Formation of Complex Organic Molecules (COMs) from Polycyclic Aromatic Hydrocarbons (PAHs): Implications for ISM IR Emission Plateaus and Solar System Organics
ACS Earth and Space Chemistry
The presence of complex organic molecules (COMs) in a variety of solar system objects has prompted various theories regarding their origins. This paper reports on a series of proton, electron, and UV radiation experiments, conducted singly and in combination, on polycyclic aromatic hydrocarbon (PAH) thin films to determine if PAH-related materials could be related to COMs. Fourier transform infrared (FTIR) and mass spectra reveal that the PAHs are fragmented and altered, producing new infrared features and a mass spectral pattern similar to those from small aliphatic and larger saturated hydrocarbon chain and ring systems. Mass spectra after proton and electron irradiation exhibit significant differences. The proton irradiation products appear to contain more oxygen-related species, possibly the result of higher H2O concentrations in the vacuum chamber, whereas electron irradiation generates a more abundant, larger mass organic species mass spectral pattern. Combined irradiation produces a superposition of the proton and electron results, with some subtle differences. Mass spectral patterns resulting from electron and combined irradiation compare favorably to Cassini ion neutral mass spectrometer (INMS) measurements of particles falling from Saturn's inner rings into its upper atmosphere. FTIR results are compared to PAH emission from interstellar sources and absorption components in IR spectra observed toward low-mass young stellar objects (YSOs). Notably, some newly reported broad IR features appear very similar in structure to the plateaus underlying the astronomical 3.2-3.6, 6-9, and 10-15 μm emission plateaus and several of the absorption components found in spectra observed toward YSOs. The studies also indicate that the presence of other species, such as H2O, may significantly impact the radiation products. These results point to the fact that a top-down synthesis of solar system COMs is possible via irradiative processing of PAHs.
National Aeronautics and Space Administration
complex organic materials (COMs), IR, irradiation, ISM, mass spectra, PAHs, Saturn
Andrew L. Mattioda, Gustavo A. Cruz-Diaz, Andrew Ging, Michael Barnhardt, Christiaan Boersma, Louis J. Allamandola, Todd Schneider, Jason Vaughn, Brandon Phillips, and Alessandra Ricca. "Formation of Complex Organic Molecules (COMs) from Polycyclic Aromatic Hydrocarbons (PAHs): Implications for ISM IR Emission Plateaus and Solar System Organics" ACS Earth and Space Chemistry (2020): 2227-2245. https://doi.org/10.1021/acsearthspacechem.0c00165