Formation of surface ternary alloys by coadsorption of alkali metals on Al(111)

Coadsorption of one-quarter monolayer Na with one-quarter monolayer K, Rb, or Cs on Al(111) at 300 K is shown to result in the formation of surface ternary alloys with (2 X 2) unit cells. Chemical shifts in core electron binding energies and quantitative analyses by low energy electron diffraction indicate that the first four surface layers consist in each case of an X-Al-Na sandwich (X = K, Rb, or Cs) on a reconstructed Al substrate layer. The lower layer of the sandwich always contains Na, irrespective of the adsorption sequence.