ALUMINUM-INDUCED SURFACE CLOCK RECONSTRUCTION OF PD(001) AND THE EFFECT OF OXYGEN-ADSORPTION

The structure of the Pd(001)-(2x2)p4g-Al surface has been investigated using low-energy ion scattering, low-energy electron diffraction, and three-dimensional classical scattering simulations. The thermal treat ment of the Al films (theta(Al)greater than or equal to 0.5 ML) trigge red the Al diffusion and reaction to form a clock rotated (001) Pd lay er above an ordered c(2x2) Al-Pd underlayer, with a stoichiometry of t he top two layers independent of the initial Al coverage. By using a r eliability R-factor analysis to compare the experimental and simulated azimuthal scans, the lateral clockwise-counterclockwise displacement of the surface Pd atoms was determined to be Delta x = 0.5 +/- 0.1 Ang strom. The driving force for this clock reconstruction is proposed to be the Al-induced interfacial strain. We have also studied the oxygen- induced lifting of the Pd(001)-(2x2)p4g-Al reconstruction. Adsorption of oxygen on the (2x2)p4g surface at room temperature induces Al segre gation and lifts the reconstruction to yield the (1x1) phase. Oxygen r emoval from the (1x1) surface by higher temperature annealing (similar to 900 K) was accompanied by depletion of Al from the surface, recove ring the reconstruction. The mechanism of a reversible conversion, (2x 2)p4g-->(1x1) by O adsorption and (1x1)-->(2x2)p4g by anneal, is discu ssed.