[1] SLAC Natl Accelerator Lab, Menlo Pk, CA USA
[2] Univ Hamburg, Inst Expt Phys, Hamburg, Germany
[3] Stockholm Univ, Dept Phys, S-10691 Stockholm, Sweden
[4] Helmholtz Zentrum Berlin, Methods & Instrumentat Synchrotron Radiat GI2, Berlin, Germany
[5] Fritz Haber Inst, Dept Phys Chem, D-1000 Berlin, Germany