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Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Photons, Electrons and Desorption An Application of Laboratory.

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Presentation on theme: "Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Photons, Electrons and Desorption An Application of Laboratory."— Presentation transcript:

1 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Photons, Electrons and Desorption An Application of Laboratory Surface Science in Astrophysics Martin McCoustra

2 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University NGC 3603 W. Brander (JPL/IPAC), E. K. Grebel (University of Washington) and Y. -H. Chu (University of Illinois, Urbana- Champaign) Diffuse ISM Dense Clouds Star and Planet Formation (Conditions for Evolution of Life and Sustaining it) Stellar Evolution and Death The Chemically-controlled Cosmos

3 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  At the most important part of the matter cycle in the Universe today, chemistry exerts a controlling influence since molecules  Maintain the current rate of star formation  Ensure the formation of small, long-lived stars such as our own Sun  Seed the Universe with the chemical potential for life  But...  There have been problems in comparing the results of chemical network simulations of the evolution of dense gas clouds with observed column densities for even relatively simple species like H 2  Chemical reactions occurring on dust grains are used to account for the discrepancy between observations and gas- phase only models of the chemical evolution of dense clouds The Chemically-controlled Cosmos

4 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University CH 4 Icy Mantle The Chemically-controlled Cosmos H H2H2 H O H2OH2O H N H3NH3N Silicate or Carbonaceous Core nm CO, N 2

5 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University The Chemically-controlled Cosmos CH 4 Icy Mantle Silicate or Carbonaceous Core nm CO N2N2 H2OH2O NH 3 Heat Input Thermal Desorption UV Light Input Photodesorption Cosmic Ray Input Sputtering and Electron- stimulated Desorption CH 3 OH CO 2 CH 3 NH 2

6 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Returning molecules to the gas phase from the icy grain mantles is an important step in the surface physics and chemistry of grain – thermal and non- thermal mechanisms can contribute to this process. The Chemically-controlled Cosmos

7 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  The model system we have chosen to study is the benzene- water ice system  C 6 H 6 may be thought of as a prototypical PAH compound and is amongst the list of known interstellar molecules  Water ice is a good representation of icy mantles on grains  C 6 H 6 does not wet the H 2 O ice and forms an islanded layer; isolated C 6 H 6 molecules can diffuse between the islands (Ostwald ripening) at temperatures around and above 120 K  Amorphous silica or sapphire substrate moves us away from metal surfaces where UV irradiation can produce lots of hot electrons that will induce chemistry A Model System

8 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University The Experimental Arrangement

9 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University The Experimental Arrangement

10 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Both C 6 H 6 and H 2 O are observed to desorb translationally hot (in excess of 1000 K) in resonance with the C 6 H 6 absorption spectrum around 250 nm  Energy release can be explained with a simple model of unimolecular decomposition of a C 6 H 6... (H 2 O) x surface cluster in which C 6 H 6 is  facially hydrogen bonded to the water cluster via a single H 2 O molecule Shining a Little Light on Icy Surfaces

11 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Shining a Little Light on Icy Surfaces  Cross-sections for C 6 H 6 and H 2 O desorption can be estimated from PSD curves to be 4  cm 2 and 1  cm 2 respectively at 250 nm

12 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Cross-sections for C 6 H 6 and H 2 O desorption can be estimated from PSD curves to be 4  cm 2 and 1  cm 2 respectively at 250 nm Shining a Little Light on Icy Surfaces

13 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Icy films of C 6 H 6 and H 2 O ice were irradiated with electrons of energies of around 100 to 300 eV  Desorption of C 6 H 6 mediated by the H 2 O ice and the formation of solvated electrons  Desorption of C 6 H 6 diffusing between islands has a massive cross-section of around 2  cm 2 in this range  Build-up and long time decay process associated with diffusion of C 6 H 6 from islands followed by ESD has a cross- section of 5  cm 2 Firing a Few Electrons at Surfaces

14 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  H 2 O ESD in this energy range was measured by a combination of TPD and RAIRS to be ca. 5  cm 2 and independent of the C 6 H 6 coverage at exposures where C 6 H 6 forms islands  Supports the idea that electron cooling and attachment to water is important Firing a Few Electrons at Surfaces

15 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Non-thermal desorption of ices mediated by  Photon-stimulated desorption involving photons from the interstellar radiation field Astrophysical Impact Photon Flux at ca. 250 nm ≈ 10 8 cm -2 s -1 J. S. Mathis, P. G. Mezger, and N. Panagia, Astron. Astrophys., 1983, 128, 212.

16 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Non-thermal desorption of ices mediated by  Photon-stimulated desorption involving photons from the interstellar radiation field Astrophysical Impact C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203  Photon-stimulated desorption involving the background VUV field produced by cosmic ray ionisation Limiting cosmic ray induced UV Flux in Dense Regions ≈ 10 3 cm -2 s -1

17 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Electron-stimulated desorption associated from secondary electrons produced by cosmic ray interactions with icy grains Astrophysical Impact C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203 For 1MeV cosmic ray protons, the secondary electron yield is around 90 cm -2 s -1 at 100 to 300 eV  Non-thermal desorption of ices mediated by  Photon-stimulated desorption involving photons from the interstellar radiation field  Photon-stimulated desorption involving the background VUV field produced by cosmic ray ionisation

18 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides Astrophysical Impact

19 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides Astrophysical Impact  Steady-state

20 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides  Steady-state Astrophysical Impact  Thermal desorption

21 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Long wavelength ISRF-driven PSD will be important in regions where this radiation penetrates dense molecular clouds  ESD is as important, if not more important, than CRRF-driven PSD in dense molecular clouds  Surface Science techniques (both experimental and theoretical) can help us understand heterogeneous chemistry in the astrophysical environment  Much more work is needed and it requires a close collaboration between laboratory surface scientists (both experimental and computational), chemical modellers and observers Conclusions

22 Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University John Thrower, Ali Abdulgalil and Dr. Mark Collings (Heriot-Watt) Farah Islam and Dr. Daren Burke (UCL) Jenny Noble and Sharon Baillie (Strathclyde) Dr. Anita Dawes, Dr. Paul Kendall and Dr. Phil Holtom (OU) Dr. Wendy Brown (UCL) Dr. Helen Fraser (Strathclyde University) Professor Nigel Mason (OU) Professor Tony Parker and Dr. Ian Clark (CLF LSF) ££ EPSRC and STFC University of Nottingham ££ Acknowledgements


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