Low temperature laser absorption spectra of methane in the near-infrared at 1.65 μm for lower state energy determination

Direct absorption spectra of the 2ν₃ band of methane (CH₄) from 6038 to 6050 cm⁻¹ were studied at different low temperatures using a newly developed cryogenic cell in combination with a distributed feedback (DFB) diode laser. The cryogenic cell can operate at any stabilized temperature ranging from room temperature down to 100 K with temperature fluctuation less than ±1 K within 1 hour. In the present work, the CH₄ spectra in the range of 6038–6050 cm⁻¹ were recorded at 296, 266, 248, 223, 198, and 176 K. The lower state energy E'' and the rotational assignment of the angular momentum J were determined by a "2-low-temperature spectra method" using the spectra recorded at 198 and 176 K. The results were compared with the data from the GOSAT and the recently reported results from Campargue and co-workers using two spectra measured at room temperature and 81 K. We demonstrated that the use of a 2-low-temperature spectra method permits one to complete the E'' and J values missed in the previous studies.