Laboratory investigation of the gas-phase formation of (poly)cyclic hydrocarbons under interstellar conditions

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Cyclic molecules composed of five or six carbon atoms, as well as small polycyclic aromatic hydrocarbons (PAHs) represent the most complex molecules unambiguously identified by radio astronomy in cold dark molecular clouds, but little is known about how they are formed under interstellar conditions. Using a combined experimental and computational approach, this research project will investigate key gas phase reaction mechanisms driving the formation of complex cyclic hydrocarbons in cold interstellar clouds. Results from the project will serve as critical input to astrochemical models, allowing astronomers to explain the presence of complex cyclic hydrocarbons in space. This project will train a graduate student in methods of advanced laboratory astrophysics and provide research opportunities to an undergraduate student. The research team will investigate the reactivity of two radical species, ortho-benzyne and 2,3-pyridyne. The investigators have shown that ortho-benzyne, a molecule recently identified in the Taurus Molecular Cloud (TMC-1), plays a crucial role in the synthesis of the complex hydrocarbons ethynylcyclopentadiene and indene which have also been identified there. This project will build upon that work by investigating reactions of ortho-benzyne and 2,3-pyridyne that potentially form the PAH naphthalene, as well as the cyanocyclopentadienes which have been detected in TMC-1. A pyrolysis microreactor coupled to a double imaging photoelectron photoion coincidence (i2PEPICO) spectrometer connected to the vacuum ultraviolet beamline of a synchrotron will be used in the study. The i2PEPICO system will be capable of measuring---with isomer specificity---the products formed in chemical reactions of the selected radical species. Quantum chemical calculations of potential energy surfaces will yield insights into the reactions and a statistical model will be employed to derive product branching fractions. Because the reactions investigated have the potential to yield products that are plausible candidates for detection in space, results from this study would also guide future radio astronomical observations. Besides astronomy, results from this project are directly applicable to PAH and soot formation mechanisms in combustion chemistry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.