A Microscopic Description of Dielectric Relaxation in Water using Quasi-Elastic Neutron Scattering*

Water exhibits an anomalously high dielectric constant, ε=78, at ambient temperature. This is caused by a Debye peak in the dielectric spectrum with a maximum at ~20 GHz. It is reported that this Debye peak reflects some hydrogen bond mediated, collective dipolar dynamics in water. However, a clear microscopic description of this phenomenon is lacking. We report our study on the microscopic picture of water’s dielectric relaxation using the time-dependent pair-distribution function, the Van Hove function. The dynamic structure factor of water, S(Q, E), is measured using quasi-elastic neutron scattering over a wide momentum transfer range by making use of recent advances in analyzer crystal options at BASIS, SNS. S(Q, E) is double Fourier transformed to obtain the Van Hove function, g(r, t), to investigate the temporal evolution of molecular correlations in real-space up to 10 picoseconds. Our findings align with previous studies on the time-scale of the Debye process and the higher frequency excess Debye process, although significantly differ in the microscopic description.