![]() ![]() 7 Methylchloride detection in the ppm range with long-baseline THz spectroscopy based on a white cell design has been also reported. A broadband trace gas sensor at 0.5 THz based on chirped-pulse THz absorption spectroscopy has been demonstrated to detect OCS, N 2 O, and methanol (CH 3 OH) with noise equivalent concentration of a few hundreds of ppm in volume. The achieved detection limits for HCN, CO, and H 2 CO were 9 parts in 10 6 (ppm), 0.1% and 114 ppm respectively. ![]() 6 reported a sensor based on photomixing techniques in the range 0.5 – 1.5 THz to detect and quantify small quantities of hydrogen cyanide, carbon monoxide, formaldehyde, and water. A number of THz sensing systems using di ff erent optical techniques have been employed for gas sensing. 5 In the last few years, the rapid growth of new technologies needed to access this region is stimulating a resurgence in THz spectroscopic application. 1 Other important applications include direct detection of traces of such gases in open air 2,3 and spectroscopic identi cation of explosives 4 and di ff erent crystalline polymorphic states of drugs. The precise knowledge of their concentrations in the stratosphere and upper troposphere areas is crucial for the study of chemical processes related to ozone depletion, pollution monitoring, and global warming. For example, gas species such as water, nitrogen compounds, oxygen, and chlorine present strong absorption bands in the TH spectral range. Many absorption and emission molecular lines of large interest fall in this spectral region, where typical absorption strengths are of the same order of magnitude of those observed in the mid-IR and from three to six orders of magnitude stronger than those measured in the microwave region. sensitivity THz molecular spectroscopy is a powerful tool both for laboratory purposes and for remote sensing.
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