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High-Resolution IR Absorption Spectroscopy of Polycyclic Aromatic Hydrocarbons in the 3-Μm Region: Role of Periphery

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High-Resolution IR Absorption Spectroscopy of Polycyclic Aromatic Hydrocarbons in the 3-Μm Region: Role of Periphery High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3-µm region: Role of periphery Elena Maltseva University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands and Annemieke Petrignani1;2, Alessandra Candian, Cameron J. Mackie Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands and Xinchuan Huang3 SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043, USA and Timothy J. Lee NASA Ames Research Center, Moffett Field, California 94035-1000, USA and Alexander G. G. M. Tielens Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands and Jos Oomens Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands and Wybren Jan Buma University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands arXiv:1609.09325v1 [astro-ph.GA] 29 Sep 2016 [email protected] ABSTRACT 1Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands 2University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands 3NASA Ames Research Center, Moffett Field, California 94035-1000, USA 1 In this work we report on high-resolution IR absorption studies that provide a detailed view on how the peripheral structure of irregular polycyclic aromatic hydrocarbons (PAHs) affects the shape and position of their 3-µm absorption band. To this purpose we present mass-selected, high-resolution absorption spectra of cold and isolated phenanthrene, pyrene, benz[a]antracene, chrysene, triphenylene, and perylene molecules in the 2950-3150 cm−1 range. The experimental spectra are compared with standard harmonic calculations, and anharmonic calculations using a modified version of the SPECTRO program that incorporates a Fermi resonance treatment utilizing intensity redistribution. We show that the 3-µm region is dominated by the effects of anharmonicity, resulting in many more bands than would have been expected in a purely harmonic approximation. Importantly, we find that anharmonic spectra as calculated by SPECTRO are in good agreement with the experimental spectra. Together with previously reported high-resolution spectra of linear acenes, the present spectra provide us with an extensive dataset of spectra of PAHs with a varying number of aromatic rings, with geometries that range from open to highly- condensed structures, and featuring CH groups in all possible edge configurations. We discuss the astrophysical implications of the comparison of these spectra on the interpretation of the appearance of the aromatic infrared 3-µm band, and on features such as the two-component emission character of this band and the 3-µm emission plateau. Subject headings: astrochemistry | ISM: molecules | methods: laboratory | techniques: spectroscopic | line: identification 1. Introduction 2000). Although such cooling conditions allow for an increase in spectral resolution as compared to Polycyclic Aromatic Hydrocarbons (PAHs) are room-temperature experiments, they lead at the a family of molecules consisting of carbon and same time to matrix-induced effects that are not hydrogen atoms combined into fused benzenoid well understood and hard to predict. Gas-phase rings. From a chemical and physical point of view studies are much preferred but have so far predom- they have properties that have led to exciting ap- inantly been restricted to IR absorption studies of plications in novel materials (Wan et al. 2012; Sul- hot (1000 K) vaporized PAHs (Joblin et al. 1995, livan & Jones 2008), but at the same time also to 1994) or at best under room-temperature condi- quite a cautious use on account of their health- tions (e.g., Pirali et al. (2009); Can´e et al. (1997)). related impact (Kim et al. 2013; Boffetta et al. Due to their low volatility, high-resolution studies 1997). For astrophysics they play a particularly of low-temperature, isolated PAH molecules has important role since PAHs have been proposed as for a long time remained out of reach with as a no- main candidates for carriers of the so-called aro- table exception the cavity ring down spectroscopy matic infrared bands (AIBs), a series of infrared (CRDS) studies of Huneycutt et al. (2004) on emission features that are ubiquitously observed small PAHs, although contaminations originating across a wide variety of interstellar objects. These from isotopologues and other PAH species or im- emission features are thought to be nonthermal in purities remained a point of concern. nature and arising from radiative cooling of iso- Recently, we have applied IR-UV double res- lated PAHs that have been excited by UV radia- onance laser spectroscopic techniques on PAHs tion (Sellgren 1984). seeded in supersonic molecular beams. In com- Since they offer such a powerful probe for car- bination with mass-resolved ion detection these bon evolution in space, these bands have been sub- techniques allow for recording of mass- and ject to extensive experimental and theoretical