Growth of polycyclic aromatic hydrocarbons by ion-molecule reactions

In this contribution we discuss a few molecular mechanisms leading to the growth of polycyclic aromatic hydrocarbons (PAHs). Such compounds have been observed in quite different gaseous environments (e.g. combustion systems, interstellar medium, hydrocarbon plasmas), thus understanding the formation of such ubiquitous molecules has become an increasingly important research topic. While great progress has been made in the knowledge of synthesis routes based on radical and neutral reactions, much less is known about ionic mechanisms leading to the synthesis of PAHs. By using a guided ion beam tandem mass spectrometer, we have explored the reactivity of C$_{10}$H$_{7}^{+}$ with C$_{6}$H$_{6}$, observing the growth of hydrocarbon ions via C-C bond forming reactions. The condensation adduct C$_{16}$H$_{13}^{+}$ is observed as the most abundant product at the smallest collision energy ($\sim $ 0.2 eV). Other products are the ions C$_{16}$H$_{n}^{+}$ (n=10-12) coming from H and H$_{2}$ elimination from the adduct, and the C$_{15}$H$_{10}^{+}$ ion formally corresponding to a CH$_{3}$ elimination. To elucidate the mechanisms responsible for such a rich chemistry we have performed \textit{ab initio} calculations.