ExoMol: molecular spectra and for exoplanet and other hot atmospheresAllis Prize: Jonathan Tennyson*

The ExoMol database (www.exomol.com) provides comprehensive line lists of molecular transitions (line lists)  which are valid over extended temperatures  ranges; currently for 91 molecules (251 isotopologues) and a total of about 10¹³ transitions. The database stores  energy levels (hence transition  frequencies) and Einstein A coefficients with other properties, including lifetimes of individual states, temperature-dependent cooling functions, Lande g-factors, partition functions, cross sections,  and transition dipoles with phase relations.

  

The methodology used starts with detailed electronic structure calculations of potential energy and dipole moment surfaces, plus spin-orbit and other couplings as required.   Variational methods are used to treat the nuclear motion and potentials are generally tuned to reproduced observed data. Conversely transition intensities can often be predicted using ab initio dipole moments to accuracies competitive with the best experiments.  While for many molecules this procedure gives excellent results, this is not true in all cases. Particularly challenging  are diatomic molecules containing a transition metal, many of which have low-lying electronic states which absorb strongly in the atmospheres of cool stars and brown dwarfs, and are thought to be important in exoplanets. Current electronic structure programs struggle to give accurate results for these species and experimental data is usually at best partial.

The recent development of cross correlation spectroscopy to identify exoplanetary molecules has emphasized the need for highly accurate transition frequencies. In general these can only  be achieved using experimental data for which we use the MARVEL (measured active vibration rotation  energy levels) procedure. For some molecules, VO for example, it is necessary to explicitly include  hyperfine effects in the spectroscopic model. We have therefore extended our diatomic nuclear motion program Duo to include a full treatment of hyperfine interactions. The ExoMol data base is being extended to ultraviolet wavelengths which involves the consideration of  continuum absorption, predissociation and photodissociation.