Strong Anisotropy in Liquid Water upon Librational Excitation using Terahertz Laser Fields.

Orienting 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, with the birefringence exceeding previously reported values by three to five orders of magnitude. Using a theory that replaces the third order 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, thereby enhancing the liquid anisotropy 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.