Photoelectron spectroscopy of size-selected transition metal clusters: Fe−<i>n</i>, <i>n</i>=3–24

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 (n=3–24) 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−3–8 show large size dependence with many resolved features. The spectra for Fe−9–15 exhibit some similarity with each other, all with a rather sharp feature near the threshold. An abrupt spectral change occurs at Fe−16, then again at Fe−19 and Fe−23. These photoelectron spectral changes coincide remarkably with changes of the cluster reactivity with H2. Extended Hückel molecular orbital (EHMO) calculations are performed for all the clusters to aid the spectral interpretations. The calculations yield surprisingly good agreement with the experiment for clusters beyond Fe9 when body-centered cubic (bcc) structures are assumed for Fe9–15 and a similarly close-packed structure with a bcc Fe15 core for the larger clusters. The EHMO calculations allow a systematic interpretation of the sharp photoelectron spectral features in Fe−9–15 and reproduced the abrupt spectral change taking place from Fe−15 to Fe−16. Most importantly, the reactivity changes of the clusters with H2 are successfully explained based on the detailed electronic structures of the clusters, as revealed from the photoelectron spectroscopy (PES) 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.