The study of the energetic processing, i.e. formation, destruction and reactivity of interstellar polycyclic aromatic hydrocarbons (PAHs) is a current extensive research area. Key questions that are addressed concern in particular the role of PAH in the formation of small molecules such as H2 and the potential link between PAH and the formation of neutral or cationic fullerenes, that have been detected in the ISM. Indeed, although astrophysical PAHs can be seen as mostly remaining at very low energy in their ground electronic and vibrational state, they may very rarely receive a large amount of energy : they can be submitted to photon irradiation coming from stars or collide with hot gas, energetic/ionized particles depending on the local environment. Improving the understanding of these processes requires the synergy of experimental, astrophysical and chemical modelling, and we will focus on the latter. Given the size of the systems, an explicit description of the electronic structure requires the use of methods based on Density Functional Theory (DFT). A common approach consists in determining DFT static reaction paths to get insights into isomerisation and dissociation mechanisms. Such an approach is often used to complement UV-photodissociation experiments. We will mention the particular the recent example of the dissociation of the dibenzopyrene cationic isomers. However, many isomerisation reactions (H-shifts, ring openings, Stone-Wales isomers…) are likely to occur and an exhaustive study with such an approach becomes out-of-reach. An interesting alternative is to run extensive molecular dynamics (MD) simulations, with the electronic structure computed on-the-fly using the Self-Consistent Charge Density Functional based Tight Binding method (SCC-DFTB), an approximate DFT scheme. Such simulations, achieved at several energies, allow to get insights into isomerisation and dissociation mechanism, and to determine kinetic rates and branching ratios. We will give some examples of MD/SCC-DFTB studies of PAHs+. We will discuss the limits of the approach and to which extent they can be used to interpret experimental results.

Theoretical studies of PAHs’ energetic processing: Isomerisation and dissociation

256th National Meeting (2018)

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