  Formation of local molecular structures in liquid water is believed to have
marked effect on the bulk properties of water, however, resolving such
structural motives in an experiment is challenging. This challenge might be
handled if the relevant low-frequency structural motion of the liquid is
directly driven with an intense electromagnetic pulse. Here, we resonantly
excite diffusive reorientational motions in water with intense terahertz pulses
and measure the resulting transient optical birefringence. The observed
response is shown to arise from a particular configuration, namely the
restricted trans-lational motion of water molecules whose motions are
predominantly orthogonal to the dipole moment of the excited neighboring water
molecules. Accordingly, we estimate the strength of the anharmonic coupling
between the rotational and the restricted translational degrees of freedom of
water.
