Laser induced fluorescence spectroscopy of jet-cooled molecular species: a tool to identify diffuse interstellar band carriers

We present the first results of an experimental method (jet expansion followed by laser induced fluorescence) that collects absorption spectra of cold, gas-phase isolated species. This method, closely simulating interstellar conditions, is a powerful tool to assert diffuse interstellar bands (DIB) identification. It only requires the species studied to be fluorescent. This latter condition is fulfilled for some DIB carriers as shown recently. Stimulated by the recent detection in rare-gas matrices of its fluorescence, we focus on the perylene cation. This molecule pertains to a family (the polycyclic aromatic hydrocarbons or PAHs) whose ions or radicals are potential DIB carriers. We failed to detect the fluorescence observed in rare-gas matrix. Rather, we observed a photofragmentation process leading to the C2 molecule.We thus obtained the absorption spectrum of a PAH radical of size comparable to its parent molecule, perylene. From a comparison with the DIB spectrum, we find that rotationally broadened bands of PAH radicals may explain the profile of broad DIBs, or regularly spaced DIBs sequences.