Chapter 8: Clusters of the Transition Metals

8.1.1 Electronic Shell Effects

The free atoms of the transition metals series have an incomplete d shell in the ground state or in low lying excited states. The d electrons are responsible for many properties of these elements as free atoms or in the bulk phase. In the same way, most properties of clusters of the transition elements reflect the rather localized behavior of the d electrons.

Cu, Ag and Au occupy a special place at the end of the 3d, 4 d and 5 d periods respectively. The d shell is filled with 10 electrons and the valence shell contains a single s electron. The d band is well buried below the Fermi level in the bulk metal and this leads one to expect some similarities between the electronic structure of noble metal clusters and the alkali metal clusters. Experiments for noble metal clusters [1, 2] indicate the existence of shell effects similar to those observed in alkali clusters. For instance, the mass spectrum of cationic clusters obtained by bombarding the metal with inert gas ions shows two types of anomalies. The first one, observed for small clusters, is an odd-even alternation of the abundance, such that the population of odd- N clusters is higher than the population of even- N clusters. The other anomaly is that a steep drop of the cluster intensities occurs after the sizes N = 3, 9, 21,...