Cluster Dynamics
Prof. Dr. Udo Buck

To investigate the reactivity of sodium clusters, a crossed molecular beam experiment is carried out with mass, angular, and energy resolution after the collision. The products are detected by by single photoionization in a time-of-flight mass spectrometer. Up to now we have measured the oxidation of sodium clusters which leads to metal rich monoxides and dioxides. The reaction of sodium clusters with water clusters produces in these single collisions experiments only products of the type Na(H₂O)n. Very recently we succeeded in observing also products of the type Na(NaOH)₂(H2O)_n. In this case a pick-up arrangement with multiple collisions was used and at least 3 Na atoms and 6 H₂O molecules were necessary, a results which was also confirmed by extensive calculations in the Parinello group. By increasing the amount of sodium we have measured the solvation properties of water and ammonia clusters which differ completely [217].

An exciting application of the cluster mass spectra of these system has been the clarification of the results of the Cassini mission on the cryovulcanic plumes of the Enceladus moon of Saturn. The results of the Heidelberg cosmic dust team agree qualitatively with our cluster experiments. Na poor ice particles exhibit Na(H₂O)_n structures, while Na rich grains contain Na(NaOH)_k(H₂O)_n species. The presence of Na is a direct indication of liquid water in the Enceladus. Therefore we repeated the experiment in a liquid beam in the Abel group and observed also the predicted sodium chloride and carbonate salts. The results were published in Nature [237] and found a lot of attention in the press in Germany .

In a further experiment the binding energy of the solvated electron was first measured in liquid water in a micro jet [241]. For the production of the solvated electron from a Kalium-Ironcyanide solution, a value of 3.3 eV was obtained for the inner part of the liquid. For the production by two-photon-excitation in pure water 1.6 eV was measured.  Because of the special form of excitation a rydberg state with a broad electron distribution is obtained. The electron stays at the surface in agreement with extrapolated values form cluster anions, which also display a broad electron distribution.