Coadsorption of Li and Na on Al(100)

The coadsorption of Li and Na on Al(100) has been studied by high-resolution core-level spectroscopy and low energy electron diffraction. Coadsorption of 0.25 ML Li and 0.25 ML Na at room temperature leads to the formation of a c(2×2) structure, which is shown to be a substitutional surface ternary alloy, in which the alkali atoms randomly occupy the vacancies in a c(2×2)-Al sublattice. Adsorption of 0.5 ML Li followed by 0.5 ML Na at room temperature leads again to the formation of a c(2×2) structure. This is shown to be a multilayer surface ternary alloy, consisting of a mixed c(2×2)-Al/Na layer, followed by a (1×1)-Al layer, followed by a mixed c(2×2)-Al/Li layer. This structure is also obtained by reversing the adsorption sequence. Adsorption of 0.5 ML Li at room temperature followed by 0.5 ML Na at 120 K leads again to the formation of a c(2×2) structure, which consists of a chemisorbed Na overlayer on a mixed c(2×2)-Al/Li layer. Surprisingly, reversing the adsorption sequence leads again to the same structure. The mechanisms of formation of the different phases are discussed.