  Energy dissipation in water is very fast and more efficient than in many
other liquids. This behavior is commonly attributed to the intermolecular
interactions associated with hydrogen bonding. Here, we investigate the dynamic
energy flow in the hydrogen-bond network of liquid water by a pump-probe
experiment. We resonantly excite intermolecular degrees of freedom with
ultrashort single-cycle terahertz pulses and monitor its Raman response. By
using ultrathin sample-cell windows, a background-free bipolar signal whose
tail relaxes mono-exponentially is obtained. The relaxation is attributed to
the molecular translational motions, using complementary experiments,
force-field and ab initio molecular dynamics simulations. They reveal an
initial coupling of the terahertz electric field to the molecular rotational
degrees of freedom whose energy is rapidly transferred, within the excitation
pulse duration, to the restricted-translational motion of neighboring
molecules. This rapid energy transfer may be rationalized by the strong
anharmonicity of the intermolecular interactions.
