Molecular spectroscopy in astrophysics: the case of polycyclic aromatic hydrocarbons

The role of molecular spectroscopy in astrophysics and astrochemistry is di scussed in the context of the study of larger complex, carbon-bearing molec ules, namely. polycyclic aromatic hydrocarbons (PAHs). These molecular spec ies are now thought to be widespread in the interstellar medium in their ne utral and ionized forms. Identifying the carriers responsible for unidentif ied interstellar spectral bands will allow to derive important information on cosmic elemental abundance as well as information on the physical condit ions (density, temperature) reigning in specific interstellar environments. These, in turn, are key elements for a correct understanding of the energe tic mechanisms that govern the origin and the evolution of the interstellar medium. A multidisciplinary approach - combining astronomical observations with laboratory simulations and theoretical modeling - is required to addr ess these complex issues. Laboratory spectra of several PAHs, isolated at l ow temperature in inert gas matrices or seeded in a supersonic jet expansio n, are discussed here and compared to the astronomical spectra of reddened. early type, stars. The electronic spectroscopy of PAHs in the ultraviolet, visible and near-infrared domains is reviewed and an assessment of the pot ential contribution of PAHs to the interstellar extinction in the ultraviol et and in the visible is discussed. (C) 2001 Elsevier Science B.V. All righ ts reserved.