Transcript Slide 1

VUV Spectroscopy of
Astrophysical Ices
Michael Davis
Introduction
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Experimental background
Apparatus and techniques
Results and spectra
Discussion
Background
Accretion of gas
phase atoms and
molecules
A mantle of simple
volatile molecules is
formed, processed
by UV/charged
pratical irradiation
Chemical explosions
and other processes
release molecules
into the gas phase
An outer volatile
mantle protects an
inner organic
mantle from
irradiation
Background
 Ion irradiation
caused by Jupiter’s
magnetosphere
 Ices exist in bulk
 Higher temperatures
(~100K) than ISM
(~10K)
Apparatus
Apparatus
 Designed to travel to
irradiation sources
 CaF/MgF2 substrate
for IR/UV
transmission
 Helium/nitrogen
cryogens, achieves
temperatures <20K
 Reaches base
pressure 10-8mbar
Cryogen inlet
Rotary feedthrough
Ion gauge
Sample
mount
ELECTRON GUN
ION SOURCE
Sample
deposition
SYNCHROTRON
Apparatus
Irradiation
SYNCHROTRON
SOURCE
VUV
Spectroscopy
PMT
FTIR
Spectroscopy
DETECTOR
Detection
methods
FTIR
SOURCE
RF DEUTERIUM
DISCHARGE
LAMP (UV)
Experiments
 VUV spectroscopy undertaken at:
 ISA Storage Ring, University of Aarhus
 SRS, Daresbury Laboratory
 Most recent beam-times September 2004
(ISA) and December 2004 (SRS)
 Looking at simple ices (NH3, CO, CO2)
and mixtures with water
 Depositions at different speeds and
temperatures
Experiments
 Why use VUV spectroscopy?
 Complements FTIR studies
 Highlight differences between gas and solid
phase VUV data
 Knowing electronic transitions allows to
know the effects of irradiation by UV
discharge lamp
Experiments
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Presenting NH3 and NH3/H2O results
Depositing at 25K, 75K, 85K and 95K
Annealing up to 120K
Depositing ~0.2µm in <2 minutes and
~30 minutes
 Mixtures at 9:1, 1:1 and 1:3 ratios
Deposition speed
2.5
2.0
Absorbance
 In ammonia, no
difference in curve
features
 Absorbance is
increased
 Other molecules
show drastic
changes
1.5
1.0
0.5
0
140
NH3 deposited at 20K in 93s
NH3 deposited at 20K in 1760s
160
180
Wavelength (nm)
200
220
Temperature Effects
3
194nm
Absorbance
 Annealing from
20K to >75K shifts
the main peak and
adds another
feature
 Depositing at 95K
has a similar
effect, but more
pronounced
NH3 dep. at 20K
NH3 ann. at 110K
NH3 dep. at 95K
2
1
0
140
160
180
Wavelength (nm)
200
Temperature Effects
3
194nm
Absorbance
 Depositing at
~75K-85K causes
major changes
 New features
seen, existing
features modified
NH3 dep. at 20K
NH3 dep. at 72.5K
NH3 dep. at 87K
NH3 dep. at 95K
2
1
0
140
160
180
Wavelength (nm)
200
Temperature Effects
2.5
2.0
Absorbance
 Some features
enhanced or
decreased
 No major changes
to curve shape
 Completely
different result
than annealing the
20K sample
1.5
1.0
0.5
0
140
NH3 dep. at 72.5K
NH3 ann. at 118K
160
180
Wavelength (nm)
200
NH3:H2O Mixtures
2.5
2.0
Absorbance
 Adding small
amounts of water
to the ammonia
does not change
the spectra
significantly
 No water features
visible
1.5
1.0
0.5
0
140
NH3 dep. at 20K
NH3:H2O 10:1 dep. at 28K
NH3 dep. at 72.5K
NH3:H2O 9:1 dep. at 72K
160
180
Wavelength (nm)
200
NH3:H2O Mixtures
Absorbance
 Additional H2O
suppresses the
crystal structure
 Some H2O
remains after NH3
has been
desorbed
 No significant
annealing effects
NH3 dep. at 72.5K
NH3:H2O 1:1 dep. at 72K
NH3:H2O 1:3 dep. at 77K
NH3:H2O 1:3 ann. at 150K
H2O dep.at 25K
3
2
1
0
140
160
180
Wavelength (nm)
200
Summary
 Outline of experimental techniques and
apparatus
 Deposition speed has little effect in NH3
 Deposition temperature has a major
effect in NH3
 Annealing has some effect on spectra
 Sample is not strongly affected by water
“impurities”
Acknowledgements
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Prof. Nigel Mason
Dr Anita Dawes
Dr Robin Mukerji
Philip Holtom
Bhalamurugan Sivaraman
Sarah Webb
David Shaw (SRS)
The Open University
EPSRC
PPARC