Electronic spectroscopy of polycyclic aromatic hydrocarbons (PAHs) at low temperature in the gas phase and in helium droplets

The electronic spectroscopy of polycyclic aromatic hydrocarbons (PAHs) is of particular interest for astrophysicists since these molecules are suggested as possible carriers of the diffuse interstellar bands. Here we report on our study of the S1←S0 transition of 2,3-benzofluorene (or benzo[b]fluorene) in the gas phase and in He droplets using cavity ring-down spectroscopy in a supersonic jet expansion and molecular beam depletion spectroscopy, respectively. The same techniques were also employed to identify the PAHs contained in the carbonaceous powder which was produced by CO2 laser pyrolysis of a mixture of ethylene and benzene vapor. Phenanthrene and anthracene were unambiguously confirmed as components of the as-synthesized soot.