LABORATORY EVIDENCE FOR IONIZED POLYCYCLIC AROMATIC-HYDROCARBONS IN THE INTERSTELLAR-MEDIUM

THE origin of the broad interstellar infrared emission bands (at 3.3, 6.2, 7.8, 8.6 and 11.3 mum) found in the vicinity of many galactic and extragalactic sources is still poorly understood. The original suggestion' that the bands are associated with aromatic species embedded in small carbon particles was later challenged by the proposal2 that they originate from vapour-phase, neutral polycyclic aromatic hydrocarbons (PAHs); Allamandola et al. independently argued3 that PAH cations are the source of the bands. This latter proposal has steadily gained acceptance, but the lack of experimentally determined emission spectra of PAH cations has made it difficult to test the idea. We have recently measured the visible and infrared spectra of the neutrals and cations of four PAHs-naphthalene4, anthracene5, pyrene6 and perylene7-dispersed in argon matrices at 12 K, to approximate the low-pressure, gas-phase conditions of the interstellar medium. Here we compare the infrared absorption from these four molecules (neutrals and cations), integrated over the spectral regions corresponding to the interstellar bands, with the astronomical observations. We rind that the interstellar bands cannot be explained solely on the basis of neutral PAH species, but that cations must be a significant, and in some cases dominant, component.