On the Shape of C ₆ H ₆ ⁺

The benzene molecule serves as a benchmark among the aromatic hydrocarbons and has been the subject of numerous experimental and theoretical studies. Despite such intensive investigations, the precise structure of the benzene cation (C 6 H 6 + ) is not known. Now, experiments measuring high-resolution state-to-state threshold photoionization spectra of benzene concretely establish the terms of vibronic levels in the distorted cation that are split by higher order Jahn-Teller coupling between its 2 E 1g electronic ground state and ν 6 e 2g in-plane ring-bending vibrational mode. This assignment, in turn, sets the absolute energy phase of the vibronic pseudorotation in this coordinate and thereby offers a definitive experimental determination of the shape of the benzene cation.