Disclosing the nature of the collective THz dynamics in hydrogen bonded liquids

The behavior of liquids has always been a subject of strong interest and still presents many challenging issues. A paradigmatic example is the understanding of their atomic-scale collective dynamics. Experimentally accessible to inelastic neutron and X-ray scattering, this regime has recently disclosed an unexpectedly rich scenario made up of more than one collective excitation. However, experiments lack a fundamental piece of information since they cannot distinguish the longitudinal or transverse nature of the detected modes. Here we present an inelastic neutron scattering study of the atomic dynamics of water and sulfuric acid, two prototypes of hydrogen bonded liquids. We propose a novel method that allows the experimental determination of the nature of the observed modes, without resorting to any indirect information. The dynamic structure factors show a complex collective dynamics, with two propagating excitations that we describe in the framework of an interacting-modes model. The first high-energy excitation is usually assumed to be the prolongation of the longitudinal acoustic mode and is characterized by a strong fast sound, which in turn we ascribe to the interaction with the second low-energy mode. Our approach confirms the main longitudinal nature of the first mode and unambiguously identifies the transverse nature of the second one. Finally, the lifetime of the modes suggests a common origin for the second mode, possibly related to the presence of the hydrogen bond network.