1. Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
2. Department of Chemistry, University of Bergen, P.O. Box 7803, N-5020 Bergen, Norway
3. Department of Chemistry, University of Milano, via C. Golgi 19, Milano 20133, Italy
4. Bulgarian Academy of Sciences, Sofia 1040, Bulgaria
5. Department of Solid State Physics, University of Sofia, J. Bourchier str. 5, Sofia 1164, Bulgaria