RAMAN-SPECTRA OF WATER TO 400-DEGREES-C AND 3000 BAR

A small-size high-temperature, high-pressure Raman cell of about 1.6 cm3 internal volume with four sapphire windows is described. It can be used to 400-degrees-C and 3000 bar. The OD-stretching bands around 2600 cm-1 of HDO diluted in H2O have been measured from 25-degrees-C to 400-degrees-C and in a density range between 0.04 g.cm-3 to 1.0 g.cm-3. The critical temperature of water T(c) is 374-degrees-C, the critical density Q(c) = 0.32 g.cm-3. At the supercritical temperature of 400-degrees-C the OD-band was determined at densities from 0.04 to 0.8 g.cm-3. At nearly constant densities between 1.0 and 0.8 g.cm-3 the band was recorded from 25 to 400-degrees-C. Thus the OD-band, which depends on hydrogen bonding, could be studied in a wide range of temperatures and pressures, avoiding the immediate critical region. - At 400-degrees-C and low density the sharp narrow OD-band has a maximum frequency of 2727 cm-1. It becomes broader and shifts to 2668 cm-1 at 0.8 g.cm-3. A very broad band occurs at 25-degrees-C and 1 g.cm-3 at 2530 cm-1. Depolarization ratios were measured, local field corrections were applied and permitted the determination of isotropic and anisotropic spectra. The transition of band shape in the wide range of temperature and density is always smooth with no prominent features. As compared with a two-component hydrogen-bonded and non-bonded "mixture" model a description with a "continuum"-model as proposed by Efimov et al. appears to be preferable. Specific interaction energies have been derived from this description.