Ab initio investigation of the multimode dynamical Jahn-Teller effect in the X̃ ²E_1g state of the benzene cation

Linear-Jahn-Teller coupling parameters for the X̃ ²E_1g state of C₆H⁺ ₆ have been determined using beyond-Hartree-Fock ab initio methods. The vibronic structure of the first band of the photoelectron spectrum of benzene has been computed including up to three nonseparable Jahn-Teller-active modes and the totally symmetric C-C stretch mode. The calculated band-shape is in excellent agreement with the best-available photoelectron spectrum. The calculations reveal that the comparatively strong Jahn-Teller activity of model v₈ is hidden in the vibronic spectrum as a consequence of strong mode mixing with lower-frequency modes.