Structure and electronic properties of ionized PAH clusters

69th ISMS, U. Illinois16-20/06/2014

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'OrsayUniversité Paris-Sud 11 & CNRS

*Synchrotron SOLEIL GASPARIM

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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

PAH+

PAH0

VSGs

UV

proc

essi

ng

7 9 11 13 15Wavelength (μm)

6 7 8 9 10 11 Wavelength (μm)

C60+C60

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

X

Z

YR

θ

φ

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 ovenup to T=400oC

4

Formation of PAH clusters – PEPICO experiments

<Toven (C24H12)>=360oC; carrier gas: Ar

Coro

BzP

x2

x3

x4

x5

5

PES – coronene monomer – DELICIOUS II

6

Properties of C24H12 clusters – Ionization potential

12

3

54

7

Modeling the properties of (ionized) PAH clustersMETHODS

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 statesDontot, Suaud, Rapacioli, Spiegelman, subm (2014)

Stacked clusters (constrained geometry) Relaxed gas-phase clustersGlobal exploration of the potential energy surface (Parallel

Tempering Monte Carlo) + Local optimisation (conjugate gradient algorithm)

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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: IPvert = E0 (R0) – E+(R0)

IPadia = E0 (R0) – 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

Lowest structures of neutral coronene (C24H12) clusters

10

Properties of C24H12 clusters – Ionization potentialExperiments (TPES) vs theory (IPver)

11Joblin, Dontot, Garcia et al. (2014)

Properties of C16H10 clusters – Ionization potentialExperiments (TPES) vs theory (IPver)

Theory - shifted

Evidence for structural changes

12Joblin, Dontot, Garcia et al. (2014)

Some structures for the hexamer of pyrene [C16H10]6

13

Dontot L., PhD, Univ. Toulouse 3 (2014)

Excited electronic states of [C24H12]2+

Experiments (TPES) vs theory

Neutral geometryRelaxed ion geometry

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Excited electronic states of [C24H12]3+

Experiments (TPES) vs theory

Trimer/ neutral geometryDimer/ neutral geometry

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Conclusions and perspectives

Validation of the theoretical approach- Very good agreeement between measured IE and calculated IPver (0 K)

strong confidence in the calculated structures and energies

- Current calculations: IPadia . Effects of isomerization and finite Tvib

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 [C24H12]n

+ species

- Photodissociation studies at threshold

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The PIRENEA set-up for astrochemistry

UV-Visible irradiation

Superconductor magnet (5T)

ICR cell

Turbo-molecular pump

OPO laser210 nm – 2 µm

Solid pellet

Ablation laser (266 nm)

Externalcold shield

Internal cold shield

zr

P ~ 10-11 mbarT=35 K

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MPD spectroscopy on C24H12+ 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. 1992Joblin, Salama, unpublished

+a)

b)

c)

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Laser desorption/ionization

Ejection of isotopes

Photofragmentation(OPO laser shots)

MPD spectroscopy of [C24H12]n+ in

PIRENEA

[C24H12]2+

C24H12+

In situ formation of clusters

C24H12+ + C24H12 [C24H12]2

+

Photodissociation

[C24H12]2+ C24H12

+ + C24H12

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Financial support

ANR GASPARIM project (2010-2014)

CNRS / Programme National Physique et Chimie du Milieu Interstellaire

University of Toulouse 3 (UPS) and Observatory Midi-Pyrénées

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GASPARIM