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. Atoms (H, N, O) and Radicals (CN, OH, CH) UV Light and Electrons Mass Spectrometer Infrared for RAIRS Gold Film Cool to Below 10 K Looking at Grain Surfaces

18. Looking at Grain Surfaces H. J. Fraser, M. P. Collings and M. R. S. McCoustra Rev. Sci. Instrum., 2002, 73, 2161

19. 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!!!

20. Water Ice Films • At temperatures around 10 K, ice grows from the vapour phase by ballistic deposition. The resulting films, pASW, are highly porous (Kay and co-workers, J. Chem. Phys., 2001, 114, 5284; ibid, 5295) • Thermal processing of the porous films results in pore collapse at temperatures above ca. 30 K to give cASW • TEM studies show the pASWcASW phase transition occurring between 30 and 80 K and the cASW Ic crystallisation process at ca. 140 K in UHV (Jenniskens and Blake, Sci. Am., 2001, 285(2), 44)

21. CO on Water Ice • CO exposure build up sequence on pASW

22. CO on Water Ice • At low exposures • CO monolayer peak at around 50 K • Volcano peak (140 K) and co-desorption peak (160 K) both observed

23. CO on Water Ice • With Increasing CO exposure • CO monolayer peak moves to lower temperature • Repulsive interactions? • Pore filling? • Volcano and co-desorption peaks saturate • Ice film can trap only a certain amount of CO

24. Extended Compact 2152 cm-1 2140 cm-1 CO on Water Ice • At sub-monolayer exposures, CO RAIR spectrum shows two features that grow in at 2152 and 2140 cm-1, respectively • Two binding sites for CO on the water surface?

25. CO on Water Ice • Two multilayer features grow on top of the monolayer features at 2142 and 2138 cm-1 • Splitting of longitudinal (LO - 2138 cm-1) and transverse optical (TO - 2142 cm-1) modes of the solid CO - LST Splitting

26. CO on Water Ice • Between 8 and 15 K, redistribution of IR intensity without significant loss to the gas phase suggests CO migration into porous ice structure. • At least two CO binding sites characterised by 2152 cm-1 and 2138 cm-1 features.

27. CO on Water Ice • High frequency feature lost as pores collapse between 30 and 80 K. • A single CO site is preferred above 80 K until volcano desorption occurs. • Single feature, 2138 cm-1, is all we observe if we adsorb on to non-porous ice grown at 80 K.

28. 160 K 135 - 140 K Temperature 30 - 70 K 10 - 20 K < 10 K CO on Water Ice M. P. Collings, H. J. Fraser, J. W. Dever, M. R. S. McCoustra and D. A. Williams Ap. J., 2003, 583, 1058-1062

29. CO on Water Ice • To go further than this qualitative picture, we must construct a kinetic model • Desorption of CO monolayer on water ice and solid CO • Porous nature of the water ice substrate and migration of solid CO into the pores - “oil wetting a sponge” • Desorption and re-adsorption in the pores delays the appearance of the monolayer feature - “sticky bouncing along pores” • Pore collapse kinetics treated as second order autocatalytic process and results in CO trapping • Trapped CO appears during water ice crystallisation and desorption

30. CO on Water Ice • The model reproduces well our experimental observations. • We are now using it in a predictive manner to determine what happens at astronomically relevant heating rates, i.e. A few nK s-1cf. 80 mK s-1 in our TPD studies Experiment Model

31. New Picture of CO Evaporation Old Picture of CO Evaporation CO on Water Ice • What do these observations mean to those modelling the chemistry of the interstellar medium? Assume Heating Rate of 1 K millennium-1

32. H2O CH3OH OCS H2S CH4 N2 Beyond CO on Water Ice • Ices in the interstellar medium comprise more than just CO and H2O. What behaviour might species such as CO2, CH4, NH3etc. exhibit? • TPD Survey of Overlayers and Mixtures

33. H2O CH3OH OCS H2S CH4 N2 Beyond CO on Water Ice • Qualitative survey of TPD of grain mantle constituents • Type 1 • Hydrogen bonding materials, e.g. NH3, CH3OH, …, which desorb only when the water ice substrate desorbs • Type 2 • Species where Tsub > Tpore collapse, e.g. H2S, CH3CN, …, have a limited ability to diffuse and hence show only molecular desorption and do not trap when overlayered on water ice but exhibit largely trapping behaviour in mixtures • Type 3 • Species where Tsub < Tpore collapse, e.g. N2, O2, …, readily diffuse and so behave like CO and exhibit four TPD features whether in overlayers or mixtures • Type 4 • Refractory materials, e.g. metals, sulfur, etc. desorb only at high temperatures (100’s of K)

34. 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

35. 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?

36. Shining a Little Light on Icy Surfaces

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

38. 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

39. 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

40. 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

41. 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

42. 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

43. Acknowledgements Professor David Williams and Dr Serena Viti (UCL) Dr. Helen Fraser (Strathclyde University) Dr. Mark Collings Rui Chen, John Dever, Simon Green and John Thrower Dr. Wendy Brown (UCL) and her group Professor Nigel Mason (OU) and his group Professor Tony Parker and Dr. Ian Clark (CLF LSF) ££ PPARC, EPSRC and CCLRC Leverhulme Trust University of Nottingham ££