Decoration of fullerenes with atoms and low-mass molecules

Laboratory measurements are proposed to investigate physisorption of metal atoms and selected molecules on nanometer sized model surfaces. Recently developed experimental techniques and instruments will be utilized and enhanced with additional methods, scanning tunneling microscopy and spectroscopy. High-end computations and simulations provided by closely collaborating theoreticians will be compared with the experimental observations and are essential to interpret and unravel the molecular processes. Physisorption is a widely observed phenomenon and occurs in any solid/fluid or solid/gas system. In contrast to chemisorption it is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption. In the presently proposed studies weakly bound complexes, consisting of one or more fullerenes and various physisorbed atoms or low-mass molecules (e.g. H2 , H2 O, CH 4 ) will be formed upon sequential pickup into superfluid helium nanodroplets. Mono- and multilayer coverage can be realized by controlled experimental settings. Decoration and the formation of molecular ad-layers around these chosen model surfaces will be studied by means of high-resolution mass spectrometry and molecular dynamics calculations. Shell closures are often expressed by magic numbers in cluster size distributions. Evolution of such magic numbers as a function of the temperature of the complex tuned by photo- and collision induced heating will be studied. The effect of the polarizability and the dipole moment of the adsorbates will be investigated and intra-cluster reactions driven by the ionization process compared to mixed clusters that do not contain fullerenes. The expected outcome of these proposed studies will contribute significantly to two scientific and technological topics: Firstly, physisorption of hydrogen and methane is of fundamental relevance for the hydrogen storage technology. The presently proposed investigations are a novel approach to probe the storage capacity and binding energy of different carbon and metal-carbon surfaces. Secondly, the expected results will be of interest for the astrochemical community. The weak binding energy of the adsorbates makes physisorption particularly important at low temperatures. In the interstellar medium, where temperatures are as low as 10K, physisorption of atoms and molecules onto dust grains, including polycyclic aromatic hydrocarbon molecules, is considered as a significant step towards the synthesis of many molecules. Thus most of the adsorbates chosen for the presently proposed experiments are highly abundant in molecular clouds of the interstellar medium.

The surface of fullerenes and clusters of fullerenes represent ideal models to study the adsorption of atoms and low-mass molecules to well-defined structures. A single fullerene resembles very well carbon nanotubes or even a piece of graphene with the advantage that fullerenes can be produced with very high purity (>99.99%). This enables mass spectrometric experiments, where the number of adsorbed species can exactly by determined by simply weighing the complex. The effect of the curvature can be tuned by the size of the fullerene. Grooves between two and dimples between three fullerenes are preferential binding sites for adorbates that interact more strongly with the fullerene surface than among each other. In contrast, species with a dominant interaction among themselves, exhibit weakly perturbed cluster growth at the fullerene surface. Carbon, gold and alkali atoms, however, form strong chemical bonds with fullerenes, leading in the former two cases to the formation of dumbbells and even chains and in the case of alkali metals to salts. High potential for hydrogen storage has been predicted for complexes of C60 with lithium based on theoretical calculations. In the present project we could verify this experimentally for the system C60 and cesium. Micro-solvation of fullerene and coronene ions with helium and hydrogen clearly exhibit geometrical shell closures where the high binding energy to the first layer often results in a solid film where the solvent atoms/molecules occupy geometrically strongly confined positions. However, optical spectroscopy of helium decorated fullerene ions indicate a partial melting of the solid layer upon occupation with additional He atoms. Laser excitation of electronic transitions of C60+ leads to the evaporation of up to several 100 adsorbed He atoms which can easily be observed mass spectrometrically. Our measurements confirm absorption lines of C60+ that were recently determined via photo dissociation of He-tagged fullerene ions in cryogenic ion traps. In this previous study these very same lines were also observed in astronomical spectra from diffuse interstellar clouds. For one of these lines we observed a clear difference for He attached to pentagonal or hexagonal faces. This leads to a pronounced line broadening due to statistical occupation of the two types of faces of the fullerene surface. This new kind of action-spectroscopy is way more efficient and provides deeper insight into the micro-solvation and matrix shifts of the first solvent layer than previous methods. Thus we plan to utilize this advantage in a follow-up project for the search of additional carriers of the 600 known diffuse interstellar bands.