11th International HITRAN Conference, 16-18 June 2010

1. N2-broadened 13CH4 at 80 to 296 KMary Ann H. Smith1, Keeyoon Sung2, Linda R. Brown2, Timothy J. Crawford2, Arlan W. Mantz3, V. Malathy Devi4, and D. Chris Benner4 1Science Directorate, NASA Langley Research Center, Hampton, VA 23681, U.S.A.2Science Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, U.S.A. 3Dept. of Physics, Astronomy and Geophysics, Connecticut College, New London, CT 06320, U.S.A. 4The College of William and Mary, Williamsburg, VA 23187, U.S.A. 11th International HITRAN Conference, 16-18 June 2010

2. Motivation • Titan atmospheric temperature range 70 - 200 K • up to 5% CH4 in N2 • 12CH4, 13CH4, and CH3D bands overlap at 7.5 μm. • Determine Titan 13C/12C and D/H ratios • Titan temperature retrievals • HITRAN CH4halfwidths and shifts based mostly on lab measurements at >200 K • Extrapolation from HITRAN can lead to inaccuracies??? Titan atmospheric spectrum Brightness T (K) vs.Wavenumber (cm-1) recordedby CIRS onboard the Cassini spacecraft (Coustenis et al. , Icarus (2007) Need a cold cell with good temperature control !!! 11th International HITRAN Conference, 16-18 June 2010

3. Cells cooled by closed-cycle helium refrigerators Heritage from collisional-cooling cells designed and used for microwave studies by DeLucia group at OSU. Cells constructed by A. Mantz have been used with infrared TDL spectrometers and with the Kitt Peak FTS. New helium-cooled cells were specifically designed to fit in the sample compartment of a Bruker IFS 120 HR or IFS 125 HR FTS. First cell was constructed at Connecticut College using Bruker drawings, shipped to JPL in April 2009, and successfully installed and tested in the Bruker IFS 125 HR. Good news: Vibrations from the helium refrigerator did not affect FTS performance. 11th International HITRAN Conference, 16-18 June 2010

4. Bruker IFS 125 HR facility at JPL 11th International HITRAN Conference, 16-18 June 2010

5. The cells and refrigerating system Cell #1 installed in the Bruker IFS 125 HR sample compartment, with compressor in the foreground. #Thermal conductivity, among the best in the temperatures 70 < T < 300 (better than Al and Au) $Achieved by PID (Proportional, Integrate and Differentiate) temperature control loop adopted in a Model 331 temperature controller supplied by Lakeshore Cryotronics, Inc. Arlan inspecting Cell #2 with vacuum shroud box installed. 11th International HITRAN Conference, 16-18 June 2010

6. Ice buildup on cell windows • Top panel (Cell #1): • Ice features grew with time • Resulting in changes in the continuum • Limiting integration time for coadding • Changing background • Diminished temperature stability • Middle panel (Cell #1): • Some features are persistent. • Windows warm up more slowly than the cell body. • Bottom panel (Cell #2) • The ice features diminished substantially • Possible gas sources of ice (outside or inside the cell): • H2O • CO2 • CH4 • C2H6 • their mixtures, and others? 11th International HITRAN Conference, 16-18 June 2010

7. Cooling performance of Cell #2 (with vacuum shroud) 11th International HITRAN Conference, 16-18 June 2010

8. Spectra of N2-broadened 13CH4 at 296 K and 80 K • Pure sample (99% 13C) spectrum at 296K • P = 1.05 Torr • T = 295.8 K • Lots of features from high J transitions • N2-broadened spectrum at 296K • Ptot = 795.6 Torr • Ps = 1.03 Torr • T = 295.8 K • The high J features broadened out. • N2-broadened spectrum at 80K • Ptot = 299.3 Torr • Ps = 1.20 Torr • T = 79.53 K • The high J features almost disappeared. 11th International HITRAN Conference, 16-18 June 2010

