H 2 AND N 2 -BROADENED C 2 H 6 AND C 3 H 8 ABSORPTION CROSS SECTIONS ROBERT J. HARGREAVES a DOMINIQUE APPADOO b BRANT E. BILLINGHURST c PETER F. BERNATH a a Department of Chemistry Old Dominion University Norfolk, VA b 800 Blackburn Road Australian Synchrotron Melbourne Victoria, Australia c Canadian Light Source Inc. Saskatoon, Saskatchewan Canada TUESDAY 23 rd JUNE 2015 Image: Cassini Team

OUTER SOLAR SYSTEM  Remote sensing of outer planets and moons  Derive physical and chemical properties of planets  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 Cassini - Huygens  Future missions will also depend on spectroscopic data  E.g., Juno mission (Jupiter)

OUTER PLANETS Atmospheric profiles of outer planets and moons Jupiter Saturn [adapted from Mueller-Wodarg et al. 2008] Jupiter  ~ 100 – 200 K  H 2 = 90%  He = 10%  Other gases  CH 4 ~ 0.3%  C 2 H 6 ~ %  C 3 H 8 trace Saturn  ~ 70 – 150 K  H 2 = 96%  He = 3%  Trace gases  CH 4 ~ 0.4%  C 2 H 6 ~ %  C 3 H 8 trace T = 70 – 200 K Broadener = H 2, He

TITAN  Largest moon of Saturn  Only moon with more than a trace atmosphere  N 2 = 98.4%  CH 4 = 1.4%  Higher in troposphere  Major discovery of Cassini-Huygens mission  Stable liquid lakes on Titan  E.g. Ligeia and Kraken  Primarily methane and ethane?  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) Kraken Mare Ligeia Mare Titan North Pole (NASA) [Flasar et al. 2005] CIRS nadir and limb profile of Titan T = 70 – 190 K Broadener = N 2

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  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 = cm -1  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 cm -1 Synchrotron Globar

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:  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 : PNNL at 5°C ν9ν9 C2H6C2H6

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 C 2 H 6 P Q 3 sub-band

C 2 H 6 RESULTS AT 90 K  Effective C 2 H 6 pressure not known…  Enclosive flow  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 K  263 K  232 K  200 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 ν 26 ν8ν8 ν 21 ν 20 ν7ν7

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 is funded by a NASA outer planets grant. We would like to thank the Australian Synchrotron for experimental support during beamtime measurements. Thanks also go to the Canadian Light Source. More information: Hargreaves et al. (2015), JMS Special Issue, in press. THANKS FOR LISTENING