Abstract
Polar fluctuations due to reorientation of the dipolar molecules in liquid water are central to its properties. Here we report a high resolution hyper-Rayleigh light-scattering spectrum for liquid water which shows collective dipole orientation fluctuations on two very different time scales: $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ and $>50\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. The $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ relaxation time for the transverse fluctuations matches the macroscopic dielectric relaxation time, but the $>50\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ relaxation time for the longitudinal fluctuations has not been previously observed and is dramatically $({10}^{5}\ifmmode\times\else\texttimes\fi{})$ slower than expected.
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