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*(2(3)S) 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 pi 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 pi and nonbonding orbitals were confirmed utilizing large-band intensity in the Penning ionization electron spectra reflecting extending molecular orbitals and an attractive interaction. (C) 2000 Elsevier Science B.V. All rights reserved.