Cluster Dynamics
Prof. Dr. Udo Buck

Single molecules can be generated, depending on the production method (adiabatic expansion or pick-up), either embedded inside or sitting on the surface of a large host cluster. Experiments have been carried out for HBr molecules and argon clusters which were dissociated by laser radiation of 193 nm and  243 nm. The formed H-atoms are detected by multi-photon ionization in a time-of-flight mass spectrometer and their kinetic energy is measured as a function of cluster size. In this way the different dissociation channels as well as their angular dependence and their branching ratios are determined. The influence of the cluster environment, e.g. the cage effect, manifests itself in totally slowed down H-atoms. The systems under investigation are  HBr, HI, and HCl molecules, and small HBr clusters adsorbed on the surface or embedded in different rare gas clusters.The cage effect depends critically on the site of the molecule and on the mass of the cage. In addition, interesting energy transfer effects in different vibrational states are observed resulting from collisions of fast H atoms with neighbored molecules. We observed an unusual behavior for Ne and Xe host clusters. In the case of Ne, the outer shell turned out to be liquid. For Xe, the time-of-flight distributions exhibited an asymmetric behaviour. This is attributed to the formation of HXeI and HXeCCH.  molecules, which were recently observed in matrices. We detect them by their orientation in coupled weak electric and strong laser fields and by their subsequent photodissociation observing the outgoing H atom. In the meantime the experiments  were continued with systems which are important in atmospheric chemistry solvated in or adsorbed on the surface of water clusters. First experiments of HCl on ice particles have already been carried out. HCl dissociates at the surface of the water clusters and after electronic excitation the radical H₃O is formed. Furthermore pyrrole, imidazol, and acetylene were investigated. The experimental arrangement has also been used for measuring the fragmentation probabilities by electron impact ionization. In this way the heavy rare gases and  methyl lactate as well as pyrrole  clusters have been investigated.