Giant splitting of the hydrogen rotational eigenenergies in the C${}_2$ filled ice

Abstract

Hydrogen hydrates present a rich phase diagram influenced by both pressure and temperature, with the so-called C${}_2$ phase emerging prominently above 2.5 GPa. In this phase, hydrogen molecules are densely packed within a cubic ice-like lattice and the interaction with the surrounding water molecules profoundly affects their quantum rotational dynamics. Herein, we delve into this intricate interplay by directly solving the Schrödinger's equation for a quantum H${}_2$ rotor in the C${}_2$ crystal field at finite temperature, generated through Density Functional Theory. Our calculations reveal a giant energy splitting relative to the magnetic quantum number of $\pm$3.2 meV for $l=1$. Employing inelastic neutron scattering, we experimentally measure the energy levels of H${}_2$ within the C${}_2$ phase at 6.0 and 3.4 GPa and low temperatures, finding remarkable agreement with our theoretical predictions. These findings underscore the pivotal role of hydrogen--water interactions in dictating the rotational behavior of the hydrogen molecules within the C${}_2$ phase and indicate heightened induced-dipole interactions compared to other hydrogen hydrates.

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