The Search for Carriers of Diffuse Interstellar Bands

In 1922 Mary Lea Heger discovered absorption lines in the light of stars passing through diffuse interstellar clouds. In the meantime, almost 600 lines have been reported and only very recently the first five of them could be assigned to C60+. In the presently proposed joint project two research groups, one from Innsbruck and one from Jena, plan to identify additional carriers for the diffuse interstellar bands (DIBs). Several astronomical observations indicate a molecular origin of the DIBs and carbon-bearing molecules, such as polycyclic aromatic hydrocarbons or fullerenes, are promising candidates. Carriers of DIBs have to withstand the harsh conditions in space and thus will not dissociate by the UV light passing through these diffuse clouds. However, these heavy molecules will be efficiently ionized by the high energy photons. Thus, molecular ions, such as C60+ are expected to be carriers for DIBs. The low density of ions requires special spectroscopic techniques. In order to probe the absorption of a photon an indirect method will be utilized. Weakly bound complexes of promising ions with a He atom or a hydrogen molecule are formed and the loss of the tag will be measured as a function of the wavelength of the light. He tagged ions will be formed upon electron ionization of doped helium nanodroplets. Besides charged dopants, also clusters and complex reaction products such as fullerene dumbbells are formed. A highly efficient technique developed in Innsbruck allows to investigate whole series of ions in a few days. Absorption features, discovered in such survey scans, will be analyzed with higher precision by the German partner in Jena and compared with astronomical observations. The background of the Jena team in astrophysics and astrochemistry and an existing collaboration between Jena and the group of Manfred Kappes in Karlsruhe are essential to identify interesting molecules that will be investigate in the presently proposed studies. Two PhD students will perform these experiments utilizing an existing instrument and a new instrument that will be constructed during the first year of this project and that is optimized for the laser spectroscopy of complex ions. The two PhD students will be supported and guided by the applicant and experienced postdoc researchers working in the group in Innsbruck, as well as the German partner in Jena.

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. Astronomers have suggested that dust or carbonaceous molecules are the most likely source of these absorption lines. 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 were 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 measured absorption spectra of several polycyclic aromatic hydrocarbon (PAH) ions by means of action spectroscopy utilizing helium tagging. Besides commercially available samples, we obtained two PAH molecules from the group of Prof. Holger Bettinger from Tübingen. Tagging of ions with helium has been achieved utilizing cryogenic ion traps, cryogenic flow tubes and helium nanodroplets. During the present project, 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. Unfortunately, none of the absorption lines measured for the selected molecules, matches DIBs reported in astronomical observations. This enables us to exclude these PAH ions as carriers of DIBs. The majority of the scientific results has been published in prestigious journals and more will be published soon. In addition, the results obtained in this project were presented due to the pandemic mostly in 2022 at several international conferences.