To see whether vibration spectroscopy confirms or contradicts the idea that the wetting layer of D@sub 2@O/Ru(0001) is half-dissociated,@footnote 2@ vibration spectra have been computed from first principles for comparison to experiment. The calculations show that dissociation of the non-hydrogen-bonding O-D bonds of a heavy-water bilayer eliminates the highest-energy O-D stretch feature, and replaces it with a lower frequency mode. This behavior agrees with recent Sum Frequency Generation observations,@footnote 3@ lending credence to the argument that a half dissociated D@sub 2@O layer is needed both to explain why water wets Ru(0001) at all, and to account for the inference drawn from Low Energy Electron Diffraction data, that the O atoms of @sr@3x@sr@3-D@sub 2@O/Ru(0001) are nearly coplanar. Beyond helping to clarify the nature of the wetting layer, the computed vibration spectra also permit an estimate of its zero-point energy relative to that of competing adsorption structures. Zero-point energy is found to favor half-dissociated adlayers because, of every four oxygen-hydrogen bonds in an intact-water-molecule structure, one is replaced by a softer metal-hydrogen bond in a half-dissociated arrangement. For @sr@3x@sr@3-D@sub 2@O/Ru(0001), this zero-point stabilization amounts to about 30 meV per D@sub 2@O. In the case of H@sub 2@O adsorption, it would be ~50 meV/ad-molecule. @FootnoteText@ @footnote 1@ Work supported by the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. DOE.@footnote 2@ P. J. Feibelman, Science 295, 99(2002).@footnote 3@ D. N. Denzler, unpublished.