  Water is vital for life, and without it biomolecules and cells cannot
maintain their structures and functions. The remarkable properties of water
originate from its ability to form hydrogen-bonding networks and dynamics,
which the connectivity constantly alters because of the orientation rotation of
individual water molecules. Experimental investigation of the dynamics of
water, however, has proven challenging due to the strong absorption of water at
terahertz frequencies. In response, by employing a high-precision terahertz
spectrometer, we have measured and characterized the terahertz dielectric
response of water from supercooled liquid to near the boiling point to explore
the motions. The response reveals dynamic relaxation processes corresponding to
the collective orientation, single-molecule rotation, and structural
rearrangements resulting from breaking and reforming hydrogen bonds in water.
We have observed the direct relationship between the macroscopic and
microscopic relaxation dynamics of water, and the results have provided
evidence of two liquid forms in water with different transition temperatures
and thermal activation energies. The results reported here thus provide an
unprecedented opportunity to directly test microscopic computational models of
water dynamics.
