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Grzegorz P. Słowik
Institute of Physics, Faculty of Physics and Astronomy, University of Zielona Góra, 4a Szafrana Str., 65-516 Zielona Góra, Poland
Andrzej B. Więckowski
Institute of Physics, Faculty of Physics and Astronomy, University of Zielona Góra, 4a Szafrana Str., 65-516 Zielona Góra, Poland and Institute of Molecular Physics of the Polish Academy of Sciences, 17 Smoluchowskiego Str., 60-179 Poznań, Poland