Knowledge Bank

Rotationally resolved state-to-state ZEKE threshold photoionization spectra of benzene concretely establish the vibronic terms of levels split by higher-order Jahn-Teller coupling between the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}E1g$ cation electronic state and the $v6e2g$ in-plane ring-bending vibrational mode. This assignment, in turn, sets the absolute energy phase of the vibronic pseudorotation in this coordinate, constituting the first definitive experimental determination of the shape of the benzene cation. In the present work, with precise knowledge of intermediate state structure afforded by $S0-S1$ spectroscopy, new ultra-high-resolution ZEKE electron spectroscopy of the $S1-D0$ transition has been used to obtain the rotational structure of benzene cation vibronic states. Assignment of this structure directly addresses the issue of modulation of the electronic potential energy surface in the region of the $\nu 6$ conical intersection, and definitively resolves the controversy concerning the relative stability of the locally distorted $(D2h)B2g$ and $B3g$ configurations.