Investigation of the temperature dependence of dielectric relaxation in liquid water by THz reflection spectroscopy and molecular dynamics simulation

TLDR

The study measured liquid water’s dielectric constant from 0.1 to 2.0 THz over 271–366 K using THz time‑domain reflection spectroscopy, fitted the data with a Debye model plus an extra relaxation time, and compared the resulting time‑correlation functions from molecular‑dynamics simulations to experiment while analyzing the Debye relaxation’s temperature dependence with transition‑state theory, a Debye–Stokes–Einstein relation, and a singularity‑based model, finding excellent agreement with the latter two. The measurements reveal a fast femtosecond and a Debye picosecond relaxation of macroscopic polarization, and the temperature dependence of the Debye time agrees with Debye–Stokes–Einstein and singularity models, though simulation data are too noisy to confirm this relation.

Abstract

We report measurements of the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been obtained with the use of THz time domain reflection spectroscopy, utilizing ultrashort electromagnetic pulses generated from a photoconductive antenna driven by femtosecond laser pulses. A Debye model with an additional relaxation time is used to fit the frequency dependence of the complex dielectric constants. We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding time correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye–Stoke–Einstein relation between the viscosity and the Debye time, and a model stating that its temperature dependence can be extrapolated from a singularity of liquid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results with too large statistical error for establishing a relation for the temperature dependence.