Electronic Spectroscopy of Medium-Sized Polycyclic Aromatic Hydrocarbons: Implications for the Carriers of the 2175 Å Uv Bump

Mixtures of polycyclic aromatic hydrocarbons (PAHs) have been produced by means of laser pyrolysis. The main fraction of the extracted PAHs was primarily medium-sized, up to a maximum size of 38 carbon atoms per molecule. The use of different extraction solvents and subsequent chromatographic fractionation provided mixtures of different size distributions. UV–VIS absorption spectra have been measured at low temperature by matrix isolation spectroscopy and at room temperature with PAHs as film-like deposits on transparent substrates. In accordance with semi-empirical calculations, our findings suggest that large PAHs with sizes around 50–60 carbon atoms per molecule could be responsible for the interstellar UV bump at 217.5 nm.