The distortion dipole rotational spectrum of [formula omitted]: A low temperature far-infrared study

Abstract

The perturbation-allowed distortion moment spectrum of CH 4 has been studied between 20 and 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 ← J ) , the tetrahedral fine structure was blended together into a single R ( J ) envelope. Six such envelopes for J = 3 – 8 were measured, the strongest having a signal-to-noise ratio ∼ 80 . From an intensity analysis of R ( 5 ) , R ( 6 ) , and R ( 7 ) , the distortion dipole moment μ D of methane was determined to be 23.82 ( 0.88 ) and 23.94 ( 1.20 ) μ 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 ) μ D obtained from the Stark effect by Ozier [Ground-state electric dipole moment of methane. Phys Rev Lett 1971;27: 1329–32]. 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 – 180 cm - 1 (with a gap between 100 and 120 cm - 1 ), and is compared with the theoretical model of Borysow and Frommhold [Collision-induced rototranslational absorption spectra of CH 4–CH 4 pairs at temperatures from 50 to 300 K. Ap J 1987;318:940–3] at a lower temperature and with higher absolute accuracy than previously possible. Two features near 125.6 and 157.3 cm - 1 , each ∼ 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.