Volume 141, 2009

Order and disorder in the wetting layer on Ru(0001)

Abstract

The growth of an intact water monolayer on Ru(0001) has been investigated by comparing the ordering of O and Ru, determined by low-energy electron diffraction (LEED), with that of the top layer of O and H, as probed by He atom scattering (HAS). Although LEED shows that water forms an extended commensurate (√3 × √3) R30° structure as the coverage approaches 0.67 monolayer, the HAS distributions are insensitive to the exact water coverage and show a very low specular reflectivity, indicating a disordered water layer. The angular profile from a D2O monolayer shows a broad diffuse peak in the angular scattering distribution at a momentum exchange similar to the position of the second-order (1/3,1/3) peaks, but the maxima show little variation with scattering azimuth. H2O shows a slightly higher He reflectivity and more clearly resolved angular structure, with broad, faint peaks appearing close to the first-order diffraction positions. The origin of this disorder is discussed based on density functional calculations for the monolayer which find that water forms chains of flat and H-down molecules within a hexagonal hydrogen-bonding network, rather than the ice bilayer usually assumed. This arrangement leads to long-range order in the O location, but disorder in the O height and the proton orientation. We discuss how this combination of lateral order in the adsorption site, but disorder in the water orientation, is reflected in the sharp √3 LEED pattern but diffuse, broad peaks in He scattering.

Article information

Article type
Paper
Submitted
09 May 2008
Accepted
11 Jun 2008
First published
30 Sep 2008

Faraday Discuss., 2009,141, 231-249

Order and disorder in the wetting layer on Ru(0001)

M. Gallagher, A. Omer, G. R. Darling and A. Hodgson, Faraday Discuss., 2009, 141, 231 DOI: 10.1039/B807809B

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