The photoionization efficiency ͑PIE͒ spectrum of HSO was measured in the spectral range ͑107-130͒ nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. HSO radicals were generated by reacting O atoms with various organothiol compounds, C 2 H 5 SH, 2-C 3 H 7 SH, or HSC 2 H 4 SH, in the flow tube. The ionization energy of HSO was determined for the first time and found to be ͑9.918Ϯ0.016͒ eV. GAUSSIAN-2 calculations predict 9.897

more »

... eV for ionization to HSO ϩ , the singlet ground state of the molecular ion, in satisfactory agreement with the experimental result. The onset to triplet HSO ϩ may occur at ͑11.15Ϯ0.04͒ eV. A vibrational frequency of HSO ϩ of (1150Ϯ160) cm Ϫ1 was derived from the separation of steps in the PIE spectrum. The heat of formation of HSO ϩ was also derived and calculated to be ⌬ f H 298 ‫ؠ‬ (HSO ϩ ͒ϭ͑228Ϯ5͒ kcal mol Ϫ1 .