AU-SEGREGATED DEALLOYING AND PD-INDUCED CLOCK RECONSTRUCTING OF CU(001)

The structure and growth of ultrathin Au and Pd films on Cu(001) have been studied by low-energy ion scattering (LEIS) and low-energy electr on diffraction (LEED). Due to the existence of a kinetic pathway for i ntermixing, both Au and Pd are incorporated into the Cu(001) surface a t room temperature, forming a c(2 x 2) surface alloy at a coverage of 0.5 monolayer (ML). At Au coverage near 1.2 ML, the surface layer is p ure Au; this is caused by Au-induced segregation from the underlying c (2 x 2) layer to the surface. No second- and/or third-layer Au is evid ent. In contrast to this, for the Pd/Cu(001) case a (2 x 2)p4g clock r econstruction is formed after deposition of 1 ML Pd. The LEIS data, th rough comparisons with 3-D computer simulation, reveal a 0.25 Angstrom lateral clockwise-anticlockwise displacement of the first-layer Pd at oms. A simple model for this clock reconstruction is proposed. The dri ving force behind the dealloying for the Cu-Au alloy and Pd-induced re constructing for the Cu-Pd surface is also discussed.