The methane absorption spectrum near 1.73 µm (5695–5850 cm−1): Empirical line lists at 80 K and 296 K and rovibrational assignments

Abstract The methane absorption spectrum is studied at 297 K and 80 K in the center of the Tetradecad between 5695 and 5850 cm −1 . The spectra are recorded by differential absorption spectroscopy (DAS) with a noise equivalent absorption of about α min ≈ 1.5 × 10 −7 cm −1 . Two empirical line lists are constructed including about 4000 and 2300 lines at 297 K and 80 K, respectively. Lines due to 13 CH 4 present in natural abundance were identified by comparison with a spectrum of pure 13 CH 4 recorded in the same temperature conditions. About 1700 empirical values of the lower state energy level, E emp , were derived from the ratios of the line intensities at 80 K and 296 K. They provide accurate temperature dependence for most of the absorption in the region (93% and 82% at 80 K and 296 K, respectively). The quality of the derived empirical values is illustrated by the clear propensity of the corresponding lower state rotational quantum number, J emp , to be close to integer values. Using an effective Hamiltonian model derived from a previously published ab initio potential energy surface, about 2060 lines are rovibrationnally assigned, adding about 1660 new assignments to those provided in the HITRAN database for 12 CH 4 in the region.