The radiative relaxation mechanism of water between its different phases is studied to understand an uncommon radiation phenomenon observed in the first-orderphase-transition process of water. This kind of radiation is often referred to as phase-transitionradiation, whose nature is different from the Planckian radiation because its strength can be even stronger than blackbody radiation at the same temperature.In the theoretical approach of this study, analytical thermodynamic models forcondensed-state water are presented to study its energetic behaviors at temperaturesranging from a few degrees K to near the critical point. Changes of energetic behaviors of water molecules during phase-transitions are of special interest and are linked to the direct emission of infrared radiation. A two-level energy transition model is proposed to investigate the characteristic radiation during vapor condensation, leading to a newly defined absorption coefficient for phase-transition radiation in the radiative transferequation. The reported characteristic radiation for vapor condensation at wavelength 4-8micron meter is attributed to the radiative relaxation with one hydrogen-bond formation in liquid-water during vapor condensation.In addition to the theoretical modeling, optical measurements are also included in this study to examine the energy transmission characteristics in vapor-liquid mixtures of water in the 3-5 micron meter spectral range. Results from the infrared transmission experiments and the associated theoretical predictions by the Monte Carlo radiative transfer analysis suggest that the probability for condensation radiation occurrence is one out of 20 million collisions between water-vapor molecules and liquid-water droplets.
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