New Nanocosmos Doctor

Some news from the young team surrounding our PI from Toulouse, Christine Joblin. Sarah Rodríguez Castillo (third on the left, first row) presented on October 30th 2018 her PhD thesis entitled “Study on the dissociation of astro-PAHs”, that was carried out at the University of Toulouse/CNRS in the framework of the Nanocosmos projet.

Congratulations on this great achievement, Doctor!


Polycyclic Aromatic Hydrocarbons (PAHs) are revealed in astrophysical environments thanks to their characteristic infrared emission after these molecules are subjected to the vacuum ultraviolet (VUV) radiation from nearby massive stars. This interaction regulates their charge state, stability and dissociation mechanisms, which in turn affect the energy balance and the chemistry of the gas in the interstellar medium. In particular, PAHs could contribute to the formation of the most abundant molecule, H2, in photodissociation regions (PDRs). This work aims at contributing to these topics by quantifying the VUV photoprocessing of specific medium-sized PAH cations through experimental studies complemented by computational investigations.The experimental results were gathered from two campaigns at synchrotron facilities: ion trap experiments allowed us to obtain the yields of ionization and fragmentation and the branching ratios between the different photoevents, while from iPEPICO spectroscopy we obtained breakdown curves and RRKM-fitted dissociation rates. We detail the case of the fragmentation processes of two isomers of dibenzopyrene cation (C24H14+) in order to assess the impact of structure on these processes. We present Density Functional Theory calculations and Molecular Dynamics simulations, which evidence the relevance of structure and planarity in these mechanisms and provide a better view on the dissociation pathways and energetics.This work brings significant new data for models that describe the chemical evolution of PAHs in astrophysical environments, including the first measurement of the ionization yield of medium-sized PAH cations as well as several dissociation rates. We also report a new mechanism, involving specific structures with bay areas, that would need to be considered while evaluating the contribution of PAHs to the formation of H2 in PDRs.

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