Experimental investigation on new absorption peaks of water vapor at high temperatures using terahertz spectroscopy

A monochromatic continuous wave terahertz source with widely tunable frequency range and high resolution has been used to measure the water absorption spectrum at temperatures up to 500 °C. Experiments are carried out based on the HITRAN simulation at different humidity levels. The whole THz spectroscopy system was validated with a strong absorption peak at 0.557 THz (18.38 cm^-1) that appears at both room temperature and high temperature. We also found that, at high temperatures, new absorption peaks at 0.667 THz (22.01 cm^-1) and 0.88 THz (29.04 cm^-1) would emerge, and grow as the water concentration goes up. It provides the opportunity to measure the high temperature water vapor (e.g. in a furnace) using these new peaks while reducing interference from ambient air humidity.