He-tagged action spectroscopy of C70 fullerene oligomers

Fullerenes are among the most abundant carbon-bearing molecules in the interstellar medium. Their ionic forms, as well as clustered structures, might exhibit strong absorption features in the infrared spectral region. Experimental spectra of cold molecules in the gas phase are crucial for matching astrophysical observations with the corresponding compounds. In this work, employing He-tagging spectroscopy, we present the first gas-phase spectra of the cationic and anionic C70 dimers, (C70)2±, in the spectral range between 4.5 and 12 μm. Quantum chemical calculations are employed to interpret the experimental spectra in terms of plausible dimer structures, indicating the coexistence of multiple isomers that contribute to the observed vibrational features. In contrast to the weakly absorbing C70± monomers, the strong absorptions, especially toward lower wavelengths below 6 μm, indicate that (C70)2± would be, in principle, detectable in astrophysical observations. While monomeric fullerenes such as C60+ and C70+ have been identified in several photodissociation regions, our data suggest that the formation of dimers is unlikely under astrophysical conditions, as their characteristic bands are absent in observational IR spectra. The experimental vibrational bands do not align with the strongest features of the unidentified infrared bands in the 6.5-9 μm range, where current assignments for PAHs and fullerenes remain debated.