Photoemission from small palladium clusters supported on various substrates.

Abstract

The core-electron binding energy obtained for small Pd clusters supported on various substrates is greater than that obtained for bulk Pd metal. The shifts of the core-electron binding energy and the core-valence-valence Auger-electron kinetic energy for small Pd clusters on the conductive amorphous carbon substrate are in good agreement with those calculated by the thermodynamic model using Miedema's semiempirical theory. Both experimentally and theoretically, the positive shift of the Pd core-electron binding energy with decreasing coverage is shown to be due to the photoemission initial-state effect. The shifts of the Pd core-electron binding energy with the coverage for small clusters on the semiconductive InSb and InP substrates are primarily due to the initial-state effect. The ratio of the photoemission initial-state-effect change to the photoemission final-state-effect change decreases with an increase of the polarizability of the substrate. The photoemission final-state effect predominantly arises from the positive shift of the Pd core-electron binding energy with decreasing coverage on the insulating ${\mathrm{SiO}}_{2}$ and ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ substrates. The changes in the terms of the extra-atomic relaxation energy for the Pd core hole and the potential energy of the Pd core electron differ for each substrate. The change in the extra-atomic relaxation energy for the Pd core hole varies with the change of the polarizability of the substrate. The change in the potential energy of the Pd core electron correlates with the difference in electronegativities of the substrate components.