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Photophysique des molécules polycycliques aromatiques hydrogénées d'intérêt interstellaire avec l'expérience PIRENEA

Abstract : One of the interesting discoveries of infrared astronomy is the ubiquitous presence of polycyclic aromatic hydrocarbons (PAHs) in interstellar and circumstellar environments. These macromolecules play a key role in the physics and chemistry of the interstellar medium (ISM). However, despite a lot of observational, laboratory and modelling studies, no definite identification of individual species has been possible yet. The identification process requires both an understanding of the nature of these large carbon molecules, which is driven by their formation and further evolution under the action of UV photodissociation and chemistry, and the search for specific spectroscopic fingerprints. These subjects are experimentally approached in this work,taking advantage of the PIRENEA set-up dedicated to astrochemistry. In the first part of this thesis a study of the photodissociation cascade of several medium-sized PAHs isolated in the ion trap of PIRENEA is performed. The aim of this study is to provide information on both destruction of PAHs by UV radiation and formation channels of small hydrocarbons and carbon clusters through destruction of a larger precursor. An inventory of the formed species is made and the main fragmentation patterns are identified. The second part of the thesis focuses on the visible spectroscopy of different PAH cations and dehydrogenated derivatives. Multiphoton dissociation spectroscopy is performed to measure the electronic spectra of these species. The experimental results are interpreted with the help of theoretical spectra calculated in the frame of the time-dependent density functional theory and laboratory data obtained in rare-gas matrices. The photophysics of the laser-irradiated ions is also modelled to derive, for some of the studied species, the absorption cross-sections of the measured electronic transitions. The purpose of this second study is to obtain gas-phase spectroscopic data on different ionised PAHs and derivatives, that can be useful for the pre-selection of the most promising candidates for some of the diffuse interstellar bands, a set of absorption bands observed in the ISM since almost a century but still unidentified.
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https://tel.archives-ouvertes.fr/tel-00469704
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Submitted on : Friday, April 2, 2010 - 12:07:04 PM
Last modification on : Thursday, October 15, 2020 - 4:07:19 AM
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Francesca Useli Bacchitta. Photophysique des molécules polycycliques aromatiques hydrogénées d'intérêt interstellaire avec l'expérience PIRENEA. Planète et Univers [physics]. Université Paul Sabatier - Toulouse III, 2009. Français. ⟨tel-00469704⟩

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