A High-Resolution Linelist for Molecular Oxygen

<p>Molecular oxygen (O<sub>2</sub>) 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.</p>

Multipolar terahertz absorption spectroscopy ignited by graphene plasmons

Terahertz absorption spectroscopy plays a key role in physical, chemical and biological systems as a powerful tool to identify molecular species through their rotational spectrum fingerprint. Owing to the sub-nanometer scale of molecules, radiation-matter coupling is typically dominated by dipolar interaction. Here we show that multipolar rotational spectroscopy of molecules in proximity of localized graphene structures can be accessed through the extraordinary enhancement of their multipolar transitions provided by terahertz plasmons. In particular, specializing our calculations to homonuclear diatomic molecules, we demonstrate that a micron-sized graphene ring with a nano-hole at the core combines a strong near-field enhancement and an inherently pronounced field localization enabling the enhancement of the dipole-forbidden terahertz absorption cross-section of $${{\rm{H}}}_{2}^{+}$$ H 2 + by 8 orders of magnitude. Our results shed light on the strong potential offered by nano-structured graphene as a robust and electrically tunable platform for multipolar terahertz absorption spectroscopy at the nanoscale.