Symmetry and electronic structure of noble-metal nanoparticles and the role of relativity

We present high resolution UV-photoelectron spectra of cold mass selected Cun, Agn, and Aun with n = 5358. The observed electron density of states is not the expected simple electron shell structure, but is strongly influenced by electron-lattice interactions. Only Cu55 and Ag55 exhibit highly degenerate states. This is a direct consequence of their icosahedral symmetry, as is confirmed by den sity functional theory calculations. Neighboring sizes exhibit perturbed electronic structures, as they are formed by removal or addition of atoms to the icosa hedron and therefore have lower symmetries. Gold clusters in the same size range show completely different spectra with almost no degeneracy, which indicates that they have structures of much lower symmetry. This behavior is related to strong relativistic bonding effects in gold, as demonstrated by ab initio calculations for Au55.