Ice surface structure and ice at interfaces

　Owing to its physical and chemical properties, ice has attracted continuing interest for a long time. Despite containing only a simple molecule, H2O, the hydrogen bond network thus formed exhibits diverse variety in structure and dynamics depending on temperature and pressure. In particular, the surface of ice plays a crucial role in the chemistry occurring in the stratosphere and interstellar space; therefore, elucidating the structures of ice at its surface and buried interfaces is vital. However, investigating the atomic structure of soft materials at their surface and buried interfaces is not straightforward, even with sophisticated surface science techniques. In a series of works, we applied phase-sensitive sum-frequency generation spectroscopy to thin films of ice grown under ultra-high vacuum conditions. Utilizing the merit of the proposed technique to determine the absolute orientation of hydroxyl groups, we successfully revealed the ice film’s proton ordering at the buried interface with a single platinum crystal, showing that a ferroelectric phase of ice emerges at much higher temperatures than in bulk. We further found that the proton order largely depends on the substrate: on Rh(111), ice Ih has a proton configuration that exhibits normal disordering. In this case, the phase-resolved sum-frequency generation spectroscopy signal yields detailed information on the subsurface structural relaxation of the ice/vacuum interface.