Adsorption, desorption, and clustering of H2O on Pt111.

The adsorption, desorption, and clustering behavior of H2O on Pt111 has been investigated by specular He scattering. The data show that water adsorbed on a clean Pt111 surface undergoes a structural transition from a random distribution to clustered islands near 60 K. The initial helium scattering cross sections as a function of temperature are found to be insensitive to the incident H2O flux over a range of 0.005 monolayers (ML)/s-0.55 ML/s indicating that the clustering process is more complex than simple surface diffusion. The coarsening process of an initially random distribution of water deposited at 25 K is found to occur over a broad temperature range, 60<T<140 K, during thermal annealing. The desorption kinetics for submonolayer water are determined to be zero order for surface coverages greater than 0.05 ML and temperatures between 145 and 172 K. The zero-order desorption kinetics are consistent with a two-dimensional two-phase coexistence between a high-density H2O condensed phase (islands) and a low-density two-dimensional gaslike phase on the Pt surface.