Low-Frequency Raman Scattering from Water at High Pressures and High Temperatures

Low-frequency isobaric Raman spectra were obtained from water at 0.5, 1.0, 1.5, and 2.0 kbar and temperatures from −18 to 342 °C. Peaks near ≈60 and ≈175 cm-1 were observed at −18 °C and 2.0 kbar, but only a single, asymmetric 50−60 cm-1 feature, skewed to ≈300 cm-1, remained near ≈170 °C and above. The low-temperature ≈60 and 175 cm-1 peaks arise respectively from transverse acoustic (TA) and longitudinal acoustic (LA) standing waves, whereas the skewed, high-temperature Raman feature is well-fitted by a relative kinetic energy distribution (RKED). The TA phonon and RKED peaks are coincident near 60 cm-1. Spectra taken at intermediate temperatures, e.g., 77 and 124 °C, were fitted by a combination of the appropriate RKED and the (−18 °C, 2.0 kbar) TA + LA phonon spectrum. Temperature rise breaks hydrogen bonds and the solidlike patches which support the TA and LA phonons. This continues until only the gaslike RKED remains at high temperatures. Librational spectra were also obtained isochorically. They indicate little change in the total integrated librational intensity from 25 to ≈200 °C, implying that strong cohesive forces continue to restrain rotations in the dense (1 g/cm3) liquid at high temperatures, even when most of the H bonds are broken.