New ultrafast nonradiative decay mechanism in the benzene radical cation

Abstract

The simultaneous vibronic interactions between the X̃ 2E 1g, B̃ 2E 2g and C̃ 2A 2u states of the benzene radical cation are investigated theoretically. A new type of conical intersection between the X̃ and B̃ state potential energy surfaces is found which is triggered by the Jahn—Teller effects in the two degenerate states, the coupling being provided by the out-of-plane mode ν 8. Extensive wavepacket dynamical calculations have been performed involving all three coupled electronic states and up to five nonseparable vibrational modes (≈2 million basis states). They reveal a decay to the X̃ 2E 1g state upon broadband excitation to the C̃ 2A 2u state within ≈250 fs. This can explain the absence of C̃ å X̃ radiative emission as well as the fact that the benzene cation fragments via the X̃ state even if higher electronic states are populated initially. A comparison with (partly) halogenated derivatives indicates that the absence of a low-lying 2E 2g ( 2E′) state is responsible for the observation of Ã→X̃ radiative emission in these species.