The theory of the second relaxation region in liquid water

Abstract

An analytical linear-response theory based on a phenomenological molecular model is proposed. The complex permittivity e*=e′+ie′’ of liquid water (H2O) in the submillimeter-wavelength region is described in terms of the harmonic oscillator model. This description is based on the assumption that the specific features of the spectrum are associated with the damped vibrations of hydrogen-bonded water molecules. The calculated frequency dependences of the real (e′) and imaginary (e′’) parts of the permittivity at room temperature are in agreement with available experimental data. The contribution of the vibrations to the absorption band located at ∼200 cm−1 is determined. The nonspecific contribution to this band due to reorientations of polar water molecules in a rectangular intermolecular potential well is evaluated. The dielectric response to reorientations is calculated within the hybrid model, which is also applied to the description of the main (Debye) relaxation region and the librational absorption band (centered at ∼650 cm−1) of liquid water. The harmonic oscillator-hybrid model proposed makes it possible, for the first time, to describe two processes in the framework of a unified approach. The first (vibrational) process in the range from 10 to ∼300 cm−1 is specific to systems with hydrogen bonds and includes the dielectric response associated with the translational degrees of freedom of dipoles in an aqueous medium. The second (nonspecific) process involves the dielectric response to orientational motion of a polar water molecule and is responsible primarily for the Debye relaxation region and the librational absorption band. The fundamental differences between D2O and H2O are considered within the molecular model.