Broadband THz time-domain spectroscopy of water: Enhanced cooperativity of hydrogen-bond dynamics in the Debye wing

Broadband terahertz time-domain spectroscopy measurements of liquid water are presented in the 0.3–12 THz frequency range from 360 K down to the supercooled regime at 255 K. The probed spectral window allows the characterization of the intermediate-frequency region linking the main Debye relaxation to intermolecular vibrational modes. The dielectric response is analysed using two alternative fitting schemes, yielding consistent fits despite the different parametrizations. Upon cooling, the dielectric response shows a progressive transfer of spectral weight from librational motions toward intermediate-frequency contributions demonstrating an enhancement of collective hydrogen-bond network dynamics. Comparison with depolarized Raman spectra reveals a separation between the fast dielectric components and the structural relaxation. The observed evolution is compatible with an increasing prevalence of tetrahedrally coordinated, LDL-like local environments. Our findings indicate that the apparent multiplicity of relaxation processes reflects a continuous spectrum of correlated hydrogen-bond network dynamics evolving toward increasingly cooperative behaviour upon cooling.