Gas Phase Absorption Spectroscopy of C+60 and C+70 in a Cryogenic Ion Trap: Comparison with Astronomical Measurements

Recent low-temperature laboratory measurements and astronomical observations have proved that the fullerene cation C60+ 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 C70+, with its origin band at 7959.2 Å, 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 C60+, one further band in the astronomical spectrum at 9348.5 Å is identified, increasing the total number of assigned DIBs to five. Numerous other C60+ 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 C60+ this directly yields a column density, N(C60+) of- 2x10^13 cm-2 in diffuse clouds, without the need to rely on theoretical oscillator strengths. The intensity of the C70+ 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 C70+ will be too weak to be detected in the astronomical spectra, which is confirmed giving an upper limit of 2 mÅ to the equivalent width.