The optical spectrum of a large isolated polycyclic aromatic hydrocarbon: hexa-peri -hexabenzocoronene, C42H18

The first optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium is reported. Vibronic bands of the first electronic transition of the all benzenoid polycyclic aromatic hydrocarbon hexa-peri -hexabenzocoronene were observed in the 4080 − 4530 Å range by resonant 2-color 2-photon ionization spectroscopy. The strongest feature at 4261 Å is estimated to have an oscillator strength of f = 1.4 × 10, placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4× 10 cm, accounting for a maximum of ∼ 0.02% of interstellar carbon. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future. Subject headings: molecular data, molecular processes, ISM: abundances, ISM: molecules