Multi-laboratory measurements of 12CH4 2ν3-band line parameters. Part I: Line intensities with relative combined standard uncertainties at the permille level

We present accurate transition intensities for the R(0) to R(10) manifolds of ¹²CH${}_4$ in the 2${\nu }_3$-band tetradecad region based on independent cavity ring-down spectroscopy and Fourier Transform spectroscopy measurements from three laboratories. Two sets of experiments involved multi-spectrum fits to low-pressure spectra acquired at Doppler-limited conditions which greatly reduced complications from effects caused by collisional broadening and line mixing. In a third experiment, cavity ring-down spectroscopy measurements of lineshapes made at elevated pressures were used constrain residual collisional broadening effects in the low-pressure spectra, thereby improving measurement precision. Intensities are reported for manifolds and specified multiplets comprising blended transitions, as well as for individual transitions. Summation of the manifold and multiplet intensities agree at the permille level, whereas highly blended individual transitions agree at the several permille level. For measurements of methane, the intensities reported here will enable more accurate spectroscopic determinations of amount-of-substance, and will support progress in remote sensing missions and future refinements of its other spectroscopic parameters.