Journal Article

Lyα-induced charge effects of polycyclic aromatic hydrocarbons embedded in ammonia and ammonia:water ice

Steven H. Cuylle, Emily D. Tenenbaum, Jordy Bouwman, Harold Linnartz and Louis J. Allamandola

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 423, issue 2, pages 1825-1830
Published in print June 2012 | ISSN: 0035-8711
Published online June 2012 | e-ISSN: 1365-2966 | DOI:
Lyα-induced charge effects of polycyclic aromatic hydrocarbons embedded in ammonia and ammonia:water ice

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Infrared emission features assigned to gas phase polycyclic aromatic hydrocarbons (PAHs) are observed in space along many lines of sight. In regions where interstellar ices are present, these emissions are largely quenched. It is here that PAHs form agglomerates covered by ice or freeze out on to dust grains, together with volatile species such as H2O, CO, CO2 and NH3. Upon exposure to the Lyα-dominated interstellar radiation field, PAHs are expected to participate in photo-induced chemical reactions, explicitly also involving the surrounding ice matrix.

In this paper, a systematic laboratory-based study is presented for the solid-state behaviour of the PAHs pyrene and benzo[ghi]perylene upon Lyα irradiation in ammonia and mixed NH3:H2O astronomical ice analogues. The results are compared to recently published work focusing on a pure water ice environment. It is found that the ice matrix acts as an ‘electronic solid-state switch’ in which the relative amount of water and ammonia determines whether positively or negatively charged PAHs form. In pure water ice, cations are generated through direct ionization, whereas in pure ammonia ice, anions form through electron donation from ammonia-related photoproducts. The solid-state process controlling this latter channel involves electron transfer, rather than acid–base type proton transfer. In the mixed ice, the resulting products depend on the mixing ratio. The astronomical consequences of these laboratory findings are discussed.

Keywords: astrochemistry; molecular processes; methods: laboratory; ISM: molecules

Journal Article.  4596 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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