PAH IR and UV_vis peak wavelength

In the CH stretching region, clustering enhances the 3.3um 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 3.3um 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 7-9um 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 6.2um 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. 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 6.2um 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.