View
17
Download
0
Category
Preview:
DESCRIPTION
- PowerPoint PPT Presentation
Citation preview
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
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
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
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
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
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
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
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
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
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!
Looking at Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
But then we need to consider how to return these molecules to the gas phase ... Desorption induced by Collisions
Looking at Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
But then we need to consider how to return these molecules to the gas phase ... Desorption induced by Collisions Desorption induced by Heating
Looking at Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
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
Thermal Processes on Grain Surfaces
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
Thermal Processes on Grain Surfaces
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 10s of pK s-1 cf. 80 mK s-1 in our TPD studies
Experiment
Model
Thermal Processes on Grain Surfaces
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
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
Thermal Processes on Grain Surfaces
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
Thermal Processes on Grain Surfaces
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
Thermal Processes on Grain Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Looking at Grain Surfaces
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
0
2
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
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
Recommended