Transcript 67OSU_Joblin12.ppt

Investigating electronic properties of ionized PAH clusters
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
67th OSU International symposium on
molecular spectroscopy
18-22/06/2012 (Columbus, OH, USA)
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
Astrophysical context: The Aromatic Infrared Bands (AIBs)
Spitzer Space Telescope IRAC image
Werner et al. 2004, ApJ Supp 154, 309
• Stochastic heating
N~50 ; T~1000 K
Sellgren (1984), ApJ 277, 623
• Candidates: PAH molecules
Léger & Puget 1984, A&A 137, L5
Allamandola, Tielens & Barker 1985, ApJ 290, L25
 10 to 20% of total carbon
XPAH~ 10-7 (NC~50)
Infrared Space Observatory SWS spectra
Vertratete et al. 2001, A&A 372, 981
Astrophysical context:
Photoprocessing of AIB carriers
Rapacioli, Joblin, Boissel, 2005, A&A 429, 193
Berné, Joblin Deville, et al. 2007, , A&A 469, 575
UV processing
VSG – PAH
clusters?
PAH
0
PAH+
Ex: NGC 7023 NW PDR
PAH cluster stability in a radiation field
Rapacioli, Calvo, Joblin et al. 2006, A&A 460, 519
Ex: NGC 7023 NW PDR
•
•
Evaporation rate from Phase Space Theory Calvo & Parneix, JPC 120, 6 (2003) – Rigid
molecules  kevap(Einter).
kIR(Eintra) of the i molecule in the cluster
~ kIR(Eintra) in the isolated molecule –
microcanonical approach - Joblin et al., Mol.
Phys. 100, 3595 (2002)
17 years
(C24H12)13
(C24H12)4
 The mean lifetime for coronene clusters (4-13 units) is less than 1.5 105 h
(17 years) much smaller than typical lifetimes for reformation by collisions
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).
Modelling the properties of ionized PAH clusters
Rapacioli & Spiegelman, Eur. Phys. J. D 52, 55 (2009)
CI-DFTB approach
+
Distribution of charge as a function of units
 Molecules at the border of the pile are
not much involved in the charge transfer
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 potentials
Excitation energies of charge
resonant states
Stability and spectroscopy of PAH clusters using
SAPHIRS/DELICIOUS II at the VUV beamline DESIRS at SOLEIL
Z
Garcia et al. Rev. Sci.
Instrum. 80, 023102
(2009)

R

Y
X
Molecular beam chamber
VUV DESIRS
•Velocity Map Imaging
PEPICO spectrometer
(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
Properties of C24H12 clusters – SOLEIL experiments
Properties of C24H12 clusters – SOLEIL experiments
Calculations: ionized stacks of
C24H12 ; good approximation up to
8 units for this PAH
Rapacioli et al., Phys. Status Solidi
B 249 (2), 245 (2012)
scaled
calc. vertical
calc. scaled
calc. adiabatic
experimental data
Experimental data
IP(C24H12)= 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
Electronic properties of C24H12 – SOLEIL experiments
Photoabsorption spectrum – C24H12+ in Ne matrices
Ehrenfreund et al., A&A259, 257 (1992)
VUV photoabsorption spectrum – C24H12
Joblin C., Salama F., unpublished
Joblin et al., ApJ 393, L79 (1992): Joblin C.,
IP
cation
D1 D0
vibrational
structure
PhD, Univ. Paris 7, 1992
Autoionizing transitions
AI
AI
AI
Electronic properties of [C24H12]20/+ - SOLEIL experiments
Calculated charge resonant states
Rapacioli et al., Phys. Status Solidi B 249 (2), 245 (2012)
+ Localized states
IP
vibrational
structure Predicted
charge
resonant state
AI
AI
D1 state of
monomer
cation
AI
AI
AI
AI
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 [C24H12]n+ species
- Characterization of the band position and cross-section (+profile?)
- Photodissociation studies at threshold
The PIRENEA set-up for astrochemistry
P ~ 10-11 mbar
T=35 K
UV-Visible irradiation
Solid pellet
Superconductor magnet (5T)
ICR cell
r
Internal cold shield
z
OPO laser
210 nm – 2 µm
Ablation laser (266 nm)
External
cold shield
Turbo-molecular pump
MPD spectroscopy of [C24H12]n+ in
PIRENEA
• MPD spectroscopy + Monte Carlo model  to measure the
band position and absorption cross-section
Useli Bacchitta et al. 2010, Chem. Phys. 371, 16
• Band profile – current effort with a 2 colour
scheme – Cf. Kokkin et al., this conference
• In situ formation of cold clusters
C24H12+
+
[C
H
]
+
+
24
12
2
[C24H12]2 C24H12 + C24H12
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

University Paul Sabatier and Observatory Midi-Pyrénées (BQR, PPF)

CESR (IRAP since 01/2011), UPS/CNRS