OBSERVATION OF A LOW-ENERGY ADSORBATE CORE-LEVEL SATELLITE FOR CO BONDED TO PALLADIUM - COORDINATION-DEPENDENT EFFECTS

A strong low-energy shake-up satellite for CO adsorbed on Pd is observ ed. The occurrence of the satellite is established for the CO/1 hit Pd /Mo(110) system at a coverage where CO adsorbs exclusively on-top. Com parisons with CO adsorbed on Pd single-crystal surfaces and small supp orted Pd particles indicate that the strongly increased satellite inte nsity is due to the decreased CO-Pd interaction strength for on-top ad sorbed CO. This can be used to get further insight into the structure and bonding properties of the adsorbate system. Since a low-energy sha ke-up feature may be misinterpreted as a chemically shifted component, the conclusion is that great care has to be taken in the evaluation o f adsorbate core-level spectra for systems with large variations in ad sorption strength depending on the adsorbate sites. Large variations i n the CO site distribution may furthermore occur depending on the natu re of the Pd substrate: Adsorption of CO on 1 ML Pd/Mo(110) leads to a n overlayer dominated by an on-top species and, likewise, the CO overl ayer formed on small Pd particles after large doses has a large fracti on of on-top bonded species. This is in strong contrast to Pd single-c rystal surfaces, where CO adsorbed in more highly coordinated sites is abundant.