With use of vacuum-UV radiation from a synchrotron, gas-phase negative ions are detected by mass spectrometry following photoexcitation of SF _5Cl. F~-, Cl~-, and SF_5~- are observed, and their ion yields recorded in the range 8-30 eV. F~- and Cl~- show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically written AB + h → C~- + D~+ (+ neutral(s)). F~- is the strongest signal, and absolute cross sections are determined by calibrating the signal intensity with that of F~- from SF_6 and CF _4. Resonances are observed and assigned to transitions to Rydberg states of SF_5Cl. The Cl~- signal is much weaker, despite the S-Cl bond being significantly weaker than the S-F bond. Appearance energies for F~- and Cl~- of 12.7 ± 0.2 and 10.6 ± 0.2 eV are determined. The spectra suggest that these ions form indirectly by crossing of Rydberg states of SF_5Cl onto an ion-pair continuum.
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