Diffusion and Island formation on the ice Ih basal plane surface

We present theoretical calculations of the adsorption, diffusion and island formation of water admolecules on the basal plane surface of an ice Ih cry stal. These are preliminary calculations based on the simple TIP4P interact ion potential, a pail wise additive potential function based on point charg es. At low coverage, we find that an admolecule prefers to sit at non-cryst allographic sites on the surface (i.e., sites that do not fit into the ice lattice). Since ice Ih is proton disordered, Ilo two sites are exactly the same and there is a wide range of binding energies. For some local environm ents the binding energy is of the order of, or even larger than, the cohesi ve energy. The proton disorder also results in a range of activation energi es for diffusion. After mapping out a large number of diffusion barriers us ing the nudged elastic band method, a kinetic Monte-Carlo calculation of th e diffusion at 140 K was performed. At early time, the mean squared displac ement has anomalous scaling with time as is common for diffusion on random lattices. But, at longer time the scaling is normal and a diffusion coeffic ient can be obtained. The diffusivity is slightly larger than a recent expe rimental upper bound given by Brown and George. The energetics and dynamics of the formation of small islands on the ice surface have also been studie d. It is found that islands up to and including pentamer are noncrystallogr aphic, but the hexamer is crystallographic. While the formation of a crysta llographic hexamer from a non-crystallographic pentamer and a new admolecul e involves a complex concerted motion of all the island molecules and a lar ge relaxation of the substrate, the activation energy for the process is es timated to be quite small, smaller than the admolecule diffusion barrier. ( C) 2001 Elsevier Science B.V. All rights reserved.