Electronic spectra of fullerenes in cryogenic radio frequency ion traps

Abstract

Over several decades interest has been devoted to the astronomical enigma of the diffuse interstellar bands (DIBs). These are hundreds of absorption features of interstellar origin seen in the spectra of stars with different strengths and widths spread over the visible and near infrared (NIR). They are typically broader than atomic lines and concluded to be of molecular nature. Polycyclic aromatic hydrocarbons, long carbon-chain molecules, and fullerenes have been suspected as their carriers. Two of the DIBs showed coincident spectral features recorded in a neon matrix experiment for the fullerene C60+. Embedding molecules in a solid matrix are known to induce perturbations of the measured spectrum and consequently, the assignment was classified as tentative. An unambiguous identification of a specific molecule as a carrier can only be made upon measurements of its laboratory gas-phase spectrum under similar conditions as they are present in the interstellar medium. Nevertheless, the recent identification of the infrared signature of C60, C60+ and C70 in the spectra of a protoplanetary and reflection nebula fueled their relevance as possible candidates. Optical and NIR spectroscopy of large molecules has strong demands on the employed method. Therefore, an existing apparatus was improved and a special spectroscopic technique was thought. The heart of the experiment was a radio-frequency ion trap in which a cryogenic bath of a neutral gas was created to confine and prepare the ionic species for further investigations. Electronic gas-phase spectra have been finally obtained by photofragmentation of weakly bound cation-helium complexes, which enabled a confident confrontation with astronomical observations. In the case of C60+, an unequivocal assignment of five DIBs has been achieved, and thus, the first identification of a carrier almost 100 years after their first detection.