Multi-state vibronic interactions in the fluorobenzene radical cation: The importance of quadratic coupling terms

The multi-mode multi-state vibronic interactions in the set of X∼2B1-D∼2A1 electronic states of the monofluoro benzene radical cation are investigated theoretically, based on a quadratic vibronic coupling approach. The underlying ionization potentials and coupling constants are obtained from ab initio coupled-cluster calculations. Previous investigations (relying on the linear coupling approach) are extended by including all (diagonal) quadratic coupling constants for the totally symmetric modes in the five coupled electronic states. The inclusion of these quadratic couplings is found to be essential to correctly reproduce the low-energy conical intersections between different sets of electronic states. The photoelectron, mass-analyzed threshold ionization and photoinduced Rydberg ionization spectra are re-analyzed. The lowering of the minimum energies of intersection is shown to be crucial to lead to an efficient population transfer from higher excited states of the cation to the ground state. The implications of these findings for understanding the fluorescence dynamics of fluorinated benzene radical cations are discussed.