Strong Anisotropy in Liquid Water upon Librational Excitation Using Terahertz Laser Fields

Area of Science:

• Physical Chemistry
• Spectroscopy
• Molecular Dynamics

Background:

• Tracking water molecule excitation is difficult due to rapid energy dissipation in the hydrogen-bonded network.
• Understanding water's dynamic response to external fields is crucial for various chemical and biological processes.

Purpose of the Study:

• To demonstrate strong transient anisotropy in liquid water using specific terahertz (THz) frequencies.
• To investigate the nonlinear optical response and molecular dynamics of water under THz excitation.
• To explore the effect of ionic environments on water's THz response.

Main Methods:

• Single-color pump-probe experiments utilizing 12.3 THz radiation.
• Deduction of third-order nonlinear susceptibility (χ(3)) values.
• Theoretical modeling integrated with molecular dynamics simulations.
• Comparative studies with and without salt (MgSO4) addition.

Main Results:

• Strong transient anisotropy was observed in homogeneous liquid water via librational excitation.
• A third-order nonlinear response (χ(3)) three orders of magnitude higher than optical ranges was deduced.
• Molecular dynamics simulations confirmed resonant driving of librational motion by the THz field.
• Addition of MgSO4 significantly reduced the dynamic perturbation of water molecules by THz pulses.

Conclusions:

• Librational excitation at 12.3 THz effectively induces strong transient anisotropy in liquid water.
• The enhanced nonlinear response is attributed to resonant driving of damped molecular motion.
• Ionic environments, like those from MgSO4, diminish the THz-induced dynamic response of hydration water.