1. Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
2. Institute of Physics, University of Tartu, W. Ostwald 1, 50411 Tartu, Estonia
3. Accelerator Laboratory, Department of Physics,
University of Helsinki, P.O. Box 43 (A.I. Virtasen aukio 1),
FI-00014 Helsinki, Finland
4. Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská Cesta 9, 841 04 Bratislava,
Slovakia