A High-Resolution Linelist for Molecular Oxygen

Molecular oxygen (O₂) is of particular interest in exoplanetary observations not least of all as an important biosignature on habitable planets. The atmospheric absorption bands are well studied, but a complete and accurate, high-resolution linelist is yet to be produced. Owing to their symmetry, the commonly employed electric dipole approximation is not valid for homonuclear diatomic molecules, and their rovibrational spectra are instead dominated by higher order transitions moments. These higher-order moments, such as the electric quadrupole and magnetic dipole, give rise to transition linestrengths that are orders of magnitude weaker than typical electric dipole transitions. Although such transitions are observable for atmospheric path lengths, their weak nature makes laboratory measurements especially challenging. In this work we develop and apply ab initio computational techniques to produce an accurate electric quadrupole spectrum of molecular oxygen presented for use in atmospheric retrievals across a range of temperatures, and made available through the ExoMol database.