Gas phase electronic spectra of pure carbon cations generated by laser vaporization of graphite in a supersonic jet and cooled to below 10 K and tagged with helium atoms in a cryogenic trap are presented. The measured C 2 n + –He with n from 6 to 14, are believed to be monocyclic ring structures and possess an origin band wavelength that shifts linearly with the number of carbon atoms, as recently demonstrated through N 2 tagging by Buntine et al. ( 34879679 J. Chem. Phys. 2021 155 214302 ). The set of data presented here further constrains the spectral characteristics inferred for the bare C 2 n + ions to facilitate astronomical searches for them in diffuse clouds by absorption spectroscopy.

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The electronic spectrum of the endohedral fullerene He@C + 60 observed by messenger spectroscopy in a cryogenic ion trap is presented. The role played by the messanger tag in the adopted experimental method is evaluated by recording spectra of He@C + 60 − He n with n = 1 − 4. The results indicate a linear shift of ∼ 0.7Å in the wavelengths allowing accurate gas phase values to be reported. The presence of the helium inside the cage shifts the absorption bands by 2 − 3Å toward shorter wavelengths compared to C + 60. The magnitude of this displacement will enable searches for the spectral signatures of this fullerene analogue in interstellar environments by absorption spectroscopy. The implications for potential astronomical detection are discussed.

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