PHOTOCHEMISTRY OF POLYCYCLIC AROMATIC HYDROCARBONS IN COSMIC WATER ICE: THE ROLE OF PAH IONIZATION AND CONCENTRATION
- NASA Ames Research Center, PO Box 1, M/S 245-6, Moffett Field, CA 94035 (United States)
- SETI Institute, 189 North Bernardo Avenue, Mountain View, CA 94043 (United States)
- Radboud University Nijmegen, Institute for Molecules and Materials, Toernooiveld 5, 6525 ED Nijmegen (Netherlands)
- Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, PO Box 9513, NL2300 RA Leiden (Netherlands)
Infrared spectroscopic studies of ultraviolet (UV) irradiated, water-rich, cosmic ice analogs containing small polycyclic aromatic hydrocarbons (PAHs) are described. The irradiation studies of anthracene:H{sub 2}O, pyrene:H{sub 2}O, and benzo[ghi]perylene:H{sub 2}O ices (14 K) at various concentrations reported by Bouwman et al. are extended. While aromatic alcohols and ketones have been reported in residues after irradiated PAH:H{sub 2}O ices were warmed to 270 K, it was not known if they formed during ice irradiation or during warm-up when reactants interact as H{sub 2}O sublimes. Recent work has shown that they form in low temperature ice. Using DFT computed IR spectra to identify photoproducts and PAH cations, we tentatively identify the production of specific alcohols [PAH(OH) {sub n} ] and quinones [PAH(O) {sub n} ] for all PAH:H{sub 2}O ices considered here. Little evidence is found for hydrogenation at 14 K, consistent with the findings of Gudipati and Yang. Addition of O and OH to the parent PAH is the dominant photochemical reaction, but PAH erosion to smaller PAHs (producing CO{sub 2} and H{sub 2}CO) is also important. DFT spectra are used to assess the contribution of PAH-related species to interstellar absorption features from 5 to 9 μm. The case is made that PAH cations are important contributors to the C2 component and PAH(OH) {sub n} and PAH(O) {sub n} to the C5 component described by Boogert et al. Thus, interstellar ices should contain neutral and ionized PAHs, alcohols, ketones and quinones at the ∼2%-4% level relative to H{sub 2}O. PAHs, their photoproducts, and ion-mediated processes should therefore be considered when modeling interstellar ice processes.
- OSTI ID:
- 22364554
- Journal Information:
- Astrophysical Journal, Vol. 799, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ANTHRACENE
CARBON DIOXIDE
CONCENTRATION RATIO
COSMOCHEMISTRY
HYDROGENATION
ICE
INFRARED SPECTRA
IONIZATION
IRRADIATION
KETONES
MOLECULES
PERYLENE
PHOTOCHEMICAL REACTIONS
PHOTOCHEMISTRY
POLYCYCLIC AROMATIC HYDROCARBONS
PYRENE
QUINONES
ULTRAVIOLET RADIATION
WATER