Vacuum-Ultraviolet Negative Photoion Spectroscopy of SF5Cl

Using 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\(v\) \(\rightarrow\) 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.