High-Resolution Terahertz Spectroscopy of Water Vapor with Different Humidity Levels under Normal Atmospheric Conditions and in a Vapor-Liquid Coexistence at Reduced Pressure

Abstract

We have developed a continuous-wave (CW) terahertz (THz) radiation source that is excited using a laser diode (LD). With this source, the radiation is generated via difference frequency generation (DFG) under non-collinear phase matching in gallium phosphide (GaP). A distributed feedback (DFB) LD and an external cavity laser diode (ECLD) were used as the excitation light sources, and these, together with a polarization maintaining (PM) ytterbium-doped fiber amplifier, had linewidths of less than 4 MHz. The narrow linewidth means that the THz radiation can be used for high resolution spectroscopy. As an example of this, the spectrum for a mixture of saturated water vapor and ethanol at room temperature was obtained, where the spectral absorption lines for water and ethanol at around 2.46 THz could be distinguished. Furthermore, the THz spectra of water vapor at different humidity levels were measured under normal atmospheric conditions. From these we obtained the dependences on humidity of the position of the absorption peak, its FWHM and the intensity of the water vapor absorption band due to the rotational transition of 4 32 ←4 23 . We also show the results for time-dependent THz spectra of water vapor in a vapor-liquid coexistence in a chamber at reduced pressure, where an absorption peak, probably due to an intermediate state of the water molecules, appeared around 2.585 THz just after reducing the pressure in the chamber.

Recent Sensing Technologies of Imperceptible Water in Atmosphere

“…Techniques used for this category required a certain threshold volume of water droplets to detect sensing. In addition, the careful determination should be realized for surface and bulk water amounts to emphasize the liquid water in the air environment and the liquid water on the target SDFS [3].…”

“…Tech niques used for this category required a certain threshold volume of water droplets t detect sensing. In addition, the careful determination should be realized for surface and bulk water amounts to emphasize the liquid water in the air environment and the liquid water on the target SDFS [3]. In this regard, the controlled regulation of humidity implies accurate monitoring o its real level via proper sensors/actuators called "hygrometers."…”