1. Department of Chemical Physics, Chemical Center, Lund University, S-221 00 Lund, Sweden, Institute of Molecular and Atomic Physics, 70 F. Skaryna Avenue, 220072 Minsk, Belarus, Department of Biochemistry and Biophysics, Göteborg University, S-413 90 Gothenburg, Sweden, and Department of Physical Chemistry, Chalmers University of Technology S-412 96 Gothenburg, Sweden