Photodissociation spectra of cobalt cluster ion-argon atom complexes, Con+Ar (n = 3-5), were measured by detecting the product ions, Con+ and Con-1+, with a tunable laser pulse from an optical parametric oscillator in the photon-energy range from 0.7 to 2.8 eV. The photodissociation spectra thus obtained are equivalent to the optical absorption spectra of the underlying cobalt cluster ions, Con+, because the argon atom is weakly bound to Con+. The spectrum was analyzed by means of a spin-polarized DV-Xα calculation, and the electronic and the geometric structures of Co3+ and Co4+ were determined. The analysis shows that all the transitions in the entire energy range studied occur between occupied and unoccupied energy levels associated with 3d atomic orbitals (AOs) having the minority spin. The spin difference (difference in population per Co atom between the majority and the minority spins) was evaluated from the electronic structures thus obtained. The spin differences of 2.00 for Co3+ and 1.75 for Co4+ indicate ferromagnetic spin coupling in these cobalt cluster ions.
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