A terahertz cavity for accurate spectroscopic measurements of atmospheric species

Terahertz (THz) spectroscopy can distinguish polar molecules in gas mixtures thanks to the narrow and intense transitions specific to this band. Thus, rotational transitions of many atmospheric species can be accurately measured. CF₄, the most abundant perfluorocarbon in the atmosphere, is a highly stable greenhouse gas, with an atmospheric lifetime of 50,000 years and a warming potential much greater than that of CO₂. Accurate quantification of CF₄ is essential for understanding its contribution to the radiative forcing budget. However, this molecule has a very weak dipolar moment induced by centrifugal distortion, which makes its spectroscopic study challenging in the THz domain.

Here, we present the specific features of an ultrasensitive, high-finesse cavity spectrometer that enables both Cavity Enhanced Absorption Spectroscopy (CEAS) and Cavity Ring-Down Spectroscopy (CRDS) measurements. This setup has enabled highly resolved spectra of the weak transitions of CF₄. More than 50 pure rotational ν₃ − ν₃ lines have been measured, yielding both position and intensity data with unequalled precision. CRDS enabled the absolute intensities, used in the global fit, to be determined. Finally, the updated TFMeCaSDa database is available for future spectroscopic and monitoring activities.

Further challenging work is underway with GeH₄, a compound primarily used for manufacturing high-performance integrated circuits in the semiconductor industry. CEAS measurements of some transitions with very low intensities are presented here. Spectroscopic analysis is ongoing.