Photochemistry of Polycyclic Aromatic Hydrocarbons in Cosmic Water Ice II
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A&A 529, A46 (2011) Astronomy DOI: 10.1051/0004-6361/201015762 & c ESO 2011 Astrophysics Photochemistry of polycyclic aromatic hydrocarbons in cosmic water ice II. Near UV/VIS spectroscopy and ionization rates J. Bouwman1,4, 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: [email protected] 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 trappedinH2O 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:H2Oice 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. Key words. astrochemistry – molecular processes – methods: laboratory – techniques: spectroscopic – ISM: molecules 1. Introduction Chiar et al. 2000; Bregman et al. 2000). Based on these obser- vations, Bouwman et al. (2011) deduced PAH:H2O ice ratios to- The presence of polycyclic aromatic hydrocarbons (PAHs) in ward MonR2/IRS3 to be about 2%, a ratio comparable to com- − many phases of the interstellar medium is evidenced by their mon interstellar ice species such as H2CO and OCN .However, strong and ubiquitous mid-infrared (mid-IR) emission features since astronomical PAHs are expected to contain on the order of (Smith et al. 2007; Draine et al. 2007; Draine & Li 2007; Tielens 50 to 100 C atoms and some 20 to 30 reactive C–H peripheral 2008). Mid-IR features are efficiently emitted by a PAH after sites, their influence on interstellar ice chemistry and ice physics excitation by an energetic photon. Toward dense clouds, how- is likely important. ever, these mid-IR features are strongly quenched. Here, most Experimental studies on the effect of vacuum ultraviolet volatile molecules are frozen out on grains forming layers of ice (VUV) irradiation of interstellar ice analogues have shown that (e.g., Pontoppidan et al. 2004; Boogert et al. 2008; Öberg et al. more complex molecules form in the simplest mixed ices (e.g. 2008; Bottinelli et al. 2010). Under such conditions, less volatile Gerakines et al. 1995; Öberg et al. 2009a; Muñoz Caro & molecules such as PAHs condense on interstellar grains as well, Dartois 2009). Laboratory studies on VUV irradiated PAH con- incorporating them in interstellar ices. taining ices indicated that PAHs are easily ionized (Gudipati & Indeed, weak mid-IR absorption features in the spectra of Allamandola 2003; Gudipati 2004; Bouwman et al. 2009, 2010). dense clouds point to the presence of PAHs in these ices as well, These experiments also showed the formation of new species. with PAH:H2O ice concentrations on the order of a few per- Kinetic information on these chemical reactions, however, is cent (Smith et al. 1989; Sellgren et al. 1995; Brooke et al. 1999; largely lacking. Article published by EDP Sciences A46, page 1 of 9 A&A 529, A46 (2011) Here, we present the time evolution of the use-up of four (Muñoz Caro et al. 2002). In the present experiment this value PAHs, anthracene (Ant, C14H10), pyrene (Py, C16H10), may be higher as a reflecting tube has been positioned behind benzo[ghi]perylene (BghiP, C 22H12), and coronene (Cor, C24H12) the lamp also directing diverging photons towards the ice sam- 15 −2 −1 in H2O ice together with the formation and destruction of the ple. As an upper limit we use a flux of 10 photons cm s in ionized PAH+ species. This paper is part II of a series which this paper. aims to quantify and understand the time-dependent chemistry Near UV/VIS absorption spectra are taken during VUV pro- of PAH:H2O ice mixtures upon VUV irradiation and the result- cessing of the samples. To this end, a Xe-lamp is used as ing photoproducts. Part I deals with the mid-IR spectroscopy of a broadband light source and a spectrometer equipped with the photoproducts and its applications to astronomical observa- a 1024 × 256 pixel CCD camera is used as detector. The tions of low and high mass objects and has been published sep- CCD camera is read out in vertical binning mode by a computer arately (Bouwman et al. 2011). Because of severe band over- on which the raw data are converted into optical depth (OD = lap, the infrared experiments are not capable of providing the ln(I/I0)). Spectra ranging from ∼280 to 800 nm are taken at a kinetic information reported here. The two papers together pro- rate of 0.1 s−1,whichissufficient to monitor chemical changes in vide a complementary (vibrational and electronic) study of PAH our ice samples. Each spectrum is the result of co-adding 229 in- containing H2O ice, as well as their photoproducts and dynamic dividual spectra, resulting in a high signal-to-noise ratio. behavior upon VUV excitation. The integrated absorbance of the deposited neutral PAH sig- Here, the chemical evolution is tracked by means of near- nal, τνdν, is converted into a PAH column density, NPAH ,via UV/VIS absorption spectroscopy at two temperatures, 25 K and the oscillator strength of the neutral PAH, f , and the conversion 125 K. This work is an extension of a previous optical study factor 8.88 × 10−13 taken from Kjaergaard et al. (2000): of pyrene in H2Oice(Bouwman et al. 2010) and draws more general conclusions on PAH photochemistry in ices based on a τνdν ff NPAH = · (1) larger set of di erent PAHs. Furthermore, the present work ex- 8.88 × 10−13 f tends the PAH:H2O photochemistry to larger and astrophysically more relevant members of the PAH family. Together with an ice thickness measurement based on the inter- The outline of this paper is as follows. In Sect. 2 the ex- ference pattern in the reflection of a HeNe laser as described in perimental setup is briefly discussed, together with details of Bouwman et al. (2009), this allows for a determination of the supporting calculations. Section 3 describes spectra of the PAH PAH:H2O concentration. Sample deposition is stopped at thick- and PAH+ cations, the assignments of the observed transitions nesses of ∼2 μm, resulting in comparable ice samples. and presents the (relative) oscillator strengths. The fitted time- In a typical 4 h experiment, as many as 1400 spectra are ob- dependent data are discussed in detail in Sect. 4,afterwhichthe tained. The spectra are all baseline corrected by fitting a second- astrophysical implications are discussed in Sect. 5. The conclu- order polynomial through points where no absorptions occur sions are summarized in Sect. 6. and subtracting the polynomial. Additionally, absorption bands are integrated and, if necessary, corrected for contributions by atomic H lines of the H2 MW discharge lamp. All the data han- 2. Experimental technique dling is performed in a LabView program. The experimental details are available from Bouwman et al. In order to support the assignment of the measured absorp- (2009). The heart of the setup is of a high-vacuum (∼10−7 mbar) tion photoproduct bands, density functional theory (DFT) calcu- lations are performed using the gaussian09 software (Frisch et al. chamber. In the center of the vacuum chamber a MgF2 sample window is suspended, which is cooled by a closed cycle He cryo- 2009). The B3LYP functional is used in conjunction with the 6- ++ stat to a temperature of 25 K. Temperatures as low as 11 K can 311 G(2d, p) basis set to determine the ground state geometry and electronic structure of PAH neutrals and cations as a starting be realized.