ABSTRACT
In early studies of small metal particles, their shapes were assumed to be irregular. These irregularities caused the electronic levels to be nondegenerate and nonuniformly distributed. The theoretical models used random-matrix theory for predicting properties of these objects. In this approach, no systematic variation of the cluster properties with their size was expected. Cini [1] and Martins et al. [2] made the rather simplifying assumption that metal clusters have spherical symmetry. The orbital degeneracy resulting as a consequence of spherical symmetry led to the prediction that small metal clusters have breaks in their ionization potential (IP) and electron afnity (EA) at certain sizes. Orbital degeneracy produces systematic sizedependent variation in cluster properties as successive bunches of degenerate levels, called shells, are lled. It is interesting to note Martins et al.’s [2, p. 267] comment, “This effect is certainly non-physical” in their paper, as real clusters need not be spherical and hence there would be no orbital degeneracy. In another similar study in 1984, Ekardt [3] reached the same conclusion that IP’s of metal clusters drop every time a new shell starts getting occupied. Till that time there was no experimental evidence in support of these predictions. A landmark experiment by Knight et al. [4] in 1984, indeed, proved that the assumptions of spherical symmetry and consequent
8.1 Introduction .................................................................................................. 137 8.2 Magic Numbers and Shell Model ................................................................. 138 8.3 Ionization Potential and Electron Afnity: Relation to Shell Model ........... 143 8.4 Transition Metal Clusters: Structure and Magnetism ................................... 145 8.5 Transition Metal-Doped Bimetallic Clusters: Interplay of
Shell Effect and Magnetism.......................................................................... 150 8.5.1 Transition Metal-Doped Noble Metal Clusters................................. 151 8.5.2 Transition Metal-Doped Alkali Metal Clusters ................................ 153
8.6 Conclusions ................................................................................................... 157 References .............................................................................................................. 158
orbital degeneracy are valid to a large extent for simple metal clusters. They measured the abundance spectrum of NaN clusters through mass spectrometry in the size range N < 100. The striking feature of this spectrum was sharp drops in abundance at denite sizes. These observations, and the accompanying theoretical analysis dramatically changed the way simple metal clusters are viewed, and their properties are understood. These rmly established the validity of the so-called electronic shell models for these clusters.
