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Astronomy Astrophysics A&A 390, 1153–1170 (2002) Astronomy DOI: 10.1051/0004-6361:20020478 & c ESO 2002 Astrophysics Spectroscopy of large PAHs Laboratory studies and comparison to the Diffuse Interstellar Bands R. Ruiterkamp1, T. Halasinski2, F. Salama2,B.H.Foing3, L. J. Allamandola2,W.Schmidt4, and P. Ehrenfreund1 1 Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands 2 Space Science Division, NASA-Ames Research Center, Moffett Field CA 94035, USA 3 ESA Research Support Division, ESTEC/SCI-SR, PO Box 229, 2200 AG, Noordwijk, The Netherlands 4 PAH Forschungs Institut, Flurstrasse 17, 86926 Greifenberg/Ammersee, Germany Received 16 January 2002 / Accepted 27 March 2002 Abstract. Polycyclic Aromatic Hydrocarbons (PAHs) are thought to be the carriers of the ubiquitous infrared emission bands (UIBs). Data from the Infrared Space Observatory (ISO) have provided new insights into the size distribution and the structure of interstellar PAH molecules pointing to a trend towards larger-size PAHs. The mid-infrared spectra of galactic and extragalactic sources have also indicated the presence of 5-ring structures and PAH structures with attached side groups. This paper reports for the first time the laboratory measurement of the UV–Vis–NIR absorption spectra of a representative set of large PAHs that have also been selected for a long duration exposure experiment on the International Space Station ISS. PAHs with sizes up to 600 amu, including 5-ring species and PAHs containing heteroatoms, have been synthesized and their spectra measured using matrix isolation spectroscopy. The spectra of the neutral species and the associated cations and anions measured in this work are also compared to astronomical spectra of Diffuse Interstellar Bands (DIBs). Key words. molecular data – methods: laboratory – ISM: lines and bands – ISM: molecules 1. Introduction Douglas & Herzberg 1941); many more molecules have been + detected in the diffuse medium, such as HCO , CO, OH, C2, A substantial fraction of the ca. 120 gas phase molecules that HCN, CN, CS and H2CO (Lucas & Liszt 1997). Models of gas- have been identified in interstellar and circumstellar regions phase reactions in diffuse clouds predict that organic molecules are organic in nature (Ehrenfreund & Charnley 2000). Most can be formed through ion-molecule reactions and neutral- of these molecules have been detected through their rotational neutral reactions in the diffuse medium (Bettens & Herbst lines. Large carbon-bearing molecules (such as polycyclic aro- 1996). Aromatic and unsaturated linear carbon molecules of matic hydrocarbons (PAHs), fullerenes and unsaturated chains) up to 64 C atoms have been predicted by these models (Herbst are also thought to be present in the interstellar medium (ISM). 1995; Ruffle et al. 1999). Reactions in circumstellar envelopes ffi These large molecules are di cult to identify in the radio range and subsequent mixing into the diffuse medium, photochemical and can only be detected through their electronic or vibrational reactions and grain collisions in carbonaceous dust are alterna- signature in the UV-Vis-IR range. tive processes that can also lead to the formation of organic The presence of large aromatic structures is evidenced by molecules. infrared observations of the ISM in our galaxy and in extra- Here, we report the spectroscopy of large PAHs (see galactic environments (see A&A Vol. 315, 1996) and by the Fig. 1). Although extensive laboratory studies have been per- observation of the Diffuse Interstellar Bands (DIBs) in the visi- formed on the spectroscopy of smaller PAHs (i.e., <25 C-atoms ble range (see Herbig 1995 for a review, Tielens & Snow 1995). molecules) in an astrophysical context, almost no information Hydrogenated amorphous carbon (HAC) is considered a po- is available regarding large PAHs (>25 C atoms, for a recent tential carrier for the 2200 Å bump observed in the interstel- review see Salama 1999). lar extinction curve (Mennella et al. 1999), while a variety of complex aromatic networks are likely to be present on car- The PAH molecules shown in Fig. 1 have also been selected bonaceous grains (see Henning & Salama 1998 for a review). for a long duration exposure experiment “Organic Matter in Since the initial discovery of simple diatomic molecules in in- Space” on the International Space Station (ISS) (Ruiterkamp terstellar space (Swings & Rosenfeld 1937; McKellar 1940; et al. 2001). This space experiment will provide quantitative data on the photochemical evolution of specific PAH species Send offprint requests to: R. Ruiterkamp, in irradiated environments. The results will help to constrain e-mail: [email protected] the formation and destruction pathways of large carbonaceous 1154 R. Ruiterkamp et al.: Spectroscopy of large PAHs O They are observed in a spectral range extending from the UV to the near-IR, and they exhibit a large diversity of band strengths and profiles (Herbig 1995). The detection of DIBs in almost all Dinphenanthro