New assignments in the 2 μm transparency window of the 12CH4 Octad band system

This paper reports new assignments of rovibrational transitions of 12CH4 bands in the range 4600-4887  cm−1 which is usually referred to as a part of the 2 μm methane transparency window. Several experimental data sources for methane line positions and intensities were combined for this analysis. Three long path Fourier transform spectra newly recorded in Reims with 1603 m absorption path length and pressures of 1, 7 and 34 hPa for samples of natural abundance CH4 provided new measurements of 12CH4 lines. Older spectra for 13CH4 (90% purity) from JPL with 73 m absorption path length were used to identify the corresponding lines. Most of the lines in this region belong to the Octad system of 12CH4. The new spectra allowed us to assign 1014 new line positions and to measure 1095 line intensities in the cold bands of the Octad. These new line positions and intensities were added to the global fit of Hamiltonian and dipole moment parameters of the Ground State, Dyad, Pentad and Octad systems. This leads to a noticeable improvement of the theoretical description in this methane transparency window and a better global prediction of the methane spectrum.

► We measure 12CH4 spectra with very long absorption path-length . ► The Octad part of the methane 2 μm transparency window is treated. ► Assignments are made using tensorial formalism. ► The global fit of the Octad has been enhanced. ► The data will be proposed for atmospheric end planetary applications.