H 2 AND N 2-BROADENED
C 2H 6 AND C 3H 8 ABSORPTION
CROSS SECTIONS
ROBERT J.
HARGREAVES a
rhargrea@odu.edu
DOMINIQUE
APPADOO b
BRANT E.
BILLINGHURST c
PETER F.
BERNATH a
Image: Cassini Team
a Department of Chemistry
Old Dominion University
Norfolk, VA 23529
b 800 Blackburn Road
Australian Synchrotron
Melbourne
Victoria, Australia
TUESDAY 23rd JUNE 2015
c Canadian Light Source Inc.
Saskatoon, Saskatchewan
Canada
OUTER SOLAR SYSTEM
 Remote sensing of outer planets and moons
 Derive physical and chemical properties of planets
Cassini - Huygens
 E.g., temperature, pressure, altitude…
 Molecular abundances
 For example: Cassini-Huygens
 Saturn and moons (e.g., Titan)
 NASA’s Composite Infrared Spectrometer (CIRS)
 Entirely dependent on spectroscopic data
 Often use molecular line databases (HITRAN)
 Intended for Earth’s atmosphere
 Air-broadened
Juno
 Future missions will also depend
on spectroscopic data
 E.g., Juno mission (Jupiter)
OUTER PLANETS
At m o s p h e r i c p ro f i l es o f o u te r p l a n et s a n d m o o n s
[adapted from Mueller-Wodarg et al. 2008]
Saturn
Jupiter
T = 70 – 200 K
Broadener = H2, He
Jupiter
 ~100 – 200 K
 H 2 = 90%
 He = 10%
 Other gases
 CH 4 ~0.3%
 C 2 H 6 ~0.0006%
 C 3 H 8 trace
Saturn
 ~70 – 150 K
 H 2 = 96%
 He = 3%
 Trace gases
 CH 4 ~0.4%
 C 2 H 6 ~0.0007%
 C 3 H 8 trace
TITAN
 Largest moon of Saturn
 Only moon with more than a trace atmosphere
 N 2 = 98.4%
 CH 4 = 1.4%
 Higher in troposphere
Titan North Pole (NASA)
Ligeia
Mare
Kraken
Mare
 Major discovery of Cassini -Huygens mission
 Stable liquid lakes on Titan
 E.g. Ligeia and Kraken
 Primarily methane and ethane?
CIRS nadir and limb profile of Titan
[Flasar et al. 2005]
 Amounts vary depending on models
 May also include propane
 Remaining atmosphere trace hydrocarbons
 UV photolysis of CH 4 and subsequent reactions
 Includes C 2 H 6 , C 3 H 8
 70 – 190 K (0 – 300 km)
T = 70 – 190 K
Broadener = N2
PROPANE AND ETHANE OVERVIEW
 Extensive previous work
 In general, does not cover full temperature/spectral range with appropriate broadeners
 Need appropriate data
 Difficult complex spectra
 Dense line structure
 Extensive perturbations
 Low lying torsion modes
 Recent work by Nixon et al. (2013) shows the importance of appropriate data
 Retrieved propene (C 3 H 6 ) in Titan
 Better C 3 H 8 cross section
 Pseudo-line list model from JPL (Sung et al. 2013)
 Pacific Northwest National Laboratory (PNNL)
 Cross sections over appropriate spectral range
 Not suitable for remote sensing of planetary atmospheres
 Relatively low resolution (0.112 cm -1 ) > under resolved
 Pressures and temperatures for Earth (1 atm N 2 )
 Useful for calibrations and validation
LOW VAPOR PRESSURE
 Low vapor pressures at low temperatures
 C 2 H 6 : 0.1 Torr at ~100 K
PNNL
 Longer path length needed
 Special cell required below 130 K
 C 3 H 8 : 0.1 Torr at ~130 K
PNNL
WHY USE A SYNCHROTRON
 Require high resolution
spectra…
 Very high brightness
 Collimated
 Very intense for small aperture
 Allows high resolution due to
point source
 Bruker FTS max = 0.00096 cm -1
Globar
Synchrotron
 Better signal to noise
 For region near 800 cm -1 the
gain is around 3 to 4 times
 Quicker experiments
 These benefits are significant
up to ~1000 cm -1
Rotationally resolved sample at 0.001 cm-1
SYNCHROTRONS
 Australian Synchrotron
 Bruker IFS 125HR
 Cell based on design by Bauerecker et al. (1995)
 Operating options
 static cell
 ‘enclosive flow cooled’ cell (EFC)
 Liquid-N 2 cooled
 Capable of He cooling
 Canadian Light Source
 Bruker IFS 125HR
 2m white cell
 Long path
 Advantage over current cell at ODU




