1. Physikalisches Institut, Universität Freiburg, D-79104 Freiburg, Germany
2. Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland
3. Department of Chemistry, University of Copenhagen, 2100 Copenhagen, Denmark
4. Grenoble High Magnetic Field Laboratory, CNRS, BP 166, 38042 Grenoble Cedex 9, France