HIGH-RESOLUTION ABSORPTION CROSS SECTIONS OF C 2 H 6 AND C 3 H 8 AT LOW TEMPERATURES ROBERT J. HARGREAVES DANIEL J. FROHMAN MICHAEL DULICK DOMINIQUE APPADOO synchrotron.org.au PETER F. BERNATH FRIDAY 16 TH JUNE 2014 Image: Titan’s atmosphere (Cassini)

 Remote sensing of outer planets and moons  NASA’s Composite Infrared Spectrometer (CIRS) on Cassini  Entirely dependent on spectroscopic data  Molecular line databases (HITRAN) intended for Earth’s atmosphere  Derive physical and chemical properties of planets  Spectroscopic data used to derive abundances and other properties such as temperature SCIENTIFIC PURPOSE Juno SOFIA Spitzer

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 [Flasar et al. 2005] CIRS nadir and limb profile of Titan  Only moon with more than a trace atmosphere  N 2 = 98.4%  CH 4 = 1.4%  Larger in troposphere  Remainder trace hydrocarbons  Includes C 2 H 6, C 3 H 8  UV photolysis of CH 4 and subsequent reactions  70 – 190 K (0 – 300 km) T = 70 – 200 K Broadener = N 2

 Difficult complex spectra  Low lying torsion modes  Extensive perturbations  Dense line structure  Extensive previous work  In general, does not cover full temperature/spectral range with appropriate gas broadeners (H 2, He and N 2 )  Recent work by Nixon et al. (2013) shows the importance of this:  Retrieved propene (C 3 H 6 ) in Titan  Better C 3 H 8 cross sections: 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 planetary atmospheres  Relatively low resolution (0.112 cm -1 ) - under resolved – see later  Pressures and temperatures for Earth (1 atm N 2 )  Useful for calibration and validation C 2 H 6 AND C 3 H 8

 Our goal is to produce cross sections for both C 2 H 6 and C 3 H 8  Appropriate temperatures and pressures  N 2, H 2 and He broadened  Down to 80 K  Far IR to 4000 cm -1  These spectra will first be represented by a set of ‘pseudo lines’  Empirically reproduce cross sections/spectra  Convenient for practical calculations  Models treat them as if they are real lines PROJECT AIM

 Long process…  Large number of spectra need to be recorded at high resolution  Utilize FTS spectrometer and cold cell at ODU  Combine with FTS spectrometer  Max resolution = cm -1  Cell based on Belgian design (J. Vander Auwera)  20 cm path length  Minimum temperature = -70 °C (ethanol liquid)  Fits within spectrometer  Does not cover complete temperature range  Could be adapted to reach even colder temperatures HOW WILL THIS BE DONE? J. Vander Awera design Coolant reservoir CaF 2 window Sample

LOW VAPOR PRESSURES  Low vapor pressures at low T  C 2 H 6 : 0.1 Torr at ~ 100 K  Longer path length needed and below 130 K a new type of cell is needed…  C 3 H 8 : 0.1 Torr at ~ 130 K PNNL

 Australian Synchrotron has unique 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  Advantage over current cell at ODU  Lower temperatures  Longer sample path length  Combine with the synchrotron  EFC ENCLOSIVE FLOW CELL

 Very high brightness  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 ALSO BENEFITS FROM USING A SYNCHROTRON Rotationally resolved sample at cm -1

 Sung et al. (2013) has completed cross sections between 700 – 1500 cm -1  T = 145 – 297 K  Wavenumber range = 690 – 1550 cm -1  Pseudo-line grid = cm -1  Used by Nixon et al. (2013) for propene retrievals  Current work is in the 3000 cm -1 region:  T = 233, 253, 273, 293 K  P = Pure (1 Torr), 10, 30, 100, 300 Torr of H 2 broadener PROPANE Sung et al. (2013) Harrison & Bernath (2010) 3 µm region Air broadened ν 15, ν 23 ν 16

TEMPERATURE DEPENDENCE OF PURE C 3 H 8 1 Torr of C 3 H 8 at -40°C (233 K) 1 Torr of C 3 H 8 at 0°C (273 K) 1 Torr of C 3 H 8 at 20°C (293 K) 64 scans, cm -1

C 3 H 8 AT -40°C 1 Torr of C 3 H 8 0 Torr of H 2 1 Torr of C 3 H 8 10 Torr of H 2 1 Torr of C 3 H 8 30 Torr of H 2 1 Torr of C 3 H Torr of H 2 1 Torr of C 3 H Torr of H 2 64 scans, cm scans, 0.01 cm scans, 0.02 cm scans, 0.04 cm -1

 Started investigating the 820 cm -1 (ν 9 band)  Main band used for atmospheric retrievals  Obtained time at the Australian Synchrotron  Benefit for low lying modes  Can the EFC cell be used for low vapor pressure? ETHANE PNNL Harrison et al. (2010) 3 µm region Air broadened 100x Wavenumber (cm -1 ) Absorbance ν9ν9 ν8ν8 ν7ν7 ν5ν5

C 2 H 6 RESULTS TO DATE Temp (K) C 2 H 6 Pressure (Torr) N 2 Pressure (Torr) C 2 H 6 Partial Pressure Resolution (cm -1 ) SourceScans Synchrotron Synchrotron MIR source MIR source Synchrotron Synchrotron MIR source MIR source Synchrotron Synchrotron MIR source MIR source400 90< Synchrotron114 90< MIR source184

 Far infrared (FIR/THz) beamline  1 of 9 beamlines AUSTRALIAN SYNCHROTRON

EXPERIMENTAL SETUP (INTERNAL SOURCE) Bruker IRS125 HR EFC apparatus Beamsplitter Mirror Detector Internal IR source EFC cell

EXPERIMENTAL SETUP (SYNCHROTRON SOURCE) FIR/THz Beamline Bruker IRS125 HR EFC apparatus Synchrotron Toward IR Microscopy beamline Beamsplitter Mirror Detector Internal IR source EFC cell

C 2 H 6 AT 200 K Pure 5 Torr 25 Torr100 Torr 176 scans cm scans cm scans 0.01 cm scans 0.04 cm -1

C 2 H 6 AT 200 K ( P Q 1 SUB-BAND) 100 Torr, 0.04 cm Torr, 0.01 cm -1 5 Torr, cm -1 Pure, cm -1

Temperature: 90 K Resolution: cm -1 Scans: 25 Pressure: 5 Torr N Torr C 2 H 6 (estimated) FIRST ENCLOSIVE RESULTS AT 90 K Non-enclosive Enclosive

 Lots still to do!  Project is in the preliminary stages  Continue investigations with cold cell at ODU  Obtain more spectra with EFC cell and synchrotron in Australia  Take measurements using Canadian Light Source to support work at ODU  These spectra could also be fit using LabFit  Multispectral fitting program (D. C. Benner)  Refine the broadening parameters over the extended pressures FUTURE WORK

THANKS FOR LISTENING This work has been funded by a NASA outer planets grant. We would like to thank the Australian Synchrotron, where some of this work has been carried out.