69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 1/12 JPL Progress Report Keeyoon Sung, Geoffrey C. Toon, Linda R. Brown Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA FT-IR measurements of cross sections of cold benzene at 7 – 15 µm for Titan

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 2/12 JPL Atmospheric C 6 H 6 in Titan, Jupiter, Saturn  Observed in their stratospheres  Possible pathways to high mass molecules  Leading to higher mass molecules (e.g. Voyager/INMS)  Spectroscopic data are demanded (e.g., Cassini/CIRS)  Status of the current database available  Mid-IR Xsec (cross section) from PNNL Coustenis et al Titan However, only at warm temperatures (278, 298, and 323 K) Sharpe et al Rinsland et al  Temperature-dependent Xsecs are needed (e.g., N 2 -broadened data for Titan) v 4 band Wilson et al Proposed pathway to C 6 H 6

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 3/12 JPL Benzene (C 6 H 6 ) in the 7 – 15 µm region  Spectroscopic properties  Totally symmetric – no dipole moment ; no rotational lines – Infrared band studies - more important  20 vibrational fundamentals  Infrared region, congested by low lying and dark states and their interactions Band centers of C 6 H 6 fundamentals (in cm -1 ) v 1 (3602)v 6 (1010)v 11 ( 849)v 16 (1596) v 2 ( 992 )v 7 ( 993)v 12 (3063) v17 ( 1178) v 3 (1326 )v 8 ( 703) v 13 (1486) v 18 ( 606) v 4 ( 674)v 9 (1310)v 14 (1038)v 19 ( 975) v 5 (3068)v 10 (1150)v 15 (3047)v 20 ( 410) Band centers from NIST Chem Web book Ten infrared active bands (in red)  Measurements and modeling for Spectroscopic Observations  Four key parameters needed: position, intensity, line width, and lower-state energy  Line-by-line calculations are desired, but only a few are available; (e.g., GEISA lists for v 4 band only)  Hamiltonian modeling is very challenging  We need a complementary approach!  We provide an intermediate solution in this work (in progress).  Measured temperature-dependent Xsecs  Pseudolines: A mathematical construct to reproduce the measured Xsecs

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 4/12 JPL Bruker 125HR + 80 cm cold cell ☼  Cooling the cell: Flowing cold N 2 gas Passive T control Cu - Cu·Ni thermocouples Developed by J. Margolis for Kitt Peak FTS Old Bruker 120 scanner arm Cell

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 5/12 JPL Experimental conditions and data acquisition  Cross section measurements for Titan  Pure and N 2 -broadened C 6 H 6 at low T’s  Xsecs can be directly obtained from transmittance at the given P and T.  Sample spectra are presented below. Experimental conditions of the cold C 6 H 6 Gas samplesC 6 H 6 (99.9%), N 2 ( %) Spectral region7 – 15 µm (620 – 1550 cm -1 ) Resolution0.01, 0.02, 0.04 cm -1 C 6 H 6 pressures10 mTorr – 5.25 Torr Total pressure80 – 596 Torr Mixing ratios1.00 (pure); – 0.02 Temperatures232 – 296 K # of spectra19 (including six pure benzene) v4v4 v 7 -v 20 v 14 v 13, v 4 +v 11 v 11 +v 20 v 17 -v 20 Residual H 2 O 232 K – 0.01 Torr Torr (N 2 ) 245 K – 0.45 Torr Torr (N 2 ) 296 K – 5.25 Torr Sample spectra of C 6 H 6 (+ N 2 )

