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(1 11) substrate utilizing Auger-electron spectroscopy (AES), low-energy electron diffraction, and x-ray photoelectron spectroscopy. The AES re sults 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) st ructure of palladium adatoms on the Pt(111) substrate, supporting the Frank-van der Merve growth mechanism. In contrast to two-dimensional p alladium clusters and palladium bimetallic systems, the Pd 3d core-lev el binding energy of palladium on Pt(111) shifts toward lower binding energy relative to the value of bulk palladium with decreasing palladi um overlayer coverage. This negative binding-energy shift of a surface adatom core level results mainly from the initial-state band-narrowin g 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) s ystem indicates that efficient screening or very fast relaxation occur s, and that hybridization of the valence bands of the palladium adlaye r and the platinum substrate plays an implortant role in the negative surface-atom binding-energy shift of the Pd 3d core level.