NEW ASPECT ON RELAXATION AND VIBRATION OF WATER AND LIQUID IN THz FREQUENCY REGION

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(1) Low-frequency Raman spectra of four water isotopes (H_2O, D_2O, H_2^<18>O, D_2^<18>O) in the THz frequency region are well fitted by a relaxation function based on a Multiple Random Telegraph (MRT) model and two damped oscillator functions. The MRT model effectively includes the inertia effect and memory effect of the heat bath. From the fitting analysis the 190 cm-1 modes are assigned to a stretching motion of oxygen atoms in a tetrahedral-like structure of four water molecules and the 50 cm-1 peaks are composed both by a bending-like motion between water molecules and a component of the MRT relaxation mode.(2) The relaxation times of H-waters (H_2O, H_2^<18>O) are always shorter than those of D-water (D_2O, D_2^<18>O). The Arrhenius plot of the relaxation times show that the slope is changed at about 300 K.(3) Concentration dependence of low-frequency Raman spectra of ethanol/water binary mixture in the THz frequency region are analyzed by a simple addition of ethanol spectrum and water spectrum. Although above 40 cm-1 each observed spectrum is well reproduced by a sum spectrum, below 40 cm-1 where a relaxation component is dominant is never fitted by a sum spectrum and shows a systematic deviation. The integrated intensity of the deviation shows a maximum at about 0.6 water mole fraction. Concentration dependence of the experimentally obtained Landau-Placzek ratio which is the ratio of the Rayleigh component against the Brillouin doublets show a maximum at about 0.8 water mole fraction. This concentration coincides to the maximum of concentration fluctuation from the ultra-sonic attenuation experiment. Comparing the results among other alcohol such as methanol, 1-propanol and 1-propanol, the microscopic homogeneity of these alcohol/water binary mixtures is discussed.