The hydrogenation of PAH cations: a journey guided by stability and magic numbers Stéphanie Cazaux Leon Boschman Thomas Schlathölter Ronnie Hoekstra Geert Reitsma Marco Spaans Nathalie Rougeau Sabine 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 Although the exact nature of carriers is unknown, The UIE bands are coomonly attributed to PAHs Once they are electronically excited by the UV, PAHs relax to the groundstate mainly by emitting infrared light that comes from their C-C and C-H vibrations. Peeters et al. 2002 A&A 390 Tielens 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 ISM key 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? mid-infrared (5.5-14 μm) spectrum can be used to trace key physical conditions along a given line of sight, such as the UV radiation field, the ionization parameter and the dust column density. These parameters are often difficult to determine independently from PDR models. The PAHTAT toolbox offers the opportunity to analyze mid-IR spectra using a limited number of parameters, that are associated with the physical properties of the dust and gas being observed.

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 +1 +3 +5 Boschman, L. et al. 2012

Hydrogenation of coronene cations +1 +3 +5 Boschman, L. et al. 2012

+17H +11H +5H

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 2.81 eV 2.14 eV 1.91 eV DFT calculations  Equilibrium geometries, binding energies and transition states Binding energy for 1st hydrogenation 2.81 eV 2.14 eV 1.91 eV Outer edge Inner Edge Center

First Hydrogenations +1H +2H 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  dominance of odd states 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 even depend on reaction H + radical /closed shell AND deformation of the system Even Odd

Next Hydrogenations Outer edge Inner Edge Center 1 2

Next Hydrogenations 6 Outer edge Inner Edge Center 3 Barrier 0.1 eV 4 5 1 2

Next Hydrogenations 6 7 Outer edge Inner Edge Center 8 3 Barrier 0.1 eV 4 5 1 2 2 11 9 10

Next Hydrogenations 6 7 Outer edge Inner Edge Center 8 24 23 3 22 4 Barrier 0.1 eV 19 18 21 20 17 5 15 16 12 1 13 2 14 11 9 10

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 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 Boschman et al. 2015

Summary and conclusions Experiments Predominant hydrogenated states = magic numbers Theory Definite sequence to hydrogenate coronene cations  stable PAHH+ associated with the magic numbers found experimentally Astrophysics H2 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!

Dominique Teilley-Billy Leon Boschman Thomas Schlathölter Ronnie Hoekstra Geert Reitsma Marco Spaans Nathalie Rougeau Sabine Morisset Dominique Teilley-Billy Thank you