  Far-infrared vibrational spectroscopy by multiple photon dissociation has
proven to be a very useful technique for the structural fingerprinting of small
metal clusters. Contrary to previous studies on cationic V, Nb and Ta clusters,
measured vibrational spectra of small cationic cobalt clusters show a strong
dependence on the number of adsorbed Ar probe atoms, which increases with
decreasing cluster size. Focusing on the series Co_4^+ to Co_8^+ we therefore
use density-functional theory to analyze the nature of the Ar-Co_n^+ bond and
its role for the vibrational spectra. In a first step, energetically low-lying
isomer structures are identified through first-principles basin-hopping
sampling runs and their vibrational spectra computed for a varying number of
adsorbed Ar atoms. A comparison of these fingerprints with the experimental
data enables in some cases a unique assignment of the cluster structure.
Independent of the specific low-lying isomer, we obtain a pronounced increase
of the Ar binding energy for the smallest cluster sizes, which correlates
nicely with the observed increased influence of the Ar probe atoms on the IR
spectra. Further analysis of the electronic structure motivates a simple
electrostatic picture that not only explains this binding energy trend, but
also why the influence of the rare-gas atom is much stronger than in the
previously studied systems.
