Photons, Electrons and Desorption

1. Photons, Electrons and Desorption An Application of Laboratory Surface Science in Astrophysics Martin McCoustra

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

3. The Chemically-controlled Cosmos • 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 H2 • 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

4. 1 - 1000 nm H2 H Icy Mantle Silicate or Carbonaceous Core H3N H H2O H CH4 CO, N2 N CO, N2 O The Chemically-controlled Cosmos

5. 1 - 1000 nm Heat Input CH3NH2 CH3OH Icy Mantle Silicate or Carbonaceous Core NH3 H2O Thermal Desorption N2 CH4 CO2 CO Cosmic Ray Input Photodesorption Sputtering and Electron-stimulated Desorption UV Light Input The Chemically-controlled Cosmos

6. The Chemically-controlled Cosmos 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.

7. A Model System • The model system we have chosen to study is the benzene-water ice system • C6H6 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 • C6H6 does not wet the H2O ice and forms an islanded layer; isolated C6H6 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

8. The Experimental Arrangement

9. The Experimental Arrangement

10. Shining a Little Light on Icy Surfaces • Both C6H6 and H2O are observed to desorb translationally hot (in excess of 1000 K) in resonance with the C6H6 absorption spectrum around 250 nm • Energy release can be explained with a simple model of unimolecular decomposition of a C6H6...(H2O)x surface cluster in which C6H6 is  facially hydrogen bonded to the water cluster via a single H2O molecule

11. Shining a Little Light on Icy Surfaces • Cross-sections for C6H6 and H2O desorption can be estimated from PSD curves to be 410-19 cm2 and 110-19 cm2 respectively at 250 nm

12. Shining a Little Light on Icy Surfaces • Cross-sections for C6H6 and H2O desorption can be estimated from PSD curves to be 410-19 cm2 and 110-19 cm2 respectively at 250 nm

13. Firing a Few Electrons at Surfaces • Icy films of C6H6 and H2O ice were irradiated with electrons of energies of around 100 to 300 eV • Desorption of C6H6 mediated by the H2O ice and the formation of solvated electrons • Desorption of C6H6 diffusing between islands has a massive cross-section of around 210-15 cm2 in this range • Build-up and long time decay process associated with diffusion of C6H6 from islands followed by ESD has a cross-section of 510-17 cm2

14. Firing a Few Electrons at Surfaces • H2O ESD in this energy range was measured by a combination of TPD and RAIRS to be ca. 510-17 cm2 and independent of the C6H6 coverage at exposures where C6H6 forms islands • Supports the idea that electron cooling and attachment to water is important

15. Photon Flux at ca. 250 nm ≈ 108 cm-2 s-1 Astrophysical Impact • Non-thermal desorption of ices mediated by • Photon-stimulated desorption involving photons from the interstellar radiation field J. S. Mathis, P. G. Mezger, and N. Panagia, Astron. Astrophys., 1983, 128, 212.

16. Limiting cosmic ray induced UV Flux in Dense Regions ≈ 103 cm-2 s-1 Astrophysical Impact • 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 C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203

17. For 1MeV cosmic ray protons, the secondary electron yield is around 90 cm-2 s-1 at 100 to 300 eV Astrophysical Impact • 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 • Electron-stimulated desorption associated from secondary electrons produced by cosmic ray interactions with icy grains C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203

18. Astrophysical Impact • Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides

19. Astrophysical Impact • Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides • Steady-state

20. Astrophysical Impact • Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides • Steady-state • Thermal desorption

21. Conclusions • 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

22. Acknowledgements 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 ££