GAS PHASE ABSORPTION SPECTROSCOPY OF AND IN A CRYOGENIC ION TRAP: COMPARISON WITH ASTRONOMICAL MEASUREMENTS*

Abstract

ABSTRACT Recent low-temperature laboratory measurements and astronomical observations have proved that the fullerene cation 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 is reported here for the full wavelength range encompassed by the electronic transition. The absorption spectrum of , with its origin band at , has been obtained under similar laboratory conditions. Observations made toward the reddened star were used in a specific search for the absorption of these fullerene cations in diffuse clouds. In the case of , one further band in the astronomical spectrum at is identified, increasing the total number of assigned DIBs to five. Numerous other 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 this directly yields a column density, , of in diffuse clouds, without the need to rely on theoretical oscillator strengths. The intensity of the 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 will be too weak to be detected in the astronomical spectra, which is confirmed giving an upper limit of to the equivalent width.