Scanning tunneling topography of long-unexplained “$\sqrt{37}$” and “$\sqrt{39}$” periodic wetting arrangements of water molecules on Pt(111) reveals triangular depressions embedded in a hexagonal ${\mathrm{H}}_2\mathrm{O}$-molecule lattice. Remarkably, the hexagons are rotated 30° 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 ${\mathrm{H}}_2\mathrm{O}$ molecules separate these clusters from surrounding “H-down” molecules.

• Received 14 April 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.026102