A helium atom scattering study of the structure and phonon dynamics of theice surface

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 (1x1) diffraction peaks characteristic of a full bilayer terminated ice Ih crystal. Additional, very broad and weak, p(2.1x2.1) peaks may indicate th e presence of small domains of antiphase oriented molecules. An eikonal ana lysis of the 1x1 peaks is compatible with either a proton disordered or a p roton ordered surface with corrugations of 0.76 Angstrom and 0.63 Angstrom, respectively. Inelastic time-of-flight spectra reveal not only a dispersio nless phonon branch reported previously at 5.9 meV, but also the first evid ence for the surface Rayleigh phonons, which are reproduced well by a Born- von Karman simulation of a full bilayer terminated ice surface using the un modified force constants derived from neutron scattering bulk phonon measur ements. Since the lattice dynamics simulations do not reproduce the dispers ionless branch, it is attributed to the vibrations of single water molecule s on the ice surface. (C) 2000 American Institute of Physics. [S0021-9606(0 0)70424-7].