National Research Council of Canada. National Science Infrastructure
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 10K is reported here for the full wavelength range encompassed by the electronic transition. The absorption spectrum of C⁺₇₀ with its origin band at 7959.2Å 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 Å 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× 10¹³cm⁻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 2mÅ to the equivalent width.