Coadsorption of Li and Na on Al(100)

The coadsorption of Li and Na on Al(100) has been studied by high-resolutio n 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 x 2) structure, which is shown to be a substitutional surface ternar y alloy, in which the alkali atoms randomly occupy the vacancies in a c(2 x 2)-Al sublattice. Adsorption of 0.5 ML Li followed by 0.5 ML Na at room te mperature leads again to the formation of a c(2 x 2) structure. This is sho wn to be a multilayer surface ternary alloy, consisting of a mixed c(2 x 2) -Al/Na layer, followed by a (1 x I)-Al layer, followed by a mixed c(2 x 2)- Al/Li layer. This structure is also obtained by reversing the adsorption se quence. Adsorption of 0.5 ML Li at room temperature followed by 0.5 ML Na a t 120 K leads again to the formation of a c(2 x 2) structure, which consist s of a chemisorbed Na overlayer on a mixed c(2 x 2)-Al/Li layer. Surprising ly, reversing the adsorption sequence leads again to the same structure. Th e mechanisms of formation of the different phases are discussed. (C) 2000 E lsevier Science B.V. All rights reserved.