A higher resolution magnetic bottle photoelectron spectrometer for the study of the electronic structure of size-selected metal clusters is presented. The initial study on Fe n Ϫ ͑nϭ3-24͒ is reported at a photon energy of 3.49 eV. The photoelectron spectra of these clusters exhibit sharp features throughout the size range. The spectra for Fe 3-8Ϫ show large size dependence with many resolved features. The spectra for Fe 9-15 Ϫ exhibit some similarity with each other, all with a rather sharp feature near the threshold. An abrupt spectral change occurs at Fe 16 Ϫ , then again at Fe 19 Ϫ and Fe 23 Ϫ . These photoelectron spectral changes coincide remarkably with changes of the cluster reactivity with H 2 . Extended Hückel molecular orbital ͑EHMO͒ calculations are performed for all the clusters to aid the spectral interpretations. The calculations yield surprisingly good agreement with the experiment for clusters beyond Fe 9 when body-centered cubic ͑bcc͒ structures are assumed for Fe 9-15 and a similarly close-packed structure with a bcc Fe 15 core for the larger clusters. The EHMO calculations allow a systematic interpretation of the sharp photoelectron spectral features in Fe 9-15 Ϫ and reproduced the abrupt spectral change taking place from Fe 15Ϫ to Fe 16 Ϫ . Most importantly, the reactivity changes of the clusters with H 2 are successfully explained based on the detailed electronic structures of the clusters, as revealed from the photoelectron spectroscopy ͑PES͒ spectra and the theoretical calculations. The calculations also correctly predict the existence of magnetism in these clusters and yield reasonable values for the cluster magnetic moments.

We investigated the electronic structure and chemical bonding of two bimetallic clusters NaGa4- and NaIn4-. Photoelectron spectra of the anions were obtained and compared with ab initio calculations. We found that the ground state of the two anions contains a square planar dianion interacting with a Na+ cation. The Ga4(2-) and In4(2-) dianions both possess two delocalized pi electrons and are considered to be aromatic, similar to that recently found in Al4(2-). Using calculations for a model compound, we showed that a recently synthesized Ga4-organometallic compound also contains an aromatic -Ga4(2-)- unit, analogous to the gaseous clusters.