The absorption spectrum of methane at 80 and 294 K in the icosad (6717-7589 cm-1): Improved empirical line lists, isotopologue identification and temperature dependence
Résumé
Using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers, new high resolution spectra of methane have been recorded at 80 K and room temperature by differential absorption spectroscopy (DAS) between 6717 and 7589 cm-1 (1.49-1.32 μm). The investigated spectral region corresponds to the very congested icosad, which is not tractable by theory. Empirical lists of 19,940 and 24,001 lines were constructed from the 80 K and room temperature spectrum, respectively. The room temperature list adds about 8500 features to the empirical list of 15,375 lines at 296 K adopted in the HITRAN database from the original work of L. Brown (Brown, L. Empirical line parameters of methane from 1.1 to 2.1 μm. JQSRT 2005;96:251-70). A number of relatively strong CH4 lines located near strong water lines were found missing in the HITRAN line list. The improved sensitivity allowed adding more than 7000 lines to our previous list of about 12,000 transitions at 80 K (Campargue A, Wang L, Kassi S, Mašát M, Votava O. Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (II) The Icosad region (1.49-1.30 μm). JQSRT 2010;111:1141-51). In order to facilitate identification of the transitions of the different methane isotopologues present in "natural" isotopic abundance, spectra of highly enriched CH3D and 13CH4 samples were recorded with the same experimental setup, both at room temperature and at 80 K.From the variation of the line strengths between 80 K and 294 K, the low energy values of about 12,000 transitions were determined. They allow accounting for the temperature dependence of 84 and 93% of the methane absorbance in the region, at room temperature and 80 K, respectively. As a result, we provide as supplementary material two complete line lists at 80 K and 294 K, including CH3D and 13CH4 identification and lower state energy values.