Surface Science Models of the Gas-Grain Interaction

1. Surface Science Models of the Gas-Grain Interaction 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 • Molecules play several key astronomical roles • Indicators of star forming regions • Probes of the local environment within such regions • May act as a chemical clock for star formation • Provide crucial radiative cooling pathways in the early stages of star formation

4. The Chemically Controlled Cosmos • Complex molecules point to a surprisingly complex chemistry • Low temperatures and pressures means that most normal chemistry is impossible • No thermal activation • No collisional activation • Gas phase chemistry involving ion-molecule reactions and some type of reaction 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

5. The Chemically Controlled Cosmos • Dust grains are believed to have several crucial roles in the clouds • Assist in the formation of small molecules including H2, N2, H2O, CH4, ... • Some of these molecules will be trapped as icy mantles on the grains that then act as a reservoir of molecules used to radiatively cool collapsing clouds as they warm and to seed the post-collapse gasphase chemistry • Reactions induced by photons and cosmic rays in these icy mantles can create complex, pre-biotic molecules Surface physics and chemistry play a key role in these processes, but the surface physics and chemistry of grains is poorly understood.

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

7. 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

8. Looking at Grain Surfaces • Reflection-Absorption Infrared Spectroscopy (RAIRS) • Thin (< 50 nm) films minimise bulk absorption • Identification of adsorbed species by their infrared spectra • Use of a metal substrate potentially allows determination of adsorbate orientation • Temperature Programmed Desorption (TPD) • Mass spectrometric detection of desorbed neutrals as film is heated • Line-of-sight geometry employed to localise region of the surface from which desorption is detected • Film composition and reaction products

9. Looking at Grain Surfaces Mass Spectrometer Infrared Beam Gold Film Cool to Below 10 K

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

11. 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)

12. CO on Gold CO on Water Ice CO on Water Ice • 20 L of CO exposed to the substrate at 7 K. • On gold we clearly have multilayer and monolayer desorption. • On water ice, TPD is much more complex with evidence for strong binding of the CO to the surface and trapping of CO in the ice matrix.

13. CO on Water Ice • To understand this difference in detail, we need to look at desorption of CO from a variety of water systems. • Note that the equivalent dose of CO is used in each case.

14. CO on Water Ice • CO desorption from Au • Sharp feature due to sublimation of solid • Zero order kinetics, cf. water ice CO on Au

15. CO on Water Ice • CO desorption from cASW@120 K • Sharp feature due to sublimation of solid • Broad feature to higher temperatures due to desorption of CO directly bound to cASW surface • Monolayer feature desorbs with first order kinetics CO on Au CO on cASW@120 K

16. CO on Water Ice • CO desorption from cASW@70 K • No sharp feature due to sublimation of solid suggests much larger surface area. Substrate is porous. • Monolayer feature delayed to even higher temperatures • Different binding site? • Pore escape time? CO on Au CO on cASW@120 K CO on cASW@70 K

17. CO on Water Ice • CO desorption from pASW • No solid feature • Delayed monolayer feature • Features above 100 K • 140 K corresponds to the cASWIc transition - volcano desorption • 160 K corresponds to sublimation of the water ice film itself - co-desorption CO on Au CO on cASW@120 K CO on cASW@70 K CO on pASW

18. CO on Water Ice • CO desorption from a CO-H2O mixture • Very similar to the previous case CO on Au CO on cASW@120 K CO on cASW@70 K CO on pASW CO in H2O

19. 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

20. 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

21. 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

22. 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

23. 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

24. H2O CH3OH OCS H2S CH4 N2 Beyond CO on Water Ice • Type 1 • Hydrogen bonding materials, e.g. NH3, CH3OH, …, which desorb only when the water ice substrate desorbs • Qualitative survey of TPD of grain mantle constituents

25. H2O CH3OH OCS H2S CH4 N2 Beyond CO on Water Ice • 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 • Qualitative survey of TPD of grain mantle constituents

26. H2O CH3OH OCS H2S CH4 N2 Beyond CO on Water Ice • 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 • Qualitative survey of TPD of grain mantle constituents

27. H2O CH3OH OCS H2S CH4 N2 Beyond CO on Water Ice • Type 4 • Refractory materials, e.g. metals, sulfur, …, desorb only at elevated temperatures (100’s of K) • Qualitative survey of TPD of grain mantle constituents

28. Beyond CO on Water Ice • A Laboratory Survey of the Desorption of Astrophysically Relevant Molecules. • M. P. Collings, M. A. Anderson, R. Chen, J. W. Dever, S. Viti, D. A. Williams • and M. R. S. McCoustra, • Mon. Not. Roy. Astron. Soc., 2004, 354, 1133-1140. • Evaporation of Ices Near Massive Stars: Models Based on Laboratory TPD Data. • S. Viti, M. P. Collings, J. W. Dever, M. R. S. McCoustra • and D. A. Williams • Mon. Not. Roy. Astron. Soc., 2004, 354, 1141-1145.

29. 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

30. Acknowledgements Professor David Williams and Dr Serena Viti (UCL) Dr. Helen Fraser (Strathclyde University) Dr. Mark Collings Rui Chen, John Dever and Simon Green ££ PPARC and EPSRC Leverhulme Trust University of Nottingham ££