Mixed longitudinal-transverse nature of collective modes in water

We report high resolution, high statistics inelastic x-ray scattering measurements of the dynamic structure factor of water as a function of momentum and energy transfer in various thermodynamic conditions, including high pressure liquid near the melting point, supercooled liquid and polycrystalline ice. For momentum transfer values below 8nm−1 two collective excitations associated with longitudinal and transverse acoustic modes were observed. Above 8nm−1, another excitation was detected in the liquid. Comparison with polycrystalline data and molecular dynamics simulations suggests that this mode is related to longitudinal-transverse mixing of mode symmetry.