Neutron spectroscopy of the intra-molecular H-O-H bending mode in different ice phases and confined water

Inelastic neutron scattering (INS) measurements reveal that the intra-molecular H-O-H bending mode (BM) in ice Ih is remarkably broad (~28 meV), whereas this mode is significantly narrow in water confined within minerals such as beryl and cordierite (~4 meV), hemimorphite, mordenite, and bassanite (~8 meV), as well as in the Ba(ClO₃)₂•H₂O compound (~4 meV). INS spectra of H-ordered ice phases II and VIII show moderately narrower BM widths (~20 meV) compared with the hydrogen-disordered ice phases Ih, III, and VI (~25 meV). These results indicate that H disorder alone cannot account for the pronounced broadening of the BM observed in ice. MD and PIMD simulations reproduce the overall features of the experimental INS spectra, though they fail to capture the full width of the BM in ices. The first overtone of the water librational band (55—130 meV) overlaps with the intra-molecular bending mode (~205 meV) in ice, but not in confined water, where the librational band and its overtone occur at significantly lower energies. We therefore propose that the large broadening of the BM in ice arises from Fermi resonance coupling between the first overtone of the librational band and the H-O-H bending mode.