Christine Joblin Institut de Recherche en Astrophysique et Planétologie Université de Toulouse [UPS] CNRS Negative ions and molecules in astrophysics Gothenburg and Onsala, Sweden 22-24/08/2011
Outline The astrophysical context : the PAH model Photophysics of an isolated PAH Statistical theories: rate constant for fragmentation A dedicated experimental set-up: PIRENEA Photodissociation studies Astrophysical implications Reactivity with H, H 2 A model for the evolution of interstellar PAHs
[Ne ] [Ne ] [S ] [Ar ] [Ar ] «Infrared Space Observatory ESA; 1995-1998 «Spitzer Space Telescope NASA; 2003-2009 H 2 H 2 H 2 H 2 SST IRAC 8 µm image NGC 7023 Werner et al. 2004, ApJ Supp 154, 309 ISO SWS spectra Vertratete et al. 2001, A&A 372, 981
...very good tracers of regions that are exposed to UV photons... Adapted from http://www.nrao.edu/pr/2006/gbtmolecules/ Bill Saxton, NRAO/AUI/NSF
Fluorescence Phosphorescence emission Vibrational emission 10-12 s - 10-8 s IVR 10-12 s - 10-10 s Radiation field UV radiation field absorption τ IR slow Broad absorption in the UV Discrete absorption features in the visible
From Désert et al. 1990, A&A 237, 215 UV-visible extinction IR emission 100 µm far-uv rise AIBs 10 µm far-ir DIBs 220nm bump radio Polycyclic Aromatic Hydrocarbons PAH Very Small Grains VSG Nano-objects (1-20 nm) composition? C, Si, Fe,. Big Grains 0.1 µm - BG silicates
Interaction of PAHs with UV photons Account for the Aromatic Infrared Bands - Léger & Puget A&A (1984) ; Allamandola et al. ApJ (1985) Account for most (all?) of the UV bump at 220 nm and non-linear far-uv rise of the extinction curve - Joblin et al. ApJ (1992); Cecchi-Pestellini et al. A&A Let. (2008); Steglich et al., ApJ 712, L16 (2010) Good carriers for the Diffuse Interstellar Bands - Léger & d Hendecourt A&A (1985),... Main contributor to the photoelectric heating of the gas - Bakes & Tielens. ApJ (1994) Could help traditional dust grains in promoting H 2 formation - Habart et al., A&A 414, 531 (2004) - Bauschlicher, ApJ 509, L125 (1998) Could produce other observed features such as the Blue Luminescence (by electronic fluorescence) and the Extended Red Emission (by phosphorescence). Compared to other molecules, PAHs have a remarkable stability against photodissociation
Photophysics of an isolated PAH / IR emission Ex: reflection nebula: <hν UV > = 10 ev Go=10 3 (1 Go =10 8 photons cm -2 s -1 ) The stochastic heating and slow IR emission... Typical timescales for IS PAH: Absorption of a UV-Vis photon every 10 hours Emission of a pure rotational photon once a year Collisions with dominant species once in 20 days Joblin et al. (2002), Mol. Phys. 100(22), 3595 Importance of the events involving IR cooling: dissociation in competition with IR cooling, radiative association
Relaxation mechanims IV. NRT, PAH et MIS ~10-12 s From short timescales (~ps) to very long timescales (~s) Energy IC IVR IC IVR Dissociation? A B D n Q 1? D 1 IR Emission AIBs 10-2 - 1 s hν interstellar D 0 PAH = AB 10
A key mechanism : IVR Single Vibronic Level Dispersed Fluorescence Spectrum of 9MA in a Supersonic Jet - IVR starts at 388 cm -1 Baba et al., J. Phys. Chem. A 113, 2366 (2009) Anthracene - W. R. Lambert, P. M. Felker, and A. H. Zewail, J. Chem. Phys. 81, 2209 (1984)
