# Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths
## 3 Results and Discussion
### Intra-Cavity THz Measurements of CF\({}_{4}\)
In this study, the observation of CF4 rotational clusters has required to co-add more than 5000 spectra measured at middle resolution (0.01 cm\({}^{-1}\)) and pressure-broadened at P = 100 mbar. Due to a lack of sensitivity and resolution, it was not possible to measure at lower pressure and to fully resolve the tetrahedral splitting of CF4 rotational lines. Moreover, the ground state transitions with absorbances lower than 1% (Figure 6, upper panel) were not observed. The first measurements of the tetrahedral split components of distortion-induced rotational lines of CF4 are presented in this work with the FP-TAS measurements performed in the 625 GHz region. The middle panel of Figure 6 targets the frequency range accessible by FP-TAS. The THz spectrum of CF4 is simulated at a resolution of 100 kHz with fully resolved and Doppler limited rotational lines at P = 100 \(\upmu\)bar. With a 1 km path-length, absorbances above 1% are reached for several transitions in the ground state and in the \(v_{3}=1\) hot band. Finally, four examples of measured lines by FP-TAS in the \(R\)(20) cluster are presented in the lower panel. The lines of the multiplet were measured individually with finesses between 2800 to 3250 and pressures between 80 \(\upmu\)bar and 150 \(\upmu\)bar. The frequency step was fixed to a few kHz and the time constant to 200 ms. The SNR for the four lines of Figure 6 are measured between 20 to 120, which shows that the measurement of very weak THz line intensities below \(10^{-28}\) cm\({}^{-1}/(\text{m{olec}}\) cm\({}^{-2})\) may be for now considered in THz spectroscopy. Considering this level of intensity now reachable, we can expect trace gas detection of N2O and O3 (Figure 4) by FP-TAS in the THz cavity at a subppm LOD, enough for a detection at atmospheric concentrations. Figure 5: Left: Scheme of the Fabry-Perot THz cavity based on a low-loss oversized corrugated waveguide with highly reflective photonic mirrors. Successive cavity modes are shown inside the cavity scheme and an achieved finesse above 3500 in the 626-629 GHz frequency range is highlighted (\(L_{eff. }>1.1\) km). Right: Picture of the FP-THz absorption spectrometer with some legends. Figure 6: Upper panel: Simulation of the CF\({}_{4}\) THz absorbance in the conditions of the synchrotron-based FT-Far-IR measurements (see Ref. [36]), namely a resolution of 0.01 cm\({}^{-1}\), a gas pressure of 100 mbar and a path-length of 150 m. Middle panel: Simulation of the CF\({}_{4}\) THz absorbance measured in the frequency range accessible by FP-TAS at a resolution of 100 kHz with a gas pressure of 100 \(\upmu\)bar and an effective path-length of 1 km. For both simulations, ground state transitions are in blue, \(\
u_{3}=1\) hot band transitions are in red. Lower panel: Zoom on experimental individual rotational lines (\(R(20)\) cluster in the \(\
u_{3}\gets v_{3}\) hot band) measured by FP-TAS. The experimental THz frequencies are determined by the microwave synthesiser used to drive the AMC. The microwave synthesiser is referenced to a time signal provided by a GPS receiver. The calculated frequencies are determined using the STDS software [27]. Dashed red line correspond to the FT-Far-IR synchrotron-based measurements at a pressure of 100 mbar and a resolution of 0.01 cm\({}^{-1}\).