AVS 45th International Symposium
    Surface Science Division Thursday Sessions
       Session SS1-ThA

Invited Paper SS1-ThA5
Diffusion and Island Formation of Water Molecules on Ice Ih Surfaces

Thursday, November 5, 1998, 3:20 pm, Room 308

Session: Surface Diffusion
Presenter: H. Jonsson, University of Washington
Authors: E.R. Batista, University of Washington
H. Jonsson, University of Washington
Correspondent: Click to Email
We present theoretical calculations of the deposition, adsorption, diffusion, and island formation of water admolecules on the basal face of ice Ih. The calculations are based on both pairwise additive interaction potentials as well as a polarizable model. The sticking coefficient is found to be near unity for vapor deposition, but drops to 0.9 for incident energy of 1.5 eV and 60 degree angle of incidence. At low coverage, an admolecule prefers to sit at non-crystallographic sites (i.e. not fitting into the ice lattice) with a large binding energy. Since ice Ih is proton disordered, there is a range of binding energies, and for some local environments the binding energy is on the order of (and even larger than) the cohesive energy. The proton disorder also results in a range of activation energies for diffusion. After mapping out a large number of diffusion barriers using the Nudged Elastic Band method, a kinetic Monte Carlo calculation of the diffusion at 140 K was performed. At short time, the mean squared displacement has anomalous scaling with time as is common for diffusion on random lattices. From the long time scaling a diffusion barrier is obtained which is not inconsistent with recent upper bound found by Brown and George. The simulated diffusion is also reasonably consistent with earli! er estimates of the diffusion leng th by Mason. Paths and energy barriers for the formation of small islands have also been studied. It is found that islands up to and including pentamer are non-crystallographic, but the hexamer is crystallographic. The dominant activation energy barrier to the formation of the hexamer is the admolecule diffusion barrier.