Elsevier

Molecular Astrophysics

Volume 2, March 2016, Pages 12-17
Molecular Astrophysics

An optical spectrum of a large isolated gas-phase PAH cation: C78H26+

https://doi.org/10.1016/j.molap.2015.11.001Get rights and content

Abstract

A gas-phase optical spectrum of a large polycyclic aromatic hydrocarbon (PAH) cation - C78H26+ in the 410610 nm range is presented. This large all-benzenoid PAH should be large enough to be stable with respect to photodissociation in the harsh conditions prevailing in the interstellar medium (ISM). The spectrum is obtained via multi-photon dissociation (MPD) spectroscopy of cationic C78H26 stored in the Fourier Transform Ion Cyclotron Resonance (FT-ICR) cell of the PIRENEA setup using the radiation from a mid-band optical parametric oscillator (OPO) laser.

The experimental spectrum shows two main absorption peaks at 431 nm and 516 nm, in good agreement with a theoretical spectrum computed via time-dependent density functional theory (TD-DFT). DFT calculations indicate that the equilibrium geometry, with the absolute minimum energy, is of lowered, nonplanar C2 symmetry instead of the more symmetric planar D2h symmetry that is usually the minimum for similar PAHs of smaller size. This kind of slightly broken symmetry could produce some of the fine structure observed in some diffuse interstellar bands (DIBs). It can also favor the folding of C78H26+ fragments and ultimately the formation of fullerenes.

This study opens up the possibility to identify the most promising candidates for DIBs amongst large cationic PAHs.

Introduction

The aromatic infrared bands (AIBs), strong emission features at 3.3, 6.2, 7.7, 8.6, 11.2 µm, dominating the near and mid infrared (IR) spectrum of many interstellar sources, are generally attributed to IR fluorescence of large (∼50 C atom) PAH molecules pumped by UV photons (Allamandola, Tielens, Barker, 1989, Genzel, et al., 1998, Puget, Léger, 1989, Sellgren, 1984, Tielens, 2013). These PAH species are found to be ubiquitous and abundant, containing ∼10% of the elemental carbon. They are expected to play an important role in the ionization and energy balance of the ISM of galaxies (Tielens, 2008 and references therein). They have also been proposed as an important catalyst for the formation of molecular H2 in photodissociation regions (Boschman et al., 2015). In addition, PAHs are also considered as potential carriers for the DIBs (Crawford, Tielens, Allamandola, 1985, Léger, d’Hendecourt, 1985, Salama, Galazutdinov, Krelowski, Allamandola, Musaev, 1999). The leading idea in this proposal is that if PAHs are as abundant as they need to be to produce the observed AIBs, and since PAHs have prominent bands in the visible when either ionized or large enough, such bands must contribute to DIBs. The main arguments for PAHs as likely DIB carriers were reviewed relatively recently by Cox (2011), and a critical review of the current status of research in this direction is available by Salama and Ehrenfreund (2013).

Over the years, many experimental, theoretical and observational studies have investigated the spectroscopic properties of PAHs in relation with DIBs, ranging from the smallest species (Salama and Allamandola, 1992) to relatively large ones (see e.g. Weisman, Lee, Salama, Head–Gordon, 2003, Huisken, Rouillé, Steglich, Carpentier, Jäger, Henning, 2013) and to PAH derivatives (see e.g. Hammonds, Pathak, Sarre, 2009, Rouillé, Jäger, Huisken, Henning, 2013). Some effort was spent in trying to study large numbers of PAHs systematically, attempting to single out general trends that would hopefully allow to choose the most promising candidate DIB carriers in the vast population of this chemical family (see e.g. Ruiterkamp, Cox, Spaans, Kaper, Foing, Salama, Ehrenfreund, 2005, Weisman, Lee, Salama, Head–Gordon, 2003, Tan, 2009). It is thus now understood that the onset of absorption due to electronic transitions tends to shift redwards with the increasing size of PAHs, and that very fast non–radiative transitions severely affect the spectral shape of PAH bands, due to lifetime broadening effects (Pino et al., 2011). This latter lifetime broadening effect tends to be much stronger for ions than for neutrals, with the former thereby exhibiting shallow, featureless bands, whereas the latter produce bands with some recognisable rotational envelopes. Notably there are DIBs qualitatively matching both kinds of spectral behaviors (see e.g. Sarre, 2013)

