Raman spectroscopic study of bulk water supercooled to − 33 °C

Abstract

We present Raman spectroscopic measurements in the OH stretch region of water (2900 to 3800 cm−1) for the temperature range 80 to −33 °C. This latter temperature represents the homogeneous limit of nucleation for our bulk samples. We find that the temperature dependence of both our intensity and depolarization ratio measurements are well described by a two-state model in which both states have frequency dependent depolarization ratios. We argue that the hydrogen bonded mode of the spectrum begins to develop a collective nature at both its low and high frequency ends. This collective nature can explain a breakdown in isosbestic behavior at low temperature. It is suggested that these collective modes may be due to in- and out-of-phase motions of OH oscillators. Their intensity approaches those seen in amorphous solid water as the liquid is cooled to the apparent singular temperature Ts≂−45 °C, for a variety of bulk properties. We use our Raman data to make an estimate of hydrogen bond probability and show that four bonded molecules would percolate near the singularity. We then argue that the collective mode grows because clusters of four bonded molecules grow in extent until at Ts percolation occurs to yield an infinite cluster with the amorphous solid’s collective spectrum.