1. Uzhhorod National University, 88000 Uzhhorod, Ukraine
2. Wigner Research Centre for Physics, Hungarian Academy of Sciences, 1121 Budapest, Hungary
3. Pavol Jozef Šafárik University in Košice, Park Angelinum 9, Košice, Slovak Republic
4. Department of Surface and Plasma Science, Charles University, 18000 Prague 8, Czech Republic
5. Institute of Physics, Academy of Science of the Czech Republic, Na Slovance 2, CZ-182 21 Prague 8, Czech Republic
6. Sincrotrone Trieste S.C.p.A., 34149 Basovizza, Trieste, Italy