Ultrafast hydrogen bond dynamics of liquid water revealed by terahertz-induced transient birefringence

Hydrogen bond dynamics: Excitations for pickin' up good vibrations A new technique uses terahertz electric pulses to detect the stretching and bending vibrations in the bonds between water molecules. Liangliang Zhang of China's Capital Normal University and colleagues found that applying a range of terahertz electric pulses on a film of free-falling water elicits measurable changes in refractive index that correspond to ultrafast hydrogen bond dynamics in liquid water. The pulses range from 1 to 10 terahertz, with the lower frequencies eliciting hydrogen bond bending vibrations and the higher frequencies eliciting stretching vibration signals. The approach enhances the relatively weak intermolecular bond vibrations, allowing scientists to distinguish them from other types of molecular motions. The method could be applied to study the physical mechanisms in water's gas and crystalline phases, as well as the complex interactions between solvent water molecules and reagents. The fundamental properties of water molecules, such as their molecular polarizability, have not yet been clarified. The hydrogen bond network is generally considered to play an important role in the thermodynamic properties of water. The terahertz (THz) Kerr effect technique, as a novel tool, is expected to be useful in exploring the low-frequency molecular dynamics of liquid water. Here, we use an intense and ultrabroadband THz pulse (peak electric field strength of 14.9 MV/cm, centre frequency of 3.9 THz, and bandwidth of 1-10 THz) to resonantly excite intermolecular modes of liquid water. Bipolar THz field-induced transient birefringence signals are observed in a free-flowing water film. We propose a hydrogen bond harmonic oscillator model associated with the dielectric susceptibility and combine it with the Lorentz dynamic equation to investigate the intermolecular structure and dynamics of liquid water. We mainly decompose the bipolar signals into a positive signal caused by hydrogen bond stretching vibration and a negative signal caused by hydrogen bond bending vibration, indicating that the polarizability perturbation of water presents competing contributions under bending and stretching conditions. A Kerr coefficient equation related to the intermolecular modes of water is established. The ultrafast intermolecular hydrogen bond dynamics of water revealed by an ultrabroadband THz pump pulse can provide further insights into the transient structure of liquid water corresponding to the pertinent modes.