A STRUCTURAL STUDY OF PD CU(100) SURFACE ALLOYS/

The structures formed by one-half and one monolayer (ML) of Pd evapora ted onto Cu(100) at 300 K were studied by low energy electron diffract ion (LEED), medium energy ion scattering (MEIS), thermal desorption sp ectroscopy (TDS), and embedded atom method (EAM) calculations. In the half monolayer case, the LEED I(E) curves are consistent with the esta blished c(2 x 2) surface alloy model. The MEIS data, however, suggest that a fraction of the Pd (similar to 1/4) is in ''second layer'' site s, in agreement with previous LEIS, TDS and XPS forward scattering mea surements. The EAM simulations support the formation of alloy islands, providing a mechanism for the covering of some Pd atoms. As the depos ition proceeds, however, this island formation is indicated to occur p referentially over clean copper. In the one monolayer case, a p(2 x 2) -p4g LEED pattern is observed. Analysis of the I(E) curves suggests th at this arises from (100) Pd packed above the c(2 x 2) alloy. EAM calc ulations confirm the stability of this model. Evidence from MEIS and T DS, however, shows that the one monolayer surface as prepared in this work is inhomogeneous. c(2 x 2) and Cu rich surface domains exist in a ddition to those having the p4g Pd/c(2 x 2)PdCu structure.