  The liquid and supercooled states of water show a series of anomalies whose
nature is debated. A key role is attributed to the formation of structural
aggregates induced by critical phenomena occurring deep in the supercooled
region; the nature of the water anomalies and of the hidden critical processes
remains elusive. Here we report a time-resolved optical Kerr effect
investigation of the vibrational dynamics and relaxation processes in
supercooled bulk water. The experiment measures the water intermolecular
vibrations and the structural relaxation process in an extended temperature
range, and with unprecedented data quality. A mode-coupling analysis of the
experimental data enables to characterize the intermolecular vibrational modes
and their interplay with the structural relaxation process. The results bring
evidence of the coexistence of two local configurations, which are interpreted
as high-density and low-density water forms, with an increasing weight of the
latter at low temperatures.
