[1] Univ Complutense Madrid, Fac Quim, Dept Quim Fis, E-28040 Madrid, Spain
[2] CSIC, Inst Estructura Mat, E-28006 Madrid, Spain
[3] Hewlett Packard Corp, Beckman Inst, Urbana, IL 61801 USA
[4] Univ Bielefeld, Fak Phys, D-33501 Bielefeld, Germany
[5] Univ Wurzburg, Inst Phys, D-97074 Wurzburg, Germany