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School of Chemistry, University of Nottingham,UK 1 Why Does Star Formation Need Surface Science? Using Laboratory Surface Science to Understand the Astronomical.

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Presentation on theme: "School of Chemistry, University of Nottingham,UK 1 Why Does Star Formation Need Surface Science? Using Laboratory Surface Science to Understand the Astronomical."— Presentation transcript:

1 School of Chemistry, University of Nottingham,UK 1 Why Does Star Formation Need Surface Science? Using Laboratory Surface Science to Understand the Astronomical Gas-Grain Interaction Martin McCoustra

2 School of Chemistry, University of Nottingham,UK 2 Gas, Dust and Grains –A Chemists Guide to Astronomy Probing the Gas-Grain Interaction –Surface Science for Astronomers Some Examples –Water Ice Film Growth and Desorption –The Carbon Monoxide - Water Ice System Where Do We Go From Here? Conclusions and Acknowledgements Outline

3 School of Chemistry, University of Nottingham,UK 3 Gas, Dust and Grains Eagle Nebula Horsehead NebulaTriffid Nebula 30 Doradus Nebula

4 School of Chemistry, University of Nottingham,UK 4 Gas, Dust and Grains Hot, Shiny Things –Stars etc. Elemental Foundaries Small molecules, e.g. H 2 O, C 2, SiO, TiO, SiC 2 …, in cooler parts of stellar atmospheres Nanoscale silicate and carbonaceous dusts

5 School of Chemistry, University of Nottingham,UK 5 Gas, Dust and Grains Cold, Dark Stuff –Interstellar Medium (ISM) Generally cold and dilute, but there are some hot and some dense regions,Photoionisation regions,Clouds Spectroscopic observations have found over 110 different types of chemical species,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, amino acids(?), nucleic acids(?)

6 School of Chemistry, University of Nottingham,UK 6 Gas, Dust and Grains Molecules are associated with star forming regions and are crucial for maintaining the current rate of star formation Thermal motion will resist further gravitational collapse unless the cloud is radiatively cooled Rovibrational transitions in complex molecules resulting in radio, microwave and infrared emission provide the means of doing so Cold Cloud Gravitational Collapse Hot Cloud

7 School of Chemistry, University of Nottingham,UK 7 Gas, Dust and Grains Complex molecules point to a surprisingly complex chemistry –Low temperatures (<20 K) mean that reactions cant be thermally activated –Low pressures (<< mbar) mean that three- body processes are unlikely The chemistry must be efficient –Ion-Molecule Reactions e.g. C + + H 2 CH + + H –Neutral Exchange/Abstraction Reactions e.g.N + OH NO + H; H 2 + OH H 2 O + H But...

8 School of Chemistry, University of Nottingham,UK 8 Gas, Dust and Grains Gas-dust interactions are invoked as a means of accounting for the discrepancy between gas- phase only chemical models and observations –Catalytic Surfaces e.g. H+H H 2 –Freeze Out Surfaces About 1% of the mass of the interstellar medium is in the form of ice-covered dust grains Ground- and space-based IR astronomy tells us its scale and composition

9 School of Chemistry, University of Nottingham,UK 9 Gas, Dust and Grains Grains are typically no more than a few 10s of nm across Have a core and mantle structure Mantle may have an onion-like layering of materials if grown by accretion The common core materials are believed to be amorphous and crystalline silicates, amorphous hydrogenated carbon materials and PAHs

10 School of Chemistry, University of Nottingham,UK 10 Gas, Dust and Grains Composition of the mantle depends on the age of the cloud containing the grain. –Young Clouds (H atom rich, [H]/[H 2 ]>1) Polar Ices H 2 O, CH 4, CH 3 OH,… –Old Clouds (H atom depleted, [H]/[H 2 ]<1) Apolar Ices CO, CO 2, N 2, O 2,...

11 School of Chemistry, University of Nottingham,UK 11 Gas, Dust and Grains Grains have several crucial roles in the clouds –Act as catalysts for the formation of H 2 from H atoms –Reservoir of molecules used to radiatively cool collapsing clouds –Chemical factories on and in which complex new chemical species are formed by reactions induced by photons and cosmic rays Surface physics and chemistry play a key role in these processes, but the surface physics and chemistry of grains is poorly understood.

