The Search for Carriers of Diffuse Interstellar Bands

More than 100 years ago the astronomer Mary Lea Heger discovered some awkward lines in the spectrum of a star. These lines were much broader and diffuse compared to lines originating from the atmosphere of a star. In the meantime, more than 500 of these so-called diffuse interstellar bands (DIBs) have been discovered and only for five of them a carrier could be assigned, i.e., the cation C60+. In the present project we are searching for additional carriers of DIBs by comparing absorption spectra of potential ions with astronomical observations. Together with our German partner, Dr. Serge Krasnokutski from Jena, we will measure absorption spectra of various ions by means of action spectroscopy utilizing helium tagging. Currently, we consider long linear Cn chains and alkenes, which are fully hydrogenated C n chains as the most promising candidates for the role of DIB carriers. Therefore, the main focus will be given to the spectroscopy of these classes of molecules. However, other molecular classes like PAHs, fullerene derivatives, COMs, and diamondoids will be also considered. Tagging of ions with helium has been achieved utilizing cryogenic ion traps, cryogenic flow tubes and helium nanodroplets. Recently, we discovered a novel method for efficient formation of helium tagged ions of both polarities. In the case of anions, this is so far the only existing method that provides ion yields sufficiently high to perform spectroscopy. The yield of He tagged ions upon pickup into highly-charged ions is several orders of magnitude higher compared to electron ionization of neutral doped helium droplets. This enables shorter acquisition times at a simultaneous increase of sensitivity. Since hundreds of ions can be formed in every helium droplet, this method is also beneficial for the formation of cluster ions and nanoparticles. This can be achieved by simply increasing the particle density of the dopants. A European patent was granted based on the results obtained from this project. Ions embedded in helium droplets can be liberated either by gentle evaporation of the droplets or by collisions with a clean surface. In both cases the majority of ions is solvated with He atoms. The low binding energy of He to ions is a measure for the temperature of the complex. For typical ions this temperature is a few Kelvin and thus similar to the temperatures measured in interstellar clouds.