Unrivaled accuracy in measuring rotational transitions of greenhouse gases: THz CRDS of CF 4 .

Tetrafluoromethane CF 4 is the most abundant perfluorocarbon in the atmosphere, where it is designated as PFC-14. This greenhouse gas is very stable, has an atmospheric lifetime of 50 000 years, and a high greenhouse warming potential 6500 times that of CO 2 . Over the last 15 years, its atmospheric concentration has increased at a rate of 0.8 ppt per year. The accurate quantification of CF 4 is key to understanding the contribution of its emissions to the radiative forcing budget, and the most precise spectroscopic parameters possible are hence required. In this study, a novel high finesse THz cavity, providing an interaction length in excess of 1 km, has enabled highly resolved spectra, and quantification of the weak transitions of CF 4 by cavity ring-down spectroscopy (CRDS). More than 50 pure rotational P 6 - P 6  :  ν 3 - ν 3 lines of CF 4 have been measured, yielding both position and intensity with unequalled precision. Several tetrahedral splittings are fully resolved and measured with sub-MHz accuracy. Moreover, CRDS-THz allows determining absolute intensities and, using a global fit of the ν 2 polyad series, a CF 4 dipole parameter, namely  3,3 , has been fitted to 106.38(53) mD. This value is in very good agreement with that of the ab initio -based parameter deduced from a dipole moment surface. For the first time, a set of ab initio effective dipole moment parameters is derived for the computation of the transitions of the type P n - P n ( n = 0,…, 8) and the resulting line list composed of 25 863 transitions can be used to model the whole CF 4 rotational spectrum. Finally, the TFMeCaSDa database is updated and is available for future spectroscopic and monitoring activities.