Complex organic molecules in space: the carriers of the interstellar infrared emission features

The Unidentified Infrared Bands (UIR bands) are a complex family of infrared emission features which are observed in a variety of astronomical sources. While these features have been known for more than twenty years, a satisfactory identification of the carriers remains elusive. While the gross appearance of the emission spectrum indicates that the molecular carriers are aromatic compounds, differences in detail between the astronomical spectra and the available laboratory spectra have prevented a more complete description of the identity and physical state of these compounds. In this paper we present the first detailed comparison between the astronomical emission spectra and the spectra of ionized polycyclic aromatic hydrocarbons (PAHs) measured in the laboratory. These spectra are found to provide the best fit to date of the astronomical spectra and demonstrate that the positions and intensities of the UIR bands are entirely consistent with the emission from a gas-phase mixture of PAH molecules dominated by PAH cations.