Multispectrum measurements of spectral line parameters including temperature dependences of N_2- and self-broadened half-width coefficients in the region of the ∨_9 band of ~(12)C_2H_6

摘要

Ethane is a prominent contributor to the spectrum of Titan, particularly in the ∨_9 region centered near 822cm~(-1). To improve the spectroscopic line parameters at 12μm, 41 high-resolution (0.0016-0.005cm~(-1)) absorption spectra of C_2H_6 were obtained at sample temperatures between 211 and 298K with the Bruker IFS 120HR at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington. Two additional spectra were later recorded at ~150K using a new temperature-stabilized cryogenic cell designed for the sample compartment of the Bruker IFS 125HR at the Jet Propulsion Laboratory (JPL) in Pasadena, California. A multispectrum nonlinear least-squares fitting program was applied simultaneously to all 43 spectra to measure the line positions, intensities, N_2- and self-broadened half-width coefficients and their temperature dependences. Reliable pressure-induced shift coefficients could not be obtained, however, because of the high congestion of spectral lines (due to torsional-split components, hot-band transitions as well as blends). Existing theoretical modeling of this very complicated ∨_9 region permitted effective control of the multispectrum fitting technique; some constraints were applied using predicted intensity ratios, doublet separations, half-width coefficients and their temperature dependence exponents in order to determine reliable parameters for each of the two torsional-split components. For ~(12)C_2H_6, the resulting retrievals included 17 ~pQ and ~rQ sub-bands of ∨_9 (as well as some ~pP, ~rR sub-bands). Positions and intensities were measured for 3771 transitions, and a puzzling difference between previously measured ∨_9 intensities was clarified. In addition, line positions and intensities were obtained for two ~(12)C_2H_6 hot bands ∨_9+∨_4-∨_4, ∨_9+2∨_4-2∨_4) and the ∨_9 band of ~(13)C~(12)CH_6, as well as several hundred presently unidentified transitions. N_2- and self-broadened half-width coefficients were determined for over 1700 transitions, along with 1350 corresponding temperature dependence exponents. Similar to N_2- and self-broadened half-width coefficients, their temperature dependence exponents were also found to follow distinctively different patterns. However, while the self- and N_2-broaded widths differed by 40%, the temperature dependence exponents of the two broadening gases were similar. The variations of the observed half-width coefficients and their temperature dependences with respect to J, K quantum numbers were modeled with a set of linear equations for each K. The present broadening coefficients compared well with some of the prior measurements.