Combined Jahn-Teller and Pseudo-Jahn-Teller effects in the benzene radical cation

The multi-state vibronic interactions in the benzene radical cation are investigated by an ab initio quantum dynamical approach, based on, and extending the work of J. Chem. Phys. 117 (2002) 2645. The X ̃2E1g ground state and several excited electronic states are included in the calculations, which focus on the vibronic structure of the photoelectron (PE) spectrum and on the internal conversion dynamics. Good agreement with the experimental PE spectrum of Baltzer et al. is achieved. New ab initio data are provided on the pseudo-rotational barrier in the ground state. The D ̃- E ̃ state pseudo Jahn-Teller interaction is shown to lead to a femtosecond population decay of the E ̃ state. The importance of stepwise internal conversion processes, involving more than two strongly coupled potential energy surfaces, in this and other medium-sized molecular systems is pointed out. © 2003.