Unrivaled accuracy in measuring rotational transitions of greenhouse gases: THz CRDS of CF4
Résumé
Tetrafluoromethane CF4 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 CO2. Over the last 15 years, its atmospheric concentration has increased at a rate of 0.8 ppt/year. The accurate quantification of CF4 is key to understanding the contribution of its emissions on the radiative forcing budget, 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 1km, has enabled highly resolved spectra, and quantification of the weak transitions of CF4 by Cavity Ring-Down Spectroscopy (CRDS). More than 50 pure rotational P6-P6: ν3-ν3 lines of CF4 have been measured, yielding both position and intensity with unequalled precision. Several tetrahedral splitting are fully resolved and measured with sub-MHz accuracy. Moreover, CRDS-THz allows determining absolute intensities and, using a global fit of the ν2 polyad series, a CF4 dipole parameter, namely µ3,3, has been fitted to 106.38(53) mD. This value is in a 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 Pn-Pn (n=0,...,8) and the resulting line list composed of 25,863 transitions can be used to model the whole CF4 rotational spectrum. Finally, the TFMeCaSDa database is updated and is available for future spectroscopic and monitoring activities.