Penning ionization of substituted benzenes (aniline, phenol and thiophenol) by collision with He*(2³S) metastable atoms

Penning ionization electron spectra of substituted benzenes (aniline, phenol and thiophenol) upon collision with He*(2³S) metastable atoms were observed, and a collision energy dependence of partial ionization cross-sections (CEDPICS) of the sample molecules was studied using collision-energy-resolved Penning ionization electron spectroscopy. CEDPICS indicates that the interaction potential between He* and the target molecule was anisotropic in the studied collision energy range (90-300 meV). The interaction potential around the hetero atoms and the out-of-plane direction of the phenyl ring is attractive, while a repulsive interaction around the hydrogen atoms was derived from the small negative slope of CEDPICS for ionization from π orbitals and model potential calculations. An attractive interaction around the nitrogen atom of aniline indicates the `lone pair' character of the nonbonding molecular orbital. For thiophenol, the interaction potential for the out-of-plane direction around the sulfur atom is attractive in contrast to the in-plane direction. In contrast, the interaction potential for the in-plane direction around the oxygen atom is attractive in the case of phenol. Assignments of photoelectron spectra for ionization from π and nonbonding orbitals were confirmed utilizing large-band intensity in the Penning ionization electron spectra reflecting extending molecular orbitals and an attractive interaction.