Knowledge Bank

The Diffuse Interstellar Bands (DIBs) are absorption features systematically observed throughout the visible and near infra-red region of the Interstellar Extinction curve. These bands represent the longest-standing mystery of astrophysical spectroscopy since no carriers have been identified after more than fifty years. The purpose of the present work is to obtain near-infrared and visible spectra of PAH cations that can be directly compared to the observational data, so that the knowledge of the electronic spectra of $PAHs+$ can provide a direct test for the proposal of these molecules as DIBs carriers. We have developed a new spectroscopic method especially adapted to obtain absorption spectra of such large aromatic cations by measuring the photodissociation spectra of aromatic-rare gas van der Walls clusters in a molecular beam. The cluster cations produced are rotationally and vibrationally cold. Because the binding energy of Ar in these van Der Walls clusters is small (of the order of 500 cm-1), they can easily be fragmented by exciting the species above the dissociation threshold (this process occurs on a subnanosecond time-scale). We have applied this method to a number of cations including PAH+ species and others, which have been measured in the near-IR and the visible region. This set of results represents the first ``high resolution'' spectroscopic database available today on such large polyatomic cations.