re- conformation-selected IR absorption spectra with search with the ultimate aim being a rigorous iden- resonance band widths down to 1 cm−1 (Malt- tification of the molecular structure of the AIB seva et al. 2015). Under such high-resolution con- carriers. Significant progress has been made in ditions, IR absorption spectra of PAHs in the this respect with infrared (IR) studies on PAH 3-µm region turn out to display an unexpected species deposited in a cold (10 K) rare-gas ma- large number of strong bands, and certainly many trix (for example, Hudgins & Sandford (1998a,b, 2 more than expected on the basis of a simple har- mandola et al. 1989). However, to what extent monic vibrational analysis. Such a conclusion is this explanation can account for the appearance more pertinent as theoretical studies of IR spec- of the entire plateau is still far from clear. tra of PAHs are typically performed at the Den- Our previous study aimed at recording spec- sity Functional Theory (DFT) level, using the tra under the highest resolution conditions possi- harmonic approximation for vibrational frequen- ble and applying the appropriate theoretical treat- cies and double harmonic approximation for in- ment including anharmonic effects and resonances. tensities, neglecting the effects of anharmonicity. For that reason, we focused on the spectra of In our previous papers we demonstrated that a the linear PAHs naphthalene (C10H8), anthracene proper treatment of anharmonicity and Fermi res- (C14H10), and tetracene (C18H12). As discussed onances indeed leads to predicted spectra that are above, astronomical spectra likely comprise the in near-quantitative agreement with the experi- contributions of a much larger variety of PAHs. mental spectra, both with respect to the frequen- To advance the interpretation and characteriza- cies of vibrational bands and their intensities. tion of these data, we therefore extend our experi- The shape of the 3-µm band recorded in as- mental and theoretical studies to a wider variety of tronomical observations has been found to vary condensed and irregular isomers containing up to within the same astronomical object and between five rings (phenanthrene C14H10, benz[a]antracene different astronomical objects. To account for C18H12, chrysene C18H12, triphenylene C18H12, these differences, several explanations have been pyrene C16H10 and perylene C20H12). The goals put forward (Sellgren et al. 1990; Tokunaga et al. of these studies are twofold. Firstly, we aim to 1991; van Diedenhoven et al. 2004). One of the understand how the effects of anharmonicity ob- suggestions for classifying the shape of this band served for the linear PAHs are affected by geo- is to interpret it as being associated with emission metrical structure and how this in turn affects the from two components (Song et al. 2003; Candian appearance of the IR absorption spectra in the 3- et al. 2012) in which there are contributions from µm region. Secondly, we aim to uncover general different groups of carriers at 3.28 and 3.30 µm. trends in band shapes that could provide spectral Song et al. (2007) propose that these two compo- signatures that would allow for a much more de- nents originate from groups of PAHs with differ- tailed description of the contribution of different ent sizes, finding support for this in the labora- PAHs. tory high-temperature gas-phase studies of pyrene (C16H10) and ovalene (C32H14) for which a blue 2. Methods shift of the 3-µm band was observed upon increas- ing the size of the PAH (Joblin et al. 1995). An- 2.1. Experimental techniques other factor that has been suggested to contribute IR spectra of cold and isolated molecules were to the apparent two-component appearance of the obtained in molecular beam setup described in emission are differences in the peripheral struc- Smolarek et al. (2011). In these experiments the ture of different PAHs, in particular steric effects sample of interest was placed in an oven attached as occurring for hydrogen atoms at so-called bay- to a pulsed valve (General Valve). Sufficient va- sites (Candian et al. 2012). The influence of the por pressure was found to be obtained when the edge structure has been investigated by means sample was heated to temperatures slightly higher of harmonic DFT calculations for large species than its melting point. Subsequent pulsed expan- (Bauschlicher et al. 2009, 2008) but systematic ex- sion with 2 bars of argon as carrier gas and a typ- perimental high-resolution studies are notoriously ical opening time of 200 µs then led to supersonic lacking. Similarly, it has been found (Geballe et al. cooling of the sample molecules. 1989) that next to the prominent emission band The ground-state vibrational manifold of PAHs at 3.29 µm, a broad plateau is present that spans was probed by UV-IR ion dip spectroscopy. To the 3.1-3.7 µm region and has been indicated as this purpose two-color resonance enhanced mul- the 3-µm plateau. It has been speculated that tiphoton ionization followed by mass-selected ion this plateau might in part derive from anharmonic detection was used to generate an ion signal.
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