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Dynamic components in THz Kerr effect signal of water. left, Square of THz field (black line) is convoluted with two exponential functions with í µí¼ 1 ≈ 0.1 ps (cyan line) and í µí¼ 2 ≈ 0.5 ps (red line). right, Subtraction of the latter two curves gives rise to the green line which captures almost all features of the TKE signal of water (blue dots).
Source publication
Formation of local molecular structures in liquid water is believed to have marked effect on the bulk properties of water, however, resolving such structural motives in an experiment is challenging. This challenge might be handled if the relevant low-frequency structural motion of the liquid is directly driven with an intense electromagnetic pulse....
Contexts in source publication
Context 1
... To unravel the underlying dynamical components of the TKE signal of water, we reproduce the TKE signal by a convolution of the driving THz intensity with exponential functions. As illustrated in Fig. 3, the square of the THz pulse, taken from the TKE signal of a thin diamond plate (black line) is convoluted by two decaying exponential functions whose time constants and relative amplitude are left as fitting parameters. The addition of the convoluted components with time constants of 0.1 ps (cyan line) and 0.5 ps (red line) reproduces ...
Context 2
... taken from the TKE signal of a thin diamond plate (black line) is convoluted by two decaying exponential functions whose time constants and relative amplitude are left as fitting parameters. The addition of the convoluted components with time constants of 0.1 ps (cyan line) and 0.5 ps (red line) reproduces the experimental results (green line in Fig. 3), reasonably well. Therefore, the TKE response of water can be understood as an instantaneous electronic response around time zero and two step-like nuclear responses with opposite sign that decay exponentially with time constants of about 0.1 ps and 0.5 ps, respectively. In the following, we take use these distinct features of the TKE ...
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Citations
... THz-related spectroscopic techniques play an important role in low-frequency molecular dynamics research [1][2][3][4][5][6], semiconductor and material property investigation [7][8][9][10][11][12][13][14][15], national defense and security [16][17][18], information technology [19][20][21][22][23][24], and medical diagnosis [25][26][27][28][29][30]. The Kerr effect describes the phenomenon of birefringence that occurs in a dielectric medium excited by an electric field [31]. After the development of the laser, the electric field of light was recognized to induce birefringence in the medium, which is called the optical Kerr effect (OKE) [32]. ...
In recent years, tremendous advancements have been made in various technologies such as far-infrared, low-frequency Raman, and two-dimensional (2D) Raman terahertz (THz) spectroscopies.
A coherent method has emerged from numerous experimental and theoretical investigations of molecular dynamics in liquids by comparing linear and non-linear spectroscopic techniques. Intermolecular hydrogen bond vibration, molecular reorientation motion, and interaction between molecule/ionic solute and hydrogen bonds have been demonstrated to occur in the THz region, which are closely related to their physical/chemical properties and structural dynamics. However, precise probing of various modes of motion is difficult because of the complexity of the collective and cooperative motion of molecules and spectral overlap of related modes. With the development of THz science and technology, current state-of-the-art THz sources can generate pulsed electric fields with peak intensities of the order of microvolts per centimeter (MV/cm). Such strong fields enable the use of THz waves as the light source for non-linear polarization of the medium and in turn leads to the development of the emerging THz Kerr effect (TKE) technique. Many low-frequency molecular motions, such as the collective directional motion of molecules and cooperative motion under the constraint of weak intermolecular interactions, are resonantly excited by an intense THz electric field. Thus, the TKE technique provides an interesting prospect for investigating low-frequency dynamics of different media. In view of this, this paper first summarizes the research work on TKE spectroscopy by taking a solid material without low-frequency molecular motions as an example.
Starting from the principle of TKE technology and its application in investigating the properties of solid matter, we have explored the low-frequency molecular dynamics of liquid water and aqueous
solutions using TKE. Liquid water is a core of life and possesses many extraordinary physical and biochemical properties. The hydrogen bond network plays a crucial role in these properties and is the main reason for its various kinetic and thermodynamic properties, which differ from those of other liquids. However, the structure of the hydrogen bond network between water and solutes is not well known. Therefore, evaluating the hydrogen bond-related kinetic properties of liquid water is important.
... THz-related spectroscopic techniques play an important role in low-frequency molecular dynamics research [1][2][3][4][5][6], semiconductor and material property investigation [7][8][9][10][11][12][13][14][15], national defense and security [16][17][18], information technology [19][20][21][22][23][24], and medical diagnosis [25][26][27][28][29][30]. The Kerr effect describes the phenomenon of birefringence that occurs in a dielectric medium excited by an electric field [31]. After the development of the laser, the electric field of light was recognized to induce birefringence in the medium, which is called the optical Kerr effect (OKE) [32]. ...
