2020ApJ...897...88C

The electronic spectrum of the endohedral fullerene He@C₆₀⁺ observed by messenger spectroscopy in a cryogenic ion trap is presented. The role played by the messenger tag in the adopted experimental method is evaluated by recording spectra of He@C₆₀⁺-He_n with n = 1-4. The results indicate a linear shift of ∼0.7 Å in the wavelengths allowing accurate gas phase values to be reported. The presence of the helium inside the cage shifts the absorption bands by 2-3 Å toward shorter wavelengths compared to C₆₀⁺. 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.