1. Department of Energy and Process Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
2. Department of Physics, University of Helsinki and Helsinki Institute of Physics, P. O. Box 64, FI-00014 Helsinki, Finland
3. Dipartimento di Fisica e Chimica Emilio Segrè, Università degli Studi di Palermo, Via Archirafi 36, I-90123 Palermo, Italy