From an interplay of time-lapsed high resolution scanning tunneling microscopy and density functional theory calculations we reveal the formation and diffusion of water dimers on hydrated rutile ${\mathrm{TiO}}_2(110)\mathrm{\text{−}}(1\times 1)$ surfaces, i.e., surfaces containing ${\mathrm{OH}}_{\mathrm{br}}$ groups. At temperatures between $\sim 150$ and $\sim 210\mathrm{K}$ water monomers diffusing along the Ti troughs were found to form stable water dimers that diffuse faster than the water monomers. An H bond mediated rollover mechanism operating for the water dimers explains the experimental findings.

• Received 8 April 2009

DOI:https://doi.org/10.1103/PhysRevLett.102.226101