METAL-METAL BONDING ON SURFACES - ZN-AU ON RU(001)

The properties of Zn. Au, and Zn-Au films on Ru(001) have been studied using thermal desorption mass spectroscopy (TDS) and core and valence level photoemission. TDS spectra show desorption of Zn from Ru(001) at 450 (multilayer), 490 (second layer), and 720 K (first layer). The monolayer of Zn desorbs following zero-order kinetics with an activation energy for desorption of 36 kcal/mol. Photoemission studies show that the Zn(3s) and Zn(3d) levels of Zn bonded to Ru are shifted 0.5 eV toward lower binding energy with respect to the corresponding levels of pure metallic Zn The direction of this binding-energy shift together with the results of CO-TDS indicate that Zn is withdrawing electrons from the Ru(001) surface. Such charge transfer is in part responsible for the large stability of the Ru-Zn bond. Au desorbs from Ru(001) at temperatures of 1175 (multilayer) and 1260 K (first layer) For a monolayer of Au deposited on Ru(001), the positions of the Au(4f) and Au(5d) levels are very close lo those of the surface atoms of Au(111), indicating that charge transfer between Au and Ru is minimal. A comparison of the desorption temperature for a monolayer of Zn or Au from Ru(001) with values previously reported for the desorption of monolayers of alkali (K and Cs), noble (Cu and Ag), and transition (Mn, Ni, and Pd) metals indicates that the strength of a metal-metal bond in a bimetallic surface depends on (1) the bulk cohesive energy of the individual metals (which gives an indication of the tendency of the element to form strong metal-metal bonds), and (2) the charge transfer within the bond. Zn and Au alloy when coadsorbed on Ru(001). Results for submonolayer coverages of Zn and Au show that the Zn-Ru and Zn-Au bonds in the trimetallic system are more stable than the corresponding bonds in Zn/Ru(001) or Zn-Au alloys. This phenomenon is probably caused by synergistic interactions in three center metal-metal bonding. The formation of ZnAu alloys induces a shift of approximately + 1.0 eV in the Au(4f) and Au(5d) levels, and -0.2 eV in the Zn(3s) and Zn(3d) levels. The shift in the core and valence levels of Au is caused by rehybridization of the Au(5d,6s,6p) levels, while the shift in the Zn levels comes from a contraction in volume.