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 treatment of the Al films (theta(Al)greater than or equal to 0.5 ML) triggered the Al diffusion and reaction to form a clock rotated (001) Pd layer above an ordered c(2x2) Al-Pd underlayer, with a stoichiometry of the top two layers independent of the initial Al coverage. By using a reliability 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 Angstrom. 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 segregation and lifts the reconstruction to yield the (1x1) phase. Oxygen removal from the (1x1) surface by higher temperature annealing (similar to 900 K) was accompanied by depletion of Al from the surface, recovering the reconstruction. The mechanism of a reversible conversion, (2x2)p4g-->(1x1) by O adsorption and (1x1)-->(2x2)p4g by anneal, is discussed.