THz dynamics of hydrogen sulfide: Search for collective modes of a non-hydrogen-bonded analog of water

Understanding the atomic-scale dynamics of liquids in the terahertz (THz) regime remains challenging because liquids sustain phonon-like collective excitations governed by complex interactions. To clarify the role of hydrogen bonding in such dynamics, we performed inelastic neutron scattering on hydrogen sulfide (H_{2}S) and its deuterated form (D_{2}S), molecular analogues of water without a hydrogen-bonded network. By combining coherent and incoherent measurements, we show that liquid H_{2}S exhibits a clear fast-sound mode, with a propagation velocity about twice the adiabatic sound speed, as in water. In contrast to hydrogen-bonded liquids, which display a secondary, nearly momentum-independent mode at 6-7 meV, no corresponding feature is observed in liquid D_{2}S. This absence highlights the central role of hydrogen bonding in enabling additional low-energy excitations. Overall, while fast sound is not exclusive to hydrogen-bonded systems, the detailed vibrational spectrum is strongly shaped by the local bonding environment.