Molecular Alignment of Bulk Water: Observing a Giant THz Kerr Effect upon Librational Excitation

Induced orientation of a molecule in real space by static and intense laser fields has been successfully employed to control reactions in the gas phase. However, for bulk water an effective alignment was not realized, yet due to the fast energy dissipation into the water network. Here we report a nonlinear Terahertz (THz) experiment carried out at the free electron laser FELIX. At 11.7 THz we observe a giant, resonance enhanced Kerr parameter which exceeds previous values by 4 orders of magnitude. Using ab initio molecular dynamics calculations, the large THz Kerr effect can be rationalized in terms of a linear response of a driven resonance orientation upon excitation of single water rotations. Our results suggest that bulk water can be efficiently aligned by THz laser fields around 12 THz.