1. Department of Physics, University of Oslo, PO Box 1048, N-0316 Oslo, Norway, Department of Physics, IFM, Chemical Physics, University of Linköping, S581-83, Sweden, Teachers College of Education, PO Box 254, Al-Rustaq 318, Sultanate of Oman, and Department of Chemistry, Lund University, PO Box 124, S-221 00 Lund, Sweden