Experimental low energy values of CH4 transitions near 1.33 μm by absorption spectroscopy at 81 K

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.36 and 1.30 μm (7351-7655 cm−1) using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. The investigated spectral range corresponds to the high energy part of the icosad dominated by the ν2+2ν3 band near 7510 cm−1. The positions and strengths at 81 K of 3473 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10−26 cm/molecule, i.e. significantly lower than the intensity cut off of the HITRAN database in the region (4×10−25 cm/molecule at 296 K). From the variation of the line strength between 81 and 296 K, the low energy values of 1273 transitions could be determined. They represent 69% and 81% of the absorbance in the region at 296 and 81 K, respectively. The obtained results are discussed in relation with the few rovibrational assignments previously reported in the region.