The hydrogenation of PAH cations: a journey guided by stability and magic numbers

Stéphanie CazauxLeon Boschman

Thomas SchlathölterRonnie HoekstraGeert Reitsma Marco Spaans

Nathalie RougeauSabine Morisset

Dominique Teillet Billy

From Clouds to Protoplanetary Disks: the Astrochemical Link

06th October 2015

VIDI fellow

PAHs in space

• IR emission spectra in X (extra) galactic objects vast array of Unidentified Infrared Emission (UIE) Aromatic Infrared Bands (AIB)

• AIB attributed to PAHs with sizes 50-100C Leger & Puget 1984 A&A; Allamandola 1984 ASSL 108

Peeters et al. 2002 A&A 390Tielens 2008 ARAA 46

PAHs in space PAHs in space

Size? Weingartner & Draine 2001 ApJ, 548; Draine & Li 2007 ApJ 657

Neutral of cation? Allamandola 1999; Oomens et al. 2001 ApJ 560

Aromatic or aliphatic? Li & Draine 2012 ApJL 760; Pilleri et al. 2015 A&A 577

Hydrogenated or dehydrogenated? Bernstein et al. 1996 ApJ 472; Montillaud et al. 2013 A&A 552; Snow 1998 Nature

Composition of ISMkey physical conditions (UV, ionisation, dust column) Pilleri et al. 2012 A&A 542, 69

Interstellar catalysts (formation of H2) Bauschlicher 1998 ApJL 509; Mennella et al. 2012 ApJL 745; Thrower et al. 2012 ApJ 752; Boschman et al. 2012, ApJL 761

Study hydrogenation of PAHs stability? H2 formation?

Hydrogenation of coronene cations

Hydrogen source

Boschman, L., Reitsma, G., Cazaux, S., Schlathölter, T., Hoekstra, R. Zernike Institute for Advanced materials in Groningen

Hydrogenation of coronene cations

Boschman, L. et al. 2012

+1 +3 +5

Hydrogenation of coronene cations

Boschman, L. et al. 2012

+1 +3 +5

+11H

+5H

+17H

Hydrogenation of coronene cations

Hydrogenation of coronene cations

• Experiments show:– Hydrogenation increases with H exposure– Odd hydrogentation states are predominant– Occurrence of magic numbers of H attached:

5 -- 11 -- 17

Why are some hydrogenated states predominant?

First Hydrogenation

DFT calculations Equilibrium geometries, binding energies and transition states Binding energy for 1st hydrogenation

2.81 eV

2.14 eV

1.91 eV

Outer edgeInner EdgeCenter

First Hydrogenations

+1H• Outer edge carbon with

Eb=2.81 eV• Radical + Radical Little

barrier of 0.01 eV motion of the CH to form a CH2 group

+2H• Neighbor outer edge carbon

with Eb=2.94 eV• Radical + close shell system

barrier of 0.07 eV torsion of the C-C bond

Next Hydrogenations• Alternance

– H attachment to a radical even odd higher binding energy / small or no barrier

– H attachment to a closed shell system odd even lower binding energy / barrier

dominance of odd states

• Binding energies DO NOT alternate from odd to evendepend on reaction

H + radical /closed shell AND deformation of the system

EvenOdd

Next Hydrogenations

1

2

Outer edgeInner EdgeCenter

Next Hydrogenations

1

2

3

4

5

6

Barrier 0.1 eV

Outer edgeInner EdgeCenter

Next Hydrogenations

2

6

1

2

3

4

5

7

11

9 10

Barrier 0.1 eV

8Outer edgeInner EdgeCenter

Next Hydrogenations

6

1

4

5

7

11

9 10

8

1213

14

15 1617

181920

Barrier 0.1 eV

21

24

22

23

Outer edgeInner EdgeCenter

2

3

The sequence to hydrogenate coronene cations

• Hydrogenation of coronene cations follow a definite sequence (from binding energies and attachment barriers) occurrence of stable states 5, 11 and 17 = Magic numbers

• For these stable closed-shell cations: further hydrogenation requires appreciable structural changes high barriers

• Barriers to add H How do PAHs contribute to the formation of H2?

PDR model: H2 formation PAHs/dust with coronene as prototypical PAH in our model

H2 formation in PDRs

Boschman et al. 2015

H2 forms on PAHs for high Tgas and Tdust

Observations from PDRs can be explain by the formation of H2 on PAHs

PAHs are a high temperature pathway to molecular hydrogen

Summary and conclusions• ExperimentsPredominant hydrogenated states = magic numbers

• TheoryDefinite sequence to hydrogenate coronene cations stable PAHH+ associated with the magic numbers found experimentally

• AstrophysicsH2 can form efficiently on PAHs at high temperatures (Tgas > 200K) explain PDRs

• Remaining questions PAHH in space? Parameter study to find PAHH in PDRs. Resistance to UV? Determine their IR spectra signatures.

Li & Draine 2012

Thank you

H2 formation in PDRs

• H2 can form on PAHs through abstraction or photodesorption

• For coronene as prototypical “PAHs”, photo processes are the most important route to form H2, and dominate the dust route in warm environments (Tgas > 200 K)

• H2 on PAHs → necessary to reproduce the observations of PDRs.

• Our model considers coronene while a distribution of sizes should be considered (do PAHs with < 40C survive?).

• Larger sizes H and H2 loss as number of C Super-hydrogenated large PAHs (Montillaud 2013) H abstraction

• Study with distribution of sizes (including addition barriers) needed!

Thank you

Leon Boschman Thomas Schlathölter

Ronnie HoekstraGeert ReitsmaMarco Spaans

Nathalie RougeauSabine Morisset

Dominique Teilley-Billy