# Terahertz-Wave Absorption Gas Sensing for Dimethyl Sulfoxide
## 3 Results and Discussion  
Spectral absorbance measurements in the 220-330 GHz frequency range for pure DMSO at 297 K and 0.4 Torr (53 Pa) are illustrated in Figure 4. For the full frequency  
Figure 3: Example experiment: 0.4 Torr (53 Pa) pure DMSO at 297 K. Top panel: reference and absorbed intensity in total frequency range (220–330 GHz); middle panel: reference and absorbed intensity in subset of frequency range (272–273 GHz); bottom panel: absorbance in subset of frequency range. range, repeating absorption bands (saw tooth structures) are observed with widths of \(\sim\)2-3 GHz. The strength of the repeating bands increased from low to high frequency. At higher resolution, individual rotational lines were observed that make up the bands, and the strength of the lines generally increased within each repeating band from low to high frequency. Many distinct rotational lines were present within the repeating absorption bands, for both the ground vibrational state as well as for some low-lying vibrationally-excited states, as demonstrated by [PERSON] et al. [26] and [PERSON] et al. [27], who previously characterized the current frequency range in high-resolution low-pressure experiments (10 kHz, 0.2 Torr or 2.6 Pa). In the present experiments, carried out at higher pressures and for faster acquisition times necessary for remote gas sensing (1.1 s acquisition time in present study versus several hours in [PERSON] et al. ), 4-6 distinct transitions within each repeating feature were resolved against the noise floor. The noise floor (approximately \(\pm 10^{-3}\) in absorbance space) contains broadband sensor noise, emanating from the THz source and detector, and weak unresolved rotational lines that provide structured features within the noise and are difficult to separate. Many of these weak unresolved lines are for five low-lying excited bending/vibrational states (200-400 cm\({}^{-1}\)) [27] that contain fractions of DMSO molecules of order 10% at 297 K.  
[PERSON] et al. [26] and [PERSON] et al. [27] carried out a fundamental characterization of DMSO rotational spectra in the 70-700 GHz range. In combination, those two studies identified and assigned approximately 7000 transitions but did not report intensities. Ad  
Figure 4: Measured absorbance for 0.4 Torr (53 Pa) pure DMSO at 297K in the frequency ranges of: (a) 220–330 GHz, (b) 260–290 GHz, (c) 270–279 GHz, and (d) 270–273 GHz. ditionally, [PERSON] et al. and [PERSON] et al. reported rotational constants and centrifugal distortion terms that allowed for prediction of the DMSO rotational spectrum. Here, calculations of the spectrum for DMSO in the ground-vibrational-state have been carried out using the PGOPHER code [33] with the rotational parameters reported by [PERSON] et al. [27]. Comparisons of the predicted and measured spectrum are illustrated in Figures 5-7. As shown in Figures 6 and 7, the agreement is very good between the current measurements and the simulations for the strongest transitions observed here experimentally. Previous work [26; 27] has reported precise measurement for the transition frequencies in the DMSO rotational spectra. Newly, intensities of the rotational absorption lines are experimentally observed and reported for DMSO  
DMSO is a slightly-asymmetric oblate top that has ground-vibrational-state rotational constants of A = 7036.58 MHz, B = 6910.83 MHz, and C = 4218.78 MHz [27]. As the PGOPHER simulations illustrate, the strongest transitions we observed in the present frequency range were _a_-type _R_-branch transitions, which obey the following selection rules: \(\Delta J\) = +1, \(\Delta K_{a}\) = 0, and \(\Delta K_{c}\) = +1, where \(J\) is the quantum number for overall angular momentum and \(K_{a}\) and \(K_{c}\) are the quantum numbers for the projection of angular momentum onto the a-axis and c-axis, respectively.