Structural Isotope Effect in Water Bilayers Adsorbed on Ru(001)

Using negligible-damage low energy electron diffraction we have found a structural isotope effect in water bilayers on Ru(001), in contrast to bulk ice where none exists. While the ${\mathrm{D}}_2$O bilayer forms an extended commensurate $p(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ structure with known geometry, the ${\mathrm{H}}_2$O bilayer is characterized by a striped domain structure 6.5 substrate lattice constants wide with essentially the same short range arrangements as for ${\mathrm{D}}_2$O. This unexpected isotope effect is explained by a difference in H and D bond lengths (an Ubbelohde effect) and potentials, induced by the interaction with the substrate, and a consequent difference in the misfit to the substrate lattice.