12 School of Chemistry, University of Nottingham,UK 12 Neutrals Electrons Ions Photons Neutrals Ions Electrons Photons Looking at Grain Surfaces Surface science attempts to paint an atomistic picture of the gas-solid interaction –Routinely achievable pressures in the UHV –Clean, well-characterised and (perhaps) well-defined surfaces with which to work –Tools that permit us to characterise surfaces either by being intrinsically surface specific or capable of operating in a surface sensitive manner

13 School of Chemistry, University of Nottingham,UK 13 Reflection-Absorption Infrared Spectroscopy (RAIRS) –Grazing incidence reflection from a metal substrate yields a 50 to 60-fold increase in sensitivity over transmission spectroscopy –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 Looking at Grain Surfaces

14 School of Chemistry, University of Nottingham,UK 14 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 –Mechanistic and kinetic information Looking at Grain Surfaces

15 School of Chemistry, University of Nottingham,UK 15 Gold Film Cool to Below 10 K Infrared Beam Mass Spectrometer Looking at Grain Surfaces

16 School of Chemistry, University of Nottingham,UK 16 Looking at Grain Surfaces H. J. Fraser, M. P. Collings and M. R. S. McCoustra Rev. Sci. Instrum., in print

17 School of Chemistry, University of Nottingham,UK 17 Direct molecular beam measurements of the condensation and evaporation of water ice films by Kay and co-workers (J. Phys. Chem., 1996, 100, 4988) Water Ice Films Cooled ( K) Ruthenium single crystal H 2 O Beam QMS

18 School of Chemistry, University of Nottingham,UK 18 Water Ice Films King and Wells method used to investigate temperature variation of condensation coefficient,.

19 School of Chemistry, University of Nottingham,UK 19 Water Ice Films Condensation coefficient reflects balance of adsorption and desorption processes occurring at the surface Input Flux, J in Reflected Flux, J ref Adsorbed Flux, J ads Desorbed Flux, J des

20 School of Chemistry, University of Nottingham,UK 20 Water Ice Films

21 School of Chemistry, University of Nottingham,UK 21 Water Ice Films

22 School of Chemistry, University of Nottingham,UK 22 Water Ice Films Sticking coefficient, S, of H 2 O is unity and independent of temperature Exponential increase in rate of desorption with temperature suggests that desorption kinetics of ice multilayers are zero order E des =48.25 ±0.80 kJ mol -1

23 School of Chemistry, University of Nottingham,UK 23 Water Ice Films TPD measurements of ice deposited at ca. 10 K (McCoustra and co- workers, Mon. Not. Roy. Astron. Soc., 2001, 327, ) Confirms zero order desorption kinetics of multilayer ice films

24 School of Chemistry, University of Nottingham,UK 24 Water Ice Films Kinetic analysis gives the rate coefficient for desorption of multilayer water ice films

25 School of Chemistry, University of Nottingham,UK 25 At temperatures around 10 K, ice grows from the vapour phase by ballistic deposition. The resulting films are highly porous (Kay and co-workers, J. Chem. Phys., 2001, 114, ; ibid ) Thermal processing of the porous films results in pore collapse at temperatures above ca. 30 K TEM studies show the I hda I lda phase transition occurring between 30 and 80 K and the I lda I c crystallisation process at ca. 140 K in UHV (Jenniskens and Blake, Sci. Am., 2001, 285(2), 44-50) Water Ice Films

26 School of Chemistry, University of Nottingham,UK 26 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. CO on Gold CO on Water Ice

27 School of Chemistry, University of Nottingham,UK 27 CO on Water Ice At low exposures, the CO monolayer peak occurs at much greater temperatures than on gold (50 K cf. 30 K). CO is much more strongly bound to the ice surface than previously thought.

28 School of Chemistry, University of Nottingham,UK 28 CO on Water Ice As the exposure increases, the monolayer peak moves to lower temperature. This suggests that the CO is perhaps sampling more weakly bound sites.

29 School of Chemistry, University of Nottingham,UK 29 CO on Water Ice High temperature volcano features associated with crystallisation (ca. 140 K) and ice film evaporation seem to saturate out.

