Gas-phase Absorption of C²⁺ ₇₀ below 10 K: Astronomical Implications

The electronic spectrum of the fullerene dication C²⁺ ₇₀ has been measured in the gas phase at low temperature in a cryogenic radiofrequency ion trap. The spectrum consists of a strong origin band at 7030 Å and two weaker features to higher energy. The bands have FWHMs of 35 Å indicating an excited state lifetime on the order of onetenth of a picosecond. Absorption cross-section measurements yield (2 ± 1) × 10^-15 cm² at 7030 Å. These results are used to predict the depth of diffuse interstellar bands (DIBs) due to the absorption by C²⁺ ₇₀ . At an assumed column density of 2 × 10¹² cm^-2 the attenuation of starlight at 7030 Å is around 0.4% and thus the detection of such a shallow and broad interstellar band would be difficult. The electronic spectrum of C²⁺ ₆₀ shows no absorptions in the visible. Below 4000 Å the spectra of C₆₀, C⁺ ₆₀ and C²⁺ ₆₀ are similar. The large intrinsic FWHM of the features in this region, ∼200 Å for the band near 3250 Å, make them unsuitable for DIB detection.