Gas-phase Electronic Spectra of Coronene and Corannulene Cations

Gas-phase electronic spectra of the coronene (C24H12+) and corannulene (C20H10+) cations complexed with helium have been recorded in a quadrupole ion trap at 5 K by photodissociation. The electronic spectrum of C20H10+ with two helium atoms was also measured to estimate the perturbation. This method is sufficient for an astronomical comparison because the shift due to the weakly bound helium is on the order of 0.2 angstrom. C24H12+-He has the origin band of the A(2)E(1g) <- X E-2(2u) transition at 9438.3 angstrom and that to a much higher state D-3 <- X E-2(2u) at 4570 angstrom. The corannulene cation is subject to a Jahn-Teller distortion in the electronic ground state, leading to the 3(2)Lambda' <- X-2 Lambda and 3(2)Lambda <- X (2)Lambda' transitions with origin band maxima when complexed with helium at 5996.1 and 5882.6 angstrom. These absorptions lie in a region where there is a congestion of diffuse interstellar bands (DIBs). However, the recorded features have no match with astronomical observations, removing coronene and corannulene cations and probably other aromatic hydrocarbons of this size as possible carriers of the DIBs.