Water orientations at stepped metal surfaces studied by PS-VSFG

　Microscopic understanding of water structure and dynamics at metal surfaces is crucial for understanding electrochemical reactions, corrosion and heterogeneous catalysis. While defect sites such as steps have been considered to provide active sites for various surface reactions, the behaviour of water at step sites has been much less explored in the past. In particular, the water orientations at the stepped surface are not known. We study the water structure at the platinum stepped surfaces using phase-sensitive sum-frequency generation spectroscopy under ultrahigh vacuum conditions. We found that the water structure depends on the step density and the crystal face orientations, and that the steps control the net water orientation at the surfaces.