X-RAY-PHOTOELECTRON-SPECTROSCOPY AND AUGER-ELECTRON-SPECTROSCOPY STUDY OF ULTRATHIN PALLADIUM FILMS ON A PT(111) SUBSTRATE

We have studied ultrathin palladium films vacuum deposited onto a Pt(111) substrate utilizing Auger-electron spectroscopy (AES), low-energy electron diffraction, and x-ray photoelectron spectroscopy. The AES results fit well to a layer-by-layer growth deposition. Below a coverage of 4 monolayers, the electron-diffraction data only show a (1 X 1) structure of palladium adatoms on the Pt(111) substrate, supporting the Frank-van der Merve growth mechanism. In contrast to two-dimensional palladium clusters and palladium bimetallic systems, the Pd 3d core-level binding energy of palladium on Pt(111) shifts toward lower binding energy relative to the value of bulk palladium with decreasing palladium overlayer coverage. This negative binding-energy shift of a surface adatom core level results mainly from the initial-state band-narrowing effect predicted by Citrin, Wertheim, and Baer. Also, the absence of the final-state effect after creating a core hole in the Pd/Pt(111) system indicates that efficient screening or very fast relaxation occurs, and that hybridization of the valence bands of the palladium adlayer and the platinum substrate plays an implortant role in the negative surface-atom binding-energy shift of the Pd 3d core level.