Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. II. OCS --> S - +CO + , O - +CS + , and CO2 --> O - +CO +

Ion-pair formation from photoexcitation of OCS and CO₂ has been studied by negative-ion mass spectrometry using synchrotron radiation in the 15-35 eV photon energy range. Negative ions S^- and O^- from OCS and O^- from CO₂ have been observed. The lowest onset energy in the photodissociation efficiency curve for each ion is in good agreement with the thermochemical threshold for the formation of the negative ion in the ground ²P_u state and its counterpart positive ion in the ground ²Σ⁺ state. There exist series of peaks with medium intensities in the efficiency curves of S^- from OCS and O^- from CO₂; they are identified as resulting from predissociation of the Rydberg states converging to OCS⁺(B∼ ²Σ⁺) and CO⁺₂(C∼ ²Σ⁺_g), respectively. Broad peaks are observed at 18.4 eV (∼675 Å) in the efficiency curves for both S^- and O^- produced from OCS. Predissociation of the excited valence state formed by the intravalence 9σ→10σ transition is considered to mainly contribute to these features. In addition, a broadband feature is present in the wavelength range of 400-620 Å in the O^- efficiency curve. The most likely candidate for the corresponding doorway state is the two-electron excited state involving simultaneous 9σ→10σ and 3π→4π transitions. In the case of the O^- efficiency curve from CO₂, two maxima observed at 21.4 (580 Å) and 23.0 eV (538 Å) are explained as resulting from the 3σ_u→5σ_g transition forming an excited valence state which effectively couples to the ion-pair continuum.