The 13CH4 absorption spectrum in the Icosad range (6600–7692 cm−1) at 80 K and 296 K: Empirical line lists and temperature dependence

Abstract

The 13CH4 absorption spectrum has been recorded at 296K and 80K in the Icosad range between 6600 and 7700cm−1. The achieved noise equivalent absorption of the spectra recorded by differential absorption spectroscopy (DAS) is about αmin≈1.5×10−7cm−1. Two empirical line lists were constructed including 17,792 and 24,139 lines at 80K and 296K, respectively. For comparison, the HITRAN database provides only 1040 13CH4 lines in the region determined from methane spectra with natural isotopic abundance. Empirical values of the lower state energy level, Eemp, were systematically derived from the intensity ratios of the lines measured at 80K and 296K. Overall 10,792 Eemp values were determined providing accurate temperature dependence for most of the 13CH4 absorption in the region (93% and 82% at 80K and 296K, respectively). The quality of the derived empirical values of the lower state rotational quantum number, Jemp, is illustrated by their clear propensity to be close to an integer. A good agreement is achieved between our small Jemp values, with previous accurate determinations obtained by applying the 2T method to jet and 80K spectra. The line lists at 296K and 80K which are provided as Supplementary material will be used for future rovibrational assignments based on accurate variational calculations.