Diffuse interstellar bands (DIBs) are mysterious absorption lines in the optical spectra of stars, and have been known for 75 years. Although it is widely believed 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 realization that many emission features in astronomical infrared spectra probably arise from polycyclic aromatic hydrocarbons (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. Laboratory investigations have revealed that small, positively charged PAHs in matrices have absorption features that bear some resemblance to DIBs, but no clear identification of any DIB with any specific 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, and are therefore unlikely to survive in high abundances in the interstellar medium. Rather, such molecules will react rapidly with hydrogen, and we therefore suggest that the resulting protonated PAH cations (and species derived from them) should become the focus of future searches for a correspondence between molecular absorption features and the DIBs.

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A model of the hydrogenation and charge states of polycyclic aromatic hydrocarbons (PAHs) in di †use clouds is presented. The main physical and chemical processes included in the model are ionization and photodissociation in the interstellar UV Ðeld, electron recombination with PAH cations, and chemistry between PAH cations and major interstellar species present in the di †use medium, such as H 2 , H, O, and N atoms. A statistical model of photodissociation is presented, which is a simpliÐed version of the Rice-Ramsperger-Kassel-Marcus theory. The predictions of this new approach have been successfully compared to experimental results for small PAH cations, justifying the application of the model to larger PAHs for which no experimental data are available. This simpliÐed statistical theory has also been used to estimate the importance of the dissociative recombination channel in the reaction between PAH cations and electrons. Recent experimental results obtained on the chemistry between PAH cations and H, O, and N atoms are discussed and included in the model. Finally, a discussion is presented on H 2 , other important processes which may a †ect the PAH distribution in the interstellar medium, such as electron attachment, photodetachment, photofragmentation with carbon loss, double ionization, and chemistry between PAH cations and minor species present in di †use clouds. The results obtained with this model for compact PAHs ranging from benzene to species bearing up to 200 carbon atoms are discussed in a separate paper.

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