One of the most fundamental issues in water science is the characterization of H-bonding configuration formed on surfaces and H-atom transfer through hydrogen bonds. Ideally, attacking this problem requires the access to the internal degrees of freedom of water molecules, i.e. the directionality of OH bonds. In this talk, I will demonstrate the possibility of discerning the O-H directionality in real space through submolecular-resolution orbital imaging of interfacial water, using a cryogenic scanning tunneling microscope (STM) [1]. The key steps are decoupling electronically the water from the metal substrate by inserting an insulating NaCl layer and tuning controllably the molecular density of states of water around the Fermi level via tip-water coupling. These techniques allow us to distinguish the detailed topology of H-bonded network, probe the dynamics of proton transfer between the water molecules, and determine the H-bonding strength at the single bond limit.

[1] J. Guo et al., “Real-space imaging of interfacial water with submolecular resolution”, Nature Materials 13, 184 (2014).