skip to main content

Title: PHOTOCHEMISTRY OF POLYCYCLIC AROMATIC HYDROCARBONS IN COSMIC WATER ICE: THE ROLE OF PAH IONIZATION AND CONCENTRATION

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 importantmore » 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.« less
Authors:
; ; ;  [1] ; ;  [2] ;  [3] ;  [4]
  1. NASA Ames Research Center, PO Box 1, M/S 245-6, Moffett Field, CA 94035 (United States)
  2. SETI Institute, 189 North Bernardo Avenue, Mountain View, CA 94043 (United States)
  3. Radboud University Nijmegen, Institute for Molecules and Materials, Toernooiveld 5, 6525 ED Nijmegen (Netherlands)
  4. Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, PO Box 9513, NL2300 RA Leiden (Netherlands)
Publication Date:
OSTI Identifier:
22364554
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 799; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, 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