Evidence of two distinct local structures of water from ambient to supercooled conditions

TL;DR

This study uses advanced optical techniques to reveal two distinct local structures in water, supporting the existence of high-density and low-density forms, especially prominent at lower temperatures.

Contribution

It provides direct experimental evidence of two coexisting local water structures through time-resolved vibrational and relaxation measurements.

Findings

Evidence of two local water configurations, high-density and low-density.

Increasing prevalence of low-density water at lower temperatures.

Characterization of intermolecular vibrational modes and their relation to structural relaxation.

Abstract

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.