1. Institute for Coastal Research/Physical and Chemical Analysis, GKSS National Research Center, D-21502 Geesthacht, Germany, Universität Rostock, Physics Department, c/o HASYLAB, D-22603 Hamburg, Germany, European Molecular Biology Laboratory, Hamburg Outstation, D-22603 Hamburg, Germany, and Universität der Bundeswehr Hamburg, D-22043 Hamburg, Germany