department of chemistry, school of engineering and physical sciences, heriot-watt university probing...

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



Hot, Shiny Things Stars etc.

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

atmospheres Nanoscale silicate and carbonaceous dusts



Cold, Dark Stuff Interstellar Medium (ISM)

Generally cold and dilute Temperatures below 10 K and densities of a few particles per cm3

Some hot regions Photoionisation regions have effective temperatures of 100’s to 1,000’s of K

Some dense regions Clouds have average densities approaching that of good quality UHV Localised densities can approach even the HV or above



Cold, Dark Stuff Interstellar Medium (ISM)

Spectroscopic observations have found over 130 different types of chemical species in the gas and solid phases

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, glycine, other amino acids and pre-biotic molecules(?)

Observations tell us that these molecules are associated with the dense regions, which are themselves known to be sites of star and planet formation




Molecules are crucial for Maintaining the current rate of star formation Ensuring the formation of small, long-lived stars such as our own Sun Seeding the Universe with the chemical potential for life


Thermal motion will resist further gravitational collapse unless the cloud is radiatively cooled

ColdCloud

Gravitational Collapse Hot Clump

in Cold Cloud

Gravitational Collapse

Star



In the early Universe Only H atoms were present and so radiative cooling would only be

possible on electronic transitions, i.e. at temperatures of 1000s of K. Collapsing gas clumps needed to be very large (100s of solar

masses) to reach the temperature necessary to excite electronic transitions by gravitational collapse alone

In the current Universe Rovibrational transitions in complex molecules result in radio,

microwave and infrared emission and so provide the radiative cooling mechanism

Collapsing gas clumps are typical much smaller, near solar mass, since much less gravitational energy is required to match temperatures of a few 10s to 100s of K.



Complex molecules point to a surprisingly complex chemistry Low temperatures and pressures mean that most normal

chemistry is impossible No thermal activation No collisional activation

Gas phase chemistry involving ion-molecule reactions and some type of reactions 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



CH4

IcyMantle


H

H2

H

O

H2O

H

N

H3N

Silicate or Carbonaceous Core

1 - 1000 nm

CO, N2

CO, N2



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


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



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



Molecular Formation Rates H2 is relatively well studied, but there is still some disagreement

For the heavier molecules (H2O, NH3 etc.) nothing is known but watch this space!!!

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

Desorption Processes Thermal desorption is increasingly well understood Cosmic ray sputtering is well understood Photon and low energy electron stimulated processes are poorly

understood, but again watch this space!!!



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 exposure build up sequence on pASW

CO on Water Ice


At low exposures CO monolayer peak at around

50 K Volcano peak (140 K) and co-

desorption peak (160 K) both observed

CO on Water Ice


With Increasing CO exposure CO monolayer peak moves to

lower temperature Repulsive interactions? Pore filling?

Volcano and co-desorption peaks saturate

Ice film can trap only a certain amount of CO

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?

Extended Compact2152 cm-1 2140 cm-1

CO on Water Ice


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


Between 8 and 15 K, redistribution of IR intensity without significant loss to the gas phase suggests CO migration into porous ice structure.

At least two CO binding sites characterised by 2152 cm-1 and 2138 cm-1 features.

CO on Water Ice


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


< 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

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

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

Experiment

Model

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

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

CO on Water Ice


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

Beyond CO on Water Ice


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

Beyond CO on Water Ice


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


Professor David Williams and Dr Serena Viti (UCL)Dr. Helen Fraser (Strathclyde University)

Dr. Mark Collings Rui Chen, John Dever, Simon Green and John Thrower

££PPARC, EPSRC and CCLRC

Leverhulme TrustUniversity of Nottingham

££

Acknowledgements

Dr. Wendy Brown (UCL) and her groupProfessor Nigel Mason (OU) and his group

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

department of chemistry, school of engineering and physical sciences, heriot-watt university probing...

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