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THE DISTORTION DIPOLE ROTATIONAL SPECTRUM OF CH$_{4}$: A LOW TEMPERATURE FAR-INFRARED STUDY

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Title: THE DISTORTION DIPOLE ROTATIONAL SPECTRUM OF CH$_{4}$: A LOW TEMPERATURE FAR-INFRARED STUDY
Creators: Wishnow, E. H.; Orton, G. S.; Ozier, I.; Gush, H. P.
Issue Date: 2006
Abstract: The perturbation-allowed distortion moment spectrum of CH$_{4}$ has been studied between $20$ to $100$ cm$^{-1}$ with a Fourier transform spectrometer at a temperature of $113.5$ K similar to that in the atmospheres of Saturn and Titan. Data were obtained at a resolution of $0.06$ cm$^{-1}$\ and of $0.24$ cm$^{-1}$ with a sample gas pressure of $794$ Torr using an absorption path length of $60.0$ m. For each $(J+1\leftarrow J)$, the tetrahedral fine structure was blended together into a single $R(J)$ envelope. Six such envelopes for $J=$ $3$ to $8$ were measured, the strongest having a signal-to-noise ratio $\sim $80. From an intensity analysis of $R(5)$, $R(6)$, and $R(7)$, the distortion dipole moment $\mu _{D}$ of methane was determined to be $23.82(0.88)$ and $23.94(1.20)$ $\mu $% D from the low and high resolution spectra, respectively, in excellent agreement with earlier less precise intensity measurements at room temperature and the value of $24.06(0.45)$ $\mu $D obtained from the Stark effect by I. Ozier, Phys. Rev. Lett. \underline{27}, 1329 (1971). Based on these results, it is recommended that the intensities for these transitions in the HITRAN/GEISA data bases be scaled upward by a factor of $1.154$. This line spectrum arising from centrifugal distortion mixing was superimposed on a broad continuum due to collision-induced translation-rotation transitions. This continuum was measured from $20$ and $180$ cm$^{-1}$ (with a gap between $100$ and $120$ cm$^{-1}$), and is compared with the theoretical model of A. Borysow and L. Frommhold, Ap. J. \underline{318}, 940 (1987) at a lower temperature and with higher absolute accuracy than previously possible. Two features near 125.6 and 157. 3 cm$^{-1}$, each $\sim $5 cm$^{-1}$ wide, are seen to arise from rotational transitions in CH$_{4}-$CH$_{4}$ dimers. The study of the distortion dipole spectrum has direct application to the measurement of the CH$_{4}$:H$_{2}$ ratio and the temperature structure in the atmospheres of the Giant Planets and Titan.
URI: http://hdl.handle.net/1811/31069
Other Identifiers: 2006-TB-01
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