Disentangling the isomeric structure of C7H7+ is a longstanding experimental issue. We report here the full mid-infrared vibrational spectrum of C7H7+ tagged with Ne obtained with infrared-predissociation spectroscopy at 10 K. Saturation depletion measurements were used to assign the contribution of benzylium and tropylium isomers and demonstrate that no other isomer is involved. Recorded spectral features compare well with density functional theory calculations. This opens perspectives for a better understanding and control of the formation paths leading to either tropylium or benzylium ions.

The first infrared predissociation spectra of the pyrene and doubly-dehydrogenated pyrene cations (C16H10˙+; Py+ and C16H8˙+; ddPy+) are presented. The vibrationally resolved spectrum of ddPy+ is dominated by absorption features from two isomers.

The so-called aromatic infrared bands (AIBs) are attributed to emission of polycyclic aromatic hydrocarbons (PAHs). The observed variations toward different regions in space are believed to be caused by contributions of different classes of PAH molecules, that is to say 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 (IR) 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 (EI) 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 (DFT) 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 µm 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.

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The Cover Feature shows C7H7+ produced by dissociative ionization of toluene. Combining cryogenic trapping with infrared pre‐dissociation spectroscopy, two structures could be identified: the benzylium and tropylium ions. More information can be found in the Communication by P. Jusko et al. on page 3182 in Issue 23, 2018 (DOI: 10.1002/cphc.201800744).

Polycyclic aromatic hydrocarbons (PAHs) are thought to be a major constituent of astrophysical environments, being the carriers of the ubiquitous aromatic infrared bands (AIBs) observed in the spectra of galactic...

Photoionization and dissociation of anthracene, phenanthrene, and pentacene in the multiphoton regime at 266 nm wavelength were performed. (n = 4–6) ions were observed as fragments, and their yields as a function of laser intensity were studied. H-migration and “roaming” mechanisms leading to the formation of these fluxional ions from polycyclic aromatic hydrocarbons (PAHs) are proposed. The present results show higher photostability of PAHs with bent structure and suggest PAHs in the UV regions of interstellar medium as a prominent source for (n = 4–6).

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