INFRARED-EMISSION SPECTRA OF CANDIDATE INTERSTELLAR AROMATIC-MOLECULES

INTERSTELLAR dust is responsible, through surface reactions, for the c reation of molecular hydrogen, the main component of the interstellar clouds in which new stars form, Intermediate between small, gas-phase molecules and dust are the polycyclic aromatic hydrocarbons (PAHs). Su ch molecules could account for 2-30% of the carbon in the Galaxy(1), a nd may provide nucleation sites for the formation of carbonaceous dust (2,3). Although PAHs have been proposed(4,5) as the sources of the uni dentified infrared emission bands that are observed in the spectra of a variety of interstellar sources(6-11), the emission characteristics of such molecules are sources still poorly understood. Here we report laboratory emission spectra of several representative PAHs, obtained i n conditions approximating those of the interstellar medium, and measu red over the entire spectral region spanned by the unidentified infrar ed bands. We find that neutral PAHs of small and moderate size can at best make only a minor contribution to these emission bands, Cations o f these molecules, as well as much larger PAHs and their cations, rema in viable candidates for the sources of these bands.