Interpretation of the GHz to THz dielectric relaxation dynamics of water in the framework of the Coupling Model

The microscopic origins of the processes in water observed at temperatures above and below room temperature by dielectric relaxation spectroscopy in the range from GHz to beyond THz are identified by three sequential and interrelated contributions in the framework of the Coupling Model. Appearing after vibration and boson peak is the caged water dynamics. Cage decays with the onset of the primitive relaxation of the Coupling Model (CM), which is practically coincident with the nearly exponential structural α-relaxation. These sequential processes explain the nearly Debye α-loss peak, the high frequency power law (HFPL) associated with it, and the higher frequency dispersion before reaching the vibrational contributions. High frequency and short time experimental data of several glass-formers obtained by different techniques are presented to show the same three processes with the same relationship between them, as proposed for water. Crucial supports of the CM explanation come from neutron scattering experiments on water showing that: (1) caging and decaging indeed occurs in the HFPL regime; (2) the dynamics of the HFPL is independent of the scattering vector Q, in accord with the HFPL is comprised of caged dynamics and decaging; and (3) the terminal process is normal diffusion, justifying the 20 GHz dielectric loss peak originates from the structural α-relaxation, which follows after the completion of decaging, i.e. the end of the HFPL regime.