Formation and diffusion of water dimers on rutile TiO2(110)

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 TiO2(110)-(1x1) surfaces, i.e., surfaces containing OH_{br} groups. At temperatures between approximately 150 and approximately 210 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.