title: VUV photo-processing of PAH cations: quantitative study on the ionization versus fragmentation processes.
abstract: Interstellar polycyclic aromatic hydrocarbons (PAHs) are strongly affected by the absorption of vacuum ultraviolet (VUV) photons in the interstellar medium (ISM), yet the branching ratio between ionization and fragmentation is poorly studied. This is crucial for the stability and charge state of PAHs in the ISM in different environments, affecting in turn the chemistry, the energy balance, and the contribution of PAHs to the extinction and emission curves. We studied the interaction of PAH cations with VUV photons in the 7 - 20 eV range from the synchrotron SOLEIL beamline, DESIRS. We recorded by action spectroscopy the relative intensities of photo-fragmentation and photo-ionization for a set of eight PAH cations ranging in size from 14 to 24 carbon atoms, with different structures. At photon energies below ~13.6 eV fragmentation dominates for the smaller species, while for larger species ionization is immediately competitive after the second ionization potential (IP). At higher photon energies, all species behave similarly, the ionization yield gradually increases, leveling off between 0.8 and 0.9 at ~18 eV. Among isomers, PAH structure appears to mainly affect the fragmentation cross section, but not the ionization cross section. We also measured the second IP for all species and the third IP for two of them, all are in good agreement with theoretical ones confirming that PAH cations can be further ionized in the diffuse ISM. Determining actual PAH dication abundances in the ISM will require detailed modeling. Our measured photo-ionization yields for several PAH cations provide a necessary ingredient for such models.
authors.affiliation: Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France; CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France.
authors.forename: Junfeng
authors.initials: J
authors.lastname: Zhen
authors.affiliation: Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France; CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France; Laboratoire de Chimie et Physique Quantiques LCPQ/IRSAMC, Université de Toulouse (UPS) and CNRS, 118 Route de Narbonne, 31062 Toulouse, France.
authors.forename: Sarah Rodriguez
authors.initials: SR
authors.lastname: Castillo
authors.affiliation: Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France; CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France.
authors.forename: Christine
authors.initials: C
authors.lastname: Joblin
authors.affiliation: Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France; CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France; Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Mulas
authors.affiliation: Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France; CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France.
authors.forename: Hassan
authors.initials: H
authors.lastname: Sabbah
authors.affiliation: Synchrotron SOLEIL, LOrme des Merisiers, 91192 Gif sur Yvette Cedex, France; INRA, UAR1008 Caractérisation et Elaboration des Produits Issus de l'Agriculture, 44316 Nantes, France.
authors.forename: Alexandre
authors.initials: A
authors.lastname: Giuliani
authors.affiliation: Synchrotron SOLEIL, LOrme des Merisiers, 91192 Gif sur Yvette Cedex, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Nahon
authors.affiliation: Institut Lumière Matière, Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne cedex, France.
authors.forename: Serge
authors.initials: S
authors.lastname: Martin
authors.affiliation: Laboratoire Collisions Agrégats Réactivité, Université de Toulouse, UPS-IRSAMC, CNRS, 118 Route de Narbonne, Bat 3R1B4, 31062 Toulouse Cedex 9, France.
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Champeaux
authors.affiliation: Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa K1N 6N5, Canada.
authors.forename: Paul M
authors.initials: PM
authors.lastname: Mayer