Raman spectra of the OH-stretching region of liquid H2O and 10% HDO have been obtained between -50 and +100°C at pressures up to 240 MPa, extending into the deeply supercooled region. Application of pressure made it possible to study liquid water at temperatures 20 K lower than reported so far.
The low-frequency shoulder of the H2O-spectra, which has been developed into a probe of inphase collective motions , grows rapidly with falling temperature. The intensity of the collective band increases monotonically to the lowest temperatures under study.
Supercooled water shows a number of thermodynamic and dynamic anomalies, which become apparent from the unusual p- and T-dependence of the compressibility and the molecular mobility in the pressure range between 0 and 200 MPa. The Raman spectra, however, do not change significantly in this pressure region. At the lowest temperatures they are practically independent of pressure.
This finding suggests that the intensity of the collective Raman band originates from short range correlations of the OH-bond vibrations, that are not connected with the more extended density fluctuations responsible for the well known anomalies of supercooled water.