Spectroscopy of the benzene cation: Resonance-enhanced multiphoton dissociation spectra of the B̃(E_2g)←X̃(E_1g) transition

We present the first well-resolved laser spectra of the lowest, dipole-forbidden electronic transition in the benzene radical cation (B̃(E_2g)←X̃(E_1g)). We employ our technique of resonance-enhanced multiphoton dissociation spectroscopy (REMPDS) which is unique in its capability of investigating bound, non-fluorescing ionic states with photofragment detection. The technique of multiphoton ionization permits the preparation of the ions either in the vibrationless ionic ground state or with one quantum of ν₁₆ excited. We thus obtained two ion spectra, representing vibronic levels of different symmetry. Most of the observed transitions are assignable to combination levels involving the inducing modes ν₁₆ and ν₁₇. Our results support recent theoretical work about Jahn-Teller and pseudo-Jahn-Teller effects in the B̃(E_2g) state.