Laboratory spectra of cold gas phase polycyclic aromatic hydrocarbon cations, and their possible relation to the diffuse interstellar bands

A novel laboratory technique is described, combining the use of supersonic expansion, laser excitation and small aromatic-rare gas van der Waals (vdW) clusters properties, which was developed to access the electronic absorpti on spectra of the polycyclic aromatic hydrocarbon (PAH) cations in the visi ble. It consists in preparing vdW complexes of the PAH molecule with a rare gas in a molecular beam, to photoionize it by resonant selective two-photo n ionization, then to photodissociate this ionic complex by means of a dela yed laser pulse in a time-of-flight mass spectrometer. The method is illust rated by presenting the visible spectra of the Naphthalene, Phenanthrene, F luorene and Phenylacetylene cations. Such spectra can be unambiguously comp ared to the astronomical spectra of reddened stars, which exhibit the so-ca lled diffuse interstellar bands (DIBs) in absorption. An interesting featur e of the technique is its ability to measure the absolute absorption cross- sections. The large values of the oscillator strengths of the transitions, which are derived, are discussed in the astrophysical context which consist s in considering that the PAH cations could be carriers for some of the DIB s. (C) 2001 Elsevier Science B.V. All rights reserved.