1. Institute for Experimental Physics, University of Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
2. Deutsches Elekronen Synchrotron (DESY), Notkestraße 85, D-22607 Hamburg, Germany
3. Paul Scherrer Institute (PSI), Ch-5232 Villigen, Switzerland
4. Institute for Physics, University of Rostock, Universitätsplatz 3, D-18051 Rostock, Germany