Recombination pathways and energy release in H 2 formation on graphitic surfaces

1. Recombination pathways and energy release in H2 formation on graphitic surfaces Liv Hornekær University of Aarhus, Denmark

2. Interstellar H2 formation Dense clouds - Diffuse clouds - PDRs Efficient recombination from physisorbed states: up to ~20K Above that more tightly bound states are needed Chemisorbed states on graphite (Cazaux & Tielens, Astrophys. J. 604, 222 (2004)) Formation pumping a possible explanation of PDR observations of vibrational state populations (NGC2023, Burton et al., MNRAS 257, 1p (1992))

3. Binding sites on graphitic surfaces Physisorption Step edges Vacancy Chemisorption Physisorption: Creighan et al, J. Chem. Phys. 124, 114701 (2006) Chemisorption at defects: Sha et al, J. Am. Chem. Soc. 126, 13095 (2004) Güttler et al, Surface Science 570, 218 (2004)

4. H chemisorbed on HOPG Eva Rauls Sha et al, Surface Science 496, 318 (2002)

5. H2 formation on graphite dQ = -k0 e- / T Qn EB kB dt n=1 => First order desorption 490 K => 1.4 eV 580 K => 1.6 eV Zecho et al, J. Chem. Phys. 117, 8486 (2002)

6. STM on graphite 2.46 Å

7. Hydrogen on graphite –Monomers Zeljko Sljivancanin 155 x 171 Å2, 180 K Vt~ -710mV, It~ -0.16nA

8. Monomer desorption Flux: 1012 cm-2s-1 STM at ~180 K RT 20% 1030 x 1140 Å2 1030 x 1140 Å2 Experiment: Upper limit: t = 6 min. Theory: Eb= 0.9 eV, n = 1013 s-1 =>t = 130 s

9. Monomer desorption Flux: 1012 cm-2s-1 STM at ~180 K RT 20% 1030 x 1140 Å2 1030 x 1140 Å2 Flux: 1014 cm-2s-1 STM at ~170 K RT 80% 1030 x 1140 Å2 515 x 570 Å2

10. 103 x 114 Å2 H-Dimers on graphite Dimer A Dimer B Vt = 884 mV, It = 0.16 nA

11. Dimers: Theory vs. Experiment Dimer A Dimer B Vt=0.9 V, LDOS=1x10-6 (eV)-1 Å-3 e. f. Vt = 884 mV, It = 0.16 nA

12. Diffusion Barrier to diffusion for an isolated H atom: 1.14 eV Barrier to desorption for an isolated H atom: 0.9 eV

13. Dimer formation Eva Rauls

14. 103 x 114 Å2 H-Dimers on graphite Dimer A Dimer B Vt = 884 mV, It = 0.16 nA

15. Dimers after Anneal 103 x 114 Å2 80 x 72 Å2 Vt = 884 mV, It = 0.36 nA Vt = 884 mV, It = 0.19 nA

16. Recombination pathways Hornekaer et al. Phys. Rev. Lett. 96, 156104 (2006)

17. Measuring the kinetic energy of formed molecules Laser Induced Thermal Desorption (LITD) Alexandrite Laser 4 mJ 100 ns pulse Time of Flight Measurement D Laser QMS D2

18. Kinetic energy distribution

19. H on graphitic surfaces under PDR conditions? Tgas ~ 600-1000K => population of the chemisorbed states on graphite Barrier to recombination: 1.4 eV => thermal desorption: 1000 years at 300K Thermal spikes? Localized heating? => Ekin ~ 1.3 eV, Surface + rovibration: 2.1 eV or high coverage of H2 => Eley Rideal

20. Eley Rideal - Abstraction Chemisorption Sha et al, J. Chem. Phys. 116, 7158 (2002) Zecho et al, Chem. Phys. Lett. 366, 188 (2002)

21. Outlook PAHs Onions

22. People involved Laser desorption: Arnd Baurichter Alan Luntz Saoud Baouche Dept. Phys. SDU Thomas Zecho Univ. Bayreuth and MPG Plasma STM group: Wei Xu Roberto Ortero Flemming Besenbacher iNANO and Dept. Phys. and Astron. University of Aarhus Surface Theory: Eva Rauls Bjørk Hammer iNANO and Dept. Phys. and Astron. University of Aarhus Zeljko Sljivancanin EPFL