Penning ionization of difluorobenzenes upon collision with metastable He*(23S) atoms was studied by two- dimensional (collision-energy/electron-energy-resolved) Penning ionization electron spectroscopy. Collision energy dependence of the partial ionization cross sections (CEDPICS), which reflects interaction potential energy between the molecule and He*(23S), showed anisotropic interaction around the molecules. Assignments of the Penning ionization electron spectra and He I ultraviolet photoelectron spectra were discussed on the basis of different behavior of CEDPICS and calculated ionization potentials by the outer valence Green's function (OVGF) method. Furthermore, the ordering of the reactivity of the n∥ and σCF orbitals and the magnitude of attractive interaction around the F atoms with the metastable atom was found to be o- > m- ∼ p-C6H4F2. It indicates that the wider potential well brings the larger attractive effect for Penning ionization reaction.
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