Modification of spectral properties of the PAH cations upon cluster formation

¹ Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria
² University of Jena, Helmholtzweg 3, 07743 Jena, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 February 2026
Accepted: 25 May 2026

Abstract

Context. Polycyclic aromatic hydrocarbons (PAHs) and their clusters are widely invoked as carriers of unidentified infrared bands (UIBs) and are abundant in the interstellar medium (ISM). Although the spectra of individual PAHs have been extensively studied, the spectroscopic properties of ionized PAH clusters remain poorly constrained.

Aims. We investigate how clustering affects the spectral properties of PAH cations and assess the implications of their contribution to the observed infrared (IR) emission and their detectability in astronomical observations.

Methods. Helium-tagging action spectroscopy was performed on cationic clusters of naphthalene, anthracene, and hexabenzocoronene in different spectral ranges. The experimental spectra were analyzed with the support of quantum chemical calculations.

Results. Cluster formation strongly modifies PAH spectra. In the CH stretching region, clustering enhances the 3.3 μηι absorption band by more than an order of magnitude while preserving its position, confirming this band as a robust tracer of both isolated and clustered PAHs. Combination bands contribute significantly to the total intensity of the 3.3 μm band and may be efficiently excited at lower internal energies than fundamental modes. In the middle IR range, the clustering enhances and broadens the vibrational bands and shifts the features in the 7–9 μm region, providing a match with the observed emission in this range. In contrast, clustering has little impact on the position of the bands near the observed 6.2 μm UIBs, which remain significantly redshifted relative to the position of the UIB. This suggests that small classical PAH cations are unlikely carriers of this feature. The match of the positions could be expected for considerably larger PAH cations. However, for such large PAHs, electronic transitions appear in the IR range even for monomeric cations, producing broad and very intense absorptions, which could contribute to the observed infrared continuum emission. Absorption in the ultraviolet (UV), visible (Vis), and near IR (NIR) ranges is strongly attenuated with clustering.

Conclusions. Ionized PAH clusters exhibit spectroscopic signatures that differ significantly from those of isolated PAHs, which must be explicitly considered in models of interstellar emission. The observed mismatch with the 6.2 μm band suggests that small classical PAH cations and their clusters are unlikely to be carriers of the UIBs. Furthermore, the absence of narrow absorption features in the UV–Vis–NIR spectral ranges indicates that PAH cation clusters are also poor candidates for carriers of the diffuse interstellar bands. Only substantially larger PAH cations than those investigated in the present study, as well as their clusters, may exhibit spectral properties consistent with the observed UIBs. Such large PAH cations are also expected to display broad continuum emission in the infrared region arising from electronic transitions.