Photoelectron spectroscopy of sodium clusters: Direct observation of the electronic shell structure

Abstract

Size selected, positively charged sodium clusters $({\mathrm{Na}}_{n}^{+},$ $n=31$--500) have been studied by photoelectron spectroscopy using photons of 6.42 eV. The spectra clearly exhibit the expected shell-like electronic density of states that in this size range up to now has only been observed indirectly. The overall agreement of the measured structure with jellium model predictions is excellent. One prominent exception is ${\mathrm{Na}}_{55}^{+}$ for which jellium models predict a prolate shape, while here strong evidence for an icosahedral structure has been found.