Origin of the fast relaxation component of water and heavy water revealed by terahertz time-domain attenuated total reflection spectroscopy

We determined the dielectric constants of water and heavy water (0.2–3.5 THz) from 270 to 362 K, and decomposed them into four components: slow relaxation, fast relaxation, intermolecular stretching vibration, and intermolecular libration. The fast component has approximately 2% of the slow relaxation strength, sub-picosecond relaxation time, and almost no temperature dependence. This component is assigned to an individual mode governed by the collision process, which is characteristic of non-hydrogen bonding structure, transiently appearing in dynamical hydrogen bonding network. This short-lived structure in sub-picosecond timescale indicates the heterogeneity of the water structure, which is consistent with the two-component model.