Structural Relaxation in Liquid Water by Inelastic UV Scattering

Using the novel synchrotron radiation based inelastic ultraviolet scattering technique, the dynamic structure factor of normal and supercooled liquid water has been measured at a momentum transfer $Q\approx 0.1{\mathrm{nm}}^{-1}$, in the temperature range 260–340 K. The structural ($\alpha$) relaxation has been observed in the supercooled temperature region ($T\le 273\mathrm{K}$), where the inverse relaxation time matches the frequency of the probed sound modes. The $T$ dependence of the relaxation time shows a diverging behavior with a critical temperature $T\approx 220\mathrm{K}$. These results provide a unique experimental opportunity to frame the dynamics of water in the mode-coupling theory.