Properties of the π* and σ* states of the chlorobenzene anion determined by electron impact spectroscopy

Abstract

The selectivity of vibrational excitation by electron impact has been used to unambiguously assign the negative ion states (resonances) of chlorobenzene and to settle a recent controversy on this subject. The excitation functions of the ring deformation vibrations exhibit bands in the 0.8–1.4 eV range, identifying them as temporary electron captures in the b₁ and a₂ π* orbitals. A broad band peaking at 2.6 eV appears in the excitation functions of the C–Cl stretch vibration but is missing in the excitation functions of the ring deformation vibrations, proving that it corresponds to a temporary electron capture in the σ^*_C–Cl orbital. A more detailed insight into the properties of the potential surfaces of the anion is gained from the excitation functions of many vibrations, and from their comparison with anion potential curves based on the Koopmans theorem. Slopes of the potential curves in the Franck–Condon region reproduce well the observed intensities of totally symmetric vibrations. Strong excitation of out-of-plane vibrations, corroborated by the calculations, reveal vibronic coupling of the b₁ π* and the σ* anion states. The a₂ π* state and the σ* state are coupled by vibrations with a₂ symmetry. Excitation of in-plane non-totally symmetric vibrations (b₂) reveals vibronic coupling between the two π* states b₁ and a₂, which is also reproduced by the calculated potential curves. The results indicate that symmetry lowering induced by vibronic coupling provides the path for dissociation of the π* states of the chlorobenzene anion.