Two highly ordered, epitaxially rotated phases of bilayer ice are observed on Pt(111) in high resolution helium atom diffraction. Analysis of helium diffraction patterns shows that the two phases differ slightly in their density and alignment with respect to the surface. The lack of any isotope effect for both phases indicates that the ice bilayers have structures and hydrogen bond lengths very similar to bulk ice.

Articles you may be interested inAdsorbed state of thiophene on Si(100)(2×1) surface studied by electron spectroscopic techniques and semiempirical methods A lowenergy electron diffraction investigation of the surface deformation induced by misfit dislocations in thin MgO films grown on Fe (001) A Heatom scattering study of the frustrated translational mode of CO chemisorbed on defects on copper surfacesThe adsorption and desorption of water on single crystal MgO(100): The role of surface defects A lowenergy electron diffraction data acquisition system for very low electron doses based upon a slow scan charge coupled device camera Rev.The interaction of water with MgO ͑100͒ single crystal surfaces cleaved in situ has been studied by low energy electron diffraction and helium atom scattering in the temperature range 80 K-230 K. At T crystal ϭ100-180 K water forms a layer with a c͑4ϫ2͒ symmetry in good agreement with previous spot profile analysis of low energy electron diffraction experiments. Adsorption at T crystal ϭ185-221 K leads to the formation of a new ordered phase. The results of the low energy electron diffraction and elastic helium atom scattering experiments show that this high-temperature phase has a ͑3ϫ2͒ symmetry, and that the unit cell contains a glide plane. The isosteric heat of adsorption at half coverage Q st ϭ85.3 kJ/mol has been determined from equilibrium adsorption isotherms measured between 210 and 221 K.

A helium atom scattering study of the structure and dynamics of a Xe monolayer on NaCl(001) Structural and dynamic properties of H 2 O adsorbed on an in situ cleaved NaCl͑100͒ crystal surface were studied by He atom scattering in the range of temperatures and partial pressures of 80-160 K and 5ϫ10 Ϫ7 -5ϫ10 Ϫ10 mbar, respectively. A well-ordered 2D H 2 O layer was formed during adsorption as well as by heating of 3D ice. The isosteric heat of adsorption in the monolayer regime was found to be in the range 58 -63 kJ/mol. The structure of the 2D condensed phase was investigated for both modes of preparation and in both cases a ͑1ϫ1͒ diffraction pattern was found. Time-of-flight spectra of the monolayer H 2 O/NaCl were measured in the ͗100͘ direction and phonon dispersion curves were determined. The observed dispersion-free vibrational mode, with បϭ5.5 meV, was identified as a frustrated translation of the water molecule parallel to the surface, via calculations for physisorption models of the H 2 O/NaCl interaction. Other salient features of the adsorption, the large heat of adsorption, and the magnitude of the frequency of molecular vibration perpendicular to the substrate, are reproduced in the calculations.

The structure and phonons of an ordered ice surface, prepared in situ under ultra high vacuum conditions, have been studied by high resolution helium atom scattering. The angular distributions are dominated by sharp hexagonal (1×1) diffraction peaks characteristic of a full bilayer terminated ice Ih crystal. Additional, very broad and weak, p(2.1×2.1) peaks may indicate the presence of small domains of antiphase oriented molecules. An eikonal analysis of the 1×1 peaks is compatible with either a proton disordered or a proton ordered surface with corrugations of 0.76 Å and 0.63 Å, respectively. Inelastic time-of-flight spectra reveal not only a dispersionless phonon branch reported previously at 5.9 meV, but also the first evidence for the surface Rayleigh phonons, which are reproduced well by a Born–von Kármán simulation of a full bilayer terminated ice surface using the unmodified force constants derived from neutron scattering bulk phonon measurements. Since the lattice dynamics simulations do not reproduce the dispersionless branch, it is attributed to the vibrations of single water molecules on the ice surface.

The structure and phonon dynamics of the ice surface, prepared by growing thick ice layers (ϳ100 Å) on Pt(111), were investigated using high resolution helium atom scattering. Ice layers grown at T s 125 K were found to be well ordered, with a complete bilayer (1 3 1) surface termination, as shown by diffraction measurements at T s 30 K. Time-of-flight spectra provide evidence for an intense longitudinal shearing mode, large multiphonon background, and enhanced vibrational amplitudes at the surface, which are consistent with dynamic disorder and a large accommodation coefficient at the surface. [S0031-9007(98)

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