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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically Controlled Cosmos
Eagle Nebula
Horsehead Nebula Triffid Nebula
30 Doradus Nebula
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
The Chemically Controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
The Chemically Controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
The Chemically Controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically Controlled Cosmos
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Thermal motion will resist further gravitational collapse unless the cloud is radiatively cooled
ColdCloud
Gravitational Collapse Hot Clump
in Cold Cloud
Gravitational Collapse
Star
The Chemically Controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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.
The Chemically Controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
The Chemically Controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
CH4
IcyMantle
The Chemically Controlled Cosmos
H
H2
H
O
H2O
H
N
H3N
Silicate or Carbonaceous Core
1 - 1000 nm
CO, N2
CO, N2
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically Controlled Cosmos
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
The Chemically Controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Surface physics and chemistry play a key role in these processes, but the surface physics and chemistry of grains was poorly understood.
The Chemically Controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Ultrahigh Vacuum (UHV) is the key to understanding the gas-grain interaction Pressures < 10-9 mbar
0
200000
400000
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1200000
0 5 10 15 20 25 30 35 40 45
Mass / mu
Sig
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/ A
rbit
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its
Pre-bake ChamberResidual Gases
Post-bake ChamberResidual Gases (x100)
Looking at Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Ultrahigh Vacuum (UHV) is the key to understanding the gas-grain interaction Pressures < 10-9 mbar Clean surfaces Controllable gas phase
Looking at Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Gold Film
Cool to Below 10 K
Infraredfor RAIRS
MassSpectrometer
Atoms (H, N, O) and Radicals (CN, OH, CH)
UV Light andElectrons
Looking at Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
H. J. Fraser, M. P. Collings and M. R. S. McCoustraRev. Sci. Instrum., 2002, 73, 2161
Looking at Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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!!!
Looking at Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
CO exposure build up sequence on pASW
CO on Water Ice
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
< 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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
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0.4
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0.8
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0 25 50 75 100 125
Temperature / K
Fra
ctio
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f C
O D
esor
bed
New Picture of CO Evaporation
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1.2
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Temperature / K
Fra
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f C
O D
esor
bed
CO on Water Ice
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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?
Shining a Little Light on Icy Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Shining a Little Light on Icy Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
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4
6
mcs
co
un
ts
time-of-flight (ms)
Photon-induced Desorption
Time of Flight (ToF)
Liquid N2
Shining a Little Light on Icy Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Shining a Little Light on Icy Surfaces
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Shining a Little Light on Icy Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Shining a Little Light on Icy Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Shining a Little Light on Icy Surfaces
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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)