PowerPoint Presentation - Nobel Symposium 133

							 Recombination pathways and
energy release in H2 formation
    on graphitic surfaces

             Liv Hornekær
     University of Aarhus, Denmark
          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))
       Binding sites on graphitic surfaces


              Step edges               Physisorption




                            Chemisorption            Vacancy


   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)
               H chemisorbed on HOPG




                                             Eva Rauls
Sha et al, Surface Science 496, 318 (2002)
H2 formation on graphite

                    dQ
                        = -k0 e- EB / kBT Qn
                     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)
         STM on graphite




2.46 Å
  Hydrogen on graphite –Monomers




Zeljko Sljivancanin
                      155 x 171 Å2 , 180 K
                      Vt~ -710mV, It~ -0.16nA
                 Monomer desorption

Flux:
1012 cm-2s-1
STM at                              RT
~180 K                             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
                Monomer desorption
Flux:
1012 cm-2s-1
STM at                           RT
~180 K                          20%


               1030 x 1140 Å2         1030 x 1140 Å2

Flux:
1014 cm-2s-1
STM at                           RT
~170 K                          80%


               1030 x 1140 Å2         515 x 570 Å2
103 x 114 Å2
               H-Dimers on graphite
                              Dimer A




                              Dimer B




                              Vt = 884 mV, It = 0.16 nA
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
                   Diffusion
Barrier to diffusion for an isolated H atom: 1.14 eV
Barrier to desorption for an isolated H atom: 0.9 eV
            Dimer formation




Eva Rauls
103 x 114 Å2
               H-Dimers on graphite
                              Dimer A




                              Dimer B




                              Vt = 884 mV, It = 0.16 nA
                Dimers after Anneal
103 x 114 Å2                80 x 72 Å2




Vt = 884 mV, It = 0.19 nA   Vt = 884 mV, It = 0.36 nA
Recombination pathways




        Hornekaer et al. Phys. Rev. Lett. 96, 156104 (2006)
Measuring the kinetic energy of
     formed molecules
                     Laser Induced
                   Thermal Desorption
                        (LITD)
      D
                       Alexandrite
    Laser                 Laser

                          4 mJ
          QMS
                       100 ns pulse
     D2

                Time of Flight Measurement
Kinetic energy distribution
          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
           Eley Rideal - Abstraction




                           Chemisorption



Sha et al, J. Chem. Phys. 116, 7158 (2002)
Zecho et al, Chem. Phys. Lett. 366, 188 (2002)
         Outlook
Onions             PAHs
                    People involved


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