# Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths
## 3 Results and Discussion  
### THz Measurements of CH\({}_{4}\) in a White Type Multi-Pass Cell  
THz long-path spectroscopic experiments were initially conducted in the 2000's using home-made or commercial White type gas cells. The White cell was first described in 1942 by [PERSON] [18] and was a significant improvement over previous long path measurement techniques used in optical spectroscopy. A White cell is constructed using three spherical, concave mirrors with the same radius of curvature. The mirrors are separated by a distance equal to their radii of curvature. FT Michelson-based interferometers have been connected by different groups to White type gas cells allowing improving the detection sensitivity in the Far-IR domain. The THz pathlengths reached vary from several tens of meters with classical sources such as Hg lamps [19; 20] up to 180 m with weakly divergent and high brigthness Far-IR synchrotron radiations [12; 21]. As example, the White type cell connected to the Bruker IFS125 high-resolution interferometer coupled to the AILES Far-IR/THz beamline of the SOLEIL synchrotron is shown in Figure 1 (right part). Optoelectronic THz sources have been also coupled to White type cells to attempt THz rotational transitions measurements with long pathlengths (see Figure 1, left part). Ppm and subppm LOD of strongly polar compounds were reached in pulsed THz-TDS and in cw-THz spectroscopy with maximal THz path-lengths of 5 m [22] and 20 m [14], respectively. To date, the spectroscopic investigation of pure rotational THz transitions of CH\({}_{4}\) provides probably the best results in terms of sensitivity obtained by coupling cw-THz sources with White-type cells. Even if CH\({}_{4}\) is not polar, centrifugal distortion and vibrational effects induce a weak dipole (\(\simeq\)1 u D) which allows transitions between rotational energy levels. So very weak rotational absorptions in the THz domain may be measured with an optimized level of sensitivity. Using the HITRAN database and its graphical tool \"HITRAN on the web\" [23; 24], the rotational absorbance of pure methane has been modelled in Figure 2 (main panel) in the 2.0-2.7 THz frequency range for a THz path length of 20 m and a pressure of 10 mbar. The THz spectroscopy of CH\({}_{4}\) requires the measurements of very weak rotational absorptions lower than \(2\times 10^{-5}\) cm\({}^{-1}\) in a THz frequency region difficult to access. FT-Far-IR synchrotron-based spectroscopy [21; 25] and cw-THz by photomixing [13] succeeded to measure these weak rotational lines (see insets Figure 2) with 150 m and 20 m pathlengths, respectively. The White type multi-pass gas cells shown in Figure 1 were used to reach these long interaction distances and to increase the THz absorbances. While the FT-Far-IR rotational lines are limited by the maximal resolution of the high-resolution interferometer (around 30 MHz), the THz CH\({}_{4}\) lines measured by cw-THz spectroscopy are fully resolved and Doppler limited. These THz frequencies resulting from the gaussian fit of the low-pressure line profile have improved the accuracies of the ground state and the \(\
u_{4}\gets\
u_{4}\) hot band molecular parameters allowing a better global modelling of the CH\({}_{4}\) emission and absorption used to determine the molecular abundance of this major GHG in Earth's atmosphere and also in various (exo)planetary upper atmospheres [21; 26]. Figure 1: Picture/Scheme of the two White-type gas cells used for the CH\({}_{4}\) rotational spectra measurements. Left: standard White-type cell (Infrared Analysis, 35-V) used in the cw-THz photomixing measurement [14]. Right: White-type cell located in the AILES beamline of the SOLEIL synchrotron [12]. Trace Gases THz Measurements of N\({}_{2}\)O and O\({}_{3}\) in an Atmospheric Simulation Chamber Equipped with a Chernin Type Multi-Pass Cell  
A ppm LOD is required to monitor CH\({}_{4}\) in our atmosphere.