# 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
Due to the weak intensities of pure rotational lines, THz spectroscopic monitoring of methane at atmospheric concentrations would prove serious difficulties even after a strong improvement of the spectrometers sensitivity. On the contrary, the THz trace gas detection of polar GHGs already proved its capability by the monitoring of N\({}_{2}\)O and O\({}_{3}\) in the stratosphere with the MLS and TALIS sounders [8; 9]. These THz heterodyne receivers are able to determine atmospheric subppm concentration profiles in the submm- and in the mm-wave domains. A possibility to get close to the detection levels of N\({}_{2}\)O and O\({}_{3}\) is demonstrated in this study by performing the first THz absorption measurements in an atmospheric simulation chamber. Compared to CH\({}_{4}\), rather strong rotational transitions of N\({}_{2}\)O and O\({}_{3}\), with intensities around \(10^{-22}\) cm\({}^{-1}/(\text{molecule cm}^{-2})\)[23] were probed in the submm-wave domain using versatile spectrometers based on AMC [16]. Nevertheless, with increasing wavelengths, the THz beam divergence rises and long path THz absorption experiments becomes challenging. Recently, promising results have been obtained with compact circular multi-pass cells able to refocus the beam at each internal reflection: (i) in Ref. [29], [PERSON] et al., using a AMC, have measured traces of acetaldehyde in methanol down to 110 ppm with an optical path of 1.9 m in the 250 GHz region; (ii) in Ref. [11], [PERSON] et al., using THz-TDS, have recorded the pure rotational spectrum of N\({}_{2}\)O at atmospheric pressure with a dilution of 1% by reaching a THz path of 18.61 m. Even if the results of these studies are interesting since they were obtained without pre-concentration in a very compact and easily transportable gas cell, the LOD reached is clearly not sufficient for trace gas detection in the atmosphere. Yet it is possible to reach lower LOD with longer THz path-lengths using a multi-pass cell based on the clever Chernin arrangement [30]. Compared to the White type arrangement discussed in the previous section, a Chernin multi-pass cell optimizes the recirculating of the beam over many focused lines on the field mirrors [31]. In particular longer wavelengths are very critical due to the overlapping between adjacent refocusing points within the cell what needs to be avoided or at least limited in order to prevent the presence of stationary waves yielding to baseline variations. Moreover the optical path-length in the Chernin arrangement is easy to adjust with a matrix distribution of the refocusing points and a variable number of rows and columns. As shown in Figure 3, we dimensioned a Chernin multi-pass cell in order to integrate it in CHARME (Chamber for the Atmospheric Reactivity and the Metrology of the Environment), a 9.2 m\({}^{3}\), evacucable cylinder used to simulate the physical-chemistry of the atmosphere in a controlled environment [32]. Figure 2: Central part: Absorption of ground state rotational THz lines of CH\({}_{4}\) induced by centrifugal distortion simulated for a pathlength of 20 m. and a pressure of 10 mbar. Left and right parts: two examples of experimental lines measured at 30 MHz resolution, \(P=10\) mbar and \(L=150\) m by FT-FIR spectroscopy using the AILES beamline of the SOLEIL synchrotron (blue solid line) and at 30 kHz resolution, \(P=1.5\) mbar and \(L=20\) m with the cw-THz photomixing spectrometer [13]. Calculations were performed using the STDS software [27], which is part of the XTDS [28] package that implements the tensorial formalism developed for spherical-top molecules in the Dijon group. The Chernin type optical setup called hereafter \"MultiCHARME\" is composed of 5 mirrors (2 rectangular field mirrors and 3 spherical objective mirrors) each with 5 m radius of curvature corresponding to the 5 m baselength of CHARME. All mirror holders are equipped with computer-controlled micrometric screws for optical adjustments and path length changes.