Anion-water hydrogen bond vibration revealed by the terahertz Kerr effect.

The microscopic mechanism for ionic influence on the hydrogen bond network of water has not been fully understood. Here we employ the terahertz Kerr effect (TKE) technique to map the intermolecular hydrogen bond dynamics in a series of aqueous halide solutions at the sub-picosecond scale. Compared with pure water, the significantly enhanced bipolar TKE response associated with polarization anisotropy in an ionic aqueous solution is successfully captured. We decompose the measured TKE response into different molecular motion modes and demonstrate that the obviously increasing positive polarity response is mainly due to the anion-water hydrogen bond vibration mode with the resonant THz electric field excitation. Our measurement results provide an experimental basis for further insight into the effects of ions on the structure and dynamics of a hydrogen bond in water.