  The existence of water dimers in equilibrium water vapor at room temperature
and their anomalous properties revealed by recent studies suggest the benchmark
role of water dimer in both experiment and theory. However, there has been a
limited observation of individual water dimers due to the challenge of water
separation and generation at the single-molecule level. Here, we achieve
real-space imaging of individual confined water dimers embedded inside
self-assembled layer of a DNA base, adenine, on Ag(111). The hydration of the
adenine layers by these water dimers causes a local surface chiral inversion in
a way that the neighboring homochiral adenine molecules become heterochiral
after hydration, resulting in a mismatched hydrogen-bond pattern between
neighboring adenine molecules. Furthermore, the mutual influence between the
adenine superstructure and these dynamic confined water dimers is corroborated
by theoretical simulation and calculations. The observation of single confined
water dimers offers an unprecedented approach to studying the fundamental forms
of water clusters and their interaction with the local chemical environment.
