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 $C_{60}^{+}$ 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 $7959.2\mathring{A}$, has been obtained under similar laboratory conditions. Observations made toward the reddened star $HD183143$ 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 $9348.5\mathring{A}$ 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, $N\left(C_{60}^{+}\right)$, of $2\times {10}^{13}{cm}^{-2}$ 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 $2m\mathring{A}$ to the equivalent width.