Difference-frequency spectroscopy of water in the 600–1300 GHz region

This paper reports high-accuracy water vapour line parameters in the 0.6–1.3 THz region, measured by direct absorption spectroscopy with a difference-frequency terahertz spectrometer. Two near-infrared lasers near 192 THz are mixed on a photomixer to produce narrow-linewidth THz radiation, tunable from about 0.5 GHz to 1.4 THz, with its absolute frequency referenced to an optical frequency comb. Ten pure water vapour lines were recorded at pressures up to 1 mbar. The detection chain was confirmed to be linear by varying the incident power with a pair of wire-grid polarizers. A multi-fit analysis using a speed-dependent Voigt profile, which explicitly models Fabry–Pérot etalon interactions with the gas, reveals beyond-Voigt line shapes. The analysis gives reliable pressure-broadening and shift coefficients, line intensities and zero-pressure transition frequencies for spectroscopic databases.