A study of the low-lying electronic states of Fe₂ and Co ₂ by negative ion photoelectron spectroscopy

The anions Fe₂^- and Co₂^- were prepared and cooled to 300 K in a flowing afterglow ion source, and the low-lying electronic states of the neutral dimers were probed by negative ion photoelectron spectroscopy. Previous ab initio studies of Fe₂ and Co₂ have predicted single 4s-4s bonds, and extremely high densities of low-lying states due to the small energy cost in transferring electrons among nonbonding 3d orbitals. In contrast to the complex photoelectron spectra implied by these calculations, the observed spectra are remarkably simple. It is argued that this spectral simplicity implies a greater role for the 3d electrons in the iron and cobalt dimer bonds. These data also provide values for the electron affinities of the neutral dimers (0.902 ± 0.008 eV Fe2, 1.110 ± 0.008 eV Co₂), the bond elongation on electron attachment (0.08 ± 0.02 Å Fe₂, Co₂), and the vibrational frequencies of the anions (250 ± 20 cm^-1 Fe ₂^-, 240 ± 15 cm^-1 Co₂ ^-). Related studies of the atomic anions yield improved values for the electron affinities of Fe (0.151 ± 0.003 eV) and Co (0.662 ± 0.003 eV).