Polycyclic aromatic hydrocarbons and the diffuse interstellar bands: A survey

We discuss the proposal relating the origin of some of the diffuse interste llar bands (DIBs) to neutral and ionized polycyclic aromatic hydrocarbons ( PAHs) present in interstellar clouds. Laboratory spectra of several PAHs, i solated at low temperature in inert gas matrices, are compared with the spe ctra of five reddened early-type stars selected from an extensive set of as tronomical spectra. From this comparison, it is concluded that PAH ions are good candidates to explain some of the DIBs. Unambiguous assignments are d ifficult, however, because of the shift in:wavelengths and the band broaden ing induced in the laboratory spectra by the solid matrix. This situation i s illustrated by a comparison with the gas-phase spectra made available rec ently for two PAH ions. Definitive band assignments and, ultimately, the te st of the proposal that PAH ions carry some of the DIBs must await the avai lability of a larger set of gas-phase measurements in the laboratory. The p resent assessment offers a guideline for future laboratory experiments by a llowing the preselection of promising PAH molecules to be studied in jet ex pansions.