Unravelling the structure of monocyclic carbon ring cations using vibrational spectroscopy

Laser ablation of graphite produces a diverse range of molecular structures including chains, rings and fullerenes. Whilst several of these species have been spectroscopically characterised and detected in the interstellar medium, until recently, the spectra of many monocyclic rings were unknown. In the infrared (IR), action spectra of C_4n⁺ rings (n = 5-11) have been shown to exhibit giant IR resonances, which arise from a Kekulé-type vibrational mode. However, the structure and spectroscopic properties of smaller C_4n⁺ rings are unexplored. In this study, the IR spectra of C_4n⁺ rings (n = 2-7) have been recorded using He-tagging spectroscopy. The most intense IR mode of the larger monocyclic rings (n = 4-7) show systematic size-dependent trends, where a linear redshift and increase in intensity is experimentally observed as the ring size increases. A breakdown in these trends is observed for smaller clusters - whilst C₁₂⁺ deviates slightly, C₈⁺ shows more substantial differences, indicating a departure from their common structure. Density functional theory calculations suggest these observations are largely due to changes to the bond length alternation of the polyynic species, however, the exact structures are challenging to model using these methods. The results provide new insights into the geometric structure of carbon cluster cations along with the requisite spectroscopic data to search for these species in astrophysical environments.