Another Look at Photoelectron Spectra of the Anion Cr₂O₂^-: Multireference Character and Energetic Degeneracy

Geometrical and electronic structures of the chromium oxide dimer in both charge states Cr₂O₂ ^-/0 were carefully and thoroughly studied using the restricted active space RASSCF/RASPT2 and density matrix renormalization group followed by second-order perturbation theory DMRG-CASPT2 methods in conjunction with large basis sets. The anionic and neutral clusters Cr₂O₂ ^-/0 are characterized by a D_2h structure and high-spin ground states (10 and 9, respectively). RASSCF wave functions point out strong multireference characters of their electronic ground state. The two lowest-lying states ¹⁰A_g and ¹⁰B_2g of the anion Cr₂O₂ ^- are nearly degenerate, and both were proven to be populated and involved in the anion photoelectron spectra of Cr₂O₂ ^-. The neutral cluster exhibits a ⁹B_2g ground state. Within the active space of 20 electrons in 18 orbitals used, 30 electronic transitions starting from both lower-lying anionic states to corresponding neutral states were predicted. Transitions from the anionic ground state ¹⁰A_g cause most of visible bands in the anion photoelectron spectra of Cr₂O₂ ^-, while the first band with low intensity is determined to arise from a transition starting from the nearly degenerate state ¹⁰B_2g. Furthermore, multidimensional Franck-Condon factor simulations were carried out to further support our band assignments.