Unsolved Problems and Future Directions

Although rather good overall understanding has been reached, there still exist some unsolved problems. Experimentally one would like to go to much lower temperatures than the present 35 K, but no idea is available today how to measure the optical response large, cold clusters. Electron diffraction experiments on mass-selected clusters will soon be available [74], and similar results for inner shell excitation will probably also be done in the near future, especially when the new generation of free-electron lasers becomes available. 

The data discussed here give a fairly complete map of the optical response of sodium cluster ions. The response has been studied as a function of three parameters temperature, size and charge of the cluster. The overall result is that good agreement with the jellium model is obtained. Exceptions are the splitting of the resonances at low temperature and the lifetime/width problem, both of which are influenced by the detailed arrangement of the atoms. The thermodynamic experiments are too new, so that no satisfactory understanding has yet evolved. 

Clearly the data presented are the work of a whole team of people. Contributors have been (in alphabetical order) Christoph Ellert, Thomas Hippier, Bernd von Issendorff, Werner Kronmuller, Robert Kusche, Walter Orlik, Thomas Reiners, Ralph Schlipper Martin Schmidt and Christina Schmitt. Financial support came from the Deutsche Forschungsgemeinschaft through SFB 276. 

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