Formation and photochemistry of covalently bonded large functional PAH clusters

Abstract

Polycyclic aromatic hydrocarbon (PAH) molecules belong to a large and diverse chemical family in the interstellar medium (ISM). We study the formation and photochemistry of covalently bonded large functional PAH clusters, dicoronylene (DC, C48H20)/9-vinylanthracene (C16H12) and dicoronylene/9-methylanthracene (C15H12) cluster cations, in the gas phase, and we offer an approach to the evolution of different types of large (covalently bonded) PAH clusters in the ISM. The experiments, which we combined with a quadrupole ion trap and time-of-flight mass spectrometry, show that large functional PAH cluster cations can form by gas-phase condensation through molecular-ion reactions. One group of functional PAH cluster cations contain the vinyl group (−CHCH2), that is, from C16H12DDC+ (e.g., C16H12C48H19+, m/z = 799) to (C16H12)2DDC+ (e.g., (C16H12)2C48H18+, m/z = 1002). The other group of functional PAH cluster cations contain the methyl group (−CH3), that is, from C15H12DDC+ (e.g., C15H12C48H19+, m/z = 787) to (C15H12)2DDC+ (e.g., (C15H12)2C48H18+, m/z = 990). With laser irradiation, the DC/9-vinylanthracene and DC/9-methylanthracene cluster cations show a very complicated dissociation process (e.g., dehydrogenation, −CH3 or −CHCH2 unit losses). We investigate the structure of newly formed PAH cluster cations, the bond energy, and the photodissociation energy for these reaction pathways with quantumchemical calculations. The obtained results provide a general molecular growth route toward large PAH cluster cations (e.g., functional PAH clusters) in a bottom-up formation process and the insight of the functional group (e.g., vinyl, −C2H3 and methyl, −CH3) effect on their evolutionary behavior. In addition, the studies of DC/9-vinylanthracene and DC/9-methylanthracene clusters (94–123 atoms, ∼2 nm in size) also provide a possible way of interpreting the formation processes of nanometer-sized grains in the ISM, especially when functional PAHs are included.