2021ApJ...913..136M

The helium-tagging technique was employed to record absorption spectra of cold anthracene cations and protonated anthracene. The evaluation of the spectra of the chromophore with a different number of attached He atoms allows getting the precise band positions of the molecular ions in the gas phase. The positions of the two most intense bands of anthracene, suitable for astrophysical detection, were found to be λ_max = 3478.9 ± 1.8 Å and λ_max = 7068.9 ± 5.7 Å. A considerable shift of the red band position compared to a previous measurement was attributed to a temperature effect. No coincidence of the absorption bands in astrophysical observational spectra was found. This allows estimating the upper limit for the abundance of anthracene cations per H nuclei <10^–9 along the HD 183143 line of sight. We discuss possible reasons for such a low abundance of this molecular ion.