the hydrogenation of pah cations: a journey guided by stability and magic numbers stéphanie cazaux...
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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