2016ApJ...822...17C

Recent low-temperature laboratory measurements and astronomical observations have proved that the fullerene cation {C}_{60}^+ is responsible for four diffuse interstellar bands (DIBs). These absorptions correspond to the strongest bands of the lowest electronic transition. The gas phase spectrum below 10 K is reported here for the full wavelength range encompassed by the electronic transition. The absorption spectrum of {C}_{70}^+ , with its origin band at 7959.2 A , has been obtained under similar laboratory conditions. Observations made toward the reddened star HD 183143 were used in a specific search for the absorption of these fullerene cations in diffuse clouds. In the case of {C}_{60}^+ , one further band in the astronomical spectrum at 9348.5 A is identified, increasing the total number of assigned DIBs to five. Numerous other {C}_{60}^+ absorptions in the laboratory spectrum are found to lie below the astronomical detection limit. Special emphasis is placed on the laboratory determination of absolute absorption cross-sections. For {C}_{60}^+ this directly yields a column density, N({C}_{60}^+) , of 2×{10}^{{13} {cm}}{-2} in diffuse clouds, without the need to rely on theoretical oscillator strengths. The intensity of the {C}_{70}^+ electronic transition in the range 7000-8000 Å is spread over many features of similar strength. Absorption cross-section measurements indicate that even for a similar column density, the individual absorption bands of {C}_{70}^+ will be too weak to be detected in the astronomical spectra, which is confirmed giving an upper limit of 2 mA to the equivalent width.