Astrochimistry – Spring 2013 Interstellar PAHs

Astrochimistry – Spring 2013 

Lecture 4: 

Interstellar PAHs 

NGC 7023 - HST 

Julien Montillaud 

8 th February 2013

Outline 

I. From Unidentified to Aromatic Infrared Bands (7 p.) 

I.1 Historical background 

I.2 Observational evidence 

I.3 The PAH hypothesis 

II. General properties of “chemical” PAHs (9 p.) 

II.1 Overview of the PAH family 

II.2 Electronic properties 

II.3 Vibrational properties 

III. Evolution of interstellar PAHs (15 p.) 

III.1 Observational evidence 

III.2 Dissociation of PAHs 

III.3 Reactivity of PAHs 

III.4 Formation of PAHs 

IV. Influence of PAHs on the ISM evolution (3 p.) 

IV.1 Extinction curve 

IV.2 Photoelectric effect 

IV.3 Formation of H 2 

V. Summary 

VI. Some bibliographic references 

2

I. From Unidentified to Aromatic Infrared Bands 

I.1 Historical background 

Gillett et al. (1967): IR excess in the ?? range compared to free-free radio continuum 

extrapolation 

Russell et al. (1977): unidentified IR bands at 3.3, 3.4, 6.2, 7.7, 8.6 & 11.3 µm from reflection 

nebulea 

Duley & Williams (1981): notice the match between some PAH spectral features and UIBs 

Sellgren et al. (1983): near IR emission from reflection nebulae inconsistant 

with UV-photons thermally excited dust grains => transient heating of small particles ? 

Léger & Puget (1984): the “first first” attribution of UIBs to PAHs 

(transient heating excitation model) 

Allamandola, Tielens & Barker (1985): the “second first” attribution of UIBs to PAHs 

(non thermal excitation model) 

Désert et al. (1990): global model of dust grains including PAHs 

1990': first big experiments and detailed theoretical models + Infrared Space Observatory era 

2000': multiplication of experiments and detailed models + Spitzer Space Telescope era 

3



NGC 7023 

4



HII region 

Edge of molecular cloud 

NGC 7023 

Proto-planetary nebula 

5 

Planetary nebula



Band positions: typical of C-H and 

C-C stretches or bends in aromatic 

material 

Plateaux at short wavelength: 

High temperature, small species 

Simple reasoning: 

→ Plateau up to ~3.4 µm 

→ T~1000 K (black body approx.) 

→ 1 photon @ E=10 eV 

→ Cv=3kN=E/T 

=> N=39 atoms 

Possible reading: Sellgren (1984) 

6



Astrophysical PAH = carrier of the Aromatic Infrared Bands 

PAH = polycyclic aromatic hydrocarbon 

“chemical” PAH = one model for “astrophysical” PAH 

Other models are reasonnable 

Graphitic grains/ 

PAH clusters 

Free flying PAH 

molecules 

Hydrogenated 

amorphos carbon 

7



Still no individual spectroscopic identification 

Pilleri et al. 2009 

Pilleri et al. 2009 

Kokkin et al. 2008 

Not symmetric 

→ permanent electric dipole 

→ strong rotational emission 

… 

Undetected in Red Rectangle 

in the millimetric range (1→3mm) 

at IRAM-30m 

Electronic transition 

in the visible 

8

Related detections 



Fullerene C60: not a PAH, but very close 

Also: 

- C 70 

- C 6 

H 6 

9

II. General properties of “chemical” PAHs 






III. Grain-catalyzed formation of H 2 

IV. From processes to interstellar H 2 

formation rate 

V. Summary 

10



5-cycles 

Catacondensed 

Irregular 

Pericondensed 

11

Electron delocalization (=resonance) and aromaticity 



Molecular orbitals = combination of atomic orbitals 

original p-atomic orbitals 

Resulting molecular 

(delocalized) orbital 

Electrostatic potential 

map of benzene 

Images from Paula Yurkanis Bruice, Cleveland, OH - 

http://wps.prenhall.com/wps/media/objects/724/741576/chapter_07.html 

12




• Bonding MO: constructive (in-phase) overlap 

• Antibonding MO: destructive (out-of-phase) overlap 

13




n(e) = 4n+2 => extra stabilisation (aromatic) 

n(e) = 4n => extra de-stabilisation (anti-aromatic) 

n(e) = 4n+1 or 4n+3 => nothing special 

NB: 

