2017ApJ...836...37H

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 A. C₂₄H₁₂⁺–He has the origin band of the A²E_1g←X²E_2u transition at 9438.3 A and that to a much higher state D₃←X²E_2u at 4570 A. The corannulene cation is subject to a Jahn-Teller distortion in the electronic ground state, leading to the 3²A^′ ←X²A^′′^ and 3²A^′′^ ←X²A^′ transitions with origin band maxima when complexed with helium at 5996.1 and 5882.6 A. 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.