Vacuum-ultraviolet negative photoion spectroscopy of SF5Cl

With use of vacuum-UV radiation from a synchrotron, gas-phase negative ions are detected by mass spectrometry following photoexcitation of SF(5)Cl. 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 nu --> 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(5)Cl. 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(5)Cl onto an ion-pair continuum.