ADSORPTION, SURFACE RESTRUCTURING AND ALLOY FORMATION IN THE NA/AU(111) SYSTEM

The adsorption of Na on the reconstructed Au(111) surface has been studied by scanning tunneling microscopy, low-energy electron diffraction (LEED), thermal desorption spectroscopy and Auger electron spectroscopy. Upon deposition at 85 K, adsorbed Na atoms order spontaneously at coverages between 0.2 and 0.6 monolayers (ML) on the reconstructed substrate. A sequence of hexagonal overlayer structures is observed by LEED. Annealing the Na-covered sample to 300 K results in irreversible changes of the surface structures: Intermixed Na-Au surface layers are formed for theta(Na) > 0.25 ML. A well-ordered c(4 X 2) substitutional structure develops at theta(Na) approximate to 0.50 ML, where every fourth Au surface atom is replaced by a Na atom. The structures formed at 300 K are metastable, and upon annealing to 600 K the equilibrium surface configurations evolve: For small Na coverages the periodicity of the Au(111) Chevron phase is reduced, At theta(Na) approximate to 0.23 ML the Na atoms induce a surface phase transformation of the Au(111) Chevron phase into a poorly ordered domain structure with an isotropically contracted surface layer and hexagonal symmetry. At even higher coverages a mixed NaAu2 structure is formed in the topmost layer which covers the entire surface at theta(Na) approximate to 0.50 ML. The NaAu, structure is arranged in a hexagonal lattice with a (1.08 root 3 X 1.08 root)R30 degrees geometry where the Au atoms form a honeycomb structure whose central sites are occupied by Na atoms. The lattice misfit between the surface and the second layer leads to a regular hexagonal Moire pattern of approximate to 38 Angstrom periodicity and approximate to 0.5 Angstrom corrugation. This phase is regarded as a two-dimensional alloy. With adsorbed Na multilayers, Na atoms penetrate into deeper substrate layers and a thin NaAu2 alloy film is formed.