# Terahertz-Wave Absorption Gas Sensing for Dimethyl Sulfoxide
## 1 Introduction  
Terahertz-wave absorption spectroscopy offers the potential for quantitative and selective gas sensing for polar molecules in practical and harsh environments. Commercial electronic terahertz (THz) wave gas sensors may be realizable in the near term due to the fundamental advantages offered by absorption spectroscopy in the THz-wave region (0.1-1.0 THz or 3.33-33.3 cm\({}^{-1}\)). This includes sensitivity, molecular selectivity, and strongly limited insensitivity to interference from water vapor and particle scattering compared to the infrared band, as well as the significant progress made in the development of THz-wave electronic sources and detectors that are miniaturized, robust, energy efficient, and inexpensive, relative to photonics devices. Due to these advantages, THz-wave gas sensing has shown recent success in analytical chemistry and engineering [1; 2; 3; 4; 5; 6; 7; 8]. In recent studies, THz-wave absorption spectroscopy based on microelectronic sources and detectors has been used to probe rotational transitions for the detection of gaseous volatile organic compounds (VOCs) [9], halogenated hydrocarbons [10], cyanides [11; 12],small sulfur-containing compounds [13; 14], and other chemical classes. However, there is minimal prior development of gas sensing for larger volatile sulfur compounds (VSCs). Both organic and inorganic sulfur-containing compounds are used across a plethora of chemical, pharmaceutical, metallurgic, textile, photographic, food, and cosmetic applications [15]. VSCs are used in polymer and other material manufacturing and processing [16]. They are often used to affect the aromas of food and beverages, such as vegetables, cheese, coffee, chocolate, UHT milk, wine, and beer [17]. VSCs are naturally produced in the ocean by marine biota, making up approximately 40% of the sulfur emissions to the atmosphere [18]. Common VSCs include dimethyl sulfide and dimethyl sulfoxide (DMSO) [17]. See Figure 1 for the organic structure of DSMO (H\({}_{3}\)CS(0)CH\({}_{3}\)). DMSO is used in medical, cleaning, and polymer and paper manufacturing applications. It is used as an anti-inflammatory drug [19], for pain reduction, in healing wounds or injuries, and other conditions. DMSO, as part of medical treatments or as a natural component of various foods, is known to penetrate tissues, allowing for greater efficacy of drug treatments. DMSO is also used as an industrial solvent in fungicides, herbicides, and antibiotics, due to its antimicrobial activity. DMSO is used in clearing solutions where it acts as a solvent for the removal of organic and polymer debris from manufacturing and other equipment. It is also used in paint stripping applications and in material processing as a reaction and processing solvent during polymer manufacturing and casting and synthetic fiber production. A gas sensor for DMSO for process and environmental monitoring and control would be particularly valuable in industrial, manufacturing, materials, and medical settings, especially in situations where DMSO poses worker and environmental safety issues. For example, DMSO has been shown to lead to undesired exothermic reactions in medical settings, when exposed to sodium hydride [20]. In calorimetry experiments, DMSO/sodium hydride mixtures have been shown to lead to explosive conditions at very modest temperatures (40-60 \({}^{\circ}\)C) [21]. Similarly, DMSO has been shown to decompose and act as an oxidizer or auto-catalyst while in contact with certain organic and inorganic acids and peroxides [22; 23]. DMSO is a health hazard to humans causing skin/eye irritation and damage depending on exposure concentrations [24]. As a slightly asymmetric oblate top molecule with relatively strong dipole moment (3.94 Deybe [25]), DMSO proves to be a good candidate for gas-phase sensing using rotational absorption spectroscopy in the THz frequency band, where DMSO exhibits spectra rich in rotational structure from both the ground vibrational state and low-lying vibrationally-excited states [26; 27]. [PERSON] et al.