Unveiling the Elusive Low-Energy Vibrational Modes of Ice I Using Resonant Inelastic X-ray Spectroscopy

Ice I is the most abundant crystalline phase of water and plays a fundamental role in nature. Understanding its complex nuclear dynamics is crucial on its own and also provides critical insights into hydrogen bonding networks as well as water’s unique properties. Here, we present high-resolution oxygen K-edge resonant inelastic X-ray scattering (RIXS) of Ice I (H₂O and D₂O) measured using intense X-ray pulses from MAX IV. While previous RIXS studies on liquid and gas phase water were dominated mostly by OH stretching vibrations, we clearly identify multiple vibrational modes (OH stretch, HOH bend, librations, and their overtones and combinations) of ice I, including a libration mode (at 684 cm^–1) that has not been clearly resolved previously using other methods. Tuning the excitation energy allowed detailed measurements based on variations in the potential energy surface of core-excited states and to resolve the pure HOH bending mode at 1730 cm^–1 by suppressing interfering modes. Furthermore, a comprehensive analysis of H₂O and D₂O spectra confirmed characteristic phenomena arising from the isotope effects.