High-Resolution Spectroscopy and Theoretical Study of C ₂ O ₂ ⁺ : Unveiling the 1 ² A _g –X ² B _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 12Ag–X̃2Bu 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.