THE ADSORPTION OF AL ON PD(001) AND OF O AND CO ON AN INTERFACIAL AL-ALLOYED PD(001) SURFACE

The adsorption of ultrathin Al films on Pd(001) and adsorption of oxyg en and carbon monoxide on an interfacial Al-alloyed Pd(001) surface ha ve been examined via low energy He+ ion scattering, tow energy electro n diffraction and work function change (Delta Phi) measurements. For l ow Al coverages at 325 K, interaction between Al and the Pd substrate is observed, resulting in the loss of near surface order by an Al cove rage of 0.5 monolayer (ML). The adsorption of Al results in a work fun ction decrease. Annealing Al films with initial coverages from 0.5 ML to larger than 1 ML at 750-950 K leads to interfacial alloying and a s table (2 x 2)p4g LEED pattern dominates the surface structure. This st ructure has been determined to be a clock rotated (001) Pd top layer a bove a mixed c(2 x 2) Al-Pd underlayer. Adsorption of oxygen on the (2 X 2)p4g surface at 325 K induces Al segregation and lifts the reconst ruction. In contrast, the interaction of CO with the (2 X 2)p4g surfac e dose not lift the reconstruction. The implications of these results for relative stability of Al-O bond strength are discussed. (C) 1998 E lsevier Science B.V.