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 cor e electron binding energies and quantitative analyses by low energy el ectron 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 reconstru cted Al substrate layer. The lower layer of the sandwich always contai ns Na, irrespective of the adsorption sequence.