An improved empirical line list for methane in the region of the 2ν3 band at 1.66 μm

The absorption spectrum of methane is recorded at room temperature by differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5855-6183 cm−1) dominated by the 2ν3 band at 1.66 μm. An empirical list of 9228 lines is constructed from a profile fitting of the very congested spectrum. The achieved noise equivalent absorption of the spectra (αmin≈1×10−7 cm−1) allowed us to add about 4000 lines to the recent empirical list constructed in relation with the project “Greenhouse Gases Observing Satellite” (Nikitin A, Lyulin O, Mikhailenko S, Perevalov V, Filippov N, Grigoriev I, et al. GOSAT-2009 methane spectral list in the 5550-6236 cm−1range. J Quant Spectrosc Radiat Transfer 2010;111:2211-24). Lines due to the 13CH4 and CH3D isotopologues present in the methane sample in “natural” abundance (0.011 and 5.0×10−4, respectively) are systematically identified by comparison with spectra of pure CH3D and 13CH4 recorded separately. In order to derive empirical values of the lower state energy level, Eemp, from the intensity ratios of the lines measured at 80 K and 296 K, the obtained 296 K line list is combined with the WKMC list of methane at 80 K (Campargue A, Leshchishina O, Wang L, Mondelain D, Kassi S, Nikitin AV. Refinements of the WKMC empirical line lists (5852-7919 cm−1) for methane between 80 K and 296 K. J Quant Spectrosc Radiat Transfer 2012;113:1855-73). Overall 4396 lower state energy values were determined extending significantly the previous set of Eemp values derived using the GOSAT list for the room temperature dataset. These improvements allow for a better accounting of the methane spectrum temperature dependence between 80 K and 300 K. A number of discrepancies between the rovibrational assignments included in the GOSAT list and our derived Eemp values is discussed. The new line dataset including a large number of empirical lower state energies provides a solid basis for future theoretical modeling of the tetradecad of 12CH4.

► New absorption spectra of methane at 296 K in the 2ν3 region have been recorded. ► An empirical list of 9228 lines at 296 K is constructed including about 4000 new lines. ► The 2-temperature method is applied to this list combined to the WKMC list at 80 K. ► 4396 empirical lower state energy values are determined. ► The obtained results improve significantly the GOSAT list in the region.