High sensitivity absorption spectroscopy of methane at 80K in the 1.58μm transparency window: Temperature dependence and importance of the CH3D contribution

Abstract: The high resolution absorption spectrum of methane in the 1.58μm transparency window has been recorded at room temperature and at 79K by CW-Cavity Ring Down Spectroscopy using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers. The achieved sensitivity (αmin ∼3×10−10 cm−1) has allowed for a detailed characterization of the 6289–6526cm−1 region which corresponds to the lowest opacity of the transparency window. A list of 6868 and 4555 transitions with intensities as weak as 1×10−29 cm/molecule was constructed from the recordings at 297 and 79K, respectively. By comparison with a spectrum of CH3D recorded separately by Fourier Transform Spectroscopy, 1282 and 640 transitions of monodeuterated methane, CH3D, in natural abundance in our sample were identified at 297 and 79K, respectively. The rotational temperature determined from the intensity distribution of the 3ν 2 band of CH3D (79.3K) was found in good agreement with the temperature value previously obtained from the Doppler line broadening. The reduction of the rotational congestion by cooling down to 79K reveals a spectral region near 6300cm−1 where CH3D transitions are dominant. The low energy values of the transitions observed both at 79K and at room temperature were derived from the variation of their line intensities. These transitions with lower energy determination represent 93.9% and 68.4% of the total absorbance in the region, at 79K and room temperature, respectively. The quality of the obtained empirical low energy values is demonstrated for CH4 by the marked propensity of the empirical low J values to be close to integers. The line lists at 79K and room temperature provided as Supplementary Material allow accounting for the temperature dependence of methane absorption between these two temperatures. The investigated region covering the 5ν 4 band of the 12CH4 isotopologue will be valuable for the theoretical treatment of this band which is the lowest energy band of the icosad. [Copyright &y& Elsevier]