1. Structure and electronic properties of ionized PAH clusters 69 th ISMS, U. Illinois 16-20/06/2014 C. Joblin, D. Kokkin, H. Sabbah, A. Bonnamy, L. Dontot, M. Rapacioli, A. Simon, F. Spiegelman, P. Parneix, T. Pino, O. Pirali, C. Falvo, A. Gamboa, P. Bréchignac, G. Garcia, L. Nahon * 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 GASPARIM 1

2. Outline The astrophysical context PEPICO experiments Modeling the electronic properties of PAH clusters The ionization potential: experiments vs theory Excited electronic states of ionized PAH clusters: PEPICO experiments vs theory Towards ion trap studies 2

3. PAH + PAH 0 VSGs UV processing 7 9 11 13 15 Wavelength (μm) 6 7 8 9 10 11 Wavelength (μm) C 60 + C 60 Rapacioli, Joblin, Boissel, 2005, A&A 429, 193 Berné, Joblin Deville, et al. 2007,, A&A 469, 575 Berné, Mulas, Joblin, 2013, A&A 550, L4 Graphene? Proposed evolutionary scenario for interstellar PAHs NGC 7023 NW PDR Electronic properties and dissociation of (ionized) PAH clusters 3

4. X Z Y R   Stability and spectroscopy of PAH clusters using SAPHIRS/DELICIOUS II-III at the VUV beamline DESIRS at SOLEIL Velocity Map Imaging (VMI): zero energy (<1 meV resolution) Wiley-McLaren TOF: mass resolution 130 at 300 amu Coincidences: Mass selected photoelectron images Selection of the relevant species (clusters) Production of ions with a defined internal state (DELICIOUS III) VUV DESIRS Molecular beam chamber PEPICO spectrometer Garcia et al. Rev. Sci. Instrum. 80, 023102 (2009) PAH oven up to T=400 o C 4

5. Formation of PAH clusters – PEPICO experiments =360 o C; carrier gas: Ar Coro BzP x2 x3 x4 x5 5

6. PES – coronene monomer – DELICIOUS II 6

7. Properties of C 24 H 12 clusters – Ionization potential 1 2 3 5 4 7

8. Modeling the properties of (ionized) PAH clusters METHODS Neutral clusters: Density Functional based Tight Binding method (DFTB) modified to treat correctly dispersion forces. Description of atomic charges : Mulliken  CM3 charges Rapacioli et al., J. Chem. Phys. 130, 244304 (2009) Ionized clusters: Implementation of a configuration interaction (CI) scheme to take into account charge resonance (DFTB-VBCI)  Distribution of charge as a function of units Rapacioli & Spiegelman, Eur. Phys. J. D 52, 55 (2009) Rapacioli et al., Phys. Status Solidi B 249 (2), 245 (2012) + Extended DFTB-CI model for charge-transfer excited states Dontot, Suaud, Rapacioli, Spiegelman, subm (2014) Stacked clusters (constrained geometry)  Relaxed gas-phase clusters Global exploration of the potential energy surface (Parallel Tempering Monte Carlo) + Local optimisation (conjugate gradient algorithm) 8

9. Modeling the properties of (ionized) PAH clusters NUMERICAL SIMULATIONS Structures: Most stable structures (lowest points in the potential energy surface) Number of structures at close energies (isomers) Ionization potential: IP vert = E 0 (R 0 ) – E + (R 0 ) IP adia = E 0 (R 0 ) – E + (R + ) Energy and most stable structure for the neutral (0) and cation (+) Excited electronic states: excitation energies for charge resonant states and local excitation states Error bars: due to the number of approximations made in DFTB (minimal basis of atomic orbitals,...). Difficult to quantify.  Benchmarking with experimental values. 9

10. Lowest structures of neutral coronene (C 24 H 12 ) clusters 10

11. Properties of C 24 H 12 clusters – Ionization potential Experiments (TPES) vs theory (IP ver ) 11 Joblin, Dontot, Garcia et al. (2014)

12. Properties of C 16 H 10 clusters – Ionization potential Experiments (TPES) vs theory (IP ver ) Theory - shifted  Evidence for structural changes 12 Joblin, Dontot, Garcia et al. (2014)

13. Some structures for the hexamer of pyrene [C 16 H 10 ] 6 13 Dontot L., PhD, Univ. Toulouse 3 (2014)

14. Excited electronic states of [C 24 H 12 ] 2 + Experiments (TPES) vs theory Neutral geometry Relaxed ion geometry 14

15. Excited electronic states of [C 24 H 12 ] 3 + Experiments (TPES) vs theory Trimer/ neutral geometry Dimer/ neutral geometry 15

16. Conclusions and perspectives  Validation of the theoretical approach - Very good agreeement between measured IE and calculated IP ver (0 K)  strong confidence in the calculated structures and energies - Current calculations: IP adia. Effects of isomerization and finite T vib  Analysis of SOLEIL data - Current analysis of the electronic structure - Last campaign: cluster fragmentation with Delicious III (imaging of both electrons and ions)  Complementary studies with the PIRENEA cold ICR cell - Action spectroscopy: electronic states of [C 24 H 12 ] n + species - Photodissociation studies at threshold 16

17. 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 17

18. MPD spectroscopy on C 24 H 12 + with PIRENEA -MPD spectrum - Useli Bacchitta et al. 2010, Chem. Phys. 371, 16 - Improved cooling of the ions + 2-color laser scheme Kokkin et al., unpublished -Ne matrix – Ehrenfreund et al. 1992 Joblin, Salama, unpublished + a) b) c) 18 Laser desorption/ ionization Ejection of isotopes Photofragmentation (OPO laser shots)

19. 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 + In situ formation of clusters C 24 H 12 + + C 24 H 12  [C 24 H 12 ] 2 + Photodissociation [C 24 H 12 ] 2 +  C 24 H 12 + + C 24 H 12 19

20. Financial support ANR GASPARIM project (2010-2014) ANR GASPARIM project (2010-2014) CNRS / Programme National Physique et Chimie du Milieu Interstellaire CNRS / Programme National Physique et Chimie du Milieu Interstellaire University of Toulouse 3 (UPS) and Observatory Midi-Pyrénées University of Toulouse 3 (UPS) and Observatory Midi-Pyrénées 20 GASPARIM