TRANSITION FROM A MOLECULAR TO A METALLIC ADSORBATE SYSTEM - CORE-HOLE CREATION AND DECAY DYNAMICS FOR CO COORDINATED TO PD

Two alternative methods to experimentally monitor the development of a CO-adsorption system that gradually changes from molecular to metalli c are presented: firstly by adsorption of CO on Pd islands of increasi ng size deposited under UHV conditions, and secondly by growth of a Pd carbonyl-like species, formed by Pd deposition in CO atmosphere. The change in screening dynamics as a function of the number of metal atom s was investigated, using x-ray photoelectron spectroscopy, x-ray abso rption spectroscopy, and core-hole-decay techniques. For CO adsorbed o n UHV-deposited islands, the electronic properties of the whole CO-Pd complex is strongly dependent on island size and CO coverage: large am ounts of CO result in a reduced screening ability, and small effects c haracteristic of molecular systems can be detected even for islands co ntaining about 100 Pd atoms. If about half of the CO overlayer is deso rbed, the CO-Pd complex exhibits a relaxation upon core ionization tha t is nearly as efficient as for metallic systems, even for the smalles t islands (of the order of 10 Pd atoms). The growth of the carbonyl-li ke compound proceeds via formation of Pd-Pd bonds and has a relatively well-defined local structure. It is demonstrated that the properties of this compound approach those of an extended system for increasing c overages, and it may therefore also serve as an important link between a carbonyl and CO adsorbed on a metallic surface. A brief discussion is also given in which the results are discussed in terms of electroni c properties of the thin alumina film versus bulk alumina and the appl icability of the forcer to the construction of model catalysts.