The distortion dipole rotational spectrum of CH4: A low temperature far-infrared study

The perturbation-allowed distortion moment spectrum of CH4 has been studied between 20 and 100cm-1 with a Fourier transform spectrometer at a temperature of 113.5K similar to that in the atmospheres of Saturn and Titan. Data were obtained at a resolution of 0.06cm-1 and of 0.24cm-1 with a sample gas pressure of 794Torr using an absorption path length of 60.0m. 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 μD of methane was determined to be 23.82(0.88) and 23.94(1.20)μD 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)μD 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 20-180cm-1 (with a gap between 100 and 120cm-1), and is compared with the theoretical model of Borysow and Frommhold [Collision-induced rototranslational absorption spectra of CH4-CH4 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 157.3cm-1, each ∼5cm-1 wide, are seen to arise from rotational transitions in CH4-CH4 dimers. The study of the distortion dipole spectrum has direct application to the measurement of the CH4:H2 ratio and the temperature structure in the atmospheres of the Giant Planets and Titan.