s-pHybridization and Electron Shell Structures in Aluminum Clusters: A Photoelectron Spectroscopy Study

Using photoelectron spectroscopy of size-selected Alₓ^- (x0ex0ex=0ex0ex1--162) clusters, we studied the electronic structure evolution of Alₓ and observed that the Al $3s$- and $3p$-derived bands evolve and broaden with cluster size and begin to overlap at Al₉. Direct spectroscopic signatures were obtained for electron shell structures with spherical shell closings at Al₁₁^-, Al₁₃^-, Al₁₉^-, Al₂₃^-, Al₃₅^-, Al₃₇^-, Al₄₆, Al₅₂, Al₅₅^-, Al₅₆, Al₆₆, and Al₇₃^-. The electron shell effect diminishes above Al₇₅ and new spectral features appearing in Alₓ^- $(x>100)$ suggest a possible geometrical packing effect in large clusters.