Deciphering the infrared emission of astro-PAHs: the interplay between laboratory experiments, quantum chemistry and modelling
The aromatic infrared bands (AIBs) are major dust emission features that Astronomers aim to use to trace variations in the physical and chemical conditions in regions of star formation from the small scales of protoplanetary disks to the large scales of galaxies. The AIBs also carry information on the chemical complexity of their emitters, which mainly consist of polycyclic aromatic hydrocarbons (PAHs) that are heated by the absorption of ultraviolet (UV) photons. A detailed analysis of the AIB spectra requires taking into account molecular diversity together with excitation conditions. In heated molecules, vibrational coupling (anharmonicity) plays a major role, affecting the band positions and widths  and leading to a number of new bands such as combination/difference bands and overtones. Disentangling temperature effects from chemical complexity requires a detailed comparison of the observed spectra with synthetic spectra that model the emission of a given PAH molecule in a given UV-visible astrophysical radiation field.
The necessary molecular data ingredients for the model are the UV-visible photoabsorption spectrum, the vibrational modes and IR absorption cross-sections, and in principle rotational constants although rotational broadening can be neglected in most cases except for low frequency bands . These data have been and are still being gathered in several dedicated databases [3,4]. However, anharmonic effects are poorly covered and need to be included to generate the simulated IR spectra that can be compared to the astronomical AIB spectra. Obtaining empirical anharmonic parameters that describe the evolution of the band positions and widths with temperature has been the focus of our recent research both using experimental methods  and quantum chemistry calculations . An emission code is also being developed in order to build a simulator for a fast calculation of the emission of a given PAH in a given radiation field. The simulator will provide a library of synthetic AIB spectra (the LAIBrary project) based on theoretical and experimental molecular data, at the accuracy and sensitivity level required to match coming James Webb Space Telescope (JWST) observations such as the Early Release Science programme “Radiative feedback from massive stars” (http://www.jwst-ism.org).
Support from CNES (ID 5830) and the Nanocosmos ERC Synergy project (G.A. 610256) is acknowledged.
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