Terahertz dynamics of nanoconfined water molecules

We performed spectroscopic studies of water-containing beryl crystals. At terahertz frequencies, we observed a strongly temperature-dependent spectral feature, corresponding to a collective excitation caused by dipole-dipole interactions between separate H
₂
O molecules. We associate the excitation with a soft mode signaling an incipient ferroelectric state within the water molecular subsystem. Single-particle excitations of nanocaged H
₂
O molecules are also observed at terahertz and infrared frequencies; using DFT analysis, their origin is assigned to translational and librational vibrational states.