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This study investigates the low-frequency dielectric response of liquid water, revealing that it can be modeled by a combination of collective Debye relaxation and a Drude-Smith term, with significant variations observed upon heating and isotopic substitution. The findings indicate that the dielectric response is linked to a transient imbalance in hydrogen-bond donor and acceptor populations, as supported by molecular dynamics simulations and ab-initio spectral calculations. These insights enhance our understanding of water's complex dielectric properties and the interplay between nuclear and electronic contributions at low frequencies.
The low-frequency THz response of water reveals a surprising link between hydrogen-bond donor-acceptor imbalances and dielectric behavior, challenging existing models.
The low-frequency dielectric response of liquid water is commonly described by a dominant Debye relaxation together with additional faster contributions whose microscopic origin remains debated. Here we show that the dielectric function of water between 0.14 and 1.21 THz can be represented by a collective Debye relaxation plus a Drude-Smith term constrained to the zero-dc-conductivity limit. The Drude-Smith spectral weight increases upon heating pure H2O from 20 C to 50 C and decreases upon isotopic substitution (D2O at 20 C vs. H2O at 20 C). Molecular dynamics simulations including nuclear quantum effects show correlated changes in the population of water molecules with unequal numbers of donated and accepted hydrogen-bonds. Ab-initio-based spectra calculations further indicate that the ~0.1-1 THz response contains both nuclear-motion and explicit electronic-polarisation/charge-redistribution contributions. We therefore interpret the excess low-frequency THz response as a localised, mixed nuclear-electronic dielectric response correlated with transient donor-acceptor imbalance in the hydrogen-bond network.
