1. Investigating electronic properties of ionized PAH clusters 67th OSU International symposium on molecular spectroscopy 18-22/06/2012 (Columbus, OH, USA) C. Joblin, D. Kokkin, A. Bonnamy, D. Toublanc, M. Rapacioli, A. Simon, L. Dontot, A. Gamboa, F. Spiegelman P. Parneix, T. Pino, O. Pirali, G. Féraud, H. Friha, C. Falvo, P. Bréchignac G. Garcia, L. Nahon ; G. Mulas * Institut de Recherche en Astrophysique et Planétologie *Laboratoire de Chimie et Physique Quantiques Université de Toulouse [UPS] & CNRS *Institut des Sciences Moléculaires d'Orsay Université Paris-Sud 11 & CNRS *Synchrotron SOLEIL *INAF; Osservatorio Astronomico di Cagliari

2. Outline The astrophysical context : from PAHs to PAH clusters Modelling the properties of ionized PAH clusters PEPICO experiments with DELICIOUS II at SOLEIL Perspectives MPD spectroscopy with PIRENEA

3. Infrared Space Observatory SWS spectra Vertratete et al. 2001, A&A 372, 981 Astrophysical context: The Aromatic Infrared Bands (AIBs) [S ++ ] [Ne ++ ] [Ne + ] H2H2 H2H2 H2H2 H2H2 [Ar ++ ] [Ar + ] Spitzer Space Telescope IRAC image Werner et al. 2004, ApJ Supp 154, 309 Stochastic heating Stochastic heating N~50 ; T~1000 K Sellgren (1984), ApJ 277, 623 Candidates: PAH molecules Candidates: PAH molecules Léger & Puget 1984, A&A 137, L5 Allamandola, Tielens & Barker 1985, ApJ 290, L25  10 to 20% of total carbon X PAH ~ 10 -7 (N C ~50) X PAH ~ 10 -7 (N C ~50)

4. Astrophysical context: Photoprocessing of AIB carriers Rapacioli, Joblin, Boissel, 2005, A&A 429, 193 Berné, Joblin Deville, et al. 2007,, A&A 469, 575 VSG – PAH clusters? PAH 0 PAH + Ex: NGC 7023 NW PDR UV processing

5. PAH cluster stability in a radiation field Evaporation rate from Phase Space Theory - Calvo & Parneix, JPC 120, 6 (2003) – Rigid molecules  k evap (E inter ). k IR (E intra ) of the i molecule in the cluster ~ k IR (E intra ) in the isolated molecule – microcanonical approach - Joblin et al., Mol. Phys. 100, 3595 (2002) Rapacioli, Calvo, Joblin et al. 2006, A&A 460, 519 17 years (C 24 H 12 ) 4 (C 24 H 12 ) 13  The mean lifetime for coronene clusters (4-13 units) is less than 1.5 10 5 h (17 years) much smaller than typical lifetimes for reformation by collisions Ex: NGC 7023 NW PDR

6. Evaporating VSGs – PAH clusters? Destruction faster than (re)formation. Could improve if they are ionised  increase of the formation rate and decrease of the dissociation rate. Effect of the size important as well. One objective for the GASPARIM ANR project Direct evidence for PAH clusters by searching for IR and electronic signatures Interactions with UV photons: ionisation, dissociation  From a theoretical point of view, address the question of charge delocalisation in charged PAH clusters  Experimental studies at SOLEIL using the PEPICO spectrometer and SAPHIRS molecular beam on the VUV beamline DESIRS  Experimental studies using the cold ion trap PIRENEA at IRAP (+ the Icare set-up at ISMO).

