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 D2O bilayer forms an extended commensurate p(3×3) R30°structure with known geometry, the H2O bilayer is characterized by a striped domain structure 6.5 substrate lattice constants wide with essentially the same short range arrangements as for D2O. 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.