Ionization of dichlorobenzenes 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 the interaction potential energy between the molecule and He*(23S), showed anisotropic interaction around the molecules. The π and n (nonbonding) orbitals regions of the molecules showed attractive interactions. It was also found that the magnitude of attractive interaction around the Cl atom lone pair region perpendicular to the phenyl ring was o- > p- > m-C6H4Cl2. Negative slopes of CEDPICS for σ type bands indicate an attractive effect for ionization reaction by a substitution of the Cl atom.
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