Statistical theories rate Cte for fragmentation Rate constant for fragmentation: The characteristic timescale for dissociation T disso depends on the internal energy (or temperature), on the dissociation energy and on the number of degrees of freedom role of the density of states Density of states ρ(e) = dn(e) de Microcanonical ensemble: enthropy / temperature S(E) = k B ln(ρ(e) 1 T(E) = S(E) E Canonical ensemble: C V (T) = E T
Unimolecular dissociation - RRKM Theory Rice-Ramsperger-Kassel and Marcus Arrhenius equation: k(t) = A e E a/rt RRKM equation: k(e) = σ N (E,E 0,R*) h ρ(e) k(t) k 4 3 k 2 k 1 k(e) M Simplified formalism: Boissel et al. (1997), J. Chem. Phys. 106, 4973 based on Forst (1972) J. Phys. Chem. 76, 342 Barker (1983) Chem. Phys. 77, 301 M
Density of quantum states (DOS) For g harmonic oscillators that are not coupled g E({n}) = ω i (n i 1 2 ) i=1 with {n} the vibrational quantum numbers The discretisation of the energy density of states can be defined Counting using the Beyer- Swinehart (1973) algorithm. Ex. calculations for C 24 H 12 Mulas et al. 2006, A&A 460, 93 More sophisticated calculations to derive the anharmonic DOS
Photodissociation rate of PAHs: experiments Usual methods: - 1- Time-resolved photo-ionisation in the VUV RRKM modelling t ~ 10 µs k~10 5-10 6 s -1 Jochims et al. (1994) ApJ 420, 307; Ling & Lifshitz (1998) J. Phys. Chem. A 102, 708-2- Time-resolved photodissociation RRKM modelling t ~ 1 ms k~10 3-10 4 s -1 Cui et al. (2000) J. Phys. Chem. A 104, 6339 C 14 H 10
Photodissociation at threshold: trapped PAHs under continuous irradiation of the Xe lamp Boissel et al., J. Chem. Phys. 106, 4973 (1997) Anthracene cation - C 14 H 10 hν abs = 1.78 ev k DISS k IR Sequential absorption of photons E thresh. Increase of internal energy a) Internal energy distribution for t irr = 3 s The ion dissociate for k diss (U) > k IR (U) at U > E threshold In the ISM where hν <13.6 ev and PAHs are larger than anthracene, photodissociation at thereshold is the dominant photodissociation process.
P ~ 10-11 mbar T=35 K UV-Visible irradiation Superconductor magnet (5T) Solid pellet Internal cold shield ICR cell r z Gas injection Laser irradiation (MPD) Ablation laser (266 nm) External cold shield Turbo-molecular pump
PIRENEA* Physics and chemistry of macromolecules and nanograins in interstellar conditions * Piège à Ions pour la Recherche et l Etude de Nouvelles Espèces Astrochimiques 1 2 3 1- Photophysics 2- Spectroscopy 3- Chemistry Photodissociation / IR Emission Isolation (P e ~10-11 mbar) Trapping --> long timescales Photodissociation spectroscopy Production and control of laboratory analogues of interstellar species Radiative association Gas-grain interactions with a truly isolated grain Low temperatures (<40K)
Photodissociation of 12 C 24 H p / Xe lamp C 24 H 4 C 24 H 6 C 24 H 5 12 C 24 H 12 C 24 H 10 C 24 H 8 C 24 H 7 C 24 H 9 12 C 24 C 24 H 2 C 24 H 4 C 24 H 6 C 24 H 8 C 24 H 10 C 24 H 12 C 24 H C 24 H 3 C 24 H 5 C 24 H 7 C 24 H 9 C 24 H 11
t irr =30s C 24 H 12-6H -1H -7H -4H -2H -3H -9H -8H -10H C 24 H 11 t irr =0.5 s C 24 H 5 t irr =0.4 s -5H -11H C 24 H 12 C 24 H 11.C 24 by successive H losses Dissociation kinetics of C 24 H 2p1 much faster than C 24 H 2p
