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

Ferdinando Formisano* and Alessio De Francesco

Francesco Sacchetti and Caterina Petrillo

Eleonora Guarini

  • CNR-IOM and INSIDE@ILL, Operative Group in Grenoble, 71 avenue des Martyrs, F-38042 Grenoble, France, and ILL, 71 avenue des Martyrs, F-38042 Grenoble, France

  • *Contact author: ferdinando.formisano@cnr.it
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Phys. Rev. E 114, 015412 – Published 15 July, 2026

DOI: https://doi.org/10.1103/ccq9-618c

Abstract

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 (H2S) and its deuterated form (D2S), molecular analogues of water without a hydrogen-bonded network. By combining coherent and incoherent measurements, we show that liquid H2S 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 D2S. 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.

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Phys. Rev. E 114, 015412– Published 15 July, 2026
  • Received 25 March 2026
  • Accepted 18 June 2026
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