Ionic Species in a Naphthalene Plasma: Understanding Fragmentation Patterns and Growth of PAHs.

Abstract

The role of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM) is a key question in astrophysics. Moreover, our limited understanding of the plasma-driven processes of PAHs in the ISM motivates the present study on naphthalene as a model compound. In this work, a mass spectrometric characterization of the positive and negative ionic species in a naphthalene plasma was carried out. According to our findings, the main naphthalene dissociation channels upon electronic excitation proceed through hydrogen or acetylene loss. In addition, we report experimental evidence of the hydrogen abstraction-acetylene addition mechanism taking place under plasma conditions, which contributes to our understanding of the growth of PAHs. Regarding negative ions, species belonging to the astrochemically relevant family of polyynyl anions were detected, i.e., C4H- and C6H-. We postulate that the latter could be formed in a "top-down" chemistry as fragments of PAHs. Finally, our results show that negative ions add to neutral naphthalene molecules yielding larger anions, which suggests that negatively charged species may also play a role in the growth of PAHs.