Electronic structure of vanadium cluster anions as studied by photoelectron spectroscopy

Photoelectron spectra of vanadium cluster anions, V^- _n (3≤n≤100), were measured at a photon energy of 3.49 eV (355 nm) by using a magnetic-bottle-type photoelectron spectrometer. The electronic density-of-state profiles and the photoelectron spectra of V^- ₄ and V^- ₃ were calculated by the spin-polarized DV (discrete variational)-Xα method for several plausible geometrical structures. The most plausible structure was determined so that the calculated photoelectron spectrum based on this structure well reproduces the observed one. The calculation showed that V^- ₄ has a square planar geometrical structure and a charge distribution of D_4h symmetry. On the other hand, V^- ₃ was found to possess an equilateral triangle geometrical structure, but C_2v symmetry for the charge distribution. The electronic states in the vicinity of the Fermi energy were found to consist of 3d atomic orbitals. The population differences between the minority and the majority spins per atom turned out to be 4.6 and 1.7 for V^- ₄ and V^- ₃, respectively. The result implies that the d-electron spins are coupled ferromagnetically in these cluster anions. In addition, the size-dependence of the electron affinity of V_n was explained by a spherical conducting droplet model in the n≳9 range.