Probing the Gas-Grain Interaction

1. Probing the Gas-Grain Interaction Applications of Laboratory Surface Science in Astrophysics Martin McCoustra

2. Horsehead Nebula Triffid Nebula Eagle Nebula 30 Doradus Nebula The Chemically Controlled Cosmos

3. 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

4. The Chemically Controlled Cosmos • Hot, Shiny Things • Stars etc. • Elemental foundries • Small molecules, e.g. H2O, C2, SiO, TiO, SiC2 …, in cooler parts of stellar atmospheres • Nanoscale silicate and carbonaceous dusts

5. The Chemically Controlled Cosmos • Cold, Dark Stuff • Interstellar Medium (ISM) • Generally cold and dilute • Temperatures below 10 K and densities of a few particles per cm3 • Some hot regions • Photoionisation regions have effective temperatures of 100’s to 1,000’s of K • Some dense regions • Clouds have average densities approaching that of good quality UHV • Localised densities can approach even the HV or above

6. The Chemically Controlled Cosmos • Cold, Dark Stuff • Interstellar Medium (ISM) • Spectroscopic observations have found over 130 different types of chemical species in the gas and solid phases • Atoms, Radicals and Ions, e.g.H, N, O, …, OH, CH, CN, …, H3+, HCO+, ... • Simple Molecules, e.g.H2, CO, H2O, CH4, NH3, … • “Complex” Molecules, e.g.HCN, CH3CN, CH3OH, C2H5OH, CH3COOH, (CH3)2CO, glycine, other amino acids and pre-biotic molecules(?) • Observations tell us that these molecules are associated with the dense regions, which are themselves known to be sites of star and planet formation

7. The Chemically Controlled Cosmos • Molecules are crucial for • Maintaining the current rate of star formation • Ensuring the formation of small, long-lived stars such as our own Sun • Seeding the Universe with the chemical potential for life

8. Gravitational Collapse Gravitational Collapse Cold Cloud Hot Clump in Cold Cloud Star The Chemically Controlled Cosmos • Thermal motion will resist further gravitational collapse unless the cloud is radiatively cooled

9. The Chemically Controlled Cosmos • In the early Universe • Only H atoms were present and so radiative cooling would only be possible on electronic transitions, i.e. at temperatures of 1000s of K. • Collapsing gas clumps needed to be very large (100s of solar masses) to reach the temperature necessary to excite electronic transitions by gravitational collapse alone • In the current Universe • Rovibrational transitions in complex molecules result in radio, microwave and infrared emission and so provide the radiative cooling mechanism • Collapsing gas clumps are typical much smaller, near solar mass, since much less gravitational energy is required to match temperatures of a few 10s to 100s of K.

10. The Chemically Controlled Cosmos • Complex molecules point to a surprisingly complex chemistry • Low temperatures and pressures mean that most normal chemistry is impossible • No thermal activation • No collisional activation • Gas phase chemistry involving ion-molecule reactions and some type of reactions involving free radicals go a long way to explain what we see • But ... Astrophysicists invoke gas-dust interactions as a means of accounting for the discrepancy between gas-phase only chemical models and observations

11. 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

12. 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

13. The Chemically Controlled Cosmos • Dust grains are believed to have several crucial roles in the clouds • Assist in the formation of small hydrogen-rich molecules including H2, H2O, CH4, NH3, ... some of which will be trapped as icy mantles on the grains • Some molecules including CO, N2, ... can condense on the grains from the gas phase • The icy grain mantle acts as a reservoir of molecules used to radiatively cool collapsing clouds as they warm • Reactions induced by UV photons and cosmic rays in these icy mantles can create complex, even pre-biotic molecules

14. The Chemically Controlled Cosmos Surface physics and chemistry play a key role in these processes, but the surface physics and chemistry of grains was poorly understood.

