School of Chemistry, University of Nottingham,UK 1

Why Does Star Formation Need Surface Why Does Star Formation Need Surface Science? Science?

Using Laboratory Surface Science to Understand the Using Laboratory Surface Science to Understand the Astronomical Gas-Grain InteractionAstronomical Gas-Grain Interaction

Martin McCoustra

School of Chemistry, University of Nottingham,UK 2

Gas, Dust and Grains– A Chemist’s 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

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Gas, Dust and Grains

Eagle Nebula

Horsehead Nebula Triffid Nebula

30 Doradus Nebula

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Gas, Dust and Grains

Hot, Shiny Things– Stars etc.

• Elemental Foundaries

• Small molecules, e.g. H2O, C2, SiO, TiO, SiC2 …, in cooler parts of stellar atmospheres

• Nanoscale silicate and carbonaceous dusts

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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, …, H3

+, HCO+, ...

Simple Molecules, e.g. H2, CO, H2O, CH4, NH3, …

“Complex” Molecules, e.g. HCN, CH3CN, CH3OH, C2H5OH, CH3COOH, (CH3)2CO, amino acids(?), nucleic acids(?)

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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

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Gas, Dust and Grains Complex molecules point to a surprisingly

complex chemistry– Low temperatures (<20 K) mean that reactions

can’t be thermally activated

– Low pressures (<<10-10 mbar) mean that three-body processes are unlikely

The chemistry must be efficient– Ion-Molecule Reactions

• e.g. C+ + H2 CH+ + H

– Neutral Exchange/Abstraction Reactions• e.g. N + OH NO + H; H2 + OH H2O

+ H

But ...

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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 H2

– “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

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Gas, Dust and Grains Grains are typically no more

than a few 10’s 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

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Gas, Dust and Grains

Composition of the mantle depends on the age of the cloud containing the grain.– Young Clouds (H atom rich,

[H]/[H2]>1)

• Polar Ices

• H2O, CH4, CH3OH ,…

– Old Clouds (H atom depleted, [H]/[H2]<1)

• Apolar Ices

• CO, CO2, N2, O2 ,...

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Gas, Dust and Grains

Grains have several crucial roles in the clouds– Act as catalysts for the formation of H2 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.

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NeutralsElectrons

Ions

Photons

Neutrals IonsElectrons

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

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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

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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

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Gold Film

Cool to Below 10 K

InfraredBeam

MassSpectrometer

Looking at Grain Surfaces

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Looking at Grain Surfaces

H. J. Fraser, M. P. Collings and M. R. S. McCoustraRev. Sci. Instrum., in print

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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 (80-250 K) Ruthenium single

crystal

H2OBeam

QMS

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Water Ice Films

King and Wells method used to investigate temperature variation of condensation coefficient, .

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Water Ice Films

Condensation coefficient reflects balance of adsorption and desorption processes occurring at the surface

Input Flux, JinReflected Flux, Jref

Adsorbed Flux, Jads

Desorbed Flux, Jdes

in

desin J

)T(k)T(S)J,T(

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Water Ice Films

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Water Ice Films

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Water Ice Films

Sticking coefficient, S, of H2O is unity and independent of temperature

Exponential increase in rate of desorption with temperature suggests that desorption kinetics of ice multilayers are zero order

Edes=48.25 ±0.80 kJ mol-1

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Water Ice Films

TPD measurements of ice deposited at ca. 10 K (McCoustra and co-workers, Mon. Not. Roy. Astron. Soc., 2001, 327, 1165-1172)

Confirms zero order desorption kinetics of multilayer ice films

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Water Ice Films

Kinetic analysis gives the rate coefficient for desorption of multilayer water ice films

1-2-RT/500000,48230)bulk(OH s cm moleculese10dt

dn2

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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, 5284-5294; ibid 5295-5303)

Thermal processing of the porous films results in pore collapse at temperatures above ca. 30 K

TEM studies show the IhdaIlda phase transition occurring between 30 and 80 K and the Ilda Ic crystallisation process at ca. 140 K in UHV (Jenniskens and Blake, Sci. Am., 2001, 285(2), 44-50)

Water Ice Films

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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

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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.

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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.

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CO on Water Ice

High temperature volcano features associated with crystallisation (ca. 140 K) and ice film evaporation seem to saturate out.

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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?

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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-1 2140 cm-1

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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

CO on Water Ice

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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

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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

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< 10 K

Tem

pera

ture

10 - 20 K

30 - 70 K

135 - 140 K

160 K

CO on Water Ice

M. P. Collings, H. J. Fraser, J. W. Dever, M. R. S. McCoustra and D. A. WilliamsAp. J. Lett., submitted

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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

)g(CO)i(CO

)g(CO)s(CO

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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

)g(CO)i(CO

)g(CO)s(CO

)pi(CO)s(CO

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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

)g(CO)i(CO

)g(CO)s(CO

)pi(CO)s(CO

)pg(CO)pi(CO

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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

)g(CO)i(CO

)g(CO)s(CO

)pi(CO)s(CO

)pg(CO)pi(CO

)pi(CO)pg(CO

)g(CO)pg(CO

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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

)g(CO)i(CO

)g(CO)s(CO

)pi(CO)s(CO

)pg(CO)pi(CO

)pi(CO)pg(CO

)g(CO)pg(CO

)pt(CO)pi(CO)pt(CO

)pt(CO)pi(CO

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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

)g(CO)i(CO

)g(CO)s(CO

)pi(CO)s(CO

)pg(CO)pi(CO

)pi(CO)pg(CO

)g(CO)pg(CO

)pt(CO)pi(CO)pt(CO

)pt(CO)pi(CO

)g(CO)pt(CO

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Using reasonable values for the various kinetic parameters, this model qualitatively reproduces our observations

CO on Water Ice

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Just like opening Pandora’s 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?

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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+HH2

• 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?

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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

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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 ££

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Why UHV?

Number densities in UHV approach those of dense clouds

Atmospheric Pressure

UHV

HV

XHV

DenseCloudsDiffuse

Clouds

GeneralISM

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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.

Tmk2

PZ

Bw

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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