Interpretation of photoelectron spectra of CoGen- (n = 4, 5) clusters by multiconfigurational RASPT2 calculations.

The low-lying electronic states CoGen-/0 (n = 4, 5) have been investigated with density functional theory and the state-of-the-art RASSCF/RASPT2 method to give assignments for the anion photoelectron spectra. The BP86 functional was employed to optimize the geometrical structures of the electronic states, while the RASSCF/RASPT2 was applied to calculate the single-point energies. With the RASSCF/RASPT2 approach, the active spaces are extended to a size of 21 orbitals for CoGe4-/0 and 24 orbitals for CoGe5-/0. The ground states of CoGe4-/0 are determined to be 3A″ and 2A″ of a trigonal bipyramidal structure in which the Co atom is situated at the equatorial corner of the bipyramid. The vertical detachment energies of the transitions from the anionic ground state to the neutral 2A″, 14A″, 2A', 24A″, 34A″, 14A', 24A', and 64A″ states are evaluated to be 2.29, 2.39, 2.60, 2.83, 3.17, 3.24, 3.47, and 4.00 eV. For the CoGe5-/0 clusters, the ground states are computed to be 1A1 and 12A2 of an octahedral structure. The vertical detachment energies of the removal of one electron from the anionic ground state to result in the 12A2, 12A1, 22A1, 12B1, 12B2, 42B1, 42B2, and 62A2 states are estimated to be 2.16, 2.79, 2.84, 3.06, 3.06, 3.59, 3.59, and 4.22 eV. All features in the photoelectron spectra of CoGe4- and CoGe5- are interpreted based on the computed electron detachment energies of the anionic ground states.