Hydrogenation and charge states of PAHs in diffuse clouds. I. Development of a model

A model of the hydrogenation and charge states of polycyclic aromatic hydrocarbons (PAHs) in diffuse clouds is presented. The main physical and chemical processes included in the model are ionization and photodissociation in the interstellar UV field, electron recombination with PAH cations, and chemistry between PAH cations and major interstellar species present in the diffuse medium, such as H-2, H, O, and N atoms. A statistical model of photodissociation is presented, which is a simplified 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 simplified 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 other important processes which may affect 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 diffuse 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.