Isotope effects on van-Hove functions for liquid H2O and D2O by inelastic X-ray scattering

Dynamics of liquid water at the molecular level is crucial to the understanding of transport properties of water, including viscosity. Despite intensive efforts over the past decades the molecular level dynamics has not been fully understood because structural relaxation and its associated hydrogen-bond dynamics have not been clearly identified. To study the structural relaxation the real-space/real-time analysis of inelastic X-ray scattering data on liquid water was carried out. We discovered a clear distinction in structural rearrangements between liquid H₂O and D₂O and showed that the characteristic time at which the first and second peaks in van-Hove function merged is associated with viscosity or local configurational excitations where an atom loses or gains one neighbor. This result also demonstrated the importance of real-space analysis in studying the local dynamics in the liquid.