Recently four DIBs were identified as due to the fullerene cation C60+ (Campbell, Holz, Gerlich, Maier, 2015, Walker, Bohlender, Maier, Campbell, 2015), which confirms that large carbonaceous molecules are good carrier candidates for the DIBs. However so far no DIBs could be identified as arising from electronic transitions in PAHs. The reason might be that spectroscopic measurements on PAH cations have concerned relatively small species, whereas chemical models predict that only large PAHs can survive the UV radiation field even in the diffuse ISM where this field is quite diluted (Le Page, Snow, Bierbaum, 2001, Montillaud, Joblin, Toublanc, 2013). Berné and Tielens (2012) have shown that closer to stars even large PAHs are expected to be efficiently photodissociated and this photoprocessing could be at the origin of the formation of C60 in these regions (Berné et al., 2015).

The very large PAH, C78H26 cation, has been studied by Zhen et al. (2014) as a possible precursor of fullerenes. It was selected because its armchair edges give it greater stability than PAHs with zigzag edges (Koskinen, Malola, Häkkinen, 2008, Poater, Visser, Solà, Bickelhaupt, 2007), thereby favoring its presence in space (Candian et al., 2014). But due to the limitations of their experimental set-up, Zhen et al. (2014) had difficulties to observe its photo-fragmentation behavior, especially the dehydrogenation.

We here describe our new study of the photo-fragmentation behavior, and a gas-phase spectrum of cationic C78H26 that we obtained in the range of 410–610 nm. The spectrum of this large PAH cation is compared with TD-DFT calculations to investigate its electronic structure. The experimental methods and results are described in Section 2, the computational methods and results are shown in Section 3. Section 4 compares the experimental and theoretical results, and finally Section 5 summarizes the main results.

Section snippets

Experimental methods and results

We have studied the C78H26 cation using PIRENEA (Piège à Ions pour la Recherche et l’Etude de Nouvelles Espèces Astrochimiques), which is an original home-built experimental set-up conceived to perform photo-physical and chemical studies on large molecules and nano-sized particles of astrophysical interest in isolation conditions approaching those of interstellar space in terms of collisions and interactions (Joblin et al., 2002). One of its advantages is that it can be used to study individual

Computational methods and results

Our theoretical calculations were carried out in the framework of the DFT, in its stationary formulation for molecular properties in the electronic ground state, and in its time-dependent formulation (TD-DFT) to study excited states. In particular, we used the B-LYP gradient-corrected exchange-correlation functional (Lee et al., 1988) in combination with the def2-TZVP Gaussian basis set (Schafer et al., 1993), under the resolution of identity approximation, as implemented in the Turbomole (

Discussion

Fig. 2 compares our MPD spectrum of C78H26+ , obtained with (7.0 ± 1.0) mJ per pulse and 2.0 s irradiation time, with the theoretical one computed by TD-DFT for the molecule in its minimum energy geometry (C2 symmetry). We identify the first band at 431 nm in the MPD spectrum with the band at 423.9 nm (f = 0.099), possibly merged with the very close ones at 426.6 nm (f = 0.019), and 429.0 nm (f = 0.044) in the TD-DFT vertical spectrum. The second band measured at 516 nm would correspond to the

Conclusion

We have succeeded in obtaining an optical spectrum of the isolated giant PAH cation C78H26+. Its strongest transitions in the region studied were found to lie at 431 nm and 516 nm, which does not correspond to any known DIB. Despite drawbacks on band profiles and intensities, the MPD technique is expected to provide a reliable band position for absorption bands and then allows us to identify candidates of interest for the DIBs that could motivate spectroscopic studies with more sophisticated

Acknowledgments

We are grateful to L. Noguès for technical support on the PIRENEA setup and to L. J. Allamandola for providing the chemical sample from the Ames collection. We acknowledge support from the European Research Council under the European Union’s Seventh Framework Programme ERC-2013-SyG, Grant Agreement no. 610256 NANOCOSMOS.

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