Spin-polarized electronic structure of cobalt cluster anions studied by photoelectron spectroscopy

The photoelectron spectra of Co_n ^- (3≤n≤70) were measured at the photon energy of 4.025 eV by use of a XeCl excimer laser. For Co_n ^- with n= 3, 4, and 6, the geometric and electronic structures were obtained from the spectra in comparison with the calculated spectra by the spin-polarized DV-Xα method. The spectra observed are reproduced reasonably well by the calculation with postulating the most probable geometrical structures. It is revealed that the 3d band with the majority spin is separated by 1.0-2.8 eV from that with the minority spin; the former is completely filled while the latter is partly filled and extends above Fermi level. The magnetic moments and the average exchange energies of these cluster anions were estimated. For Co_n ^- with n≥7, the observed electron affinity depends linearly on the reciprocal of the cluster radius and approach the work function of a cobalt metal, as n increases. Below n = 6, the electron affinity deviates from the linear dependence. This finding indicates that a size-dependent transition in the electronic structure occurs at n≃-7. The spherical conducting drop model suggests the presence of mobile electrons in Co_n ^- with n≥7.