Magnetic spin and orbital moments of size-selected free iron cluster ions Fe{n}{+} (n=3-20) have been determined via x-ray magnetic circular dichroism spectroscopy. Iron atoms within the clusters exhibit ferromagnetic coupling except for Fe{13}{+}, where the central atom is coupled antiferromagnetically to the atoms in the surrounding shell. Even in very small clusters, the orbital magnetic moment is strongly quenched and reduced to 5%-25% of its atomic value while the spin magnetic moment remains at 60%-90%. This demonstrates that the formation of bonds quenches orbital angular momenta in homonuclear iron clusters already for coordination numbers much smaller than those of the bulk.
Size-selected cationic transition-metal-doped silicon clusters have been studied with x-ray absorption spectroscopy at the transition-metal L 2,3 edges to investigate the local electronic structure of the dopant atoms. For VSi 16 + , the x-ray absorption spectrum is dominated by sharp transitions which directly reveal the formation of a highly symmetric silicon cage around the vanadium atom. In spite of their different number of valence electrons, a nearly identical local electronic structure is found for the dopant atoms in TiSi 16 + , VSi 16 + , and CrSi 16 +. This indicates strongly interlinked electronic and geometric properties: while the transition-metal atom imposes a geometric rearrangement on the silicon cluster, the interaction with the highly symmetric silicon cage determines the local electronic structure of the transition-metal dopant.
An experiment was designed to perform x-ray and VUV spectroscopy on size-selected clusters in the gas phase. Using a radio frequency ion trap and a quadrupole mass filter combined with an intense magnetron sputter source made it possible to record x-ray absorption spectra of mass-selected clusters in ion yield mode. These measurements clearly reveal the development from richly structured atomic spectra to bulk-like line shapes in transition metal clusters.
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