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 4264 Å is estimated to have an oscillator strength of f = 1.4 \times 10 ^ { -3 } , placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4 \times 10 ^ { 12 } cm ^ { -2 } , accounting for a maximum of \sim 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 .