Abstract
The orientational ordering behavior of water molecules in crystalline-ice thin films was investigated using
sum-frequency generation (SFG) spectroscopy. Homodyne-and heterodyne-detected SFG measurements
revealed that net-H-down ferroelectric ice films were grown on a model platinum substrate, where
water molecules orient their protons toward the substrate. Temperature dependent measurement shows
that the order-disorder transition temperature is significantly enhanced in comparison with bulk ice due
to the strong orientational pinning of the first-layer water molecules to net-H-down configuration. Our
results demonstrate the possibility of SFG for sensitively evaluating the proton configuration of even a
single molecular layer. This evaluation method would be also applicable to other hydrogen-bonded nanoscale
systems.
