Terahertz-induced local-field dynamics and transient birefringence in aqueous electrolytes
Terahertz Kerr-effect (TKE) spectroscopy provides a time-domain optical probe of the intermolecular structural dynamics of liquids, but the measured birefringence can only be interpreted microscopically if transient molecular structure, local electric fields, and nonlinear optical response are treated on the same footing. We combine terahertz-driven non-equilibrium molecular dynamics with a hyperpolarizability-enhanced dipole-induced-dipole (DID) response model to connect the measured TKE signal of water and aqueous electrolytes to molecular-scale hydration dynamics. Starting from a multipolar expansion of the field-dependent molecular energy, the optical response is written as a differential polarizability in which first and second molecular hyperpolarizabilities are coupled to the instantaneous local field generated by the evolving liquid structure. For neat water, these local-field terms convert an insufficiently structured intramolecular response into the experimentally observed bipolar TKE line shape. For aqueous MgCl2, polarizable force-field trajectories combined with the extended optical response reproduce the concentration-dependent enhancement of the negative birefringence, whereas non-polarizable trajectories yield the wrong trend. Species-resolved analysis assigns the high-concentration response to strongly field-polarized anions and, more importantly, to water molecules bridging cations and anions. The work identifies TKE birefringence as a sensitive readout of ion-hydration local-field dynamics and shows that induced polarization in both the trajectory and the optical response is required to recover the structural dynamics encoded in electrolyte TKE measurements.