15. Looking at Grain Surfaces • Ultrahigh Vacuum (UHV) is the key to understanding the gas-grain interaction • Pressures < 10-9 mbar

16. Looking at Grain Surfaces • Ultrahigh Vacuum (UHV) is the key to understanding the gas-grain interaction • Pressures < 10-9 mbar • Clean surfaces • Controllable gas phase

17. Looking at Grain Surfaces • Molecular Formation Rates • H2 is relatively well studied, but there is still some disagreement • For the heavier molecules (H2O, NH3etc.) nothing is known but watch this space!!! • Solid state synthesis in icy matrices using photons and low energy electrons is thought to be well understood but there are problems! • Desorption Processes • Thermal desorption is increasingly well understood • Cosmic ray sputtering is well understood • Photon and low energy electron stimulated processes are poorly understood, but again watch this space!!!

18. Shining a Little Light on Icy Surfaces • Many existing studies of photochemistry in icy mixtures (e.g. the work of the NASA Ames and Leiden Observatory groups) done at high vacuum • Such studies cannot answer the fundamental question of how much of the photon energy goes into driving physical (desorption, phase changes etc.) versus chemical processes • Measurements utilising the CLF UHV Surface Science Facility by a team involving Heriot-Watt, UCL and the OU seek to address this

19. Shining a Little Light on Icy Surfaces • Model system we have chosen to study is the water-benzene system • C6H6 may be thought of as a prototypical (poly)cyclic aromatic (PAH) compound • C6H6 is amongst the list of known interstellar molecules and heavier PAHs are believed to be a major sink of carbon in the ISM (and may account for the Diffuse Interstellar Bands and Unidentified Infrared Bands) • PAHs likely to be incorporated into icy grain mantles and are strongly absorbing in the near UV region • Can we detect desorption of C6H6 or even H2O following photon absorption? Is there any change in the ice morphology following photon absorption? Is there chemistry?

20. Shining a Little Light on Icy Surfaces

21. Shining a Little Light on Icy Surfaces Liquid N2 QMS Photon-induced Desorption trigger MCS Doubled Dye Laser Nd3+:YAG Time of Flight (ToF)

22. C6H6 H2O C6H6 H2O H2O C6H6 Sapphire Sapphire Sapphire Sapphire Shining a Little Light on Icy Surfaces • Sapphire substrate • Easily cooled to cryogenic temperatures by Closed Cycle He cryostat to around 60-80 K • Eliminate metal-mediated effects (hot electron chemistry) • Ices deposited by introducing gases into chamber via a fine leak valve to a consistent exposure (200 nbar s) • Irradiate at 248.8 nm (on-resonance), 250.0 nm (near-resonance) and 275.0 nm (off-resonance) at “low” (1.1 mJ/pulse) and “high” (1.8 mJ/pulse) laser pulse energies

23. Shining a Little Light on Icy Surfaces • C6H6 desorption observed at all wavelengths • Substrate-mediated desorption weakly dependent on wavelength • Adsorbate-mediated desorption reflects absorption strength of C6H6 • Yield of C6H6 is reduced by the presence of a H2O capping layer

24. Shining a Little Light on Icy Surfaces • H2O desorption echoes that of C6H6 • H2O does not absorb at any of these wavelengths and so desorption is mediated via the substrate and the C6H6 • Yield of H2O is increased by the presence of a C6H6 layer

25. Shining a Little Light on Icy Surfaces • Analysis of the ToF data using single and double Maxwell distributions for a density sensitive detector is on going • Preliminary results suggest that both the benzene and the water leave the surface hot • C6H6 in the substrate-mediated desorption channel has a kinetic temperature of ca. 550 K • C6H6 in the self-mediated desorption channel has a kinetic temperature of ca. 1100 K • H2O appears to behave similarly Photon- and Low Energy Electron-induced Desorption of hot molecules from icy grain mantles will have implications for the gas phase chemistry of the interstellar medium

26. Conclusions • 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, chemical modellers and observers

27. Acknowledgements John Thrower 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 CCLRC University of Nottingham ££