In recent years, various optical measurement technologies such as far-infrared spectroscopy, low-frequency Raman spectroscopy, optical Kerr effect spectroscopy, and two-dimensional Raman-terahertz spectroscopy have developed vigorously. By comparing the complementary aspects detected by various linear and nonlinear spectroscopic techniques, a coherent picture emerges from studies of dynamics in water. Numerous molecular dynamics experiments and theoretical investigations have demonstrated low-frequency molecular motions in liquid water. For example, intermolecular hydrogen bond vibration, molecular reorientation motion, and the interaction between molecule/ionic solute and hydrogen bond all occur in the THz region, which are closely related to their physical/chemical properties and structural dynamics. However, precise probing of various modes of motion is difficult due to the complexity of the collective and cooperative motion of molecules and the spectral overlap of related modes. With the development of THz optical technology, current state-of-the-art THz sources can generate pulsed electric fields with peak intensities on the order of ~MV/cm. Such strong fields make it possible to use THz waves as the driving light source for nonlinear polarization of the medium, which in turn leads to the development of the emerging terahertz Kerr effect (TKE) spectroscopy technique. Many low-frequency molecular motion modes, such as the collective directional motion of molecules and the cooperative motion under the constraint of weak intermolecular interactions, are resonantly excited to unprecedentedly strong amplitudes driven by the THz electric field. Thereby the collected responses are increased. The TKE technique thus creates an interesting prospect for investigating low-frequency dynamics in these media. In view of this, this paper firstly summarizes the research work on the measurement mechanism of TKE spectroscopy by taking the solid material without low-frequency molecular dynamics process as an example. Starting from the principle of TKE technology and the exploration of the properties of solid matter using this technology, its application in the exploration of low-frequency molecular dynamics of liquid water and aqueous solutions is introduced. Liquid water is considered the building block of life and possesses many extraordinary physical and biochemical properties. Its hydrogen bonding network plays a crucial role in these properties. It is generally believed that the ability of water to form complex hydrogen-bonding networks is the main reason for its various kinetic and thermodynamic properties that are different from other liquids. However, the exact structure of the hydrogen-bonding network, the spatial extent of its existence, and the associated timescale are not known. And the relevant spectral information of the basic properties of the reaction water is still not known. Therefore, it is of great significance to evaluate the hydrogen bond-related kinetic properties of liquid water by optical means.
... Recently, researchers have successfully used THz pulses to drive liquids, such as dimethyl sulfoxide (DMSO), chloroform and liquid water, and tracked the molecular dynamics by detecting transient birefringence 24,[27][28][29][30][31][32][33] . In particular, in 2018, Zalden et al. used a low-frequency single-cycle THz pulse (centre frequency of 0.25 THz) generated by optical rectification in a LiNbO 3 crystal to stimulate liquid water and observed the time-resolved birefringent signal induced by the transient orientation of dipole moments. ...
... Compared with the negative polarity signal with an obvious relaxation tail extending over a few picoseconds in a previous study 32 , our birefringent signals should have different causes than the molecular orientational relaxation of liquid water. Taking into account the dielectric spectrum of water 15,16,18,19,31,34 , the THz band in our work mainly covers the two modes of intermolecular hydrogen bond motion, which indicates that the transient birefringence is mainly attributed to the intermolecular structure dynamics of water. ...
... Moreover, the latest experiment on THz-driven liquid water shows that with a lower centre frequency excitation (~0.25 THz), the orientation process guides the anisotropic distribution, showing a negative TKE response 32 . However, Kampfrath et al. observed a bipolar Kerr effect of liquid water with añ 1 THz centre frequency for excitation 31 , which suggested that the molecular response mechanism depends critically on the driving frequency. For a relatively high THz frequency excitation (such as~3.9 ...
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.
... Recent TKE experiments on water vapor show that Δα > 0 due to the positive sign of the Kerr effect 23 . The application of this technique to liquid water and alcohols is challenging due to their orders of magnitude higher absorption and lower Kerr coefficients 36 . Therefore, these experiments require THz sources of highest field strengths and lowest pulseto-pulse fluctuations at the same time. ...
Reaction pathways of biochemical processes are influenced by the dissipative electrostatic interaction of the reagents with solvent water molecules. The simulation of these interactions requires a parametrization of the permanent and induced dipole moments. However, the underlying molecular polarizability of water and its dependence on ions are partially unknown. Here, we apply intense terahertz pulses to liquid water, whose oscillations match the timescale of orientational relaxation. Using a combination of terahertz pump / optical probe experiments, molecular dynamics simulations, and a Langevin dynamics model, we demonstrate a transient orientation of their dipole moments, not possible by optical excitation. The resulting birefringence reveals that the polarizability of water is lower along its dipole moment than the average value perpendicular to it. This anisotropy, also observed in heavy water and alcohols, increases with the concentration of sodium iodide dissolved in water. Our results enable a more accurate parametrization and a benchmarking of existing and future water models.
