Refinements of the WKMC empirical line lists (5852-7919 cm-1) for methane between 80 K and 296 K
Résumé
During the last 4 years, empirical line lists for methane at room temperature and at 80 K were constructed from spectra recorded by (i) differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5852-6195 cm-1) and in the icosad (6717-7589 cm-1) and (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58 μm and 1.28 μm transparency windows (6165-6750 cm-1 and 7541-7919 cm-1, respectively). We have recently constructed the global line lists for methane in "natural" isotopic abundance, covering the spectral region from 5854 to 7919 cm-1 (Campargue A, Wang L, Kassi S, Mondelain D, Bézard B, Lellouch E, et al., An empirical line list for methane in the 1.26-1.71 μm region for planetary investigations (T=80-300 K). Application to Titan, Icarus 219 (2012) 110-128). These WKMC (Wang, Kassi, Mondelain, Campargue) empirical lists include about 43,000 and 46,420 lines at 80±3 K and 296±3 K, respectively. The "two temperature method" provided lower state energy values, Eemp, for about 24,000 transitions allowing us to account satisfactorily for the temperature dependence of the methane absorption over the considered region. The obtained lists have been already successfully applied in a large range of temperature conditions existing on Titan, Uranus, Pluto, Saturn and Jupiter.In the present contribution, we provide some improvements to our lists by using literature data to extend the set of lower state energy values and by correcting the distortion of the high Eemp values (J>10) due to the temperature gradient existing in the cryogenic cell used for the recordings. The proposed refinements are found to have an overall limited impact but they may be significant in some spectral intervals below 6500 cm-1.The new version of our lists at 80 K and 296 K is provided as Supplementary Material: the WKMC@80K+ and WKMC@296K lists are adapted for planetary and atmospheric applications, respectively. The WKMC@80K+ list is made applicable over a wider range of temperatures and shows satisfactory extrapolation capabilities up to room temperature. It was obtained by transferring to the 80 K list the 27,580 single lines present only in the 296 K list, with corresponding lower state energy values chosen to make them below the detectivity limit at 80 K.In the discussion, the different line lists and databases available for methane in the near infrared are compared and some suggestions are given.