Superhydrogenated PAHs: Catalytic formation of H2

A possible route to H 2 formationAndrew Cassidy(John Thrower)Department of Physics and Astronomy, Aarhus University, Denmark

Superhydrogenated PAHs• Evidence in IR emission• C-H stretching mode• 3.3 μm – aromatic• 3.4 μm – aliphatic• High UV flux (Orion bar)• Limited excess hydrogen• Low UV flux (IRAS 05341)• Significant excesshydrogen• -CH 3 or -HM. P. Bernstein, et al., ApJ, 472, L127 (1996).

Hydrogenation of Coronene• DFT Calculations• Addition of 1 st H• Outer edge site• Small barrier• Graphite -> 200 meV• Strongly bound• Graphite -> 700 meV• Lower gastemperatures viable60 meV-1.4 eVC 24 H 13C 24 H 12 + HE. Rauls & L. Hornekær, Astrophys. J., 679, 531 (2008).

H 2 formation via abstraction• DFT Calculations• Barrierless addition of 2 ndH• Large binding energy• Barrierless H abstractionby 2 nd H• H 2 formation• Competition betweenabsraction and addition(low/vanishing barriers)• Higher degrees ofhydrogenation, includingcenter sites, alsofavourable

Experimental arrangement

TPD: C 24 H 12 from graphite• Graphite (HOPG) exposedto C 24 H 12• Thermal desorption @ 1K/s• Monitor m/z=300 (parention)• Resolve mono- andmultilayers• Multilayers consistentwith previous study †• Desorption energy ca. 1.5eV †R. Zacharia, et al., Phys. Rev. B, 69, 155406 (2004).

Hydrogenation of CoroneneCoronene TPDCoronene exposed to 5 min Dφ=10 16 D atoms cm -2Evidence for hydrogenation ofcoronene through detection of C 24 H x D y

Hydrogenation of Coronene• Hydrogenated coronenedesorbs at slightlylower temperature• Cf. C 6 H 12 desorbing atlower temperature thanC 6 H 6††M. Xi, et al., J. Vac. Sci. Technol. B, 10, 2440 (1992).

Hydrogenation of Coronene• Mass spectra of desorbedspecies• Integrated over TPD peak• Expected fragmentation /isotope peaks for coroneneCoronene

Hydrogenation of Coronene• Mass spectra of desorbedspecies• Integrated over TPD peak• Expected fragmentation /isotope peaks for coronene• Exposure to atomic D• Clear evidence forsuperhydrogenated species• Simultaneous loss ofparent coroneneCoroneneCoronene exposed to 5 min D

Decay of Coronene Yield• Significant loss of coronene• Single layer• Efficient hydrogenation• σ=0.4 Å 2 300exp( )C A m

Hydrogenation of Coronene• Long exposure times• Extensive hydrogenation• Dominated by high m/zCoronene exposed to 180 min D• Close to limiting case(m/z=360)Perhydrocoronene

Hydrogenation of Coronene• Long exposure times• Close to limiting case(m/z=360)• Above m/z=336confirms hydrogenationof center sites

Hydrogenation: STM imagesCoroneneSubstrate: Cu(100)70 x 70 Å

Hydrogenation: STM imagesCoroneneCoronene exposed to DSubstrate: Cu(100)70 x 70 Å

Hydrogenation of Coronene• Long exposure times• Close to limiting case(m/z=360)• Above m/z=336 confirmshydrogenation ofcenter sites• Above m/z=348 requiresH-D exchange• Implies abstractionreactions• => HD (H 2 formation)

Thank you for your attention

Interstellar H 2 formation• The “well-known”problem

Interstellar H 2 formation• The “well-known”problem• Surface reactions• Low T• Physisorption on icesurfaces~30 K

Interstellar H 2 formation• The “well-known”problem• Surface reactions• Low T• Physisorption on icesurfaces• High T• Chemisorption oncarbonaceous surfaces~30 K

Interstellar H 2 formation• The “well-known”problem• Surface reactions• Low T• Physisorption on icesurfaces• High T• Chemisorption oncarbonaceous surfaces• Intermediate T• PAHS?~30 K

Conclusions• DFT calculations and experimental observations:• Show efficient superhydrogenation (includingcenter sites) of coronene• Indirect evidence for HD (H 2 ) formation throughabstraction reactions

Conclusions• DFT calculations indicate efficientsuperhydrogenation of coronene• Experimental observation of superhydrogenatedcoronene• Clear evidence for extensive hydrogenation• Indirect evidence for HD (H 2 ) formation throughabstraction reactions3.12 3.31 3.64See posters by Andrew Cassidy, Emil Friis and Vito MennellaSTM TPD IR

STM parameters (Cu)• Coronene• V t =-3500 mV• I t =-0.1 nA• Coronene exposed to D• V t =-1900 mV• I t =-0.2 nA

Thermal desorption (TPD)ThermocoupleAmplifierControllerPCQMS Signal desorption rateExpose to Coronene to D - Hot (~2100 K)

Experimental arrangement

Interstellar PAHs• High UV flux• Small PAHs dissociated• Intermediate UV flux• Dehydrogenation• IonizationLow UV flux• Higher hydrogenationstates for large PAHsstablePAHhProductsPAH h Product HPAH h PAH ePAH H HPAHV. Le Page, et al., ApJSS, 132, 233 (2001).

Hydrogenation: STM imagesCoroneneSubstrate: HOPG200 x 200 Å

Hydrogenation: STM imagesCoroneneCoronene exposed to DSubstrate: HOPG200 x 200 Å

Hydrogenation: STM imagesCoroneneCoronene exposed to DSubstrate: Cu(100)70 x 70 Å