UV absorption rate k UV = σ UV F UV IR emission rate k i IR,v =A i v, v-1 p i v p iv = ρ*(u-vhνi) / ρ(u) density of states Dissociation rates k(- H, - H 2, - C 2 H 2 ) k d = A ρ(u- E d ) H(U- E d d ) ρ(u) /simplified statistical theory Forst 1972, JPC 76, 342 Exact stochastic method (Monte Carlo kinetics) Gillespie 1976, J. Comp. Phys. 22,403 Barker 1983, Chem. Phys. 77,301 Joblin et al. 2003, SF2A, EDP Sciences, p.84 <E diss (C 24 H 2p ) > = 10.5 ev <E diss (C 24 H 2p1 ) > = 5.7 ev
Dissociation rates k(- H, - H k d = A ρ(u- 2, - C E 2 H d ) 2 ) H(U- E d d ) ρ(u) /simplified statistical theory Forst 1972, JPC 76, 342 Joblin et al. 2003, SF2A, EDP Sciences, p.84 E d_hduo = 4.48 ev (RRKM fit C 10 H 8 ) Ho et al. 1995, JACS (4.8 ev DFT larger PAHs) A d calibration Photoion appearance potential/ 10 4 s -1 Jochims et al. 1994, ApJ k IR E d_hsolo = 3.0-3.2 ev derived from PIRENEA measurements, assuming same A d as for H duo allows to probe dissociation at threshold (E crit ) E crit
Astrophysical implications: E crit (H loss) multiple photon events C 16 H 10 C 24 H 12 C 10 H 8 Simple extrapolation towards larger sizes: if no multiple photon events, no dissociation (H loss) for N<58 (ex: C 42 H 16 ) Need to study the competition with H recombination
Reactivity of coronene C 24 H 12 with H, O, N, H 2 Selected Ion Flow Tube apparatus, Boulder (CO, USA) 298 K 0.4 Torr He Coronene cations are formed via Ar M C 14 H 12 Ar C 14 H 12 e where metastable argon is generated by a cold cathode discharge N atoms are formed by N 2 dissociation through a microwave discharge N NO O N 2 H atoms by thermal dissociation of H 2
Reactivity of coronene C 24 H 12 with H, O, N, H 2 H atoms Betts et al., ApJ 651, L129 (2006) No size dependency of the reactivity with H and O Relatively fast reactions O atoms N atoms Association reactions are favoured and the branching ratio increases with the PAH size for O Reactivity of N atoms decreases when the PAH size increases No reactions with H 2
Reactivity of PAHs with H, H 2 C 24 H p H 2 (1 p 11) experiments in progress (PIRENEA) Montillaud, Joblin, Toublanc, PAHs and the Universe, EAS Publications Series vol. 46, 2011
Modeling the evolution of PAHs in photodissociation regions PhD thesis J. Montillaud, Toulouse University Realistic astrophysical conditions (Meudon PDR code; Le Petit et al. 2006, ApJ) C N C H NH q (E) Species modeled: Charge state: q = 2, 1, 0, -1 Hydrogenation state: N H N 0 H 1 & N H > N 0 H 1 ( method Internal energy (discrete Processes: Photoionization vs electron recombination Photodissociation (H-loss) vs reactivity (H, H 2 ( T n tot n H n H2 n e Competition between IR emission & dissociation Possible successive absorption of several photons Pseudo-time dependence: time evolution in fixed conditions characteristic timescales steady state
G 0 = 2200 (UV flux [6-13.6 ev] x 10 8 photons cm -2 s -1 ) n tot = 2 10 4 cm -3 ; Montillaud et al. C 24 C 24 C 24 H 12 C 24 H 12 C 54 C 54 H 18 C 54 C 54 H 18 Model by Le Page et al. 2003, ApJ 584, 316 N c < 20 - small PAHs are destroyed N c ={20,30} - fully dehydrogenated carbon clusters C n larger N c - fully hydrogenated / surhydrogenated more modelling effort required (laboratory data)
Photoprocessing of AIB carriers Ex: NGC 7023 NW PDR Rapacioli, Joblin, Boissel, 2005, A&A 429, 193 Sellgren et al. 2010, ApJ 722, L54 Presence of C 60 close to the star VSG PAH 0 PAH C 60
Thanks
To learn more about interstellar PAHs