The experimental setup consists of a CW THz system (CW Spectra 400, TeraView) and a gas cell. The THz system consists of a THz emitter and receiver. The THz radiation from the THz emitter is a CW wave and can be tuned from 0.1 THz to 1.85 THz with a frequency resolution of 0.1 GHz and a power of 1 µW. The THz radiation goes through the gas sample and is detected coherently by a THz receiver. The gas cell is made of a one meter long, two-inch-diameter stainless steel tube. The two end windows are 5 mm-thick disks made of z-cut quartz. The temperature, pressure, and gas composition inside the gas cell can be controlled.