school of chemistry, university of nottingham 1 surface science models of the gas-grain interaction...

School of Chemistry, University of Nottingham 1

Surface Science Models of the Surface Science Models of the Gas-Grain InteractionGas-Grain Interaction

Martin McCoustra


NGC 3603W. 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



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



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



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.


Ultrahigh Vacuum (UHV) is the key to understanding the gas-grain interaction– Pressures < 10-9 mbar

Looking at Grain Surfaces

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Mass / mu

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Pre-bake ChamberResidual Gases

Post-bake ChamberResidual Gases (x100)


Ultrahigh Vacuum (UHV) is the key to understanding the gas-grain interaction– Pressures < 10-9 mbar


– Clean surfaces

– Controllable gas phase


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



Gold Film

Cool to Below 10 K

InfraredBeam

MassSpectrometer




H. J. Fraser, M. P. Collings and M. R. S. McCoustraRev. Sci. Instrum., 2002, 73, 2161


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 pASWcASW 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)

Water Ice Films


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


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.


CO on Water Ice

CO desorption from Au– Sharp feature due to

sublimation of solid

– Zero order kinetics, cf. water ice

CO on Au

1-2-RT/200900,626)bulk(CO s cm moleculese10)27(dt

dn


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

1-2-)monolayer(CO

RT/200800,914)monolayer(CO s cm moleculesne10)25(dt

dn

CO on cASW@120 K


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@70 K

CO on cASW@120 K


CO on Water Ice

CO desorption from pASW– No solid feature

– Delayed monolayer feature

– Features above 100 K• 140 K corresponds to the

cASWIc transition - volcano desorption

• 160 K corresponds to sublimation of the water ice film itself - co-desorption

CO on Au

CO on cASW@70 K

CO on cASW@120 K

CO on pASW


CO on Water Ice

CO desorption from a CO-H2O mixture

– Very similar to the previous case

CO on Au

CO on cASW@70 K

CO on cASW@120 K

CO on pASW

CO in H2O


< 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., 2003, 583, 1058-1062


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


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

CO on Water Ice

Experiment

Model


What do these observations mean to those modelling the chemistry of the interstellar medium?

CO on Water Ice

Assume Heating Rate of 1 K millennium-1

Old Picture of CO Evaporation

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New Picture of CO Evaporation

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Ices in the interstellar medium comprise more than just CO and H2O. What behaviour might species such as CO2, CH4, NH3 etc. exhibit?– TPD Survey of Overlayers and

Mixtures

Beyond CO on Water Ice

H2O

CH3OH

OCS

H2S

CH4

N2



H2O

CH3OH

OCS

H2S

CH4

N2

– 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



H2O

CH3OH

OCS

H2S

CH4

N2

– 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




H2O

CH3OH

OCS

H2S

CH4

N2

– 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




H2O

CH3OH

OCS

H2S

CH4

N2

– Type 4• Refractory materials, e.g. metals,

sulfur, …, desorb only at elevated temperatures (100’s of K)




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. WilliamsMon. Not. Roy. Astron. Soc., 2004, 354, 1141-1145.


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


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 EPSRCLeverhulme Trust

University of Nottingham££

school of chemistry, university of nottingham 1 surface science models of the gas-grain interaction...

Documents

school of chemistry

university of nottingham

chemistry of grains

water ice slide

normal chemistry

grain surfaces slide

water ice films

controlled cosmos slide