9. Initial retrievals: 13CH4 R(2) manifold • Why choose the R(2) manifold? • Two lines only, well isolated at low P • Low J lines are persistent at low T. • No line mixing is expected • between F and E symmetry species. • Voigt profile is good enough. • Selected 9 low-abundance spectra to avoid self-broadening. • Retrievals for three T ranges • Subset#1: 181 – 296 K • Subset#2: 80 – 181 K • Entire Set: 80 – 296 K • Retrievals at individual temperatures to examine power-law T-dependence. 11th International HITRAN Conference, 16-18 June 2010

10. 13CH4 R(2) Multispectrum Fitting Residuals • Fitting residuals • from the Entire set(►) • from Subset #1 (▼) 11th International HITRAN Conference, 16-18 June 2010

11. Temperature Dependences • Lorentz line widths γo(T) = γo(To) × (To/T)n γo(T) = half width at T at 1 atm To = reference T (296 K unless otherwise noted) n = temperature dependence Power law • Pressure-induced line shifts δo(T) = δo(To) + δ'×(T-To) δo(T) = half width at T at 1 atm To = reference T (296 K unless otherwise noted) δ' = temperature dependence Note that we do not use a power law here. 11th International HITRAN Conference, 16-18 June 2010

12. R(2) Preliminary Fit Results Note: Units of γ and δ are cm-1 atm-1 at 296K, units of δ′ are cm-1 atm-1 K-1, and n is unitless. 11th International HITRAN Conference, 16-18 June 2010

13. Evidence for departure from power law 13CH4/N2 fit to the empirical power law, Sung et al., JMS in press (2010). Extra term proposed by Mondelain et al. for 12CH4/N2 (JMS, 2007) and 13CO/He, 13CO/Ar (APB, 2008) γo(T) = γo(To) × (To/T)n ln(γoT) = ln(γoTo) +n1 ln(To/T) + n2ln2(To/T) n2 is the non-linear term (smaller that n1 by a factor of 12) 11th International HITRAN Conference, 16-18 June 2010

14. 13CH4 R(2) Results Comparison • HITRAN values are based on measurements from 210K to room temperature; we measured widths and shifts from 80K to 296K. • Widths are about 3x greater 80K than at 296K. Extrapolation using HITRAN08 parameters results in a 6 to 10% underestimate of the 80K line width. • The frequency shift for the E line is smaller than that for the F line at room temperature, but the different temperature dependences result in a 2x greater E line shift at 80 K. • At Titan’s surface, atmospheric temperature is ~93K and pressure is ~1.5 bar. 11th International HITRAN Conference, 16-18 June 2010

15. R(0) – R(3) Fit Results, 80 – 296 K Preliminary, work in progress Note: Units of γ and δ are cm-1 atm-1 at 296K, units of δ′ are cm-1 atm-1 K-1, and n is unitless. *Retrieved with line mixing. 11th International HITRAN Conference, 16-18 June 2010

16. Summary and Conclusions New experimental capability for low-T high-resolution spectroscopy • Closed-cycle He-cooled cell (single path) designed for the Bruker IFS 120/125 HR sample compartment. • Tested successfully with the Bruker IFS 125 HR at JPL. • Temperature range achieved with the FTS: 79.3 – 296 K. • Temperature stability 0.01 K for several days. Measured and observed • Line width and pressure-induced shifts for 13CH4/N2 R(2) manifold. • Temperature dependences in 80 – 296 K range. • Non-linearity in the T-dependence of the widths. Continuing analysis of other 13CH4 manifolds • R(0) through R(3) done; more to come. • Line mixing and speed-dependence to be considered. • Self-broadening must be quantified to obtain accurate low-T results for N2-broadening. • More gases at low temperatures (e.g., C2H6 at this conference). 11th International HITRAN Conference, 16-18 June 2010

17. The Team and Acknowledgements Tim Linda Arlan Keeyoon Malathy Mary Ann Chris Acknowledgements Research described in this talk was performed at Connecticut College, the College of William and Mary, NASA Langley Research Center and the Jet Propulsion Laboratory, California Institute of Technology, under contracts and cooperative agreements with the National Aeronautics and Space Administration. 11th International HITRAN Conference, 16-18 June 2010