Discovery of benzene cation in a very long-lived excited electronic state

Presence of benzene cation in a long-lived excited electronic state at ∼2.3 eV above the ground state was found through photodissociation kinetics and charge exchange ionization mass spectrometry. The lifetime of this state seems to be longer than 10 μs, maybe much longer. The experimental energy level of this state suggests that B̃ ²E_2g is the best candidate. The radiative transition from the latter state to the ground state, B̃ ²E_2g→X̃²E_1g, is electric dipole forbidden. Then, a very long lifetime requires an inefficient radiationless decay of the state. This is in contrast with the very fast decay proposed previously to account for the lack of fluorescence from the B̃ ²E_2g state. The present finding opens the possibility of studying a new excited state chemistry.