Combined ion scattering, electron diffraction and work function change study on growth, alloying and initial oxygen adsorption of ultrathin Al films in Pd(001)

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