[1] Tech Univ Dresden, Inst Oberflachen & Mikrostrukturphys, D-01062 Dresden, Germany
[2] St Petersburg State Univ, Inst Phys, St Petersburg 198904, Russia
[3] Free Univ Berlin, Inst Expt Phys, D-14195 Berlin, Germany
[4] BESSY GmbH, D-12489 Berlin, Germany