Theoretical investigation on the valence ionization spectra of $Cl2O,$ ClOOCl, and $F2O$ by correlation-based configuration interaction methods

We report on theoretical valence ionization spectra of molecules relevant in the stratosphere photochemistry obtained by all-electron SAC–CI (symmetry-adapted-cluster–configuration-interaction) calculations. Vertical ionization energies and pole strengths of the one- and two-electron processes of $Cl2O$ and $F2O$ were calculated beyond the energy region so far explored in the few other available theoretical and experimental studies to interpret the electronic structure of these molecules. Early and recently proposed incomplete assignments of the available He I photoelectron spectra are discussed and completed at least up to 20–21 eV binding energy on firmer grounds relying on valuable and accurate results based on different basis sets and an adequate treatment of electron correlations. Our theoretical data predict satellite states of $Cl2O$ starting already in the outer-valence region because of strong correlation effects; the $(2b1−1),$ $(8a1−1),$ and $(6b2−1)$ states interact with the two-electron processes and they split into more-than-two peaks. On the other hand, Koopmans’ picture is valid for the main peaks of $F2O$ and no prominent satellites with strong intensity were found in the outer-valence region. The $(4b2−1),$ $(6a1−1),$ and $(1a2−1)$ were attributed to the second band of $F2O,$ for which different assignments or orderings of the states have previously been proposed in some experimental and theoretical works. Differences of the valence-ionization spectra of $Cl2O$ and $F2O$ for the appearance of the satellites in the intermediate energy region have been discussed with the aid of the calculated ionization potentials and excitation energies. For ClOOCl, we have presented the first theoretical low-energy ionization spectrum and discussed the character of the calculated states referring to the available ionization spectra of ClO radical.