Terrylene Ovalene regions of the diffuse ISM indicates that the carriers are chem- (9,10-b;9'-10'-d) ically stable. C30H16 C32H14 furan High resolution surveys of the DIBs indicate that the band strengths depend on the environmental conditions that prevail C H O 28 16 in the ISM (Herbig 1975; Cami et al. 1997; Sonnentrucker et al. 1997). The DIB carriers are strongly influenced by the UV field. The shielding of UV radiation by the outer layers of the clouds leads to a progressive reduction of DIB strength towards the interior of the cloud (Cami et al. 1997). The current consensus based on both astronomical obser- vations and laboratory studies is that the DIB carriers are Decacylene 2,2'-Bis-triphenylyl Quaterrylene gas phase molecules that are chemically stable and carbona- ceous in nature (Herbig 1995; Tielens & Snow 1995; Henning C H C H C H 36 18 36 22 40 20 & Salama 1998; Ehrenfreund & Charnley 2000; Ehrenfreund et al. 2001). The observation of some DIBs in emission in the Red Rectangle (Sarre et al. 1995) and the detection of intrinsic substructures in several of the stronger DIBs marked a breakthrough in the identification of the molecular nature of the DIB carriers (Sarre et al. 1995; Ehrenfreund & Foing 1996; Krelowski et al. 1998; Le Coupanec et al. 1999; Walker et al. Hexabenzo Dicoronylene 2001). (a,cd,fgh,jk,m,qrs) C48H20 peropyrene 2.2. Proposed carbonaceous carriers C44H20 Among the potential DIB carriers are stable carbonaceous Fig. 1. Structural formulae of the Polycyclic Aromatic Hydrocarbon molecules such as PAHs, fullerenes, carbon chains and rings. molecules studied by UV-Vis-NIR spectroscopy. In order to identify the carrier molecules of DIBs, laboratory studies have been carried out on PAHs (Salama et al. 1996; molecules in interstellar and interplanetary media. The sam- Salama et al. 1999) and carbon chains (Freivogel et al. 1994; ple set includes regular compact and non-compact PAHs (i.e., Tulej et al. 1998). There is, however, no definitive identifica- structures made only of benzenoid rings), PAH structures with tion of one specific carrier for any of the 300 known DIBs. It 5-membered rings as well as one oxygen-bearing PAH. seems logical to seek the carriers of the DIBs among interstellar Thus, the objectives of this work are: (i) to explore for the molecules that are both stable and cosmically abundant. In this first time the electronic spectroscopy of large PAH molecules report we focus on PAHs. The spectroscopy of large fullerenes and ions in an environment that is astrophysically relevant for will be discussed in a separate paper. PAHs are believed to comparison with the spectra of DIBs; (ii) to identify promising be the most abundant free organic molecules in space (Puget target molecules for further studies in gas-phase, jet expansion, & Leger 1989; Allamandola et al. 1989) and to be formed in experiments; (iii) to study the photo-stability of these selected the outer atmosphere of carbon stars, or by shock fragmen- species in the laboratory for a comparison with samples to be tation of carbonaceous solid material. The infrared emission exposed on the ISS. bands (UIBs) measured at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7 µm In Sect. 2 we briefly review the evidence pointing toward are ubiquitous in space. The bands are observed in the diffuse the presence of PAHs in the ISM and the potential connection interstellar medium, in circumstellar environments and even in to the DIBs. In Sect. 3 we describe the laboratory setup used external galaxies (Tielens et al. 1999). UIBs have been success- in this study. In Sect. 4 we report and discuss the UV-Vis-NIR fully modeled by combining the infrared laboratory absorption absorption spectra of selected large, neutral and ionized, PAH spectra of neutral and positively charged PAHs (Hudgins & molecules in the astrophysical context. The spectra are finally Allamandola 1999a, b). compared with high resolution DIB data in Sect. 5. PAHs play an important role in the evolution of interstellar gas phase chemistry (Bakes & Tielens 1994). Gaseous PAH 2. The carrier molecules of the diffuse interstellar molecules react according to the environmental conditions. bands Neutral and hydrogenated PAHs are present in dense regions 2.1. The Diffuse Interstellar Bands and regions of high electron density. The local ultraviolet ra- diation field determines their charge and their hydrogenation Diffuse Interstellar Bands (DIBs) are absorption lines that are states (Salama et al. 1996; Sonnentrucker et al. 1997; Vuong & detected in the spectra of reddened stars throughout our galaxy. Foing 2000). Furthermore, the UV-Vis spectra of PAH cations R. Ruiterkamp et al.: Spectroscopy of large PAHs 1155 with up to 100 C atoms show strong absorption bands. This spectral features in the UV-NIR range (180–1200 nm). The new makes PAHs promising candidates for the molecular carriers features are associated with the formation of ions in the matrix of the diffuse interstellar bands.
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