Complementary to crossed beam experiments, we developed in collaboration with Dr. Ahmed (Lawrence Berkeley National Lab) an experimental protocol to investigate the formation mechanisms of PAHs within a high temperature chemical reactor by simulating combustion and astrophysically relevant conditions and probing isomer distributions by tunable vacuum ultraviolet (VUV) radiation from the Advanced Light Source. So far, molecular beam experiments in conjunction with electronic structure calculations by Prof. Mebel (Florida International University) unraveled via five elementary mechanisms leading to PAHs: Hydrogen Abstraction – Acetylene Addition (HACA), Hydrogen Abstraction – Vinylacetylene Addition (HAVA), Phenyl Addition – DehydroCyclization (PAC), Radical-Radical Reactions (RRR), and Methylidyne Addition - Cyclization - Aromatization (MACA). These concepts provide a solid, experimentally and computationally constrained framework to PAHs in extreme environments from low temperatures in molecular clouds (10 K) and hydrocarbon-rich atmospheres of planets and their moons (35–150 K) to high temperature environments like circumstellar envelopes of carbon rich AGB stars and combustion systems at a few 1,000 K. Our newly developed high-temperature chemical reactor presents a versatile approach to study the formation of combustion-relevant polycyclic aromatic hydrocarbons (PAHs) under well-defined and controlled conditions eventually leading to 2D and 3D nanostructures such as nanosheets and fullerenes in extreme environments.
Selective molecular mass growth processes leading to aromatic systems with up to six rings based on our studies. Pathways color coded in blue represent barrierless reaction mechanisms. Multiple pathways leading to identical products like naphthalene (8), triphenylene (30), and indene (56) highlight the complexity of prospective chemical reaction networks for combustion systems and in extraterrestrial environments.