Terahertz Wave Absorption Spectroscopy of Gases

Abstract

Abstract This article discusses gas analysis using terahertz (THz) wave spectroscopy in scientific research and industry. THz wave spectroscopy is performed with electromagnetic radiation in the THz frequency range. THz wave spectroscopy has attracted great interest in recent years due to the development of improved THz wave radiation sources and THz wave radiation detectors as well as novel THz wave spectroscopy systems. THz wave spectroscopy is a powerful method for trace gas analysis. THz wave absorption measurements provide selective identification of polar gas molecules through their characteristic rotational spectra. The quantum mechanics of molecular rotations for spherical molecular rotors, linear molecular rotors, and symmetric rotors is outlined. Exemplary measurements of the THz wave absorption spectra of dinitrogen oxide (N 2 O), chloromethane (CH 3 Cl), and methanol (CH 3 OH) recorded between 220 GHz and 330 GHz are presented and discussed. The approach to and performance of a state‐of‐the‐art electronic THz wave absorption spectrometer operating in the 220–330 GHz frequency band are explained and illustrated. The capability of electronic THz wave spectrometers to detect a gas with high sensitivity and selectivity in a gas mixture is described for samples of gas isotopes and volatile organic compounds.