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

The low-lying electronic states CoGe (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 CoGe and 24 orbitals for CoGe. The ground states of CoGe are determined to be A″ and A″ 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 A″, 1A″, A', 2A″, 3A″, 1A', 2A', and 6A″ 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 CoGe clusters, the ground states are computed to be A and 1A of an octahedral structure. The vertical detachment energies of the removal of one electron from the anionic ground state to result in the 1A, 1A, 2A, 1B, 1B, 4B, 4B, and 6A 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 CoGe and CoGe are interpreted based on the computed electron detachment energies of the anionic ground states.