C₈₄: A prototype of larger fullerenes. laboratory spectroscopy and astronomical relevance

We present for the first time the UV/Vis spectrum of the neutral and ionized fullerene C₈₄ isolated in a neon matrix at low temperature. After ionization with high energy (10.2 eV) UV photons, we are able to identify a new spectroscopic absorption band at 718.2 nm that we tentatively attribute to the C₈₄⁺ ion. We compare the optical spectrum to the diffuse interstellar absorption bands (DIBs) and find eight possible DIBs that fall within the expected shift induced by the solid matrix (± 5 nm) of the assigned C₈₄⁺ absorption band at 713.6, 713.8, 715.4, 716.1, 718.0, 722.3, 722.4, and 722.8 nm. The astronomical C₈₄ ⁺ column density that is required to account for the observed strength of these DIBs is calculated assuming the oscillator strength of the C₈₄⁺ transition obtained from the laboratory spectrum (f = 0.004). We find that 0.08% and 0.06% of the cosmic carbon abundance would be required to reproduce the 713.6 and 715.4 nm DIB, respectively. A ratio of cosmic C₈₄⁺ to C₆₀⁺ of ∼7% or ∼5% is derived based on the assignment of C₈₄⁺ to the weakest DIBs at 713.6 or 715.4 nm, respectively. Both ratios are in good agreement with the C₈₄⁺ to C₆₀⁺ ratio measured in the laboratory under various experimental conditions.