Cluster Dynamics
Prof. Dr. Udo Buck

The neutral molecular clusters are size selected by momentum transfer from an atomic beam and dissociated by the excitation of intra-molecular vibrations. The resulting depletion spectra are used to determine the underlying structure and binding forces of the clusters. Up to now mainly hydrogen bonded systems of methanol, hydrazine and acetonitrile up to n=13 were investigated. They exhibit cyclic structures, 3D-networks, and antiparallel dipole arrangements, respectively. Then  water clusters  from n=6 to n=10 were studied. This size range allowed us for the first time to measure 3D structures of 3-coordinated water molecules, which can all be derived from the cubic octamer structure.  For the nonamer, also a spectrum of a liquid-like cluster was observed at a temperature of 186 K. For the hexamer according to new calculations the cage and the book isomer were observed in the temperature range of the experiment from 40 to 60 K. Recently in cooperation with the Suhm group, these measurement have been extended to glycolate and lactate clusters which play a role in chiral recognition.

The pick-up arrangement also allows us to obtain nearly fragmentation free mass spectra by doping the clusters with one sodium atom. With the help of this method we determined reliable size distributions for rare gas, water, and ammonia clusters which were correlated by scaling laws [198]. When these signals are coupled with IR-excitation so that signal enhancement results, size selected IR-spectra of Na(H₂O)n clusters result [230]. We continued the experiments of the ionization potential of sodium doped ammonia and methanol clusters as function of size.  While methanol behaves in the same way as water, a decrease of the ionization potential with increasing size up to the closing of the first solvation shell and then a constant behavior, ammonia continues to decrease up to the bulk limit. For water a further isomers with lower binding energy was discovered [242]. Together with the results from the dissociation and the IR-spectra a complete picture of the different behaviour of the solvated electron results [248].

Very recently, we succeeded in developing a new method by extending the IR-UV arrangement which allows us to measure size selective IR spectra of water clusters up to n=500. In this way the crystallization was determined to start around n=275 and and to finish at n=475. The results were published in Science [249].