N 2 -broadened 13 CH 4 at 80 to 296 K Mary Ann H. Smith 1, Keeyoon Sung 2, Linda R. Brown 2, Timothy J. Crawford 2, Arlan W. Mantz 3, V. Malathy Devi 4, and D. Chris Benner 4 1 Science Directorate, NASA Langley Research Center, Hampton, VA 23681, U.S.A. 2 Science Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, U.S.A. 3 Dept. of Physics, Astronomy and Geophysics, Connecticut College, New London, CT 06320, U.S.A. 4 The College of William and Mary, Williamsburg, VA 23187, U.S.A. 11th International HITRAN Conference, June 2010

Motivation 11th International HITRAN Conference, June 2010 Titan atmospheric spectrum Brightness T (K) vs.Wavenumber (cm -1 ) recorded by CIRS onboard the Cassini spacecraft (Coustenis et al., Icarus (2007) Need a cold cell with good temperature control !!!  Titan atmospheric temperature range K  up to 5% CH 4 in N 2  12 CH 4, 13 CH 4, and CH 3 D bands overlap at 7.5 μm.  Determine Titan 13 C/ 12 C and D/H ratios  Titan temperature retrievals  HITRAN CH 4 halfwidths and shifts based mostly on lab measurements at >200 K  Extrapolation from HITRAN can lead to inaccuracies???

Cells cooled by closed-cycle helium refrigerators 11th International HITRAN Conference, June 2010 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.

Bruker IFS 125 HR facility at JPL 11th International HITRAN Conference, June 2010

The cells and refrigerating system 11th International HITRAN Conference, June 2010 Cell #1Cell #2 Body Oxygen Free High Conductivity copper # Path length cm cm Cell WindowsZnSe (wedged, 30‘) ZnSe (wedged, 30') Window clearance4.45 cm (dia.) Vacuum box (Windows) none (none) Installed (KBr, wedged, 30') Total weight6.45 kg2.75 kg Cooling system CTI Cryogenics Model 22 He-refrigerator Refrig. agentHelium ( %) Heater capacity50 W Temp. sensorsilicon diode (accuracy of 0.125K) Temp. achieved180 K79.3 K Temp. control $ < 0.01 K for 7 hrs< 0.01 K for 24 hrs 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.

Ice buildup on cell windows 11th International HITRAN Conference, June 2010  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):  H 2 O  CO 2  CH 4  C 2 H 6  their mixtures, and others?

Cooling performance of Cell #2 (with vacuum shroud) 11th International HITRAN Conference, June 2010

Spectra of N 2 -broadened 13 CH 4 at 296 K and 80 K 11th International HITRAN Conference, June 2010  Pure sample (99% 13 C) spectrum at 296K P = 1.05 Torr T = K Lots of features from high J transitions  N 2 -broadened spectrum at 296K P tot = Torr P s = 1.03 Torr T = K The high J features broadened out.  N 2 -broadened spectrum at 80K P tot = Torr P s = 1.20 Torr T = K The high J features almost disappeared.

Initial retrievals: 13 CH 4 R(2) manifold 11th International HITRAN Conference, June 2010  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.

13 CH 4 R(2) Multispectrum Fitting Residuals 11th International HITRAN Conference, June 2010 Fitting residuals from the Entire set(►) from Subset #1 (▼)

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

R(2) Preliminary Fit Results 11th International HITRAN Conference, June 2010 Transitionγ0γ0 nδ0δ0 δ′ No. Spectra R2 F2 1 ( cm -1 ) Subset#1: 181 – 296 K (1)0.855(6)−0.0017(1)3(2)E-065 Subset#2: 80 – 181 K (3)0.898(5)−0.0012(6)7(4)E-065 Entire Set: 80 – 296 K (1)0.887(3)−0.0017(1)7(1)E-069 R2 E 1 ( cm -1 ) Subset#1: 181 – 296 K (1)0.801(7)−0.0019(1)1.3(2)E-055 Subset#2: 80 – 181 K (3)0.890(7) (7)4.3(5)E-055 Entire Set: 80 – 296 K (1)0.860(4)−0.0013(2)2.3(2)E-059 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.

Evidence for departure from power law 11th International HITRAN Conference, June CH 4 /N 2 fit to the empirical power law, Sung et al., JMS in press (2010). γ o (T) = γ o (T o ) × (T o /T) n Extra term proposed by Mondelain et al. for 12 CH4/N 2 (JMS, 2007) and 13 CO/He, 13 CO/Ar (APB, 2008) ln(γ o T ) = ln(γ o To ) +n 1 ln(T o /T) + n 2 ln 2 (T o /T) n 2 is the non-linear term (smaller that n 1 by a factor of 12)

13 CH 4 R(2) Results Comparison 11th International HITRAN Conference, June 2010  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.

R(0) – R(3) Fit Results, 80 – 296 K Preliminary, work in progress 11th International HITRAN Conference, June 2010 Transitionγ0γ0 nδ0δ0 δ′ No. Spectra R0 A1 1 ( cm -1 ) (1)0.679(3)−0.0013(1)+5.2(1)E-059 R1 F1 1 ( cm -1 ) (1)0.869(2)−0.0006(1)+2.3(1)E-059 R2 F2 1 ( cm -1 ) (1)0.887(3)−0.0017(1)+7(1)E-069 R2 E 1 ( cm -1 ) (1)0.860(4)−0.0013(2)+2.3(2)E-059 R3 A2 1 ( cm -1 ) (1)0.773(4)−0.0011(1)+1.6(1)E (1)0.787(4)−0.0013(1)+1.0(2)E-059* R3 F2 1 ( cm -1 ) (2)0.805(6) (2)−1(3)E (2)0.828(5)−0.0044(2)−2.1(3)E-059* R3 F1 1 ( cm -1 ) (2)0.782(5)−0.0040(2)+7.1(2)E (2)0.842(2) (2)+5.3(2)E-059* 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.

Summary and Conclusions 11th International HITRAN Conference, June 2010 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 13 CH 4 /N 2 R(2) manifold. Temperature dependences in 80 – 296 K range. Non-linearity in the T-dependence of the widths. Continuing analysis of other 13 CH 4 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 N 2 -broadening. More gases at low temperatures (e.g., C 2 H 6 at this conference).

The Team and Acknowledgements 11th International HITRAN Conference, June 2010 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. Malathy Keeyoon Linda Tim Arlan Mary Ann Chris