Aspects of molecules, grains and clusters in space, with special reference to polycyclic aromatic hydrocarbons

The concept of clusters is extended to cover all types from molecules up to clusters of galaxies. Astrophysical ices and dust particles are defined. The sites and the nature of molecules and dust grains are explicited in the solar system, stellar atmospheres, interstellar clouds, and regions subjected to high UV fields. One class of molecules, the polyatomic aromatic hydrocarbons (PAHs) which are considered to be formed in the outflows of red giant stars, are discussed in detail. Modelling the formation and destruction of PAHs, and the assignment of observed spectral features in the interstellar medium: require knowledge of the spectroscopy and photophysics of these species. A review is given which details the results of research on the spectroscopy of neutral PAHs, their ionization yields, the spectroscopy, photophysics and photofragmentation of PAH monocations and anions, and the formation and fragmentation of PAH dications. Some astrophysical implications of these results are discussed, in particular with respect to UV and visible absorption features of the interstellar extinction curve and to the unidentified infrared emission bands observed for many lines of sight.