First assignment of the 5ν4 and ν2+4ν4 band systems of 12CH4 in the 6287-6550 cm−1 region

This paper reports the first assignment of rovibrational transitions of the 5ν4 and ν2+4ν4 band systems of 12CH4 in the 6287-6550 cm−1 region, which is usually referred to as part of the 1.58 μm methane transparency window. The analysis was based on two line lists previously obtained in Grenoble by cavity ring down spectroscopy at T=297 and 79 K completed by three long-path Fourier transform spectra recorded in Reims (at 290 K, L=1603 m, P=1-34 mbar). In order to determine the dipole transition moment parameters and quantify the intensity borrowing due to the resonance interactions, we had to include in the fit of the effective Hamiltonian model some lines of the stronger ν1+3ν4 and ν2+4ν4 bands. For this purpose, intensities of 179 additional lines were retrieved from FTS spectra above 6550 cm−1 though the analysis of these higher bands is not complete. About 1955 experimental line positions and 1462 line intensities were fitted with RMS standard deviations of 0.003 cm−1 and 13.1%, respectively. A line list of 8029 calculated and observed transitions which are considered as dominant was constructed for 12CH4 in the 6287-6550 cm−1 region. This is the first high-resolution analysis and modelling of 5-quanta band systems of 12CH4.

Research Highlights►Assignment of rovibrational transitions of the 5ν4 and ν2+4ν4 band systems of 12CH4. ►Analysis was based on CRDS (Genoble) and FTS (Reims) spectra. ►1955 positions and 1462 intensities were fitted with RMS 0.003 cm−1 and 13.1%. ►Line list of 8029 calculated and observed transitions was constructed.