1. Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Sts. Cyril and Methodius University, POB 162, 1000 Skopje, Macedonia, Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, 1784 Sofia, Bulgaria, and Space Research Institute, Bulgarian Academy of Sciences, P. O. Box 799, 1000 Sofia, Bulgaria