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Water: monomers and dimers

The structure and dynamics of water monomers and dimers on Cu(110) is reported in this paper. We used a scanning tunneling microscope (STM) and density-functional theory (DFT) for the study. The monomer is adsorbed on top of Cu atom and thermally hops on the surface. The dimer consists of hydrogen-bond donor and acceptor molecules, which were observed to dynamically exchange their roles via hydrogen-bond rearrangement.

(a) Typical STM image of water monomers at 6 K. They are observed as round protrusions. A fractional image (upper right) indicates that the monomer hops along the [1-10] direction. The inset is a separately taken image that shows relative position of water molecule to coadsorbed CO molecules. White grid lines show the lattice of Cu(110).
(b) Side view of the calculated structure for monomer on Cu(110).
(c) The same as (b) from top view.

The dynamics of water monomers was investigated by using atom tracking techniques.
(a) Typical trace of the monomer at Vs = 24 mV and It = 0.5 nA. The traces are represented by the displacement from the original position in the [1-10] (red) and [001] (blue) directions.
(b) Distribution of the time intervals between the hopping events. The fitting to an exponential function gives a hopping rate of 0.13 s-1. The inset shows the hopping rates as a function of tunneling current at Vs= 24 mV.

Sequential images of two interacting H2O dimers. Water dimer diffuses the substrate accompanied with interchange motion.
(a) Two dimers show dynamical fluctuation; details of dimer’s dynamics are in [Phys. Rev. Lett. 100, 166101 (4pp) (2008).]
(b) As they migrate and come near each other, one of the two configurations is strongly preferred due to the interaction.
(c) They pass each other.
(d) The two dimers restart their inherent interchange motion.