15min:

E. H. WISHNOW, Space Sciences Laboratory, University of California, Berkeley, CA 94720; and Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada V6T 1Z1; G. S. ORTON, Jet Propulsion Laboratory, Pasadena, CA 91109; I. OZIER, Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada V6T 1Z1; H. P. GUSH, Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada V6T 1Z1.

The perturbation-allowed distortion moment spectrum of CH₄ 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 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 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 I. Ozier, Phys. Rev. Lett. \underline27, 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. \underline318, 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 5 cm^-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.