Douglas M. Hudgins

Since their discovery by Gillett, Forrest, and Merrill in 1973, the nature of the so-called Unidentified Infrared or UIR bands - a ubiquitous family of interstellar infrared emission features - has remained one of the central issues challenging our understanding of the chemistry and physics of the interstellar medium. Laboratory efforts in the ensuing years have been directed primarily toward the chemical identification of suitable candidate materials which could accommodate both the spectral and photophysical constraints which characterize the interstellar emission zones.

This talk will summarize the current research into the infrared spectroscopic properties of neutral and ionized polycyclic aromatic hydrocarbons (PAHs) -research which directly addresses the spectroscopic criticisms previously leveled at the PAH hypothesis and which demonstrates that PAH-related molecular species are indeed responsible for the widespread UIR emission. Global fits will be presented for a selection of objects which span the evolutionary range of interstellar material and which illustrate the variability in the appearance of the interstellar infrared emission spectrum. These fits reflect the structure, abundance, and ionization state of the interstellar PAHs which, in turn, provide direct insight into the processes of carbon nucleation, growth and evolution in circumstellar shells and the interstellar medium. These will be followed by a more in depth look at some specific features within the UIR spectrum, considering both what we have learned about them and what we learn from them. Finally, directions for future research in the field are considered - research which should include the first exploitation of PAHs as probes of a new and heretofore largely unexplored facet of astrochemistry and which has the potential to make PAHs the probe of the next Millennium much as CO has been for the last quarter century.

D.M. Hudgins, L.J. Allamandola, and S.A. Sandford, "Complex Organic Molecules in Space: The Carriers of the Interstellar Infrared Emission Features",

Adv.Space Res., 19(7), 999 (1997).