Photoemission study of Au overlayers on Pd(111) and the formation of a Pd-Au(111) alloy surface

Abstract

The techniques of photoemission spectroscopy (PES), Auger spectroscopy, and low-energy electron diffraction were used to examine the interaction of Au and Pd for the following: (1) overlayers of Au on Pd(111), ranging from 0.1 to 10 monolayers, (2) Au diffused into the Pd matrix, and (3) a stabilized Pd-Au alloy surface, formed by diffusing a thick Au layer into the Pd until the composition stabilized. An ordered Au(111) layer formed on the Pd. For the Au overlayers, three effects were seen by PES: (1) structures appeared which merged smoothly into the metallic Au structure, (2) structures at the energies of the clean Pd peaks were sharpened when submonolayers of Au were evaporated, and (3) a new structure appeared which could not be definitely attributed to Pd or Au. Au diffused into the Pd evidenced a more atomiclike structure than the overlayers. The stabilized alloy, which was determined to have 30 at.% Au in the surface region, evidenced PES structure strikingly similar to the 0.3 monolayer overlayer. Direct transition effects were seen for the alloy. The Au-derived structure was found to narrow and shift to slightly lower binding energy for the alloy as compared to the 0.3 monolayer overlayer. Comparison is made between the thin layers of Au on Pd and thin layers of Cu on Zn. Comparison is also made between the stabilized Pd-Au alloy and the Cu-Ni alloy system. Theoretical support for direct transitions in alloys is cited.