Photochemistry of polycyclic aromatic hydrocarbons in cosmic water ice

II. Near UV/VIS spectroscopy and ionization rates

J. Bouwman^1,4, H. M. Cuppen¹, M. Steglich², L. J. Allamandola³ and H. Linnartz¹

¹ Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: bouwman@berkeley.edu
² Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
³ NASA-Ames Research Center, Space Science Division, Mail Stop 245-6, Moffett Field, CA 94035, USA
⁴ Current address: Departments of Chemistry and Physics, University of California, Berkeley, California 94720, USA

Received: 15 September 2010
Accepted: 6 February 2011

Abstract

Context. Mid-infrared emission features originating from polycyclic aromatic hydrocarbons (PAHs) are observed towards photon dominated regions in space. Towards dense clouds, however, these emission features are quenched. Observations of dense clouds show that many simple volatile molecules are frozen out on interstellar grains, forming thin layers of ice. Recently, observations have shown that more complex non-volatile species, presumably including PAHs, also freeze out and contribute to the ongoing solid-state chemistry.

Aims. The study presented here aims at obtaining reaction rate data that characterize PAH photochemistry upon vacuum ultraviolet (VUV) irradiation in an interstellar H₂O ice analogue to explore the potential impact of PAH:H₂O ice reactions on overall interstellar ice chemistry. To this end, the experimental results are implemented in a chemical model under simple interstellar cloud conditions.

Methods. Time-dependent near-UV/VIS spectroscopy on the VUV photochemistry of anthracene, pyrene, benzo[ghi]perylene and coronene containing interstellar H₂O ice analogs is performed at 25 and 125 K, using an optical absorption setup.

Results. Near-UV/VIS absorption spectra are presented for these four PAHs and their photoproducts including cationic species trapped in H₂O ice. Oscillator strengths of the cation absorption bands are derived relative to the oscillator strength of the neutral parent PAH. The loss of the parent and growth of PAH photoproducts are measured as a function of VUV dose, yielding solid state reaction constants. The rate constants are used in an exploratory astrochemical model, to assess the importance of PAH:H₂O ice photoprocessing in UV exposed interstellar environments, compared with the timescales in which PAH molecules are incorporated in interstellar ices.

Conclusions. All four PAHs studied here are found to be readily ionized upon VUV photolysis when trapped in H₂O ice and exhibit similar rates for ionization at astronomically relevant temperatures. Depending on the relative efficiency of H₂O photodesorption and PAH photoionization in H₂O ice, the latter may trigger a charge induced aromatic solid state chemistry, in which PAH cations play a central role.