- changing the charge changes the aromaticity 

- changing the number of H-atoms changes the aromaticity 

Le Page et al. 2001 

14



Ionization potential 

Empirical classical model: PAH = thin conducting disk 

Reasonable agreement with detailed quantum calculations (Density Functional Theory) 

Z=1 Z=2 

Malloci et al. 2007 

15



Electronic spectrum 

Extinction curve 

Visible 

UV 

→ * 

Lyman cut (13.6 eV) 

→ * 

→ * 

→ * 

16



Electronic spectrum 

E n 

~ n / L 

n=level number 

E n 

= energy of level n 

L = well width 

17



Experimental spectra 

Theoretical spectra 

Joblin et al. 1994 

http://www.astrochem.org/pahdb/ 

18



PAH spectroscopy: only little variations 

Neutral 

Anion 

- 

- 

- 

- 

19

III. Evolution of interstellar PAHs 

I. From Unidentified to Aromatic Infrared bands 




III.2 Photodissociation of PAHs 





V. Summary 

20

In HII regions and reflection nebulae: from region to region 



Peeters et al. 2004 & 2002 

21

In the reflection nebulae NGC 7023: within one region 



Berné et al. 2007 

Peeters et al. 2004 & 2002 

22



Direct photodissociation ? 

→ apparently not efficient 

(Buch 1989, Jochims et al. 1994) 

Other possibility: 

→ statistical photodissociation 

23



Relaxation processes of an isolated PAH 

IVR: internal vibrational redistribution 

IC: internal conversion 

24



(diagram for a PAH cluster, but similar to H-loss or C-loss for an isolated PAH molecule) 

25



Arrhenius law 

k diss 

(T) = A diss 

exp(-E 0 

/kT) 

k diss 

= dissociation rate coefficient 

T = vibrational temperature 

→ also kinetic temperature if enough 

collisions for thermalization 

A diss 

= prefactor 

E 0 

= dissociation energy 

Laplace 

transform 

Approximated microcanonical expression 

k diss 

= dissociation rate coefficient 

E = internal (vibrational) energy 

A diss 

= prefactor 

→ vibrational frequency 

→ entropy cost (structural changes) 

→ difficult to estimate theoretically, 

but not impossible 

E 0 

= dissociation energy 

= vibrational density of state(VDOS) of 

the parent molecule => one has to know 

26 

the vibrational modes



Anharmonicity 

= different from a parabolic potential well 

→ always the case for dissociation 

→ mode frequency vary 

with internal energy 

→ much more difficult to 

find the frequencies, and 

to compute VDOS from 

frequencies 

→ VDOS directly 

accessible from 

molecular dynamics 

simulations 

In practice, astronomers still use the harmonic VDOS 

27



General trends 

Ekern et al. 1998 

Experimental results: 

More compact => 

more photostable 

Bigger => 

more photostable 

Loss E 0 

[eV] A diss 

[s -1 ] 

H(even) 4.5-4.8 6.8e17 

H(odd) 3.2 6.8e17 

C 7.4 6.2e15 

C2 8.5 3.5e17 

C3 8.0 1.5e18 

(Joblin et al. 2013, Léger et al. 1989) 

28



Reactivity with hydrogen 

Montillaud et al. 2013 

No data on neutral 

Only qualitative data on super-hydrogenated (more H-atoms than in “normal” PAHs) 

Measurement/calculation for dehydrogenated only for small PAHs 

Fast reactions with H 

No or slow reactions with H 2 

29



Reactivity with other atoms 

Le Page et al. (1999) 

Fast reactions for normal and dehydrogenated cations with O and N 

=> it is likely that a more general population than strict PAHs populate the ISM 

Tentative “family identification”, using NASA Ames database, but not convincing (so far...) 

