When water is cooled, it becomes increasingly tetrahedral, and yet various lines of evidence suggest that local tetrahedral order in cold water is not the same as that of hexagonal ice (or any other solid phase of ice). In order to probe the structure of cold water we have performed experimental Raman multivariate curve resolution (Raman-MCR) measurements and both fully classical and quantum mechanical simulations. In the experiments, we have used self-modeling-curve-resolution (SMCR) to decompose the Raman spectrum of pure water into high and low temperature components over a temperature range of 240 K to 370 K. The low temperature component has a prominent lower frequency OH stretch band, consistent with a more tetrahedral structure, as well as a smaller high frequency feature with a non-monotonic temperature dependence. In an effort to assign this high frequency feature we have obtained vibrational density of states and electric field distributions pertaining to various local H-bond configurations in liquid water as a function of temperature. Our results point to a close connection between the high frequency feature and a particular non-ice-like configuration whose population initially increases with decreasing temperature and then decreases upon further super-cooling of liquid water.

Emergent structures in cold water

2019 Fall National Meeting

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