High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: Temperature dependence and importance of the CH3D contribution

The low energy values of the transitions observed both at 79 K and at room temperature were derived from the variation of their line intensities. These transitions with lower energy determination represent 93.9% and 68.4% of the total absorbance in the region, at 79 K and room temperature, respectively. The quality of the obtained empirical low energy values is demonstrated for CH4 by the marked propensity of the empirical low J values to be close to integers. The line lists at 79 K and room temperature provided as Supplementary Material allow accounting for the temperature dependence of methane absorption between these two temperatures. The investigated region covering the 5ν4 band of the 12CH4 isotopologue will be valuable for the theoretical treatment of this band which is the lowest energy band of the icosad.