30



Physisorption of bigger atoms (Si, Fe, ...) 

Joalland et al. (2009) 

Thermodynamically favorable 

Weak spectral signature => not impossible but difficult 

E 0 

= 1.5 eV => easily photodissociated 

31



Aggregation of PAHs 

(C 24 

H 12 

) 11 

(C 96 

H 24 

) 4 

32



Aggregation of PAHs 

Formation of PAH clusters: 

→ dilution of the excess energy within the numerous vibrational degrees of freedom 

(see movie, credit: Mathias Rapacioli, IRSAMC, Toulouse, France) 

Destruction of PAH clusters: 

→ analogous to PAH photodissociation 

→ difficulty to deal with all the degrees of freedom => rigid molecules approximation ? 

Spectroscopy of PAH clusters: 

→ are they responsible for continuum emission in the mid-IR ? 

33



Asymptotic Giant Branch Stars (AGB) 

IRC 10216 

Cherchneff et al. 1992 

Shocks 

→ local dense and cold points 

→ condensation of some dust grains 

→ acceleration of grains by radiation pressure 

→ wind acceleration by dragging 

⇒ coupling stellar wind dynamics – dust formation 

34 

Model of AGB circumstellar material



Most classical model: the HACA mechanism 

in circumstellar regions of AGB stars 

(H-abstraction, C-addition) 

Frenklach et al. 1984 

Consistent with observations ? 

CRL 2688 

IRC 10216 

AGB stars 

No UV 

Protoplanetary nebula: some UV 

IC 418 

Planetary nebula 

Intense UV RF 

35






IV. Influence of PAHs on ISM evolution 

IV.1 Photoelectric heating 


IV.3 Formation of H2 

V. Summary 

36

IV. Influence of PAHs on the ISM evolution 

IV.1 Photoelectric heating 

PDR model for NGC 7023 

37



38


IV.3 Formation of H2 

Rauls & Hornekaer 2008 

39

V. Summary 

“Astronomical” PAHs are defined as the carriers of the AIBs 

“Chemical” PAHs are one good model for astronomical PAHs 

→ no individual spectroscopic identification 

→ but identification of 3 large aromatic molecules: C 6 

H 6 

, C 60 

, C 70 

Many theoretical and experimental studies provide molecular data, but still not enough 

→ Theoretical spectral database of PAHs (Cagliari, Italy) 

→ NASA Ames PAH IR Spectroscopic Database 

Understanding is limited by: 

- confusion in mid-IR spectroscopy 

- scarcity in UV-visible spectroscopy 

- the huge number of parameters (e.g. molecule size, structure, charge, ...) 

- the absence of emission in UV-poor environments 

- the large size of interstellar PAHs 

=> difficult (if not impossible) for both theoretical and experimental studies 

Deep coupling between PAH and ISM evolutions through photoelectric heating, extinction 

curve and H 2 

formation 

In the future, progresses should be achieved from on-going laboratory and theoretical studies 

40 

for large species + spectroscopic observations (DIBs, AIBs, far-IR rotation lines) + coupling 

with chemistry of other species (H 2 

formation/excitation, small hydrocarbons, evaporating VSGs)

VI. Some bibliographic references 

PAH review: 

- Tielens, A. G. G. M., The Physics and Chemistry of the Interstellar Medium, Cambridge University Press (2005) 

- Tielens, A. G. G. M., ARA&A, (2008) 

Chemical evolution: 

- Le Page et al., ApJSS, 132: 233-251, (2001) 

- Le Page et al., ApJ, 584: 316-330 (2003) 

- 

- 

Theoretical chemistry aspects (mainly for astronomers) 

- Malloci et al., A&A, 462: 627-635, (2007) 

- Léger et al. A&A, 213: 351-359 (1989) 

- 

Experimental chemistry aspects 

- 

- 

Modelling aspects 

- 

- 

- 

- 

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Astrochimistry – Spring 2013 Interstellar PAHs

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