Photoionization spectra and ionization thresholds of CH₃SO, CH₃SOH, and CH₃SS(O)CH₃

Photoionization efficiency (PIE) spectra of CH₃SO, CH₃SOH, and CH₃SS(O)CH₃ were measured in the spectral range 110-150 nm by means of a discharge flow system coupled to a photoionization mass spectrometer that employs a synchrotron radiation source. These species were generated by reacting O atoms with CH₃SSCH₃ in the flow tube; a microwave discharge was employed to produce O atoms from O₂. Alternatively, C₂H₅SCH₃ replaced CH₃SSCH₃ to form CH₃SO, CH₃SOH, and other products. Ionization thresholds of 8.99 ± 0.02, 8.67 ± 0.03, and 8.82 ± 0.05 eV were determined for the first time for CH₃SO, CH₃SOH, and CH₃SS(O)CH₃, respectively. The former two values agree well with theoretical calculations with the G2 method. The latter value fits satisfactorily with those predicted for CH₃SS(O)CH₃ with the G2MP2 method; the G2 method was not employed in this case due to the numerous atoms in this molecule. lonization thresholds of CH₃S and CH₃S₂ were also determined to be 9.27 ± 0.03 and 8.62 ± 0.05 eV, respectively, in agreement with recent experimental reports.