Fig. 1.

The THz-induced Kerr effect in water and reference liquids. The refinement (red curve) is the sum of two contributions: An instantaneous electronic birefringence (orange curve) and a delayed, molecular birefringence (purple curve). While in the non-polar molecules carbon disulfide (CS₂) and benzene (a, b), a positive molecular alignment effect is observed together with a positive electronic response, the polar water molecules in regular (H₂O; c) and heavy (D₂O; d) water as well as aqueous solutions of sodium iodide (NaI; e, f) reveal a negative molecular orientation effect. The electronic Kerr effect background of the cuvette was subtracted. We also isolate the molecular orientation mechanism of water based on its temperature dependence $-\mathrm{d}\Delta \varphi ∕\mathrm{d}T$ (purple curve) in a background-free measurement. Error bars correspond to the standard deviation. In c, d, additional dotted curves, offset by −0.03 mrad, correspond to a convolution with a Gaussian σ = 1.4 ps. Error bars represent the standard deviation