SHELL STRUCTURE AND S-P HYBRIDIZATION IN SMALL ALUMINUM CLUSTERS

Photoelectron spectra of Al-n(-) (n = 2-20) clusters are presented. Due to the improved energy resolution and the relatively high photon energy (h nu = 6.424 eV, ArF laser radiation) available in our experiment the spectra reveal the mechanism of the onset of the s-p hybridization occuring in these particles. Between Al-6(-)-Al-12(-) a mixing of the uppermost antibonding 3s derived orbital with the 3p band takes place which is observed simultaneously with the 2D-->3D structural transition predicted for Al clusters in this size range. Starting from Al-13(-) a pattern resembling an electronic shell structure is identified.