Gas-phase Absorption of C702+ Below 10 K: Astronomical Implications

The electronic spectrum of the fullerene dication C-70(2+) 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 angstrom and two weaker features to higher energy. The bands have FWHMs of 35 angstrom indicating an excited state lifetime on the order of one-tenth of a picosecond. Absorption cross-section measurements yield (2 +/- 1) x 10(-15) cm(2) at 7030 angstrom. These results are used to predict the depth of diffuse interstellar bands (DIBs) due to the absorption by C-70(2+). At an assumed column density of 2 x 10(12) cm(-2) the attenuation of starlight at 7030 angstrom is around 0.4% and thus the. detection of such a shallow and broad interstellar band would be difficult. The electronic spectrum of C-60(2+) shows no absorptions in the visible. Below 4000 angstrom the spectra of C-60, C-60(+) and C-60(2+) are similar. The large intrinsic FWHM of the features in this region, similar to 200 angstrom for the band near 3250 angstrom, make them unsuitable for DIB detection.