Line Positions and Intensities for the 12 Band of 13 C 12 CH 6 V. Malathy Devi 1, D. Chris Benner...

Line Positions and Intensitiesfor the ν12 Band of 13C12CH6

V. Malathy Devi1, D. Chris Benner1, Keeyoon Sung2, Timothy J. Crawford2, Arlan W. Mantz3, and Mary Ann H. Smith4

1The College of William and Mary, Williamsburg, VA 23187, 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.4Science Directorate, NASA Langley Research Center, Hampton, VA 23681, U.S.A.

1FE08-69th International Symposium on Molecular Spectroscopy, June 16-20, 2014

Ethane in Earth and Planetary Atmospheres

The most abundant ethane isotopologue, 12C2H6, is easily detected in spectra of Earth and planetary atmospheres and in comet volatiles.

13C12CH6 appears in atmospheric spectra of Jupiter, Saturn, Neptune and Titan.

Spectral observations of both isotopologues at 12 µm enable 13C/12C ratios to be determined for Jupiter, Saturn, and Titan.

Recent high-resolution studies to improve 12-µm ethane parameters:

12C2H6 ν9 band [Malathy Devi et al., JQSRT 111 (2010) 1234-1251; Malathy Devi et al., JQSRT 111 (2010) 2481-2504 and references therein] updated in HITRAN 2012.

13C12CH6 ν12 based on Kurtz et al. (1991) and Weber et al. (1993,1994) in GEISA 2009 ; more recent ν12 and ν12+ν6−ν6 positions and assignments [Borvayeh et al., J. Mol. Spectrosc. 255 (2009) 157-163; Moazzen-Ahmadi et al., JQSRT 129 (2013) 316-323] not yet in GEISA or HITRAN.

Using low-temperature spectra to minimize hot-band transitions, we have measured line positions and absolute intensities for 1660 ν12 transitions.


13C12CH6 in the spectrum of JupiterP.V. Sada, G.H. McCabe, G.L. Bjoraker, D.E. Jennings, D.C. Reuter. Astrophys. J. 472 (1996) 903-907.


Recording the Spectra

The spectra of 13C12CH6 were recorded with the 20.38 cm coolable cell [Sung et al., J.

Mol. Spectrosc. 262 (2010) 122-134] installed inside the sample compartment of the Bruker IFS 125 HR spectrometer at JPL. The cell body is oxygen free high conductivity (OFHC) copper with wedged ZnSe windows; the vacuum shroud box has wedged KBr windows.

• KBr beamsplitter, MCT detector, resolution ~0.003 cm-1

• Temperatures: 130 K, 178 K, 208 K, and 294 K.

• Total pressures: 2 – 5 torr (2.7 – 6.7 hPa) at 130 – 208 K; 9.5 torr (12.6 hPa) at 294 K.

• Gas sample: 13C-enriched ethane (13C, 99%) from Cambridge Isotope Labs.


Laboratory spectra of 13C12CH6 at two temperatures

a 1 atm = 101.3 kPa = 760 Torr


Laboratory spectra of 13C12CH6 at two temperatures

a 1 atm = 101.3 kPa = 760 Torr


RQ(J,7) RQ(J,8)

● RR(5,4) ● RR(6,5)

Multispectrum Fitting Analysis Only the 4 low-temperature spectra 130–208 K were included in the fits to minimize

interferences from weak high-J and hot-band transitions. Short spectral intervals (1‒3 cm−1) were fit in all 4 spectra simultaneously.

Important: All spectra are calibrated to the same reference standard (H2O ν2 line positions [R.A.Toth, J. Opt. Soc. Am. B 8 (1991) 2236-2255]).

Initially 13C12CH6 line list from GEISA 2009 (also available as a supplement to HITRAN 2008). Weak features missing from the original list were added interactively.

HITRAN partition functions [A. L. Laraia et al., Icarus 215 (2011) 391-400] were applied in the fitting algorithm to obtain corresponding line intensities at 296 K.

Intensity ratios of torsionally split components and their separations, were at first constrained to predicted values, but some were later adjusted in the fitting process.

Lorentz half-width, pressure-induced shift coefficients and their temperature dependences were fixed to appropriate default values using 12C2H6 measured values [Malathy Devi et al., JQSRT 111 (2010) 1234-1251; JQSRT 111 (2010) 2481-2504].


Observed torsional splitting in ν12 of 13C12CH6, 2 Torr at 130 K


13C12CH6 multispectrum fit and residuals

4 self-broadened spectra

The same color codes are used for the observed spectra and residuals:

Red (2.05 Torr at 130 K)Blue (4.99 Torr at 178 K)Green (2.95 Torr at 178 K)Black (4.94 Torr at 208 K)


PP transitions

Torsional splitting not resolved






PP, RP, and PQ transitions

Torsional splitting not resolved






PP, RP, and PQ transitions

Torsional splitting is resolved for high-J PQ(K=6) lines






PR and RR transitions

Torsional splitting is resolved only for PR(18,6) line ~830.1 cm-1

Results

Four spectra recorded at 130 to 208 K were included in the analysis. Of the 2948 spectral lines measured, 34% were weak, unidentified features; however, approximate lower state energies could be estimated for some of these lines.15% of the remaining ν12 transitions were not reported because of blends or unsatisfactory fits.Line positions and intensities are reported for 1660 ν12 transitions of 13C12CH6 with known assignments.

• Positions accurate to ± 1 to 4 × 10−4 cm−1

• Intensities accurate to ± 2 to 3%Good agreement with HITRAN 2008, but not HITRAN 2012.[13C12CH6 intensities in HITRAN 2012 were inadvertently re-scaled by the same factor (−15%) as those for 12C2H6.]


13C12CH6 ν12 Positions and Intensities Compared to HITRAN08 Supplement (same as GEISA09)

PS/HITRAN08 intensity ratio Mean ± Standard Deviation: 1.09 ± 0.10


Most position differences are within ± 0.0005 cm−1

To agree with the present study, the 13C12CH6 ν12

intensities in HITRAN 2012 should be scaled by 1.28

Summary

From multispectrum fits of low-temperature (130 – 208 K) spectra we have determined

• Line positions and absolute intensities for 1660 ν12 transitions of 12C13CH6.

• Positions and intensities for approximately 1000 other unassigned lines in the 12.2 µm region.

• Approximate lower state energies for some of these unassigned lines.

Results were recently published:V. Malathy Devi et al., J. Mol. Spectrosc. 301 (2014) 28-38.

http://dx.doi.org/10.1016/j.jms.2014.05.005


The Team and Acknowledgements

AcknowledgementsResearch 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. We thank Linda Brown for technical assistance and helpful comments.

Malathy Keeyoon LindaTim

Arlan Mary Ann Chris


Line Positions and Intensities for the 12 Band of 13 C 12 CH 6 V. Malathy Devi 1, D. Chris Benner...

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