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Photo physics and photo chemistry of ice films on graphite

Photo physics and photo chemistry of ice films on graphite. Department of Applied Physics Chalmers and Göteborg University. Dinko Chakarov Johan Bergeld Michael Gleeson Bengt Kasemo ……………. Photon induced processes (UV - visible – IR) Water ice (amorphous, crystalline, reactions)

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Photo physics and photo chemistry of ice films on graphite

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  1. Photo physics and photo chemistry of ice films on graphite Department of Applied Physics Chalmers and Göteborg University Dinko Chakarov Johan Bergeld Michael Gleeson Bengt Kasemo ……………

  2. Photon induced processes (UV - visible – IR) Water ice (amorphous, crystalline, reactions) Surfaces Graphite Spectroscopy Key words:

  3. Experimental conditions:° Atomically clean and ordered surfacesIce (H2O/D2O) on HOPG (XYA)° UHV (below 10-10 torr)° Temperature range 25 - 1500 K° Photon fluxes (cw and pulsed)1012 - 1029 photons.s-1° Arc lamps + filters/monochromator and/or Nd:YAG based OPO (220-1600 nm)Experimental methods:° HREELS ° TDS/ITS° PID ° QCM

  4. Ice - Optical / Electronic properties

  5. Graphite: Electronic/Optical Properties electronic configuration: 2-4 or 1s22s22p2 Extinction coefficient Photon Energy (eV)

  6. A A A A PHOTO DESORPTION OF ALKALI ATOMS Graphite J. Chem. Phys. 106 (3), 982, 1997

  7. Phenomena/Examples: • Photoinduced structural changes in amorphous ice • Photoejection of water molecules from amorphous ice • Photoreactions: H2O & coadsorbates on graphite

  8. Water on Graphite TPD; binding energy lateral interactions (UHV and Low Temperatures) HREELS; energetics orientation ITD; structre coverages

  9. Structure of Ice (Ic and Ih) Hydrogen bonding: Ideal ice structures obey the so-called Bernal-Fowler rules: • each hydrogen atom (or proton) is situated on the • line joining each pair of oxygens; • each oxygen atom has two hydrogen atoms attached • to it at distances of about 1Å, • thereby forming a water molecule H2O.

  10. Photoinduced crystallization • Experimental observations

  11. Re-crystallization Mechanism PRL, 81, 5181 (1999)

  12. Defects annealing

  13. Structural changes of ice films

  14. Balance between adsorbed and gas phase water Reactivity of the ice surface Importance and Consequences

  15. Substrates: Graphite, Si(001), Pt(111), … Coadsorbates: Metal ions and clusters: Na, K, Cs, Ag, Au Simple molecules: CO, NO, H2S Observed products: H2, CO, CO2, CH4, NH3, … Photoreactions with ice; investigated systems:

  16. AM-stabilized H2O : TPD

  17. Photoreactions of water and carbon at 90 K - C - H - O Carbon/Graphite J. Chem. Phys. 115 (20) 2001

  18. 18000 16000 Irradiation: 355 nm / 5.3 mJ (10x10 sec exposures) 14000 12000 NO exposure ~12 L Ag coverage 10000 0,0 ML 0,3 ML NO desorption 1,3 ML 8000 2,2 ML 6000 4000 2000 0 -25 0 25 50 75 100 125 150 175 200 225 Time (sec) Photo-desorption as a function of Ag coverage

  19. Summary: • Photoinduced structural changes in amorphous ice • Photoejection of water molecules from amorphous ice • Photoreactions: H2O & coadsorbates on graphite • Demonstrate the richness of the processes and phenomena; • Knowing the conditions is possible to • identify the predominant process.

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