Earth and Planetary Astrophysics