MRM_Apchem_Jan07

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

Probing the Gas-Grain InteractionProbing the Gas-Grain Interaction

Applications of Laboratory Surface Science in Astrophysics

Martin McCoustra


The Chemically Controlled Cosmos

Eagle Nebula

Horsehead Nebula Triffid Nebula

30 Doradus Nebula


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



Astrophysicists invoke gas-dust interactions as a means of accounting for the discrepancy between gas-phase only chemical

models and observations


Understanding the Chemical Evolution of the Universe, which we observe through the remote eyes of molecular spectroscopy, helps

us to understand the Physical Evolution of the Universe


CH4

IcyMantle


H

H2

H

O

H2O

H

N

H3N

Silicate or Carbonaceous Core

1 - 1000 nm

CO, N2

CO, N2


Dust grains are believed to have several crucial roles in the clouds Assist in the formation of small hydrogen-rich molecules including H2,

H2O, CH4, NH3, ... some of which will be trapped as icy mantles on the grains

Some molecules including CO, N2, ... 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




CH4

IcyMantle

Silicate or Carbonaceous Core

1 - 1000 nm

CO

N2

H2O

NH3

HeatInput

ThermalDesorption

UV LightInput

PhotodesorptionCosmic RayInput Sputtering and Electron-

stimulated Desorption

CH3OH

CO2

CH3NH2


Surface physics and chemistry play a key role in these processes, but the surface physics and chemistry of grains was poorly understood.



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

0

200000

400000

600000

800000

1000000

1200000

0 5 10 15 20 25 30 35 40 45

Mass / mu

Sig

nal

/ A

rbit

rary

Un

its

Pre-bake ChamberResidual Gases

Post-bake ChamberResidual Gases (x100)

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



Gold Film

Cool to Below 10 K

Infraredfor RAIRS

MassSpectrometer

Atoms (H, N, O) and Radicals (CN, OH, CH)

UV Light andElectrons



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



First we have to make molecules ... H2 is relatively well studied, but there is still some disagreement

For the heavier molecules (H2O, NH3 etc.) little is currently known, but watch this space as groups in Paris and Edinburgh are starting to work on this problem

Solid state synthesis in icy matrices using photons and low energy electrons is thought to be well understood but there are problems!



But then we need to consider how to return these molecules to the gas phase ... Desorption induced by Collisions



Thermal collision energies too small to induce desorption of anything but the weakest of physisorbed atoms and molecules

Cosmic rays with kinetic energies in excess of the surface binding energy, typically a few eV, can induce desorption At energies in the 100s of eV and above, simple billiard ball dynamics

apply and we can very successfully use molecular dynamics simulations to investigate the desorption and ionisation processes associated with cosmic ray sputtering

At energies in the range of a few eV, chemical energies, processes associated with electron exchange, charge neutralisation and chemical reactions cannot be modelled simply using classical molecular dynamics simulations

Grain-grain collisions as a desorption mechanism?

Molecular Pinball on Surfaces


But then we need to consider how to return these molecules to the gas phase ... Desorption induced by Collisions Desorption induced by Heating



Experimentally the simplest of measurements combining solid state (RAIRS) and gas phase (QMS) probes

Investigate increasing complex systems as you learn more about the simpler ones Water Ice

Kinetic order of desorption for solids is zero NOT one!

Water/Carbon Monoxide Morphological changes in the water ice play an important role in promoting

trapping of volatiles beyond their normal desorption temperature

Water/Methanol/Carbon Monoxide Clathrate formation?

Thermal Processes on Grain Surfaces


< 10 K

Tem

pera

ture

10 - 20 K

30 - 70 K

135 - 140 K

160 K

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



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 10s of pK s-1 cf. 80 mK s-1 in our TPD studies

Experiment

Model



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

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 25 50 75 100 125

Temperature / K

Fra

ctio

n o

f C

O D

esor

bed

New Picture of CO Evaporation

0

0.2

0.4

0.6

0.8

1

1.2

0 25 50 75 100 125

Temperature / K

Fra

ctio

n o

f C

O D

esor

bed



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

H2O

CH3OH

OCS

H2S

CH4

N2



Qualitative survey of TPD of grain mantle constituents Type 1

Hydrogen bonding materials, e.g. NH3, CH3OH, …, which desorb only when the water ice substrate desorbs

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

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

Type 4 Refractory materials, e.g. metals, sulfur, etc.

desorb only at high temperatures (100’s of K)

H2O

CH3OH

OCS

H2S

CH4

N2



But then we need to consider how to return these molecules to the gas phase ... Desorption induced by Collisions Desorption induced by Heating Desorption induced by Electronic Excitation

Photon Absorption (Secondary) Electron Attachment



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


Model system we have chosen to study is the water-benzene system C6H6 may be thought of as a prototypical (poly)cyclic aromatic (PAH)

compound C6H6 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 C6H6 or even H2O following photon absorption? Is there any change in the ice morphology following photon absorption? Is there chemistry?





DoubledDyeLaser

Nd 3+:YAG

QMS

MCS

trigger

30 40

0

500

1000

1500

Photon Induced Desorption Curves

Mas

s 78

SE

M c

ou

nts

/s

Time (s)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

2

4

6

mcs

co

un

ts

time-of-flight (ms)

Photon-induced Desorption

Time of Flight (ToF)

Liquid N2



Sapphire substrate Easily cooled to cryogenic temperatures by Closed Cycle He cryostat

to around 60-80 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 Sapphire Sapphire Sapphire

C6H6

C6H6

C6H6H2O H2O

H2O


Irradiate at 248.8 nm (on-resonance), 250.0 nm (near-resonance) and 275.0 nm (off-resonance) at “low” (1.1 mJ/pulse) and “high” (1.8 mJ/pulse) laser pulse energies


C6H6 desorption observed at all wavelengths Substrate-mediated

desorption weakly dependent on wavelength

Adsorbate-mediated desorption reflects absorption strength of C6H6

Yield of C6H6 is reduced by the presence of a H2O capping layer



H2O desorption echoes that of C6H6

H2O does not absorb at any of these wavelengths and so desorption is mediated via the substrate and the C6H6

Yield of H2O is increased by the presence of a C6H6 layer



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 C6H6 in the substrate-mediated desorption channel has a kinetic

temperature of ca. 550 K C6H6 in the self-mediated desorption channel has a kinetic

temperature of ca. 1100 K H2O appears to behave similarly


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


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


John Thrower and Dr. Mark Collings (Heriot-Watt)Farah Islam and Dr. Daren Burke (UCL)

Jenny Noble and Sharon Baillie (Strathclyde)Dr. Anita Dawes, Dr. Paul Kendall and Dr. Phil Holtom (OU)

Dr. Wendy Brown (UCL)Dr. Helen Fraser (Strathclyde University)

Professor Nigel Mason (OU)

Professor Tony Parker and Dr. Ian Clark (CLF LSF)

££EPSRC and CCLRC

University of Nottingham££

Acknowledgements

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MRM_Apchem_Jan07