[1] Katholieke Univ Leuven, Lab Solid State Phys & Magnetism, B-3001 Louvain, Belgium
[2] Katholieke Univ Leuven, Dept Chem, B-3001 Louvain, Belgium
[3] Max Planck Gesell, Fritz Haber Inst, D-14195 Berlin, Germany
[4] Clayton State Univ, Dept Nat Sci, Morrow, GA 30260 USA
[5] Tech Univ Berlin, Inst Opt & Atomare Phys, D-10623 Berlin, Germany