Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Probing the Gas-Grain Interaction Applications of Laboratory Surface Science in Astrophysics Martin McCoustra

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University The Chemically Controlled Cosmos Eagle Nebula Horsehead NebulaTriffid Nebula 30 Doradus Nebula

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

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Astrophysicists invoke gas-dust interactions as a means of accounting for the discrepancy between gas-phase only chemical models and observations The Chemically Controlled Cosmos Understanding the Chemical Evolution of the Universe, which we observe through the remote eyes of molecular spectroscopy, helps us to understand the Physical Evolution of the Universe

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

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Dust grains are believed to have several crucial roles in the clouds  Assist in the formation of small hydrogen-rich molecules including H 2, H 2 O, CH 4, NH 3,... some of which will be trapped as icy mantles on the grains  Some molecules including CO, N 2,... 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 The Chemically Controlled Cosmos

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

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Surface physics and chemistry play a key role in these processes, but the surface physics and chemistry of grains was poorly understood. The Chemically Controlled Cosmos

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Ultrahigh Vacuum (UHV) is the key to understanding the gas-grain interaction  Pressures < mbar Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Ultrahigh Vacuum (UHV) is the key to understanding the gas-grain interaction  Pressures < mbar  Clean surfaces  Controllable gas phase Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Gold Film Cool to Below 10 K Infrared for RAIRS Mass Spectrometer Atoms (H, N, O) and Radicals (CN, OH, CH) UV Light and Electrons Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University H. J. Fraser, M. P. Collings and M. R. S. McCoustra Rev. Sci. Instrum., 2002, 73, 2161 Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  First we have to make molecules...  H 2 is relatively well studied, but there is still some disagreement  For the heavier molecules (H 2 O, NH 3 etc.) little is currently known, but watch this space as groups in Paris and Edinburgh are starting to work on this problem  Solid state synthesis in icy matrices using photons and low energy electrons is thought to be well understood but there are problems! Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  But then we need to consider how to return these molecules to the gas phase...  Desorption induced by Collisions Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Thermal collision energies too small to induce desorption of anything but the weakest of physisorbed atoms and molecules  Cosmic rays with kinetic energies in excess of the surface binding energy, typically a few eV, can induce desorption  At energies in the 100s of eV and above, simple billiard ball dynamics apply and we can very successfully use molecular dynamics simulations to investigate the desorption and ionisation processes associated with cosmic ray sputtering  At energies in the range of a few eV, chemical energies, processes associated with electron exchange, charge neutralisation and chemical reactions cannot be modelled simply using classical molecular dynamics simulations  Grain-grain collisions as a desorption mechanism? Molecular Pinball on Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  But then we need to consider how to return these molecules to the gas phase...  Desorption induced by Collisions  Desorption induced by Heating Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Experimentally the simplest of measurements combining solid state (RAIRS) and gas phase (QMS) probes  Investigate increasing complex systems as you learn more about the simpler ones  Water Ice  Kinetic order of desorption for solids is zero NOT one!  Water/Carbon Monoxide  Morphological changes in the water ice play an important role in promoting trapping of volatiles beyond their normal desorption temperature  Water/Methanol/Carbon Monoxide  Clathrate formation? Thermal Processes on Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University < 10 K Temperature K K K 160 K M. P. Collings, H. J. Fraser, J. W. Dever, M. R. S. McCoustra and D. A. Williams Ap. J., 2003, 583, Thermal Processes on Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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 Thermal Processes on Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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 10s of pK s -1 cf. 80 mK s -1 in our TPD studies Experiment Model Thermal Processes on Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  What do these observations mean to those modelling the chemistry of the interstellar medium? Assume Heating Rate of 1 K millennium -1 Old Picture of CO Evaporation New Picture of CO Evaporation Thermal Processes on Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Ices in the interstellar medium comprise more than just CO and H 2 O. What behaviour might species such as CO 2, CH 4, NH 3 etc. exhibit?  TPD Survey of Overlayers and Mixtures H2OH2O CH 3 OH OCS H2SH2S CH 4 N2N2 Thermal Processes on Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Qualitative survey of TPD of grain mantle constituents  Type 1  Hydrogen bonding materials, e.g. NH 3, CH 3 OH, …, which desorb only when the water ice substrate desorbs  Type 2  Species where T sub > T pore collapse, e.g. H 2 S, CH 3 CN, …, 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 T sub < T pore collapse, e.g. N 2, O 2, …, 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) H2OH2O CH 3 OH OCS H2SH2S CH 4 N2N2 Thermal Processes on Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  But then we need to consider how to return these molecules to the gas phase...  Desorption induced by Collisions  Desorption induced by Heating  Desorption induced by Electronic Excitation  Photon Absorption  (Secondary) Electron Attachment Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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 Shining a Little Light on Icy Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Model system we have chosen to study is the water-benzene system  C 6 H 6 may be thought of as a prototypical (poly)cyclic aromatic (PAH) compound  C 6 H 6 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 C 6 H 6 or even H 2 O following photon absorption? Is there any change in the ice morphology following photon absorption? Is there chemistry? Shining a Little Light on Icy Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Shining a Little Light on Icy Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Doubled Dye Laser Nd 3+ :YAG QMS MCS trigger Photon-induced Desorption Time of Flight (ToF) Liquid N 2 Shining a Little Light on Icy Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Sapphire substrate  Easily cooled to cryogenic temperatures by Closed Cycle He cryostat to around 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) Sapphire C6H6C6H6 C6H6C6H6 C6H6C6H6 H2OH2OH2OH2O H2OH2O Shining a Little Light on Icy Surfaces  Irradiate at nm (on-resonance), nm (near- resonance) and nm (off-resonance) at “low” (1.1 mJ/pulse) and “high” (1.8 mJ/pulse) laser pulse energies

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  C 6 H 6 desorption observed at all wavelengths  Substrate-mediated desorption weakly dependent on wavelength  Adsorbate-mediated desorption reflects absorption strength of C 6 H 6  Yield of C 6 H 6 is reduced by the presence of a H 2 O capping layer Shining a Little Light on Icy Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  H 2 O desorption echoes that of C 6 H 6  H 2 O does not absorb at any of these wavelengths and so desorption is mediated via the substrate and the C 6 H 6  Yield of H 2 O is increased by the presence of a C 6 H 6 layer Shining a Little Light on Icy Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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  C 6 H 6 in the substrate-mediated desorption channel has a kinetic temperature of ca. 550 K  C 6 H 6 in the self-mediated desorption channel has a kinetic temperature of ca K  H 2 O appears to behave similarly Shining a Little Light on Icy Surfaces 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

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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 Conclusions

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University 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 ££ Acknowledgements