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 6/12 JPL Pseudolines for measured Xsecs  Benzene Xsecs measured at T = 232 – 296 K  Xsec vs. Transmittance vs. Molecular line shape profile Transmittance, τ(v) = exp[ ‒ S·ψ(v-v 0 )·n·l] Cross sections, κ(v) = S·ψ(v-v 0 ) also, κ(v) ≈ - ln[τ(v)]/[nl]  Measured C 6 H 6 Xsecs in the 630–1530 cm -1  Limitations on the Xsec measurements - “Line profile information is lost”. - Integrated Xsec vs. Band intensity - Need measurements at various T and P.  Pseudolines to reproduce the lab spectra 1) Assumption: “Valid average transmission can be defined for a given frequency interval (i.e., uniformly-spaced frequency bins as ‘pseudolines’), having valid mean values of strength, pressure-broadened widths, and lower state energy, E″ at the frequency.” 2) Fit lab spectra simultaneously to derive S and E″ (while holding widths best estimates) 3) Their applicability has been proven in Earth remote sensing (For details, visit

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 7/12 JPL Benzene pseudolines – Empirical S and E″  Deriving the C 6 H 6 pseudoline parameters at T o = 296 K  Line spacing chosen to be cm -1 (fully resolves structure in lab spectra)  N 2 - & self-broadened half widths, adopted to be 0.12 & 0.17 [Waschull et al. 1990].  Temperature dependence exponent for widths, n = 0.7 in b(T) = b(T o )×(T o /T) n  Adopted Voigt line profile convolved with ILS, i.e., ψ D ●ψ L ●ψ ILS ●ψ FOV.  Vibrational partition function was computed from the band centers  Rotational partition function at T, Q rot (T), was estimated by Q rot (T o )×(T/T o ) 2  Fit all laboratory spectra simultaneously recorded at various T’s and P’s  Retrieve line strengths and lower state energies of the individual pseudolines. H2OH2O JPL(Obs.), 296K C 6 H 6 (1.93 Torr + N 2 (78Torr)

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 8/12 JPL Preliminary results C 6 H 6 pseudolines Spectrum fitting residuals from pseudoline fits  Compile the pseudoline parameters in HITRAN format  Position, intensity, self-width, N 2 - width, lower state energies (PLL: PseudoLine List)  HITRAN-type molecule and isotope index, e.g., 70 and 0  As a result, they can be treated in the same way as for spectroscopic line parameters in the Radiative Transfer line-by-line calculations  However, they should also be treated as one entity as a whole.

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 9/12 JPL Preliminary Comparisons Linelist comparison with GEISA  Observed(JPL)  Synthetic(GEISA) – cold band only  Synthetic(Pseudoline) – all bands observed hot & under- lying bands 0.78 Torr 252.3K 80 cm 0.01 cm -1 At the same Temp. PNNL no data below 278K Cold data is more appropriate for Cassini/CIRS. Showing temperature-dependence ? Comparison with PNNL All contributions are needed all at once.

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 10/12 JPL Comparison of band strength estimates  Integrated intensity (cm/molec) for three spectral regions 1)∑(pseudoline intensities) 2)∑(PNNL cross section) cf.) ∑(GEISA intensity for v 4 )  Band intensities for other spectral regions are under way. Integrated intensity comparison Note: ~3% uncertainties are too small to show up.

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 11/12 JPL Summary and on-going work The 2 cm long coolable cell at JPL  Obtained cold spectra of C 6 H 6 by using a 80 cm path long cold cell  Measured temperature-dependent Xsecs at T = 232 – 296 K  Made progress in deriving pseudoline parameters (line strengths and lower state energies) in the 1000 – 1530 cm -1  Continue for the v 4 band region in the 630 – 750 cm -1. New spectra needed with a shorter cell.  Final results will be compiled in HITRAN format and distributed.

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 12/12 JPL Acknowledgements Research at Jet Propulsion Laboratory, California Institute of Technology, was performed under contracts and cooperative agreements with the National Aeronautics and Space Administration. Also, K. Sung thanks Timothy Crawford, the master technician from our FT-IR laboratory, for modifying the Bruker IFS with the extension chamber from the old Bruker 120HR and the sample cell.

69th Meeting - Champaign-Urbana, Illinois, 2014 FE11 13/12 JPL Distribution of the derived lower state energy, E ʺ