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

2.  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

3. 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 ~ 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 T = 70 – 200 K Broadener = H 2, He

4. 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

5.  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

6.  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

7.  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 = 0.002 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

8. 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

9.  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

10.  Very high brightness  Very intense for small aperture  Allows high resolution due to point source  Bruker FTS max = 0.00098 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 0.001 cm -1

11.  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 = 0.005 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

12. 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, 0.005 cm -1

13. 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 8 100 Torr of H 2 1 Torr of C 3 H 8 300 Torr of H 2 64 scans, 0.005 cm -1 128 scans, 0.01 cm -1 256 scans, 0.02 cm -1 512 scans, 0.04 cm -1

14.  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 100x10 -6 80 60 40 20 0 800 100012001400 1600 Wavenumber (cm -1 ) Absorbance ν9ν9 ν8ν8 ν7ν7 ν5ν5

15. 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 2000.301.000.003Synchrotron176 2000.550.090.003Synchrotron175 2001.0250.040.01MIR source400 2001.01000.010.04MIR source400 1600.201.000.002Synchrotron118 1600.550.090.002Synchrotron119 1600.8250.030.01MIR source442 1601.01000.010.04MIR source400 1200.101.000.001Synchrotron294 1200.150.020.002Synchrotron322 1200.5250.020.01MIR source500 1200.61000.0060.04MIR source400 90<0.0550.010.002Synchrotron114 90<0.5250.020.01MIR source184

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

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

18. 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

19. C 2 H 6 AT 200 K Pure 5 Torr 25 Torr100 Torr 176 scans 0.003 cm -1 175 scans 0.003 cm -1 400 scans 0.01 cm -1 176 scans 0.04 cm -1

20. C 2 H 6 AT 200 K ( P Q 1 SUB-BAND) 100 Torr, 0.04 cm -1 25 Torr, 0.01 cm -1 5 Torr, 0.003 cm -1 Pure, 0.003 cm -1

21. Temperature: 90 K Resolution: 0.002 cm -1 Scans: 25 Pressure: 5 Torr N 2 + 0.05 Torr C 2 H 6 (estimated) FIRST ENCLOSIVE RESULTS AT 90 K Non-enclosive Enclosive

22.  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

23. 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.