The 1.58 μm transparency window of methane (6165–6750 cm −1): Empirical line list and temperature dependence between 80 and 296 K

Abstract

The empirical line parameters of over 12,000 methane transitions have been obtained at 80 K in the 1.58 μm transparency window (6165–6750 cm −1) which is of importance for planetary applications. This line list (WKC-80K) was constructed from high sensitivity spectra of normal abundance methane recorded by CW-Cavity Ring Down Spectroscopy at low temperature. The minimum intensity reported is on the order of 5×10 −30 cm/molecule. High resolution Fourier transform spectra have also been recorded using enriched CH 3D samples at 90–120 K in order to facilitate identification of monodeuterated methane features in the methane line list at 80 K. The CH 3D relative contribution in the considered region is observed to be much larger at 80 K than at room temperature. In particular, CH 3D is found dominant in a narrow spectral window near 6300 cm −1 corresponding to the highest transparency region. Using a similar line list constructed at room temperature (Campargue A, Wang L, Liu AW, Hu SM and Kassi S. Empirical line parameters of methane in the 1.63–1.48 μm transparency window by high sensitivity Cavity Ring Down Spectroscopy. Chem Phys 2010;373:203–10.), the low energy values of the transitions observed both at 80 K and at room temperature were derived from the variation of their line intensities. Empirical lower states and J-values have been obtained for 5671 CH 4 and 1572 CH 3D transitions representing the most part of the absorbance in the region. The good quality of these derived energy values is demonstrated by the marked propensity of the corresponding CH 4 lower state J values to be close to integers. The WKC line lists at 80 K and room temperature provided as Supplementary Material allow one accounting for the temperature dependence of methane absorption between these two temperatures. The importance of the 80 K line list for the study of Titan and other methane containing planetary atmospheres is underlined and further improvements are proposed. The resulting information will advance the theoretical modeling of the methane spectrum in the 1.58 μm transparency window.