[1] Univ Cologne, Inst Phys 2, D-50937 Cologne, Germany
[2] Univ Augsburg, D-86135 Augsburg, Germany
[3] Univ Cologne, Inst Theoret Phys, D-50937 Cologne, Germany
[4] Univ Paris 11, Lab Physicochim, F-91405 Orsay, France
[5] Rhein Westfal TH Aachen, Inst Phys 2, D-52056 Aachen, Germany