Empirical determination of low J values of 13 CH 4 transitions from jet cooled and 80 K cell spectra in the icosad region (7170–7367 cm −1 )

The absorption spectrum of 13 CH 4 was recorded at two low temperatures in the icosad region near 1.38 µm, using direct absorption tunable diode lasers. Spectra were obtained using a cryogenic cell cooled at liquid nitrogen temperature (80 K) and a supersonic jet providing a 32 K rotational temperature in the 7173–7367 cm −1 and 7200–7354 cm −1 spectral intervals, respectively. Two lists of 4498 and 339 lines, including absolute line intensities, were constructed from the 80 K and jet spectra, respectively. All the transitions observed in jet conditions were observed at 80 K. From the temperature variation of their line intensities, the corresponding lower state energy values were determined. The 339 derived empirical values of the J rotational quantum number are found close to integer values and are all smaller than 4, as a consequence of the efficient rotational cooling. Six R (0) transitions have been identified providing key information on the origins of the vibrational bands which contribute to the very congested and not yet assigned 13 CH 4 spectrum in the considered region of the icosad.