OPTICAL-ABSORPTION SPECTRA OF AU-7, AU-9, AU-11 AND AU-13 AND THEIR CATIONS - GOLD CLUSTERS WITH 6, 7, 8, 9, 10, 11, 12, AND 13 S-ELECTRONS

The optical absorption spectra of a series of small gold clusters and their cations have been measured, between 1.9 and 5.6 eV, using a method based upon the photodepletion of a molecular beam of their van der Waals complexes containing one and two xenon atoms. This method provides size-specific information even though the molecular beam contains a wide range of cluster sizes. There is little difference between the spectra of complexes containing one or two xenon atoms. However there is a pronounced odd-even alternation in the spectra of gold clusters with differing numbers of valence s electrons. This alternation is described in terms of a simple electron pairing scheme. The spectrum for Au-13 is in reasonable agreement with Dirac scattered-wave molecular orbital considerations for icosahedral Au-13 [A.F. Ramos, R. Arratia-Perez, and G. L. Malli, Phys. Rev. B 35, 3790 (1987)]. This description of the molecular and electronic structure of small gold clusters in terms of localized molecular orbitals is contrasted with other models based upon jellium potentials and delocalized excitations that have been used to describe small clusters of alkali metals and silver. The bonding in gold clusters is influenced by relativistic effects that increase the degree of sd hybridization in the molecular orbitals. Even though gold clusters can be described in this way, some evidence for electron shells is also presented. Thus, it is concluded that structural motifs other than jellium potentials can lead to shell structure in cluster properties.