Penning ionization of CH₃SCH₃, CH₃SSCH₃, and CH₃CH₂SH by collision with He*(2³S) metastable atoms

Penning ionization of CH₃SCH₃ (dimethyl thioether), CH₃SSCH₃ (dimethyl disulfide), and CH₃CH₂SH (ethyl thioalcohol) upon collision with He*(2³S) metastable atoms was studied by two-dimensional (electron-energy/ collision-energy-resolved) Penning ionization electron spectroscopy. A collision energy dependence of the partial ionization cross sections has indicated that the interaction potentials are anisotropic between He*(2³S) and the target molecules. For CH₃SCH₃ and CH₃CH₂SH, the interaction potential around the sulfur atom was found to be more attractive for the out-of-plane direction vertical to the CSC or CCS plane than for the in-plane direction, while the opposite trend was observed around the oxygen atom of CH₃CH₂OH. Anisotropic potential for CH₃SSCH₃ was found to be similar to that for CH₃SCH₃. Strongly enhanced bands in Penning ionization electron spectra of sulfur compounds, which were assigned to ionization from molecular orbitals having sulfur 3s (S_3s) atomic orbital character, were found to be associated with an intermolecular excitation transfer from He* to the target molecule and also with the following intramolecular Auger-like autoionization process.