1. Contribution from the Department of Molecular Spectroscopy, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands, Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, Department of Biochemistry, University of Oslo, P.O. Box 1041 Blindern, N-0316 Oslo, Norway, Department of Biophysics, Stockholm University, S-106 91 Stockholm, Sweden, and High Magnetic Field Laboratory, CNRS, B.P. 166, F-38042 Grenoble Cedex 9, France