Vibrational study of water dimers on Pt(1 1 1) using a scanning tunneling microscope

Adsorption and diffusion of isolated water molecules such as monomers and dimers on Pt(1 1 1) surface were directly observed by the scanning tunneling microscopy (STM) at very low coverage and at low temperature. A water dimer appears as a 6-fold symmetric “flower-like” protrusion, which can be explained based on the model that a hydrogen bond acceptor molecule in the dimer is rotating around a donor molecule. We obtained vibrational spectrum of individual water dimers by means of single-molecule vibrational spectroscopic method from the vibrationally-induced lateral hopping motions. Analyzing the spectra suggests a detailed model for adsorption structure of a water dimer where an acceptor molecule of the dimer points toward the surface to form O–H–Pt hydrogen bonding.