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

Abstract

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.

Planar-to-tubular structural transition in boron clusters: B 20 as the embryo of single-walled boron nanotubes

“…The tubular structure gives rise to 20 strong peripheral B-B bonds, more than any planar isomers. Furthermore, although -bonding in smaller B clusters indeed provides additional stability to the planar structure, our previous work on B 10 to B 15 revealed that in larger clusters the orbitals tend to fragment (localize) in different parts of the 2D structures (14), weakening the contributions of the -bonding to the stability of planar isomers and hinting possible 2D-to-3D transitions with increasing cluster sizes. On the other hand, despite the curvature there is still considerable -bonding in the tubular structure, similar to that in fullerenes or carbon nanotubes.…”

“…The experiments were carried out by using a magnetic-bottle time-of-flight PES apparatus equipped with a laser vaporization supersonic cluster source (15,17). B n Ϫ cluster anions were produced by laser vaporization of a disk target made of enriched 10 B isotope (99.75%) in the presence of a helium carrier gas and were analyzed with a time-of-flight mass spectrometer.…”

“…But small boron clusters were predicted to be planar (9)(10)(11), in stark contrast to the bulk three-dimensional (3D) cages. Planar boron clusters have been recently produced in the gas phase and experimentally confirmed up to B 15 (12)(13)(14). However, it is still unclear at what critical size the 2D-to-3D structural transition occurs.…”

“…However, it is still unclear at what critical size the 2D-to-3D structural transition occurs. We show from concerted photoelectron spectroscopy (PES) and global geometry optimization theoretical studies (15)(16)(17) that the transition occurs at the size of 20 atoms. The B 20 neutral cluster is found to overwhelmingly favor a double-ring tubular-type structure over any 2D isomers, whereas in the anion the tubular and several 2D structures are close in energy.…”

Tuning the electronic properties of the golden buckyball by endohedral doping: M@Au16− (M=Ag,Zn,In)

“…15 The MAu 16 − ͑M =Ag,Zn,In͒ clusters were produced by laser vaporization of Au/ M composite disk targets containing about 7% Ag, 8% Zn, and 4% In, respectively. Negatively charged clusters were extracted from the cluster beam and analyzed using a time-of-flight mass spectrometer.…”