COMBINED ION-SCATTERING, ELECTRON-DIFFRACTION AND WORK FUNCTION CHANGE STUDY ON GROWTH, ALLOYING AND INITIAL OXYGEN-ADSORPTION OF ULTRATHINAL FILMS IN PD(001)

We have investigated the oxygen-induced lifting of the Pd(001)-(2 x 2) p4g-Al clock reconstruction using low-energy He+ ion scattering, law-e nergy electron diffraction and work function change measurements. The structure was prepared by annealing Al films with initial coverages fr om 0.5 ML (monolayers) to larger than 1 ML at similar to 900 K. This s urface was determined to consist of a clock-rotated (001) Pd top layer above a mixed c(2 x 2) Al-Pd underlayer. Adsorption of oxygen on the (2 x 2)p4g surface at room temperature induces Al segregation and lift s the reconstruction to yield (1 x 1) phase. Oxygen removal from the ( 1 x 1) surface by higher-temperature annealing is accompanied by deple tion of Al from the surface, recovering the reconstruction. The drivin g mechanism for the reversible phase transition process is discussed.