7. Modelling the properties of ionized PAH clusters CI-DFTB approach Distribution of charge as a function of units + Rapacioli & Spiegelman, Eur. Phys. J. D 52, 55 (2009)  Molecules at the border of the pile are not much involved in the charge transfer

8. Modelling the properties of ionised PAH clusters: Electronic structure Rapacioli et al., Phys. Status Solidi B 249 (2), 245 (2012) Properties of singly charged stacks of coronene clusters Ionization potentialsExcitation energies of charge resonant states

9. X Z Y R   Stability and spectroscopy of PAH clusters using SAPHIRS/DELICIOUS II at the VUV beamline DESIRS at SOLEIL Velocity Map Imaging (VMI): energy and ejection angle (multiplex), fast particles (<=17 eV KE, dE/E=5%), and zero energy (<1 meV resolution) Wiley-McLaren TOF: mass resolution 130 à 300 amu Coincidences: Mass selected photoelectron images Selection of the relevant species (clusters) Production of ions with a defined internal state VUV DESIRS Molecular beam chamber PEPICO spectrometer Garcia et al. Rev. Sci. Instrum. 80, 023102 (2009)

10. Properties of C 24 H 12 clusters – SOLEIL experiments

11. calc. vertical calc. adiabatic scaled experimental data calc. scaled Calculations: ionized stacks of C 24 H 12 ; good approximation up to 8 units for this PAH Rapacioli et al., Phys. Status Solidi B 249 (2), 245 (2012) Experimental data IP(C 24 H 12 )= 7.30 eV (7.29 eV; Clar et al. 1981, J. Am. Chem. Soc. 103, 1320) Calculated values scaled to the IP of the monomer

12. Electronic properties of C 24 H 12 – SOLEIL experiments IP vibrational structure cation D1  D0 AI Autoionizing transitions VUV photoabsorption spectrum – C 24 H 12 Joblin et al., ApJ 393, L79 (1992): Joblin C., PhD, Univ. Paris 7, 1992 Photoabsorption spectrum – C 24 H 12 + in Ne matrices Ehrenfreund et al., A&A259, 257 (1992) Joblin C., Salama F., unpublished

13. Electronic properties of [C 24 H 12 ] 2 0/+ - SOLEIL experiments IP vibrational structure D1 state of monomer cation Calculated charge resonant states Rapacioli et al., Phys. Status Solidi B 249 (2), 245 (2012) + Localized states Predicted charge resonant state AI

14. Conclusions and perspectives  Validation of the theoretical approach - Calculations on relaxed clusters; constrained one-pile geometry is however good for coronene clusters up to 8 units - Current effort to obtain an approximate equation of the PES for molecular dynamics simulations.  Analysis of SOLEIL data - Detailed analysis of the monomer spectrum to derive the internal temperature of the molecular species in the beam - Analysis of auto-ionizing states with results from the last campaign - Study of the cluster fragmentation with Delicious III (imaging of both electrons and ions)  Complementary studies with the PIRENEA set-up - Identification of electronic states of [C 24 H 12 ] n + species - Characterization of the band position and cross-section (+profile?) - Photodissociation studies at threshold

15. The PIRENEA set-up for astrochemistry UV-Visible irradiation Superconductor magnet (5T) ICR cell Turbo-molecular pump OPO laser 210 nm – 2 µm Solid pellet Ablation laser (266 nm) External cold shield Internal cold shield z r P ~ 10 -11 mbar T=35 K

16. MPD spectroscopy of [C 24 H 12 ] n + in PIRENEA MPD spectroscopy of [C 24 H 12 ] n + in PIRENEA [C 24 H 12 ] 2 + C 24 H 12 + Band profile – current effort with a 2 colour scheme – Cf. Kokkin et al., this conference Band profile – current effort with a 2 colour scheme – Cf. Kokkin et al., this conference In situ formation of cold clusters In situ formation of cold clusters MPD spectroscopy + Monte Carlo model  to measure the band position and absorption cross-section MPD spectroscopy + Monte Carlo model  to measure the band position and absorption cross-section Useli Bacchitta et al. 2010, Chem. Phys. 371, 16 [C 24 H 12 ] 2 +  C 24 H 12 + + C 24 H 12

17. Thanks Technical team of PIRENEA (present and past) A. Bonnamy, L. Nogues, D. Murat, O. Coeur-Joly, M. Armengaud, P. Frabel Financial support ANR GASPARIM project (2010-2013) CNRS / Programme National Physique et Chimie du Milieu Interstellaire CNRS / Programme National Physique et Chimie du Milieu Interstellaire University Paul Sabatier and Observatory Midi-Pyrénées (BQR, PPF) University Paul Sabatier and Observatory Midi-Pyrénées (BQR, PPF) CESR (IRAP since 01/2011), UPS/CNRS CESR (IRAP since 01/2011), UPS/CNRS