High-Resolution Spectroscopy and Theoretical Study of C₂O₂⁺: Unveiling the 1²A_g-X̃²B_u Electronic Transition

The vibronic spectrum of mass-selected C₂O₂⁺ in the 293-310 nm region has been recorded using photodissociation spectroscopy in combination with a cryogenic ion trap. We performed linearized pair-density functional theory (L-PDFT) calculations on potential energy surfaces of six lowest-lying electronic states to elucidate this spectrum. The results indicate that the six low-lying states originate from splitting of three degenerate ²Π states due to the Renner-Teller effect. Combined with the theoretical excitation energies, the spectrum corresponds to the dipole-allowed 1²A_g-X̃²B_u electronic transition. Calculations on the Franck-Condon factors and vibrational anharmonicity have yielded detailed assignments of the vibronic bands, revealing significant Fermi resonance between ν₃ and 2ν₆ vibronic levels. Partially resolved rotational features in the 0₀⁰ band have also been analyzed to determine the accurate electronic transition energy. Bond order and spin density have been obtained by the analysis of multiconfigurational wave functions, providing a more in-depth understanding of the electronic structure and bonding characteristics of C₂O₂⁺. This work highlights L-PDFT as a highly effective method for studying the excited states of open-shell molecules with strong electronic correlations.