Tracking the excitation of water molecules in the homogeneous liquid is
challenging due to the ultrafast dissipation of rotational excitation energy
through the hydrogen-bonded network. Here we demonstrate strong transient
anisotropy of liquid water through librational excitation using single-color
pump-probe experiments at 12.3 THz. We deduce a third order response of chi^3
exceeding previously reported values in the optical range by three orders of
magnitude. Using a theory that replaces the nonlinear response with a material
response property amenable to molecular dynamics simulation, we show that the
rotationally damped motion of water molecules in the librational band is
resonantly driven at this frequency, which could explain the enhancement of the
anisotropy in the liquid by the external Terahertz field. By addition of salt
(MgSO4), the hydration water is instead dominated by the local electric field
of the ions, resulting in reduction of water molecules that can be dynamically
perturbed by THz pulses
