Photoionization efficiency spectrum and ionization energy of HSO studied by discharge flow-photoionization mass spectrometry

The photoionization efficiency ~PIE! spectrum of HSO was measured in the spectral range ~107– 130! 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, C2H5SH, 2-C3H7SH, or HSC2H4SH, in the flow tube. The ionization energy of HSO was determined for the first time and found to be ~9.91860.016! eV. GAUSSIAN-2 calculations predict 9.897 eV for ionization to HSO, the singlet ground state of the molecular ion, in satisfactory agreement with the experimental result. The onset to triplet HSO may occur at ~11.1560.04! eV. A vibrational frequency of HSO of (11506160) cm was derived from the separation of steps in the PIE spectrum. The heat of formation of HSO was also derived and calculated to be D fH298 + (HSO!5~22865! kcal mol. © 1997 American Institute of Physics. @S0021-9606~97!03423-5#