Unrivaled accuracy in measuring rotational transitions of greenhouse gases: THz CRDS of CF4

Abstract

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 per year. The accurate quantification of CF₄ 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 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 splittings 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 _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 CF₄ rotational spectrum. Finally, the TFMeCaSDa database is updated and is available for future spectroscopic and monitoring activities.