The He i photoelectron spectra of CO2, CS2, and OCS: Vibronic coupling

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Abstract
High resolution He i photoelectron (PE) spectra of CO2, CS2, and OCS have been recorded and re‐examined. Resolved spin‐orbit splittings in the first doubly degenerate electronic states of CO+2, CS+2, and OCS+ are 160, 440, and 350 cm−1, respectively, and are in good agreement with the results obtained from emission spectra analysis. The fine structure analyses of the four lower systems reveal that nontotally symmetric modes are active upon ionization in addition to the totally symmetric ones. This indicates that the Born–Oppenheimer approximation is not valid in these cases, i.e., that nonvanishing interactions between and within electronic states take place. One quantum of bending mode couples X̃ (2Πig) with B̃ (2Σ+u), à (2Πiu) with C̃ (2 Σ+g) in cases of CO+2 and CS+2, and 2Πi with 2Σ+ states in OCS+. The à (2Πi) and B̃(2Σ+) states of OCS+ deserve special attention since considerable mixing of states is expected in the overlapping region.