Pentagons and Heptagons in the First Water Layer on Pt(111)

Scanning tunneling topography of long-unexplained root 37 and root 39 periodic wetting arrangements of water molecules on Pt(111) reveals triangular depressions embedded in a hexagonal H(2)O-molecule lattice. Remarkably, the hexagons are rotated 30 degrees relative to the classic bilayer'' model of water-metal adsorption. With support from density functional theory energetics and image simulation, we assign the depressions to clusters of flat-lying water molecules. 5- and 7-member rings of H(2)O molecules separate these clusters from surrounding H-down'' molecules.