2023A&A...672A...4K

Context. The absorption bands of the interstellar medium (ISM) in the optical and near-infrared regions called the diffuse interstellar bands (DIBs) have been known for almost a century, yet their origins remain largely unknown. Knowledge of molecular carriers of DIBs would allow for a much better understanding of the chemistry and physics of the ISM. Polycyclic aromatic hydrocarbons (PAHs) and, among them, polyacenes have been suggested as promising candidates for carriers of DIBs.
Aims. In this paper, we report on the spectroscopy of heptacene (Hep), the polyacene molecule consisting of seven aromatic rings in a linear arrangement, in its cationic and anionic forms (Hep^±). The performed spectroscopic studies made it possible to accurately determine the Hep^± absorption band positions and to conduct a direct comparison of laboratory and observational spectra.
Methods. We utilized helium-tagging action spectroscopy to measure the spectra of Hep^± in a wide spectral range of 3000-13 000 Å. In most cases, the spectra obtained by this method can be directly compared with the observational spectra. By analyzing the spectral shift as a function of the number of attached helium atoms, we obtained precise estimates of the gas-phase band positions. Quantum-chemical computations were used to support and interpret the findings. Matrix isolation spectroscopy provided information on the spectral properties of neutral Hep and extended the spectral range for Hep⁺.
Results. We found several absorption bands characterized by a rather large full width at half maximum in the spectra of Hep^±. The two most intense bands were found at 4714 ± 5 Å and 12 250 ± 12 Å for Hep⁺ and at 4673 ± 14 Å and 11326 ± 4 Å for Hep^–. We did not find any good match between laboratory and observational spectra. In particular, the intrinsic width of the absorption bands of Hep^± is much higher than that of most observed DIBs.
Conclusions. The non-detection of Hep^± in the observational spectra excludes the bottom-up formation route for polyacenes in the ISM. Larger polyacene molecules could still be considered as potential carriers of DIBs in the case of an efficient top-down formation route. All currently measured polyacene ions exhibit relatively broad absorption bands. Therefore, additional spectroscopy studies of neutral polyacenes and larger polyacene ions as well as the study of possible top-down formation routes are suggested.