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Étude de la dissociation des hydrocarbures aromatiques polycycliques dans un contexte astrophysique

Abstract : Polycyclic Aromatic Hydrocarbons (PAHs) are revealed in astrophysical environments thanks to their characteristic infrared features that are emitted following the absorption of vacuum ultraviolet (VUV) photons 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 dissociation rates from their fitting using an RRKM model. 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 importance of structure and non-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 that would need to be considered while evaluating the contribution of PAHs to the formation of H2 in PDRs. This mechanism involves specific structures with bay areas that would need to be studied on a larger sample of PAHs.
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Sarah Rodriguez Castillo. Étude de la dissociation des hydrocarbures aromatiques polycycliques dans un contexte astrophysique. Astrophysique [astro-ph]. Université Toulouse 3 – Paul Sabatier, 2018. Français. ⟨tel-02174638⟩



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