All-electronic THz-wave gas sensing via absorption spectroscopy

Abstract

Gas sensing via THz-wave absorption spectroscopy is developed for the detection of severalvolatile organic compounds, halogenated hydrocarbons, and nitrogen-containing compounds using a broadband electronics-based THz wave spectrometer. Spectral absorption is characterized in the frequency range from 220 to 330 GHz, a region where atmospheric attenuation is minimal, scattering from particles and aerosols is negligible, spectral selectivity is high for the chosen compounds, and microelectronic radiation sources and detectors are being developed. The target compounds of the present study are important in industrial, chemical, combustion, environmental, agricultural, and medical processes in which gas sensors are desired. Experiments are conducted at room temperature and at pressures of 0.25−16 Torr, conditions where pressure-broadening (collisional line-broadening) often results in complex blended spectra. The observed transitions mostly exist for rotational absorption bands for ground vibrational states but some transitions are also observed for low-lying vibrationally-excited states. Where available, the measurements agree well with spectral simulations and documented line positions; however, most of the present experiments are the first-of-their-kind and there are not suitable experimental comparisons. Detection limits for remote gas sensing based on the electronic spectrometer are estimated to be of the order 10^(12)-10^(13) molecules cm^(-3) per meter pathlength. For dilute gases in air at 1 atm, detection limits range from 5 to 1,000 ppm per meter pathlength. The present study illustrates the potential for THz-wave quantitative gas sensing using all-electronic miniaturized systems for the polar gases of industrial relevance. As humanity becomes increasingly interested in the mitigation of anthropogenic greenhouse gases, local and global air pollution, and industrial air safety, inexpensive remote gas sensors, such as the type pursued in this thesis, should be in critical demand.