1. Universität des Saarlandes, Im Stadtwald 38, Postfach 15 11 50, 66041 Saarbrücken, Germany.
2. Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, CH-5232 Villingen-PSI, Switzerland.
3. European Synchrotron Radiation Facility, Boite Postale 220, F-38043 Grenoble Cedex, France.