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

Abstract

This paper reports the first assignment of rovibrational transitions of the 5 ν 4 and ν 2+4 ν 4 band systems of 12CH 4 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 ν 1+3 ν 4 and ν 2+4 ν 4 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 12CH 4 in the 6287–6550 cm −1 region. This is the first high-resolution analysis and modelling of 5-quanta band systems of 12CH 4.