30 School of Chemistry, University of Nottingham,UK 30 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?

31 School of Chemistry, University of Nottingham,UK 31 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? CO on Water Ice Extended Compact ? 2152 cm cm -1

32 School of Chemistry, University of Nottingham,UK 32 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

33 School of Chemistry, University of Nottingham,UK 33 Between 8 and 15 K, redistribution of IR intensity without significant loss to the gas phase suggests CO diffusion into porous ice structure. At least two CO binding sites characterised by 2152 cm -1 and 2138 cm -1 features. CO on Water Ice

34 School of Chemistry, University of Nottingham,UK 34 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

35 School of Chemistry, University of Nottingham,UK 35 < 10 K Temperature K K K 160 K CO on Water Ice M. P. Collings, H. J. Fraser, J. W. Dever, M. R. S. McCoustra and D. A. Williams Ap. J. Lett., submitted

36 School of Chemistry, University of Nottingham,UK 36 We have constructed a kinetic model that reproduces our TPD observations –CO is deposited as a monolayer (i) and multilayers (s) at the water ice interface CO on Water Ice

37 School of Chemistry, University of Nottingham,UK 37 We have constructed a kinetic model that reproduces our TPD observations –Warming results in diffusion from the multilayer material to monolayer sites in pores (i-p) CO on Water Ice

38 School of Chemistry, University of Nottingham,UK 38 We have constructed a kinetic model that reproduces our TPD observations –Monolayer CO desorption is the same from the open surface and from pore surfaces CO on Water Ice

39 School of Chemistry, University of Nottingham,UK 39 We have constructed a kinetic model that reproduces our TPD observations –There is a bottleneck restricting escape of gas phase molecules from pores allowing re- adsorption to compete CO on Water Ice

40 School of Chemistry, University of Nottingham,UK 40 We have constructed a kinetic model that reproduces our TPD observations –Pore collapse, trapping CO in the ice matrix, is assumed to be autocatalytic CO on Water Ice

41 School of Chemistry, University of Nottingham,UK 41 We have constructed a kinetic model that reproduces our TPD observations –Trapped CO desorbs with the same kinetics as bulk water ice CO on Water Ice

42 School of Chemistry, University of Nottingham,UK 42 Using reasonable values for the various kinetic parameters, this model qualitatively reproduces our observations CO on Water Ice

43 School of Chemistry, University of Nottingham,UK 43 Just like opening Pandoras Box In terms of work at Nottingham –Short Term Tie up the loose ends on the Water-Carbon Monoxide System,Kinetics,Spectroscopy TPD survey of other molecules in dilute mixtures with water –Medium Term Water-Carbon Dioxide, Water-Methane and Water-Ammonia Systems Others ? –Longer Term Atoms, Radicals, Ions and Photons Where Do We Go From Here?

44 School of Chemistry, University of Nottingham,UK 44 Growing number of European groups working in the area –Chalmers University Photon-induced processes on ice –Leiden Studies on the O+CO reaction – UCL Theory and Experiment of state-resolved studies on H+H H 2 Studies on the H+CO etc. –Université de Provence IR studies of small molecules in/on ice Theoretical modelling of ice surfaces –…–… Where Do We Go From Here?

45 School of Chemistry, University of Nottingham,UK 45 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 Traditional astronomy funding agencies have to appreciate the differences between observational/theoretical work and laboratory work Framework 6? Conclusions

46 School of Chemistry, University of Nottingham,UK 46 Acknowledgements Professor David Williams (UCL) Dr. Helen Fraser (University of Leiden) John Dever and Dr. Mark Collings (University of Nottingham) ££PPARC, EPSRC and the University of Nottingham££

47 School of Chemistry, University of Nottingham,UK 47

48 School of Chemistry, University of Nottingham,UK 48 Why UHV? Number densities in UHV approach those of dense clouds Atmospheric Pressure UHV HV XHV Dense Clouds Diffuse Clouds General ISM

49 School of Chemistry, University of Nottingham,UK 49 Why UHV? UHV helps us to keep a surface clean. Assuming each molecule striking a surface sticks, the time it takes to fill the surface can be estimated from the equation below.

50 School of Chemistry, University of Nottingham,UK 50

51 School of Chemistry, University of Nottingham,UK 51 Surface Probes Over 50 different experimental probes available to the surface scientist –Surface Atomic and Molecular Structure/Composition AES, EELS, LITD, UPS, RAIRS, SFG, SIMS, TPD, VEELS, XPS, … –Surface Structure and Geometry HAD, LEED, SRXD, XAFS, XSW, … –Gas-Solid Interactions and Reactions Atomic/Molecular Beam Scattering, State-resolved Techniques, Time-resolved Techniques


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