THE INTERSTELLAR CHEMISTRY OF PAH CATIONS

Diffuse interstellar bands (DIBs) are mysterious absorption lines in t he optical spectra of stars, and have been known for 75 years(1). Alth ough it is widely believes(2-4) that they arise from gas-phase organic molecules (rather than from dust grains) in the interstellar medium, no consensus has been reached regarding their precise cause. The reali zation that many emission features in astronomical infrared spectra pr obably arise from polycyclic aromatic hydrocarbons(5-8) (PAHs), which may themselves be very abundant in the interstellar medium, has led to the suggestion that ionized PAHs might be the source of the DIBs(9-12 ). Laboratory investigations have revealed that small, positively char ged PAHs in matrices have absorption features that bear some resemblan ce to DIBs(13-15), but no clear identification of any DIB with any spe cific PAH cation has yet been made. Here we report a laboratory study of the chemical reactivity of PAH cations (C6H6+, C10H8+ and C16H10+) in the gas phase. We find that these PAH cations are very reactive, an d are therefore unlikely to survive in high abundances in the interste llar medium. Rather, such molecules will react rapidly with hydrogen, and we therefore suggest that the resulting protonated PAH cations (an d species derived from them) should become the focus of future searche s for a correspondence between molecular absorption features and the D IBs.