The strong interplay between geometry and electronic structure in clusters consisting of a countable number of atoms critically governs their properties, making precise knowledge of geometry essential. This challenge is particularly acute for transition-metal clusters, where many unpaired electrons lead to multiple low-lying spin and geometric isomers, implying cumbersome and contradicting theoretical predictions of the ground state. In this Letter, we conclusively assign geometries and spin states of iron clusters of 3 to 12 atoms, which significantly reduces uncertainties of spin magnetic moments inferred from x-ray magnetic circular dichroism experiments [Niemeyer et al. Phys. Rev. Lett. 108, 057201 (2012)]. Hereto we use infrared multiple photon dissociation spectroscopy with Ar as messenger atom to measure the vibrational spectra of cationic iron clusters in the $110–400{\mathrm{cm}}^{-1}$ range and compare those with density functional theory calculations. We show that in addition to direct structural information the spectra provide an accurate determination of spin multiplicities. This methodology is broadly applicable to transition-metal clusters of any charge state and provides a general route to benchmark theory.

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• Atomic & molecular clusters
• Electronic & magnetic properties of clusters
• Magnetism
• Spectroscopy of clusters
• Density functional theory
• Infrared spectroscopy
• Time-of-flight mass spectrometry