Origins of fast diffusion of water dimers on surfaces

Scanning-tunnelling microscopy (STM) is an amazing experimental tool that can “see” individual molecules on surfaces. It has been widely used to observe how molecules adsorb and move on surfaces. Typically, it has been found that molecules or atoms that stick stronger to the surface diffuse slower. Is it also the case for clusters? Not quite, experiments have revealed that water dimers can diffuse as fast as or even faster than water monomers. In our work we wanted to develop a theory for understanding this strange behavior. Since the experiments under consideration were performed at temperatures close to absolute zero we also accounted for quantum mechanical effects, such as tunnelling in our theories.

From our ab initio simulations an incredibly rich variety of dynamical behaviour is revealed with the key findings being: i) rapid water dimer diffusion is more widespread than previously anticipated; and ii) the physical origin of rapid water diffusion lies in a delicate interplay of hydrogen bonding, water-surface bonding, and tunneling effects. One highlight is that the water dimer has a unique way of ‘waltzing’ on the surface, and we find that tunnelling can make the water dimer waltz faster than the monomer on certain surfaces.


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