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 432←423. 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.