Interactions of ultrathin Pb films with Ru(0001) and Pd(111)

The interaction of ultrathin Pb films with Ru(0001) and Pd(111) has been studied with x-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). XPS and TDS show that at room temperature Pb grows on Ru(0001) via a Stranski-Krastanov mechanism, i.e., after completion of the first adlayer of Pb on Ru(0001), further Pb deposition leads to the formation of three-dimensional islands. Upon annealing to 500 K, the overlayer Pb undergoes significant clustering with the extent dictated by the initial coverages. XPS results show that the Pb 4f(7/2) core level binding energy (BE) increases slightly (+0.3 eV) as the Pb coverage increases from 0.07 ML to multilayers. When the first monolayer of Pb/Ru(0001) has been completed, the Pb 4f7/2 BE has nearly reached the BE value for bulk Pb; it is only 0.1 eV lower. At similar to 2 ML, the Pb 4f(7/2) BE is equivalent to that of bulk Pb. In contrast, at room temperature, Pb alloys with the Pd(111) surface, the Pd 3 d(5/2) BE increasing similar to0.8 eV with coverage from 0.07- to 4.3-ML Pb. The Pb 4f(7/2) BEs also increase with increasing coverage reaching the bulk value of Pb at a coverage of similar to2 ML. The Pb 4f(7/2) BE at 0.07 ML is 0.55 eV smaller than that of bulk Pb. The lineshape and position of the x-ray induced Pd Auger features (M4.5VV) also display significant changes upon alloying. For Pb/Pd alloys, annealing to 500 K does not induce further changes in the Pd (M4,5VV) Auger features. The core level BE shifts are discussed in terms of possible initial and final state contributions. In particular, the initial state BE shift mechanisms of hybridization and environmental conduction band charge density are explained in the context of prior theoretical studies. It is shown that 6s--> 6p hybridization in Pb can be expected to be sufficiently large to contribute to the observed BE shifts.