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The so-called aromatic infrared bands are attributed to emission of polycyclic aromatic hydrocarbons. The observed variations toward different regions in space are believed to be caused by contributions of different classes of PAH molecules, i.e. with respect to their size, structure, and charge state. Laboratory spectra of members of these classes are needed to compare them to observations and to benchmark quantum-chemically computed spectra of these species. In this paper we present the experimental infrared spectra of three different PAH dications, naphthalene$^{2+}$, anthracene$^{2+}$, and phenanthrene$^{2+}$, in the vibrational fingerprint region 500-1700~cm$^{-1}$. The dications were produced by electron impact ionization of the vapors with 70 eV electrons, and they remained stable against dissociation and Coulomb explosion. The vibrational spectra were obtained by IR predissociation of the PAH$^{2+}$ complexed with neon in a 22-pole cryogenic ion trap setup coupled to a free-electron infrared laser at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. We performed anharmonic density-functional theory calculations for both singly and doubly charged states of the three molecules. The experimental band positions showed excellent agreement with the calculated band positions of the singlet electronic ground state for all three doubly charged species, indicating its higher stability over the triplet state. The presence of several strong combination bands and additional weaker features in the recorded spectra, especially in the 10-15~$\mu$m region of the mid-IR spectrum, required anharmonic calculations to understand their effects on the total integrated intensity for the different charge states. These measurements, in tandem with theoretical calculations, will help in the identification of this specific class of doubly-charged PAHs as carriers of AIBs., Comment: Accepted for publication in A&A

The mid-infrared emission spectra of a large variety of astronomical objects are dominated by the aromatic infrared bands (AIBs), which are now widely accepted to originate from gaseous polycyclic aromatic hydrocarbons (PAHs). It is believed that the astrophysically most relevant molecules are at least 40–50 carbon atoms in size. Still, the large majority of laboratory experiments have been performed on smaller PAHs, mainly for reasons of experimental limitations and availability. Here, we show that combination of atmospheric pressure chemical ionization (APCI) with a direct insertion probe (DIP) inlet gives efficient access to larger, ionic PAHs for action spectroscopy studies. We present the gaseous IR spectra of two astrophysically relevant large PAHs, hexa-peri-hexabenzocoronene ( C 42 H 19 + ) and dicoronylene ( C 48 H 21 + ) in their protonated form. Compared to their radical cation analogs, the protonated species have a lower dissociation threshold as they can expel a neutral hydrogen radical leaving behind the resonance-stabilized radical cation; provided that the mass spectrometer can resolve precursor and product ions at one amu difference, this generates good quality spectra under multiple-photon dissociation conditions. Quantum-chemical computations at the density functional level are used to support experiments. Despite the apparent similarity of different protonation isomers, their IR spectra are predicted to be remarkably distinct. This facilitates a straightforward identification of the isomers formed experimentally. For both species studied, protonation occurs on the peripheral CH moiety in the ’bay region’ of the molecules. We compare the spectra of the protonated species with those of their radical cation analogs reported previously and briefly discuss the astrophysical relevance.