  Mixtures of polycylic aromatic hydrocarbons (PAHs) have been produced by
means of laser pyrolysis. The main fraction of the extracted PAHs were
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 to 60 carbon atoms per
molecule could be responsible for the interstellar UV bump at 217.5 nm.
