Gas-phase Electronic Spectra of Coronene and Corannulene Cations
Gas-phase electronic spectra of the coronene (C{sub 24}H{sub 12}{sup +}) and corannulene (C{sub 20}H{sub 10}{sup +}) cations complexed with helium have been recorded in a quadrupole ion trap at 5 K by photodissociation. The electronic spectrum of C{sub 20}H{sub 10}{sup +} 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 Å. C{sub 24}H{sub 12}{sup +}--He has the origin band of the A{sup 2}E{sub 1g}←X{sup 2}E{sub 2u} transition at 9438.3 Å and that to a much higher state D{sub 3}←X{sup 2}E{sub 2u} at 4570 Å. The corannulene cation is subject to a Jahn–Teller distortion in the electronic ground state, leading to the 3{sup 2}A{sup ′}←X{sup 2}A{sup ′′} and 3{sup 2}A{sup ′′}←X{sup 2}A{sup ′} transitions with origin band maxima when complexed with helium at 5996.1 and 5882.6 Å. 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.