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Ion-pair formation from photoexcitation of SF6 has been studied by negative-ion mass spectrometry using synchrotron radiation in the 11.27–31.0 eV photon energy range. Negative ions F−, SF−6, and SF−5 have been observed. The appearance energy of the F− ion is about 1 eV higher than the thermochemical threshold for the formation of the pair of the ground state ions F−(1Sg) and SF+5(X̃1A1). The peak features observed in the F− efficiency curve are interpreted as resulting from transitions to neutral excited states with the 1T1u symmetry which effectively couple with ion-pair states through avoided potential surface crossings. The peaks assigned to diffuse Rydberg states are distinctively enhanced in the F− efficiency curve, probably because of large transition probabilities from the dissociative Rydberg states to the ion-pair states. In contrast, the excited states of valence type autoionize in a short period and have quite small branching to the ion-pair channel. Consequently, the corresponding peaks are markedly suppressed in the F− spectrum. Assignments of the peak features in the previous photoabsorption spectra are also performed by using the term values for related Rydberg and virtual valence orbitals. Other negative ions observed, SF−6 and SF−5, are produced by resonance capture of low energy electrons emitted by photoionization of the parent molecules, and are not of major concern of the present study.

Ion-pair formation from photoexcited halomethanes, CH3X*→X−+CH+3 (X=F, Cl, Br) has been studied by measuring photodissociation efficiency curves of X− using synchrotron radiation in the 9.9–27.5 eV photon energy range. A new spectral feature is observed in each of the curves near the threshold for the removal of an na1 electron from CH3X (n=4, 6, and 8, respectively, for CH3F, CH3Cl, and CH3Br ). This feature, composed of two or three peaks in each case, is interpreted as resulting from photoexcitation to the Rydberg states converging to CH3X+(C̃ 2A1), which then predissociate into ion pairs through avoided potential energy surface crossings. The interpretation is based on the results of the inner-shell electron energy loss study by Brion and co-workers and the photoabsorption study by Hochmann and co-workers. Peak features are also observed in the X− efficiency curves near the ionization threshold for CH3X+(X̃ 2E). The origins of these peaks are also discussed.

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 2Pu 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+(B̃ 2Σ+) and CO+2(C̃ 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 3σu→5σg transition forming an excited valence state which effectively couples to the ion-pair continuum.