Core-level photoelectron spectra, in excellent agreement with ab initio calculations, confirm that the stable wetting layer of water on Ru{0001} contains O-H and ${\mathrm{H}}_2\mathrm{O}$ in roughly $3:5$ proportion, for ${\mathrm{O}\mathrm{H}}_x$ coverages between 0.25 and 0.7 ML, and $T<170\mathrm{K}$. Proton disorder explains why the wetting structure looks to low energy electron diffraction (LEED) to be an ordered $p(\sqrt{3}\times \sqrt{3})R30°$ adlayer, even though $\approx \frac{3}{8}$ of its molecules are dissociated. Complete dissociation to atomic oxygen starts near 190 K. Low photon flux in the synchrotron experiments ensured that the diagnosis of the nature of the wetting structure quantified by LEED is free of beam-induced damage.

• Received 5 May 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.196102