# Terahertz-Wave Absorption Gas Sensing for Dimethyl Sulfoxide
## 2 Experimental Methods  
For the interrogation of rotational absorption spectra for the purposes of gas sensing, THz-wave radiation was generated with an electronic source, comprised of a radio frequency (RF) synthesizer (HP model 83,752, 10 MHz-20 GHz) coupled to a Virginia Diodes (VDi Model WR-3.4, power \(\sim\)0.6 mW, resolution \(\sim\)1 kHz, dimensions: 20 \(\times\) 13 \(\times\) 8 cm) signal generator extension (SGX) module that frequency multiplies the RF signal to generate THz-wave radiation. The output of the SGX module was coupled to a diagonal horn antenna to generate diverging free-space THz radiation. The diverging radiation was collimated using a plano-convex Teflon lens. The collimated radiation then passed through a high-density polyethylene (HDPE) gas cell assembled with two 45-degree HDPE windows. The absorption pathlength of the gas cell was 21.6 cm and was connected to a gas and vacuum management system. Downstream to the cell, a second plano-convex Teflon lens focused the radiation onto a Schottky diode detector (VDi Model QOD 3-15, sensitivity \(\sim\)0.5 V/mW, NEP \(\sim\)10 pW/rtHz, sub-ns time response, dimensions: 3.8 \(\times\) 3.8 \(\times\) 1.1 cm) for monitoring the transmitted radiation intensity. See Figure 2 for a schematic of the experimental hardware.  
The electronic source producing coherent radiation in a frequency range of 220-330 GHz was continuously scanned in frequency space at a fast rate (\(\sim\)100 GHz/s), providing rapid measurements that can also be averaged to increase the signal-to-noise ratio. The spectral absorbance, \(A\), is described using the Beer-Lambert Law:  
\[A=\varepsilon cl=-\ln\left(\frac{l}{l_{0}}\right) \tag{1}\]  
Figure 2: Schematic of the experimental setup.  
where \(\varepsilon\) is the spectral absorption coefficient, \(c\) is the concentration of the absorbing gas (DMSO), and \(l\) is the absorption pathlength (21.6 cm). The reference and transmitted intensities are \(l_{0}\) and \(l\), respectively, where \(I_{0}\) is measured while the cell is under vacuum and \(I\) is measured with the gas of interest in the cell. The absorption coefficient, \(\varepsilon\), is a function of the spectroscopic parameters for the probed absorption transitions (i.e., line position, strengths, and shapes), which are a function of frequency, temperature and pressure, and the gas mixture.  
Spectral absorption for pure DMSO was measured at room temperature (297 K) and 0.4 Torr (53 Pa), using the methods described by [PERSON] et al. [9]. First, a reference intensity was obtained by completing a frequency sweep of the radiation source while the gas cell was evacuated. To flush contaminants out of the gas cell, the cell was purged three times using pure N\({}_{2}\) and then evacuated with a turbomolecular pump prior to each experiment. Following the purge-vacuum process, the reference intensity was measured. Next, the cell was filled with DMSO, by vaporizing liquid DMSO (purity of 99.7+%) contained in attached glassware into the evacuated cell. DMSO has quite a low vapor pressure at room temperature (0.41 Torr (55 Pa) [32]); hence, measurements were carried out only at pressures of 0.4 Torr (53 Pa). Once the gas cell was filled, the radiation source was again swept in frequency, the transmitted intensity recorded, and the absorbance determined using the Beer-Lambert law. The signals for an example measurement are illustrated in Figure 3. The resulting absorbance measurements have a noise floor of approximately \(\pm 10^{-3}\) in absorbance and have frequency resolution of 0.5 MHz.