Summary of the project

  • Polycyclic Aromatic Hydrocarbons Reactivity in Cryogenic Solids / Réactivité des PAH en matrice cryogénique et sur la glace
  • ANR-13-BS08-0005

Polycyclic aromatic hydrocarbons (PAHs) are organic macromolecules that have an astrophysical interest, since the discovery of the AIBs (aromatic interstellar bands), assigned to vibration modes of PAHs, which emit in the infrared (IR) after absorption of star UV light. PAHs are also candidates for diffuse interstellar bands (DIBs), which are weak absorption bands measured in the visible range. However, no specific PAH has been identified yet, and a variety of PAH-derived species (ionized, substituted, coordinated) have been proposed. The ambition of the PARCS project is to bring new elements for the understanding of these assignments through the study of: 1) Reactions of PAHs with Fe atoms and aggregates. The hypothesis of the formation of metal complexes Fex(PAH)y was proposed to account for the depletion of Fe from the gaseous phase of the interstellar medium, but no study has validated this hypothesis for neutral Fex(PAH)y complexes yet. 2) Photo-reactions of PAHs embedded in or trapped on water ice. In dense molecular clouds, atoms and molecules are condensed on cold dust and ice particles, where they are transformed by thermal and photochemical processes, leading to the formation of more complex molecules, some of them being of astrobiological interest. IR spectroscopy, associated to matrix isolation techniques, is the main tool used to study the composition of interstellar ices, and has allowed to identify simple molecules (NH3, CO2, OCS, CH3OH) and to understand the formation of more complex species. Although IR spectroscopy of ice has been performed for a long time, studies of ices containing PAHs have just begun and have shown that neutral PAHs may play an important and unexpected role in cosmic ice chemistry. For instance, IR identification of quinones, alcohols and ketones through photo-irradiation of coronene or pyrene on amorphous solid water has been published in 2011 by Partner 1. The originality of this project is to propose pioneering joint theoretical and experimental treatments of these reactions in order to understand how these molecules are formed. In the experimental part, the challenging novelty is the study of ternary PAH/Fe/H2O systems at cryogenic temperatures, that will be made by means of a specially designed furnace fixed to the cryostat, allowing co-deposition of Fe and PAH, and of two spectrometers, allowing to run both IR and UV spectroscopy on the same sample.In the theoretical part (Partners 2 and 3), the challenging goal is to describe the dynamic processes that drive these reactions, namely (i) characterize the reaction products, through their IR and electronic spectra, and (ii) achieve molecular dynamics (MD) simulations to characterize the mechanisms, in the ground- and excited-state after electronic excitation. This implies the elaboration of a multiscale-multimethod approach that uses some of the best current available techniques with various levels of sophistication and efficiency. The difficulty arises from the large size of the systems, their complex electronic structure (open-shell), the nature of the interactions (van der Waals, H-bonds, …) and the efficiency of the MD schemes to achieve statistical significance. Ab initio wavefunction methods (with local orbitals), DFT and approximate-DFT approaches will be used with cross-benchmarking. Efforts will also be dedicated to describe anharmonic and nuclear quantum effects on IR spectra using classical and quantum dynamics, and to the elucidation of reactive mechanisms via MD simulations. Finally, the influence of the environment (matrix, ice) will be focused on via combined orbital/force field methods. Thanks to the synergy of both approaches, this project will contribute to the assignment of the AIBs and DIBs and to the understanding of the role of water-ice in the photochemical reactions of PAHs with species of astrophysical and environmental interest (Fe, H2O). New experimental and theoretical developments, mandatory to achieve these goals, will be proposed.