Abstract
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.
Highlights
Liquid water is considered the cornerstone of life and has many extraordinary physical and biochemical properties[1,2]
We use a THz electric field to resonantly excite the intermolecular modes of liquid water
We believe that the transient rotation of a molecule produces an induced dipole moment, which immediately transfers the momentum driven by the THz field to the restricted translational motion of adjacent water molecules
Summary
Liquid water is considered the cornerstone of life and has many extraordinary physical and biochemical properties[1,2]. The time-resolved optical Kerr effect (OKE) technique[20,21,22,23] is a powerful experimental tool enabling accurate. When the OKE technique is used to investigate intermolecular hydrogen bond motions, the birefringence signal is always positive, and the molecular response is extremely weak. An enhanced molecular response can be obtained when using a THz electric field, as the field is in resonance with the rotational transitions of a single molecule or the cooperative low-frequency molecular motions[24,25,26]. The time-resolved THz Kerr effect (TKE) response is expected to be a powerful phenomenon for exploring the low-frequency dynamics of water molecules
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