Photoionization Efficiency Spectrum and Ionization Energy of HSSH Produced from Gaseous Self-Reaction of HS Radicals

The spectral dependence of the photoionization efficiency of HSSH was obtained over the wavelength range 110−140 nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. The HSSH was generated in the flow tube at room temperature via the sequential reactions: O + H 2 S → OH + HS; HS + HS (+ M) → HSSH (+ M), M being any third body. The ionization energy of HSSH was determined to be 9.40 ± 0.02 eV. This result agrees satisfactorily with the reported value measured from photoelectron spectra. The heat of formation of the ion HSSH +, Δ f H ° 298 (HSSH + ), was calculated to be 220.6 ± 2 kcal mol -1 . The generation of HSSH from the self-reaction of HS radicals might indicate a previously neglected role in the atmospheric sulfur cycle.