ESCA and molecular orbital studies of small silver particles

Abstract

The results of x-ray photoelectron spectroscopy studies of evaporated silver clusters on carbon are reported. The samples were prepared and the particle-size distribution was determined. Spectra have been recorded at coverages from 2.7×1013 to 4×1016 atoms cm−2. The very low coverages yield particle distributions dominated by single-atom centers, whereas the high coverages represent bulk metallic silver. Low-coverage spectra show low binding energy states that are thought to result either from interactions with the carbon substrate or reaction with sulfur contamination. CNDO calculations show low binding energy states to exist for Ag4 on a C16 model substrate. At higher coverages the Ag 4d states broaden and split, and the threshold moves 2.5 eV to lower binding energy. The splitting of the Ag 4d band is attributed to spin–orbit and crystal field effects. The change in threshold can be attributed to the filling and spreading of the Ag 5s states to form the s–p conduction band of the bulk metal. The measured threshold shift of 2.5 eV (Fermi-level reference) is to be compared with 1.5 eV (vacuum-level reference) predicted by extended Hückel calculations. Small shifts in the Ag 3d core levels are also observed and follow the trends predicted by CNDO charge calculations.