A HELIUM ATOM SCATTERING STUDY OF WATER-ADSORPTION ON THE NACL(100) SINGLE-CRYSTAL SURFACE

Structural and dynamic properties of H2O adsorbed on an in situ cleaved NaCl(100) crystal surface were studied by He atom scattering in the range of temperatures and partial pressures of 80-160 K and 5x10(-7)-5x10(-10) mbar, respectively. A well-ordered 2D H2O 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 (1X1) diffraction pattern was found. Time-of-flight spectra of the monolayer H2O/NaCl were measured in the (100) direction and phonon dispersion curves were determined. The observed dispersion-free vibrational mode, with HBAR omega=5.5 meV, was identified as a frustrated translation of the water molecule parallel to the surface, via calculations for physisorption models of the H2O/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. (C) 1995 American Institute of Physics.