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  Hot, Shiny Things  Stars etc.  Elemental foundries  Small molecules, e.g. H 2 O, C 2, SiO, TiO, SiC 2 …, in cooler parts of stellar atmospheres  Nanoscale silicate and carbonaceous dusts The Chemically Controlled Cosmos

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Cold, Dark Stuff  Interstellar Medium (ISM)  Generally cold and dilute  Temperatures below 10 K and densities of a few particles per cm 3  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 The Chemically Controlled Cosmos

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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, …, H 3 +, HCO +,...  Simple Molecules, e.g. H 2, CO, H 2 O, CH 4, NH 3, …  “Complex” Molecules, e.g. HCN, CH 3 CN, CH 3 OH, C 2 H 5 OH, CH 3 COOH, (CH 3 ) 2 CO, 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 The Chemically Controlled Cosmos

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

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Thermal motion will resist further gravitational collapse unless the cloud is radiatively cooled Cold Cloud Gravitational Collapse Hot Clump in Cold Cloud Gravitational Collapse Star The Chemically Controlled Cosmos

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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. The Chemically Controlled Cosmos

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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 The Chemically Controlled Cosmos

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 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  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 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  Molecular Formation Rates  H 2 is relatively well studied, but there is still some disagreement  For the heavier molecules (H 2 O, NH 3 etc.) 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!!! Looking at Grain Surfaces

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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  I c crystallisation process at ca. 140 K in UHV (Jenniskens and Blake, Sci. Am., 2001, 285(2), 44) Water Ice Films

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  CO exposure build up sequence on pASW CO on Water Ice

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  At low exposures  CO monolayer peak at around 50 K  Volcano peak (140 K) and co- desorption peak (160 K) both observed CO on Water Ice

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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 CO on Water Ice

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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? Extended Compact 2152 cm cm -1 CO on Water Ice

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  Two multilayer features grow on top of the monolayer features at 2142 and 2138 cm -1  Splitting of longitudinal (LO cm -1 ) and transverse optical (TO cm -1 ) modes of the solid CO - LST Splitting CO on Water Ice

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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. CO on Water Ice

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University  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. CO on Water Ice

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, CO on Water Ice

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 CO on Water Ice

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 nK s - 1 cf. 80 mK s -1 in our TPD studies Experiment Model CO on Water Ice

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 CO on Water Ice

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 Beyond CO on Water Ice

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 Beyond CO on Water Ice

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 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 ££ PPARC, EPSRC and CCLRC Leverhulme Trust University of Nottingham ££ Acknowledgements Dr. Wendy Brown (UCL) and her group Professor Nigel Mason (OU) and his group Professor Tony Parker and Dr. Ian Clark (CLF LSF)