Can neutral and ionized PAHs be carriers of the UV extinction bump and the diffuse interstellar bands?

TL;DR

This study investigates whether neutral and ionized PAHs can explain the UV extinction bump and DIBs, finding they likely account for the UV bump but not for the DIBs, based on experimental and theoretical analyses.

Contribution

The paper provides the first laboratory and theoretical analysis of medium-sized PAHs in both neutral and ionized states as carriers of interstellar UV features.

Findings

UV absorption bands broaden and shift slightly upon ionization.

The UV bump position remains stable across different PAH states.

PAHs are unlikely carriers of the DIBs.

Abstract

Up to now, no laboratory-based study has investigated polycyclic aromatic hydrocarbon (PAH) species as potential carriers of both the diffuse interstellar bands (DIBs) and the 2175 A UV bump. We examined the proposed correlation between these two features by applying experimental and theoretical techniques on two specific medium-sized/large PAHs (dibenzorubicene C30H14 and hexabenzocoronene C42H18) in their neutral and cationic states. It was already shown that mixtures of sufficiently large, neutral PAHs can partly or even completely account for the UV bump. We investigated how the absorption bands are altered upon ionization of these molecules by interstellar UV photons. The experimental studies presented here were realized by performing matrix isolation spectroscopy with subsequent far-UV irradiation. The main effects were found to be a broadening of the absorption bands in the UV combined with slight red shifts. The position of the complete pi - pi* absorption structure around 217.5 nm, however, remains more or less unchanged which could explain the observed position invariance of the interstellar bump for different lines of sight. This favors the assignment of this feature to the interstellar PAH population. As far as the DIBs are concerned, neither our investigations nor the laboratory studies carried out by other research groups support a possible connection with this class of molecules. Instead, there are reasonable arguments that neutral and singly ionized cationic PAHs cannot be made responsible for the DIBs.