Experimental Evidence for a Three-Body Interaction between Diffusing CO Molecules

Common structures of CO2 on structurally different coin metal surfaces

We investigate superstructures formed by CO2 on Ag(100) and Cu(111) from small clusters forming at 21 K up to multilayers grown at 43 K by low temperature scanning tunneling microscopy. On both surfaces, CO2 nucleates only at defects, here at co-adsorbed CO. At the lower adsorption temperature, superstructures of different symmetry coexist on both surfaces at submonolayer coverage, while the superstructures formed at the higher adsorption temperature differ largely for the two surfaces. On Ag(100), the CO2 monolayer exhibits a long-range order interrupted by antiphase domain boundaries. On Cu(111), a random distribution of domain structures of different symmetry leads to a monolayer without long-range order. Surprisingly, the degree of ordering is inverted for the 2nd layer of CO2. On Ag(100), the coexistence of different superstructures in the 2nd layer leads to reduced long-range order. On Cu(111), a hexagonal 2nd layer exhibits long-range order. A layer of a similar superstructure, hexagonal with long-range order, exists as the 3rd layer of Ag(100). Despite the different substrates, a multitude of common structural features of CO2 exist. Hexagonal layers grow with a long-range order on less ordered layers on both surfaces. Our results suggest that the preferred structure of a CO2 layer is hexagonal.

Anomalously Low Barrier for Water Dimer Diffusion on Cu(111)

A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone.