Determination of the low energy values of 13CH 4 transitions in the 2 ν3 region near 1.66 μm from absorption spectra at 296 and 81 K

Abstract

The high resolution absorption spectra of 13CH 4 were recorded at 81 K by differential absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers and at room temperature by Fourier transform spectroscopy. The investigated spectral region corresponds to the high energy part of the 13CH 4 tetradecad dominated by the 2 ν 3 overtone near 5988 cm −1. Empirical line lists were constructed containing, respectively, 1629 13CH 4 transitions detected at 81 K (5852–6124 cm −1) and 3481 features (including 85 lines of 12CH 4) measured at room temperature (5850–6150 cm −1); the smallest measured intensities are about 3 × 10 −26 and 4 × 10 −25 cm/molecule at 81 and 296 K, respectively. The lower state energy values were derived for 1196 13CH 4 transitions from the variation of the line intensities between 81 and 296 K. These transitions represent 99.2% and 84.6% of the total absorbance in the region, at 81 and 296 K, respectively. Over 400 additional weak features were measured at 81 K and could not be matched to lines observed at room temperature. The quality of the resulting empirical low energy values is demonstrated by the excellent agreement with the already-assigned transitions and the clear propensity of the empirical low J values to be close to integers. The two line lists at 81 and at 296 K provided as Supplementary material will enable future theoretical analyses of the upper 13CH 4 tetradecad.