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Abstract No.: ThP-118
Session: FT-ICR Instrumentation and Methods
Presentation date: Thu, Aug 31, 2006
Presentation time: 14:30 – 16:00

Photophysics and Chemistry of Interstellar PAH Candidates in PIRENEA, a FTICR-MS Set-up for Astrochemistry

Christine Joblin1, Aude Simon1, Natacha Bruneleau1, Dominique Toublanc1, Michel Armengaud1, Patrick Frabel1, Loic Nogues1

1 CESR CNRS-UPS, Toulouse, France

Correspondence address: Christine Joblin, CESR CNRS-UPS, 9 Av. du Colonel Roche, Toulouse, 31028 France.

Keywords: Fourier Transform ICR; Photodissociation; Polycyclic Aromatic Hydrocarbons; Reaction, Ion/Molecule.

Novel aspect: We present experimental results obtained on the photodissociation and reactivity of polycyclic aromatic hydrocarbons in a cold ICR cell. This fundamental data is used as input in models of interstellar chemistry.


A remaining challenge for laboratory experiments is to approach the physical conditions that prevail in interstellar space: cold environment (10-50 K), absence of collisions on long timescales and presence of stellar UV photons with energy less than 13.6 eV at the surface of clouds. PIRENEA, “Piege a Ions pour la Recherche et l'Etude de Nouvelles Especes Astrochimiques”, is an experimental set-up which has been developed according to these specifications. Using the trapping, and the mass spectrometry analysis of an ion cyclotron resonance (ICR) cell that is connected to cryogenic shields, studies can be performed on processes which occur on long timescales and which are of interstellar relevance: IR radiative cooling, photodissociation in low energy channels, radiative association. Photodissociation spectroscopy can also be performed with the advantage of an easy mass selection of the studied species. Infrared spectroscopy in particular can provide structural information but is also a direct way for comparison with astronomical observations.

Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed as important species in astrochemistry but the model suffers from the lack of identification of any individual molecule. The true nature of interstellar PAHs has therefore to be questioned, considering the formation mechanism of these species, the processing by UV photons and the reactivity with gas-phase species. We will illustrate how the PIRENEA set-up can be used to address these questions. Detailed studies were performed on the photodissociation and the photodissociation spectroscopy of C24Hn+ species (n=[1,12]). From these results, we were able to calculate the hydrogenation state of C24Hn+ species in interstellar clouds. We also studied the reactivity of these species with small molecules such as H2, O2, H2O, CO, CH4, CH3OH both at room temperature and at low temperatures (~40K). Particular emphasis will be given to the photodissociation of the formed complexes, which can lead to the production of pentagonal rings in the aromatic species and the release of new molecules in the gas-phase. These results give new insights into the chemical evolution of PAH species in the interstellar medium and their role in the synthesis of new molecules.