Lower temperatures
Combine with the synchrotron
Enclosive flow
Longer sample path length
CROSS SECTIONS
 Transmission ( τ) spectra recorded between 700 and 1200 cm -1
 ν 9 band of C 2 H 6 near 820 cm -1
 Numerous bands of C 3 H 8 (ν 26 , ν 8 , ν 21 , ν 20 , ν 7 )
 Cross section calculated from:
104 𝑘𝐵 𝑇
𝜎 𝜈, 𝑇 = −𝜉
ln 𝜏(𝜈, 𝑇)
𝑃𝑙
 Integrated cross section is constant over isolated band
 Demonstrated by numerous studies (e.g., Harrison et al. 2012)
 E.g., PNNL integrated between 700 – 960 cm -1 :
960 cm−1
𝜎 𝜈, 𝑇 𝑑 𝜈 = 1.014 ±0.003 × 10−18 cm molecule−1
700 cm−1
C2H6
ν9
PNNL at 5°C
C 2 H 6 OVERVIEW
 Measurements made at the
Australian synchrotron
 Far IR/THz beamline
 February 2015
 36 shifts (12 days)
 Apparatus
 MCT narrow (>700 cm -1 )
 Bruker IFS 125HR
 EFC cell
 H 2 and N 2 -broadened C 2 H 6
 Three temperatures
 150 K
 120 K
 90 K – Enclosive conditions
 Each temperature used to measure
broadening pressures




Pure ethane
20 Torr
60 Torr
200 Torr
Total acquisition
time ~ 85 hours
C 2 H 6 CROSS SECTIONS AT 150 K
C 2 H 6 CROSS SECTIONS AT 120 K
C2H6 PQ3 sub-band
C 2 H 6 RESULTS AT 90 K
 Effective C 2 H 6 pressure not known…
 Enclosive flow
104 𝑘𝐵 𝑇
𝑃=−
ln 𝜏(𝜈, 𝑇)
𝜎𝑃𝑁𝑁𝐿 𝜈, 𝑇 𝑙
 Non-equilibrium
 Deduce sample effective pressure
 assuming the integrated cross section is
correct
 Enclosive flow effect
 Increases transmittance by approx. 4 times
 Difficult to maintain
 More vital for propane
 Similar strength
 Vapor pressure much lower
 ~500 times lower at 100 K
Non-enclosive
Enclosive
C 2 H 6 CROSS SECTIONS AT 90 K
Ethane vapor pressure at 90 K ~ 0.02 Torr
C 3 H 8 OVERVIEW
 Similar measurements made at the
Canadian Light Source
 Far IR beamline
 Also in February 2015 (Cycle 21)
 60 shifts (20 days)
 Similar apparatus
 MCT narrow (>700 cm -1 )
 Bruker IFS 125HR
 2 m white cell
 H 2 -broadened C 3 H 8
 Four temperatures




295
263
232
200
K
K
K
K
 Each temperature used to measure
broadening pressures




Pure propane
10 Torr
30 Torr
100 Torr
Total acquisition time ~ 130 hours
C 3 H 8 CROSS SECTIONS AT 232 K
ν26
ν21
ν8
ν7
ν20
PROBLEMS / FUTURE
 Analysis is currently ongoing
 Propane (CLS) data has issues of channelling
 Known causes include windows of cell/ beamline
 Post-removal is never 100%
 Best solution is to not have it in the first place
 Synchrotron measurements to be supported by those made with globar
 Not dependent on the time limitations
 Only beneficial above 1000 cm -1
 Synchrotrons remain vital for measurements below 1000 cm -1
Sample channelling at 233 K
SUMMARY
 Providing cross sections for ethane and propane
 Appropriate for Outer Planets and Titan
 Make use of synchrotron benefits
 Important for bands below 1000 cm -1
 Significant time savings
 Work to improve range of CLS cell
 Reach lower temperatures (~150 K)
 Also channelling issue
 Very large project (Australia, Canada, ODU)
 Complement synchrotron observations with globar data
 Globar data for bands over 1000 cm -1
Acknowledgements
The work i s funded by a NASA outer planet s grant. We
would like to thank the Aust ralian Synchrot ron for
experi mental support during beamtime measurement s .
Thanks also go to the Canadian Light Source.
More informatio n:
Hargreaves et al. ( 2015), JMS Special Issue , in press.
THANKS FOR LISTENING