The absorption spectrum of 13CH4 at 80 K and 296 K near 1.73 µm

Abstract

The absorption spectrum of 13CH4 is recorded in the center of the tetradecad near 1.73 μm (5695–5850 cm−1) at 296 K and 80 K by direct absorption spectroscopy (DAS). The achieved noise equivalent absorption of the spectra is αmin ≈ 5 × 10−8 cm−1 leading to line intensity detectivity threshold on the order of 5 × 10−26 and 5 × 10−27 cm/molecule at 296 K and 80 K, respectively. As a result, two empirical line lists are constructed including about 3350 and 2070 lines at room temperature and liquid nitrogen temperature, respectively. For comparison, the HITRAN database provides in the region only the strongest 13CH4 lines (about 440 lines in total). From the intensity ratios of the lines measured at the two temperatures, 1529 empirical values of the lower state energy level (Eemp) were derived. Their accuracy is reflected by the clear propensity of the corresponding empirical values of the lower state rotational quantum number, Jemp, to be close to integer values. They provide accurate temperature dependence for 91% and 99% of the total 13CH4 absorption in the region at 296 K and 80 K, respectively.The comparison to the TheoReTS ab initio line list shows significant deviations for line positions. The obtained experimental data will be valuable to tune ab initio line positions to empirical values in future versions of the theoretical line lists.