On the superhalogen character of Al13 in Al13I-

It is shown that the reaction of aluminum cluster anions with HI in molecular beams leads to the generation of Al$_{n}$I$_{m}^{-}$ clusters. The stability of the clusters, examined by reacting the cluster products with oxygen, led to the observation of a magic cluster corresponding to the composition Al$_{13}$I$^{-.}$. Ab initio calculations indicate that the cluster features a structurally unperturbed Al$_{13}^{-}$ icosahedric core with I occupying an on-top site. An examination of the electronic charge density indicates a region of high charge density on the aluminum vertex opposite from the I atom. As we discuss, this ionically bound magic cluster can be understood by considering that Al$_{13}$ has an electronic structure reminiscent of a halogen atom. Comparisons to polyhalides are shown to provide a sound explanation for our chemical observations. $^{\ast }$Supported by DOE Grant No. DE-FG02-02ER46009