ON THE SHAPE OF C6H6+

The benzene molecule serves as a benchmark among the aromatic hydrocar bons and has been the subject of numerous experimental and theoretical studies. Despite such intensive investigations, the precise structure of the benzene cation (C6H6+) is not known. Now, experiments measurin g high-resolution slate-to-state threshold photoionization spectra of benzene concretely establish the terms of vibronic levels in the disto rted cation that are split by higher order Jahn-Teller coupling betwee n its (2)E(1g) electronic ground state and v(6)e(2g) in-plane ring-ben ding vibrational mode. This assignment, in turn, sets the absolute ene rgy phase of the vibronic pseudorotation in this coordinate and thereb y offers a definitive experimental determination of the shape of the b enzene cation.