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

Using photoelectron spectroscopy of size-selected ${\mathrm{Al}}_x^{-}$ $(x=1\mathrm{}–162)$ clusters, we studied the electronic structure evolution of ${\mathrm{Al}}_x$ and observed that the Al $3s$- and $3p$-derived bands evolve and broaden with cluster size and begin to overlap at ${\mathrm{Al}}_9$. Direct spectroscopic signatures were obtained for electron shell structures with spherical shell closings at ${\mathrm{Al}}_{11}^{-}$, ${\mathrm{Al}}_{13}^{-}$, ${\mathrm{Al}}_{19}^{-}$, ${\mathrm{Al}}_{23}^{-}$, ${\mathrm{Al}}_{35}^{-}$, ${\mathrm{Al}}_{37}^{-}$, ${\mathrm{Al}}_{46}$, ${\mathrm{Al}}_{52}$, ${\mathrm{Al}}_{55}^{-}$, ${\mathrm{Al}}_{56}$, ${\mathrm{Al}}_{66}$, and ${\mathrm{Al}}_{73}^{-}$. The electron shell effect diminishes above ${\mathrm{Al}}_{75}$ and new spectral features appearing in ${\mathrm{Al}}_x^{-}$ $(x>\mathrm{}100\mathrm{})$ suggest a possible geometrical packing effect in large clusters.